Aircraft observations of carbon monoxide (CO) from the ICARTT campaign over the eastern United States in summer 2004 (July 1 -August 15), interpreted with a global 3-D model of tropospheric chemistry (GEOS-Chem), show that the national anthropogenic emission inventory from the U.S. Environmental Protection Agency (93 Tg CO y-1) is too high by 60% in summer. Our best estimate of the CO anthropogenic source for the ICARTT period is 6.4 Tg CO, including 4.6 Tg from direct emission and 1.8 Tg CO from oxidation of anthropogenic volatile organic compounds (VOCs). The biogenic CO source for the same period from the oxidation of isoprene and other biogenic VOCs is 8.3 Tg CO, and is independently constrained by ICARTT observations of formaldehyde (HCHO). Anthropogenic emissions of CO in the U.S. have decreased to the point that they are now lower than the biogenic source in summer. © Copyright 2008 by the American Geophysical Union.

We analyze aircraft observations obtained during INTEX-A (1 July to 14 August 2004) to examine the summertime influence of Asian pollution in the free troposphere over North America. By applying correlation analysis and principal component analysis (PCA) to the observations between 6 and 12 km, we find dominant influences from recent convection and lightning (13% of observations), Asia (7%), the lower stratosphere (7%), and boreal forest fires (2%), with the remaining 71 % assigned to background. Asian air masses are marked by high levels of CO, O3, HCN, PAN, C2H2, C6H6. methanol, and SO4^2-. The partitioning of NOy species in the Asian plumes is dominated by PAN (∼600 pptv), with varying NOx/HNO3 ratios in individual plumes, consistent with individual transit times of 3-9 days. Export of Asian pollution occurred in warm conveyor belts of midlatitude cyclones, deep convection, and in typhoons. Compared to Asian outflow measurements during spring, INTEX-A observations display lower levels of anthropogenic pollutants (CO, C3H8, C2H6, C6H6) due to shorter summer lifetimes; higher levels of biogenic tracers (methanol and acetone) because of a more active biosphere; and higher levels of PAN, NOx, HNO3, and O3 reflecting active photochemistry, possibly enhanced by efficient NOy export and lightning. The high DO3/ΔCO ratio (0.76 mol/mol) in Asian plumes during INTEX-A is due to strong photochemical production and, in some cases, mixing with stratospheric air along isentropic surfaces. The GEOS-Chem global model captures the timing and location of the Asian plumes. However, it significantly underestimates the magnitude of observed enhancements in CO, O3, PAN and NOx. Copyright 2007 by the American Geophysical Union.

A comprehensive group of reactive nitrogen species (NO, NO2, HNO3, HO2NO2, PANs, alkyl nitrates, and aerosol-NO3^-) were measured over North America during July/August 2004 from the NASA DC-8 platform (0.1-12 km). Nitrogen containing tracers of biomass combustion (HCN and CH3CN) were also measured along with a host of other gaseous (CO, VOC, OVOC, halocarbon) and aerosol tracers. Clean background air as well as air with influences from biogenic emissions, anthropogenic pollution, biomass combustion, convection, lightning, and the stratosphere was sampled over the continental United States, the Atlantic, and the Pacific. The North American upper troposphere (UT) was found to be greatly influenced by both lightning NOx and surface pollution lofted via convection and contained elevated concentrations of PAN, ozone, hydrocarbons, and NOx. Observational data suggest that lightning was a far greater contributor to NOx in the UT than previously believed. PAN provided a dominant reservoir of reactive nitrogen in the UT while nitric acid dominated in the lower troposphere (LT). Peroxynitric acid (HO2NO2) was present in sizable concentrations peaking at around 8 km. Aerosol nitrate appeared to be mostly contained in large soil based particles in the LT. Plumes from Alaskan fires contained large amounts of PAN and aerosol nitrate but little enhancement in ozone. A comparison of observed data with simulations from four 3-D models shows significant differences between observations and models as well as among models. We investigate the partitioning and interplay of the reactive nitrogen species within characteristic air masses and further examine their role in ozone formation. Copyright 2007 by the American Geophysical Union.

A model intercomparison activity was inspired by the large suite of observations of atmospheric composition made during the International Polar Year (2008) in the Arctic. Nine global and two regional chemical transport models participated in this intercomparison and performed simulations for 2008 using a common emissions inventory to assess the differences in model chemistry and transport schemes. This paper summarizes the models and compares their simulations of ozone and its precursors and presents an evaluation of the simulations using a variety of surface, balloon, aircraft and satellite observations. Each type of measurement has some limitations in spatial or temporal coverage or in composition, but together they assist in quantifying the limitations of the models in the Arctic and surrounding regions. Despite using the same emissions, large differences are seen among the models. The cloud fields and photolysis rates are shown to vary greatly among the models, indicating one source of the differences in the simulated chemical species. The largest differences among models, and between models and observations, are in NOy partitioning (PAN vs. HNO3) and in oxygenated volatile organic compounds (VOCs) such as acetaldehyde and acetone. Comparisons to surface site measurements of ethane and propane indicate that the emissions of these species are significantly underestimated. Satellite observations of NO2 from the OMI (Ozone Monitoring Instrument) have been used to evaluate the models over source regions, indicating anthropogenic emissions are underestimated in East Asia, but fire emissions are generally overestimated. The emission factors for wildfires in Canada are evaluated using the correlations of VOCs to CO in the model output in comparison to enhancement factors derived from aircraft observations, showing reasonable agreement for methanol and acetaldehyde but underestimate ethanol, propane and acetone, while overestimating ethane emission factors.