Abstract
Abstract. Secondary organic aerosol (SOA) in the southeastern US is investigated by analyzing the spatial-temporal distribution of water-soluble organic carbon (WSOC) and other PM2.5 components from 900 archived 24-h Teflon filters collected at 15 urban or rural EPA Federal Reference Method (FRM) network sites throughout 2007. Online measurements of WSOC at an urban/rural-paired site in Georgia in the summer of 2008 are contrasted to the filter data. Based on FRM filters, excluding biomass-burning events (levoglucosan < 50 ng m−3), WSOC and sulfate were highly correlated with PM2.5 mass (r2~0.7). Both components comprised a large mass fraction of PM2.5 (13% and 31%, respectively, or ~25% and 50% for WSOM and ammonium sulfate). Sulfate and WSOC both tracked ambient temperature throughout the year, suggesting the temperature effects were mainly linked to faster photochemistry and/or synoptic meteorology and less due to enhanced biogenic hydrocarbon emissions. FRM WSOC, and to a lesser extent sulfate, were spatially homogeneous throughout the region, yet WSOC was moderately enhanced (27%) in locations of greater predicted isoprene emissions in summer. A Positive Matrix Factorization (PMF) analysis identified two major source types for the summer WSOC; 22% of the WSOC were associated with ammonium sulfate, and 56% of the WSOC were associated with brown carbon and oxalate. A small urban excess of FRM WSOC (10%) was observed in the summer of 2007, however, comparisons of online WSOC measurements at one urban/rural pair (Atlanta/Yorkville) in August 2008 showed substantially greater difference in WSOC (31%) relative to the FRM data, suggesting a low bias for urban filters. The measured Atlanta urban excess, combined with the estimated boundary layer heights, gave an estimated Atlanta daily WSOC production rate in August of 0.55 mgC m−2 h−1 between mid-morning and mid-afternoon. This study characterizes the regional nature of fine particles in the southeastern US, confirming the importance of SOA and the roles of both biogenic and anthropogenic emissions.
Highlights
The US EPA reported that the annual mean temperature anomalies over the southern and southeastern US in the past century (1901 to 2005) were significantly lower than other parts of the country
The findings in this paper add to these ambient results by investigating the spatial and seasonal variability of secondary aerosols over the southeastern US based on measurements of water-soluble organic carbon (WSOC), sulfate and other PM2.5 chemical components extracted from 900 Federal Reference Method (FRM) filters collected at 15 sites throughout the year of 2007. These results demonstrate the potential of the large array of FRM filters collected by various regulatory agencies for investigating fine aerosol sources and properties of the non-volatile components
To minimize the contribution of biomass burning to primary WSOC (WSOCBB) and focus on secondary WSOC, a surrogate of secondary organic aerosols (SOA) carbon (Sullivan et al, 2004, 2006; Hennigan et al, 2009; Miyazaki et al, 2009), filter samples with levoglucosan concentration greater than 50 ng m−3 were not included in the following analysis and discussion
Summary
The US EPA reported that the annual mean temperature anomalies over the southern and southeastern US in the past century (1901 to 2005) were significantly lower than other parts of the country (http://www.epa.gov/climatechange/ science/recenttc.html). The direct effects of aerosols on radiative forcing are estimated to produce a summertime cooling of −11 ± 6 W m−2 (Carrico et al, 2003) in urban Atlanta, and a greater radiative cooling of −3.9 W m−2 in summer than in winter over the entire southeastern US (Goldstein et al, 2009). These estimates are significant compared to the global mean CO2 radiative forcing of 1.66 W m−2 and the global mean aerosol direct radiative forcing of −0.5 ± 0.4 W m−2 (IPCC, 2007). Other processes may play a role in SOA formation, such as synergistic interactions between biogenic and anthropogenic emissions (de Gouw et al, 2005; Weber et al, 2007) as well as SOA formation through cloud processing (Ervens et al, 2008) and condensed aqueous phase chemistry (Hennigan et al, 2009; Lim et al, 2010; Tan et al, 2010; Ervens et al, 2011)
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