Aerosol and precipitation composition at a coastal New England site (Acadia National Park): implications for air quality and aerosol composition during cold air outbreaks†

Abstract

This study investigates aerosol and wet deposition chemistry at Acadia National Park (Maine, U.S.) using data between 1 January 2001 and 31 December 2021. Results show that PM_2.5 is highest in summer and dominated by sulfate salts and organics (less contribution from elemental carbon), whereas nitrate salts and sea salt were highest in winter. Fine soil is most pronounced from March through August due most likely to long-range transport. Residual mass (PM_2.5 – reconstructed PM_2.5) was negative from November–March, with reasons discussed for its seasonal changes. Major regional sources of pollution are upwind from populated cities generally to the southwest of Acadia. Extreme PM_2.5 events are mostly driven by regional pollution events with others due to transported summertime biomass burning plumes that increased in frequency in the most recent years. Aerosol composition on cold air outbreak days showed that ammonium sulfate and organics dominated PM_2.5, which provides useful information for studies focused on understanding the formation and evolution of offshore cloud decks during the winter. Monthly mean pH in wet deposition ranges from 4.8 to 5.1 with the lowest values in July when contributions from acidic ions are highest (sulfate, nitrate). Average annual pH increased from 4.64 to 5.23 over the study period coincident with reductions in sulfate and nitrate levels. Sea salt constituents dominated the wet deposition aqueous ion concentrations from November to March, whereas in the other months sulfate and nitrate were highest. Interrelationships between aerosol and wet deposition species relevant to secondarily produced species, dust, and sea salt provide support for aerosol–precipitation interactions that warrant a further look with more robust methods.