Abstract：Different transport mechanisms affect the background concentration of PM2.5 in Beijing. In this study, two different transport pathways around Beijing (i.e., transport from northern areas and transport from southern areas) are classified using a synoptic-airmass diagnose approach. PM2.5 concentrations at Shangdianzi (a background station in Beijing) were examined for each of the transport pathways. During 2006~2012, when the airmasses were from the northern areas (northern transport), background concentrations of PM2.5 varied from 10.3 to 13.5μg/m3, with the mean value of (11.7±1.3)μg/m3, while the concentrations varied from 60.2 to 92.6μg/m3, with the mean value of (76.2±12.8)μg/m3when the airmasses were from the southern areas(southern transport). Southern transport contributed 60% to total PM2.5 concentration from 2006 to 2009 and such contribution decreased gradually to 44% in 2012. Background concentrations of PM2.5 at Shangdianzi in the presence of northern transport did not show obvious inter-annual trend. On the contrary, background concentrations of PM2.5 in the presence of southern transport decreased substantially during 2006~2012 with a rate of -5.5μg/(m3×a). Increased contribution from secondary PM2.5 (i.e., produced by local photochemical reactions) during 2009~2012 offset the decreasing contribution of southerly regional transport to a certain extent, leading to a slight change of the annual averaged PM2.5.
