Synergistic use of OMI NO2 tropospheric columns and LOTOS-EUROS to evaluate the NOx emission trends across Europe

In this study trends in the tropospheric NO2 concentrations during 2005-2010 across Europe were derived from the synergistic use of OMI NO2 tropospheric columns and the chemistry transport model LOTOS-EUROS and were compared to reported NO emissions. The chemistry transport model captures a large fraction of the variability in NO2 columns at a synoptic timescale, although a seasonal signal in the bias between the modelled and retrieved column data remains. Using a simulation with constant emissions in time, trends were derived on the basis of the systematically changing bias between the modelled and retrieved columns. Significant negative trends of 5-6% a(-1) were found in highly industrialized areas across Western Europe. Strongest decreases in NO2 concentrations are observed over a region with many power plants in Northern Spain (10-20% a(-1)) and over the Po Valley (11% a(-1)). A source apportionment simulation was performed to evaluate the sensitivity of the Ozone Monitoring Instrument (OMI) to NO emission sources across Europe, identifying the importance of changes in the energy sector in Northern Spain. A comparative study on in-situ NO2 measurements shows that annual reductions at the surface are of 2-3% a(-1). This trend increases from rural polluted sites to remote areas (-4.5% a(-1)). The observed variability with station type may be explained by the increase in primary NO2 emissions combined with the representativeness of the measurement sites. Comparing country average trends in NO2 columns with national NO emission totals shows that these are generally within a factor 2 of each other. A better agreement was found for western European countries than for eastern European countries. The method described here is a promising methodology to complement and evaluate trends in NO2 columns and indirectly emission strengths. A strong advantage is the fact that the methodology using satellite data is in principle consistent throughout the entire domain. (C) 2014 Elsevier Inc. All rights reserved.