Progress of the stable carbon and radiocarbon isotopes of black carbon aerosol

Abstract

Black carbon (BC) aerosol derived from incomplete combustion of fossil fuel (e.g., coal and liquid petroleum) and biomass (e.g., agricultural residue, biofuel, and forest) is an important atmospheric pollutant globally. Besides presenting potential risks to public health, BC strongly absorbs solar radiation, decreases atmospheric visibility, heats the air, and is thought to be a key driver of climate warming. However, there is a large discrepancy between observations and models of the source apportionment and climate forcing of atmospheric BC globally, due to uncertain emission estimates. Stable carbon (13C) and radiocarbon (14C) isotope measurements are some of the most objective and accurate tools for apportioning atmospheric BC into key emission sources due to their unique fingerprint features. This review focuses on the principle of source apportionment for BC using 13C and 14C measurements, and the recent important progress in China, one of the largest emitters of BC. As expected, fossil fuel combustion is the largest contributor (70%−90%) of BC in many Chinese cities, mainly due to the huge consumption of coal and petroleum. However, the impact of biomass burning on BC can reach ~50% in some cities and even higher (~70%) in some remote sites, strongly reflecting the regional features of BC emission sources in China. Moreover, the largest and smallest contributions of fossil fuel combustion to BC are generally in summer and winter, respectively, highlighting the important influence of biomass burning on atmospheric BC during the cold season. The combined 13C and 14C measurements show that atmospheric BC in Northern China is mainly associated with coal combustion, followed by the burning of petroleum and biomass, which is very different from other regions where BC is typically dominated by petroleum burning. This implies that coal combustion is much more common and intensive in Northern China than in other regions. A strong correlation ( R 2 = 0.81) is observed between the 14C signal and atmospheric levoglucosan concentration in winter, while the correlation is very weak ( R 2 = 0.19) in summer. These results indicate that levoglucosan is chemically stable in the cold, but is degraded to a large extent in the atmosphere in summer. Compared with the levoglucosan concentration in winter, we roughly estimate that ~24% and ~70% of levoglucosan would be chemically degraded in spring and summer, respectively, which is consistent with chamber-based results (30%–75%). Therefore, we confirm that levoglucosan cannot quantify the impact of biomass burning on atmospheric BC in hot seasons. The source apportionment of BC based on emission-inventory technology is variable. In China, the contribution of biomass burning to BC could be as low as ~20% or as high as ~50%, depending on the emission inventory. Given the excellent ability of 13C and 14C isotopes to quantify atmospheric BC, this review notes that a combined 13C and 14C measurement technique can serve as a good “referee” to guide the improvement of the BC emission inventory and deepen our understanding of the environmental and climatic effects of BC aerosols. Future research should (1) establish a 13C-BC database for different combustion-related emission sources and regions, (2) bridge the emission inventory of BC and isotope-based observations using atmospheric chemistry models, (3) comprehensively explore the atmospheric stability of levoglucosan using carbon isotopes, and (4) build an atmospheric observation network for BC carbon isotopes covering urban, rural, and remote areas in China.