Person: Le Sager, P
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Le Sager
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Le Sager, P
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Publication Chemistry of Hydrogen Oxide Radicals \((HO_x)\) in the Arctic Troposphere in Spring(European Geosciences Union, 2010) Mao, Jialin; Jacob, Daniel; Evans, M. J.; Olson, J. R.; Ren, X.; Brune, W. H.; St. Clair, J. M.; Crounse, J. D.; Spencer, K. M.; Beaver, M. R.; Wennberg, P. O.; Cubison, M. J.; Jimenez, J. L.; Fried, A.; Weibring, P.; Walega, J. G.; Hall, S. R.; Weinheimer, A. J.; Cohen, R. C.; Chen, G.; Crawford, J. H.; McNaughton, C.; Clarke, A. D.; Jaeglé, L.; Fisher, J. A.; Yantosca, R. M.; Le Sager, P; Carouge, C.We use observations from the April 2008 NASA ARCTAS aircraft campaign to the North American Arctic, interpreted with a global 3-D chemical transport model (GEOS-Chem), to better understand the sources and cycling of hydrogen oxide radicals \((HO_x≡H+OH+\)peroxy radicals) and their reservoirs \((HO_y≡HO_x+\)peroxides) in the springtime Arctic atmosphere. We find that a standard gas-phase chemical mechanism overestimates the observed \(HO_2\) and \(H_2O_2\) concentrations. Computation of \(HO_x\) and \(HO_y\) gas-phase chemical budgets on the basis of the aircraft observations also indicates a large missing sink for both. We hypothesize that this could reflect \(HO_2\) uptake by aerosols, favored by low temperatures and relatively high aerosol loadings, through a mechanism that does not produce H2O2. We implemented such an uptake of \(HO_2\) by aerosol in the model using a standard reactive uptake coefficient parameterization with \(\gamma(HO_2)\) values ranging from 0.02 at 275 K to 0.5 at 220 K. This successfully reproduces the concentrations and vertical distributions of the different \(HO_x\) species and \(HO_y\) reservoirs. \(HO_2\) uptake by aerosol is then a major \(HO_x\) and \(HO_y\) sink, decreasing mean OH and \(HO_2\) concentrations in the Arctic troposphere by 32% and 31% respectively. Better rate and product data for \(HO_2\) uptake by aerosol are needed to understand this role of aerosols in limiting the oxidizing power of the Arctic atmosphere.Publication Source Attribution and Interannual Variability of Arctic Pollution in Spring Constrained by Aircraft (ARCTAS, ARCPAC) and Satellite (AIRS) Observations of Carbon Monoxide(European Geosciences Union, 2010) Fisher, J. A.; Jacob, Daniel; Purdy, M. T.; Kopacz, M.; Le Sager, P; Carouge, C.; Holmes, C. D.; Yantosca, Robert; Batchelor, R. L.; Strong, K.; Diskin, G. S.; Fuelberg, H. E.; Holloway, J. S.; Hyer, E. J.; McMillan, W. W.; Warner, J.; Streets, D. G.; Zhang, Q.; Wang, Y.; Wu, S.We use aircraft observations of carbon monoxide (CO) from the NASA ARCTAS and NOAA ARCPAC campaigns in April 2008 together with multiyear (2003–2008) CO satellite data from the AIRS instrument and a global chemical transport model (GEOS-Chem) to better understand the sources, transport, and interannual variability of pollution in the Arctic in spring. Model simulation of the aircraft data gives best estimates of CO emissions in April 2008 of \(26 Tg month^{−1}\) for Asian anthropogenic, 9.4 for European anthropogenic, 4.1 for North American anthropogenic, 15 for Russian biomass burning (anomalously large that year), and 23 for Southeast Asian biomass burning. We find that Asian anthropogenic emissions are the dominant source of Arctic CO pollution everywhere except in surface air where European anthropogenic emissions are of similar importance. Russian biomass burning makes little contribution to mean CO (reflecting the long CO lifetime) but makes a large contribution to CO variability in the form of combustion plumes. Analysis of two pollution events sampled by the aircraft demonstrates that AIRS can successfully observe pollution transport to the Arctic in the mid-troposphere. The 2003–2008 record of CO from AIRS shows that interannual variability averaged over the Arctic cap is very small. AIRS CO columns over Alaska are highly correlated with the Ocean Niño Index, suggesting a link between El Niño and Asian pollution transport to the Arctic. AIRS shows lower-than-average CO columns over Alaska during April 2008, despite the Russian fires, due to a weakened Aleutian Low hindering transport from Asia and associated with the moderate 2007–2008 La Niña. This suggests that Asian pollution influence over the Arctic may be particularly large under strong El Niño conditions.Publication Sources of carbonaceous aerosols and deposited black carbon in the Arctic in winter-spring: implications for radiative forcing(European Geosciences Union, 2011) Wang, Qiaoqiao; Jacob, Daniel; Fisher, Jenny; Mao, Jialin; Leibensperger, Eric Michael; Carouge, C. C.; Le Sager, P; Kondo, Y.; Jimenez, J. L.; Cubison, M. J.; Doherty, S. J.We use a global chemical transport model (GEOS-Chem CTM) to interpret observations of black carbon (BC) and organic aerosol (OA) from the NASA ARCTAS aircraft campaign over the North American Arctic in April 2008, as well as longer-term records in surface air and in snow (2007–2009). BC emission inventories for North America, Europe, and Asia in the model are tested by comparison with surface air observations over these source regions. Russian open fires were the dominant source of OA in the Arctic troposphere during ARCTAS but we find that BC was of prevailingly anthropogenic (fossil fuel and biofuel) origin, particularly in surface air. This source attribution is confirmed by correlation of BC and OA with acetonitrile and sulfate in the model and in the observations. Asian emissions are the main anthropogenic source of BC in the free troposphere but European, Russian and North American sources are also important in surface air. Russian anthropogenic emissions appear to dominate the source of BC in Arctic surface air in winter. Model simulations for 2007–2009 (to account for interannual variability of fires) show much higher BC snow content in the Eurasian than the North American Arctic, consistent with the limited observations. We find that anthropogenic sources contribute 90% of BC deposited to Arctic snow in January-March and 60% in April–May 2007–2009. The mean decrease in Arctic snow albedo from BC deposition is estimated to be 0.6% in spring, resulting in a regional surface radiative forcing consistent with previous estimates.Publication Regional CO pollution and export in China simulated by the high-resolution nested-grid GEOS-Chem model(Copernicus GmbH, 2009) Chen, D.; Wang, Yichao; McElroy, Michael; He, K.; Yantosca, Robert; Le Sager, PAn updated version of the nested-grid GEOS-Chem model is developed allowing for higher horizontal (0.5°×0.667°) resolution as compared to global models. CO transport over a heavily polluted region, the Beijing-Tianjin-Hebei (BTH) city cluster in China, and the pattern of outflow from East China in summertime are investigated. Comparison of the nested-grid with global models indicates that the fine-resolution nested-grid model is capable of resolving individual cities with high associated emission intensities. The nested-grid model indicates the presence of a high CO column density over the Sichuan Basin in summer, attributable to the low-level stationary vortex associated with the Basin's topographical features. The nested-grid model provides good agreement also with measurements from a suburban monitoring site in Beijing during summer 2005. Tagged CO simulation results suggest that regional emissions make significant contributions to elevated CO levels over Beijing on polluted days and that the southeastward moving cyclones bringing northwest winds to Beijing are the key meteorological mechanisms responsible for dispersion of pollution over Beijing in summer. Overall CO fluxes to the NW Pacific from Asia are found to decrease by a factor of 3–4 from spring to summer. Much of the seasonal change is driven by decreasing fluxes from India and Southeast Asia in summer, while fluxes from East China are only 30% lower in summer than in spring. Compared to spring, summertime outflow from Chinese source regions is strongest at higher latitudes (north of 35° N). The deeper convection in summer transporting CO to higher altitudes where export is more efficient is largely responsible for enhanced export in summer.Publication Regional CO Pollution in China Simulated by the High-Resolution Nested-Grid GEOS-Chem Model(Copernicus Publications, 2009) Chen, Dan; Yuxuan, Wang; McElroy, Michael; He, Kebin; Yantosca, Robert; Le Sager, PAn updated version of the nested-grid GEOS-Chem model is developed allowing for higher horizontal (0.5°×0.667°) and vertical resolution as compared to global models. CO transport over a heavily polluted region, the Beijing-Tianjin-Hebei (BTH) city cluster in China, and the pattern of outflow from East China in summertime are investigated. Comparison of the nested-grid with global models indicates that the fine-resolution nested-grid model is capable of resolving individual cities with high associated emission intensities. The nested-grid model indicates the presence of a high CO column density over the Sichuan Basin in summer, attributable to the low-level stationary vortex associated with the Basin's topographical features. The nested-grid model provides good agreement also with measurements from a suburban monitoring site in Beijing during summer 2005. Tagged CO simulation results suggest that regional emissions make significant contributions to elevated CO levels over Beijing on polluted days and that the southeastward moving cyclones bringing northwest winds to Beijing are the key meteorological mechanisms responsible for dispersion of pollution over Beijing in summer. Overall CO fluxes to the NW Pacific from Asia are found to decrease by a factor of 3–4 from spring to summer. Much of the seasonal change is driven by decreasing fluxes from India and Southeast Asia in summer, while fluxes from East China are only 30% lower in summer than in spring. Compared to spring, summertime outflow from Chinese source regions is strongest at higher latitudes (north of 35° N). The deeper convection in summer transporting CO to higher altitudes where export is more efficient is largely responsible for enhanced export in summer.