Factors affecting pollutant concentrations in the near-road environment

An improved understanding of traffic-related air pollutants is needed to estimate exposures and adverse health impacts in traffic corridors and near-road environments. In this study, concentrations of black carbon (BC), nitrogen oxides (NO, NO2, NOx), sulfur dioxide (SO2), and particulate matter (PM2.5, PM10, ultrafine particles, and accumulation mode particles, AMP) were measured using a mobile air pollutant laboratory along nine transects across major roads in Detroit, MI in winter 2012. Repeated measurements were taken during rush-hour periods at sites in residential neighborhoods located 50-500 m from both sides of the road. Concentration gradients attributable to on-road emissions were estimated by accounting for traffic volume and mix, wind speed, wind direction, and background concentrations. BC, NO, NOx, and UFP had the strongest gradients, and elevated concentrations of NOx, NO2, PM2.5 and PM10, as well as decreased particle size, were found at the 50 m sites compared to background levels. Exponential models incorporating effects of road size, wind speed, and up- and downwind distance explained from 31 to 53% of the variability in concentration gradients for BC, NO, NOx, UFP and particle size. The expected concentration increments 50 m from the study roads were 17.0 ppb for NO, 17.7 ppb for NOx, 2245 particles/cm3 for UFP, and 0.24 μg/m3 for BC, and the expected distance to decrease increments by half was 89-129 m in the downwind direction, and 14-20 m in the upwind direction. While accounting for portion of the temporal and spatial variability across transects and measurement periods, these results highlight the influence of road-to-road differences and other locally-varying factors important in urban and industrial settings. The study demonstrates a methodology to quantify near-road concentrations and influences on these concentrations while accounting for temporal and spatial variability, and it provides information useful for estimating exposures of traffic-related air pollutants in urban environments.