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

摘要

We deployed a High-Resolution Time-of-Flight Aerosol Mass Spectrometer(HR-ToF-AMS) and an Aerosol Chemical Speciation Monitor (ACSM) tocharacterize the chemical composition of submicron non-refractory particulate matter(NR-PM$_{1}$) in the southeastern USA. Measurements were performed in bothrural and urban sites in the greater Atlanta area, Georgia (GA), andCentreville, Alabama (AL), for approximately 1 year as part ofSoutheastern Center for Air Pollution and Epidemiology study (SCAPE) andSouthern Oxidant and Aerosol Study (SOAS). Organic aerosol (OA) accounts formore than half of NR-PM mass concentration regardless of samplingsites and seasons. Positive matrix factorization (PMF) analysis ofHR-ToF-AMS measurements identified various OA sources, depending on locationand 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 enhancementin winter than summer. We find a good correlation between BBOA and browncarbon, indicating biomass burning is an important source for brown carbon,although an additional, unidentified brown carbon source is likely presentat the rural Yorkville site. Isoprene-derived OA factor (isoprene-OA) is onlydeconvolved in warmer months and contributes 18–36 % of total OA. Thepresence of isoprene-OA factor in urban sites is more likely from localproduction in the presence of NO than transport from rural sites.More-oxidized and less-oxidized oxygenated organic aerosol (MO-OOA andLO-OOA, respectively) are dominant fractions (47–79 %) of OA in all sites.MO-OOA correlates well with ozone in summer but not in winter, indicatingMO-OOA sources may vary with seasons. LO-OOA, which reaches a daily maximumat night, correlates better with estimated nitrate functionality fromorganic nitrates than total nitrates.Based on the HR-ToF-AMS measurements, we estimate that the nitratefunctionality from organic nitrates contributes 63–100 % to the totalmeasured nitrates in summer. Furthermore, the contribution of organic nitratesto total OA is estimated to be 5–12 % in summer, suggesting that organicnitrates are important components in the ambient aerosol in the southeasternUSA. The spatial distribution of OA is investigated by comparing simultaneousHR-ToF-AMS measurements with ACSM measurements at two different samplingsites. OA is found to be spatially homogeneous in summer due possibly tostagnant air mass and a dominant amount of regional secondary organicaerosol (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 clearurban/rural contrast. While OA exhibits weak seasonal variation in the urbansites, its concentration is higher in summer than winter for rural sites.This observation from our year-long measurements is consistent with 14 yearsof organic carbon (OC) data from the SouthEastern Aerosol Research andCharacterization (SEARCH) network. The comparison between short-termmeasurements with advanced instruments and long-term measurements of basicair quality indicators not only tests the robustness of the short-termmeasurements but also provides insights in interpreting long-termmeasurements. We find that OA factors resolved from PMF analysis onHR-ToF-AMS measurements have distinctly different diurnal variations. Thecompensation of OA factors with different diurnal trends is one possiblereason for the repeatedly observed, relatively flat OA diurnal profile inthe southeastern USA. In addition, analysis of long-term measurements showsthat the correlation between OC and sulfate is substantially stronger insummer than winter. This seasonality could be partly due to the effects ofsulfate on isoprene SOA formation as revealed by the short-term intensivemeasurements.