Chemical and Physical Properties of Aerosols in Regional Background Air masses and Free Troposphere in the Western U.S.

Understanding the sources, evolution, and physicochemical properties of aerosols in regional background air masses and from the free troposphere (FT) is crucial for constraining the climate impacts that aerosols have on a global scale. In summer 2013, we conducted a field study at a remote high elevation site (2.8 km a.s.l.) in Central Oregon - the Mt. Bachelor Observatory (MBO) - to characterize non-refractory submicron aerosols (NR-PM1) using a High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-AMS) coupled with a thermodenuder (TD) system. During this study, MBO was frequently impacted by transported wildfire plumes but there were two periods (July 25 - 30 and August 17 - 21) when concentrations of air pollutants were very low (e.g., average NR-PM1 = 3.7 ± 4.2 μg m-3; CO = 87.8 ± 17.9 ppb). Here we focus on examining the chemical and physical properties of aerosol particles during 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. Major aerosol components showed clear diurnal variations with higher concentrations occurring during daytime driven by the evolution of the BL, consistent with previous studies at MBO. Conversely, lower concentrations of aerosol were observed at night when MBO resided in the FT and the aerosols appeared to be acidic with high mass fraction of ammonium sulfate (up to 90% of NR-PM1) and organonitrate. Nevertheless, organic aerosol (OA), which tend to be highly oxidized (O/C = 0.84), on average accounted for 85% of NR-PM1 during the clean periods. Positive Matrix Factorization (PMF) analysis of the HR-AMS data determined a BL-OOA (O/C = 0.69; 70% of OA mass) representing oxygenated OA (OOA) influenced by BL dynamics and a FT-OOA (30% of OA) comprised of highly oxidized low-volatility organics (O/C = 1.09). These results highlight the significant compositional differences between FT and BL aerosols in that the FT aerosols are significantly more oxidized and contain a higher fraction of sulfate. These observations may have important implications for understanding the climate effects of aerosols in remote regions.