Effects of anthropogenic emissions on biogenic organic aerosol formation in the southeastern United States

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Xu, Lu
Ng, Nga Lee
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Atmospheric particulate matter (PM) has substantial impacts on climate, air quality, and human health. A substantial fraction of atmospheric PM is constituted of secondary organic aerosol (SOA), which is formed in the atmosphere through the oxidation of volatile organic compounds (VOCs). Formulating strategies to control SOA is highly challenging, in part, because of the numerous sources and complex formation mechanisms of SOA. In particular, to what extent human activities alter SOA formation from biogenic emissions? Although a number of mechanisms about the interactions of anthropogenic and biogenic emissions on SOA formation have been proposed from prior laboratory studies, only a few have been directly observed in the ambient environment. Moreover, the extent of such interactions in the atmosphere is unknown. This question is investigated in depth in this dissertation based on comprehensive ambient measurements and complementary laboratories studies. The southeastern US is an ideal region to study the effects of anthropogenic emissions on biogenic organic aerosol formation because this region is characterized by large emissions from both biogenic and anthropogenic sources. In this study, we applied a suite of instruments, with a focus on a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS), to extensively characterize the composition of organic aerosol (OA) in the southeastern United States (US). Multiple measurements were obtained during different seasons at both urban sites (Jefferson Street and Georgia Tech Campus in Georgia) and rural sites (Yorkville in Georgia and Centreville in Alabama), as part of Southeastern Center of Air Pollution and Epidemiology Study (SCAPE) and Southern Oxidant and Aerosol Study (SOAS). Positive Matrix Factorization (PMF) analysis was performed on the high-resolution organic mass spectra obtained by the HR-ToF-AMS for OA source apportionment. We identified various OA sources, the contribution of which to OA concentration depends on location and season. Hydrocarbon-like OA (HOA, surrogate of OA emitted directly from vehicle emissions) 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. Isoprene SOA formed via the reactive uptake of isoprene epoxydiols (denoted as Isoprene-OA) is only resolved in warmer months and contributes 18-36% of total OA. 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 well with estimated nitrate functionality from organic nitrates. These findings significantly improve our understanding of OA sources in the southeastern US and provide suggestions for implementing effective regulations to reduce ambient PM level. After identifying OA sources in the southeastern US, we further investigated the effects of anthropogenic influences on SOA formation from biogenic VOCs (denoted as biogenic SOA) based on complementary laboratory studies and ambient measurements. Among various interactions between anthropogenic and biogenic emissions, we probed (1) the effects of NOx on the SOA formation from isoprene photooxidation, (2) the effects of sulfate on the reactive uptake of isoprene epoxydiols (IEPOX), which are oxidation products of isoprene under low-NOx oxidation conditions, and (3) the oxidation of monoterpenes by nitrate radical (a product of anthropogenic NOx and ozone). Firstly, the effects of NOx on the SOA from isoprene photooxidation were investigated in laboratory chamber experiments. We found that the yield, volatility, and oxidation state of isoprene SOA are sensitive to and exhibit a non-linear dependence on NOx levels. The non-linear dependence likely arises from gas-phase organic peroxy radical (RO2) chemistry and succeeding particle-phase oligomerization reactions. Our results suggest that it is not proper to treat the effects of NOx on SOA properties as a linear combination of SOA formation under two extremes (“low-NOx” and “high-NOx” conditions) as currently done in regional and global atmospheric SOA models. Secondly, based on ambient measurements in the southeastern US, we demonstrated that the isoprene SOA formed via reactive uptake of IEPOX (denoted as isoprene-OA), which accounts for 18-36% of total OA mass in summer time, is directly modulated by the abundance of anthropogenic sulfate. This contradicts with prior laboratory studies, which suggest the process is controlled by the particle water content and/or particle acidity. Based on both surface and flight measurements, we estimate that 1 µg m-3 reduction of sulfate would decrease the isoprene-derived OA by 0.23-0.42 µg m-3. Thirdly, SOA from monoterpenes, which was shown to account for 19-34% of total OA mass throughout the year, was enhanced at night via oxidation of monoterpenes by nitrate radical. Taken together, we present direct observational evidence on the magnitude of anthropogenic influence on biogenic SOA formation in the southeastern US. That is, anthropogenic sulfate and NOx can potentially modulate 43-70% of total measured OA in the southeastern US during summer. Long-term measurements (1999-2013) at the Southeastern Aerosol Research and Characterization (SEARCH) network have revealed that the emissions of SO2 and NOx decrease by about 65% and 52%, respectively, in the southeastern US, which is caused by regulations of anthropogenic emissions from power plants and vehicles as well as the switch from coal to natural gas in many power plants. Meanwhile, the OA concentration has also decreased significantly in the same region. Part of the observed decrease in OA concentration can be explained by our proposed mechanisms regarding anthropogenic pollutants modulating biogenic SOA formation. The continual decrease in SO2 and NOx emissions may not only reduce the OA burden, but also have impacts on climate and human health, considering biogenic SOA formed under lower sulfate and NOx environments could have substantially different properties than those formed in polluted environments. At last, updating current modeling frameworks by including anthropogenic-biogenic interactions will also lead to more accurate treatment of aerosol formation and consequently improve air quality and climate simulations.
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