Impact of Formaldehyde on Oxidant Production in the Greater Chicago Area

Formaldehyde (HCHO) is formed in the oxidation of nearly every volatile organic compound (VOC), such that HCHO observations provide a product-based and integrated perspective on the total mass sum of emitted reactive carbon. HCHO photolysis is a major source of hydroperoxyl (HO₂) radicals, which react with NO to generate hydroxyl radicals (OH) and further promote VOC oxidation. In this work, we characterize the impact of HCHO on the gas-phase chemical evolution of plumes originating in the greater Chicago area. We leverage aircraft observations from the Atmospheric Emissions and Reactions from Megacities and Marine Areas (AEROMMA) campaign during the 01 August 2023 flight. Analysis will focus on a plume originating near Gary, Indiana, where steel, paper, and plastics manufacturing is a large source of anthropogenic emissions, and on a plume originating closer to Chicago, more characterized by urban emissions. We then evaluate the sensitivity of oxidant (HO_x, O₃) production to HCHO as the plumes evolve.