Multiscale Modeling of Ozone Formation and Chemical Regimes in Arizona during the North American Monsoon

Abstract Ground-level ozone (O3) remains a persistent air-quality challenge in the Southwest U.S. desert, including Arizona, despite nationwide emission reductions. Here, we apply the variable-resolution global chemistry–climate model MUSICAv0 to investigate O3 formation, transport, and chemical regime variability across Phoenix, Tucson, Yuma, and surrounding rural regions during July 2020 monsoon-onset conditions. Model performance is evaluated against surface observations, satellite retrievals, and regional WRF-Chem simulations. MUSICAv0 captures the observed diurnal variability of O3 and its precursors across all regions, with generally good agreement in Tucson, Yuma, and rural sites. However, a persistent afternoon overestimation is evident in Phoenix. The updated MOZART-TS2 chemistry reduces the Phoenix MDA8 bias, while budget analysis shows that the remaining bias cannot be explained by chemistry alone and reflects combined transport, deposition, and boundary-layer influences. Chemical regime diagnostics indicate that Phoenix exhibits more VOC-sensitive chemistry with shifts toward transition conditions during high-O3 events, Tucson operates largely within a transition regime, and Yuma and rural regions remain predominantly NOx-limited. These findings demonstrate that for the July 2020 monsoon-onset conditions, variable-resolution global models provide a unified framework for diagnosing chemical regime sensitivity, model biases, and their interaction with transport processes in arid urban environments, supporting future O3 mitigation strategies that integrate updated emissions, high-resolution satellite observations, and field campaign constraints.