Enhancements of airborne particulate arsenic over the subtropical free troposphere: impact of southern Asian biomass burning

Abstract

Arsenic (As) has long been recognized as a toxic element of mainly anthropogenic origins, having adverse effects on human health. However, there is insufficient understanding regarding As released into atmosphere from biomass burning (BB). To this end, daily airborne As concentrations in total particulate matter (TSP) were determined at Mount Hehuan (24.16° N, 121.29° E, 3001 m a.s.l.), Taiwan from September 2011 to September 2012. During the sampling period, As concentrations varied from 0.02 to 5.9 ng m−3, with a mean value of 0.5±1.0 ng m−3. Significantly seasonal variations of As were found over the subtropical free troposphere, and higher As concentrations were observed in the southern (S) and southeastern (SE) Asian BB seasons (from January to May). Principal component analysis (PCA) results showed that BB activities seemed to be a major source of As during the S and SE Asian BB periods, which were very distinct from the major source of coal-fired power plant during the periods between July and December. Based on backward trajectory analyses and WRF-Chem model simulations, we found that the high As concentrations during the BB periods were attributed to the biomass burning activities over S Asia where groundwater, soil and crops are severely contaminated by arsenic. A strong correlation (r = 0.73 p < 0.05) between As and potassium ion (K+, a chemical tracer of BB activities) in S Asian BB events also supported this hypothesis. During the S Asian BB events, the high As ∕ Pb ratios ( > 0.2) were also observed, indicating that burning crops contaminated by lead arsenate might be a crucial candidate for high As concentrations at Mount Hehuan. Nevertheless, the net influence of S Asian BB activities on airborne As concentrations has been estimated by comparing the differences of As concentrations on BB and non-BB days. On average, the difference in As concentrations was 1.0 ng m−3, which accounted for 63 % of the average As concentration on BB days. Moreover, a ratio of ΔAs∕ΔCO ( ∼ 0.00001) in the S Asian BB events was obtained. Using this value, arsenic emissions from S Asian BB activities were estimated to be 0.17 tons yr−1, resulting in high airborne As concentrations over the subtropical free troposphere and impacting As cycles on a regional scale in the S and SE Asian BB seasons.