Trends of particulate matter (PM_2.5) and chemical composition at a regional background site in the Western Mediterranean over the last nine years (2002–2010)

Abstract. The time variability and long term trends of PM_2.5 (particulate matter of diameter < 2.5 μm) at various regional background (RB) sites across Europe are studied and interpreted in this work. Data on mean annual levels of PM_2.5 measured at Montseny (MSY, North East Spain) and various RB sites in Spain and Europe are evaluated and compared, and subsequently analysed for statistically significant trends. The MSY site registered higher average PM_2.5 levels than those measured at a selection of other RB sites across Spain, Portugal, Germany and Scandinavia by percentage compared to the mean of all the stations in these countries, but lower than those measured in Switzerland, Italy and Austria.

Reductions in PM_2.5 were observed across all stations in Spain and Europe to varying degrees (7–49%). MSY underwent a statistically significant reduction since measurements began, indicating a year-on-year gradual decrease (−3.7 μg m⁻³, calculated from the final year of data compared to the mean). Similar trends were observed in other RB sites across Spain (−1.9 μg m⁻³). Reductions recorded in PM_2.5 across Europe were varied, with many experiencing gradual, year-on-year decreases (−1.8 μg m⁻³). These reductions have been attributed to various causes: the introduction and implementation of pollution abatement strategies in EU member states, the effect of the current economic crisis on emissions of PM_2.5 and the influence of meteorology observed during the winters of 2009 and 2010. In addition, the North Atlantic Oscillation (NAO), a large scale meteorological phenomenon most prevalent during winter, was observed to influence the frequency of Saharan dust intrusions across the Iberian Peninsula.

Chemical composition of PM_2.5 at MSY is characterised by high levels of organic matter (OM) and sulphate, followed by crustal material, nitrate and ammonia. Sea Spray and elemental carbon (EC) comprised a minor part of the total PM_2.5 mass. Statistical trend analysis was performed on the various chemical components of PM_2.5 recorded at MSY to determine which components were accountable for the decrease in PM_2.5 concentration. It is shown that OM underwent the largest decrease over the time period with a statistically significant trend (−1.3 μg m⁻³ compared to the mean), followed by sulphate (−0.8 μg m⁻³), ammonium (−0.5 μg m⁻³) and nitrate (−0.4 μg m⁻³). Conversely, sea spray, EC and crustal material reductions were found to be negligible.