Abstract
Abstract. Filter-based PM2.5 samples were chemically analyzed to investigate secondary organic aerosol (SOA) formation from isoprene in a rural atmosphere of the southeastern US influenced by both anthropogenic sulfur dioxide (SO2) and ammonia (NH3) emissions. Daytime PM2.5 samples were collected during summer 2010 using conditional sampling approaches based on pre-defined high and low SO2 or NH3 thresholds. Known molecular-level tracers for isoprene SOA formation, including 2-methylglyceric acid, 3-methyltetrahydrofuran-3,4-diols, 2-methyltetrols, C5-alkene triols, dimers, and organosulfate derivatives, were identified and quantified by gas chromatography coupled to electron ionization mass spectrometry (GC/EI-MS) and ultra performance liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-Q-TOFMS). Mass concentrations of six isoprene low-NOx SOA tracers contributed to 12–19% of total organic matter (OM) in PM2.5 samples collected during the sampling period, indicating the importance of the hydroxyl radical (OH)-initiated oxidation (so-called photooxidation) of isoprene under low-NOx conditions that lead to SOA formation through reactive uptake of gaseous isoprene epoxydiols (IEPOX) in this region. The contribution of the IEPOX-derived SOA tracers to total organic matter was enhanced by 1.4% (p = 0.012) under high-SO2 sampling scenarios, although only weak associations between aerosol acidity and mass of IEPOX SOA tracers were observed. This suggests that IEPOX-derived SOA formation might be modulated by other factors simultaneously, rather than only aerosol acidity. No clear associations between isoprene SOA formation and high or low NH3 conditional samples were found. Positive correlations between sulfate aerosol loadings and IEPOX-derived SOA tracers for samples collected under all conditions indicates that sulfate aerosol could be a surrogate for surface accommodation in the uptake of IEPOX onto preexisting aerosols.
Highlights
Hydrology and Tropospheric fine aerosolEs have been recognSizcedietno chaevse significant inof six isoprene low-NOx secondary organic aerosol (SOA) tracers contributed to 12–19 % fluences on regional air quality, climate change, and human of total organic matter (OM) in PM2.5 samples collected dur- health (Kanakidou et al, 2005; Hallquist et al, 2009)
in limited understanding of their epoxydiols (IEPOX)-derived SOA tracers to total organic matter was en- uncertainties for air quality modeling and human health risk hanced by 1.4 % (p = 0.012) under high-SO2 sampling scenarios, only weak associations between aerosol acidity and mass of IEPOX SOA tracers were observed
Even though some of these BSOA tracers have been previously characterized from PM2.5 samples collected from the SEARCH network in a time-integrated manner (Chan et al, 2010b; Gao et al, 2006; Surratt et al, 2007a, 2008), using conditional sampling approaches to collect PM2.5 in this study is to our knowledge one of the first attempts to systematically examine if BSOA formation is enhanced or suppressed due to anthropogenic emissions in this region
Summary
Hydrology and Tropospheric fine aerosolEs (aPrMth2.5,Swyitshtaeermodynamic diameter ≤ 2.5 μm) have been recognSizcedietno chaevse significant inof six isoprene low-NOx SOA tracers contributed to 12–19 % fluences on regional air quality, climate change, and human of total organic matter (OM) in PM2.5 samples collected dur- health (Kanakidou et al, 2005; Hallquist et al, 2009). Organosulfate formation was reported through reactive uptake of BVOC oxidation products onto acidified sulfate seed aerosols, providing a likely link between anthropogenic pollutants and the enhanced BSOA formation (Iinuma et al, 2007, 2009; Surratt et al, 2007a, 2008). In the presence of anthropogenic pollutants, such as nitric acid and sulfuric acid produced from the oxidation of NOx and SO2, SOA mass yields from isoprene under high- and low-NOx conditions, respectively, have been shown to increase substantially (i.e., from 1–3 % to 3–30 %) with preexisting acidified sulfate aerosols in the laboratory (Chan et al, 2010a; Surratt et al, 2010). Even though some of these BSOA tracers have been previously characterized from PM2.5 samples collected from the SEARCH network in a time-integrated manner (Chan et al, 2010b; Gao et al, 2006; Surratt et al, 2007a, 2008), using conditional sampling approaches to collect PM2.5 in this study is to our knowledge one of the first attempts to systematically examine if BSOA formation is enhanced or suppressed due to anthropogenic emissions in this region
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