Factors affecting the seasonal variation of mass and ionic composition of PM2.5 at a central Mediterranean coastal site

Abstract

Mass and ionic composition of PM2.5 were determined for 1-year period (11/2004–10/2005) in a rural coastal Mediterranean site. The annual mean PM2.5 concentration measured was found to be 13.9 μg m −3. Smaller values were observed during the cold and rainy period compared to the warm and dry one. This may be ascribed to the scavenging of PM2.5 by rainfall during the cold period, in conjunction with the enhanced resuspension of soil dust and increased secondary aerosol formation during the warm period. Weekday/weekend variations and segregation of PM values according to wind direction, showed that local sources are not significant factors controlling the particle levels. Long-range transport is the dominant factor. Short distance to sampling site air paths traveling near ground resulted to larger PM values. Sulfate was found to be the main ionic species with annual mean concentration of 3.2 μg m −3, contributing 23.3% to the measured PM2.5 mass. It exhibited a seasonal variation with higher values in the summer months. The contributions of nitrate and chloride to the PM2.5 mass were 3.3% and 1.2%, respectively. Ammonium had an annual mean concentration of 1.6 μg m −3 and Ca 2+ 0.33 μg m −3. They were the dominant cations contributing 11.2% and 2.7%, respectively, to the measured PM mass. In total the ionic species accounted for 45% of the PM2.5 mass. The calcium ions indicate that crustal materials are also important contributors to PM2.5 in this region, even though their major occurrence would be in the coarse mode. Finally Na +, K + and Mg 2+ had annual mean concentrations 0.24, 0.19 and 0.02 μg m −3, respectively. They contributed 3.1%, 0.34% and 0.18%, respectively, to the measured particulate mass. Anthropogenic contribution was calculated to account for 40% of the PM2.5 mass, crustal material (dust) for 25% and marine aerosol 4.3%.