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Accurate fire emissions inventories are crucial to predict the impacts of wildland fires on air quality and atmospheric composition. Two traditional approaches are widely used to calculate fire emissions: a satellite-based top-down approach and a fuels-based bottom-up approach. However, these methods often considerably disagree on the amount of particulate mass emitted from fires. Previously available observational datasets tended to be sparse, and lacked the statistics needed to resolve these methodological discrepancies. Here, we leverage the extensive and comprehensive airborne in situ and remote sensing measurements of smoke plumes from the recent Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign to statistically assess the skill of the two traditional approaches. We use detailed campaign observations to calculate and compare emission rates at an exceptionally high-resolution using three separate approaches: top-down, bottom-up, and a novel approach based entirely on integrated airborne in situ measurements. We then compute the daily average of these high-resolution estimates and compare with estimates from lower resolution, global top-down and bottom-up inventories. We uncover strong, linear relationships between all of the high-resolution emission rate estimates in aggregate, however no single approach is capable of capturing the emission characteristics of every fire. Global inventory emission rate estimates exhibited weaker correlations with the high-resolution approaches and displayed evidence of systematic bias. The disparity between the low-resolution global inventories and the high-resolution approaches is likely caused by high levels of uncertainty in essential variables used in bottom-up inventories and imperfect assumptions in top-down inventories.

International audience; Satellite ammonia (NH3) observations provide unprecedented insights into NH3 emissions, spatiotemporal variabilities and trends, but validation with in‐situ measurements remains lacking. Here, total columns from the Infrared Atmospheric Sounding Interferometer (IASI) were intercompared to boundary layer NH3 profiles derived from aircraft‐ and surface‐based measurements primarily in Colorado, USA, in the summer of 2014. IASI‐NH3 version 3 near real‐time dataset compared well to in‐situ derived columns (windows ±15 km around centroid, ±1 hour around overpass time) with a correlation of 0.58, a slope of 0.78±0.14, and an intercept of 2.1×1015±1.5×1015 molecules cm‐2. Agreement degrades at larger spatiotemporal windows, consistent with the short atmospheric lifetime of NH3. We also examined IASI version 3R data, which relies on temperature retrievals from the ERA Reanalysis, and a third product generated using aircraft‐measured temperature profiles. The overall agreement improves slightly for both cases, and neither is biased within their combined measurement errors. Thus, spatiotemporal averaging of IASI over large windows can be used to reduce retrieval noise. Nonetheless, sampling artifacts of airborne NH3 instruments result in significant uncertainties of the in‐situ‐derived columns. For example, large validation differences exist between ascent and descent profiles, and the assumptions of the free tropospheric NH3 profiles used above the aircraft ceiling significantly impact the validation. Because short‐lived species like NH3 largely reside within the boundary layer with complex vertical structures, more comprehensive validation is needed across a wide range of environments. More accurate and widespread in‐situ NH3 datasets are therefore required for improved validations of satellite products.

Satellite-based measurements of the column CH2O/NO2 ratio have previously been used to estimate near-surface ozone (O3) sensitivity (i.e., NOx or VOC limited), and the forthcoming launch of air quality-focused geostationary satellites provides a catalyst for reevaluating the ability of satellite-measured CH2O/NO2 to be used in this manner. In this study, we use a 0-D photochemical box model to evaluate O3 sensitivity and find that the relative rate of radical termination from radical-radical interactions to radical-NOx interactions (referred to as LROx/LNOx) provides a good indicator of maximum O3 production along NOx ridgelines. Using airborne measurements from NASA's Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relative to Air Quality (DISCOVER-AQ) deployments in Colorado, Maryland, and Houston, we show that in situ measurements of CH2O/NO2 can be used to indicate O3 sensitivity, but there is an important “transition/ambiguous” range whereby CH2O/NO2 fails to categorize O3 sensitivity, and the range and span of this transition/ambiguous range varies regionally. Then, we apply these findings to aircraft-derived column density measurements from DISCOVER-AQ and find that inhomogeneities in vertical mixing in the lower troposphere further degrades the ability of column CH2O/NO2 to indicate near-surface O3 sensitivity (i.e., the transition/ambiguous range is much larger than indicated by in situ data alone), and we hypothesize that the global transition/ambiguous range is sufficiently large to make the column CH2O/NO2 ratio unuseful for classifying near-surface O3 sensitivity. Lastly, we present a case study from DISCOVER-AQ-Houston that suggests that O3 sensitivity on exceedance days may be substantially different than on nonexceedance days (which may be observable from space) and explore the diurnal evolution of O3 sensitivity, O3 production, and the column CH2O/NO2 ratio. The results of these studies suggest that although satellite measurements of CH2O/NO2 alone may not be sufficient for accurately classifying near-surface O3 sensitivity, new techniques offered by geostationary platforms may nonetheless provide methods for using space-based measurements to develop O3 mitigation strategies.

Strong correlation between O3 and CO was observed during the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) aircraft experiment in July 2011 over the Washington-Baltimore area. The observed correlation does not vary significantly with time or altitude in the boundary layer. The observations are simulated well by a regional chemical transport model. We analyze the model results to understand the factors contributing to the observed O3-CO regression slope, which has been used in past studies to estimate the anthropogenic O3 production amount. We trace separately four different CO sources: primary anthropogenic emissions, oxidation of anthropogenic volatile organic compounds, oxidation of biogenic isoprene, and transport from the lateral and upper model boundaries. Modeling analysis suggests that the contribution from biogenic isoprene oxidation to the observed O3-CO regression slope is as large as that from primary anthropogenic CO emissions. As a result of decrease of anthropogenic primary CO emissions during the past decades, biogenic CO from oxidation of isoprene is increasingly important. Consequently, observed and simulated O3-CO regression slopes can no longer be used directly with an anthropogenic CO emission inventory to quantify anthropogenic O3 production over the United States. The consistent enhancement of O3 relative to CO observed in the boundary layer, as indicated by the O3-CO regression slope, provides a useful constraint on model photochemistry and emissions.

Nitrogen dioxide (NO2) is a short-lived atmospheric pollutant that serves as an air quality indicator, and is itself a health concern. The Airborne Compact Atmospheric Mapper (ACAM) was flown on board the NASA UC-12 aircraft during the DISCOVER-AQ Maryland field campaign in July 2011. The instrument collected hyperspectral remote sensing measurements in the 304-910 nm range, allowing day-time observations of several tropospheric pollutants, including nitrogen dioxide (NO2), at an unprecedented spatial resolution of 1.5 × 1.1 km2. Retrievals of slant column abundance are based on the Differential Optical Absorption Spectroscopy (DOAS) method. For the Air Mass Factor (AMF) computations needed to convert these retrievals to vertical column abundance, we include high resolution information for the surface reflectivity by using bidirectional reflectance distribution function (BRDF) data from the Moderate Resolution Imaging Spectroradiometer (MODIS). We use high-resolution simulated vertical distributions of NO2 from the Community Multiscale Air Quality (CMAQ) and Global Modeling Initiative (GMI) models to account for the temporal variation in atmospheric NO2 to retrieve middle- and lower-tropospheric NO2 columns (NO2 below the aircraft). We compare NO2 derived from ACAM measurements with in-situ observations from NASA's P-3B research aircraft, total column observations from the ground-based Pandora spectrometers, and tropospheric column observations from the space-based OMI instrument. The high-resolution ACAM measurements not only give new insights into our understanding of atmospheric composition and chemistry through observation of sub-sampling variability in typical satellite and model resolutions, but they also provide opportunities for testing algorithm improvements for forthcoming geostationary air quality missions.

We have developed semi-independent methods for determining CH2O scavenging efficiencies (SEs) during strong midlatitude convection over the western, south-central Great Plains, and southeastern regions of the United States during the 2012 Deep Convective Clouds and Chemistry (DC3) Study. The Weather Research and Forecasting model coupled with chemistry (WRF-Chem) was employed to simulate one DC3 case to provide an independent approach of estimating SEs and the opportunity to study CH2O retention in ice when liquid drops freeze. Measurements of CH2O in storm inflow and outflow were acquired on board the NASA DC-8 and the NSF/National Center for Atmospheric Research Gulfstream V (GV) aircraft employing cross-calibrated infrared absorption spectrometers. This study also relied heavily on the nonreactive tracers i-/n-butane and i-/n-pentane measured on both aircraft in determining lateral entrainment rates during convection as well as their ratios to ensure that inflow and outflow air masses did not have different origins. Of the five storm cases studied, the various tracer measurements showed that the inflow and outflow from four storms were coherently related. The combined average of the various approaches from these storms yield remarkably consistent CH2O scavenging efficiency percentages of: 54% ± 3% for 29 May; 54% ± 6% for 6 June; 58% ± 13% for 11 June; and 41 ± 4% for 22 June. The WRF-Chem SE result of 53% for 29 May was achieved only when assuming complete CH2O degassing from ice. Further analysis indicated that proper selection of corresponding inflow and outflow time segments is more important than the particular mixing model employed.

An often used assumption in air pollution studies is a well-mixed boundary layer (BL), where pollutants are evenly distributed. Because of the difficulty in obtaining vertically resolved measurements, the validity of the assumption has not been thoroughly evaluated. In this study, we usemore than 200 vertical profiles observed in the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) aircraft campaign in July 2011 to examine the vertical distributions of pollutants over the Washington-Baltimore area. While many long-lived species are well mixed in daytime, the observed average vertical profile of NOx shows a large negative gradient with increasing altitude in the BL. Our analysis suggests that the magnitude of the NOx gradient is highly sensitive to atmospheric stability. We investigate how parameterizations of the BL and land-surface processes impact vertical profiles in a 1-D chemical transport model, using three BL schemes (Asymmetric Convective Model version 2 (ACM2), Yonsei University (YSU), and Mellor-Yamada-Janjic (MYJ)) and two land-surface schemes (Noah and Rapid Update Cycle (RUC)). The model reasonably reproduces the median vertical profiles of NOx under different BL stability conditions within 30% of observations, classified based on potential temperature gradient and BL height. Comparisons with NOx observations for individual vertical profiles reveal that while YSU performs better in the turbulent and deep BL case, in general, ACM2 (RMSE=2.0ppbv) outperforms YSU (RMSE=2.5ppbv) and MYJ (RMSE=2.2ppbv). Results also indicate that the land-surface schemes in the Weather Research and Forecasting (WRF) model have a small impact on the NOx gradient. Usingmodel simulations, we analyze the impact of BL NOx gradient on the calculation of the ozone production rate and satellite NO2 retrieval. We show that using surface measurements and the well-mixed BL assumption causes a~45%highbias in the estimated BL ozoneproduction rate and that the variability of NO2 vertical profiles is responsible for 5–10% variability in the retrieved NO2 tropospheric vertical columns.

The San Joaquin Valley (SJV) of California experiences high concentrations of particulate matter NH(4)NO(3) during episodes of meteorological stagnation in winter. A rich data set of observations related to NH(4)NO(3) formation was acquired during multiple periods of elevated NH(4)NO(3) during the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) field campaign in SJV in January and February 2013. Here NH(4)NO(3) is simulated during the SJV DISCOVER-AQ study period with the Community Multiscale Air Quality (CMAQ) model, diagnostic model evaluation is performed using the DISCOVER-AQ data set, and integrated reaction rate analysis is used to quantify HNO(3) production rates. Simulated NO(3)(−) generally agrees well with routine monitoring of 24-hr average NO(3)(−), but comparisons with hourly average NO(3)(−) measurements in Fresno revealed differences at higher time resolution. Predictions of gas-particle partitioning of total nitrate (HNO(3) + NO(3)(−)) and NHx (NH(3) + NH(4)(+)) generally agree well with measurements in Fresno, although partitioning of total nitrate to HNO(3) is sometimes overestimated at low relative humidity in afternoon. Gas-particle partitioning results indicate that NH(4)NO(3) formation is limited by HNO(3) availability in both the model and ambient. NH(3) mixing ratios are underestimated, particularly in areas with large agricultural activity, and additional work on the spatial allocation of NH(3) emissions is warranted. During a period of elevated NH(4)NO(3), the model predicted that the OH + NO(2) pathway contributed 46% to total HNO(3)production in SJV and the N(2)O(5) heterogeneous hydrolysis pathway contributed 54%. The relative importance of the OH + NO(2) pathway for HNO(3) production is predicted to increase as NOx emissions decrease.

Analysis of sun photometer measured and satellite retrieved aerosol optical depth (AOD) data has shown that major aerosol pollution events with very high fine mode AOD (>1.0 in mid-visible) in the China/Korea/Japan region are often observed to be associated with significant cloud cover. This makes remote sensing of these events difficult even for high temporal resolution sun photometer measurements. Possible physical mechanisms for these events that have high AOD include a combination of aerosol humidification, cloud processing, and meteorological co-variation with atmospheric stability and convergence. The new development of Aerosol Robotic network (AERONET) Version 3 Level 2 AOD with improved cloud screening algorithms now allow for unprecedented ability to monitor these extreme fine mode pollution events. Further, the Spectral Deconvolution Algorithm (SDA) applied to Level 1 data (L1; no cloud screening) provides an even more comprehensive assessment of fine mode AOD than L2 in current and previous data versions. Studying the 2012 winter-summer period, comparisons of AERONET L1 SDA daily average fine mode AOD data showed that Moderate Resolution Imaging Spectroradiometer (MODIS) satellite remote sensing of AOD often did not retrieve and/or identify some of the highest fine mode AOD events in this region. Also, compared to models that include data assimilation of satellite retrieved AOD, the L1 SDA fine mode AOD was significantly higher in magnitude, particularly for the highest AOD events that were often associated with significant cloudiness.

A significant source of ozone in the troposphere is transport from the stratosphere. The stratospheric contribution has been estimated mainly using global models that attribute the transport process largely to the global scale Brewer-Dobson circulation and synoptic scale dynamics associated with upper tropospheric jet streams. We report observations from research aircraft that reveal additional transport of ozone-rich stratospheric air downward into the upper troposphere by a leading-line-trailing-stratiform (LLTS) mesoscale convective system (MCS) with convection overshooting the tropopause altitude. The fine-scale transport demonstrated by these observations poses a significant challenge to global models that currently do not resolve storm scale dynamics. Thus the upper tropospheric ozone budget simulated by global chemistry-climate models where large-scale dynamics and photochemical production from lightning-produced NO are the controlling factors may require modification.

Global atmospheric concentrations of methane (CH4), a powerful greenhouse gas, are increasing, but because there are many natural and anthropogenic sources of CH4, it is difficult to assess which sources may be increasing in magnitude. Here we present a dataset of δ2H-CH4 measurements of individual sources and air in the Colorado Front Range, USA. We show that δ2H-CH4, but not δ13C, signatures are consistent in air sampled downwind of landfills, cattle feedlots, and oil and gas wells in the region. Applying these source signatures to air in ground and aircraft samples indicates that at least 50% of CH4 emitted in the region is biogenic, perhaps because regulatory restrictions on leaking oil and natural gas wells are helping to reduce this source of CH4. Source apportionment tracers such as δ2H may help close the gap between CH4 observations and inventories, which may underestimate biogenic as well as thermogenic sources.

As part of the Deep Convective Cloud and Chemistry (DC3) experiment, the National Science Foundation/National Center for Atmospheric Research (NCAR) Gulfstream-V (GV) and NASA DC-8 research aircraft probed the chemical composition of the inflow and outflow of two convective storms (north storm, NS, south storm, SS) originating in the Colorado region on 22 June 2012, a time when the High Park wildfire was active in the area. A wide range of trace species were measured on board both aircraft including biomass burning (BB) tracers hydrogen cyanide (HCN) and acetonitrile (ACN). Acrolein, a much shorter lived tracer for BB, was also quantified on the GV. The data demonstrated that the NS had ingested fresh smoke from the High Park fire and as a consequence had a higher VOC OH reactivity than the SS. The SS lofted aged fire tracers along with other boundary layer ozone precursors and was more impacted by lightning NO_x (LNO_x) than the NS. The NCAR master mechanism box model was initialized with measurements made in the outflow of the two storms. The NS and SS were predicted to produce 11 and 14 ppbv of O_3, respectively, downwind of the storm over 2 days. Sensitivity tests revealed that the ozone production potential of the SS was highly dependent on LNO_x. Normalized excess mixing ratios, ΔX/ΔCO, for HCN and ACN were determined in both the fire plume and the storm outflow and found to be 7.0 ± 0.5 and 2.3 ± 0.5 pptv ppbv^(−1), respectively, and 1.4 ± 0.3 pptv ppbv^(−1) for acrolein in the outflow only.

In support of future satellite missions that aim to address the current shortcomings in measuring air quality from space, NASA’s Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) field campaign was designed to enable exploration of relationships between column measurements of trace species relevant to air quality at high spatial and temporal resolution. In the DISCOVER-AQ data set, a modest correlation (r(2) = 0.45) between ozone (O(3)) and formaldehyde (CH(2)O) column densities was observed. Further analysis revealed regional variability in the O(3)-CH(2)O relationship, with Maryland having a strong relationship when data were viewed temporally and Houston having a strong relationship when data were viewed spatially. These differences in regional behavior are attributed to differences in volatile organic compound (VOC) emissions. In Maryland, biogenic VOCs were responsible for ~28% of CH(2)O formation within the boundary layer column, causing CH(2)O to, in general, increase monotonically throughout the day. In Houston, persistent anthropogenic emissions dominated the local hydrocarbon environment, and no discernable diurnal trend in CH(2)O was observed. Box model simulations suggested that ambient CH(2)O mixing ratios have a weak diurnal trend (±20% throughout the day) due to photochemical effects, and that larger diurnal trends are associated with changes in hydrocarbon precursors. Finally, mathematical relationships were developed from first principles and were able to replicate the different behaviors seen in Maryland and Houston. While studies would be necessary to validate these results and determine the regional applicability of the O(3)-CH(2)O relationship, the results presented here provide compelling insight into the ability of future satellite missions to aid in monitoring near-surface air quality.

Ten-day backward trajectories are used to determine the origins of air parcels arriving at locations of airborne DC-8 chemical measurements during NASA's second Pacific Exploratory Mission in the Tropics B that was conducted during February-April 1999. Chemical data at sites where the trajectories had a common geographical origin and transport history are grouped together, and statistical measures of chemical characteristics are computed. Temporal changes in potential temperature are used to determine whether trajectories experienced a significant convective influence during the 10-day period. Trajectories describing the aged marine Southern Hemispheric category remain over the South Pacific Ocean during the 10-day period, and their corresponding chemical signature indicates very clean air. The category aged marine air in the Northern Hemisphere is found to be somewhat dirtier. Subdividing its trajectories based on the direction from which the air had traveled is found to be important in explaining the various chemical signatures. Similarly, long-range northern hemispheric trajectories passing over Asia are subdivided depending on whether they had followed a mostly zonal path, had originated near the Indian Ocean, or had originated near Central or South America and subsequently experienced a stratospheric influence. Results show that the chemical signatures of these subcategories are different from each other. The chemical signature of the southern hemispheric long-range transport category apparently exhibits the effects of pollution from Australia, southern Africa, and South America. Parcels originating over Central and northern South America are found to contain the strongest pollution signature of all categories, due to biomass burning and other sources. The convective category exhibits enhanced values of nitrogen species, probably due to emissions from lightning associated with the convection. Values of various species, including peroxides and acids, confirm that parcels were influenced by the removal of soluble gas and particle species due to precipitation. Finally, current results are compared with those from the first PEM-Tropics mission that was conducted in the same region during the southern hemispheric dry season (August-October 1996) when extensive biomass burning occurred. Results show that air samples during PEM-Tropics B are considerably cleaner than those of its dry season counterpart.

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 HO2 were larger than those observed in the upper troposphere, in contrast to previous studies which show a general underprediction Of HO2 at upper altitudes. Some characteristics of the budgets Of HOx, NOx, and peroxides are discussed. The integrated net tendency for O3 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 O3 into the Pacific region throughout the year must exist in order to maintain O3 levels. Integrated net tendencies for equatorial O3 showed a seasonality, with a net loss of 1.06×1011 molecules cm−2 s−1 during PEM-Tropics B (March) increasing by 50% to 1.60×1011 molecules cm−2 s−1 during PEM-Tropics A (September). The seasonality over the southern subtropical Pacific was somewhat lower, with losses of 1.21×1011 molecules cm−2 s−1 during PEM-Tropics B (March) increasing by 25% to 1.51×1011 molecules cm−2 s−1 during PEM-Tropics A (September). While the larger net losses during PEM-Tropics A were primarily driven by higher concentrations of O3, the ability of the subtropical atmosphere to destroy O3 was ∼30% less effective during the PEM-Tropics A (September) campaign due to a drier atmosphere and higher overhead O3 column amounts.

Analysis of chemical transport on Flight 10 of the 1999 Pacific Exploratory Mission (PEM) Tropics B mission clarifies the role of the South Pacific Convergence Zone (SPCZ) in establishing ozone and other trace gas distributions in the southwestern tropical Pacific. The SPCZ is found to be a barrier to mixing in the lower troposphere but a mechanism for convective mixing of tropical boundary layer air from northeast of the SPCZ with upper tropospheric air arriving from the west. A two-dimensional cloud-resolving model is used to quantify three critical processes in global and regional transport: convective mixing, lightning NOx production, and wet scavenging of soluble species. Very low NO and O3 tropical boundary layer air from the northeastern side of the SPCZ entered the convective updrafts and was transported to the upper troposphere where it mixed with subtropical upper tropospheric air containing much larger NO and O3 mixing ratios that had arrived from Australia. Aircraft observations show that very little NO appears to have been produced by electrical discharges within the SPCZ convection. We estimate that at least 90% of the HNO3 and H2O2 that would have been in upper tropospheric cloud outflow had been removed during transport through the cloud. Lesser percentages are estimated for less soluble species (e.g.

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 km). Similarly, only modest variations were found (factors of 1.5 to 3.5) when the data were examined as a function of latitude (30°N to 30°S). Using simultaneously recorded data for CO, O3, H2O, 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.

During the NASA Pacific Exploratory Mission - Tropics B (PEM-Tropics B) the NO2 photolysis frequency was simultaneously measured by colocated National Center for Atmospheric Research scanning actinic flux spectroradiometer (SAFS) and commercial filter radiometer (FR) instruments on the NASA DC-8 and the NASA P-3B research aircraft. The measurements were collected from early March to mid-April 1999 over the Pacific Ocean, spanning the region between 35°N-25°S latitude and 80°W-170°E Longitude. Over the course of the mission the SAFS and FR instruments performed quite well with both systems reporting data for more than 85% of the PEM-Tropics B flight hours. For both the SAFS and FR systems the reported totaljNO2 is the sum of the signals from independent zenith and nadir viewing instruments. The 1-min average SAFS and FRjNO2 values were quite highly correlated (r2>0.98) for both the zenith and nadir systems in this complex radiation environment. During PEM-Tropics B these instruments exhibited a difference ranging from 5 to 40% (depending on the instrument pair), with the jNO2 FR measurements always being higher. In contrast, a consistent 30% disagreement was observed for the SAFS and FR instruments during PEM-Tropics A. Use of newer NO2 cross-section data would increase the SAFS jNO2 by 4%, while the larger difference between one of the FR and SAFS instrument pairs could be related to a change in the FR instrument sensitivity.

Author(s): Chen, G; Davis, D; Crawford, J; Nowak, JB; Eisele, F; Mauldin, RL; Tanner, D; Buhr, M; Shetter, R; Lefer, B; Arimoto, R; Hogan, A; Blake, D | Abstract: Unexpected high levels of OH and NO were recorded at the South Pole (SP) Atmospheric Research Observatory during the 1998-99 ISCAT field study. Model simulations suggest a major photochemical linkage between observed OH and NO. A detailed comparison of the observations with model predictions revealed good agreement for OH at NO levels between 120 and 380 pptv. However, the model tended to overestimate OH for NO levels l 120 pptv, while it underestimated OH at levels g 380 pptv. The reasons for these deviations appear not to involve NO directly but rather HOx radical scavenging for the low NO conditions and additional HOx sources for the high NO conditions. Because of the elevated levels of NO and highly activated HOx photochemistry, the SP was found to be a strong net source of surface ozone. It is quite likely that the strong oxidizing environment found at the South Pole extends over the entire polar plateau.

Observations of OH, NO, and actinic flux at the South Pole surface during December 1998 suggest a surprisingly active photochemical environment which should result in photochemical production of ozone. Long-term South Pole in situ ozone data as well as sonde data also appear to support this conclusion. Other possible factors contributing to ozone variability such as stratospheric influence and the origin of air transported to the South Pole are also explored. Based on box model calculations it is estimated that photochemistry could add 2.2 to 3.6 ppbv/day of ozone to surface air parcels residing on the Antarctic polar plateau. Although the oxidizing potential of the polar plateau appears to be exceptionally high for a remote site, it is unlikely that it has a significant impact on surrounding regions such as the Southern Ocean and the Antarctic free troposphere. These new findings do suggest, however, that the enhanced oxidizing power of the polar plateau may need to be considered in interpreting the chemical history of climate proxy species in ice cores.

The impact of ship emissions on marine boundary layer (MBL) NOx and SO2 levels over the Pacific Ocean has been explored by comparing predictions (with and without ships) from a global chemical transport model (GCTM) against compiled airborne observations of MBL NOx and SO2. For latitudes >15°N, which define that part of the Pacific having the heaviest shipping, this analysis revealed significant model over prediction for NOx and a modest under prediction for SO2 when ship emissions were considered. Possible reasons for the difference in NOx and SO2 were explored using a full-chemistry box model. These results revealed that for an actual plume setting the NOx lifetime could be greatly shortened by chemical processes promoted by ship plume emissions themselves. Similar chemical behavior was not found for SO2.

Airborne observations of NO during the Subsonics Assessment Ozone and Nitrogen Oxides Experiment (SONEX) reveal episodes of high NOx in the upper troposphere believed to be associated with lightning. Linkage to specific periods of lightning activity is possible through back trajectories and data from the National Lightning Detection Network. Lagrangian model calculations are used to explore the evolution of these high NOx plumes over the 1–2 days between their introduction and subsequent sampling by NASA's DC-8 aircraft. Simulations include expected changes in HNO3, H2O2, CH3OOH, HO2, and OH. Depending on the time of injection and dilution rate, initial NOx concentrations are estimated to range from 1 to 7 ppbv. Similar to many previous studies, simulated HNO3 concentrations tend to be greater than observations. Several possible explanations for this difference are explored. H2O2 observations are shown to be consistent with removal in convective activity. While it is possible that upper tropospheric CH3OOH is enhanced by convection, simulations show such increases in CH3OOH can be short-lived (e.g.

Interactions between atmospheric hydrogen oxides and aircraft nitrogen oxides determine the impact of aircraft exhaust on atmospheric chemistry. To study these interactions, the Subsonic Assessment: Ozone and Nitrogen Oxide Experiment (SONEX) assembled the most complete measurement complement to date for studying HOx (OH and HO2) chemistry in the free troposphere. Observed and modeled HOx agree on average to within experimental uncertainties (±40%). However, significant discrepancies occur as a function of NO and at solar zenith angles >70°. Some discrepancies appear to be removed by model adjustments to HOx-NOx chemistry, particularly by reducing HO2NO2 (PNA) and by including heterogeneous reactions on aerosols and cirrus clouds.

Data for the tropical upper troposphere (8–12 km, 20°N-20°S) 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., H2O2 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, CH3O2, CH2O, 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 HOx due to peroxides appears to be less than previously estimated. In particular, observations of elevated H2O2 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.

Advanced very high resolution radiometer 1.1 km resolution satellite radiance data were used to locate active fires throughout much of the tropical region during NASA's Global Tropospheric Experiment (GTE) Pacific Exploratory Mission-Tropics (PEM-Tropics A) aircraft campaign, held in September and October 1996. The spatial and temporal distributions of the fires in Australia, southern Africa, and South America are presented here. The number of fires over northern Australia, central Africa, and South America appeared to decrease toward the end of the mission period. Fire over eastern Australia was widespread, and temporal patterns showed a somewhat consistent amount of burning with periodic episodes of enhanced fire counts observed. At least one episode of enhanced fire counts corresponded to the passage of a frontal system which brought conditions conducive to fire to the region, with strong westerlies originating over the hot, dry interior continent. Regions that were affected by lower than normal rainfall during the previous wet season (e.g., northern Australia and southwestern Africa) showed relatively few fires during this period. This is consistent with a drought-induced decrease in vegetation and therefore a decreased availability of fuel for burning. Alternatively, a heavier than normal previous wet season along the southeastern coast of South Africa may have contributed to high fuel loading and an associated relatively heavy amount of burning compared to data from previous years.

An assessment ofthe effects of clouds on photolysis rate coefficients was carried out using three different experimental methods involving two different aircraft platforms. This evaluation was based on data recorded during NASA's Pacific Exploratory Mission (PEM)-Tropics A program in August-October 1996. On the NASA DC-8, upward and downward looking J(NO 2 ) filter radiometers and spectroradiometers were employed. For the NASA P-3B, the instrumentation consisted of Eppley radiometers. Although each aircraft typically sampled the same geographic region, coincident measurements occurred for only one brief period in the marine boundary layer near Christmas Island (2°N, 157°W). All three methods were compared for this flight period; however, only the J(NO 2 ) filter radiometers and spectroradiometers could be compared for the entire campaign. For the Christmas Island sampling period, all three radiometric measurements disagreed in magnitude but exhibited trends consistent with model-calculated photolysis rate coefficients. Overall, the results showed that the J(NO 2 ) filter radiometers and spectroradiometers exhibited a consistent disagreement of 30%, the J(NO 2 ) filter radiometers being higher. Eppley-derived values of J(NO 2 ) fell between those of the J(NO 2 ) filter radiometers and spectroradiometers. An examination of the variation in J(O 1 D) and J(NO 2 ) based on the output from the spectroradiometers and J(NO 2 ) filter radiometers suggests that differences between each photolysis rate coefficient's response to cloud effects tend to be smaller than model uncertainties. Thus J(O 1 D) and other photolysis rate coefficients can be corrected for cloud effects based on the response of J(NO 2 ).

Measurements of NO and NO2 are reported using a highly modified photofragmentation two-photon laser-induced fluorescence (PF-TP-LIF) instrument. Available evidence suggests that the changes made substantially reduced wall decomposition of labile NOy species. In sharp contrast to the results reported from NASA's 1991 PEM-West A program where the median value for the ratio (NO2)meas/(NO2)calc was 3.36, the PEM-Tropics A observations produced a value for this ratio of 0.93. This represents the first time that remote upper tropospheric NO-NO2 data have shown a high degree of correspondence with current photochemical mechanisms.

Studies of dimethylsulfide (DMS) oxidation chemistry were conducted at Palmer Station on Anvers Island, Antarctica, during the austral summer of 1993/1994. Part of the study involved gas phase measurements of OH, methane sulfonic acid (MSA), and H2SO4 using a chemical ionization mass spectrometer, as well as measurements of the NO, CO, and 03 concentrations. Mean 24 hour concentrations from February 16-23 of OH, MSA, and H2SO4 were 1.1 x 105, 9.5 x 105, and 1.61 x 106 molecules cm -3, respectively. Model calculations of OH compared well with observed levels (e.g., within 30%). The modeling results suggest that the dominant source of OH is from the reaction of O(1D) with H20, where O(1D) is the product of 03 photolysis. Because of the clean atmospheric environment and predicted low nonmethyl hydrocarbon levels in Antarctica, the dominant OH sink was found to be reaction with CO and CH4. Particulate levels of MSA were higher than could be attributed to condensation of boundary layer (BL) gas phase MSA on to the aerosol surface. Alternate mechanisms for generating MSA in the particle phase were speculated to involve either in-cloud oxidation of dimethylsulfoxide or OH oxidation of DMS in the atmospheric buffer layer above the boundary layer followed by condensation of gas phase MSA on aerosols and transport back to the B L (Davis et al., this issue).

A major observation recorded during NASA's western Pacific Exploratory Mission (PEM-West B) was the large shift in tropical NO levels as a function of geographical location. High-altitude NO levels exceeding 100 pptv were observed during portions of tropical flights 5-8, while values almost never exceeded 20 pptv during tropical flights 9 and 10. The geographical regions encompassing these two flight groupings are here labeled "high" and "low" NOx regimes. A comparison of these two regimes, based on back trajectories and chemical tracers, suggests that air parcels in both were strongly influenced by deep convection. The low NO(x) regime appears to have been predominantly impacted by marine convection, whereas the high NO(x) regime shows evidence of having been more influenced by deep convection over a continental land mass. DMSP satellite observations point strongly toward lightning as the major source of NOx in the latter regime. Photochemical ozone formation in the high NOx regime exceeded that for low NO(x) by factors of 2 to 6, whereas O3 destruction in the low NO(x)regime exceeded that for high NOx by factors of up to 3. Taking the tropopause height to be 17 km, estimates of the net photochemical effect on the O3 column revealed that the high NO(x) regime led to a small net production. By contrast, the low NOx regime was shown to destroy O3 at the rate of 3.4 % per day. One proposed mechanism for off-setting this projected large deficit would involve the transport of 03 rich midlatitude air into the tropics. Alternatively, it is suggested that O3 within the tropics may be overall near self-sustaining with respect to photochemical activity. This scenario would require that some tropical regions, unsampled at the time of PEM-B, display significant net column O3 production, leading to an overall balanced budget for the "greater" tropical Pacific basin. Details concerning the chemical nature of such regimes are discussed.

This study examines the influence of photochemical processes on tropospheric ozone distributions over the extratropical western North Pacific. The analysis presented here is based on data collected during the Pacific Exploratory Mission-West Phase B (PEM-West B) field study conducted in February-March 1994. Sampling in the study region involved altitudes of 0-12 km and latitudes of 10deg S to 50deg N. The extratropical component of the data set (i.e., 20-50deg N) was defined by markedly different photochemical environments north and south of 30deg N. This separation was clearly defined by an abrupt decrease in the tropopause height near 30deg N and a concomitant increase in total O3 column density. This shift in overhead O3 led to highly reduced rates of O3 formation and destruction for the 30-50deg N latitude regime. Both latitude ranges, however, still exhibited net O3 production at all altitudes. Of special significance was the finding that net O3 production prevailed even at boundary layer and lower free tropospheric altitudes (e.g., less than 4 km), a condition uncommon to Pacific marine environments. These results reflect the strong impact of continental Outflow of O3 precursors (e.g., NO and NMHCS) into the northwestern Pacific Basin. Comparisons with PEM-West A, which sampled the same region in a different season (September-October), revealed major differences at altitudes below 4 km, the altitude range most influenced by continental outflow. The resulting net rate of increase in the tropospheric O3 column for PEM-West B was 1-3 % per day, while for PEM-West A it was approximately zero. Unique to the PEM-West B study is the finding that even under wintertime conditions substantial column production of tropospheric O3 can occur at subtropical and mid-latitudes. While such impacts may not be totally unexpected at near coast locations, the present study suggests that the impact from continental outflow on the marine BL could extend out to distances of more than 2000 km from the Asian Pacific Rim.

On the basis of measurements taken during the NASA Global Tropospheric Experiment (GTE) Pacific Exploratory Mission-West A (PEM-West A), photostationary state model calculations were carried out for approximately 1300 three-minute sample runs. The objective of this study was to look at a subset of this processed data to assess the level of agreement between observed ratios of NO2 to NO and those estimated using current photochemical theory. This filtered data subset consisted of 562 NO2-NO data pairs. The comparison between observations and predictions was based on the use of the photochemical test ratio (NO2)expt/(NO2)calc, designated here as Re/ Rc. Although the expected median value for this test ratio was unity, for the PEM-West A data set it was found to be 3.36. The value of the ratio Re/Rc showed a general trend of increasing magnitude with increasing altitude and decreasing latitude. Attempts to understand the sizable discrepancy between observation and prediction (especially for the high-altitude and low-latitude data) were explored in the context of two hypotheses: (1) incomplete model chemistry and (2) interferences in the measurement of NO2. Efforts to quantify the levels of HO2, CH3O2, RO2, and/or ClOx needed to correct the Re/Rc discrepancy led to major inconsistencies in the predicted levels of other chemical species. Bromine and iodine chemistries were also investigated with results requiring Brx and Ix radical levels well in excess of what would seem reasonable given our current understanding of the source strengths for these elements. This suggests that incompleteness in the model's chemistry was unlikely the major cause of the discrepancy. The second hypothesis, involving interference in the measurement of NO2, now appears to be the most likely explanation for the largest component of the deviation in Re/Rc from unity. For example, the disagreement between (NO2)expt and (NO2)calc was found to be a strong function of the NOx/NOy ratio. Also, the magnitude of the discrepancy between (NO2)expt and (NO2)calc fell within the possible limits defined by other reactive nitrogen species (e.g., ΔNOy) available to generate the interference. These results suggest that the further development of a new direct measurement technique for NO2, involving a wall collision-free inlet system, should be considered a high priority. We should also continue, however, to examine the chemical basis of current photochemical models to assess whether yet untested mechanisms might not provide an explanation for these observations.

The DC-8 mission of September 27, 1991, was designed to sample air flowing into Typhoon Mireille in the boundary layer, air in the upper tropospheric eye region, and air emerging from the typhoon and ahead of the system, also in the upper troposphere. The objective was to find how a typhoon redistributes trace constituents in the West Pacific region and whether any such redistribution is important on the global scale. The boundary layer air (300 m), in a region to the SE of the eye, contained low mixing ratios of the tracer species 03, CO, C2H6, C2H2, C3H8, C6H6 and CS2 but high values of dimethylsulfide (DMS). The eye region relative to the boundary layer, showed somewhat elevated levels of CO, substantially increased levels of 03, CS2 and all nonmethane hydrocarbons (NMHCs), and somewhat reduced levels of DMS. Ahead of the eye, CO and the NMHCs remained unchanged, 03 and CS2 showed a modest decrease, and DMS showed a substantial decrease. There was no evidence from lidar cross sections of ozone for the downward entrainment of stratospheric air into the eye region; these sections show that low ozone values were measured in the troposphere. The DMS data suggest sub- stantial entrainment of boundary layer air into the system, particularly into the eye wall region. Estimates of the DMS sulphur flux between the boundary layer and the free tro- posphere, based on computations of velocity potential and divergent winds, gave values of about 69 gg S m -2 d-averaged over a 17.5 o grid square encompassing the typhoon. A few hours aer sampling with the DC-8, Mireille passed over Oki Island, just to the north of Japan, producing surface values of ozone of 5.5 ppbv. These 03 levels are consistent with the low tropospheric values found by lidar and are more typical of equatorial regions. We suggest that the central eye region may act like a Taylor column which has moved poleward from low latitudes. The high-altitude photochemical environment within Typhoon Mireille was found to be quite active as evidenced by significant levels of measured gas phase H202 and CH300H and model-computed levels of OH.

Aircraft measurements of ozone, its key precursors, and a variety of chemical tracers were made in the troposphere of the western and central Pacific in October 1991. These data are presented and analyzed to examine the occurrence of low ozone concentrations in the remote marine boundary layer of the tropical and equatorial Pacific Ocean. The data from these flights out of Guam, covering an area extending from the equator to 20°N and from south of the Philippines to Hawaii, show average O 3 concentrations as low as 8-9 ppb (ppb = 10 -9 v/v) at altitudes of 0.3-0.5 km in the boundary layer. Individual measurements as low as 2-5 ppb were recorded. Low O 3 concentrations do not always persist in space and time. High O 3 , generally associated with the transport of upper tropospheric air, was also encountered in the boundary layer. In practically all cases, O 3 increased to values as large as 25-30 ppb within 2 km above the boundary layer top. Steady state model computations are used to suggest that these low O 3 concentrations are a result of net photochemical O 3 destruction in a low NO environment, sea surface deposition, and low net entrainment rates (3.6 ± 1.7 mm s -1 ) from the free troposphere. Day/night measurements of select organic species (e.g., ethane, propane, C 2 Cl 4 ) suggest that Cl atom concentrations in the vicinity of 10 5 molecules cm -3 may be present in the marine boundary layer in the early morning hours. This Cl atom abundance can only be rationalized if sea-salt aerosols release active chlorine (Cl 2 or HOCl) to the gas phase when exposed to sunlight. These Cl atom concentrations, however, are still insufficient, and halogen chemistry is not likely to be an important contributor to the observed low O 3 .

We examine the characteristics of the spatial distribution and variability of tropospheric ozone (O3) by analysis of 93 ozonesonde profiles obtained at five stations over south China (18–30 N) during a field campaign in April–May 2004. We use a global 3-D chemical transport model (GEOS-Chem) to interpret these characteristics and to quantify the sources of tropospheric O3 over south China during this period. The observed tropospheric O3 mixing ratios showed strong spatiotemporal variability due to a complex interplay of various dynamical and chemical processes. A prominent feature in the upper and middle troposphere (UT/MT) was the frequent occurrence of high O3 mixing ratios shown as tongues extending down from the lower stratosphere or as isolated layers at all stations. The model largely captured the observed pattern of day-to-day variability in tropospheric O3 mixing ratios at all stations, but often underestimated those tongues or isolated layers of O3 enhancements observed in the UT/MT, especially at low-latitude stations. We found that tropospheric O3 along the southeast China coast was mainly produced within Asia. Lightning NOx emissions (over South Asia and equatorial Africa) and/or stratospheric influences were responsible for major events of high O3 observed in the UT/MT at all stations. Underestimated contributions of these sources likely led to the model’s underestimate in the low-latitude UT/MT O3. This study emphasizes the need for improved understanding of lightning NOx emissions and stratospheric influences over the Eurasian and African continents and for better representation of these processes in current global models.

[1] Emission of bromine from sea-salt aerosol, frost flowers, ice leads, and snow results in the nearly complete removal of surface ozone during Arctic spring. Regions of enhanced total column BrO observed by satellites have traditionally been associated with these emissions. However, airborne measurements of BrO and O3 within the convective boundary layer (CBL) during the ARCTAS and ARCPAC field campaigns at times bear little relation to enhanced column BrO. We show that the locations of numerous satellite BrO “hotspots” during Arctic spring are consistent with observations of total column ozone and tropopause height, suggesting a stratospheric origin to these regions of elevated BrO. Tropospheric enhancements of BrO large enough to affect the column abundance are also observed, with important contributions originating from above the CBL. Closure of the budget for total column BrO, albeit with significant uncertainty, is achieved by summing observed tropospheric partial columns with calculated stratospheric partial columns provided that natural, short-lived biogenic bromocarbons supply between 5 and 10 ppt of bromine to the Arctic lowermost stratosphere. Proper understanding of bromine and its effects on atmospheric composition requires accurate treatment of geographic variations in column BrO originating from both the stratosphere and troposphere.

One of the major goals of the 2005 Antarctic Tropospheric Chemistry Investigation (ANTCI) was to bridge the information gap between current knowledge of South Pole (SP) chemistry and that of the plateau. The former has been extensively studied, but its geographical position on the edge of the plateau makes extrapolating these findings across the plateau problematic. The airborne observations reported here demonstrate that, as at SP, elevated levels of nitric oxide (NO) are a common summertime feature of the plateau. As in earlier studies, planetary boundary layer (PBL) variations were a contributing factor leading to NO fluctuations. Thus, extensive use was made of in situ measurements and models to characterize PBL depths along each flight path and over broader areas of the plateau. Consistent with earlier SP studies that revealed photolysis of nitrate in surface snow as the source of NO x , large vertical gradients in NO were observed over most plateau areas sampled. Similar gradients were also found for the nitrogen species HNO3 and HO2NO2 and for O3. Thus, a common meteorological-chemical feature found was shallow PBLs associated with nitrogen species concentrations that exceeded free tropospheric levels. Collectively, these new results greatly extend the geographical sampling footprint defined by earlier SP studies. In particular, they suggest that previous assessments of the plateau as simply a chemical depository need updating. Although the evidence supporting this position comes in many forms, the fact that net photochemical production of ozone occurs during summer months over extensive areas of the plateau is pivotal.

[1] We report measurements of the atmospheric plume emitted by Erebus volcano, Antarctica, renowned for its persistent lava lake. The observations were made in December 2005 both at source, with an infrared spectrometer sited on the crater rim, and up to 56 km downwind, using a Twin Otter aircraft; with the two different measurement platforms, plume ages were sampled ranging from

Clouds affect tropospheric photochemistry through modification of solar radiation that determines photolysis frequencies. As a follow-up study to our recent assessment of the radiative effects of clouds on tropospheric chemistry, this paper presents an analysis of the sensitivity of such effects to cloud vertical distributions and optical properties (cloud optical depths (CODs) and cloud single scattering albedo), in a global 3-D chemical transport model (GEOS-Chem). GEOS-Chem was driven with a series of meteorological archives (GEOS1- STRAT, GEOS-3 and GEOS-4) generated by the NASA Goddard Earth Observing System data assimilation system. Clouds in GEOS1-STRAT and GEOS-3 have more similar vertical distributions (with substantially smaller CODs in GEOS1-STRAT) while those in GEOS-4 are optically much thinner in the tropical upper troposphere. We find that the radiative impact of clouds on global photolysis frequencies and hydroxyl radical (OH) is more sensitive to the vertical distribution of clouds than to the magnitude of column CODs. With random vertical overlap for clouds, the model calculated changes in global mean OH (J(O1D), J(NO2)) due to the radiative effects of clouds in June are about 0.0% (0.4%, 0.9%), 0.8% (1.7%, 3.1%), and 7.3% (4.1%, 6.0%), for GEOS1-STRAT, GEOS-3 and GEOS-4, respectively; the geographic distributions of these quantities show much larger changes, with maximum decrease in OH concentrations of approx.15-35% near the midlatitude surface. The much larger global impact of clouds in GEOS-4 reflects the fact that more solar radiation is able to penetrate through the optically thin upper-tropospheric clouds, increasing backscattering from low-level clouds. Model simulations with each of the three cloud distributions all show that the change in the global burden of ozone due to clouds is less than 5%. Model perturbation experiments with GEOS-3, where the magnitude of 3-D CODs are progressively varied from -100% to 100%, predict only modest changes (

[1] This study explores the upper tropospheric anticyclone above eastern North America and its influence on the summertime buildup and decay of lightning NOx (LNOx) and thunderstorm outflow. LNOx transport is simulated with a particle dispersion model that releases a LNOx tracer from the locations of millions of cloud-to-ground lightning flashes during May–September 2004 and 2006. On average, upper tropospheric zonal flow in May transitions to a closed anticyclone above northern Mexico and the southern United States in July that strengthens in August and rapidly decays in September. Concentrations of the LNOx tracer reach a maximum above the southern United States and Gulf of Mexico in July and August. Fourteen study sites across North America exhibit high day-to-day variability of the LNOx tracer in the upper troposphere during summer, with the sites most heavily influenced by the North American summer monsoon having the greatest background concentrations. During late spring and September the western sites have low concentrations with little variability. In general, the west coast sites plus Barbados have the most aged thunderstorm outflow, while the east coast sites have the least aged outflow. More than 80% of summertime upper tropospheric NOx above the eastern United States is produced by lightning. To produce the best available observation-based view of upper troposphere NOx above North America, measurements from six aircraft campaigns are combined in a single composite plot. The modeled upper tropospheric NOx matches the general continental-scale distribution of NOx in the composite plot, supporting the dominant role of LNOx in the simulations.

Measurements of CH2O from a tunable diode laser absorption spectrometer (TDLAS) were acquired onboard the NASA DC-8 during the summer 2004 INTEX-NA (Intercontinental Chemical Transport Experiment - North America) campaign to test our understanding of convection and production mechanisms in the upper troposphere (UT, 6-12-km) over continental North America and the North Atlantic Ocean. Point-by-point comparisons with box model calculations, when MHP (CH3OOH) measurements were available for model constraint, resulted in a median CH2O measurement/model ratio of 0.91 in the UT. Multiple tracers were used to arrive at a set of UT CH2O background and perturbed air mass periods, and 46% of the TDLAS measurements fell within the latter category. At least 66% to 73% of these elevated UT observations were caused by enhanced production from CH2O precursors rather than direct transport of CH2O from the boundary layer. This distinction is important, since the effects from the former can last for over a week or more compared to one day or less in the case of convective transport of CH2O itself. In general, production of CH2O from CH4 was found to be the dominant source term, even in perturbed air masses. This was followed by production from MHP, methanol, PAN type compounds, and ketones, in descending order of their contribution. In the presence of elevated NO from lightning and potentially from the stratosphere, there was a definite trend in the CH2O discrepancy, which for the highest NO mixing ratios produced a median CH2O measurement/model ratio of 3.9 in the 10-12-km range. Discrepancies in CH2O and HO2 in the UT with NO were highly correlated and this provided further information as to the possible mechanism(s) responsible. These discrepancies with NO are consistent with additional production sources of both gases involving CH3O2 + NO reactions, most likely caused by unmeasured hydrocarbons.

[1] A tunable diode laser absorption spectrometer (TDLAS) was operated on the NASA DC-8 aircraft during the summer INTEX-NA study to acquire ambient formaldehyde (CH2O) measurements over North America and the North Atlantic Ocean from ∼0.2 km to ∼12.5 km altitude spanning 17 science flights. Measurements of CH2O in the boundary layer and upper troposphere over the southeastern United States were anomalously low compared to studies in other years, and this was attributed to the record low temperatures over this region during the summer of 2004. Formaldehyde is primarily formed over the southeast from isoprene, and isoprene emissions are strongly temperature-dependent. Despite this effect, the median upper tropospheric (UT) CH2O mixing ratio of 159 pptv from the TDLAS over continental North America is about a factor of 4 times higher than the median UT value of 40 pptv observed over remote regions during TRACE-P. These observations together with the higher variability observed in this study all point to the fact that continental CH2O levels in the upper troposphere were significantly perturbed during the summer of 2004 relative to more typical background levels in the upper troposphere over more remote regions. The TDLAS measurements discussed in this paper are employed together with box model results in the companion paper by Fried et al. to further examine enhanced CH2O distributions in the upper troposphere due to convection. Measurements of CH2O on the DC-8 were also acquired by a coil enzyme fluorometric system and compared with measurements from the TDLAS system.

OH and HO2 were measured with the Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) as part of a large measurement suite from the NASA DC-8 aircraft during the Intercontinental Chemical Transport Experiment - A (INTEX-A). This mission, which was conducted mainly over North America and the western Atlantic Ocean in summer 2004, was an excellent test of atmospheric oxidation chemistry. Throughout the troposphere, observed OH was generally 0.60 of the modeled OH; below 8 km, observed HO2 was generally 0.78 of modeled HO2. If the over-prediction of tropospheric OH is not due to an instrument calibration error, then it implied less global tropospheric oxidation capacity and longer lifetimes for gases like methane and methyl chloroform than currently thought. This discrepancy falls well outside uncertainties in both the OH measurement and rate coefficients for known reactions and points to a large unknown OH loss. If the modeled OH is forced to agree with observed values by introducing of an undefined OH loss that removed HOx (HOx=OH+HO2), the observed and modeled HO2 and HO2/OH ratios are largely reconciled within the measurement uncertainty. HO2 behavior above 8 km was markedly different. The observed-to-modeled ratio correlating with NO. The observed-to-modeled HO2 ratio increased from approximately 1 at 8 km to more than approximately 2.5 at 11 km with the observed-to-modeled ratio correlating with NO. The observed-to-modeled HO2 and NO were both considerably greater than observations from previous campaigns. In addition, the observed-to-modeled HO2/OH, which is sensitive to cycling reactions between OH and HO2, increased from approximately 1.2 at 8 km to almost 4 above 11 km. In contrast to the lower atmosphere, these discrepancies above 8 km suggest a large unknown HOx source and additional reactants that cycle HOx from OH to HO2. In the continental planetary boundary layer, the OH observed-to-modeled ratio increased from 0.6 when isoprene was less than 0.1 ppbv to over 3 when isoprene was greater than 2 ppbv, suggesting that forests throughout the United States are emitting unknown HOx sources. Progress in resolving these discrepancies requires further examinations of possible unknown OH sinks and HOx sources and a focused research activity devoted to ascertaining the accuracy of the OH and HO2 measurements.