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.

Highlights

  • The southeastern USA is an intriguing region to study aerosol formation

  • The authors hypothesized that the differences between gas-phase aαn-pdinpeanrteicalne-dphisaospereWneSObyCNaOre3q caused by the oxidation of radicals producing substantWpiraeSlnOaemC+.oNTunhOtos3qugroehfacigttaiioss-nppslhapausrsoeidbWulecSethOmaCtoαrbe-upvtinolelianttteille+e ppNarroOtdi3cqulacent-spdhtihasaosnelow-volatility products, our study shows that there is substantial nocturnal secondary organic aerosol (SOA) production (i.e., less-oxidized oxygenated OA (LO-OOA)), which likely corresponds to the nocturnal increase in gas-phase

  • 1 year of measurements were performed across multiple sites in the southeastern USA with a variety of online instruments, with the focus on HR-ToF-Aerosol Mass Spectrometer (AMS) data in this study

Read more

Summary

Introduction

The southeastern USA is an intriguing region to study aerosol formation. The southeastern USA is characterized by large emissions from both biogenic and anthropogenic sources, which makes it an ideal region to study the effects of interactions between biogenic/anthropogenic emissions on organic aerosol formation and air quality. Recent studies revealed that the formation of SOA from biogenic VOCs is largely controlled by anthropogenic emissions in the southeastern USA (Weber et al, 2007; Xu et al, 2015). A wide range of air quality data has been routinely collected by the SEARCH (SouthEastern Aerosol Research and Characterization) network, including multiple rural and urban sites in the southeastern USA from 1999 to 2013 (Edgerton et al, 2005; Hansen et al, 2003; Hidy et al, 2014). Long-term measurements of basic air quality parameters are helpful when testing the robustness of shortterm field campaign results (Hidy et al, 2014)

Methods
Results
Discussion
Conclusion
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call