Ozone responses to reduced precursor emissions: A modeling analysis on how attainable goals can improve air quality in the Mexico City Metropolitan Area.

High tropospheric ozone (O) concentrations prevent the improvement of the air quality in the Mexico City Metropolitan Area (MCMA). Although the problem has improved considerably since the 1990s, a rebound in O levels in recent years has raised concerns about the deteriorating air quality. The nonlinear relationship between O formation and the emissions of its main precursors, i.e., volatile organic compounds (VOCs) and nitrogen oxides (NO), is a challenge when measures are enacted for effective mitigation of the O problem. This study evaluated the reduction in precursors, VOCs and NO, using an up-to-date regional air quality model (HERMES-Mex-WRF-CMAQ). For evaluating realizable scenarios, the decline in VOC achieved in Japan after policy implementation was the targeted VOC reduction (40 % from area sources), and the NO reduction observed in the MCMA during the COVID-19 pandemic was the targeted NO reduction (40 % from mobile sources). The analysis evaluated the O responses to changes in a single precursor and a combination of both during a period of high O concentrations (April 2019). The results showed that 40 % reduction in VOC emissions would decrease the O 8-h maximum concentrations by 16 %. However, 40 % reduction in NO emissions would increase O by >15 %. The simultaneous reduction of both precursors did not significantly affect O levels. The diagnosis of ozone sensitivity using the HO/HNO ratios reinforced the simulation findings, indicating that VOC emissions limited ozone formation in most MCMA areas. As the simulated scenarios were based on factual case studies, our research offers insights into the realistic aims of MCMA policies to reduce O levels.