Effects of SO2 and NH3 on Biogenic SOA
Chemical Characterization of Ambient PM2.5 Collected by Conditional Sampling Strategies from the Southeastern United States: Influences of SO2 and NH3 on Ambient Biogenic SOA Formation
Recent laboratory chamber experiments have shown that increased particle-phase acidity influences biogenic secondary organic aerosol (SOA) formation with significant enhancements of SOA yields. Acid-catalyzed heterogeneous reactions, such as those resulting in organosulfate and small oligomer formation, are considered as possible atmospherically relevant pathways. In this study, chemical constituents of ambient fine aerosol (PM2.5) collected from Yorkville, GA, a rural site within the Southeastern Aerosol Research and Characterization Study (SEARCH) network, during the summer of 2010 are analyzed to investigate the influence of acidity-dependent heterogeneous reactions on ambient organic aerosol formation, as well as to evaluate the importance of brown carbon formation in this region.
Filter samples were collected with conditional sampling strategies, which depended upon either ambient sulfur dioxide (SO2), but not time-integrated periods. The intent was to elucidate the main SOA contributors under different environmental conditions. More specifically, filter samples were collected with two high-volume samplers operated side by side in the field under pre-defined environmental thresholds during the day (i.e., 0900 – 1900 EST). High SO2 condition samples were only collected when ambient SO2 concentrations were ≥0.50 ppbv, while low SO2 condition samples were only collected when SO2 concentrations were ≤ 0.25 ppbv. Additional non-concentration dependent (i.e., no threshold) nighttime samples during the night (i.e., 2300 – 0500 EST) were also collected to correct for other chemistry, such as ozone and traffic effects, in order to investigate the role of NO3-intiatied reactions in forming SOA. In addition, pairs of filters were collected using NH3 as the threshold as well, because recent laboratory studies have shown that NH3 or NH4+ reactions with carbonyl organics lead to “brown” carbon constituents that absorb lights in the UV/near UV spectrum. For example, de Haan et al. (2009) examined the reactions of glyoxal with five amino acids common in clouds, and found all reactions were accompanied by browning. Shapiro et al. (2009) reported that light-absorbing and high-molecular-weight secondary organic products were observed to result from the reaction of glyoxal in mildly acidic (pH=4) aqueous inorganic salt solutions mimicking aqueous tropospheric aerosol particles. It is worth noting that these prior studies have only used bulk solution to look at this potential heterogeneous chemistry route. If similar products are formed in atmospheric aerosol particles, they could change the optical properties of the seed aerosol over its lifetime.
Filter samples are extracted and analyzed using advanced off-line analytical techniques to characterize and quantify the SOA constituents. Together with meteorological data and other complementary chemical measurements, such as OC/EC ratios, associations between SOA constituents and relevant environmental factors will be discussed to examine the atmospheric significance of known laboratory parameters that have been shown to enhance SOA formation.