Relationships between columnar aerosol optical properties and surface particulate matter observations in north-central Spain from long-term records (2003–2011)

Abstract. This work examines the relationships between Aerosol Optical Depth (AOD) and Particulate Matter (PM_X) parameters, based on long records (2003–2011) of two nearby sites from the AERONET and EMEP networks in the north-central area of Spain. The climatological annual cycle of PM₁₀ and PM_2.5 present a bimodality which might be partly due to desert dust intrusions, a pattern which does not appear in the annual cycle of the AOD. In the case of the AOD, this bimodality is likely to be masked because of the poor sampling of sunphotometer data as compared to PM_X (67% of days against 90%), and this fact stresses the necessity of long-term observations. In monthly series, significant interannual variations are observed and most extrema coincide, however the bimodal shape remains relatively stable for PM_X. Significant and consistent trends were found for both datasets likely associated to a decrease of desert dust apportionment until 2009. PM₁₀ and AOD daily data are moderately correlated (0.56), a correlation improving for monthly means (0.70). In the case of strong desert dust events day-to-day correlation is not systematic, therefore an extensive analysis on PM_X, fine-PM ratio, AOD and associated Ångström exponent (α) is carried out.