Characterization of winter air pollutant gradients near a major highway.

Traffic-related air pollutants (TRAP) including nitric oxide (NO), nitrogen oxide (NO), carbon monoxide (CO), ultrafine particles (UFP), black carbon (BC), and fine particulate matter (PM) were simultaneously measured at near-road sites located at 10 m (NR10) and 150 m (NR150) from the same side of a busy highway to provide insights into the influence of winter time meteorology on exposure to TRAP near major roads. The spatial variabilities of TRAP were examined for ambient temperatures ranging from -11 °C to +19 °C under downwind, upwind, and stagnant air conditions. The downwind TRAP concentrations at NR10 were higher than the upwind concentrations by a factor of 1.4 for CO to 13 for NO. Despite steep downwind reductions of 38 % to 75 % within 150 m, the downwind concentrations at NR150 were still well above upwind concentrations. Near-road concentrations of NO and UFP increased as ambient temperatures decreased due to elevated emissions of NO and UFP from vehicles under colder temperatures. Traffic-related PM sources were identified using hourly PM chemical components including organic/inorganic aerosol and trace metals at both sites. The downwind concentrations of primary PM species related to tailpipe and non-tailpipe emissions at NR10 were substantially higher than the upwind concentrations by a factor of 4 and 32, respectively. Traffic-related PM sources accounted for almost half of total PM mass under downwind conditions, leading to a rapid change of PM chemical composition. Under stagnant air conditions, the concentrations of most TRAP and related PM including tailpipe emissions, secondary nitrate, and organic aerosol were comparable to, or even greater than, the downwind concentrations under windy conditions, especially at NR150. This study demonstrates that stagnant air conditions further widen the traffic-influenced area and people living near major roadways may experience increased risks from elevated exposure to traffic emissions during cold and stagnant winter conditions.