PM levels in the Basque Country (Northern Spain): analysis of a 5-year data record and interpretation of seasonal variations

Levels of PM observed at the air quality network from the Basque Country in 1996–2000 ranged from 16 μg PM10/m3 at regional background sites, to 35–40 μg TSP/m3 (equivalent to 25–30 μg PM10/m3) at urban background sites, to 40–48 μg TSP/m3 (30–40 μg PM10/m3) at roadside sites; to 50–64 μg TSP/m3 (35–50 μg PM10/m3) at industrial and heavy traffic sites. The EU daily and annual PM10 limit values for 2005 are not equivalent for the Basque Country, and consequently only the mean 1996–2000 PM levels from one station would exceed the 2005 annual limit value but most of them surpass n=35 exceedances of the daily limit value. The equivalent n to the 2005 annual limit value is around 80. Four major processes exert an influence on PM levels throughout the Basque territory: local and regional anthropogenic contributions, precipitation, African dust and European transport. PM at Llodio (an urban background site under industrial influence and mean PM10, PM2.5 and PM1 levels for 2001 of 34, 25 and 21 μg/m3) is mainly distributed in the fine mode: 74% of PM10 is constituted by PM2.5, and 64% of PM2.5 presents a diameter <1 μm. The particle size distribution of PM varies seasonally with the fine fractions prevailing in summer (PM2.5/PM10=80–90%) and the coarser increasing in winter (PM2.5/PM10=60–70%). Meso- and synoptic scale processes affecting global PM levels in the Basque Country have been identified (mainly pollution episodes, African, Atlantic and EU transport). The results obtained allowed us to evaluate the impact of the different types of PM episodes on ambient PM levels and particle size fractions.