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Abstract. This study describes a modeling framework, model evaluation, and source apportionment to understand the causes of Los Angeles (LA) air pollution. A few major updates are applied to the Community Multiscale Air Quality (CMAQ) model with a high spatial resolution (1 km × 1 km). The updates include dynamic traffic emissions based on real-time, on-road information and recent emission factors and secondary organic aerosol (SOA) schemes to represent volatile chemical products (VCPs). Meteorology is well predicted compared to ground-based observations, and the emission rates from multiple sources (i.e., on-road, volatile chemical products, area, point, biogenic, and sea spray) are quantified. Evaluation of the CMAQ model shows that ozone is well predicted despite inaccuracies in nitrogen oxide (NOx) predictions. Particle matter (PM) is underpredicted compared to concurrent measurements made with an aerosol mass spectrometer (AMS) in Pasadena. Inorganic aerosol is well predicted, while SOA is underpredicted. Modeled SOA consists of mostly organic nitrates and products from oxidation of alkane-like intermediate volatility organic compounds (IVOCs) and has missing components that behave like less-oxidized oxygenated organic aerosol (LO-OOA). Source apportionment demonstrates that the urban areas of the LA Basin and vicinity are NOx-saturated (VOC-sensitive), with the largest sensitivity of O3 to changes in VOCs in the urban core. Differing oxidative capacities in different regions impact the nonlinear chemistry leading to PM and SOA formation, which is quantified in this study.
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This content will become publicly available on December 28, 2025
Emissions and Chemistry of Volatile Organic Compounds in the Los Angeles Basin in Summer 2022
The ozone air quality standard is regularly surpassed in the Los Angeles air basin, and efforts to mitigate ozone production have targeted emissions of precursor volatile organic compounds (VOCs), especially from mobile sources. In order to assess how VOC concentrations, emissions, and chemistry have changed over the past decade, VOCs were measured in this study using a Vocus‐2R proton‐transfer reaction time‐of‐flight mass spectrometer in Pasadena, California, downwind of Los Angeles, in summer 2022. Relative to 2010, ambient concentrations of aromatic hydrocarbons have declined at a similar rate as carbon monoxide, suggesting reduced overall emissions from mobile sources. However, the ambient concentrations of oxygenated VOCs have remained similar or increased, suggesting a greater relative importance of oxidation products and other emission sources, such as volatile chemical products whose emissions are largely unregulated. Relative to 2010, the range of measured VOCs was expanded, including higher aromatics and additional volatile chemical products, allowing a better understanding of a wider range of emission sources. Emission ratios relative to carbon monoxide were estimated and compared with 2010 emission ratios. Average measured ozone concentrations were generally comparable between 2022 and 2010; however, at the same temperature, daytime ozone concentrations were lower in 2022 than 2010. Faster photochemistry was observed in 2022, with average hydroxyl radical exposure being ∼68% higher during midday (statistically significant at 95% confidence), although this difference reduces to ∼35% when comparing observations at ambient temperatures of 25–30°C only. Future trends in temperature are important in predicting ozone production.
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- Award ID(s):
- 2206655
- PAR ID:
- 10588854
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 129
- Issue:
- 24
- ISSN:
- 2169-897X
- Page Range / eLocation ID:
- e2024JD041812
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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