Aerosol characterization over the southeastern United States using high-resolution aerosol mass spectrometry: spatial and seasonal variation of aerosol composition and sources with a focus on organic nitrates

Abstract

Abstract. We deployed a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and an Aerosol Chemical Speciation Monitor (ACSM) to characterize the chemical composition of submicron non-refractory particulate matter (NR-PM$_{1}$) in the southeastern USA. Measurements were performed in both rural and urban sites in the greater Atlanta area, Georgia (GA), and Centreville, Alabama (AL), for approximately 1 year as part of Southeastern Center for Air Pollution and Epidemiology study (SCAPE) and Southern Oxidant and Aerosol Study (SOAS). Organic aerosol (OA) accounts for more than half of NR-PM1 mass concentration regardless of sampling sites and seasons. Positive matrix factorization (PMF) analysis of HR-ToF-AMS measurements identified various OA sources, depending on location and season. Hydrocarbon-like OA (HOA) and cooking OA (COA) have important, but not dominant, contributions to total OA in urban sites (i.e., 21–38 % of total OA depending on site and season). Biomass burning OA (BBOA) concentration shows a distinct seasonal variation with a larger enhancement in winter than summer. We find a good correlation between BBOA and brown carbon, indicating biomass burning is an important source for brown carbon, although an additional, unidentified brown carbon source is likely present at the rural Yorkville site. Isoprene-derived OA factor (isoprene-OA) is only deconvolved in warmer months and contributes 18–36 % of total OA. The presence of isoprene-OA factor in urban sites is more likely from local production in the presence of NOx than transport from rural sites. More-oxidized and less-oxidized oxygenated organic aerosol (MO-OOA and LO-OOA, respectively) are dominant fractions (47–79 %) of OA in all sites. MO-OOA correlates well with ozone in summer but not in winter, indicating MO-OOA sources may vary with seasons. LO-OOA, which reaches a daily maximum at night, correlates better with estimated nitrate functionality from organic nitrates than total nitrates. Based on the HR-ToF-AMS measurements, we estimate that the nitrate functionality from organic nitrates contributes 63–100 % to the total measured nitrates in summer. Furthermore, the contribution of organic nitrates to total OA is estimated to be 5–12 % in summer, suggesting that organic nitrates are important components in the ambient aerosol in the southeastern USA. The spatial distribution of OA is investigated by comparing simultaneous HR-ToF-AMS measurements with ACSM measurements at two different sampling sites. OA is found to be spatially homogeneous in summer due possibly to stagnant air mass and a dominant amount of regional secondary organic aerosol (SOA) in the southeastern USA. The homogeneity is less in winter, which is likely due to spatial variation of primary emissions. We observe that the seasonality of OA concentration shows a clear urban/rural contrast. While OA exhibits weak seasonal variation in the urban sites, its concentration is higher in summer than winter for rural sites. This observation from our year-long measurements is consistent with 14 years of organic carbon (OC) data from the SouthEastern Aerosol Research and Characterization (SEARCH) network. The comparison between short-term measurements with advanced instruments and long-term measurements of basic air quality indicators not only tests the robustness of the short-term measurements but also provides insights in interpreting long-term measurements. We find that OA factors resolved from PMF analysis on HR-ToF-AMS measurements have distinctly different diurnal variations. The compensation of OA factors with different diurnal trends is one possible reason for the repeatedly observed, relatively flat OA diurnal profile in the southeastern USA. In addition, analysis of long-term measurements shows that the correlation between OC and sulfate is substantially stronger in summer than winter. This seasonality could be partly due to the effects of sulfate on isoprene SOA formation as revealed by the short-term intensive measurements.

Highly functionalized organic nitrates in the southeast United States: Contribution to secondary organic aerosol and reactive nitrogen budgets

“…1C). This contribution is comparable with that reported recently for the Centreville, Alabama site, where unspeciated pONs were 6-10% of OA (27). To investigate further the nature of this diurnal cycle in the pON contribution to OA, we separated the pON species into those having 5 or 10 carbon atoms.…”

Composition and Sources of Particulate Matter Measured near Houston, TX: Anthropogenic-Biogenic Interactions

“…Vegetation type may play a large role in this as the Conroe area is where the ecosystem transforms from prairie and marsh to the piney woods of the US Southeast, which are known to have higher biogenic emissions [58]. Xu et al [19] saw a similar diurnal pattern for a less oxidized OA factor in PMF analysis from measurements in Alabama and Georgia, places that have a piney woods ecosystem.…”

“…Either estimate would suggest that organic nitrates play a larger role in Conroe than has been measured in other areas. Using the same estimate for molecular weight of organic nitrates, Xu et al [19] estimated that organic nitrates constitute 5%-16% of OA during the summer in Alabama and Georgia locations. Mylones et al [49] assumed an average molecular weight of 150 g·mol −1 for organic nitrates and calculated that they are 13% of OA in Los Angeles.…”

“…The strong diurnal cycle in Conroe may in part be due to the interaction of anthropogenic oxidants with Observations from the HR-ToF-CIMS support the hypothesis that biogenic VOCs are an important contributor to the diurnal cycle seen in OA in Conroe. Recent work [19,20] has suggested that monoterpene reactions with the NO 3 radical at night are a significant source of SOA. Gas-phase organic nitrates observed during DISCOVER-AQ, which likely formed from monoterpenes (C 10 ), also exhibited a diurnal trend of elevated concentrations at night.…”

“…Recent work [19,20] has suggested that monoterpene reactions with the NO3 radical at night are a significant source of SOA. Gas-phase organic nitrates observed during DISCOVER-AQ, which likely formed from monoterpenes (C10), also exhibited a diurnal trend of elevated concentrations at night.…”