Changes in ozone production and VOC reactivity in the atmosphere of the Mexico City Metropolitan Area

The introduction of aggressive emission control policies propelled three decades ago has led to significant reductions of ozone (O₃) levels in the Mexico City Metropolitan Area (MCMA). However, in recent years the levels have stalled, and there is evidence that the production of secondary pollutants may have started to rebound. Similar responses to changes in O₃ precursors observed in other urban areas suggest major changes in atmospheric reactivity and O₃ production P(O₃), prompting the introduction of new emission control policies. In this study, we evaluate the changes in VOC-OH reactivity and P(O₃) using data of O₃, volatile organic compounds (VOCs), carbon monoxide (CO), and nitrogen oxides (NO_x) collected from the 1990s to 2019 by the local air quality monitoring network and during two dedicated 6-week field measurement campaigns in the MCMA. The results show a reduction of VOC-OH reactivity from 38 s⁻¹ in 2003–2006 to about 19 s⁻¹ in 2018 within the urban core, a slight increase from around 8 s⁻¹ to 9 s⁻¹ for the northern peripheral urban sites in the same period, and a small decrease from about 18 s⁻¹ to 15 s⁻¹ during 2014–2016 in the south of the city. Alkanes are still key contributors to VOC-OH reactivity, whereas the contribution from aromatic and alkene species has decreased, in agreement with reductions of VOC emissions from mobile sources. Changes in P(O₃) suggest that increases in the relative contributions from highly oxygenated volatile chemical products from solvents consumption, personal care and other products are responsible for sustained high O₃ levels in recent years. We also found that there is a marked spatial dependency of P(O₃) in the northern, central, and southern areas of the city, suggesting spatially different VOC and NO_x sensitivity regimes. Finally, significant increases in NO₂/NO ratios suggest changes in the pathways for the nighttime accumulation of HO_x radicals and NO_y species that could impact the morning photochemistry and radical budgets for secondary aerosol formation. These results strongly suggest a need for new measurements of radical budgets in the MCMA, the expansion of a continuous measurement network of VOCs with the addition of oxygenated species, and the use of these data to support comprehensive modeling studies for the design of effective emission control strategies.