A long-term (2001–2022) examination of surface ozone concentrations in Tucson, Arizona

Abstract

Ground-level ozone (O₃) pollution in semi-arid regions like Tucson, Arizona, presents unique challenges due to the interplay of anthropogenic emissions, biogenic volatile organic compounds (BVOCs), meteorological conditions, and regional transport. Tucson is the second-largest city in Arizona and has received comparatively less attention than the most populated city of Phoenix despite experiencing elevated O₃ levels amid rapid population growth. This study provides a comprehensive 22 year analysis (2001–2022) of O₃ trends in Tucson using a combination of ground-based monitoring data, satellite observations, NEI emissions inventories, land cover classification and meteorological datasets. The findings reveal no statistically significant long-term trend in O₃ levels at northwest (NW), urban core, and south/southeast (S/SE) monitoring sites despite regulatory actions to reduce precursor levels. However, spatial differences persist with one S/SE site (Saguaro National Park) consistently exhibiting the highest O₃ concentrations and an urban core site (Rose Elementary) usually exhibiting the lowest values across all seasons. Satellite and surface-based data reveal a decline in NO₂ across the study period, in contrast to HCHO levels that show little long-term change, with a brief increase in 2020 likely linked to regional fire activity and higher temperatures, particularly in June. Consequently, FNR values (formaldehyde-to-NO₂ ratio) increased after 2005–2009, indicating a regional shift influenced by reductions in NO_x emissions, especially during fall/winter and spring. This shift helps explain the weakening of the weekend effect (i.e., higher weekend levels versus weekdays) over time and the emergence of the weekday effect earlier in the summer (June) in contrast to the late 1990s. Generalized additive model meteorology normalization suggests that 79% of the O₃ variability is attributed to interannual weather variability. FNR started to decline post-2020, suggesting changes in O₃ responsiveness to further NO₂ reductions, particularly in cooler months. These dynamics, along with recent fall/winter O₃ increases, highlight the complex, chemical regime-dependent response of O₃ to precursor changes. This study recommends improved VOC characterization to inform future air quality strategies in the region.