I. Faloona - Academia.edu (original) (raw)

Papers by I. Faloona

Research paper thumbnail of Seasonal differences in the photochemistry of the South Pacific: A comparison of observations and model results from PEM-Tropics A and B

Journal of Geophysical Research: Atmospheres, 2001

A time-dependent photochemical box model is used to examine the photochemistry of the equatorial ... more A time-dependent photochemical box model is used to examine the photochemistry of the equatorial and southern subtropical Pacific troposphere with aircraft data obtained during two distinct seasons: the Pacific Exploratory Mission-Tropics A (PEM-Tropics A) field campaign in September and October of 1996 and the Pacific Exploratory Mission-Tropics B (PEM-Tropics B) campaign in March and April of 1999. Model-predicted values were compared to observations for selected species (e.g., NO2, OH, HO2) with generally good agreement. Predicted values of HO 2 were larger than those observed in the upper troposphere, in contrast to previous studies which show a general underprediction of HO 2 at upper altitudes. Some characteristics of the budgets of HOx, NOx, and peroxides are discussed. The integrated net tendency for 03 is negative over the remote Pacific during both seasons, with gross formation equal to no more than half of the gross destruction. This suggests that a continual supply of 03 into the Pacific region throughout the year must exist in order to maintain 03 levels. Integrated net tendencies for equatorial 03 showed a seasonality, with a net loss of 1.06x 10 • molecules cm -2 s '1 during PEM-Tropics B (March) increasing by 50% to 1.60x 10 TM molecules cm -2 s -1 during PEM-Tropics A (September). The seasonality over the southern subtropical Pacific was somewhat lower, with losses of 1.21 x 10 • molecules cm -2 s -1 during PEM-Tropics B (March) increasing by 25% to 1.51 x 10 • molecules cm '2 s -• during PEM-Tropics A (September). While the larger net losses during PEM-Tropics A were primarily driven by higher concentrations of 03, the ability of the subtropical atmosphere to destroy 03 was -30% less effective during the PEM-Tropics A (September) campaign due to a drier atmosphere and higher overhead 03 column amounts. tropospheric ozone (03) from local to global scales. High concentrations of 03 near the earth's surface have deleterious impacts on human health [Horvath and McKee, 1994] and on vegetation and crop yields [e.g., Chameides et al., 1994], and 03 is an important species in consideration of the Earth's energy budget [Ramanathan et al., 1976; Ramanathan and Dickenson, 1979; Fishman et al., 1979]. In addition, the abundance and distribution of 03 throughout the troposphere governs the oxidative capacity of the troposphere. Photodissociation of 03 in the presence of water vapor initiates the formation of the hydroxyl (OH) radical, which is responsible for the chemical removal of atmospheric pollutants. Most oxidation of long-lived gases such as CO and CH 4 occurs at tropical latitudes, where conditions for OH formation are favorable because of the moist atmosphere and abundant sunlight. Global sources of tropospheric 03 include transport from the stratosphere and photochemical production within the troposphere [Danielson, 1968; Crutzen, 1973; Chameides and Walker, 1973]. Quantification of these two sources has been a major goal of many studies [e.g., Chatfield and Harrison, 1976; Fabian and Pruchniewicz, 1977; Liu et al., 1980; Gidel and Shapiro, 1980; Logan, 1985; RoeloiS et al., 1997; Wang et al., 1998; Lelieveld and Dentenet, 2000]. Tropospheric photochemical 03 production occurs via oxidation of CO, CH4, 32,749 32,750 OLSON ET AL.: PHOTOCHEMISTRY OF THE SOUTH PACIFIC or nonmethane hydrocarbons (NMHC) in the presence of nitrogen oxides (NOx = NO + NO2) and solar radiation. The global distribution of tropospheric NOx, which is the critical limiting precursor for 03 production, is highly variable and is dependent on local photochemical loss and cycling processes as well as on the magnitude of various sources which include transport from the stratosphere, natural emissions (lightning, soils, biomass burning) and anthropogenic emissions (industrial, aircraft, ships) [Hameed et al.Thus much effort has been invested in field campaigns to characterize the regional distribution and chemical interactions of tropospheric trace species that are components of the photochemistry involving 03 and its precursors (see Emmons et al. [2000] and references therein for examples). Assessments of these regional chemical environments via techniques such as diel steady state box model analyses give values for regional photochemical tendencies of O3, which is an important component in the overall 03 budget. While human populations in tropical regions do not generate the magnitude of fossil fuel industrial emissions that are prevalent at northern hemispheric (NH) middle latitudes, the tropical troposphere is significantly impacted by large-scale seasonal biomass burning. The local rate of 03 production near regions of large-scale burning can be comparable to that over industrialized regions at temperate middle latitudes during summertime [Fishman et al., 1985; Fishman et al., 1996; Jacob et al., 1996]. Recognizing the important contribution of tropical regions to global photochemistry, several NASA Global Tropospheric Experiment (GTE) field campaigns have been conducted over tropical and subtropical regions to assess the chemical state of the tropical troposphere, including studies both over areas far from human influence and intensive field campaigns over continental regions directly impacted by human activity[

Research paper thumbnail of OH and HO 2 in the tropical Pacific: Results from PEM-Tropics B

Journal of Geophysical Research: Atmospheres, 2001

OH and HO2 data collected on NASA's Pacific Exploratory Mission-Tropics B (PEM-Tropics B) are pre... more OH and HO2 data collected on NASA's Pacific Exploratory Mission-Tropics B (PEM-Tropics B) are presented here and compared to results from a photochemical box model. PEM-Tropics B took place in the tropical Pacific in March and April of 1999 and examined photochemistry and sulfur chemistry in the remote tropical atmosphere. Altitude-resolved HOx budgets are presented. The model showed good overall agreement with the data, with a mean model to observed ratio of 0.86 for OH and 1.03 for HO2. The model tends to underpredict OH at higher altitudes and overpredict at low altitudes. The model agrees well with the H O2 observations at middle altitudes but tends to overpredict slightly at high and low altitudes.

Research paper thumbnail of Sources of HO x and production of ozone in the upper troposphere over the United States

Geophysical Research Letters, 1998

The sources of HO x (OH+peroxy radicals) and the associated production of ozone at 8-12 km over t... more The sources of HO x (OH+peroxy radicals) and the associated production of ozone at 8-12 km over the United States are examined b y modeling observations of OH, HO 2 , NO, and other species during the SUCCESS aircraft campaign in April-May 1996. The HO x concentrations measured in SUCCESS are up to a factor of 3 higher than can be calculated from oxidation of water vapor and photolysis of acetone. The highest discrepancy was seen in the outflow of a convective storm. We show that convective injection of peroxides (CH 3 OOH and H 2 O 2 ) and formaldehyde (CH 2 O) from the boundary layer to the upper troposphere c ould resolve this discrepancy. More generally, the data c ollected o ver the central United States during SUCCESS suggests that l ocal convection was a major source of HO x and NO x to the upper troposphere. The OH and HO 2 observations together with the observations of NO allow us to directly calculate the ozone production in the upper troposphere a nd its dependence on NO x . We find an average net ozone production of 2 ppbv day -1 between 8 and 12 km over the c ontinental United States in the spring. Ozone production was NO x -limited under essentially all t he c onditions encountered in SUCCESS. The high levels of HO x present i n the upper troposphere stimulate ozone production and increase the sensitivity of ozone to NO x emissions from aircraft and other sources.

Research paper thumbnail of Airborne tunable diode laser measurements of formaldehyde during TRACE-P: Distributions and box model comparisons

Journal of Geophysical Research, 2003

1] Airborne measurements of CH 2 O were acquired employing tunable diode laser absorption spectro... more 1] Airborne measurements of CH 2 O were acquired employing tunable diode laser absorption spectroscopy during the 2001 Transport and Chemical Evolution Over the Pacific (TRACE-P) study onboard NASA's DC-8 aircraft. Above 2.5km,awayfromthemostextremepollutioninfluencesandheavyaerosolloadings,comprehensivecomparisonswithasteadystateboxmodelrevealedagreementtowithin±37pptvinthemeasurementandmodelmediansbinnedaccordingtoaltitudeandlongitude.Likewise,anearunityslope(0.98±0.03)wasobtainedfromabivariatefitofthemeasurements,averagedinto25pptvmodelbins,versusthemodeledconcentrationsforvaluesupto2.5 km, away from the most extreme pollution influences and heavy aerosol loadings, comprehensive comparisons with a steady state box model revealed agreement to within ±37 pptv in the measurement and model medians binned according to altitude and longitude. Likewise, a near unity slope (0.98 ± 0.03) was obtained from a bivariate fit of the measurements, averaged into 25 pptv model bins, versus the modeled concentrations for values up to 2.5km,awayfromthemostextremepollutioninfluencesandheavyaerosolloadings,comprehensivecomparisonswithasteadystateboxmodelrevealedagreementtowithin±37pptvinthemeasurementandmodelmediansbinnedaccordingtoaltitudeandlongitude.Likewise,anearunityslope(0.98±0.03)wasobtainedfromabivariatefitofthemeasurements,averagedinto25pptvmodelbins,versusthemodeledconcentrationsforvaluesupto450 pptv. Both observations suggest that there are no systematic biases on average between CH 2 O measurements and box model results out to model values 450pptv.However,themodelresultsprogressivelyunderpredicttheobservationsathigherconcentrations,possiblyduetotransporteffectsunaccountedforinthesteadystatemodelapproach.Theassumptionofsteadystatealsoappearstocontributetothescatterobservedinthepoint−by−pointcomparisons.Themeasurement−modelvariancewasfurtherstudiedemployinghorizontalflightlegs.Forbackgroundlegsscreenedusingavarietyofnonmethanehydrocarbon(NMHC)tracers,measurementandmodelvarianceagreedtowithin15450 pptv. However, the model results progressively underpredict the observations at higher concentrations, possibly due to transport effects unaccounted for in the steady state model approach. The assumption of steady state also appears to contribute to the scatter observed in the point-by-point comparisons. The measurement-model variance was further studied employing horizontal flight legs. For background legs screened using a variety of nonmethane hydrocarbon (NMHC) tracers, measurement and model variance agreed to within 15%. By contrast, measurement variance was 450pptv.However,themodelresultsprogressivelyunderpredicttheobservationsathigherconcentrations,possiblyduetotransporteffectsunaccountedforinthesteadystatemodelapproach.Theassumptionofsteadystatealsoappearstocontributetothescatterobservedinthepoint−by−pointcomparisons.Themeasurement−modelvariancewasfurtherstudiedemployinghorizontalflightlegs.Forbackgroundlegsscreenedusingavarietyofnonmethanehydrocarbon(NMHC)tracers,measurementandmodelvarianceagreedtowithin1560% to 80% higher than the model variance, even with small to modest elevations in the NMHC tracers. Measurement-model comparisons of CH 2 O in clouds and in the lower marine troposphere in the presence of marine aerosols suggest rather significant CH 2 O uptake by as much as 85% in one extreme case compared to expectations based on modeled gas phase processes. INDEX TERMS: 0394 Atmospheric Composition and Structure: Instruments and techniques; 0368 Atmospheric Composition and Structure: Troposphere-constituent transport and chemistry; KEYWORDS: tunable diode laser, TRACE-P formaldehyde, airborne formaldehyde measurements Citation: Fried, A., et al., Airborne tunable diode laser measurements of formaldehyde during TRACE-P: Distributions and box model comparisons,

Research paper thumbnail of Ozone production in the upper troposphere and the influence of aircraft during SONEX: approach of NO x -saturated conditions

Geophysical Research Letters, 1999

During October/November 1997, simultaneous observations of NO, HO 2 and other species were obtain... more During October/November 1997, simultaneous observations of NO, HO 2 and other species were obtained as part of the SONEX campaign in the upper troposphere. We use these observations, over the North Atlantic (40-60°N), to derive ozone production rates, P(O 3 ), and to examine the relationship between P(O 3 ) and the concentrations of NO x (= NO + NO 2 ) and HO x (= OH + peroxy) radicals. A positive correlation is found between P(O 3 ) and NO x over the entire data set, which reflects the association of elevated HO x with elevated NO x injected by deep convection and lightning. By filtering out this association we find that for NO x >70 pptv, P(O 3 ) is nearly independent of NO x , showing the approach of NO x -saturated conditions. Predicted doubling of aircraft emissions in the future will result in less than doubling of the aircraft contribution to ozone over the North Atlantic in the fall. Greater sensitivity to aircraft emissions would be expected in the summer.

Research paper thumbnail of HO x budgets in a deciduous forest: Results from the PROPHET summer 1998 campaign

Journal of Geophysical Research, 2001

Results from a tightly constrained photochemical point model for OH and HO2 are compared to OH an... more Results from a tightly constrained photochemical point model for OH and HO2 are compared to OH and HO2 data collected during the Program for •e•earch on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer 1998 intensive campaign held in northern Michigan. The PROPHET campaign was located in a deciduous forest marked by relatively low NOz levels and high isoprene emissions. Detailed HOz budgets are presented. The model is generally unable to match the measured OH, with the observations 2.7 times greater than the model on average. The model HO2, however, is in good agreement with the measured HO2. Even with an additional postulated OH source from the ozonolysis of unmeasured terpenes, the measured OH is 1.5 times greater than the model; the model HO2 with this added source is 15% to 30% higher than the measured HO2. Moreover, the HO2/OH ratios as modeled are 2.5 to 4 times higher than the measured ratios, indicating that the cycling between OH and HO2 is poorly described by the model. We discuss possible reasons for the discrepancies.

Research paper thumbnail of A reevaluation of airborne HOx observations from NASA field campaigns

Journal of Geophysical Research-Atmospheres, 2006

1] In situ observations of tropospheric HO 2 obtained during four NASA airborne campaigns (SUCCES... more 1] In situ observations of tropospheric HO 2 obtained during four NASA airborne campaigns (SUCCESS, SONEX, PEM-Tropics B and TRACE-P) are reevaluated using the NASA Langley time-dependent photochemical box model. Special attention is given to previously diagnosed discrepancies between observed and predicted HO 2 which increase with higher NO x (NO + NO 2 ) levels and at high solar zenith angles. This analysis shows that much of the model discrepancy at high NO x during SUCCESS can be attributed to modeling observations at timescales too long to capture the nonlinearity of HO x (OH+HO 2 ) chemistry under highly variable conditions for NO x . Discrepancies at high NO x during SONEX can be moderated to a large extent by complete use of all available precursor observations. Differences between kinetic rate coefficients and photolysis frequencies available for previous studies versus current recommendations also explain some of the disparity. Each of these causes is shown to exert greater influence with increasing NO x because of both the chemical nonlinearity between HO x and NO x and the increased sensitivity of HO x to changes in sources at high NO x . In contrast, discrepancies at high solar zenith angles will persist until an adequate nighttime source of HO x can be identified. It is important to note that other data sets from ground-based field studies show a similar discrepancy between observed and predicted HO 2 for high NO x environments, and that the analysis presented here cannot resolve differences from those additional ground studies. Nevertheless, results from this study highlight important considerations in the application of box models to observationally based predictions of HO x radicals.

Research paper thumbnail of OH and HO 2 chemistry in the North Atlantic free troposphere

Geophysical Research Letters, 1999

... Abstract. OH and HO 2 Chemistry in the North Atlantic Free Troposphere. OH and HO2 Chemistry ... more ... Abstract. OH and HO 2 Chemistry in the North Atlantic Free Troposphere. OH and HO2 Chemistry in the North Atlantic Free Troposphere. WH Brune. ...

Research paper thumbnail of Deep convection as a source of new particles in the midlatitude upper troposphere

Journal of Geophysical Research, 2002

1] A case study of new particle formation in the region downwind of a mesoscale convective system... more 1] A case study of new particle formation in the region downwind of a mesoscale convective system stretching across much of the central United States is presented. Airborne measurements were made of condensation nuclei (CN), cloud particle surface area, water vapor, and other gases. CN concentrations were greatly enhanced above and downwind of the cirrus anvil, with maximum concentrations of 45,000 per standard cm 3 . Volatility and electron microscope measurements indicated that most of the particles were likely to be small sulfate particles. The enhancement extended over at least a 600-km region. Multivariate statistical analysis revealed that high CN concentrations were associated with surface tracers, as well as convective elements. Convection apparently brings gas-phase particle precursors from the surface to the storm outflow region, where particle nucleation is favored by the extremely low temperatures. Simple calculations showed that deep convective systems may contribute to a substantial portion of the background aerosol in the upper troposphere at midlatitudes. Citation: Twohy, C. H., et al., Deep convection as a source of new particles in the midlatitude upper troposphere,

Research paper thumbnail of Sources of upper tropospheric HO x over the South Pacific Convergence Zone: A case study

Journal of Geophysical Research, 2003

1] A zero-dimensional (0-D) model has been applied to study the sources of hydrogen oxide radical... more 1] A zero-dimensional (0-D) model has been applied to study the sources of hydrogen oxide radicals (HO x = HO 2 + OH) in the tropical upper troposphere during the Pacific Exploratory Mission in the tropics (PEM-Tropics B) aircraft mission over the South Pacific in March-April 1999. Observations made across the Southern Pacific Convergence Zone (SPCZ) and the southern branch of the Intertropical Convergence Zone (ITCZ) provided the opportunity to contrast the relative contributions of different sources of HO x , in a nitrogen oxide radical (NO x )-limited regime, in relatively pristine tropical air. The primary sources of HO x vary significantly along the flight track, in correlation with the supply of water vapor. The latitudinal variation of HO x sources is found to be controlled also by the levels of NO x and primary HO x production rates P(HO x ). Budget calculations in the 8-to 12-km altitude range show that the reaction O( 1 D) + H 2 O is a major HO x source in the cloud region traversed by the aircraft, including SPCZ and the southern branch of the ITCZ. Production from acetone becomes significant in drier region south of 20°S and can become dominant where water vapor mixing ratios lie under 200 ppmv. Over the SPCZ region, in the cloud outflow, CH 3 OOH transported by convection accounts for 22% to 64% of the total primary source. Oxidation of methane amplifies the primary HO x source by 1-1.8 in the dry regions.

Research paper thumbnail of Marine latitude/altitude OH distributions: Comparison of Pacific Ocean observations with models

Journal of Geophysical Research: Atmospheres, 2001

Reported here are tropical/subtropical Pacific basin OH observational data presented in a latitud... more Reported here are tropical/subtropical Pacific basin OH observational data presented in a latitude/altitude geographical grid. They cover two seasons of the year (spring and fall) that reflect the timing of NASA's PEM-Tropics A (1996) and B (1999) field programs. Two different OH sensors were used to collect these data, and each instrument was mounted on a different aircraft platform (i.e., NASA's P-3B and DC-8). Collectively, these chemical snapshots of the central Pacific have revealed several interesting trends. Only modest decreases (factors of 2 to 3) were found in the levels of OH with increasing altitude (0-12 krn). Similarly, only modest variations were found (factors of 1.5 to 3.5) when the data were examined as a function of latitude (30øN to 30øS). Using simultaneously recorded data for CO, 03, H20, NO, and NMHCs, comparisons with current models were also carried out. For three out of four data subsets, the results revealed a high level of correspondence. On average, the box model results agreed with the observations within a factor of 1.5. The comparison with the three-dimensional model results was found to be only slightly worse. Overall, these results suggest that current model mechanisms capture the major photochemical processes controlling OH quite well and thus provide a reasonably good representation of OH levels for tropical marine environments. They also indicate that the two OH sensors employed during the PEM-Tropics B study generally saw similar OH levels when sampling a similar tropical marine environment. However, a modest altitude bias appears to exist between these instruments. More rigorous instrument intercomparison activity would therefore seem to be justified. Further comparisons of model predictions with observations are also recommended for nontropical marine environments as well as those involving highly elevated levels of reactive non-methane hydrocarbons.

Research paper thumbnail of On the relative role of convection, chemistry, and transport over the South Pacific Convergence Zone during PEM-Tropics B: A case study

Journal of Geophysical Research, 2003

1] A mesoscale 3D model (Meso-NH) is used to assess the relative importance of convection (transp... more 1] A mesoscale 3D model (Meso-NH) is used to assess the relative importance of convection (transport and scavenging), chemistry, and advection in the vertical redistribution of HO x and their precursors in the upper tropical troposphere. The study is focused on marine deep convection over the South Pacific Convergence Zone (SPCZ) during the PEM-Tropics B Flight 10 aircraft mission. The model reproduces well the HO x mixing ratios. Vertical variations and the contrast between north and south of the SPCZ for O 3 are captured. Convection uplifted O 3 -poor air at higher altitude, creating a minimum in the 9-12 km region, in both modeled and observed profiles. The model captured 60% of the observed HCHO variance but fails to reproduce a peak of HCHO mixing ratio at 300 hPa sampled during the northern spirals. Simulated HCHO mixing ratios underestimate observations in the marine boundary layer. In the model, convection is not an efficient process to increase upper tropospheric HCHO, and HCHO is unlikely to serve as a primary source of HO x . Convection plays an important role in the vertical distribution of CH 3 OOH with efficient vertical transport from the boundary layer to the 10-15 km region where it can act as a primary source of HO x . The SPCZ region acts as a barrier to mixing of tropical and subtropical air at the surface and at high altitudes (above 250 hPa). The 400-270 hPa region over the convergence zone was more permeable, allowing subtropical air masses from the Southern Hemisphere to mix with tropical air from NE of the SPCZ and to be entrained in the SPCZ-related convection. In this altitude range, exchange of subtropical and tropical air also occurs via airflow, bypassing the convective region SW and proceeding toward the north of the SPCZ.

Research paper thumbnail of Photochemistry of HO x in the upper troposphere at northern midlatitudes

Journal of Geophysical Research, 2000

The factors controlling the concentrations of HOx radicals (= OH + peroxy) in the upper troposphe... more The factors controlling the concentrations of HOx radicals (= OH + peroxy) in the upper troposphere (8-12 km) are examined using concurrent aircraft observations of OH, HO2, H20:, CH3OOH, and CH:O made during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) at northern midlatitudes in the fall. These observations, complemented by concurrent measurements of 03, H:O, NO, peroxyacetyl nitrate (PAN), HNO3, CH4, CO, acetone, hydrocarbons, actinic fluxes, and aerosols, allow a highly constrained mass balance analysis of HOx and of the larger chemical family HO>. (= HO.• + 2 H•O: + 2 CH3OOH + HNO2 + HNO4). Observations of OH and HO: are successfully simulated to within 40% by adiel steady state model constrained with observed H:O: and CH3OOH. The model captures 85% of the observed HOx variance, which is driven mainly by the concentrations of NO.• (= NO + NO:) and by the strength of the HO., primary sources. Exceptions to the good agreement between modeled and observed HOx are at sunrise and sunset, where the model is too low by factors of 2-5, and inside cirrus clouds where the model is too high by factors of 1.2-2. Heterogeneous conversion of NO: to HO•O on aerosols (¾NO2=10 -3) during the night followed by photolysis of HONO could explain part of the discrepancy at sunrise. Heterogeneous loss of HO: on ice crystals ( ', e Ho:=0.025) could explain the discrepancy in cirrus. Primary sources of HO.• from (•(f/5)+H:O and acetone photolysis were of comparable magnitude during SONEX. The dominant sinks of HO.• were OH+HO: (NO•. <50 parts per trillion by volume (pptv)) and OH+HNO4 (NO.• >50 pptv). Observed H202 concentrations are reproduced by model calculations to within 50% if one allows in the model for heterogeneous conversion of HO• to H:O: on aerosols (¾Ho2=0.2). Observed CH3OOH concentrations are underestimated by a factor of 2 on average. Observed CH20 concentrations were usually below the 50 pptv detection limit, consistent with model results; however, frequent occurrences of high values in the observations (up to 350 pptv) are not captured by the model. These high values are correlated with high CH3OH and with cirrus clouds. Heterogeneous oxidation of CH3OH to CH:O on aerosols or ice crystals might provide an explanation (¾ice_CH3OH'"'0.01 would be needed). radicals (HO• = OH + peroxy) and of the ensemble of species thought to control HO,c production and loss: H202, CH3OOH, CH20, 03, H20, HNO•, CH4, acetone and hydrocarbons. The goal of SONEX was to assess the impact of aircraft emissions on the concentrations of nitrogen oxides (NO,c = NO + NO2) and ozone production in the upper troposphere [Singh et al., this issue]. A major step toward that goal was to understand the chemistry of HO,, which drives ozone production. An analysis of the photochemistry of ozone production during SONEX, based on the concurrent observations of HO,c and NO,, is presented by Jaegld et al. [1999]. We use here the SONEX observations to evaluate our current understanding of HOx chemistry in the upper troposphere and to introduce some new ideas regarding the role of heterogeneous chemistry. We define the chemical family HO,. including HO, radicals and their non radical reservoirs (HO,. = HO• + 2 H202 + 2 CH•OOH + HNO2 + HNO4). The factors controlling HO, concentrations in the upper troposphere can then be separated into four elements [Jaegld et al., 1997]: (1) primary HO,c sources (H20, acetone, and convective injection of HO, precursors [Chatfield and

Research paper thumbnail of Measurements of isoprene nitrates above a forest canopy

Journal of Geophysical Research, 2001

Measurements of atmospheric organic nitrates derived from isoprene, i.e., "isoprene nitrates", we... more Measurements of atmospheric organic nitrates derived from isoprene, i.e., "isoprene nitrates", were conducted from July 14 to August 19, 1998, as part of the 1998 summer intensive measurement campaign of the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) at the University of Michigan Biological Station in Pellston, Michigan. The measurements were conducted using on-line chromatography in conjunction with a nitrate-selective detection scheme. Measured concentrations of isoprene nitrates ranged from 0.5 parts per trillion (ppt), the detection limit of the method employed, to 35 ppt. In this paper we discuss the contribution of the isoprene nitrates to NOy, which was typically 0.5 -1.5% of total odd nitrogen, but up to -4% for well-aged air. Concentrations of isoprene nitrates exhibited a strong diurnal variation consistent with their expected chemical and physical removal rates. In this work we also discuss the chemistry of the precursor peroxy radicals and the NOx dependence of isoprene nitrate formation.

Research paper thumbnail of Airborne in-situ OH and HO 2 observations in the cloud-free troposphere and lower stratosphere during SUCCESS

Geophysical Research Letters, 1998

The hydroxyl (OH) and hydroperoxyl (HO2) radicals were measured for the first time throughout the... more The hydroxyl (OH) and hydroperoxyl (HO2) radicals were measured for the first time throughout the troposphere and in the lower stratosphere with a new instrument aboard the NASA DC-8 aircraft during the 1996 SUCCESS mission. Typically midday OH was 0.1-0.5 pptv and HO 2 was 3-15 pptv. Comparisons with a steady-state model yield the following conclusions. First, even in the lower stratosphere OH was sensitive to the albedo of low clouds and distant high clouds. Second, although sometimes in agreement with models, observed OH and HO 2 were more than 4 times larger at other times. Evidence suggests that for the California upper troposphere on 10 May this discrepancy was due to unmeasured HO x sources from Asia. Third, observed HO2/OH had the expected inverse dependence with NO, but was inexplicably higher than modeled HO2/OH by an average of 30%. Finally, small-scale, midday OH and HO 2 features were strongly linked to NO variations. Introduction The hydroxyl radical (OH) is the atmosphere's most important oxidizer and cleansing agent. The hydroperoxyl radical (HO2) is a major source of tropospheric ozone. It reacts with NO to form NO 2, which is then photolyzed, creating ozone. Because OH and HO 2, collectively called HO x, both initiate and participate in almost all of the atmosphere's complex chemical pathways (Ehhalt et al., 1991; Logan et al., 1981), factors that influence OH and HO 2 must be well understood. HO x photochemistry consists of sources, sinks, and exchange between OH and HO 2. An important HO x source is the ozone photolysis to O(1D), followed by the reaction of the O(1D) with H20 to form OH. This source was thought to be the only HO x source until recently; acetone photolysis and CH3OOH lifted by convection have now also been proposed for dry upper troposphere (Singh et al., 1995; Jaegl6 et al., this issue; M.J. Prather and D.J. Jacob, A persistent imbalance in HO x and NO x photochemistry of the upper troposphere driven by deep convection, submitted to Geophys. Res. Lett., 1997). The three main HO x sink reactions are: OH+HO•_--•H•_O+O•_, OH+NO•_+M--•HNO3+M, and HOe+HOe --• H•_O•_+O•, where OH+HO•_--•H•_O+O•_ dominates except where NO•_ is large. If the reactions that exchange HOx between OH and HOe are faster than the source and sink reactions, then

Research paper thumbnail of Interference Testing for Atmospheric HO x Measurements by Laser-induced Fluorescence

Journal of Atmospheric Chemistry, 2000

Accurate OH and HO 2 (collectively called HO x ) measurements by laser-induced fluorescence (LIF)... more Accurate OH and HO 2 (collectively called HO x ) measurements by laser-induced fluorescence (LIF) may be contaminated by spurious signals from interfering atmospheric chemicals or from the instrument itself. Interference tests must be conducted to ensure that observed OH signal originates solely from ambient OH and is not due to instrument artifacts. Several tests were performed on the Penn State LIF HO x instrument, both in the laboratory and in the field. These included measurements of the instrument's zero signal by using either zero air or perfluoropropylene to remove OH, examination of spectral interferences from naphthalene, sulfur dioxide, and formaldehyde, and tests of interferences by addition of suspected interfering atmospheric chemicals, including ozone, hydrogen peroxide, nitrous acid, formaldehyde, nitric acid, acetone, and organic peroxy radicals (RO 2 ). All tests lacked evidence of significant interferences for measurements in the atmosphere, including highly polluted urban environments.

Research paper thumbnail of Investigation of the nighttime decay of isoprene

Journal of Geophysical Research, 2001

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. D20, PAGES 24,335-24,346, OCTOBER 27, 2001 Investi... more JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. D20, PAGES 24,335-24,346, OCTOBER 27, 2001 Investigation of the nighttime decay of isoprene Julia M. Hurst,1 Dennis J. Barket Jr.,1 Orlando Herrera-Gomez,&amp;amp;amp;amp;amp;#x27; Tara L. Couch,1 Paul B. Shepson1-2,1. Faloona,3 D. Tan,J ...

Research paper thumbnail of Measurement and interpretation of isoprene fluxes and isoprene, methacrolein, and methyl vinyl ketone mixing ratios at the PROPHET site during the 1998 Intensive

Journal of Geophysical Research, 2002

1] Mixing ratios of isoprene, methyl vinyl ketone (MVK), and methacrolein (MACR) were determined ... more 1] Mixing ratios of isoprene, methyl vinyl ketone (MVK), and methacrolein (MACR) were determined continuously during an 8-day period in the summer of 1998 at a rural forested site located within the University of Michigan Biological Station (UMBS). The measurements were obtained as part of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) study. Fluxes of isoprene were concurrently measured at a nearby tower (AmeriFlux, located 132 m north-northeast of the PROPHET tower). Following the study, 1-kmresolution emission estimates were derived for isoprene within a 60-km radius of the tower using forest density estimates (Biogenic Emissions Inventory System (BEIS3) model). Measured isoprene fluxes at the site compared well with modeled isoprene fluxes when using BEIS3 and a detailed leaf litter-fall data set by tree species from the UMBS site. Mean midday (1000 -1400 LT) mixing ratios for isoprene, MACR, and MVK were 1.90 ± 0.43, 0.07 ± 0.01, and 0.14 ± 0.04 ppbv, respectively. Median midday mixing ratios of these compounds were 1.96 ± 0.26, 0.06 ± 0.02, and 0.10 ± 0.02 ppbv, respectively. Ratios of the isoprene oxidation products to isoprene are understood in the context of previous laboratory and field measurement studies of these compounds and a simple consecutive reaction scheme model. Results of the model indicate that the air masses studied represented relatively fresh emissions with a photochemical age of measured isoprene between 3.6 and 18 min, which is significantly less than the photochemical lifetime of isoprene (t = 45 min at [OH] = 3.35 Â 10 6 molecules cm À3 ). Thus a large portion of the isoprene that reaches the manifold has not had time to react completely with OH, yielding lower than expected ratios based on model calculations that do not explicitly take this into account. A rapid decrease in isoprene mixing ratios was observed soon after sunset, followed by a slower decay throughout the rest of the night. Emission maps were generated indicating that isoprene fluxes are highest in the immediate vicinity of the tower compared to the surrounding area of the site. Thus vertical diffusion and advection from the surrounding region are postulated to cause the observed initial rapid decrease in isoprene at the site. The second isoprene decay may be due to chemistry and/or dynamics, but the effects cannot be separated with the available data.

Research paper thumbnail of Measurement of NO and NO y emission indices during SUCCESS

Geophysical Research Letters, 1998

Exhaust measurements in the wake vortex regime of the NASA Boeing 757 aircraft were made during t... more Exhaust measurements in the wake vortex regime of the NASA Boeing 757 aircraft were made during the SUbsonic aircraft: Contrails and Cloud Effects Special Study. Emission indices for NO and Nay were calculated from in situ measurements taken on board the NASA DC-8 for plumes aged 20 -300 seconds. The average NO emission index is 7.5 g Na2/kg fuel for mean conditions of 37 kft altitude, 0.7 Mach and 0.34 kg/s fuel flow rate. Comparison is made between measured indices and predictions based on ground engine test data and a fuel flow model. Measurements are positively correlated but are on average 22% higher than predictions, with considerable scatter and systematic deviations in measurements made under low thrust conditions. These conditions are lower than typical in commercial cruise operation of the 757, for which the model was optimized. No statistically significant change in nitrogenous emissions is observed for an order of magnitude change in fuel sulfur content. Estimation of the Na2 from photochemical calculations implies a contribution to NOx of 5-19%. Examination of exhaust composition shows that 95% of the Nay is in the form of Nax.

Research paper thumbnail of A Laser-induced Fluorescence Instrument for Detecting Tropospheric OH and HO 2 : Characteristics and Calibration

Journal of Atmospheric Chemistry, 2004

Measurement capability for the detection of atmospheric OH and HO 2 has been developed at the Pen... more Measurement capability for the detection of atmospheric OH and HO 2 has been developed at the Pennsylvania State University over the last decade. The instrument is used in two forms: an aircraft configuration, Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS), and the configuration used on towers, Ground-based Tropospheric Hydrogen Oxides Sensor (GTHOS). The instrument uses ultraviolet laser induced fluorescence (LIF) to detect OH in air that is pulled by a vacuum pump through a small inlet into a low-pressure detection chamber; HO 2 is detected by reacting it with NO to form OH, which is detected by LIF in a second detection chamber. In the calibration, equal amounts of OH and HO 2 ranging from 0.15 pptv to 100 pptv are produced via photolysis of water vapor by the 185 nm emission from a low-pressure Hg lamp. Estimated absolute uncertainty at the 2σ confidence level is ±32% for both OH and HO 2 . The dependence of the instrument detection sensitivity has been quantified for changes in ambient water vapor, pressure, laser power, and the flow velocity of ambient air past the inlet. During the last 7 years, the instrument has been deployed in multi-investigator intensive field studies 5 times on the NASA DC-8 aircraft and 8 times on groundbased towers. The descriptions in this manuscript detail our cumulative wisdom of the instrumental response and calibration techniques developed over this time.

Research paper thumbnail of Seasonal differences in the photochemistry of the South Pacific: A comparison of observations and model results from PEM-Tropics A and B

Journal of Geophysical Research: Atmospheres, 2001

A time-dependent photochemical box model is used to examine the photochemistry of the equatorial ... more A time-dependent photochemical box model is used to examine the photochemistry of the equatorial and southern subtropical Pacific troposphere with aircraft data obtained during two distinct seasons: the Pacific Exploratory Mission-Tropics A (PEM-Tropics A) field campaign in September and October of 1996 and the Pacific Exploratory Mission-Tropics B (PEM-Tropics B) campaign in March and April of 1999. Model-predicted values were compared to observations for selected species (e.g., NO2, OH, HO2) with generally good agreement. Predicted values of HO 2 were larger than those observed in the upper troposphere, in contrast to previous studies which show a general underprediction of HO 2 at upper altitudes. Some characteristics of the budgets of HOx, NOx, and peroxides are discussed. The integrated net tendency for 03 is negative over the remote Pacific during both seasons, with gross formation equal to no more than half of the gross destruction. This suggests that a continual supply of 03 into the Pacific region throughout the year must exist in order to maintain 03 levels. Integrated net tendencies for equatorial 03 showed a seasonality, with a net loss of 1.06x 10 • molecules cm -2 s '1 during PEM-Tropics B (March) increasing by 50% to 1.60x 10 TM molecules cm -2 s -1 during PEM-Tropics A (September). The seasonality over the southern subtropical Pacific was somewhat lower, with losses of 1.21 x 10 • molecules cm -2 s -1 during PEM-Tropics B (March) increasing by 25% to 1.51 x 10 • molecules cm '2 s -• during PEM-Tropics A (September). While the larger net losses during PEM-Tropics A were primarily driven by higher concentrations of 03, the ability of the subtropical atmosphere to destroy 03 was -30% less effective during the PEM-Tropics A (September) campaign due to a drier atmosphere and higher overhead 03 column amounts. tropospheric ozone (03) from local to global scales. High concentrations of 03 near the earth's surface have deleterious impacts on human health [Horvath and McKee, 1994] and on vegetation and crop yields [e.g., Chameides et al., 1994], and 03 is an important species in consideration of the Earth's energy budget [Ramanathan et al., 1976; Ramanathan and Dickenson, 1979; Fishman et al., 1979]. In addition, the abundance and distribution of 03 throughout the troposphere governs the oxidative capacity of the troposphere. Photodissociation of 03 in the presence of water vapor initiates the formation of the hydroxyl (OH) radical, which is responsible for the chemical removal of atmospheric pollutants. Most oxidation of long-lived gases such as CO and CH 4 occurs at tropical latitudes, where conditions for OH formation are favorable because of the moist atmosphere and abundant sunlight. Global sources of tropospheric 03 include transport from the stratosphere and photochemical production within the troposphere [Danielson, 1968; Crutzen, 1973; Chameides and Walker, 1973]. Quantification of these two sources has been a major goal of many studies [e.g., Chatfield and Harrison, 1976; Fabian and Pruchniewicz, 1977; Liu et al., 1980; Gidel and Shapiro, 1980; Logan, 1985; RoeloiS et al., 1997; Wang et al., 1998; Lelieveld and Dentenet, 2000]. Tropospheric photochemical 03 production occurs via oxidation of CO, CH4, 32,749 32,750 OLSON ET AL.: PHOTOCHEMISTRY OF THE SOUTH PACIFIC or nonmethane hydrocarbons (NMHC) in the presence of nitrogen oxides (NOx = NO + NO2) and solar radiation. The global distribution of tropospheric NOx, which is the critical limiting precursor for 03 production, is highly variable and is dependent on local photochemical loss and cycling processes as well as on the magnitude of various sources which include transport from the stratosphere, natural emissions (lightning, soils, biomass burning) and anthropogenic emissions (industrial, aircraft, ships) [Hameed et al.Thus much effort has been invested in field campaigns to characterize the regional distribution and chemical interactions of tropospheric trace species that are components of the photochemistry involving 03 and its precursors (see Emmons et al. [2000] and references therein for examples). Assessments of these regional chemical environments via techniques such as diel steady state box model analyses give values for regional photochemical tendencies of O3, which is an important component in the overall 03 budget. While human populations in tropical regions do not generate the magnitude of fossil fuel industrial emissions that are prevalent at northern hemispheric (NH) middle latitudes, the tropical troposphere is significantly impacted by large-scale seasonal biomass burning. The local rate of 03 production near regions of large-scale burning can be comparable to that over industrialized regions at temperate middle latitudes during summertime [Fishman et al., 1985; Fishman et al., 1996; Jacob et al., 1996]. Recognizing the important contribution of tropical regions to global photochemistry, several NASA Global Tropospheric Experiment (GTE) field campaigns have been conducted over tropical and subtropical regions to assess the chemical state of the tropical troposphere, including studies both over areas far from human influence and intensive field campaigns over continental regions directly impacted by human activity[

Research paper thumbnail of OH and HO 2 in the tropical Pacific: Results from PEM-Tropics B

Journal of Geophysical Research: Atmospheres, 2001

OH and HO2 data collected on NASA's Pacific Exploratory Mission-Tropics B (PEM-Tropics B) are pre... more OH and HO2 data collected on NASA's Pacific Exploratory Mission-Tropics B (PEM-Tropics B) are presented here and compared to results from a photochemical box model. PEM-Tropics B took place in the tropical Pacific in March and April of 1999 and examined photochemistry and sulfur chemistry in the remote tropical atmosphere. Altitude-resolved HOx budgets are presented. The model showed good overall agreement with the data, with a mean model to observed ratio of 0.86 for OH and 1.03 for HO2. The model tends to underpredict OH at higher altitudes and overpredict at low altitudes. The model agrees well with the H O2 observations at middle altitudes but tends to overpredict slightly at high and low altitudes.

Research paper thumbnail of Sources of HO x and production of ozone in the upper troposphere over the United States

Geophysical Research Letters, 1998

The sources of HO x (OH+peroxy radicals) and the associated production of ozone at 8-12 km over t... more The sources of HO x (OH+peroxy radicals) and the associated production of ozone at 8-12 km over the United States are examined b y modeling observations of OH, HO 2 , NO, and other species during the SUCCESS aircraft campaign in April-May 1996. The HO x concentrations measured in SUCCESS are up to a factor of 3 higher than can be calculated from oxidation of water vapor and photolysis of acetone. The highest discrepancy was seen in the outflow of a convective storm. We show that convective injection of peroxides (CH 3 OOH and H 2 O 2 ) and formaldehyde (CH 2 O) from the boundary layer to the upper troposphere c ould resolve this discrepancy. More generally, the data c ollected o ver the central United States during SUCCESS suggests that l ocal convection was a major source of HO x and NO x to the upper troposphere. The OH and HO 2 observations together with the observations of NO allow us to directly calculate the ozone production in the upper troposphere a nd its dependence on NO x . We find an average net ozone production of 2 ppbv day -1 between 8 and 12 km over the c ontinental United States in the spring. Ozone production was NO x -limited under essentially all t he c onditions encountered in SUCCESS. The high levels of HO x present i n the upper troposphere stimulate ozone production and increase the sensitivity of ozone to NO x emissions from aircraft and other sources.

Research paper thumbnail of Airborne tunable diode laser measurements of formaldehyde during TRACE-P: Distributions and box model comparisons

Journal of Geophysical Research, 2003

1] Airborne measurements of CH 2 O were acquired employing tunable diode laser absorption spectro... more 1] Airborne measurements of CH 2 O were acquired employing tunable diode laser absorption spectroscopy during the 2001 Transport and Chemical Evolution Over the Pacific (TRACE-P) study onboard NASA's DC-8 aircraft. Above 2.5km,awayfromthemostextremepollutioninfluencesandheavyaerosolloadings,comprehensivecomparisonswithasteadystateboxmodelrevealedagreementtowithin±37pptvinthemeasurementandmodelmediansbinnedaccordingtoaltitudeandlongitude.Likewise,anearunityslope(0.98±0.03)wasobtainedfromabivariatefitofthemeasurements,averagedinto25pptvmodelbins,versusthemodeledconcentrationsforvaluesupto2.5 km, away from the most extreme pollution influences and heavy aerosol loadings, comprehensive comparisons with a steady state box model revealed agreement to within ±37 pptv in the measurement and model medians binned according to altitude and longitude. Likewise, a near unity slope (0.98 ± 0.03) was obtained from a bivariate fit of the measurements, averaged into 25 pptv model bins, versus the modeled concentrations for values up to 2.5km,awayfromthemostextremepollutioninfluencesandheavyaerosolloadings,comprehensivecomparisonswithasteadystateboxmodelrevealedagreementtowithin±37pptvinthemeasurementandmodelmediansbinnedaccordingtoaltitudeandlongitude.Likewise,anearunityslope(0.98±0.03)wasobtainedfromabivariatefitofthemeasurements,averagedinto25pptvmodelbins,versusthemodeledconcentrationsforvaluesupto450 pptv. Both observations suggest that there are no systematic biases on average between CH 2 O measurements and box model results out to model values 450pptv.However,themodelresultsprogressivelyunderpredicttheobservationsathigherconcentrations,possiblyduetotransporteffectsunaccountedforinthesteadystatemodelapproach.Theassumptionofsteadystatealsoappearstocontributetothescatterobservedinthepoint−by−pointcomparisons.Themeasurement−modelvariancewasfurtherstudiedemployinghorizontalflightlegs.Forbackgroundlegsscreenedusingavarietyofnonmethanehydrocarbon(NMHC)tracers,measurementandmodelvarianceagreedtowithin15450 pptv. However, the model results progressively underpredict the observations at higher concentrations, possibly due to transport effects unaccounted for in the steady state model approach. The assumption of steady state also appears to contribute to the scatter observed in the point-by-point comparisons. The measurement-model variance was further studied employing horizontal flight legs. For background legs screened using a variety of nonmethane hydrocarbon (NMHC) tracers, measurement and model variance agreed to within 15%. By contrast, measurement variance was 450pptv.However,themodelresultsprogressivelyunderpredicttheobservationsathigherconcentrations,possiblyduetotransporteffectsunaccountedforinthesteadystatemodelapproach.Theassumptionofsteadystatealsoappearstocontributetothescatterobservedinthepoint−by−pointcomparisons.Themeasurement−modelvariancewasfurtherstudiedemployinghorizontalflightlegs.Forbackgroundlegsscreenedusingavarietyofnonmethanehydrocarbon(NMHC)tracers,measurementandmodelvarianceagreedtowithin1560% to 80% higher than the model variance, even with small to modest elevations in the NMHC tracers. Measurement-model comparisons of CH 2 O in clouds and in the lower marine troposphere in the presence of marine aerosols suggest rather significant CH 2 O uptake by as much as 85% in one extreme case compared to expectations based on modeled gas phase processes. INDEX TERMS: 0394 Atmospheric Composition and Structure: Instruments and techniques; 0368 Atmospheric Composition and Structure: Troposphere-constituent transport and chemistry; KEYWORDS: tunable diode laser, TRACE-P formaldehyde, airborne formaldehyde measurements Citation: Fried, A., et al., Airborne tunable diode laser measurements of formaldehyde during TRACE-P: Distributions and box model comparisons,

Research paper thumbnail of Ozone production in the upper troposphere and the influence of aircraft during SONEX: approach of NO x -saturated conditions

Geophysical Research Letters, 1999

During October/November 1997, simultaneous observations of NO, HO 2 and other species were obtain... more During October/November 1997, simultaneous observations of NO, HO 2 and other species were obtained as part of the SONEX campaign in the upper troposphere. We use these observations, over the North Atlantic (40-60°N), to derive ozone production rates, P(O 3 ), and to examine the relationship between P(O 3 ) and the concentrations of NO x (= NO + NO 2 ) and HO x (= OH + peroxy) radicals. A positive correlation is found between P(O 3 ) and NO x over the entire data set, which reflects the association of elevated HO x with elevated NO x injected by deep convection and lightning. By filtering out this association we find that for NO x >70 pptv, P(O 3 ) is nearly independent of NO x , showing the approach of NO x -saturated conditions. Predicted doubling of aircraft emissions in the future will result in less than doubling of the aircraft contribution to ozone over the North Atlantic in the fall. Greater sensitivity to aircraft emissions would be expected in the summer.

Research paper thumbnail of HO x budgets in a deciduous forest: Results from the PROPHET summer 1998 campaign

Journal of Geophysical Research, 2001

Results from a tightly constrained photochemical point model for OH and HO2 are compared to OH an... more Results from a tightly constrained photochemical point model for OH and HO2 are compared to OH and HO2 data collected during the Program for •e•earch on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer 1998 intensive campaign held in northern Michigan. The PROPHET campaign was located in a deciduous forest marked by relatively low NOz levels and high isoprene emissions. Detailed HOz budgets are presented. The model is generally unable to match the measured OH, with the observations 2.7 times greater than the model on average. The model HO2, however, is in good agreement with the measured HO2. Even with an additional postulated OH source from the ozonolysis of unmeasured terpenes, the measured OH is 1.5 times greater than the model; the model HO2 with this added source is 15% to 30% higher than the measured HO2. Moreover, the HO2/OH ratios as modeled are 2.5 to 4 times higher than the measured ratios, indicating that the cycling between OH and HO2 is poorly described by the model. We discuss possible reasons for the discrepancies.

Research paper thumbnail of A reevaluation of airborne HOx observations from NASA field campaigns

Journal of Geophysical Research-Atmospheres, 2006

1] In situ observations of tropospheric HO 2 obtained during four NASA airborne campaigns (SUCCES... more 1] In situ observations of tropospheric HO 2 obtained during four NASA airborne campaigns (SUCCESS, SONEX, PEM-Tropics B and TRACE-P) are reevaluated using the NASA Langley time-dependent photochemical box model. Special attention is given to previously diagnosed discrepancies between observed and predicted HO 2 which increase with higher NO x (NO + NO 2 ) levels and at high solar zenith angles. This analysis shows that much of the model discrepancy at high NO x during SUCCESS can be attributed to modeling observations at timescales too long to capture the nonlinearity of HO x (OH+HO 2 ) chemistry under highly variable conditions for NO x . Discrepancies at high NO x during SONEX can be moderated to a large extent by complete use of all available precursor observations. Differences between kinetic rate coefficients and photolysis frequencies available for previous studies versus current recommendations also explain some of the disparity. Each of these causes is shown to exert greater influence with increasing NO x because of both the chemical nonlinearity between HO x and NO x and the increased sensitivity of HO x to changes in sources at high NO x . In contrast, discrepancies at high solar zenith angles will persist until an adequate nighttime source of HO x can be identified. It is important to note that other data sets from ground-based field studies show a similar discrepancy between observed and predicted HO 2 for high NO x environments, and that the analysis presented here cannot resolve differences from those additional ground studies. Nevertheless, results from this study highlight important considerations in the application of box models to observationally based predictions of HO x radicals.

Research paper thumbnail of OH and HO 2 chemistry in the North Atlantic free troposphere

Geophysical Research Letters, 1999

... Abstract. OH and HO 2 Chemistry in the North Atlantic Free Troposphere. OH and HO2 Chemistry ... more ... Abstract. OH and HO 2 Chemistry in the North Atlantic Free Troposphere. OH and HO2 Chemistry in the North Atlantic Free Troposphere. WH Brune. ...

Research paper thumbnail of Deep convection as a source of new particles in the midlatitude upper troposphere

Journal of Geophysical Research, 2002

1] A case study of new particle formation in the region downwind of a mesoscale convective system... more 1] A case study of new particle formation in the region downwind of a mesoscale convective system stretching across much of the central United States is presented. Airborne measurements were made of condensation nuclei (CN), cloud particle surface area, water vapor, and other gases. CN concentrations were greatly enhanced above and downwind of the cirrus anvil, with maximum concentrations of 45,000 per standard cm 3 . Volatility and electron microscope measurements indicated that most of the particles were likely to be small sulfate particles. The enhancement extended over at least a 600-km region. Multivariate statistical analysis revealed that high CN concentrations were associated with surface tracers, as well as convective elements. Convection apparently brings gas-phase particle precursors from the surface to the storm outflow region, where particle nucleation is favored by the extremely low temperatures. Simple calculations showed that deep convective systems may contribute to a substantial portion of the background aerosol in the upper troposphere at midlatitudes. Citation: Twohy, C. H., et al., Deep convection as a source of new particles in the midlatitude upper troposphere,

Research paper thumbnail of Sources of upper tropospheric HO x over the South Pacific Convergence Zone: A case study

Journal of Geophysical Research, 2003

1] A zero-dimensional (0-D) model has been applied to study the sources of hydrogen oxide radical... more 1] A zero-dimensional (0-D) model has been applied to study the sources of hydrogen oxide radicals (HO x = HO 2 + OH) in the tropical upper troposphere during the Pacific Exploratory Mission in the tropics (PEM-Tropics B) aircraft mission over the South Pacific in March-April 1999. Observations made across the Southern Pacific Convergence Zone (SPCZ) and the southern branch of the Intertropical Convergence Zone (ITCZ) provided the opportunity to contrast the relative contributions of different sources of HO x , in a nitrogen oxide radical (NO x )-limited regime, in relatively pristine tropical air. The primary sources of HO x vary significantly along the flight track, in correlation with the supply of water vapor. The latitudinal variation of HO x sources is found to be controlled also by the levels of NO x and primary HO x production rates P(HO x ). Budget calculations in the 8-to 12-km altitude range show that the reaction O( 1 D) + H 2 O is a major HO x source in the cloud region traversed by the aircraft, including SPCZ and the southern branch of the ITCZ. Production from acetone becomes significant in drier region south of 20°S and can become dominant where water vapor mixing ratios lie under 200 ppmv. Over the SPCZ region, in the cloud outflow, CH 3 OOH transported by convection accounts for 22% to 64% of the total primary source. Oxidation of methane amplifies the primary HO x source by 1-1.8 in the dry regions.

Research paper thumbnail of Marine latitude/altitude OH distributions: Comparison of Pacific Ocean observations with models

Journal of Geophysical Research: Atmospheres, 2001

Reported here are tropical/subtropical Pacific basin OH observational data presented in a latitud... more Reported here are tropical/subtropical Pacific basin OH observational data presented in a latitude/altitude geographical grid. They cover two seasons of the year (spring and fall) that reflect the timing of NASA's PEM-Tropics A (1996) and B (1999) field programs. Two different OH sensors were used to collect these data, and each instrument was mounted on a different aircraft platform (i.e., NASA's P-3B and DC-8). Collectively, these chemical snapshots of the central Pacific have revealed several interesting trends. Only modest decreases (factors of 2 to 3) were found in the levels of OH with increasing altitude (0-12 krn). Similarly, only modest variations were found (factors of 1.5 to 3.5) when the data were examined as a function of latitude (30øN to 30øS). Using simultaneously recorded data for CO, 03, H20, NO, and NMHCs, comparisons with current models were also carried out. For three out of four data subsets, the results revealed a high level of correspondence. On average, the box model results agreed with the observations within a factor of 1.5. The comparison with the three-dimensional model results was found to be only slightly worse. Overall, these results suggest that current model mechanisms capture the major photochemical processes controlling OH quite well and thus provide a reasonably good representation of OH levels for tropical marine environments. They also indicate that the two OH sensors employed during the PEM-Tropics B study generally saw similar OH levels when sampling a similar tropical marine environment. However, a modest altitude bias appears to exist between these instruments. More rigorous instrument intercomparison activity would therefore seem to be justified. Further comparisons of model predictions with observations are also recommended for nontropical marine environments as well as those involving highly elevated levels of reactive non-methane hydrocarbons.

Research paper thumbnail of On the relative role of convection, chemistry, and transport over the South Pacific Convergence Zone during PEM-Tropics B: A case study

Journal of Geophysical Research, 2003

1] A mesoscale 3D model (Meso-NH) is used to assess the relative importance of convection (transp... more 1] A mesoscale 3D model (Meso-NH) is used to assess the relative importance of convection (transport and scavenging), chemistry, and advection in the vertical redistribution of HO x and their precursors in the upper tropical troposphere. The study is focused on marine deep convection over the South Pacific Convergence Zone (SPCZ) during the PEM-Tropics B Flight 10 aircraft mission. The model reproduces well the HO x mixing ratios. Vertical variations and the contrast between north and south of the SPCZ for O 3 are captured. Convection uplifted O 3 -poor air at higher altitude, creating a minimum in the 9-12 km region, in both modeled and observed profiles. The model captured 60% of the observed HCHO variance but fails to reproduce a peak of HCHO mixing ratio at 300 hPa sampled during the northern spirals. Simulated HCHO mixing ratios underestimate observations in the marine boundary layer. In the model, convection is not an efficient process to increase upper tropospheric HCHO, and HCHO is unlikely to serve as a primary source of HO x . Convection plays an important role in the vertical distribution of CH 3 OOH with efficient vertical transport from the boundary layer to the 10-15 km region where it can act as a primary source of HO x . The SPCZ region acts as a barrier to mixing of tropical and subtropical air at the surface and at high altitudes (above 250 hPa). The 400-270 hPa region over the convergence zone was more permeable, allowing subtropical air masses from the Southern Hemisphere to mix with tropical air from NE of the SPCZ and to be entrained in the SPCZ-related convection. In this altitude range, exchange of subtropical and tropical air also occurs via airflow, bypassing the convective region SW and proceeding toward the north of the SPCZ.

Research paper thumbnail of Photochemistry of HO x in the upper troposphere at northern midlatitudes

Journal of Geophysical Research, 2000

The factors controlling the concentrations of HOx radicals (= OH + peroxy) in the upper troposphe... more The factors controlling the concentrations of HOx radicals (= OH + peroxy) in the upper troposphere (8-12 km) are examined using concurrent aircraft observations of OH, HO2, H20:, CH3OOH, and CH:O made during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) at northern midlatitudes in the fall. These observations, complemented by concurrent measurements of 03, H:O, NO, peroxyacetyl nitrate (PAN), HNO3, CH4, CO, acetone, hydrocarbons, actinic fluxes, and aerosols, allow a highly constrained mass balance analysis of HOx and of the larger chemical family HO>. (= HO.• + 2 H•O: + 2 CH3OOH + HNO2 + HNO4). Observations of OH and HO: are successfully simulated to within 40% by adiel steady state model constrained with observed H:O: and CH3OOH. The model captures 85% of the observed HOx variance, which is driven mainly by the concentrations of NO.• (= NO + NO:) and by the strength of the HO., primary sources. Exceptions to the good agreement between modeled and observed HOx are at sunrise and sunset, where the model is too low by factors of 2-5, and inside cirrus clouds where the model is too high by factors of 1.2-2. Heterogeneous conversion of NO: to HO•O on aerosols (¾NO2=10 -3) during the night followed by photolysis of HONO could explain part of the discrepancy at sunrise. Heterogeneous loss of HO: on ice crystals ( ', e Ho:=0.025) could explain the discrepancy in cirrus. Primary sources of HO.• from (•(f/5)+H:O and acetone photolysis were of comparable magnitude during SONEX. The dominant sinks of HO.• were OH+HO: (NO•. <50 parts per trillion by volume (pptv)) and OH+HNO4 (NO.• >50 pptv). Observed H202 concentrations are reproduced by model calculations to within 50% if one allows in the model for heterogeneous conversion of HO• to H:O: on aerosols (¾Ho2=0.2). Observed CH3OOH concentrations are underestimated by a factor of 2 on average. Observed CH20 concentrations were usually below the 50 pptv detection limit, consistent with model results; however, frequent occurrences of high values in the observations (up to 350 pptv) are not captured by the model. These high values are correlated with high CH3OH and with cirrus clouds. Heterogeneous oxidation of CH3OH to CH:O on aerosols or ice crystals might provide an explanation (¾ice_CH3OH'"'0.01 would be needed). radicals (HO• = OH + peroxy) and of the ensemble of species thought to control HO,c production and loss: H202, CH3OOH, CH20, 03, H20, HNO•, CH4, acetone and hydrocarbons. The goal of SONEX was to assess the impact of aircraft emissions on the concentrations of nitrogen oxides (NO,c = NO + NO2) and ozone production in the upper troposphere [Singh et al., this issue]. A major step toward that goal was to understand the chemistry of HO,, which drives ozone production. An analysis of the photochemistry of ozone production during SONEX, based on the concurrent observations of HO,c and NO,, is presented by Jaegld et al. [1999]. We use here the SONEX observations to evaluate our current understanding of HOx chemistry in the upper troposphere and to introduce some new ideas regarding the role of heterogeneous chemistry. We define the chemical family HO,. including HO, radicals and their non radical reservoirs (HO,. = HO• + 2 H202 + 2 CH•OOH + HNO2 + HNO4). The factors controlling HO, concentrations in the upper troposphere can then be separated into four elements [Jaegld et al., 1997]: (1) primary HO,c sources (H20, acetone, and convective injection of HO, precursors [Chatfield and

Research paper thumbnail of Measurements of isoprene nitrates above a forest canopy

Journal of Geophysical Research, 2001

Measurements of atmospheric organic nitrates derived from isoprene, i.e., "isoprene nitrates", we... more Measurements of atmospheric organic nitrates derived from isoprene, i.e., "isoprene nitrates", were conducted from July 14 to August 19, 1998, as part of the 1998 summer intensive measurement campaign of the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) at the University of Michigan Biological Station in Pellston, Michigan. The measurements were conducted using on-line chromatography in conjunction with a nitrate-selective detection scheme. Measured concentrations of isoprene nitrates ranged from 0.5 parts per trillion (ppt), the detection limit of the method employed, to 35 ppt. In this paper we discuss the contribution of the isoprene nitrates to NOy, which was typically 0.5 -1.5% of total odd nitrogen, but up to -4% for well-aged air. Concentrations of isoprene nitrates exhibited a strong diurnal variation consistent with their expected chemical and physical removal rates. In this work we also discuss the chemistry of the precursor peroxy radicals and the NOx dependence of isoprene nitrate formation.

Research paper thumbnail of Airborne in-situ OH and HO 2 observations in the cloud-free troposphere and lower stratosphere during SUCCESS

Geophysical Research Letters, 1998

The hydroxyl (OH) and hydroperoxyl (HO2) radicals were measured for the first time throughout the... more The hydroxyl (OH) and hydroperoxyl (HO2) radicals were measured for the first time throughout the troposphere and in the lower stratosphere with a new instrument aboard the NASA DC-8 aircraft during the 1996 SUCCESS mission. Typically midday OH was 0.1-0.5 pptv and HO 2 was 3-15 pptv. Comparisons with a steady-state model yield the following conclusions. First, even in the lower stratosphere OH was sensitive to the albedo of low clouds and distant high clouds. Second, although sometimes in agreement with models, observed OH and HO 2 were more than 4 times larger at other times. Evidence suggests that for the California upper troposphere on 10 May this discrepancy was due to unmeasured HO x sources from Asia. Third, observed HO2/OH had the expected inverse dependence with NO, but was inexplicably higher than modeled HO2/OH by an average of 30%. Finally, small-scale, midday OH and HO 2 features were strongly linked to NO variations. Introduction The hydroxyl radical (OH) is the atmosphere's most important oxidizer and cleansing agent. The hydroperoxyl radical (HO2) is a major source of tropospheric ozone. It reacts with NO to form NO 2, which is then photolyzed, creating ozone. Because OH and HO 2, collectively called HO x, both initiate and participate in almost all of the atmosphere's complex chemical pathways (Ehhalt et al., 1991; Logan et al., 1981), factors that influence OH and HO 2 must be well understood. HO x photochemistry consists of sources, sinks, and exchange between OH and HO 2. An important HO x source is the ozone photolysis to O(1D), followed by the reaction of the O(1D) with H20 to form OH. This source was thought to be the only HO x source until recently; acetone photolysis and CH3OOH lifted by convection have now also been proposed for dry upper troposphere (Singh et al., 1995; Jaegl6 et al., this issue; M.J. Prather and D.J. Jacob, A persistent imbalance in HO x and NO x photochemistry of the upper troposphere driven by deep convection, submitted to Geophys. Res. Lett., 1997). The three main HO x sink reactions are: OH+HO•_--•H•_O+O•_, OH+NO•_+M--•HNO3+M, and HOe+HOe --• H•_O•_+O•, where OH+HO•_--•H•_O+O•_ dominates except where NO•_ is large. If the reactions that exchange HOx between OH and HOe are faster than the source and sink reactions, then

Research paper thumbnail of Interference Testing for Atmospheric HO x Measurements by Laser-induced Fluorescence

Journal of Atmospheric Chemistry, 2000

Accurate OH and HO 2 (collectively called HO x ) measurements by laser-induced fluorescence (LIF)... more Accurate OH and HO 2 (collectively called HO x ) measurements by laser-induced fluorescence (LIF) may be contaminated by spurious signals from interfering atmospheric chemicals or from the instrument itself. Interference tests must be conducted to ensure that observed OH signal originates solely from ambient OH and is not due to instrument artifacts. Several tests were performed on the Penn State LIF HO x instrument, both in the laboratory and in the field. These included measurements of the instrument's zero signal by using either zero air or perfluoropropylene to remove OH, examination of spectral interferences from naphthalene, sulfur dioxide, and formaldehyde, and tests of interferences by addition of suspected interfering atmospheric chemicals, including ozone, hydrogen peroxide, nitrous acid, formaldehyde, nitric acid, acetone, and organic peroxy radicals (RO 2 ). All tests lacked evidence of significant interferences for measurements in the atmosphere, including highly polluted urban environments.

Research paper thumbnail of Investigation of the nighttime decay of isoprene

Journal of Geophysical Research, 2001

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. D20, PAGES 24,335-24,346, OCTOBER 27, 2001 Investi... more JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. D20, PAGES 24,335-24,346, OCTOBER 27, 2001 Investigation of the nighttime decay of isoprene Julia M. Hurst,1 Dennis J. Barket Jr.,1 Orlando Herrera-Gomez,&amp;amp;amp;amp;amp;#x27; Tara L. Couch,1 Paul B. Shepson1-2,1. Faloona,3 D. Tan,J ...

Research paper thumbnail of Measurement and interpretation of isoprene fluxes and isoprene, methacrolein, and methyl vinyl ketone mixing ratios at the PROPHET site during the 1998 Intensive

Journal of Geophysical Research, 2002

1] Mixing ratios of isoprene, methyl vinyl ketone (MVK), and methacrolein (MACR) were determined ... more 1] Mixing ratios of isoprene, methyl vinyl ketone (MVK), and methacrolein (MACR) were determined continuously during an 8-day period in the summer of 1998 at a rural forested site located within the University of Michigan Biological Station (UMBS). The measurements were obtained as part of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) study. Fluxes of isoprene were concurrently measured at a nearby tower (AmeriFlux, located 132 m north-northeast of the PROPHET tower). Following the study, 1-kmresolution emission estimates were derived for isoprene within a 60-km radius of the tower using forest density estimates (Biogenic Emissions Inventory System (BEIS3) model). Measured isoprene fluxes at the site compared well with modeled isoprene fluxes when using BEIS3 and a detailed leaf litter-fall data set by tree species from the UMBS site. Mean midday (1000 -1400 LT) mixing ratios for isoprene, MACR, and MVK were 1.90 ± 0.43, 0.07 ± 0.01, and 0.14 ± 0.04 ppbv, respectively. Median midday mixing ratios of these compounds were 1.96 ± 0.26, 0.06 ± 0.02, and 0.10 ± 0.02 ppbv, respectively. Ratios of the isoprene oxidation products to isoprene are understood in the context of previous laboratory and field measurement studies of these compounds and a simple consecutive reaction scheme model. Results of the model indicate that the air masses studied represented relatively fresh emissions with a photochemical age of measured isoprene between 3.6 and 18 min, which is significantly less than the photochemical lifetime of isoprene (t = 45 min at [OH] = 3.35 Â 10 6 molecules cm À3 ). Thus a large portion of the isoprene that reaches the manifold has not had time to react completely with OH, yielding lower than expected ratios based on model calculations that do not explicitly take this into account. A rapid decrease in isoprene mixing ratios was observed soon after sunset, followed by a slower decay throughout the rest of the night. Emission maps were generated indicating that isoprene fluxes are highest in the immediate vicinity of the tower compared to the surrounding area of the site. Thus vertical diffusion and advection from the surrounding region are postulated to cause the observed initial rapid decrease in isoprene at the site. The second isoprene decay may be due to chemistry and/or dynamics, but the effects cannot be separated with the available data.

Research paper thumbnail of Measurement of NO and NO y emission indices during SUCCESS

Geophysical Research Letters, 1998

Exhaust measurements in the wake vortex regime of the NASA Boeing 757 aircraft were made during t... more Exhaust measurements in the wake vortex regime of the NASA Boeing 757 aircraft were made during the SUbsonic aircraft: Contrails and Cloud Effects Special Study. Emission indices for NO and Nay were calculated from in situ measurements taken on board the NASA DC-8 for plumes aged 20 -300 seconds. The average NO emission index is 7.5 g Na2/kg fuel for mean conditions of 37 kft altitude, 0.7 Mach and 0.34 kg/s fuel flow rate. Comparison is made between measured indices and predictions based on ground engine test data and a fuel flow model. Measurements are positively correlated but are on average 22% higher than predictions, with considerable scatter and systematic deviations in measurements made under low thrust conditions. These conditions are lower than typical in commercial cruise operation of the 757, for which the model was optimized. No statistically significant change in nitrogenous emissions is observed for an order of magnitude change in fuel sulfur content. Estimation of the Na2 from photochemical calculations implies a contribution to NOx of 5-19%. Examination of exhaust composition shows that 95% of the Nay is in the form of Nax.

Research paper thumbnail of A Laser-induced Fluorescence Instrument for Detecting Tropospheric OH and HO 2 : Characteristics and Calibration

Journal of Atmospheric Chemistry, 2004

Measurement capability for the detection of atmospheric OH and HO 2 has been developed at the Pen... more Measurement capability for the detection of atmospheric OH and HO 2 has been developed at the Pennsylvania State University over the last decade. The instrument is used in two forms: an aircraft configuration, Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS), and the configuration used on towers, Ground-based Tropospheric Hydrogen Oxides Sensor (GTHOS). The instrument uses ultraviolet laser induced fluorescence (LIF) to detect OH in air that is pulled by a vacuum pump through a small inlet into a low-pressure detection chamber; HO 2 is detected by reacting it with NO to form OH, which is detected by LIF in a second detection chamber. In the calibration, equal amounts of OH and HO 2 ranging from 0.15 pptv to 100 pptv are produced via photolysis of water vapor by the 185 nm emission from a low-pressure Hg lamp. Estimated absolute uncertainty at the 2σ confidence level is ±32% for both OH and HO 2 . The dependence of the instrument detection sensitivity has been quantified for changes in ambient water vapor, pressure, laser power, and the flow velocity of ambient air past the inlet. During the last 7 years, the instrument has been deployed in multi-investigator intensive field studies 5 times on the NASA DC-8 aircraft and 8 times on groundbased towers. The descriptions in this manuscript detail our cumulative wisdom of the instrumental response and calibration techniques developed over this time.