The objectives of this paper are to:(1)quantify variability in hourly utility NO_x emission factors,activity factors,and total emissions;(2)investigate the autocorrelation structure and evaluate cyclic effects at short and long scales of the time series of total hourly emissions;(3)compare emissions for the ozone season versus the entire year to identify seasonal differences,if any;and(4)evaluate inter-annual variability.Continuous Emissions Monitoring(CEM)data were analyzed for 1995 and 1998 for 32 units from 9 baseload power plants in the Charlotte,NC airshed.Unit emissions have a strong 24 hour cycle attributable primarily to the capacity factor.Typical ranges of the coefficient of variation for emissions at a given hour of the day were from 0.2 to 0.45.Little difference was found when comparing weekend emissions with the entire week or when comparing the ozone season with the entire year.There were substantial differences in the mean and standard deviation of emissions when comparing 1995 and 1998 data,indicative of the effect of retrofits of control technology during the intervening time.The wide range of variability and its autocorrelation should be accounted for when developing probabilistic utility emission inventories for analysis of near-term future air quality episodes.
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