Atmospheric patterns in relationship with observed ozone concentrations in the Phoenix, Arizona, metropolitan area during the North American Monsoon

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Phoenix, Arizona is a designated ozone nonattainment area by the United States Environmental Protection Agency (EPA) and is susceptible to unhealthy ozone during the summer North American Monsoon. This study identifies common July and August synoptic environments linked with daily exceedances and non-exceedances of the EPA National Ambient Air Quality Standard (NAAQS). Results indicate a common weather pattern for exceeding the NAAQS consisting of 1) an amplified high-pressure ridge over the Four Corners region causing 500 hPa heights to often exceed 5910 m, 2) surface afternoon temperatures typically rising over 40 °C, 3) lighter wind speeds in the planetary boundary layer (PBL) under four ms−1, and 4) a distinct and persistent light easterly flow regime from 700 to 500 hPa. The last feature would counter local mountain-valley winds in the upper PBL to promote ozone accumulation aloft and subsequent fumigation. Conversely, non-exceedance days are associated with 1) West Coast troughing and 500 hPa geopotential heights between 5.9 and 21.4 m lower, 2) afternoon PBL temperatures on average 0.8 °C–2.2 °C cooler, 3) faster afternoon westerly flow at 925 and 850 hPa over four ms−1, and 4) westerly flow in general at and above 925 hPa, aligning with the daytime mountain-valley circulation. Furthermore, a secondary non-exceedance pattern was identified. This less frequent pattern has moist southeast flow and is recognized for bringing widespread convective storm activity to central Arizona, including the PNA, based on prior research.