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

Abstract

Abstract. Understanding the properties and life cycle processes of aerosol particles in regional air masses is crucial for constraining the climate impacts of aerosols on a global scale. In this study, characteristics of aerosols in the boundary layer (BL) and free troposphere (FT) of a remote continental region in the western US were studied using a high-resolution time-of-flight aerosol mass 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 wildfire influence, the average (±1σ) concentration of non-refractory submicrometer particulate matter (NR-PM1) at MBO was 2.8 (±2.8) µg m−3 and 84 % of the mass was organic. The other NR-PM1 components were sulfate (11 %), ammonium (2.8 %), and nitrate (0.9 %). The organic aerosol (OA) at MBO from these clean periods showed clear diurnal variations driven by the boundary layer dynamics with significantly higher concentrations occurring during daytime, upslope conditions. NR-PM1 contained a higher mass fraction of sulfate and was frequently acidic when MBO resided in the FT. In addition, OA in the FT was found to be highly oxidized (average O∕C of 1.17) with low volatility while OA in BL-influenced air masses was moderately oxidized (average O∕C of 0.67) and semivolatile. There are indications that the BL-influenced OA observed at MBO was more enriched in organonitrates and organosulfur compounds (e.g., MSA) and appeared to be representative of biogenic secondary organic aerosol (SOA) originated in the BL. A summary of the chemical compositions of NR-PM1 measured at a number of other high-altitude locations in the world is presented and similar contrasts between FT and BL aerosols were observed. The significant compositional and physical differences observed between FT and BL aerosols may have important implications for understanding the climate effects of regional background aerosols.

Interferences with aerosol acidity quantification due to gas-phase ammonia uptake onto acidic sulfate filter samples

“…For many of these sites, ammonia denuders are used to minimize biases of ammonium on filters (e.g., Baltensperger et al, 2003). Data from remote highaltitude locations have indicated that the ammonium balance is less than 1 (Cozic et al, 2008;Sun et al, 2009;Freney et al, 2016;Zhou et al, 2019), similar to the observations from the AMS shown in Fig. 3.…”

Interferences on Aerosol Acidity Quantification due to Gas-phase Ammonia Uptake onto Acidic Sulfate Filter Samples

“…For many of these sites, ammonia denuders are used to minimize biases of ammonium on filters (e.g, (Baltensperger et al, 2003) ). Data from remote, high altitude locations have indicated that the ammonium balance is less than one (Cozic et al, 2008;Sun et al, 2009;Freney et al, 2016;Zhou et al, 2019) , similar to the observations from the AMS shown in Fig. 3 .…”

Characterization of aerosol particles during a high pollution episode over Mexico City

“…The present results are consistent with those reported by Decarlo et al (2008) 11 , who observed elevated sulfate at the higher altitudes above MC. Similar studies of aerosol chemical composition conducted at other high-altitude sites in different parts of the world (in the absence of BB emissions) reveal that aerosols in the FT contain a high fraction of sulfates 11,43,44 .…”