Atmospheric Chemistry and Physics, vol. 13, issue 15 (2013) pp. 7859-7874
Trends in the composition of the lower atmo-sphere (0–1500 m altitude) and surface air quality over the Baltimore/Washington area and surrounding states were in-vestigated for the period from 1997 to 2011. We examined emissions of ozone precursors from monitors and invento-ries as well as ambient ground-level and aircraft measure-ments to characterize trends in air pollution. The US EPA Continuous Emissions Monitoring System (CEMS) program reported substantial decreases in emission of summertime nitrogen oxides (NO x) from power plants, up to ∼ 80 % in the mid-Atlantic States. These large reductions in emis-sion of NO x are reflected in a sharp decrease of ground-level concentrations of NO x starting around 2003. The de-creasing trend of tropospheric column CO observed by air-craft is ∼ 0.8 Dobson unit (DU) per year, corresponding to ∼ 35 ppbv yr −1 in the lower troposphere (the surface to 1500 m above ground level). Satellite observations of long-term, near-surface CO show a ∼ 40 % decrease over west-ern Maryland between 2000 and 2011; the same magnitude is indicated by aircraft measurements above these regions upwind of the Baltimore/Washington airshed. With decreas-ing emissions of ozone precursors, the ground-level ozone in the Baltimore/Washington area shows a 0.6 ppbv yr −1 de-crease in the past 15 yr. Since photochemical production of ozone is substantially influenced by ambient temperature, we introduce the climate penalty factor (CPF) into the trend analysis of long-term aircraft measurements. After compen-sating for inter-annual variations in temperature, historical aircraft measurements indicate that the daily net produc-tion of tropospheric ozone over the Baltimore/Washington area decreased from ∼ 20 ppbv day −1 in the late 1990s to ∼ 7 ppbv day −1 in the early 2010s during ozone season. A decrease in the long-term column ozone is observed as ∼ 0.2 DU yr −1 in the lowest 1500 m, corresponding to an improve-ment of ∼ 1.3 ppbv yr −1 . Our aircraft measurements were conducted on days when severe ozone pollution was fore-casted, and these results represent the decreasing trend in high ozone events over the past 15 yr. Back trajectory cluster analysis demonstrates that emissions of air pollutants from Ohio and Pennsylvania through Maryland influence the col-umn abundances of downwind ozone in the lower atmo-sphere. The trends in air pollutants reveal the success of reg-ulations implemented over the past decades and the impor-tance of region-wide emission controls in the eastern United States.
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