Assessment of the level of sea salt in PM10 in the Netherlands

Assessment of the level of sea salt in PM10 in the Netherlands Effect on yearly average and exceedance days The European air quality directive allows the subtraction of natural contributions from the levels of PM10. In 2005 a regulation for sea salt was implemented in Dutch legislation. The sea salt levels in this regulation now appear to be too high and will be adjusted. This is the result of an evaluation on the basis of new measurement data on sea salt. Sea salt contributes to the level of particulate matter in the air. Lower sea salt levels in The Netherlands The new data on the estimated amount of sea salt in the air are based on measured concentrations of sodium. Sodium is a more reliable source of information than the chloride concentrations that are used in the current Dutch method. Measurements of sodium in PM10 reference samples indicate that sea salt concentrations are nearly 50% lower than the levels estimated in the current regulation. The new estimate is based on the reference method for the sampling of particulate matter (PM10) and complies with the European regulations. Adjustment of the correction According to the directive the number of days with PM10 levels above 50 µg/m3 may not be larger than 35. Due to the natural contribution this number of days may be corrected. Currently a flat number of six exceedance days can be discarded for the whole country. With the new assessment this number of days will be lower. Also allows the new assessment a differentiation over the various Dutch provinces. Near the coast the number of days, that can be discarded due to the amount of sea salt, will now be four instead of six. In the eastern part of the country the number of days reduces from six to two. The effect of changing the method for sea salt reduction on the number of PM10 exceedances is presumably small. Measurement results in 2010 show no limit value exceedances even without sea salt correction. For 2011 some exceedances may be measured due to other weather conditions.