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Abstract Coastal regions are frequently subject to higher concentrations of ozone (O 3) than areas farther inland due to thermally-direct recirculation (e.g., sea/bay breezes) of pollutants during summer months. Along with high O 3 concentrations on polluted days, a bay breeze (in this case, the Chesapeake Bay breeze) causes low-level convergence over the land, which can lead to deep convection even on days without any large-scale convective forcing, altering both the local flow pattern and the concentrations and distributions of pollutants. Presented here is a bay breeze and thunderstorm climatology for Edgewood, MD, a coastal site along the Chesapeake Bay, from June, July, and August 2011–2016 between 11 and 19 EST (UTC-5). Using meteorological data from the Maryland Department of the Environment (MDE), bay breezes are identified by an automated detection algorithm, the Bay-breeze Identification Algorithm (BIA), customized for the complex coastline of river inlets within the Chesapeake Bay area. Additionally, thunderstorm vs. non-thunderstorm days are analyzed using gridded Earth Networks lightning data (ENTLN) within an influential radius of the site. The effects of bay breezes and deep convection on local O 3 is quantified. The highest daily conditional mean O 3 for all years was observed on bay breeze days, and the lowest on days with thunderstorms only (no bay breeze). Additionally, 39% of O 3 exceedance days within the analysis time period were also bay breeze days. However, as ozone precursor emissions have fallen, the relationship between bay breezes and high O 3 has diminished. While days with thunderstorms only (no bay breeze) were associated with the lowest daily mean O 3 , there were O 3 events on some thunderstorm days, especially if there was also a bay breeze. Twenty-four percent of the total ozone exceedance days had a thunderstorm, demonstrating that thunderstorm days should not be assumed to be clean days, as boundary-layer venting and downdraft mixing depends on the thunderstorm duration and type (e.g., non-frontal/pop-up convection allowed slightly higher O 3 concentrations than did frontal convection). These results illustrate the significant role that localized meteorological events that are difficult to predict play in modulating air pollution where many of the world's cities lie – near bodies of water. Highlights • Mean O 3 concentrations are higher on bay breeze days than on non-bay breeze days. • Thunderstorms do not always terminate a pollution episode. • The relationship between bay breezes and O 3 episodes diminishes with cleaner air. [ABSTRACT FROM AUTHOR]

Highly-resolved vertical profiles of ozone and reactive nitrogen in the lower troposphere were obtained using Millersville University's tethered balloon system and NASA's P-3B aircraft during the July 2011 Baltimore, MD/Washington DC and the September 2013 Houston, TX deployments of the NASA DISCOVER-AQ air quality field mission. The tethered balloon and surface measurement sites were located at Edgewood, MD and Smith Point, TX. The balloon profiles are used to connect aircraft data from the lowest portion of NASA's P-3B spirals (300 m AGL) to the surface thus creating complete profiles from the surface to 3–5 km AGL. The highest concentrations of surface ozone at these coastal sites resulted from mean flow transport of polluted air over an adjacent body of water followed by advection back over land several hours later, due to a bay or gulf breeze. Several meteorological processes including horizontal advection, vertical mixing, thermally direct circulation (i.e., bay, gulf, and, sea breezes) combined with chemical processes like photochemical production and deposition played a role in the local ozone maxima. Several small-scale, but highly polluted layers from the Chesapeake Bay advected landward over Edgewood, MD. The Houston Metro area was subject to large-scale recirculation of emissions from petrochemical sources by the Gulf of Mexico and Galveston Bay breezes. [ABSTRACT FROM AUTHOR]

Different atmospheric mercury forms have been measured at a suburban site in Beltsville, Maryland in the Mid-Atlantic United States since 2007 to investigate their inter-annual, seasonal and diurnal variabilities. Average concentrations and standard deviations of hourly measurements from 2007 to 2015 were 1.41 ± 0.23 ng m −3 for gaseous elemental mercury (GEM), 4.6 ± 33.7 pg m −3 for gaseous oxidized mercury (GOM), and 8.6 ± 56.8 pg m −3 for particulate-bound mercury (PBM). Observations show that on average, the rates of decrease were 0.020 ± 0.007 ng m −3 yr −1 (or 1.3 ± 0.5% yr −1 , statistically significant, p-value < 0.01) for GEM, 0.54 ± 0.19 pg m −3 yr −1 (or 7.3 ± 2.6% yr −1 , statistically significant, p-value < 0.01) for GOM, and 0.15 ± 0.35 pg m −3 yr −1 (or 1.6 ± 3.8% yr −1 , statistically insignificant, p-value > 0.01) for PBM over this nine-year period. In addition, the collocated annual mercury wet deposition decreased at a rate of 0.51 ± 0.24 μg m −2 yr −2 (or 4.2 ± 1.9% yr −1 , statistically insignificant, p-value > 0.01). Diurnal variation of GEM shows a slight peak in the morning, likely due to the shallow boundary layer. Seasonal variation of GEM shows lower levels in fall. Both diurnal variations of GOM and PBM show peaks in the afternoon likely due to the photochemical production of reactive mercury from the oxidation of GEM and the influence of boundary layer processes. Seasonally, GOM measurements show high levels in spring and constant low levels in the other three seasons, while PBM measurements exhibit higher levels from late fall to early spring and lower levels from late spring to fall. These measurement data were analyzed using the HYSPLIT back trajectory model in order to examine possible source-receptor relationships at this suburban site. Trajectory frequency analysis shows that high GEM/GOM/PBM events were generally associated with high frequencies of the trajectories passing through areas with high mercury emissions, while low GEM/GOM/PBM levels were largely associated the trajectories passing through relatively clean areas. This study indicates that local and regional sources appear to have a significant impact on the site and these impacts appear to have changed over time, as the local/regional emissions have been reduced. [ABSTRACT FROM AUTHOR]