Atmospheric trace metals over the south-west Indian Ocean: Total gaseous mercury, aerosol trace metal concentrations and lead isotope ratios

摘要

Atmospheric concentrations of trace metals (Al, Fe, Ba, Mn, Co, V, Cr, Mo, Sr, Pb, Cd, As, Zn, Cu and Ni) and major ions (Na~+, Mg~(2+), Ca~(2+), K~+, NH_4~+, Cl~-, NO_3~-, SO_4~(2-) and oxalate) were analysed in aerosol and rain samples collected during an Indian Ocean cruise from Seychelles to Mauritius. Semi-continuous total gaseous mercury (TGM) measurements were also made during the cruise. Data for trace metals in the marine atmosphere is sparse for the southern hemisphere, particularly for Hg, and these are the first reported TGM concentrations for this part of the Indian Ocean. TGM was 1.05-1.51 ng m~(-3) during the cruise (average 1.2 ng Hg m~(-3)), towards the lower end of mercury levels observed over oceans in the northern hemisphere. The TGM data appears to show a concentration gradient over the cruise with increasing concentrations at more northern locations closer to the inter-tropical convergence zone. This trend was also observed in some of the trace metals in aerosols. Air mass back trajectories suggest the air encountered had spent at least five days over the ocean prior to collection. Enrichment factors were calculated for the metals in the aerosols which suggested As, Pb, Ni, Cu, Cd, Zn and to some extent Cr were enriched above both crustal and oceanic sources. The total metal concentrations were measured in the aerosol samples along with the fraction soluble in pH 4.7 buffer and pH 1 nitric acid. For Fe these fractions were <5% and~13% respectively. The solubility of Al and Fe was significantly higher in pH 1 nitric acid than pH 4.7 buffer, but for the other metals studied there was no significant difference. Most of the Cd, Zn and As in the aerosols was readily soluble in both pH 1 nitric and pH 4.7 buffer. Nickel, Cu and Pb solubility in these solutions was more limited (in pH 4.7 buffer 63, 72 and 69%; in pH 1 nitric acid 60,72 and 57% respectively). Stable lead isotope ratios were also measured in the aerosol samples to investigate their origin. For most of the aerosol samples, the Pb isotopes clustered within a small range of fairly radiogenic values (~(207)Pb/~(206)Pb 0.84-0.85.~(208)Pb/~(206)Pb 2.03-2.07). No significant trend was found for Pb isotopes with aerosol size fractions. The isotope ratios are consistent with an increased importance of coal combustion as a source of Pb over the Indian Ocean.