Regional and global effects of mega-city pollution
What are the regional and global effects of mega-city pollution and how well can satellite observations in particular TES observations be used for studying outflow of urban pollution? The objective of this study is to characterize Mexico City pollution plumes by examining the vertical distributions of TES ozone, carbon monoxide and their correlation along with aircraft observations from the recent MILAGRO/INTEX-B aircraft campaign.

Direct Radiative Effect of aerosols estimated with MISR Observations
What is the global direct radiative effect of smoke particles from forest fires? The presence of smoke aerosol particles from forest fires significantly changes the amount of solar radiation reflected back into space. MISR provides identification of smoke aerosols from other types of surface as well as the associated albedo. This provides the basis for quantifying the global direct radiative effect of smoke aerosols.

Long-range transport of aerosols to the northern polar region and its impact on snow albedo
What is the relative importance of different source types/regions on black carbon concentrations in northern polar region and how the influx of black carbon impacts snow/ice albedo in the Arctic? Our proposed study will quantify from where and how much black carbon aerosols are transported to the northern high-latitudes and to what extent changes in the Arctic snow/ice albedo are due to the deposited black carbon.

Intercontinental transport of pollution in the upper troposphere constrained by MLS observations
Tans-Pacific transport of Asian pollution has received considerable attention because of its global air quality and climate implications. It has been difficult to investigate long-range transport of pollution in the upper troposphere because of the lack of suitable observations. The launch of Aura-MLS, with its measurements of upper tropospheric carbon monoxide (CO) and ozone among other species, makes it possible for the first time to systematically examine long-range transport in the upper troposphere. In this study we present an analysis of the first two years (September 2004-September 2006) of Aura-MLS upper tropospheric observations of CO and ozone, in conjunction with a global 3-D chemical transport model (GEOS-CHEM), with a focus to (1) quantify the temporal variability (i.e., frequency and strength) of trans-Pacific transport of Asian pollution in the upper troposphere, to (2) delineate transport events in the upper troposphere from the ones in the middle and lower troposphere by combining and contrasting MLS and MOPITT observations of CO, and to (3) examine the preferred meteorological conditions and associated processes including warm conveyor belt (WCB) that are responsible for these upper troposphere trans-Pacific transport.

This study is led by Nathaniel Livesey of the MLS team.

Interpreting MLS Tropical Upper Tropospheric Ozone Observations with GEOS-Chem
What are the factors controlling the distribution of tropical upper tropospheric ozone? Our focus is on better understanding the processes controlling the seasonal and interannual variability of tropical upper tropospheric ozone, including biomass burning, deep convection, and long-range transport. We interpret the distribution of tropical upper tropospheric ozone from MLS by using the GEOS-Chem model and additional information from in situ observations.

Large-scale Atmospheric Variability of AIRS CO₂ and O₃
This is an ongoing modeling analysis of carbon dioxide (CO2) and ozone (O3) from AIRS with results from two atmospheric chemistry and transport models (CTMs), in the context of the large-scale atmospheric transport. AIRS data in January, May, July, and October 2003 are retrieved applying the Vanishing Partial Derivative (VPD) method (Chahine et al. [GRL, 2005]; Chahine et al. [AGU, 2006]). Corresponding model results are simulated by 2-D and 3-D atmospheric CTMs. The AIRS retrieved and model simulated CO2 mixing ratios are compared with the Matsueda et al. observations in the tropics between 9 and 13 km (Jiang et al. [AGU, 2006]). Comparison of the simulated vertical profiles of CO2 between the two models reveals that the stratosphere and troposphere exchange in the 3-D model is likely too strong in the winter and spring. The latitudinal distributions of O3, both retrieved and simulated, are compared with ozonesonde data. Both comparisons show reasonable agreement. We then examine the spatiotemporal variabilities of CO2 and O3 and their correlation, both in the AIRS data and model results. Our objective is to better understand the AIRS observed atmospheric variability in CO2 that is associated with underlying large-scale atmospheric transport, particularly the stratosphere-troposphere-exchange (STE) at northern high latitudes in spring and the Asian monsoon summer circulation over South Asia.

This study is led by Postdoc Xun Jiang, in close collaboration with the AIRS team: Mous Chahine, Ed Olsen, Luke Chen, and with Yuk Yung of Caltech.

Chemical Data Assimilation and Inverse Modeling with the CMAQ Model
Satellite observations are revolutionizing atmospheric science in various areas such as quantifying long-range transport of pollution, and estimating gaseous/aerosol emission strengths. The use of satellite observations in regional air quality studies, however, is rather limited. Integrating satellite observations into atmospheric chemical transport models for improved predictions remains a scientific frontier. This, we believe, provides a great opportunity for exploring the potential for exploiting satellite observations from existing NASA instruments such as OMI, TES, MISR, AIRS, and MISR, as well as future GEO and LEO/MEO missions in the context of regional air quality. In collaboration with John Seinfeld and Amir Hakami of Caltech, we have been developing an adjoint version of the CMAQ model. An important application of sensitivity analysis methods is in inverse modeling and data assimilation. Among various sensitivity methods, adjoint analysis is particularly attractive for such applications due to its receptor-based nature. In the adjoint method, sensitivities of a cost function with respect to a large number of input parameters are calculated efficiently. Therefore, adjoint-based inversions are capable of adjusting very large number of parameters, and providing high spatial and/or temporal resolution for such adjustments. As part of this study, we use the newly developed adjoint version of CMAQ for assimilating NO2 column observations of SCIAMACHY over a regional domain.

Chemical Data Assimilation of TES CO Observations with the GEOS-Chem Model
We have applied a sequential sub-optimal Kalman filter assimilation scheme [Khattatov et al., 2000] to assimilate TES CO profiles during November 2004 into the GEOS-Chem global 3D CTM. The assimilation results were compared with MOPITT and MOZAIC observations. The assimilation significantly improves model simulation of CO in the middle to upper troposphere, where the MOPITT versus model bias was reduced by up to two-thirds. Assimilation results show higher levels of CO in the southern tropics, consistent with MOPITT observations. We find good agreement between the TES assimilated model estimates of CO and in situ measurements from the MOZAIC program, which shows a negative bias of up to 10 ppbv in middle and upper tropospheric TES CO. The results demonstrate how assimilation can be used for non-coincident validation of TES CO profile retrievals.

This study was led by Postdoc Nigel Richards, in close collaboration with the TES team.

Tropical Atlantic tropospheric ozone observed by TES during Nothern Africa biomass burning season

This study was led by Postdoc Line Jourdain, in close collaboration with the TES team.

Trapping of South Asian convective outflow by the Tibetan anticyclone
A global 3-D chemical transport model (GEOS-CHEM) is used to analyze observations of carbon monoxide (CO) and upper tropospheric clouds from the EOS Microwave Limb Sounder (MLS). MLS observations during 25 August-6 September 2004 reveal elevated CO concentrations and dense high-altitude clouds in the upper troposphere over the Tibetan plateau and southwest China, collocating with the upper-level Tibetan anticyclone. Model simulations indicate the transport of boundary layer pollution by Asian summer monsoon (ASM) convection and orographic lifting to the upper troposphere over South Asia, where the simulated distributions of CO closely resemble the MLS observations. Model results also show elevated aerosols and ozone in the anticyclone region. Analysis of simulated CO and aerosols indicate that the upper-level Tibetan anticyclone effectively 'traps' anthropogenic emissions lifted from northeast India and southwest China. These aerosols may be responsible for the formation of some of the dense high-altitude clouds.

Transport pathways for North American pollution in summer
We examine the major outflow pathways for North American pollution to the Atlantic in summer by conducting a 4-year simulation with the GEOS-CHEM global chemical transport model, including a coupled ozone-aerosol simulation with 1x1 horizontal resolution for summer 2000. The outflow is driven principally by cyclones tracking eastward across North America at 45-55°N, every 5 days on average during 2000. The cold fronts associated with these cyclones sweep across the northeastern United States, and the warm conveyor belts (WCBs) ahead of the fronts lift U.S. pollution to the upper troposphere on a northeastward track toward Newfoundland. Anthropogenic and fire effluents from western North America are mostly transported north and east, eventually merging with the eastern U.S. pollution outflow to the Atlantic. The WCBs typically do not reach the southeastern United States unless associated with occasional Atlantic cyclones originating along the east coast (only three in 2000). Deep convection is a more important pathway for ventilation of the southeastern and central United States. A semi-permanent upper-level anticyclone traps the convective outflow and allows it to age in the upper troposphere over the United States for several days. Rapid ozone production takes place in this outflow driven in part by anthropogenic and lightning NO_x, and in part by HO_x radicals produced from convectively lifted CH₂O that originates from biogenic isoprene. This mechanism could explain ozonesonde observations of elevated ozone in the upper troposphere over the southeastern United States. Asian and European pollution influences in the North American outflow to the Atlantic are found to be dispersed into the background and do not generate distinct plumes. Satellite observations of CO columns from MOPITT and of aerosol optical depths (AODs) from MODIS provide useful mapping of outflow events despite their restriction to clear-sky scenes.

Export efficiency for NO_y from continental boundary layer
Fossil fuel combustion accounts for > 50% of the global atmospheric emission of NO_x, but this source is concentrated in the polluted continental boundary layer (CBL) and only a small fraction is exported to the global troposphere. Better quantification is needed of the export efficiency of NO_y (NO_x and its oxidation products) out of the CBL. A recent Lagrangian analysis of the NO_y-CO correlations observed from aircraft in the North Atlantic Regional Experiment (September 1997, NARE'97) downwind of eastern North America indicated a NO_y export efficiency of < 10%, with < 10% of the exported NO_y present as NO_x. However, previous 3-D model Eulerian budget analyses for the North American boundary layer indicated NO_y export efficiencies of 25-30% with 30-35% of the exported NO_y present as NO_x. We investigated this apparent discrepancy by simulating the NARE'97 aircraft observations with a global 3-D model of tropospheric chemistry (GEOS-CHEM) and using the model to calculate the NO_y export efficiency both through a Lagrangian analysis of the NO_y-CO correlations along the aircraft flight tracks and through a Eulerian budget analysis for the North American boundary layer. The model reproduces the variability and NO_y-CO correlations observed in the aircraft data and also at the Harvard Forest surface site in the northeastern United States. We show that the previous Lagrangian analyses of the NO_y export efficiency during NARE'97 were probably biased low due to underestimate of the CO background. Correcting for this bias we find a NO_y export efficiency of 17±7% in the model and 15±11% in the observations. A similar NO_y export efficiency (20%) in the model is obtained from the Eulerian budget analysis, demonstrating that the Lagrangian and Eulerian approaches are in fact consistent. Export efficiencies of NO_y in previous 3-D model Eulerian budget analyses were probably too high because of insufficient scavenging out of the CBL. Model results indicate that only 6% of the exported NO_y is present as NO_x along the aircraft flight tracks, in agreement with the observations, but that 40% of the NO_y export flux is as NO_x,in agreement with the previous 3-D model analyses. This result reflects the fast oxidation of NO_x during transport downwind of North America. The eventual ozone production in the global troposphere due to the exported NO_x and PAN, with equal contributions from each, is estimated to be comparable to the direct export of ozone pollution from the North American boundary layer.

Atmospheric budgets for biomass burning tracers HCN and CH₃CN
We construct global atmospheric budgets of HCN and CH₃CN through a 3-D simulation of the HCN-CH₃CN-CO system constrained and evaluated with aircraft observations from the TRACE-P mission over the NW Pacific in February-April 2001. Observed background vertical gradients of HCN and CH₃CN imply a dominant ocean sink for both gases, with deposition velocity of 0.13 cm s^-1 for both and saturation ratios of 0.79 for HCN and 0.88 for CH₃CN. Observations for both gases in the free troposphere implied a dominant source from biomass burning. Enhancement of HCN observed in Chinese urban plumes is attributed tentatively to residential coal burning. Biomass burning and residential coal burning emission ratios relative to CO of 0.27% and 1.6% respectively for HCN, and of 0.20% and 0.25% respectively for CH₃CN, are consistent with observations in biomass burning and Chinese urban plumes, and provide the best fit in the model for simulation of observed TRACE-P vertical profiles, HCN-CH₃CN-CO correlations, as well as long-term records of HCN columns and CH₃CN observations over the northern Indian Ocean. Biomass burning and residential coal burning contribute 0.63 and 0.2 Tg N yr^-1 respectively to global HCN and 0.47 and 0.03 Tg N yr^-1 respectively to CH₃CN. Ocean uptake is the dominant sink for both gases, with oxidation by OH representing an additional minor sink. The resulting tropospheric lifetimes are 5.3 months for HCN and 5.8 months for CH₃CN. The model predicts very low HCN and CH₃CN concentrations at high southern latitudes, reflecting the assumption of uniform saturation ratio; observations in that region are needed. In the free troposphere, the dominance of biomass burning sources (70-85% for HCN and 90-95% for CH₃CN) implies that both gases can be used as biomass burning tracers. More work is needed to identify the urban source apparent in the Chinese plume observations.

Springtime ozone maximum at northern midlatitudes: Origin of ozone at Bermuda
Conflicting interpretations of the spring ozone maximum observed at Bermuda (32°N, 65°W) have fueled the debate on stratospheric influence versus tropospheric production as sources of tropospheric ozone. In this study we use the GEOS-CHEM global 3-D model of tropospheric ozone-NO_x-hydrocarbon chemistry driven by assimilated meteorological observations to reconcile these past interpretations. The model reproduces the observed seasonal cycle of surface ozone at Bermuda and captures the springtime day-to-day variability (r = 0.82, n = 122, p < 0.001) driven by high-ozone events. We find that boundary layer transport of North American pollution behind cold fronts is the principal contributor to springtime surface ozone at Bermuda and is responsible for all the high-ozone events. The model reproduces the observed positive correlations of surface ozone with ⁷Be and ²¹⁰Pb at Bermuda; the correlation with ⁷Be reflects the strong subsidence behind cold fronts, resulting in the mixing of middle-tropospheric air with continental boundary layer outflow in the air arriving at Bermuda, as indicated by the positive ⁷Be-²¹⁰Pb correlation. This mixing appears to have been an obfuscating factor in past interpretations of subsiding back-trajectories at Bermuda as evidence for a stratospheric or upper-tropospheric origin for ozone. Isentropic back-trajectories computed in our model reproduce the previously reported subsidence associated with high-ozone events. Even in the free troposphere we find that the stratosphere contributes less than 5 ppbv (< 10%) to spring ozone over Bermuda. Positive O₃-⁷Be and negative O₃-²¹⁰Pb correlations observed at Tenerife (28°N, 16°W, 2.4 km) in summer are reproduced by the model and are consistent with a middle-tropospheric source of ozone, not an upper-tropospheric or stratospheric source as previously suggested. A regional budget for the North Atlantic in spring indicates that the stratosphere contributes less than 10 ppbv ozone (< 5%) below 500 hPa, while the lower troposphere contributes 20-40 ppbv ozone throughout the troposphere.

Trans-Atlantic transport of pollution and implications for air quality
We examine the transatlantic transport of anthropogenic ozone and its impact on surface ozone in Europe and North America by using a 5-year (1993-1997) simulation with the GEOS-CHEM global 3-D model of tropospheric chemistry. Long-term time series of ozone and CO at Mace Head (Ireland), Sable Island (Canada), and Westman Island (Iceland) are used to evaluate transatlantic transport in the model. North American anthropogenic emissions contribute on average 5 ppbv to surface ozone at Mace Head, and up to 10-20 ppbv during transatlantic transport events which are forerunners of broader events in Europe. These events are associated with low-level westerly flow driven by an intense Icelandic low between Iceland and the British Isles. North American influence on ozone at Mace Head is strongly correlated with the North Atlantic Oscillation (NAO), implying that the NAO index can be used to forecast transatlantic transport of North American pollution to Europe. European anthropogenic emissions contribute on average less than 2 ppbv to surface ozone at Sable Island but up to 5-10 ppbv during transatlantic transport events. These events are associated with low-level easterly flow established by anomalous low pressure at 45°N over the North Atlantic. North American anthropogenic emissions enhance surface ozone in continental Europe by 2-4 ppbv on average in summer, and by 5-10 ppbv during transatlantic transport events; transport in the boundary layer and subsidence from the free troposphere are both important mechanisms. We find in the model that 20% of the violations of the European Council ozone standard (55 ppbv, 8-hour average) in the summer of 1997 over Europe would not have occurred in the absence of anthropogenic emissions from North America. North American influence on surface ozone in Europe is particularly strong at the thresholds used for the European standards (55-65 ppbv).

An ozone maximum in the middle troposphere over the Middle East
The GEOS-CHEM global 3-D model of tropospheric chemistry driven by assimilated meteorological observations predicts a pronounced summertime ozone maximum over the Middle East, with mean mixing ratios in the middle and upper troposphere in excess of 80 ppbv. This model feature is consistent with the few observations in the region from commercial aircraft. We investigated the factors that lead to the maximum in the model with tagged ozone tracer simulation and sensitivity simulations. Its origin in the model reflects a complex interplay of dynamical and chemical factors, and of anthropogenic and natural influences. The anticyclonic circulation in the middle and upper troposphere over the Middle East funnels northern midlatitudes pollution transported in the westerly subtropical jet as well as lightning outflow from the Indian monsoon and pollution from eastern Asia transported in an easterly tropical jet. Strong large-scale subsidence over the region takes place with continued net production of ozone and little mid-level outflow. Transport from the stratosphere does not contribute significantly to the ozone maximum. Sensitivity simulations with anthropogenic or lightning emissions omitted indicate maximum effects on ozone over the Middle East with decreases of 25-30% and 10-15% respectively in the tropospheric ozone column. More observations in this region are needed to confirm the presence of the ozone maximum, which is of interest both as a test of our understanding of tropospheric ozone chemistry and because of its implications for anthropogenic climate forcing.

Atmospheric budget for HCN: Biomass burning source, ocean sink?
The observed seasonal amplitude of atmospheric HCN concentrations measured in tropical regions and at northern midlatitudes implies an atmospheric lifetime of a few months for HCN, much shorter than is commonly assumed from oxidation by OH (a few years). We propose that ocean uptake may provide the missing sink, and show with the global 3-D model simulation that the observations of atmospheric HCN can be roughly reproduced in a scenario where biomass burning provides the main source (1.4-2.9 Tg N yr^-1) and ocean uptake provides the main sink (HCN atmospheric lifetime of 2-4 months). Such a budget would imply that HCN is a sensitive tracer of biomass burning on large scales, of particular value because of it is readily observed from space. The ocean sink hypothesis can be tested with measurements of HCN concentrations in marine air and seawater.