The Impact of the Bay Breeze on Ammonium Nitrate Formation in the Baltimore-Metropolitan Area

The chemical composition of particle and gas-phase constituents in the Baltimore area is strongly impacted by its close proximity to the Chesapeake Bay. In particular, the varying meteorological conditions associated with the bay breeze circulation can strongly affect the concentration and composition of pollutants. As part of the OWLETS-2 field campaign in summer 2018, measurements of PM_2.5 inorganic aerosols and gas-phase ammonia concentrations were carried out at Hart Miller Island, a site near the land-water transition in Baltimore. Meteorological data were collected along with the highly time-resolved chemical measurements to characterize the connection between the bay breeze and chemical processes occurring in the Baltimore urban airshed. High aerosol nitrate concentrations were frequently observed, an unusual characteristic for summers in the Eastern United States. Elevated gas-phase ammonium concentrations corresponded to the high nitrate concentrations. Ammonia demonstrated a very distinct diurnal profile, with a strong increase from sunrise to a mid-afternoon peak. Pollutant sources were investigated through analysis of wind direction and wind speed. Winds were predominantly from the south and the east – differences in pollutant concentrations were compared in these different regimes.