HR: 09:30h
AN: A41D-07    [Abstracts]
TI: Increase in Surface Ozone at Rural Sites in the Western U.S.
AU: * Jaffe, D A
EM: djaffe@u.washington.edu
AF: University of Washington, Bothell, 18115 Campus Way NE, Bothell, WA 98011 United States
AU: Chand, D
EM: dchand@uwb.edu
AF: University of Washington, Bothell, 18115 Campus Way NE, Bothell, WA 98011 United States
AB: We evaluated O₃ data from 12 relatively unpolluted sites in the north and western U.S., including 2 sites in Alaska, for the period 1987-2004. At the 2 sites in Alaska, no trend was present. At 9 out of 10 sites in the continental U.S. there is a statistically significant increase in O₃ with a mean trend of 0.35 ppbv/year (range 0-0.70 ppbv/year). At 4 of these sites, we examined the data seasonally and in relationship to temperature variations. We find that the trends are statistically significant in all seasons. Temperature changes can explain a small fraction of the surface O₃ trend. The change in O₃ at these sites has occurred both at the high and low end of the frequency distributions. At the one site in the western U.S. with long-term vertical profile data (Boulder, Colorado), no significant O₃ trend was identified in the troposphere. However, the statistical power in the ozonesonde analysis is limited due to the small number of data points. Over the same time period, anthropogenic NOx and VOC emissions in the western U.S. peaked in the 1990s and have fallen by approximately 15% since then, based on two separate emission inventories. Thus increasing regional NOx and VOC emissions do not appear to explain the positive O₃ trend. We hypothesize that changes in regional photochemistry, and/or increasing global background O₃ are responsible for these trends. While most sites we examined were relatively free from nearby human influence, at two sites, Rocky Mountain National Park (RMNP) in Colorado and Lassen Volcanic National Park (LVNP) in California there are some days with 8-hour averages in excess of the 0.08 ppmv air quality standard. These occur when regional photochemical pollution is transported to the sites. At RMNP, most of these days have occurred since 1998. However, it is likely that the trend in regional or global background O₃ is contributing to the increasing frequency of days in excess of the 0.08 ppmv level.
DE: 0340 Middle atmosphere: composition and chemistry
DE: 0345 Pollution: urban and regional (0305, 0478, 4251)
DE: 0365 Troposphere: composition and chemistry
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005