Anthropogenic VOCs in the Long Island Sound, NY Airshed and their role in ozone production

Abstract

Cities like New York and areas downwind have poor air quality due to pollution generated by human activity. Ozone (O3), a harmful pollutant, is produced in the atmosphere from photochemical reactions involving volatile organic compounds (VOCs) and nitrogen oxides (NOx). Aircraft measurements of VOCs obtained during an O3 event in May 2017 over the New York City metropolitan area, Long Island Sound, and Connecticut show concentrations of O3 exceeding 100 ppb between ∼200 and 500 m above the surface. During this campaign, 35 whole air canisters were collected, and 50 VOC species were measured. Biogenic isoprene often dominates VOC reactivity in summer months, but analysis of the chemistry in the middle of the Planetary Boundary Layer (PBL) indicates that primary anthropogenic VOCs such as propylene and isopentane played a major, possibly dominant role in this Spring 2017 O3 event and are suitable targets for control.The most important measured anthropogenic VOCs ranked by OH reactivity were propylene and isopentane, with n-butane and 1-pentene also playing a substantial role. The most important anthropogenic VOCs ranked by Ozone Formation Potential (OFP) were again propylene and isopentane, but aromatic species like toluene were also large contributors. Isoprene ranked tenth in importance for OFP. For VOC species with short lifetimes, like isoprene and propylene, mixing of air parcels throughout the PBL means air above the surface may quickly become depleted of these VOCs, allowing longer-lived, anthropogenic VOCs to have a proportionally greater impact on photochemical O3 production aloft. Our observations indicate that VOC reactivity was much larger in the morning than in the afternoon, suggesting controls of propylene, isopentane, and aromatics overnight may be useful for reducing morning concentrations of VOCs and therefore possibly reduce maximum concentrations of O3 observed in the afternoon. Finally, because formaldehyde (HCHO) was not measured during these flights, and since oxygenates also contribute to O3 formation, we estimate the relative contribution of isoprene and propylene to HCHO formation using a box model. Propylene produces ∼25% and isoprene produces ∼4% of HCHO within the box model.