Constraining long-term NO_<i>x</i> emissions over the United States and Europe using nitrate wet deposition monitoring networks

Abstract

Nitrogen oxides (NOx= NO + NO2) play a critical role in regulating tropospheric chemistry, yet NOx emission estimates are subject to large uncertainties, casting doubt on our ability to accurately model secondary pollutants such as ozone. Bottom-up emissions inventories are subject to a number of uncertainties related to estimates of emission activities, scaling factors, and fuel sources. Here, we provide an additional constraint on NOx emissions and trends using nitrate wet deposition (NWD) fluxes from the United States National Atmospheric Deposition Program (NADP) and the European Monitoring and Evaluation Programme (EMEP). We use these NWD measurements to evaluate anthropogenic and total NOx trends and magnitudes in the global Community Emissions Data System (CEDS) emissions inventory and the GEOS-Chem chemical transport model from 1980–2020. Over both the United States and Europe, observed NWD trends track well with anthropogenic NOx emissions from the CEDS inventory until 2010, after which NWD trends level out in contrast to continued decreases in CEDS. After 2010, NWD trends are able to reproduce total NOx emissions trends when the influences of both anthropogenic and background sources are considered. Observed NWD fluxes are also able to capture NOx emissions decreases over the 2020 COVID-19 lockdown period and are consistent with satellite and surface measurements of NO2. These results suggest that NWD fluxes constrain total NOx emissions well, whether trends are driven by anthropogenic or background sources. We further compare modeled and observed NWD to provide an additional line of evidence for potential overestimates of anthropogenic NOx in emissions inventories. Over the United States, we find that NWD is overestimated in summer from 1980–2017 by 15 %–20 % on average (interquartile range: 11 %–31 %), with overestimates most prominent in the eastern US after 2000 (20 % on average), implying an overestimate of NOx emissions in the CEDS inventory (0.5×0.5° resolution). Over Europe, we find that modeled NWD is overestimated in all seasons from 1980–2017, with the strongest average overestimates occurring in summer and fall (175 % and 170 %, respectively). These overestimates may be reduced by cutting anthropogenic NOx emissions by 50 % in CEDS over Europe (i.e., cutting the 1980–2017 average annual emissions from 2.6 to 1.3 Tg N), but summertime and fall NOx may still need to be reduced further for observations and models to align. Overestimates may extend to other inventories such as the EMEP inventory, which estimates comparable but lower emissions than CEDS, with a 1990–2017 average of 2.1 Tg N relative to the CEDS 1990–2017 average of 2.4 Tg N. We find that NOx emission reductions over Europe improve model ozone at the surface, reducing the model summertime ozone overestimate from 14 % to 2 %.