Acid-extractable heavy metals in PM2.5 over Xi'an, China: seasonal distribution and meteorological influence.

To investigate the acid-extractable heavy metals in fine particulate matter (PM2.5) over Xi'an, China, 24-h PM2.5 samples were collected every 3 days from December 2015 through November 2016. The bioavailable fraction, termed here the bioavailability index (BI), of PM2.5-bound metal (As, Ba, Cd, Co, Cu, Mn, Ni, Pb, Ti, V, and Zn) and potential influencing factors, including relative humidity, temperature, air pressure, wind speed, visibility, PM2.5, and SO2 concentrations, were assessed in this study. The annual average PM2.5 concentration was 50.6 ± 35.6 μg m-3, 1.5 times higher than the Chinese national secondary standard. Zn, Ti, and As were the most abundant elements of those analyzed in the PM2.5 samples, accounting for 72.1% of total quantity. The seasonal variations and enrichment factor analysis of heavy metals revealed that coal combustion in winter was a crucial source of Pb, Co, Cu, and Zn; and dust resuspension in spring contributed considerable Mn, Ti, and V. The acid-extractable fractions of the measured metals varied. Pb, Cu, Mn, and Zn exhibited relatively high acid-extractable concentrations and BI values. Pb was mostly in the acid-extractable fraction in PM2.5, with a mean BI value of 66.7%, the highest in summer (69.8%) and lowest in winter (63.7%). Moreover, the BIs of PM2.5-bound heavy metals were inversely related to temperature and wind speed, whereas positively correlated with relative humidity, SO2, and PM2.5 concentration in this study. This study assessed the seasonal distribution and meteorological influence of acid-extractable heavy metals, providing a deeper understanding of atmospheric heavy metal pollution in Xi'an, China.