Constraining long-term NOx emissions over the United States and Europe to improve model simulations of tropospheric ozone

Abstract:

Tropospheric ozone impacts air quality, public and vegetative health, radiative forcing, and the lifetimes of other air pollutants through its role in hydroxyl radical formation. Understanding tropospheric ozone historical trends is thus crucial for accurate predictions of future air quality and climate. However, current models are not able to explain observed tropospheric ozone trends over the past few decades, and there are large discrepancies in model ozone budgets and long-term decadal changes. Nitrogen oxides (NO_x) play a critical role in regulating ozone levels, yet NO_x emissions estimates are subject to large uncertainties. Over the United States, NO_x emissions may be overestimated in inventories, while over Europe, NO_x emissions may be underestimated; these mismatches likely reflect the challenge in capturing the rapid changes in transportation emissions in inventories. Here we provide an additional constraint on NO_x emissions and trends using nitrate wet deposition fluxes from the US National Atmospheric Deposition Program (NADP) and the European Monitoring and Evaluation Programme (EMEP). We evaluate observed nitrate wet deposition flux seasonal and annual trends from 1980-2014 and use them to assess simulated fluxes from the GEOS-Chem chemical transport model driven by a global inventory. We then relate observed nitrate wet deposition fluxes to NO_x emissions and compare to GEOS-Chem to assess to what extent current inventories are biased with respect to NO_X emissions or trends over the US and Europe. We implement these newly constrained NO_x emissions estimates into GEOS-Chem to quantify their impact on long-term trends of tropospheric ozone.