On Ozone’s Weekly Cycle in a Monsoon-Impacted Arid Region: A Close Look at Seasonal Influence in Arizona

Abstract

This study advances insights on ozone (O3) concentration temporal cycles in Arizona, exploring differences between weekdays and weekends to understand the impact of human activities on O3 fluctuations. By examining data from six sites across Arizona, this study compares O3 and NO2 levels based on day of the week and time of day as well as during five distinct seasons: fall (Sep-Nov), winter (Dec-Feb), spring (Mar-May), dry summer (June), and monsoon summer (Jul-Aug). Results highlight the correlation of O3 formation on O3 precursor levels, particularly nitrogen oxides (NOx) and volatile organic compounds (VOCs). It was found that O3 levels are contingent on diurnal, day-of-week, and seasonally-dependent factors. This dependence influences the phenomena called the "weekend effect", wherein O3 concentrations exhibit higher levels during weekends compared to weekdays, which contrasts with the more intuitive weekly cycle of particulate matter. This study reveals opposing features during Arizona’s two summer seasons in which O3 regimes reverse from VOC-sensitive (in dry summer) to NOx-sensitive (in monsoon summer) emphasizing the importance of dividing the Arizona summer season in future studies. A specific case study for Phoenix during the first COVID-19 lockdown from 15 March to 31 August (2020) shows that the area does not mimic an extended weekend scenario, where the O3 response to reduced NOx due to lockdown is similar to typical weekend trends seen in VOC-limited regimes at urban sites. Instead, we see a decrease in O3 levels in Phoenix typical of NOx-limited regime in rural sites. We attribute this decrease to the regional O3 response as most areas surrounding western United States are typically either transitioning to, or already under, a NOx-limited regime. These findings provide essential insights into understanding the intricate relationship among human activities, seasonal fluctuations, and pollutant emissions and transport that influence O3 levels across Arizona, potentially informing effective control strategies aimed at reducing O3 exceedances in the region.