HOx measurements in the summertime upper troposphere over Europe: a comparison of observations to a box model and a 3-D model (original) (raw)
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Atmospheric Chemistry and Physics
2016
In situ airborne measurements of OH and HO 2 with the HORUS (HydrOxyl Radical measurement Unit based on fluorescence Spectroscopy) instrument were performed in the summertime upper troposphere across Europe during the HOOVER 2 (HO x OVer EuRope) campaign in July 2007. Complementary measurements of trace gas species and photolysis frequencies were conducted to obtain a broad data set, which has been used to quantify the significant HO x sources and sinks. In this study we compare the in situ measurement of OH and HO 2 with simulated mixing ratios from the constrained box model CAABA/MECCA (Chemistry As A Box Model Application/Module Efficiently Calculating the Chemistry of the Atmosphere), and the global circulation model EMAC (ECHAM5/MESSy Atmospheric Chemistry Model). The constrained box model reproduces the observed OH and HO 2 mixing ratios with better agreement (obs/mod median 98 % OH, 96 % HO 2) than the global model (median 76 % OH, 59 % HO 2). The observations and the computed HO x sources and sinks are used to identify deviations between the models and their impacts on the calculated HO x budget.
Atmospheric Chemistry and Physics, 2012
A photochemical box model constrained by ancillary observations was used to simulate OH and HO 2 concentrations for three days of ambient observations during the HO x Comp field campaign held in Jülich, Germany in July 2005. Daytime OH levels observed by four instruments were fairly well reproduced to within 33 % by a base model run (Regional Atmospheric Chemistry Mechanism with updated isoprene chemistry adapted from Master Chemical Mechanism ver. 3.1) with high R 2 values (0.72-0.97) over a range of isoprene (0.3-2 ppb) and NO (0.1-10 ppb) mixing ratios. Daytime HO 2 (*) levels, reconstructed from the base model results taking into account the sensitivity toward speciated RO 2 (organic peroxy) radicals, as recently reported from one of the participating instruments in the HO 2 measurement mode, were 93 % higher than the observations made by the single instrument. This also indicates an overprediction of the HO 2 to OH recycling. Together with the good modelmeasurement agreement for OH, it implies a missing OH source in the model. Modeled OH and HO 2 (*) could only be matched to the observations by addition of a strong unknown loss process for HO 2 (*) that recycles OH at a high yield. Adding to the base model, instead, the recently proposed isomerization mechanism of isoprene peroxy radicals (Peeters and Müller, 2010) increased OH and HO 2 (*) by 28 % and 13 % on average. Although these were still only 4 % higher than the OH observations made by one of the instruments, larger overestimations (42-70 %) occurred with respect to the OH observations made by the other three instruments. The overestimation in OH could be diminished only when reactive alkanes (HC8) were solely introduced to the model to explain the missing fraction of observed OH reactivity. Moreover, the overprediction of HO 2 (*) became even larger than in the base case. These analyses imply that the rates of the isomerization are not readily supported by the ensemble of radical observations. One of the measurement days was characterized by low isoprene concentrations (∼0.5 ppb) and OH reactivity that was well explained by the observed species, especially before noon. For this selected Published by Copernicus Publications on behalf of the European Geosciences Union. 2568 Y. Kanaya et al.: HO x Comp: observed and modeled ambient OH and HO comparisons period, as opposed to the general behavior, the model tended to underestimate HO 2 (*). We found that this tendency is associated with high NO x concentrations, suggesting that some HO 2 production or regeneration processes under high NO x conditions were being overlooked; this might require revision of ozone production regimes.
2009
Measurements of hydroxyl (OH) and hydroperoxy (HO 2) radicals were made during the Mexico City Metropolitan Area (MCMA) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These measurements provide a unique opportunity to test current models of atmospheric RO x (OH + HO 2 + RO 2) photochemistry under polluted conditions. A zero-dimensional box model based on the Regional Atmospheric Chemical Mechanism (RACM) was constrained by 10-min averages of 24 J-values and the concentrations of 97 chemical species. Several issues related to the RO x chemistry under polluted conditions
Geophysical Research Letters, 1997
ER-2 aircraft observations of OH and HO2 concentrations in the upper troposphere during the NASA/STRAT campaign are interpreted using a photochemical model constrained by local observations of 03, H20, NO, CO, hydrocarbons, albedo and overhead ozone column. We find that the reaction O(•D) + H20 is minor compared to acetone photolysis as a primary source of HOx (= OH + peroxy radicals) in the upper troposphere. Calculations using a diel steady state model agree with observed HOx concentrations in the lower stratosphere and, for some flights, in the upper troposphere. However, for other flights in the upper troposphere, the steady state model underestimates observations by a factor of 2 or more. These model underestimates are found to be related to a recent (< 1 week) convective origin of the air. By conducting time-dependent model calculations along air trajectories determined for the STRAT flights, we show that convective injection of CH3OOH and H202 from the boundary layer to the upper troposphere could resolve the discrepancy. These injections of HOx reservoirs cause large HOx increases in the tropical upper troposphere for over a week downwind of the convective activity. We propose that this mechanism provides a major source of HOx in the upper troposphere. Simultaneous measurements of peroxides, formaldehyde and acetone along with OH and HO2 are needed to test our hypothesis. Thompson, A.M., The oxidizing capacity of the Earth's atmosphere: Probable past and future changes, Science, 256, 1157-1165, 1992. Wennberg, P.O., et al., In situ measurements of OH and HO2 in the upper troposphere and stratosphere, J. Atmos. Sci., 52, 3413-3420, 1995.
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 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
Chemistry of hydrogen oxide radicals (HOx) in the Arctic troposphere in spring
Atmospheric Chemistry and Physics, 2010
We use observations from the April 2008 NASA ARCTAS aircraft campaign to the North American Arctic, interpreted with a global 3-D chemical transport model (GEOS-Chem), to better understand the sources and cycling of hydrogen oxide radicals (HO x ≡H+OH+peroxy radicals) and their reservoirs (HO y ≡HO x +peroxides) in the 5 springtime Arctic atmosphere. We find that a standard gas-phase chemical mechanism overestimates the observed HO 2 and H 2 O 2 concentrations. Computation of HO x and HO y gas-phase chemical budgets on the basis of the aircraft observations also indicates a large missing sink for both. We hypothesize that this could reflect HO 2 uptake by aerosols, favored by low temperatures and relatively high aerosol loadings, 10 through a mechanism that does not produce H 2 O 2 . Such a mechanism could involve HO 2 aqueous-phase reaction with sulfate (58% of the ARCTAS submicron aerosol by mass) to produce peroxymonosulfate (HSO − 5 ) that would eventually convert back to sulfate and return water. We implemented such an uptake of HO 2 by aerosol in the model using a standard reactive uptake coefficient parameterization with γ(HO 2 ) values rang-15 ing from 0.02 at 275 K to 0.5 at 220 K. This successfully reproduces the concentrations and vertical distributions of the different HO x species and HO y reservoirs. HO 2 uptake by aerosol is then a major HO x and HO y sink, decreasing mean OH and HO 2 concentrations in the Arctic troposphere by 48% and 45% respectively. Circumpolar budget analysis in the model shows that transport of peroxides from northern mid-latitudes 20 contributes 50% of the HO y source above 6 km, and cloud chemistry and deposition of H 2 O 2 account together for 40% of the HO y sink below 3 km. Better rate and product data for HO 2 uptake by aerosol are needed to understand this role of aerosols in limiting the oxidizing power of the Arctic atmosphere. 6957 Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion 25 other volatile organic compounds (VOCs) yields formaldehyde (HCHO), which photolyzes to produce additional HO x radicals and amplify the original source: 6958 Abstract 20 Chen et al., 2004). Another unique aspect of HO x chemistry in the boundary layer is the interaction with halogen radicals. These interactions include HO x production from Br+HCHO (Evans et al., 2003), additional HO y reservoirs such as HOBr (Bloss et al., 2005), and additional processes for cycling between HO 2 and OH (Simpson et al., 2007). Abstract make reference to the P-3 aerosol data. The DC-8 conducted nine flights in the North 6960 Abstract ACPD 10, 6955-6994, 2010 Abstract ACPD 10, 6955-6994, 2010 Abstract ACPD 10, 6955-6994, 2010 Abstract ACPD 10, 6955-6994, 2010 Abstract ACPD 10, 6955-6994, 2010 Abstract ACPD 10, 6955-6994, 2010 Abstract 0.07-0.2) except for concentrated H 2 SO 4 (γ(HO 2 )< 0.01). However, γ(HO 2 ) for con-6967 Abstract − 2 reaction at an assumed pH 5, producing H 2 O 2 that 6968 Abstract ACPD 10, 6955-6994, 2010 Abstract 10, 6955-6994, 2010 Abstract 10, 6955-6994, 2010 Abstract 10, 6955-6994, 2010 Abstract 10, 6955-6994, 2010 Abstract 10, 6955-6994, 2010 Abstract References Allen, D., Pickering, K., Stenchikov, G., Thompson, A., and Kondo, Y.: A three-dimensional total odd nitrogen (NO y ) simulation during sonex using a stretched-grid chemical transport model, 25 Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion data for atmospheric chemistry: Volume II -gas phase reactions of organic species, Atmos. Global modeling of tropospheric chemistry with assimilated meteorology: Model description and evaluation, J. Geophys. Res.-Atmos., 106, 23073-23095, 2001. Bian, H. S. and Prather, M. J.: FAST-J2: Accurate simulation of stratospheric photolysis in 10 global chemical models, Impact of halogen monoxide chemistry upon boundary layer OH and HO 2 concentrations at a coastal site, Geophys.
Geophysical Research Letters, 1998
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
Journal of Geophysical Research, 1999
Data for the tropical upper troposphere (8-12 km, 20øN-20øS) collected during NASA's Pacific Exploratory Missions have been used to carry out a detailed examination of the photochemical processes controlling HOx (OH+HO2). Of particular significance is the availability of measurements of nonmethane hydrocarbons, oxygenated hydrocarbons (i.e., acetone, methanol, and ethanol) and peroxides (i.e., H202 and CH3OOH). These observations have provided constraints on model calculations permitting an assessment of the potential impact of these species on the levels of HOx, CH302, CH20, as well as ozone budget parameters. Sensitivity calculations using a time-dependent photochemical box model show that when constrained by measured values of the above oxygenated species, model estimated HOx levels are elevated relative to unconstrained calculations. The impact of constraining these species was found to increase with altitude, reflecting the systematic roll-off in water vapor mixing ratios with altitude. At 11-12 km, overall increases in HOx approached a factor of 2 with somewhat larger increases being found for gross and net photochemical production of ozone. While significant, the impact on HO• due to peroxides appears to be less than previously estimated. In particular, observations of elevated H202 levels may be more influenced by local photochemistry than by convective transport. Issues related to the uncertainty in high-altitude water vapor levels and the possibility of other contributing sources of HOx are discussed. Finally, it is noted that the uncertainties in gas kinetic rate coefficients at the low temperatures of the upper troposphere and as well as OH sensor calibrations should be areas of continued investigation. These comparisons provided important diagnostics for indirectly assessing model estimates for the fast photochemistry of HOx (OH+HO2). More recently, direct observations of riO 2 and OH in the upper troposphere and lower stratosphere have become available (e.g., Stratospheric Tracers of Atmospheric Transport (STRAT), [Wennberg et al., 1998] and Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS), [Brune et al., 1998]). These observations have offered an opportunity for more direct tests of HOx photochemical theory in the upper troposphere. With the availability of these new HOx measurements has come the realization that the photochemistry of the upper troposphere may be more complex than previously thought. For example, model calculations of upper tropospheric HO 2 and OH levels during both the STRAT and SUCCESS campaigns have revealed that model estimated values often fall well below those 16,255 16,256 CRAWFORD ET AL.: ASSESSMENT OF UPPER TROPOSPHERIC HOx SOURCES measured [Wennberg et al., 1998; Jaegld et aL, 1997, 1998; McKeen et al., 1997; Brune et a1.,1998]. This disagreement has led to the suggestion that there may be gaps in our understanding of the chemistry in this region of the atmosphere. Such problems could exist with current mechanisms being used to model atmospheric chemistry. Alternatively, the databases being used to carry out the modeling may be inadequate either from the point of view of their being statistically non-representative of the environment being examined or in terms of their not providing the necessary constraints on model calculations. The above cited HOx investigations focused on evaluating potential sources of HOx previously unaccounted for in photochemical models. Sources identified in these studies that might explain the discrepancies were acetone and peroxides. The importance of acetone as a source of HOx in the upper troposphere was first identified by Singh et al. [1995] based on measurements taken during NASA' s Pacific Exploratory Mission (PEM)-West B. The photolysis of acetone produces the radical species CH3CO 3 and CH302 which, upon further reaction, can yield up to 3.2 HOx radicals, given a high NOx-tO-HOx environment. Additional oxygenated hydrocarbons measured by Singh et al. [1995] during PEM-West B were methanol and ethanol. These compounds are lost primarily through reaction with OH, and similar to many other nonmethane hydrocarbons, they can potentially serve as a net source of HOx via further degradation of the primary radical species. Sources of oxygenated hydrocarbons are not well quantified, but include primary anthropogenic and biogenic emissions as well as secondary chemical sources from hydrocarbon oxidation [Singh et al., 1994, and references therein]. Recent studies have provided evidence of particularly strong biogenic sources [Goldan, et al., 1995; Kirstine et al., 1998].
OH and HO2 Concentrations during PROPHET 2008: Measurement and Theory
2009
Hydroxyl (OH) and hydroperoxy (HO2) radicals are key species driving the gas-phase oxidation of organic trace gases that lead to the formation of ozone and secondary organic aerosols in the troposphere. Previous measurements of OH and HO2 radicals in forest environments have shown serious discrepancies with modeled concentrations of these radicals. These discrepancies bring into question our understanding of the