On ozone's weekly cycle for different seasons in Arizona

This study advances insights on ozone (O₃) concentration temporal cycles in Arizona, exploring differences between weekdays and weekends to understand the impact of anthropogenic activities on O₃ fluctuations. By examining data from six areas across Arizona and with a more detailed look across the Phoenix metropolitan area, this study compares O₃ and NO₂ levels based on day of the week and time of day as well as during five distinct seasons between January 2015 and December 2021: fall (Sep–Nov), winter (Dec–Feb), spring (Mar–May), dry summer (June), and monsoon summer (Jul–Aug). Results highlight associations between O₃ levels and O₃ precursor levels, particularly for nitrogen oxides (NO_x) and volatile organic compounds (VOCs). Results show that O₃ levels are contingent on diurnal, day-of-week, and seasonally-dependent factors. This dependence influences the phenomena called the “weekend effect”, which is most clearly observed at urban sites across Arizona in fall and winter seasons, wherein O₃ concentrations exhibit higher levels during weekends compared to weekdays. This study reveals opposing features during Arizona's warmer months and especially the monsoon summer period in which O₃ regimes reverse from VOC-sensitive (in fall and winter) to transition and NO_x-sensitive regimes. A specific case study for Phoenix during the first COVID-19 lockdown (spring 2020) accounting for planetary boundary layer height normalization of gas concentrations shows that although NO₂ was reduced in March–April compared to other years, O₃ was also reduced due to a regional O₃ response as most areas surrounding western U.S. were typically either transitioning to, or already under, a NO_x-limited regime. These findings provide insights into understanding the intricate relationship among anthropogenic emissions, seasonal fluctuations, and pollutant emissions and transport that influence O₃ levels across Arizona, potentially providing guidance for future study designs focused on effective control strategies for the sensitive months with most exceedances (∼June–August).