Box-modelling of HOx in Mexico City

This study presents an analysis of hydroxyl (OH) and hydroperoxyl (HO₂) radicals collectively referred to as HO_x and their role in atmospheric chemistry within the Mexico City Metropolitan Area (MCMA). HO_x radicals are paramount to the oxidative capacity of the atmosphere driving the formation of secondary pollutants such as ozone (O₃) and secondary organic aerosols (SOA). A 0-D chemical box model (AtChem-2) was employed to simulate in situ production of OH and HO₂ constrained by measurements of volatile organic compounds (VOCs), nitrogen oxides (NO_x), O₃, and other trace gases along with meteorological data collected during the campaign during the dry season. The analysis compares two distinct events: a pre-ozone episode with background conditions and an ozone episode characterized by strong photochemical activity confined to the MCMA. The results indicate significantly higher concentrations of OH and HO₂ of 0.49 ppt and 25.95 ppt respectively during the ozone episode driven by enhanced photolysis of O₃, HONO, and HCHO under clear sky conditions. HONO was identified as the dominant contributor with at least 2/3 to HO_x production averaged over daytime, followed by O₃ and HCHO. CO and VOCs have comparable effects on the loss rate of HO_x while NO_x plays the predominant role in the OH reactivity. The study reveals that elevated radical concentrations during the ozone episode corresponded to stronger and more persistent photochemical reactions by facilitating increased ozone formation of about 163 ppb. These findings underscore the importance of understanding radical production mechanisms in polluted urban environments particularly for the development strategies aimed at mitigating ozone and SOA levels.