Comparative Impacts of Freight and Non-truck Traffic on NOx and Ozone Concentrations in the Los Angeles Basin

The Los Angeles (LA) metropolitan region remains in nonattainment for ozone despite decades of reductions of ozone precursors, nitrogen oxides (NOx) and volatile organic compounds (VOCs). NOx emissions from freight vehicles (ships, heavy duty trucks, trains, and airplanes) are expected to exceed emissions from passenger vehicles in southern California by 2030. Here, we use random forest machine learning to estimate the impact of freight activity on hourly NOx concentrations and determine summertime ozone production regimes across the LA basin. We find that freight activity contributes over half of weekday NOx impacts relative to non-truck traffic. During peak ozone hours, coastal areas, south LA, areas downwind (east) of downtown LA, and downtown San Bernardino are VOC-limited. Our results suggest that as of 2021, the Los Angeles urban core and nearby downwind areas have not transitioned to a NOx-limited regime on most days during the May to September ozone season. This study shows the applicability of machine learning to estimate concentration impacts from specific sources in the face of uncertain emission inventories and to analyze current ozone production regimes in areas with hourly ground observations.