Abstract
We use observations from the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) campaign over eastern North America in summer 2004, interpreted with a global 3‐D model of tropospheric chemistry (GEOS‐Chem), to improve and update estimates of North American influence on global tropospheric ozone and the effect of recent U.S. anthropogenic reductions on surface ozone pollution. We find that the 50% decrease in U.S. stationary NOx sources since 1999 has decreased mean U.S. boundary layer ozone concentrations by 6–8 ppbv in the southeast and 4–6 ppbv in the Midwest. The observed dO3/dCO molar enhancement ratio in the U.S. boundary layer during ICARTT was 0.46 mol mol−1, larger than the range of 0.3–0.4 from studies in the early 1990s, possibly reflecting the decrease in the NOx/CO emission ratio as well as an increase in the ozone production efficiency per unit NOx. North American NOx emissions during summer 2004 as constrained by the ICARTT observations (0.72 Tg N fossil fuel, 0.11 Tg N biomass burning, 0.28 Tg N lightning for 1 July to 15 August) enhanced the hemispheric tropospheric ozone burden by 12.4%, with comparable contributions from fossil fuel and lightning (5–6%), but only 1% from biomass burning emissions despite 2004 being a record fire year over Alaska and western Canada.
Highlights
[2] Ozone (O3) is produced in the troposphere by photochemical oxidation of methane (CH4), carbon monoxide (CO) and nonmethane volatile organic compounds (NMVOCs) in the presence of nitrogen oxides (NOx = NO + NO2)
[4] Hudman et al [2007, 2008] previously applied GEOS-Chem to the ICARTT data to derive constraints on U.S emissions of CO and NOx from combustion and lightning. They found that the ICARTT data implied a 50% reduction in anthropogenic NOx emissions from stationary sources relative to the National Emission Inventory of the U.S Environmental Protection Agency (EPA), constructed for 1999 (NEI 99), consistent with 1999– 2004 regulations of stationary NOx sources
We find in the GEOS-Chem simulation that the Midwest boundary layer has ozone production efficiency (OPE) values in the range 2.5– 3.5, whereas the southeast has values in the range 4– 5.5, consistent with the simulated pattern of ozone decreases
Summary
North American influence on tropospheric ozone and the effects of recent emission reductions: Constraints from ICARTT observations.
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