Ozone concentrations and influential variables during heat waves over two desert cities in the Southwest U.S.

This study investigates the impact of heatwaves (HWs) on ground-level ozone (O₃) concentrations over two urbanized desert cities in Arizona (Phoenix and Tucson) across 11 years (2012–2022). HWs were defined as periods of four or more consecutive days with daily maximum temperatures exceeding 44 °C in Phoenix and 40 °C in Tucson during June–August. Of 1,012 total days, Phoenix experienced 81 HW days (8%), of which 19.7% were classified as O₃ exceedance days compared to 9.6% of NHW days. Tucson, by contrast, had 72 HW days with none qualifying as O₃ exceedances. Comparing HWs and NHWs with/without O₃ exceedances, HWs with O₃ exceedances over Phoenix exhibited the highest O₃, NO₂, temperature, and vertical pressure velocity values (i.e., sinking motion), alongside the lowest RH, wind speed, and boundary layer height. The rate of increase in O₃ with temperature (so-called ‘climate penalty factor’) is consistently higher during HWs (2.51 ppb °C⁻¹ at Phoenix; 2.11 ppb °C⁻¹ at Tucson) compared to NHWs (2.23 ppb °C⁻¹ at Phoenix; 1.96 ppb °C⁻¹ at Tucson), for the temperature range between 22 °C and 43 °C, and with steep growth up to as high as 49 °C in Phoenix. This suggests that O₃ concentrations not only rise with temperature but do so more steeply during HWs and possibly even most at the highest temperatures. The results demonstrate the significance of mitigating measures during HWs to protect well-being in these hot southwestern cities.