Source contribution to ozone pollution during June 2021 fire events in Arizona: insights from WRF-Chem-tagged O₃ and CO

This study quantifies wildfire contributions to O₃ pollution in Arizona, relative to local and regional emissions. Using WRF-Chem with O₃ and CO tags, we analyzed emissions during June 2021, a period of drought, extreme heat, and wildfires. Our results show that background O₃ accounted for ∼ 50 % of total O₃, while local anthropogenic emissions contributed 24 %–40 %, consistent with recent estimates for Phoenix. During peak smoke conditions, fire-related O₃ ranged from 5 to 23 ppb (5 %–21 % of total O₃), averaging 15 ppb (15 %). These estimates were compared with model sensitivity tests excluding fire emissions, which confirmed the spatiotemporal pattern of fire-driven O₃, though the model underestimated the magnitude by a factor of 1.4. The results further demonstrate that wildfires exacerbate O₃ exceedances over urban areas. Our analysis reveals key differences in O₃ sources: Phoenix’s O₃ was mainly driven by local emissions, while Yuma’s was heavily influenced by transboundary transport from California and Mexico. Wildfires not only boosted O₃ formation but also altered winds and atmospheric chemistry in Phoenix and downwind areas. O₃ increases along the smoke plume resulted from NO_X and volatile organic compound (VOC) interactions, with fire-driven O₃ forming in NO_X-limited zones near the urban interface. Downwind, O₃ chemistry shifted, shaped by higher NO_X in central Phoenix and more VOCs in suburban and rural areas. Winds weakened and turned westerly near fire-affected areas. This study highlights the value of high-resolution modeling with tagging to disentangle wildfire and regional O₃ sources, particularly in arid regions, where extreme heat intensifies O₃ pollution, making accurate source attribution essential.