Concentrations and light absorption characteristics of carbonaceous aerosol in PM2.5 and PM10 of Lhasa city, the Tibetan Plateau

Light absorption properties of carbonaceous aerosol strongly influence the Earth's radiative balance, yet the related knowledge is limited for the Tibetan Plateau (TP), the highest and largest plateau in the world. In this study, organic carbon (OC), elemental carbon (EC) and water soluble organic carbon (WSOC) of PM2.5 and PM10 of Lhasa collected from May 2013 to March 2014 were studied. It showed that daily-average concentrations of OC, EC and WSOC of PM2.5 and PM10 were lower than those of other megacities. Lhasa PM2.5 was characterized by low OC/EC ratio (1.46 ± 0.55), which was similar to that of Lhasa roadside PM2.5 (1.25 ± 0.45), reflecting mainly direct influence of primary emissions and less secondary formation. Hence, although Lhasa atmosphere is relatively clean, it is intensively influenced by local vehicle emissions. Mass absorption cross-section of EC (MACEC) for both PM2.5 and PM10 at 632 nm were 7.19 ± 1.19 m2 g−1 and 7.98 ± 2.32 m2 g−1, respectively, both of which had similar variation patterns to OC/EC and secondary OC (SOC)/OC, indicating that the increase of MACEC might be caused by coating with organic aerosol. Additionally, the loading of EC for both PM2.5 and PM10 showed logarithmic relationships with those of optical attenuation (ATN) of EC, implying that the shadowing effect enhanced logarithmic with increased EC concentration. MAC of WSOC at 365 nm for PM2.5 (0.74 ± 0.22 m2 g−1) and PM10 (0.78 ± 0.21 m2 g−1) were also close to reported values of other cities mainly influenced by fossil combustion. Additionally, attenuation at 365 nm of WSOC of both PM2.5 and PM10 showed the same relationship with their WSOC concentrations, implying no difference for light absorption properties of WSOC for these two grain sizes.