Trends in the aerosol load properties over south eastern Italy

Abstract

The long-term (2003-2013) variations in columnar aerosol properties at Lecce, a site representative of the central Mediterranean, have been analysed for trend assessment. The study focuses on aerosol optical thickness (AOT) at 340, 440, 500 and 1020 nm and Ångström exponent (AE) for the pair 440-870 nm, retrieved from a sun photometer operating within the Aerosol Robotic Network (AERONET). A non-parametric trend analysis of the monthly mean, median and upper and lower tails (90th and 10th percentiles) suggests that the aerosol load has decreased during the study period, while the mean particle size remained unchanged. The characteristic advections reaching the study site were found by clustering analysis of back trajectories at 500, 1500 and 3000 m. Despite the strong influence they have on aerosol load and particle size, neither of the trends in advection routes could explain the tendencies found in the columnar aerosol properties. However, trends in aerosol data by advection type allow understanding the overall trends. Aerosol properties under flows with high residence time over continental Europe present differences according to the specific residing area. More specifically, no trend is found when flows arrive from Ukraine and the Balkans, while under advections from north-western/central Europe there are downward trends in the background levels and a reduction of the fine fraction. Negative trends are also found under flows with high residence time over the Mediterranean and northern Africa, again with differences according to the residing area.