Investigation of airborne lead concentrations in relation to Asian Dust events and air mass transport pathways

In order to explain the influence of Asian Dust (AD) on metal concentration levels in different sized particles, the analysis of Pb in both particulate matters with an aerodynamic diameter less than 2.5 (PM2.5) and 10μm (PM10) fractions was made consecutively for every spring season during a four-year study period between 2001 and 2004. To explore the impact of AD, a comparative analysis was made to compile Pb measurement data in two different categories: (1) between the AD and non-AD (NAD) periods and (2) between fine and coarse fractions. The results of our analysis indicated that the concentrations of coarse PM fraction increased significantly during the AD period, whereas an increase in the fine PM fraction was of moderate degree. However, when Pb concentrations were concerned, the patterns tended to change quite considerably in both the time-period (AD and NAD) and the particle fractions group. The distribution of atmospheric Pb in the AD period can be explained in two contrasting manners: either preferential enrichment of Pb due to the intrusion of Pb-polluted fine particles or relative depletion of Pb due to the dominance of coarse particles with poor Pb contents. Comparison of the Pb data based on the back trajectory analysis indicates that the effect of AD can be quite selective between pollutant species. If Pb data collected during the entire study period are divided by transport pathways, the patterns derived for each individual route suggest that air masses retained in the local area are more effective to build up Pb concentration levels than those affected by long range transport process.