Urban public transit systems provide an ideal, cost-effective platform for urban atmospheric monitoring. Public transit covers large spatial domains across divergent urban typologies and were operational through the COVID-19 lockdown period. Measurements on the light rail system in Salt Lake City (TRAX) include greenhouse gases (CO2 & CH4) that can be used to evaluate primary combustion emission inventories and further develop modeling techniques to understand the spatiotemporal patterns of emissions. TRAX also measures air pollutants (PM2.5 and O3) that form from secondary chemical reactions. The dramatic change in emissions during the COVID-19 lockdown period and the resulting shifts in urban atmospheric composition observed by TRAX provides a unique opportunity to examine the relationship between primary combustion and secondary pollutants. It will also accelerate our fundamental understanding of urban atmospheric chemistry. This project will increase understanding of the spatiotemporal and sectoral changes in emissions, how changes in primary combustion affected secondary air pollutants, and what the urban atmospheric implications are for policies that target emission reductions.