Comparison of the MOVES2010a, MOBILE6.2, and EMFAC2007 mobile source emission models with on-road traffic tunnel and remote sensing measurements

Abstract

The Desert Research Institute conducted an on-road mobile source emission study at a traffic tunnel in Van Nuys, California, in August 2010 to measure fleet-averaged, fuel-based emission factors. The study also included remote sensing device (RSD) measurements by the University of Denver of 13,000 vehicles near the tunnel. The tunnel and RSD fleet-averaged emission factors were compared in blind fashion with the corresponding modeled factors calculated by ENVIRON International Corporation using U.S. Environmental Protection Agency's (EPA's) MOVES2010a (Motor Vehicle Emissions Simulator) and MOBILE6.2 mobile source emission models, and California Air Resources Board's (CARB's) EMFAC2007 (EMission FACtors) emission model. With some exceptions, the fleet-averaged tunnel, RSD, and modeled carbon monoxide (CO) and oxide of nitrogen (NO x ) emission factors were in reasonable agreement (AE25%). The nonmethane hydrocarbon (NMHC) emission factors (specifically the running evaporative emissions) predicted by MOVES were insensitive to ambient temperature as compared with the tunnel measurements and the MOBILE-and EMFAC-predicted emission factors, resulting in underestimation of the measured NMHC/NO x ratios at higher ambient temperatures. Although predicted NMHC/NO x ratios are in good agreement with the measured ratios during cooler sampling periods, the measured NMHC/NO x ratios are 3.1, 1.7, and 1.4 times higher than those predicted by the MOVES, MOBILE, and EMFAC models, respectively, during high-temperature periods. Although the MOVES NO x emission factors were generally higher than the measured factors, most differences were not significant considering the variations in the modeled factors using alternative vehicle operating cycles to represent the driving conditions in the tunnel. The three models predicted large differences in NO x and particle emissions and in the relative contributions of diesel and gasoline vehicles to total NO x and particulate carbon (TC) emissions in the tunnel.Implications: Although advances have been made to mobile source emission models over the past two decades, the evidence that mobile source emissions of carbon monoxide and hydrocarbons in urban areas were underestimated by as much as a factor of 2-3 in past inventories underscores the need for on-going verification of emission inventories. Results suggest that there is an overall increase in motor vehicle NMHC emissions on hot days that is not fully accounted for by the emission models. Hot temperatures and concomitant higher ratios of NMHC emissions relative to NO x both contribute to more rapid and efficient formation of ozone. Also, the ability of EPA's MOVES model to simulate varying vehicle operating modes places increased importance on the choice of operating modes to evaluate project-level emissions.

Evaluating Summer-Time Ozone Enhancement Events in the Southeast United States

“…Scaling factors were applied to all emitted chemical species to represent anthropogenic emissions in the year 2013 using the ratio of national annual totals [30]. Recent studies have shown that NO x emissions from sources other than power plants (i.e., mobile sources) in this inventory, which contribute significantly to total NO x emissions, are overestimated and in this study the NEI emission rates are reduced by 50% similar to many recent studies (e.g., [29,[31][32][33][34]). …”

Resolving ozone vertical gradients in air quality models

“…This finding was subsequently supported by SAS observations (Miller et al, 2017). Previous studies had documented such a NOx NEI bias in urban areas (Fujita et al, 2012;Yu et al, 2012; 20 Brioude et al, 2013;Anderson et al, 2014), but our results suggest that the bias is national in extent. After correcting this NOx emission overestimate in GEOS-Chem, we found that we could match the SEAC 4 RS aircraft observations below 1.5 km altitude, but the model mean bias against surface network observations was still 6 ± 14 ppb.…”

Precursor reductions and ground-level ozone in the Continental United States

“…Some investigators have suggested that mobile source NO x emissions are overestimated after EPA began using MOVES in place of MOBILE6 (see Figure 1). For example, Fujita et al (2012) found that mobile source NO x emissions from the MOVES model overestimated NO x emissions that had been determined from measurements made in a highway tunnel. A fuel-based emission inventory by McDonald et al (2012) yielded relatively constant on-road mobile source NO x emissions from 1990 to 2000 (compared with a 12% reduction in EPA's trend inventory), but comparable (within 7%) mobile source NO x emissions in 1999 and 2008.…”