Monitoring of greenhouse gases and pollutants across an urban area using a light-rail public transit platform

Anthropogenic emissions within urban environments are characterized by spatial heterogeneity and temporal variability that present challenges for measuring urban greenhouse gases and air pollutants. To address these challenges, we mounted instruments on public transit light-rail train cars that traverse the metropolitan Salt Lake Valley (SLV) in Utah, USA to observe the temporal and spatial variability of atmospheric species including carbon dioxide (CO₂), methane (CH₄), ozone (O₃), fine particulate matter (PM_2.5), and nitrogen dioxide (NO₂). Utilizing electrified light-rail public transit as an observational platform enables real-time measurements with low operating costs while avoiding self-contamination from vehicle exhaust. We examine temporal averages and case studies of each species that reveal gradients, intermittent point sources, seasonal and diel changes, and complex relationships resulting from emissions, atmospheric chemistry, and meteorological conditions. CO₂ and NO₂ are related through the combustion of fossil fuel and we observed a broad spatial gradient across the city as well distinct plumes at traffic intersections and, for NO₂, a large plume adjacent to a locomotive rail yard. Distributions of O₃ were strongly correlated with NO₂ due to atmospheric photochemical and titration processes. Episodes of high PM_2.5 had distinct spatial patterns depending on meteorological conditions during wintertime persistent cold-air pool episodes. The spatial pattern of CH₄ was characterized by distinct plumes associated with industrial and commercial facilities, some of which followed temporal patterns indicative of daytime working hours; other plumes were persistent throughout the whole day, suggestive of leak-related fugitive emissions. The ongoing multi-year record of spatial and temporal air quality observations provides a valuable data set for future air quality exposure studies. Our results suggest pollution and greenhouse gas emission monitoring and exposure assessment could be greatly enhanced by deploying instruments on public transit systems in urban centers worldwide.