1Department of Applied Physics, Eindhoven University of Technology, Eindhoven, the Netherlands
2Climate Observations, Royal Netherlands Meteorological Institute, De Bilt, the Netherlands
3Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Canada
4Department of Atmospheric and Oceanic Sciences & Laboratory for Climate and Ocean-Atmosphere Studies, School of Physics, Peking University, Beijing 100871, China
Abstract. We present a top-down ship NOx emission inventory for the Baltic Sea, North Sea, Bay of Biscay and Mediterranean Sea, based on satellite observed tropospheric NO2 columns of the Ozone Monitoring Instrument (OMI) for 2005–2006. We improved the representation of ship emissions in the GEOS-Chem chemistry transport model, and compared simulated NO2 columns to consistent satellite observations. Relative differences between simulated and observed NO2 columns have been used to constrain ship emissions in four European seas (Baltic Sea, North Sea, Bay of Biscay and Mediterranean Sea). The constrained ship tracks account for 39% of total top-down European ship NOx emissions, which amounts to 0.96 Tg N for 2005, and 1.0 Tg N for 2006 (11–15% lower than the bottom-up EMEP ship emission inventory). Our results indicate that EMEP emissions in the Mediterranean Sea are too high (by 60%) and misplaced by up to 150 km, which can have important consequences for local air quality simulations. In the North Sea, our top-down emissions amount to 0.05 Tg N for 2005 (35% lower than EMEP). Increased top-down emissions were found for the Baltic Sea and Bay of Biscay, with emission totals of 0.05 Tg N (131% higher than EMEP) and 0.08 Tg N for 2005 (128% higher than EMEP), respectively. Our study explicitly accounts for the (non-linear) sensitivity of satellite retrievals to changes in the a priori NO2 profiles. Although the effect of this sensitivity might be minor for small emission increments, our findings stress the need for consistent information in satellite retrieval and model, as satellite observations are never fully independent of model information (i.e. assumptions on vertical NO2 profiles). Our study provides for the first time a space-based top-down ship NOx emission inventory, and can serve as a framework for future studies to constrain ship emissions using satellite NO2 observations in other seas.