Characteristics and Processing of Aerosols in Regional Air Masses and Free Troposphere in the Western U.S.

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Description

Understanding the properties and lifecycle processes of aerosols in regional air masses and the free troposphere (FT) is crucial for constraining the climate impacts of aerosols on a global scale. During July and August, 2013, an intensive field campaign for characterizing aerosol chemical and physical properties was conducted at Mt. Bachelor Observatory in Oregon (MBO; 2763 m a.s.l., 43.981°N, 121.691°W), as part of the DOE-sponsored Biomass Burning Observation Project (BBOP). During this campaign, MBO was frequently impacted by transported wildfire plumes but there were two periods, July 25 – 30 and August 17 – 21, when the influences of wildfires were not detected and concentrations of air pollutants remained low. Here, we focus on analyzing observations from these clean periods in order to understand the characteristics of aerosols in regional background air masses and their evolution in association with boundary-layer (BL) dynamics in the Western US. In the absence of wildfire influence, the average concentration of non-refractory submicrometer aerosol (NR-PM1) at MBO was 3.77 g m-3 and 85% of the mass was organic. Organic aerosols (OA) showed clear diurnal variations driven by the BL dynamics with significantly higher concentrations occurring during daytime. More oxidized OA were observed at night when MBO resided in the FT and the aerosols were composed of higher mass fractions of ammonium sulfate and organonitrates. Two sources of OA were identified, including a BL-OOA (O/C = 0.67; 70% of OA mass) representing oxygenated OA (OOA) that mainly originated from the BL and a FT-OOA (30% of OA) comprised of highly oxidized (O/C = 1.17), low-volatility organics. These results highlight the significant compositional and physical differences between FT and BL aerosols, which may have important implications for understanding the climate effects of aerosols in remote regions.