Airborne Measurements of OH Reactivity over Urban Megacities

The production of organic peroxy radicals from the reactions of the hydroxyl radical (OH) and organic compounds is frequently a rate-limiting step in the formation of ground-level ozone.  OH reactivity, the inverse of the OH radical’s lifetime, gives a measure of the production rate of peroxy radicals. A custom-built instrument that can directly measure the total OH reactivity was developed at the Forschungszentrum Jülich and employed on the AEROMMA measurement campaign, organized by the US National Oceanic and Atmospheric Administration in the summer of 2023. The measurement campaign utilized a variety of aircraft and an array of advanced instrumentation to investigate the chemical composition of urban pollution outflows. It is expected that the importance of vehicle emissions is decreasing and that of other emissions, like volatile chemical products, are increasing.  OH reactivity was measured over major urban areas, including New York City, Los Angeles, Chicago, and Toronto.           A suite of instrumentation measured the OH reactants, such as inorganics, nitrogen oxides, alkanes/alkenes, aromatics, and biogenic organic compounds. In the presentation, the sum of OH reactivity from these species is compared to the measured reactivity, to explore the closure of reactivity budget.  From the measurement campaign, the relative contributions of different emission sources to the total OH reactivity including sources for VCPs are analyzed.