Modelling of sea salt pollution over Europe: key uncertainties and comparison with observations

Abstract

Abstract. Sea spray can significantly affect the air quality. Sea salt can cause enhanced concentrations of particulate matter and change particle chemical composition, in particular in coastal areas, and therefore should be accounted for in air quality modelling. We have used an EMEP Unified model to calculate sea salt concentrations and depositions over Europe, focusing on studying the effects of uncertainties in sea salt production and lifetime on calculation results. Model calculations of sea salt have been compared with EMEP observations of sodium concentrations in air and precipitation for a four year period, from 2004 to 2007, including size (fine/coarse) resolved EMEP intensive measurements in 2006 and 2007. In the presented calculations, sodium air concentrations are between 8 and 46% overestimated, whereas concentrations in precipitation are systematically underestimated by 65–70% for years 2004–2007. Unfortunately, thus far performed tests have failed to give a clear answer regarding the reason for this underestimation and further studies are needed. The model is found to reproduce fairly well the spatial distribution of Na+ in air and precipitation over Europe, and to capture most of sea salt episodes. The paper presents the main findings from a series of tests in which we compare several different sea spray source functions and also look at the effects of meteorological input and the efficiency of removal processes on calculated sea salt concentrations. Finally, sea salt calculations with the EMEP model have been compared with results from the SILAM model and observations for 2007. While the models produce fairly close results for Na+ at most of 26 measurement sites, discrepancies in terms of bias and temporal correlation are also found. Those differences are probably due to differences in the representation of source function and treatment of sea salt aerosol, and also due to different meteorology used for model runs and due to a finer grid resolution of the SILAM model compared to EMEP. This study contributes to getting a better insight on uncertainties associated with sea salt calculations with the EMEP model and towards further improvement of EMEP aerosol modelling.