The effects of global changes upon regional ozone pollution in the United States


Chen, J., Avise, J., Lamb, B., Salathé, E.P., Mass, C.F., Guenther, A., Wiedinmyer, C., Lamarque, J-F., O’Neill, S., McKenzie, D., Larkin, N. 2009. The effects of global changes upon regional ozone pollution in the United States. Atmospheric Chemistry & Physics 9: 1125-1141.


A comprehensive numerical modeling framework was developed to estimate the effects of collective global changes upon ozone pollution in the U.S. in 2050. The framework consists of the global climate and chemistry models, PCM (Parallel Climate Model) and MOZART-2 (Model for Ozone and Related Chemical Tracers v.2), coupled with regional meteorology and chemistry models, MM5 (Mesoscale Meteorological model) and CMAQ (Community Multi-scale Air Quality model). The modeling system was applied for two 10-year simulations: 1990-1999 as a present-day base case and 2045-2054 as a future case. The regional simulations employed 36-km grid cells covering the continental U.S. with boundary conditions taken from the global models.

For the current decade, the distributions of summer daily maxima 8-hour (DM8H) ozone showed good agreement with observed distributions throughout the U.S. The future case simulation followed the Intergovernmental Panel on Climate Change (IPCC) A2 scenario together with business-as-usual U.S. emission projections and projected alterations in land use, land cover (LULC) due to urban expansion and changes in vegetation. For these projections, U.S. anthropogenic Nox (NO + NO2) and VOC (volatile organic carbon) emissions increased by approximately 8% and 50%, respectively, while biogenic VOC emissions decreased, in spite of warmer temperatures, due to decreases in forested lands and expansion of croplands, grasslands and urban areas. A stochastic model for wildfire emissions was applied that projected 25% higher VOC emissions in the future.

For the global and U.S. emission projection used here, regional ozone pollution becomes worse in the 2045-2054 period for all months. Annually, the mean DM8H ozone was projected to increase by 9.6 ppbv (22%). The changes were higher in the spring and winter (25%) and smaller in the summer (17%). The area affected by elevated ozone within the U.S. continent was projected to increase; areas with levels exceeding the 75 ppbv ozone standard at least once a year increased by 38%. In addition, the length of the ozone season was projected to increase with more pollution episodes in the spring and fall. For selected urban areas, the system projected a higher number of pollution events per year and these events had more consecutive days when DM8H ozone exceed 75 ppbv.