Intra-urban spatial variability of traffic-related air pollutants from highway to downtown
Epidemiological and toxicological studies have demonstrated that exposure to traffic-related air pollutants (TRAP) has a wide range of adverse health impacts. The concentrations of TRAP (e.g. ultrafine particles (UFP), black carbon (BC), nitrogen oxides (NOx), and carbon monoxide (CO)) can vary spatially with distance from roadways, through atmospheric processes that can depend on meteorological conditions. In a busy downtown area, concentrations of TRAP show more localized gradients due to frequent vehicle congestions, the urban street canyon effect, and compact land and building development. Comprehensive stationary and mobile measurements are required to better characterize the temporal and spatial variability of TRAP and to evaluate the effect of long-term and/or acute TRAP exposure in various urban environments. A near roadway monitoring study was conducted at two near-road and two urban background sites in Toronto. The differences in UFP, BC, NOx, and CO between urban background and near-road environments were evaluated by comparing the concentrations measured at these monitoring stations with atmospheric conditions. In addition to the stationary air quality monitoring stations, mobile measurements were made around busy downtown streets, major roadways, and residential areas. A suite of portable devices was carried along timed and planned routes through traffic dense downtown streets up to a few kilometers, while a mobile laboratory equipped with laboratory scale instruments was used to measure the levels of TRAP over a larger geographical scale. This work provides a better understanding of the spatial heterogeneity of TRAP measured using various air quality monitoring approaches from city-wide conventional stationary monitoring to walking measurements in compact downtown areas. The influences of source proximity, geographical scale of sources, and meteorological conditions on the spatial extent of the elevated concentrations of TRAP will also be discussed.