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Abstract. Understanding the properties and life cycle processes of aerosol particles inregional air masses is crucial for constraining the climate impacts ofaerosols on a global scale. In this study, characteristics of aerosols in theboundary layer (BL) and free troposphere (FT) of a remote continental regionin the western US were studied using a high-resolution time-of-flight aerosolmass spectrometer (HR-AMS) deployed at the Mount Bachelor Observatory (MBO;2763 m a.s.l.) in central Oregon in summer 2013. In the absence of wildfireinfluence, the average ( ± 1 σ ) concentration of non-refractorysubmicrometer particulate matter (NR- PM1 ) at MBO was 2.8 ( ± 2.8) µ g m −3 and 84 % of the mass was organic. The otherNR- PM1 components were sulfate (11 %), ammonium (2.8 %),and nitrate (0.9 %). The organic aerosol (OA) at MBO from these cleanperiods showed clear diurnal variations driven by the boundary layer dynamicswith significantly higher concentrations occurring during daytime, upslopeconditions. NR- PM1 contained a higher mass fraction of sulfate andwas frequently acidic when MBO resided in the FT. In addition, OA in the FTwas found to be highly oxidized (average O∕C of 1.17) with lowvolatility while OA in BL-influenced air masses was moderately oxidized(average O∕C of 0.67) and semivolatile. There are indications thatthe BL-influenced OA observed at MBO was more enriched in organonitrates andorganosulfur compounds (e.g., MSA) and appeared to be representative ofbiogenic secondary organic aerosol (SOA) originated in the BL. A summary ofthe chemical compositions of NR- PM1 measured at a number of otherhigh-altitude locations in the world is presented and similar contrastsbetween FT and BL aerosols were observed. The significant compositional andphysical differences observed between FT and BL aerosols may have importantimplications for understanding the climate effects of regional backgroundaerosols.

Free tropospheric aerosols at the Mt. Bachelor Observatory: more oxidized and higher sulfate content compared to boundary layer aerosols