(270d) The Sensitivity of Modeled Ozone to Changes In the Temporal Distribution of Area, Point, Mobile and Non-Road Emissions

High ozone concentrations that occur via a photochemical mechanism involving nitrogen oxides (NOx) and volatile organic compounds (VOCs) and exceed the 80 ppb 8-hour average National Ambient Air Quality Standard (NAAQS) are a longstanding problem in many Northeast urban/suburban areas (U.S. Environmental Protection Agency 2004). Eulerian grid models or air quality models (AQMs) that simulate atmospheric chemistry, transport, and removal processes of multiple pollutants including ozone, aerosols, and other oxidants that are used as tools in air quality decision making often satisfactorily replicate ozone when compared to measurements (U.S. Environmental Protection Agency 2001; Hogrefe 2007; Godowitch 2008). Regardless, the simulations are subject to uncertainty as a result of parameterizations and approximations embedded in the model algorithms as well as inaccuracies in the meteorological and emissions inputs as a result of measurement uncertainty, missing data, etc. (Placet 2000; Fine 2003; Mallet 2006). By understanding the sensitivities in the model ? the models' response to changes in inputs, parameters, or resolution ? one can identify areas that contribute to the simulation uncertainty and therefore require additional scientific analysis.

In this work we analyzed the effects of altering emissions temporal distributions on simulations of ozone with a focus on the Northeast. We conducted a summer simulation with the Community Multi-Scale Air Quality Model (CMAQ) v4.5 using a 12 km grid size with the 2002 National Emissions Inventory. We applied three temporal distributions to the four major source categories of emissions and simulated a total of nine cases: 1) four uniform cases, where either point, area, mobile, or non-road emissions remained the same hour-to-hour and day to day, 2) a base case, where all emissions varied according to 2002 NEI estimates, and 3) four high variability cases, where 50% of either point, area, mobile, or non-road nighttime emissions were added to the daytime. We find that the largest sensitivities occurred in urban areas both in the Northeast and Southeast when mobile sources were altered. For a summer simulation, on average, the ozone sensitivity was +/- 1-3 ppb. The largest sensitivities, +/- 2-7 ppb, occurred when an 8-hour average concentration greater than 80 ppb was simulated in the base case. When moderate, 50-80 ppb, 8-hour averages were simulated, the sensitivity was less than 2 ppb. Furthermore, the number of days with 80 ppb 8-hour average exceedances decreased by 2-10 days when mobile sources were altered. The sensitivities from the other source categories, area, point, and non-road sources, were not significant in comparison.

REFERENCE

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