Characterization of an eastern U.S. severe air pollution episode using WRF/Chem

On 8–11 July 2007 the eastern United States experienced a severe heat wave and smog event with maximum temperatures approaching 38°C and maximum 8 h average ozone mixing ratios of 125 ppbv. We examine this episode with observations and numerical simulations using the Weather Research and Forecasting model with online chemistry (WRF/Chem with RADM2). The general features of this severe smog event–a broad area of high pressure, weak winds and heavy pollution, terminated by the passage of a cold front–were well simulated by the model. WRF/Chem underpredicted O3 maxima by 5–8 ppbv where air quality was poor, usually in the northeast, but overpredicted maxima by up to 16 ppbv where ozone amounts were low, usually in the southeast. Simulated O3 vertical profiles over Beltsville, Maryland, showed good agreement with ozonesonde measurements, but the model boundary layer was too deep on 9 July, contributing to the low bias over this region. The representation of NOx chemistry in RADM2 may lead to an underestimation of NOx lifetime and is likely partially responsible for low O3 biases in the most polluted area in the northeast. To simulate the maximum effect of nighttime multiphase NOy loss, we set the N2O5 heterogeneous hydrolysis reaction rate constant to zero. This increased the mean bias outside the area of highest ozone concentration but substantially improved O3 and NOy over most of the domain, especially in smoggy areas such as the rural, Pinnacles site.