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Organic aerosol (OA) constitutes a substantial fraction of ambient particulate matter and is harmful to human health. OA is a complex mixture with a multitude of organic species from diverse sources and with evolving properties. These challenges cause the understanding on the sources and formation mechanisms of OA highly uncertain, but make it fun to study OA. My research aims to unravel the OA puzzles based on a combination of approaches (i.e., chamber experiments, field measurements, numerical modeling, etc).
OA sources in Houston, US
OA sources in Houston, US
In collaboration with Prof. Jian Wang's group, we studied the sources of organic aerosol in a costal-rural environment near Houston, US. We found that a portion of OA, which was previously attributed to cooking, actually arises from shipping. We also found that the diurnal variation of the aged OA is due to wind shift between land breezes and sea/bay breezes, instead of local secondary production.
Related Publications:
Li, J.G; Zhang, J.; Gong, X.; Spielman, S.; Kuang, C.; Singh, A.; Zawadowicz, M. A.; Xu, L.*; Wang, J. The sources and diurnal variations of submicron aerosols in a coastal–rural environment near Houston, US. Atmos. Chem. Phys. 2025, 25 (20), 13975-13993. DOI: 10.5194/acp-25-13975-2025.
Figure source: Xu et al. 2015 PNAS.
OA Source Apportionment
OA Source Apportionment
OA has diverse sources: primary emissions from vehicles, power plants, biomass burning, cooking, and secondary production from both anthropogenic and biogenic emissions. Understanding the sources of organic aerosol is the first and key step to formulate efficient strategies to reduce the organic aerosol burden and improve air quality.
I extensively characterized the chemical compositions of ambient organic aerosol with a suite of instruments in multiple field campaigns. For OA source apportionment, I applied statistical positive matrix factorization analysis on the OA composition measured by High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). I showed that primary emissions account for roughly 30% of OA in the southeastern U.S. and the rest 70% is from secondary production (i.e., via oxidation of gas species and subsequent gas/particle conversion). More importantly (and somewhat surprisingly), I found that more than half of OA originates from vegetation emissions. How to regulate OA from vegetation without cutting down trees? Some suggestions are provided from my work in the next section regarding the effects of anthropogenic emissions on biogenic OA formation.
Related Publications:
Xu, L.; Pye, H. O. T.; He, J.; Chen, Y.; Murphy, B. N.; Ng, N. L., Experimental and model estimates of the contributions from biogenic monoterpenes and sesquiterpenes to secondary organic aerosol in the southeastern United States. Atmos. Chem. Phys. 2018, 18 (17), 12613-12637 (link).
Xu, L.; Suresh, S.; Guo, H.; Weber, R. J.; Ng, N. L., 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. Atmos. Chem. Phys. 2015, 15 (13), 7307-7336 (link).
Xu, L.; Guo, H.; et al., Effects of anthropogenic emissions on aerosol formation from isoprene and monoterpenes in the southeastern United States. Proceedings of the National Academy of Sciences 2015, 112 (1), 37-42 (link).
Field Campaigns:
Figure source: Xu et al. 2015 PNAS
Effects of Anthropogenic Sulfate on Biogenic Isoprene SOA Formation.
Effects of Anthropogenic Sulfate on Biogenic Isoprene SOA Formation.
Biogenic isoprene (C5H8) accounts for about one third of total emissions of volatile organic compounds (VOCs). Based on Positive Matrix Factorization (PMF) analysis on HR-ToF-AMS data, I showed that isoprene SOA formed via reactive uptake of isoprene epoxydiols accounts for 18-36% of total OA mass in summer time. Further, I found that the amount of isoprene SOA is directly modulated by the abundance of sulfate, which is mainly from the oxidation of anthropogenic SO2. The complicated effects of sulfate on isoprene SOA formation are elucidated in complementary laboratory studies and ambient measurements. In addition to investigating the mechanism, I also estimated that 1 µg/m3 reduction of sulfate would decrease the isoprene SOA by 0.23-0.42 µg/m3 based on both surface and flight measurements. These findings suggest that reduction in anthropogenic SO2 emissions can considerably reduce the OA formation from biogenic isoprene.
Related Publications:
Xu, L.; Guo, H.; et al., Effects of anthropogenic emissions on aerosol formation from isoprene and monoterpenes in the southeastern United States. Proceedings of the National Academy of Sciences 2015, 112 (1), 37-42 (link).
Xu, L.; Middlebrook, A. M.; Liao, J.; de Gouw, J. A.; Guo, H.; Weber, R. J.; Nenes, A.; Lopez-Hilfiker, F. D.; Lee, B. H.; Thornton, J. A.; Brock, C. A.; Neuman, J. A.; Nowak, J. B.; Pollack, I. B.; Welti, A.; Graus, M.; Warneke, C.; Ng, N. L., Enhanced formation of Isoprene-derived Organic Aerosol in Sulfur-rich Power Plant Plumes during Southeast Nexus (SENEX). Journal of Geophysical Research: Atmospheres 2016 (link).
Figure source: Xu et al. 2015 ACP
Figure source: Pye et al. 2015 EST
Effects of Anthropogenic NOx on Biogenic Monoterpenes SOA Formation.
Effects of Anthropogenic NOx on Biogenic Monoterpenes SOA Formation.
The oxidation of biogenic VOCs by nitrate radical (a product of anthropogenic NOx and ozone) or in the presence of NOx is an important pathway to produce secondary organic aerosol (SOA). This pathway produces significant amount of organic nitrates, the chemistry of which largely influences ozone level and air quality. Based on complementary ambient measurements and laboratory studies, I showed for the first time that organic nitrates formed by nitrate radical oxidation of monoterpenes contribute to LO-OOA in the southeastern U.S. The updated knowledge is implemented in atmospheric chemical transport models to predict the future roles of NOx reduction in improving air quality.
Related Publications:
Xu, L.; Suresh, S.; Guo, H.; Weber, R. J.; Ng, N. L., 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. Atmos. Chem. Phys. 2015, 15 (13), 7307-7336 (link).
Xu, L.; Guo, H.; et al., Effects of anthropogenic emissions on aerosol formation from isoprene and monoterpenes in the southeastern United States. Proceedings of the National Academy of Sciences 2015, 112 (1), 37-42 (link).
Pye, H. O. T.; Luecken, D. J.; Xu, L.; Boyd, C. M.; Ng, N. L.; Baker, K. R.; Ayres, B. R.; Bash, J. O.; Baumann, K.; Carter, W. P. L.; Edgerton, E.; Fry, J. L.; Hutzell, W. T.; Schwede, D. B.; and Shepson, P. B., Modeling the Current and Future Roles of Particulate Organic Nitrates in the Southeastern United States, Environ Sci Technol, 49, 14195-14203,10.1021/acs.est.5b03738, 2015 (link).
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