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Papers by David Tan
Journal of Geophysical Research, 2001
Observations over the tropical Pacific during the Pacific Exploratory Mission (PEM)-Tropics B exp... more Observations over the tropical Pacific during the Pacific Exploratory Mission (PEM)-Tropics B experiment (March-April 1999) are analyzed. Concentrations of CO and long-lived nonmethane hydrocarbons in the region are significantly enhanced due to transport of pollutants from northern industrial continents. This pollutant import also enhances moderately 03 concentrations but not NOx concentrations. It therefore tends to depress OH concentrations over the tropical Pacific. These effects contrast to the large enhancements of 03 and NOx concentrations and the moderate increase of OH concentrations due to biomass burning outflow during the PEM-Tropics A experiment (September-October 1996). Observed CH3I concentrations, as in PEM-Tropics A, indicate that convective mass outflux in the middle and upper troposphere is largely independent of altitude over the tropical Pacific. Constraining a one-dimensional model with CH31 observations yields a 1 O-day timescale for convective turnover of the free troposphere, a factor of 2 faster than during PEM-Tropics A. Model simulated HO2, CH20, H202, and CH3OOH concentrations are generally in agreement with observations. However, simulated OH concentrations are lower (~25%) than observations above 6 km. Whereas models tend to overestimate previous field measurements, simulated HNO3 concentrations during PEM-Tropics B are too low (a factor of 2-4 below 6 km) compared to observations. Budget analyses indicate that chemical production of 03 accounts for only 50% of chemical loss; significant transport of 03 into the region appears to take place within the tropics. Convective transport of CH3OOH enhances the production of HOx and 03 in the upper troposphere, but this effect is offset by HOx loss due to the scavenging of H202. Convective transport and scavenging of reactive nitrogen species imply a necessary source of 0.4-1 Tg yr -1 of NO., in the free troposphere (above 4 km) over the tropics. A large fraction of the source could be from marine lightning. Oxidation of DMS transported by convection from the boundary layer could explain the observed free tropospheric SO2 concentrations over the tropical Pacific. This source of DMS due to convection, however, would imply in the model free tropospheric concentrations much higher than observed. The model overestimate cannot be reconciled using recent kinetics measurements of the DMS-OH adduct reaction at low pressures and temperatures and may reflect enhanced OH oxidation of DMS during convection.
Journal of Geophysical Research, 2002
1] A one-dimensional Lagrangian model for atmospheric transport and photochemistry has been devel... more 1] A one-dimensional Lagrangian model for atmospheric transport and photochemistry has been developed and used to interpret measurements made at Pellston, Michigan, during the summer of 1998. The model represents a moving vertical column of air with vertical resolution of 25 m near the ground. Calculations have been performed for a series of trajectories, with representation of emissions, vertical mixing, and photochemistry for a 3-day period ending with the arrival of the air column at Pellston. Results have been used to identify causes of the observed decrease in isoprene at night, to investigate causes of high nighttime OH. Significant OH can be generated at night by terpenes if it is assumed that some fast-reacting monoterpenes are emitted at rates comparable to inventory emissions for terpenes. However, this nighttime OH is confined to a shallow surface layer (0 -25 m) and has little impact on nighttime chemistry. The observed decrease in isoprene at night can be reproduced in models with low OH, and is attributed primarily to vertical dilution. There is also evidence that transport from Lake Michigan contributes to low nighttime isoprene at Pellston. Model results compare well with measured isoprene, NO x , and with isoprene vertical profiles. Significant model-measurement discrepancies are found for OH, HO 2 , methylvinylketone, and formaldehyde.
Journal of Geophysical Research, 2000
Simultaneous measurements of the oxides of hydrogen and nitrogen made during the NASA Subsonic As... more Simultaneous measurements of the oxides of hydrogen and nitrogen made during the NASA Subsonic Assessment, Ozone and Nitrogen Oxide Experiment (SONEX) afforded an opportunity to study the coupling between these two important families throughout the free troposphere and lowermost stratosphere. Moreover, the suite of measurements made during the campaign was unprecedented in its completeness, thus providing a uniquely detailed picture of the radical photochemistry that drives oxidation and ozone production in this part of the atmosphere. On average, observed hydrogen oxides (HOx = OH + HO2) agree well with both instantaneous and diel steady-state models; however, there is a persistent deviation of the observations that correlates with the abundance of nitrogen oxides (NOx = NO + NO2) in the sampled air mass. Specifically, the observed HO• tends to exceed the model predictions in the presence of high NO• concentrations, by as much as a factor of 5 (>500 pptv NOx), and is sometimes as little as half that expected by steady state at lower NOj levels. While many possibilities for these discrepancies are discussed, it is argued that an instrumental artifact is not probable and that the discrepancy may bespeak a shortcoming of our understanding of HO• chemistry. The consistently elevated HO• in the presence of elevated NOx leads directly to greater ozone production than expected, thereby extending the NOx-limited regime of the upper troposphere. These results could thus have bearing on the predicted impacts of increasing NO• emissions into this region of the atmosphere from, for example, the growth of global air traffic. DC-8 aircraft was the main platform for the study, and it operated for over 120 hours, including two transcontinental flights and 12 others based from Bangor, Maine, Shannon, Ireland, and the Azores. A summary of the mission can be found in the work of Singh et al. [1999]. The central importance of OH and HO2 in the chemistry of the atmosphere has been known for several decades [Levy, 1972; Logan et al., 1981], but it is only recently that in situ, simultaneous measurements of both of these extremely reactive and, consequently, very short-lived species have become available to test our understanding of the oxidative cycles in the atmosphere [Wennberg et al., 1995; Poppe et al., 1997; Folkins et al., 1997; Brune et al., 1998]. Comparisons of measurements made in the planetary boundary layer with steady-state models have been somewhat inconclusive in assessing whether or not an accurate understanding of the chemical cycles that give rise to the instantaneous levels of HOx in the troposphere is extant, or for that matter, if steady state is an applicable assumption [Crosley, 1995]. Experiments in the planetary boundary layer have shown that in general, the observations of OH are lower than expected [Perner et al., 1987; Eisele et al., 1996; Comes et al., 1997; Armerding et al., 1997]. This led many researches to invoke the presence of some unbeknown chemical sink, most probably hydrocarbon in form, which was not included in the chemical box models. Comes et al. [ 1997] even notice that the deviations of the model in their experiment in the Canary Islands seem to correspond to the NO:NO2 ratio, but the relationship is not fully developed, and they too suggest a missing biogenic hydrocarbon sink of OH. 3771 3772 FALOONA ET AL.: HOx AND ITS RELATIONSHIP TO NOx IN THE UT Meanwhile, stratospheric observations have shown the budget of HOx to be fairly well understood in that region of the atmosphere [Stimpfie et al., 1989; Wennberg et al., 1995; Pickett and Peterson, 1996]. The former two in situ studies did, however, help confirm the absence of two important heterogeneous processes in photochemical models at the time: the hydrolysis of N205 to HNO3 and the nighttime conversion of NOx species to nitrous acid (HONO) on sulfate aerosols. The latter mechanism provides a longer wavelength photolysis source of HOx in the early morning at high solar zenith angles, and tentative evidence of this was observed in the upper troposphere during SONEX as well [Jaegld et al., this issue]. Further measurements of HO• in the upper troposphere and lower stratosphere (UT/LS) provided evidence of other previously unrecognized players in the HOx budget in these dry environments, namely, the photolysis of acetone and peroxides [Singh et al., 1995; Jaegld et al., 1997; Folkins et al., 1998]. Brune et al. [1998] observed evidence of these precursors convectively lofted from Asia and transported to the upper troposphere over northern California. Jaegld et al. [1997] demonstrated this effect over the central United States in the data from the NASA Subsonic Aircraft Contrail and Cloud
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.
Journal of Geophysical Research, 2001
Observations over the tropical Pacific during the Pacific Exploratory Mission (PEM)-Tropics B exp... more Observations over the tropical Pacific during the Pacific Exploratory Mission (PEM)-Tropics B experiment (March-April 1999) are analyzed. Concentrations of CO and long-lived nonmethane hydrocarbons in the region are significantly enhanced due to transport of pollutants from northern industrial continents. This pollutant import also enhances moderately 03 concentrations but not NOx concentrations. It therefore tends to depress OH concentrations over the tropical Pacific. These effects contrast to the large enhancements of 03 and NOx concentrations and the moderate increase of OH concentrations due to biomass burning outflow during the PEM-Tropics A experiment (September-October 1996). Observed CH3I concentrations, as in PEM-Tropics A, indicate that convective mass outflux in the middle and upper troposphere is largely independent of altitude over the tropical Pacific. Constraining a one-dimensional model with CH31 observations yields a 1 O-day timescale for convective turnover of the free troposphere, a factor of 2 faster than during PEM-Tropics A. Model simulated HO2, CH20, H202, and CH3OOH concentrations are generally in agreement with observations. However, simulated OH concentrations are lower (~25%) than observations above 6 km. Whereas models tend to overestimate previous field measurements, simulated HNO3 concentrations during PEM-Tropics B are too low (a factor of 2-4 below 6 km) compared to observations. Budget analyses indicate that chemical production of 03 accounts for only 50% of chemical loss; significant transport of 03 into the region appears to take place within the tropics. Convective transport of CH3OOH enhances the production of HOx and 03 in the upper troposphere, but this effect is offset by HOx loss due to the scavenging of H202. Convective transport and scavenging of reactive nitrogen species imply a necessary source of 0.4-1 Tg yr -1 of NO., in the free troposphere (above 4 km) over the tropics. A large fraction of the source could be from marine lightning. Oxidation of DMS transported by convection from the boundary layer could explain the observed free tropospheric SO2 concentrations over the tropical Pacific. This source of DMS due to convection, however, would imply in the model free tropospheric concentrations much higher than observed. The model overestimate cannot be reconciled using recent kinetics measurements of the DMS-OH adduct reaction at low pressures and temperatures and may reflect enhanced OH oxidation of DMS during convection.
Journal of Geophysical Research, 2002
1] A one-dimensional Lagrangian model for atmospheric transport and photochemistry has been devel... more 1] A one-dimensional Lagrangian model for atmospheric transport and photochemistry has been developed and used to interpret measurements made at Pellston, Michigan, during the summer of 1998. The model represents a moving vertical column of air with vertical resolution of 25 m near the ground. Calculations have been performed for a series of trajectories, with representation of emissions, vertical mixing, and photochemistry for a 3-day period ending with the arrival of the air column at Pellston. Results have been used to identify causes of the observed decrease in isoprene at night, to investigate causes of high nighttime OH. Significant OH can be generated at night by terpenes if it is assumed that some fast-reacting monoterpenes are emitted at rates comparable to inventory emissions for terpenes. However, this nighttime OH is confined to a shallow surface layer (0 -25 m) and has little impact on nighttime chemistry. The observed decrease in isoprene at night can be reproduced in models with low OH, and is attributed primarily to vertical dilution. There is also evidence that transport from Lake Michigan contributes to low nighttime isoprene at Pellston. Model results compare well with measured isoprene, NO x , and with isoprene vertical profiles. Significant model-measurement discrepancies are found for OH, HO 2 , methylvinylketone, and formaldehyde.
Journal of Geophysical Research, 2000
Simultaneous measurements of the oxides of hydrogen and nitrogen made during the NASA Subsonic As... more Simultaneous measurements of the oxides of hydrogen and nitrogen made during the NASA Subsonic Assessment, Ozone and Nitrogen Oxide Experiment (SONEX) afforded an opportunity to study the coupling between these two important families throughout the free troposphere and lowermost stratosphere. Moreover, the suite of measurements made during the campaign was unprecedented in its completeness, thus providing a uniquely detailed picture of the radical photochemistry that drives oxidation and ozone production in this part of the atmosphere. On average, observed hydrogen oxides (HOx = OH + HO2) agree well with both instantaneous and diel steady-state models; however, there is a persistent deviation of the observations that correlates with the abundance of nitrogen oxides (NOx = NO + NO2) in the sampled air mass. Specifically, the observed HO• tends to exceed the model predictions in the presence of high NO• concentrations, by as much as a factor of 5 (>500 pptv NOx), and is sometimes as little as half that expected by steady state at lower NOj levels. While many possibilities for these discrepancies are discussed, it is argued that an instrumental artifact is not probable and that the discrepancy may bespeak a shortcoming of our understanding of HO• chemistry. The consistently elevated HO• in the presence of elevated NOx leads directly to greater ozone production than expected, thereby extending the NOx-limited regime of the upper troposphere. These results could thus have bearing on the predicted impacts of increasing NO• emissions into this region of the atmosphere from, for example, the growth of global air traffic. DC-8 aircraft was the main platform for the study, and it operated for over 120 hours, including two transcontinental flights and 12 others based from Bangor, Maine, Shannon, Ireland, and the Azores. A summary of the mission can be found in the work of Singh et al. [1999]. The central importance of OH and HO2 in the chemistry of the atmosphere has been known for several decades [Levy, 1972; Logan et al., 1981], but it is only recently that in situ, simultaneous measurements of both of these extremely reactive and, consequently, very short-lived species have become available to test our understanding of the oxidative cycles in the atmosphere [Wennberg et al., 1995; Poppe et al., 1997; Folkins et al., 1997; Brune et al., 1998]. Comparisons of measurements made in the planetary boundary layer with steady-state models have been somewhat inconclusive in assessing whether or not an accurate understanding of the chemical cycles that give rise to the instantaneous levels of HOx in the troposphere is extant, or for that matter, if steady state is an applicable assumption [Crosley, 1995]. Experiments in the planetary boundary layer have shown that in general, the observations of OH are lower than expected [Perner et al., 1987; Eisele et al., 1996; Comes et al., 1997; Armerding et al., 1997]. This led many researches to invoke the presence of some unbeknown chemical sink, most probably hydrocarbon in form, which was not included in the chemical box models. Comes et al. [ 1997] even notice that the deviations of the model in their experiment in the Canary Islands seem to correspond to the NO:NO2 ratio, but the relationship is not fully developed, and they too suggest a missing biogenic hydrocarbon sink of OH. 3771 3772 FALOONA ET AL.: HOx AND ITS RELATIONSHIP TO NOx IN THE UT Meanwhile, stratospheric observations have shown the budget of HOx to be fairly well understood in that region of the atmosphere [Stimpfie et al., 1989; Wennberg et al., 1995; Pickett and Peterson, 1996]. The former two in situ studies did, however, help confirm the absence of two important heterogeneous processes in photochemical models at the time: the hydrolysis of N205 to HNO3 and the nighttime conversion of NOx species to nitrous acid (HONO) on sulfate aerosols. The latter mechanism provides a longer wavelength photolysis source of HOx in the early morning at high solar zenith angles, and tentative evidence of this was observed in the upper troposphere during SONEX as well [Jaegld et al., this issue]. Further measurements of HO• in the upper troposphere and lower stratosphere (UT/LS) provided evidence of other previously unrecognized players in the HOx budget in these dry environments, namely, the photolysis of acetone and peroxides [Singh et al., 1995; Jaegld et al., 1997; Folkins et al., 1998]. Brune et al. [1998] observed evidence of these precursors convectively lofted from Asia and transported to the upper troposphere over northern California. Jaegld et al. [1997] demonstrated this effect over the central United States in the data from the NASA Subsonic Aircraft Contrail and Cloud
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.