COVID-19 Lockdowns Afford the First Satellite-Based Confirmation That Vehicles Are an Under-recognized Source of Urban NH₃ Pollution in Los Angeles

High Resolution Image Download MS PowerPoint Slide In situ measurements have suggested vehicle emissions may dominate agricultural sources of NH 3 in many cities, which is alarming given the potential for urban NH 3 to significantly increase human exposure to ambient particulate matter. However, confirmation of the prevalence of vehicle NH 3 throughout a city has been challenging because of mixing with agricultural sources, and the latter are thus routinely assumed to dominate. Here we report vehicle NH 3 emissions based on TROPOMI NO 2 and CrIS NH 3 (0.152 kg s –1 ) that are consistent with a model-based estimate (0.178 kg s –1 ) and show that COVID-19 lockdowns provide a unique opportunity for making the first satellite-based constraints on vehicle NH 3 emissions for an entire urban region (western Los Angeles), which we find make up 60–95% of total NH 3 emissions, substantially higher than the values of 13–22% in state and national inventories. This provides a new means of constraining a component of transportation emissions whose impacts may rival those of NO x yet which has been largely under-recognized and uncontrolled.