Uncertainties in predicted ozone concentrations due to input uncertaintiesfor the UAM-V photochemical grid model applied to the July 1995 OTAG domain

The photochemical grid model, UAM-V, has been used by regulatory agencies t o make decisions concerning emissions controls, based on studies of the Jul y 1995 ozone episode in the eastern US. The current research concerns the e ffect of the uncertainties in UAM-V input variables (emissions, initial and boundary conditions, meteorological variables, and chemical reactions) on the uncertainties in UAM-V ozone predictions. Uncertainties of 128 input va riables have been estimated and most range from about 20% to a factor of tw o. 100 Monte Carlo runs, each with new resampled values of each of the 128 input variables, have been made for given sets of median emissions assumpti ons. Emphasis is on the maximum hourly-averaged ozone concentration during the 12-14 July 1995 period. The distribution function of the 100 Monte Carl o predicted domain-wide maximum ozone concentrations is consistently close to log-normal with a 95% uncertainty range extending over plus and minus a factor of about 1.6 from the median. Uncertainties in ozone predictions are found to be most strongly correlated with uncertainties in the NO2 photoly sis rate. Also important are wind speed and direction, relative humidity, c loud cover, and biogenic VOC emissions. Differences in median predicted max imum ozone concentrations for three alternate emissions control assumptions were investigated, with the result that (1) the suggested year-2007 emissi ons changes would likely be effective in reducing concentrations from those for the year-1995 actual emissions, that (2) an additional 50% NOx emissio ns reductions would likely be effective in further reducing concentrations, and that (3) an additional 50% VOC emission reductions may not be effectiv e in further reducing concentrations. (C) 2001 Elsevier Science Ltd. All ri ghts reserved.