Investigating the Acidity and Liquid Water Content of Atmospheric Particles on the Chesapeake Bay during the OWLETS-2 Campaign

The purpose of this study was to characterize the impact of the Chesapeake Bay on aerosol acidity during the Ozone Water-Land Environmental Transition Study (OWLETS-2). Measurements of inorganic PM_2.5 composition and gas-phase ammonia were taken at the Hart-Miller Island supersite, which lies east of Baltimore on the Chesapeake Bay. The location on the Chesapeake Bay was important for multiple reasons (e.g. local bay-breeze circulations, ammonia source). Acidity is an important characteristic of atmospheric particles, having the ability to drive the partitioning of semi-volatile aerosol species, determine solubilization of trace metals in aerosols, and impact the health of both humans and various ecosystems. The acidity (i.e. pH) of a particle is a measure of the hydronium ion activity in the aqueous aerosol phase. Therefore, the aerosol pH is tightly linked with the liquid water content of the particle. The liquid water content and pH of particles measured during OWLETS-2 were predicted with the ISORROPIA II thermodynamic equilibrium model. ISORROPIA II incorporates measurements of temperature, relative humidity, and concentrations of both gases and particles. The median aerosol pH for this study was 2.31, which was higher (less acidic) than the median aerosol pH predicted for a site in downtown Baltimore also during the summer, which was 1.27. Additionally, pH and aerosol liquid water did not exhibit strong diurnal variations, in contrast to other sites in the eastern U.S. during the summer. This suggests a strong influence of the Chesapeake Bay on aerosol chemistry.