Dependence of Daily Aerosol Wet Deposition on Precipitation at Appalachian Mountains Site in the United States

ABSTRACT

The wet removal of airborne particulates is a significant component of atmospheric deposition in the Eastern United States. This study analyzed the daily wet deposition of major ions at the Canaan Valley site in Appalachian Mountains in Eastern US, for the time interval 2000–2014. The site is part of the Atmospheric Integrated Research Monitoring Network (AIRMoN), and is significantly impacted by acid precipitation, caused largely by anthropogenic sources of SO₂ and NO_x. Results show that the precipitation rate, R, varies mainly in the interval [0.01–100] mm day^–1, and the daily wet deposition flux, F, varies about two orders of magnitude for most ions. The largest daily wet depositions are for SO₄^2– and NO₃^– with extreme values over 30 mg m^–2 day^–1. In the case of NH₄⁺, the largest daily wet depositions are over 10 mg m^–2 day^–1. Seasonal variations are illustrated by contrasting the winter and summer. In general, there are much larger daily wet deposition fluxes in summer than in winter. For SO₄^2– there is more conversion of SO₂ to SO₄^2– in the gas phase and in cloud droplets during summer. Similarly, NH₄⁺ has a distinct seasonal variation with a maximum in summer, consistent with larger sources of NH₃ during the growth season. NO₃^– has a maximum concentration in precipitation during winter, and a maximum daily wet deposition flux during summer, especially during the most intense rain events. The Na⁺ and Cl^– ions have the highest wet deposition in winter due to storms that bring air masses from Atlantic. Analysis shows that precipitation events are more frequent and more intense in summer than in winter. For the Canaan Valley, the summer precipitation events are effective in wet removal of aerosols providing episodes with some of the highest rates of acid deposition.