Sources and processes affecting levels and composition of atmospheric aerosol in the western Mediterranean

[1] Airborne particulate composition in the western Mediterranean atmosphere was investigated by means of simultaneous sampling of PM10 (particulate matter <10 μm) at a rural site (Ebro basin) and an urban one (Barcelona) in eastern Spain in order to evaluate the natural and anthropogenic contributions. The major regional particulate pollutants were nitrate, sulfate, and carbonaceous aerosols, whereas sea salt and mineral dust accounted for the main natural contribution. Owing to the thermodynamic properties of nitrogenous species, the load of nitrate in PM10 attains a maximum in the cold season (30% of bulk PM10 as rural and urban means in winter and <15% in summer). Conversely, in summer, sulfate attains a maximum load in PM10 (15–35% of bulk PM10 in summer and 5–20% in winter at two sites). High-sulfate events in summer were associated with ozone episodes at the rural site, which suggests that photochemical processes play a significant role in the production of particulate pollutants. High mineral dust concentrations were simultaneously recorded at the two sites during African dust outbreaks, when the dust load accounted for 50–80% of bulk PM10 at the rural site and 20–45% at the urban site. Daily dust concentrations in the range 20–25 μg/m3 (40–50% of the daily EU limit value for PM10) are frequently recorded during these African events. Analysis of satellite observations and meteorology shows that African dust outbreaks occur over large areas in southern Europe and interferes with the monitoring of levels of anthropogenic PM10 aerosols regularly performed in the national air quality networks.