Size fractionated impactor sampling of aerosol particles over the Atlantic Ocean from Europe to Antarctica as a methodology for source identification of Cd, Pb, Tl, Ni, Cr, and Fe

Abstract

Four different samples of aerosol particles over the North Atlantic, South Atlantic, an area near the equator influenced by Saharan mineral dust, and the Antarctic Ocean were collected on board of the German research vessel “Polarstern” by a six stage cascade impactor system. A continental sample, typical in its size distribution pattern and heavy metal enrichment factors (relative to crust material) for industrialized areas, was used for comparison. To analyse the elements of interest, isotope dilution mass spectrometry (IDMS) using the thermal ionization technique was applied. The samples were digested with nitric acid followed by an electrodeposition of the heavy metals to be analysed in alkaline solution. Source identification could be carried out by the distribution of the heavy metals and enrichment factors on the different impactor stages using iron as a reference element for crustal origin. Two opposite types of size distribution patterns were obtained over the Atlantic Ocean. On the one hand, the main heavy metal fraction was found to be associated with the smallest particles collected on the last two impactor stages and the back-up filter with aerodynamic diameters (AD) of less than 0.95 μm. This pattern together with the high enrichment factors of up to several thousand indicates combustion processes and biogenic emissions as possible sources and were typical for cadmium and lead. On the other hand, chromium and iron were preferably associated with the larger particles of >1.5 μm AD. This and the low chromium enrichment factor demonstrate that the earth crust is the major source for these two elements in marine aerosol particles. Thallium and nickel could not be classified by one of these two size distribution patterns, which indicates that at least two different primary sources contribute to the content of these heavy metals in marine aerosol particles depending on the region investigated. Contrary to that, the sample collected over the Antarctic Ocean showed some significant differences. Here, a substantial amount of the total cadmium and lead was associated with the larger particles. However, relatively high enrichment factors found for cadmium, nickel, lead, and thallium in the smallest particles suggest a natural source, probably biogenic activities, in the Antarctic Ocean.