Abstract
Abstract. Trends in the composition of the lower atmosphere (0–1500 m altitude) and surface air quality over the Baltimore/Washington area and surrounding states were investigated for the period from 1997 to 2011. We examined emissions of ozone precursors from monitors and inventories as well as ambient ground-level and aircraft measurements to characterize trends in air pollution. The US EPA Continuous Emissions Monitoring System (CEMS) program reported substantial decreases in emission of summertime nitrogen oxides (NOx) from power plants, up to ∼80% in the mid-Atlantic States. These large reductions in emission of NOx are reflected in a sharp decrease of ground-level concentrations of NOx starting around 2003. The decreasing trend of tropospheric column CO observed by aircraft 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 western Maryland between 2000 and 2011; the same magnitude is indicated by aircraft measurements above these regions upwind of the Baltimore/Washington airshed. With decreasing emissions of ozone precursors, the ground-level ozone in the Baltimore/Washington area shows a 0.6 ppbv yr−1 decrease 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 compensating for inter-annual variations in temperature, historical aircraft measurements indicate that the daily net production 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 improvement of ∼1.3 ppbv yr−1. Our aircraft measurements were conducted on days when severe ozone pollution was forecasted, 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 column abundances of downwind ozone in the lower atmosphere. The trends in air pollutants reveal the success of regulations implemented over the past decades and the importance of region-wide emission controls in the eastern United States.
Highlights
Ocean Science in the Baltimore/Washington area shows a 0.6 ppbv yr−1 decrease 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
We have investigated the long-term trends of air pollution in the lower troposphere in the Baltimore/Washington airshed through a comprehensive study using emissions, aircraft, ground-based, and satellite measurements, as well as a clustering back trajectory analysis
Summary
Ocean Science in the Baltimore/Washington area shows a 0.6 ppbv yr−1 decrease in the past 15 yr. Long-term records of tropospheric ozone in the US have been investigated through ground-level observations (Oltmans et al, 2006; Jaffe and Ray, 2007; Lefohn et al, 2008, 2010; Oltmans et al, 2013), trends of ozone are not monotonic (Cooper et al, 2012; Oltmans et al, 2013) Analysis of these long-term ozone measurements can shed light on the change of “baseline” ozone in order to estimate the flux of air pollutants entering and exiting North America, relevant to regulation of ambient air quality (Oltmans et al, 2008; Parrish et al, 2009; Chan and Vet, 2010; Cooper et al, 2010; Lin et al, 2012). The RAMMPP aircraft program started with measurements of tropospheric O3 and CO in 1997, providing a 15 yr record for investigating summertime air quality trends in the Baltimore/Washington airshed. We quantify trends in summertime air pollution over the past 15 yr using measurements of tropospheric ozone and its precursors (CO and NOx) in the mid-Atlantic region and upwind states, as well as national and regional emissions data. We discuss the importance of emission regulation for improving ozone pollution in the nonattainment Baltimore/Washington airshed, as well as implications for future control measures
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