Urban ozone formation and sensitivities to volatile chemical products, cooking emissions, and NO_x across the Los Angeles Basin

Abstract. Volatile chemical products (VCPs) and other non-traditional anthropogenic sources, such as cooking, contribute substantially to the volatile organic compound (VOC) budget in urban areas. The impact of these emissions on ozone formation and urban atmospheric chemistry is uncertain. This study employs detailed Lagrangian box modeling and sensitivity analyses to evaluate ozone response to sector-specific VOC and nitrogen oxide (NO_x) emissions in the Los Angeles (LA) Basin during the summer of 2021. The model simulated the photochemical processing and transport of temporally and spatially gridded emissions from the FIVE-VCP-NEI17NRT inventory that combines emissions from fossil fuels, VCPs, and other point sources and included updates to cooking emissions based on recent field observations. The model accurately simulates the variability and magnitude of O₃ (hourly normalized mean bias = -0.03; R² = 0.83), NO_x, and speciated VOCs measured at a ground site in Pasadena, CA. VOC sensitivity analyses show that anthropogenic VOCs (AVOC) enhance daily maximum 8-hour average ozone in Pasadena by 12 ppb, whereas biogenic VOCs (BVOCs) contribute 8 ppb. Of the ozone influenced by AVOCs, VCPs represent the largest fraction at 44 % while cooking and fossil fuel VOCs are comparable at 28 % each. This study is the first to quantify the contribution of cooking emissions to urban ozone. NO_x sensitivity analyses along trajectory paths indicate the photochemical regime of ozone varies spatially and temporally. The modeled ozone response is primarily NO_x–saturated across the dense urban core and during peak ozone production in Pasadena, but transitions back to NO_x–limited chemistry briefly during late afternoon hours. Lowering the inventory emissions of NO_x by 25 % moves Pasadena to NO_x–limited chemistry throughout the majority of the day and shrinks the spatial extent of NO_x‒saturation towards downtown LA. Further sensitivity analyses in Pasadena show that using VOCs represented by a separate state inventory requires steeper NO_x reductions to transition to NO_x‒sensitivity, further suggesting that accurately representing VOC reactivity in inventories is critical to determine the effectiveness of future NO_x‒reduction policies.