The distribution of 222Rn over North America is simulated with a 3-d chemical tracer model (CTM) based on the meteorology of the GISS general circulation model (GCM). Model results are compared to surface observations from 5 sites in the United States. It is found that the 222Rn concentrations are regulated primary by dry convection. At night, the model underpredicts observations because it does not resolve the sharp 222Rn concentration gradient which forms near the surface. In daytime, the predicted and observed concentrations are usually in good agreement. Inspection of seasonal trends reveals, however, several significant discrepancies which are traced to anomalies in the GCM meteorology. -from Authors

We quantify the tropospheric ozone budget over remote high northern latitudes in summer using chemical and meteorological measurements between 0 and 6-km made during the summer of 1990 Arctic Boundary Layer Expedition (ABLE 3B). We include all components of the ozone budget, both sinks (in situ photochemical loss and deposition); and sources (in situ photochemical production, advection of pollution ozone into the region, production in biomass wildfire plumes, and downwards transport from the upper troposphere/stratosphere). In situ production and loss of ozone are calculated with a photochemical model. The net influx of pollution ozone from North America and Eurasia is estimated from the average enhancement ratio of ΔO3/ΔC2Cl4 observed in pollution plumes and scaled by the net influx of C2Cl4. The contribution of ozone produced in biomass wildfire plumes is estimated from the average enhancement ratio of ΔO3/ΔCO in aged fire plumes. Regional photochemical production and loss in the 0-6 km column are found to be approximately equal; hence, net photochemical production is near zero. However, when ozone production and loss terms are separated, we find that dispersed in situ photochemical production driven by background NO levels (5-10 pptv) is the largest source term in the ozone budget (62%). Influx of stratospheric ozone is of secondary importance (27%), long-range transport of pollution ozone makes a small contribution (9%), and photochemical production of ozone within biomass wildfire plumes is a relatively negligible term (2%) in the budget. Biomass fires and transport of anthropogenic pollution into the region may however have a major effect on the ozone budget through enhancement of background NOx mixing ratios which increase dispersed photochemical production. Using a I-D time-dependent photochemical model between 0 and 6 km, we obtain good agreement between the observed and model-generated vertical ozone profiles. We find that in situ photochemistry within the 0-6 km column accounts for nearly 90% of the ozone mixing ratio within the boundary layer, while above 5 km it accounts for only about 40%. Although photochemical production of ozone within the 0-6 km column is larger than the other source terms combined, the 1-D model results indicate that influx from above is necessary to account for the observed increase in ozone mixing ratios with altitude. Copyright 1996 by the American Geophysical Union.

The global burden of atmospheric methane has been increasing over the past decade, but the causes are not well understood. National inventory estimates from the U.S. Environmental Protection Agency indicate no significant trend in U.S. anthropogenic methane emissions from 2002 to present. Here we use satellite retrievals and surface observations of atmospheric methane to suggest that U.S. methane emissions have increased by more than 30% over the 2002-2014 period. The trend is largest in the central part of the country, but we cannot readily attribute it to any specific source type. This large increase in U.S. methane emissions could account for 30-60% of the global growth of atmospheric methane seen in the past decade.

We develop a daily-resolved global emission inventory for biomass burning using AVHRR satellite observations of fire activity corrected for data gaps and scan angle biases. We implemented this inventory in a global three-dimensional model (GEOSCHEM) to simulate aircraft CO observations during the TRACE-P mission over the NW Pacific in February-April 2001. Seasonal biomass burning in SE Asia was a major contributor to the outflow of Asian pollution observed in TRACE-P and shows large day-to-day fluctuations that vary depending on location. Three simulations were conducted with the same 3-month total (February-April) emissions but different temporal distributions: 2001 daily resolved, 2001 monthly resolved, and climatological monthly resolved. The effect of daily resolved versus monthly resolved 2001 emissions in the simulation of CO is less than 8 ppbv in Asian outflow over the NW Pacific but can exceed 100 ppbv over source regions. The relatively small effect in Asian outflow reflects spatial and temporal averaging of emissions during ageing in the continental boundary layer. Significant improvement in the simulation of TRACE-P observations (as diagnosed by the resolved variance) is found when using 2001 monthly versus climatological monthly emissions, but using 2001 daily emissions does not offer further improvement. Copyright 2003 by the American Geophysical Union.

We use a 2005-2009 record of isoprene emissions over Africa derived from Ozone Monitoring Instrument (OMI) satellite observations of formaldehyde (HCHO) to better understand the factors controlling isoprene emission in the continent and evaluate the impact on atmospheric composition. OMI-derived isoprene emissions show large seasonality over savannas driven by temperature and leaf area index (LAI), and much weaker seasonality over equatorial forests driven by temperature. The commonly used MEGAN (Model of Emissions of Gases and Aerosols from Nature, version 2.1) global isoprene emission model reproduces this seasonality but is biased high, particularly for equatorial forests, when compared to OMI and relaxed-eddy accumulation measurements. Isoprene emissions in MEGAN are computed as the product of an emission factor Eo, LAI, and activity factors dependent on environmental variables. We use the OMI-derived emissions to provide improved estimates of Eo that are in good agreement with direct leaf measurements from field campaigns (r = 0.55, bias Combining double low line-19%). The largest downward corrections to MEGAN Eo values are for equatorial forests and semi-arid environments, and this is consistent with latitudinal transects of isoprene over western Africa from the African Monsoon Multidisciplinary Analysis (AMMA) aircraft campaign. Total emission of isoprene in Africa is estimated to be 77 Tg C ag-1, compared to 104 Tg C ag-1 in MEGAN. Simulations with the GEOS-Chem oxidant-aerosol model suggest that isoprene emissions increase mean surface ozone in western Africa by up to 8 ppbv, and particulate matter by up to 1.5 μg mg-3, due to coupling with anthropogenic influences. © 2014 Author(s).

[1] We simulate the oceanic and atmospheric distribution of methyl iodide (CH3I) with a global 3-D model driven by assimilated meteorological observations from the Goddard Earth Observing System of the NASA Data Assimilation Office and coupled to an oceanic mixed layer model. A global compilation of atmospheric and oceanic observations is used to constrain and evaluate the simulation. Seawater CH3I(aq) in the model is produced photochemically from dissolved organic carbon, and is removed by reaction with Cl- and emission to the atmosphere. The net oceanic emission to the atmosphere is 214 Gg yr-1. Small terrestrial emissions from rice paddies, wetlands, and biomass burning are also included in the model. The model captures 40% of the variance in the observed seawater CH3I(aq) concentrations. Simulated concentrations at midlatitudes in summer are too high, perhaps because of a missing biological sink of CH3I(aq). We define a marine convection index (MCI) as the ratio of upper tropospheric (8-12 km) to lower tropospheric (0-2.5 km) CH3I concentrations averaged over coherent oceanic regions. The MCI in the observations ranges from 0.11 over strongly subsiding regions (southeastern subtropical Pacific) to 0.40 over strongly upwelling regions (western equatorial Pacific). The model reproduces the observed MCI with no significant global bias (offset of only +11%) but accounts for only 15% of its spatial and seasonal variance. The MCI can be used to test marine convection in global models, complementing the use of radon-222 as a test of continental convection. Copyright 2002 by the American Geophysical Union.

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.

Concurrent measurements of OH, HO2, H2O2, and CH3OOH concentrations were made during ari aircraft flight over the tropical South Pacific that followed a back-and-forth pattern at constant 10 km altitude for 4 hours. One end of the pattern sampled an aged convective outflow, while the other end sampled the background atmosphere. Concentrations of HO2 and CH3OOH in the convective outflow were elevated by 50 and 350% relative to background, respectively, while concentrations of OH and H2O2 were not elevated. The high CH3OOH concentrations in the outflow were due to convective pumping from the marine boundary layer. In contrast to CH3OOH, H2O2 was not enhanced in the outflow because its high water solubility allows efficient scavenging in the convective updraft. A photochemical model calculation constrained with the ensemble of aircraft observations reproduces the HO2 enhancement in the convective outflow and attributes it to the enhanced CH3OOH; the calculation also reproduces the lack of OH enhancement in the outflow and attributes it to OH loss from reaction with CH3OOH. Further analysis of model results shows substantial evidence that the rate constant used in standard mechanisms for the CH3O2 + HO2 reaction is about a factor of 3 too low at the low temperatures of the upper troposphere. A sensitivity simulation using a value of 3.4 × 10-11 cm3 molecule-1 s-1 at 233 K for this rate constant yields better agreement with observed HO2 concentrations and better closure of the chemical budgets for both CH3OOH and H2O2. The CH3O2 + HO2 reaction then becomes the single most important loss pathway for HOx radicals (HOx = OH+ peroxy radicals) in the upper troposphere. Copyright 2001 by the American Geophysical Union.

The late winter/early spring (February/March, 1994) measurements of Pacific Exploratory Mission-West (PEM-W) B have been analyzed to show latitudinal distributions (45°N to 10°S) of the mixing ratios of reactive nitrogen species (NO, peroxyacetylnitrate (PAN), HNO3, and NOy), ozone, and chemical tracers (CO, NMHCs, acetone, and C2Cl4) with a focus on the upper troposphere. Mixing ratios of all species are relatively low in the warm tropical and subtropical air south of the polar jetstream (≈28°N) but increase sharply with latitude in the cold polar air north of the jetstream. Noteworthy is the continuous increase in reservoir species (PAN and HNO3) and the simultaneous decrease in NOx toward the northern midlatitudes. The Harvard global three-dimensional model of tropospheric chemistry has been used to compare these observations with predictions. In the upper troposphere the magnitude and distribution of measured NOy and PAN as a function of latitude is well represented by this model, while NOx (measured NO + model calculated NO2) is underpredicted, especially in the tropics. Unlike several previous studies, where model-predicted HNO3 exceeded observations by as much as a factor of 10, the present data/model comparison is improved to within a factor of 2. The predicted upper tropospheric HNO3 is generally below or near measured values, and there is little need to invoke particle reactions as a means of removing or recycling HNO3. Comparison between measured NOy and the sum of its three main constituents (PAN + NOx + HNO3) on average show a small mean shortfall (<15%). This shortfall could be attributed to the presence of known but unmeasured species (e.g., peroxynitric acid and alkyl nitrates) as well as to instrument errors. Copyright 1998 by the American Geophysical Union.