An Evaluation of Real-time Air Quality Forecasts and Their Urban Emissions Over Eastern Texas During The Summer of 2006 Texaqs Field Study

Abstract

Forecasts of ozone (O3) and particulate matter (diameter less than 2.5 µm, PM2.5) from seven air-quality forecast models (AQFMs) are statistically evaluated against observations collected during August and September of 2006 (49 days) through the AIRNow network (Aerometric Information Retrieval Now) throughout Eastern Texas and adjoining states. Ensemble O3 and PM2.5 forecasts created by combining the seven separate forecasts with equal weighting, and simple bias-corrected forecasts, are also evaluated in terms of standard statistical measures, threshold statistics and variance analysis. These surface statistical evaluations provide a context for model comparisons with the comprehensive suite of chemical and aerosol measurements collected aboard the NOAA WP-3 aircraft during the summer 2006 TexAQS/GoMACCS field study. Aircraft flights specifically designed for sampling of Houston and Dallas urban plumes are used to determine model and observed up-wind or backgrou! nd biases, and down-wind excess concentrations that are used to infer relative emission rates. Relative emissions from the U.S. EPA NEI-99 version 3 emissions inventory (used in two of the model forecasts) are evaluated based on comparisons between observed and model concentration difference ratios. Model comparisons demonstrate that concentration difference ratios yield a reasonably accurate measure (within 25%) of relative input emissions. Boundary layer height and wind data are combined with the observed up-wind and down-wind concentration differences to estimate absolute emissions. Inventory emissions modified to include observed NOy emissions from continuous monitors are shown to be consistent with those derived from the observations for Houston. However the emission inventories consistently over-predict the ratio of CO to NOy. The ratios of ethylene and aromatics to NOy are reasonably consistent with observations over Dallas, but are significantly underpredicted for Houston. Excess ratios of PM2.5 to NOy reasonably match observations for most models but the organic carbon fraction of PM2.5 is significantly underpredicted, pointing to compensating error between secondary organic aerosol (SOA) formation and primary emissions within the models’ photochemistry and emissions. Rapid SOA formation associated with both Houston and Dallas is inferred to occur within 1 to 3 hours downwind of the urban centers, and none of the models reproduce this feature.