Introduction Air stagnation is a meteorological condition when the same air mass remains over an area for several days to a week. Light winds during air stagnation and atmospheric inversion that usually accompanies stagnation prevent transport and dispersion of air pollutants, leading to major buildup of air pollutants and severe pollution episodes. This poster presents a 30-year ( ) climatology of air stagnation conditions for the United States. Different from previous studies of similar scope, the new climatology takes advantage of the long-term, high spatial and temporal resolution gridded reanalysis data that has become available relatively recently. The climatology identifies areas favorable for air stagnation and the seasonal/interannual variability of the stagnation conditions. Data The climatological analyses carried out in this study utilized the North American Regional Reanalysis (NARR) dataset (Mesinger et al. 2006), a gridded and dynamically consistent atmospheric and land-surface hydrology dataset covering the period of 1979 to present. The data are available eight times per day for the North American domain at 32-km horizontal grid spacing and 29 vertical levels. Method Thirty year (from 0000 UTC on 1 January 1979 through 2100 UTC on 31 December 2008) NARR 10-m wind speed data are used in this analyses. Air stagnation is defined objectively as daily maximum 10-m wind speed is 4 m/s or less. Climatology of air stagnation is calculated for each grid point, which includes monthly and annual frequency, seasonal and inter-annual variability, and trends. The results indicate preferred locations and periods for air stagnation in the U.S. and provides a climatological basis for forecasting the occurrence of stagnation. Reference Mesinger F., G. DeMego, E. Kalnay, K. Mitchell, P. C. Shafran, W. Ebisuzaki, D. Jovic, and J. Woollen, (2006): North American regionall reanalysis. Bulletin of the American Meteorological Society, 87, Acknowledgements This research is supported by Michigan Agricultural Station and USDA Forest Service Northern Research Station under agreement 07-JV Spatial Distribution of Air Stagnation Frequency A33B-0156 The Climatology of Air Stagnation Conditions over U.S. as Derived from the NARR Data Randy X. Bian*^, Jay J. Charney^, and Warren E. Heilman^, USDA Forest Service, Northern Research Station, East Lansing, MI Aaron M. Pollyea, Jeffrey A. Andresen, and Sharon S. Zhong, Michigan State University, East Lansing, MI * Correspondence to: ^ The contributions of Randy X. Bian, Jay J. Charney, and Warren E. Heilman to this work were prepared as part of their official duties as United States Federal Government employees. Selected Regional Air Stagnation Frequency The results above reveal in details seasonal and inter-annual variability of air stagnation frequency for various regions in the U.S. Work is on-going to relate interannual variability to changes in large- scale circulations. The maps above show that Air stagnation frequency varies significantly across U.S.: much more frequent in western U.S. and southeastern U.S. highest frequency in mountain basins and valleys least frequent over the Central Plains, the Great Lakes region, and the northeast U.S. There is a large seasonal variation, which is region-dependent in western U.S., high frequency occurs in winter season (November through February) and lower frequency in spring. in southeastern U.S., high frequency in summer season (June-August) and lower in winter and early spring. October November December July August September April May June January February March