THE ROLE OF NITROGEN-OXIDES IN OXIDANT PRODUCTION AS PREDICTED BY THEREGIONAL ACID DEPOSITION MODEL (RADM)

Regional oxidant distributions produced under various atmospheric cond itions and emission scenarios are investigated using the Regional Acid Deposition Model (RADM), RADM is a complex, evolving three-dimensiona l Eulerian model that describes the chemistry, transport and depositio n of tropospheric trace species including SO(x), sulfate, NO(x) and vo latile organic compounds as well as O3, other major oxidants and acids . The model calculates the short-term temporal evolution of atmospheri c trace gas concentrations and their deposition on the regional scale. This study is focused on oxidant production in the eastern United Sta tes and southeastern Canada. The influence of atmospheric conditions i s explored by comparing three characteristic winter, summer and spring /fall cases. Base-case 1985 emissions of SO(x), NO(x), volatile organi c compounds (VOCs), NH3 and CO are specified using the comprehensive p ollutant emissions inventory developed as part of the National Acid Pr ecipitation Assessment Program (NAPAP). The perturbed case, which repr esents projected anthropogenic emission changes for 2010, indicates ch anges in daily total 80 km grid average NO(x) emissions ranging from i ncreases of 75% to decreases of 45% and VOC emission changes ranging f rom increases of 65% to decreases of 20%. The largest NO(x) emission c hanges occur in the northeast, and the largest VOC changes occur in th e Gulf Coast area. Ground level grid average midday O3 concentrations for the 1985 emission cases are highest (on the order of 70 to 100 ppb ) in the New York City and Houston metropolitan areas for the summer a nd spring cases; the summer case also indicates relatively high grid a verage O3 concentrations of greater than 80 ppb in the southeast. Wint er case values are much lower than summer O3 values throughout the reg ion, with highs of 40 to 50 ppb occurring in the southeast and the Gre at Lakes area. Changes in NO(x) and other emissions under the complex 2010 emissions scenario for the summer case result in maximum O3 conce ntration reductions of 10% in the Houston area and increases in O3 of a few percent in some rural areas of the southeast. This study undersc ores the need for more comprehensive assessment of the complex relatio nships among regional emission changes, oxidant production and atmosph eric conditions.