Using 137Cs and 40K to identify natural Saharan dust contributions to PM10 concentrations and air quality impairment in the Canary Islands

Tenerife (Canary Islands) is often affected by Saharan mineral dust outbreaks. These events result in high PM10 concentrations in ambient air, well above the limitingvalues adopted by the European Union (Directive EU/1999/30). To comply with the EU Air Quality Directive, a quantitative proxy for the characterisation of high PM10 events is required. Furthermore, this proxy must be capable of differentiating natural sources (as defined by the Directive), such as Saharan mineral dust outbreaks, from other anthropogenic sources. In this study, we have analysed the time series of Cs-137, K-40 and PM10 recorded at the island ofTenerife (2000-2006) in the Marine Boundary Layer (MBL), to test the possible usefulness of the two mentioned radiotracers as markers of Saharan mineral dust events. The results of the analysis showed that this is indeed the case for 137Cs and that there are, at least, two different sources of 4 K concentrations in the collected atmospheric aerosol filters. While the detection of (CS)-C-137 in atmospheric aerosol filters could be directly associated with the arrival of low altitude (< 2000 m.a.s.l.) dust intrusions to the study site, 4 K concentrations could be associated, in most cases, with either low or high altitude (> 2000 m.a.s.l) dust intrusions. Moreover, two different mechanisms of aerosol loading in the MBL were, also, identified with the mentioned radiotracers. Good correlations (R-2 > 0.6) were found between the three parameters when all the compiled data was considered. The correlation values increased to nearly 0.8 when only those samples that had Cs-137 above detection limits were considered. The highest concentrations of Cs-137, K-40 and PM10 were recorded in March 2004 in connection with a very intense low altitude Saharan dust intrusion. The results of this study, also, indicated that the (CS)-C-137, (40) K ratio could be potentially useful to differentiate low from high altitude dust intrusions at this site. (c) 2008 Elsevier Ltd. All rights reserved.