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

Abstract. This study reports the contribution of fire emissions on ozone (O₃) pollution in Arizona compared to local and regional anthropogenic emissions. Using the WRF-Chem modeling system with different O₃ and CO tags, we quantified the contributions of these emissions to O₃ levels during June 2021, a period when the region was experiencing both drought conditions and extreme heat. Our findings indicate that background O₃ levels accounted for about 50 % of the total O₃, with local anthropogenic emissions contributing between 24 % and 40 %. During the peak smoky time period, fire-contributed O₃ was significant across the Phoenix metropolitan area, ranging from 5 to 23 ppb or 5 % to 21 % of total O₃ levels, with an average of 15 ppb or 15 %. We verify these O₃ fire tags by conducting a model sensitivity test that excluded fire emissions, which showed strong agreement on the spatiotemporal pattern of O₃ due to fire emissions, although the magnitude of the contribution is underestimated by a factor of 1.4. This further demonstrates that wildfires exacerbate O₃ exceedances over urban areas. Our analysis also showed that the O₃ levels in Yuma are significantly influenced by transboundary pollution from California and Mexico, whereas Phoenix's O₃ levels are mainly driven by local anthropogenic emissions, with much smaller contributions from external sources during the study period. Consistent with previous reports, our findings highlight the role of wildfires and regional emissions in confounding the assessment of local O₃ pollution in urban environments, especially during dry and extremely hot summer in semi-arid/arid regions.