On ozone's weekly cycle for different seasons in Arizona.

This study advances insights on ozone (O 3) concentration temporal cycles in Arizona, exploring differences between weekdays and weekends to understand the impact of anthropogenic activities on O 3 fluctuations. By examining data from six areas across Arizona and with a more detailed look across the Phoenix metropolitan area, this study compares O 3 and NO 2 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 O 3 levels and O 3 precursor levels, particularly for nitrogen oxides (NO x) and volatile organic compounds (VOCs). Results show that O 3 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 O 3 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 O 3 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 NO 2 was reduced in March–April compared to other years, O 3 was also reduced due to a regional O 3 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 O 3 levels across Arizona, potentially providing guidance for future study designs focused on effective control strategies for the sensitive months with most exceedances (∼June–August). • Weekend effect characteristics most evident in winter and fall seasons. • Monsoon period shown to be distinct from other periods for O 3 characteristics. • Regional O 3 reductions are influential for O 3 levels during Mar–Apr of COVID lockdown period. [ABSTRACT FROM AUTHOR]