Impact of organic nitrates on urban ozone production

Abstract

Urban O₃ is produced by photochemical chain reactions that amplify background O₃ in mixtures of gaseous nitrogen oxides (NO_x) and organic molecules. Current thinking treats NO_x and organics as independent variables that limit O₃ production depending on the NO_x to organic ratio; in this paradigm, reducing organics either has no effect or reduces O₃. We describe a theoretical counterexample where NO_x and organics are strongly coupled and reducing organics increases O₃ production, and illustrate the example with observations from Mexico City. This effect arises from chain termination in the HO_x and NO_x cycles via organic nitrate production. We show that reductions in VOC reactivity that inadvertently reduce organic nitrate production rates will be counterproductive without concurrent reductions in NO_x or other organics.