Examining the Effects of Variability in Average Link Speeds on Estimated Mobile Source Emissions and Air Quality

Running Stabilized Emissions from vehicle exhausts are estimated by combining travel activity quantified as vehicle miles of travel (VMT) or vehicle hours of travel (VHT) with emissions factors which are adjusted for speeds on facility links. These speeds which are used for adjusting emissions factors are averaged for fixed periods (typically for one hour intervals) and across the lanes of multi-lane links. In real world traffic conditions, however, average speeds are variable for higher time resolutions and across lanes. Incorporating the variability in average speeds during calculation will result in different magnitudes of estimated running stabilized emissions for a given period of time and a facility link. Lane volume and occupancy measurements from 1376 single loop detector stations on 830 freeway links in Los Angeles were used to estimate flow rates, hourly average link speeds, 15-minute average link speeds and hourly average lane speeds. Then, these flow rates and average speeds were used to estimate hourly link CO, CO2, NOx, PMx and TOG emissions and hourly gridded emissions based on different average speed resolutions for the summer of 1997. This study statistically examines the hourly differences in running stabilized emissions when 15-minute link and hourly lane average speeds are used in contrast to hourly link average speeds for running stabilized emissions estimation. Moreover, effects of speed variability on regional air quality are evaluated by examining the differences in estimated gridded ozone (O3) and fine particulate matter (PM25) concentrations based on different gridded emissions inventories (estimated using hourly link, 15-minute link and hourly lane average speeds). The results show that the magnitudes of estimated hourly link running stabilized emissions are different especially when hourly lane average speeds are used in contrast to the hourly link average speeds. Estimated O3 and PM25 concentrations also differ, however, the differences in these estimated secondary pollutant concentrations show different patterns when compared to the differences in estimated emissions.