Significant impact of carbonates on the concentration and light absorption properties of black carbon in Lanzhou, northwest China

Abstract

The concentrations of organic carbon (OC) and elemental carbon (EC, also known as black carbon) are often overestimated by carbonates during the thermal-optical measurement, leading to an underestimation of EC's light absorption ability, especially in northwest China. To investigate this influence, a comparison between untreated and acid-treated total suspended particle (TSP) samples was investigated in Lanzhou. The overestimation of EC concentration due to acid treatment was approximately 17.47 ± 1.09 %. After correction, the overestimations of OC and EC concentrations by carbonates were approximately 9.26 ± 10.76 % and 19.68 ± 14.34 %, respectively. This overestimation of EC concentration by carbonates was highest in spring, driven by dust storms, and lowest values in winter, revealing a contrasting trend to atmospheric temperature. The actual concentrations of OC and EC in Lanzhou were the lowest in summer and highest in winter, respectively. OC/EC ratios and EC concentrations overestimated by carbonates were significantly positively correlated, indicating that mineral dust played an important role in the composition of carbonaceous aerosols, especially during spring dust storm. After correcting for the overestimated EC concentrations, the mass absorption cross-section of EC (MAC_EC) in untreated samples was only about 72.97 ± 11.48 % of that in acid treated samples at 660 nm, indicating a significant underestimation of MAC_EC values due to carbonates. This study provides the first investigation of actual carbonaceous aerosols concentrations and MAC_EC values in northwest China, providing a more accurate understanding for the climatic effects of carbonaceous aerosols.