Ozone production in the New York City urban plume

Tn the summer of 1996 the Department of Energy G-1 aircraft was deployed in the New York City metropolitan area as part of the North American Research Strategy for Tropospheric Ozone-Northeast effort to determine the causes o f elevated O-3 levels in the northeastern United States. Measurements of O- 3, O-3 precursors, and other photochemically active trace gases were made u pwind and downwind of New York City with the objective of characterizing th e O-3 formation process and its dependence on ambient levels of NO, and vol atile organic compounds (VOCs). Four flights are discussed in detail. On tw o of these flights, winds were from the W-SW, which is the typical directio n for an O-3 episode. On the other two flights, winds were from the NW, whi ch puts a cleaner area upwind of the city. The data presented include plume and background values of O-3, CO, NOx, and NOy concentration and VOC react ivity. On the W-SW flow days 0, reached 110 ppb. According to surface obser vations the G-1 intercepted the plume close to the region where maximum 0, occurred. At this point the ratio NOx/NOy was 20-30%, indicating an aged pl ume. Plume values of CO/NOy agree to within 20% with emission estimates fro m the core of the New York City metropolitan area. Steady state photochemic al calculations were performed using observed or estimated trace gas concen trations as constraints. According to these calculations the local rate of 0, production P(O-3) in all four plumes is VOC sensitive, sometimes strongl y so. The local sensitivity calculations show that a specified fractional d ecrease in VOC concentration yields a similar magnitude fractional decrease in P(O-3). Imposing a decrease in NOx, however, causes P(O-3) to increase. The question of primary interest from a regulatory point of view is the se nsitivity of O-3 concentration to changes in emissions of NO, and VOCs. A q ualitative argument is given that suggests that the total O-3 formed in the plume, which depends on the entire time evolution of the plume, is also VO C sensitive. Indicator ratios O-3/NOz and H2O2/NOz mainly support the concl usion that plume O-3 is VOC sensitive.