Did the COVID-19 lockdown allow for the first identification of NH3 transportation emissions using satellite-based observations?

Abstract:

While the largest sources of NH3 are agricultural, in situ measurements suggest vehicles are the dominant source in urban areas throughout the US, at levels more than two times higher than national emission inventories, making them an important and under-recognized contributor to aerosol-related human health impacts through formation of secondary inorganic PM2.5. Since the first satellite-based NH3 observations more than a decade ago, multiple satellite-based instruments have been used to identify NH3 emissions from agriculture, industry, biomass burning, and natural sources. However, there has not yet been a satellite-based measurement of NH3 specifically linked to transportation emissions. The drastic reductions in transportation emissions associated with the COVID-19 lockdown in Los Angeles during the early spring of 2020 provide a unique opportunity for examining the impact of transportation emissions on atmospheric composition. Here we show CrIS NH3 retrievals from the NASA TRopospheric Ozone and is Precursors from Earth System Sounding (TROPESS) project reveal a significant decrease in NH3 concentrations over the downtown area and substantial increases in the eastern part of the broader LA airshed. Coupled with evaluation of NO2 concentrations from TROPOMI, a bottom-up inventory of traffic emissions, changes in meteorology, and comparison to similar months in previous years, we hypothesize this unique pattern js due to reduction in NH3 associated with transportation emissions in the urban core, coupled with increased NH3 concentrations from agricultural emissions to the east (that were relatively unaffected by the lockdown) owing to reduced emissions of transportation NOx. The latter led to decreased formation of ammonium nitrate, and hence a decrease in the atmospheric sink of NH3. This hypothesis is further evaluated by estimating transportation NH3 emissions using the satellite data and comparing these to a new bottom-up estimate from the NOAA Fuel-Based Inventory of Vehicle Emissions (FIVE). Overall, the work provides a new means of constraining a highly uncertain yet significant component of urban emission inventories and highlights the value of multi-species remote observations for identifying sector-specific emissions of reactive nitrogen.