Impact of aging process on atmospheric black carbon aerosol properties and climate effects

Abstract

Black carbon (BC) is a key driver of global warming, exerting a strong but highly uncertain effect on the global climate. A key challenge of quantifying BC’s radiative effect is predicting the impact of aging process on BC’s properties. BC is mainly emitted from the incomplete combustion of fossil fuel and biomass burning. After being emitted, it undergoes complex chemical and physical transformations in the atmosphere, through condensation, coagulation and heterogeneous oxidation, which is referred to as BC aging. During aging, BC is gradually coated by other chemical species, which dramatically affects BC properties and alters the corresponding climate effects. The objective of this review is to discuss the current knowledge of BC aging from both laboratory and field measurements. This review first summarizes the methods used in the laboratory experiments and field observations. Then it reviews the effects of aging process on BC mixing state, which comprises both morphology and chemical composition. On this basis, changes to BC light absorption, hygroscopicity, cloud condensation nuclei (CCN) and ice nucleation (IN) activity during the aging process are discussed; these properties are closely related to BC climate effects. Laboratory studies on BC aging are always conducted using single-precursor systems, which do not capture the complex composition of the real atmosphere. In field studies, some state-of-the-art online instruments have been deployed to measure BC properties. However, since the field studies are usually conducted at fixed stations, it is difficult to dynamically track the entire BC aging process. Atmospheric aging process converts BC particles from externally mixed to internally mixed, with other chemical species coated on them. The morphology evolution in the aging process contains three stages: void filling, collapse and subsequent growth, which results in BC morphology changing from the initially chain-like shapes to spherical core-shell structures. The chemical composition of coatings is affected by both emission source and extent of aging, showing an increasing proportion of secondary species during the aging process. BC light absorption is enhanced because of coatings that accumulate during aging. However, the absorption enhancement factors reported in previous studies show large discrepancy because of differences in aging degree, morphology and chemical composition. The particle-to-particle heterogeneity in mixing state can also lead to different enhancement factors for particle populations. Along with the enhancement of light absorption, BC particles hygroscopicity and CCN activity are also enhanced during aging process. However, observational evidence is still lacking. The IN activity of BC is still an open question, and whether aging process enhances or suppresses the IN activity of BC is still debatable. Based on this review of previous studies, we recommend further research on the following topics: (1) Characterize the aging process for BC from different emission sources under realistic combustion conditions. (2) Identify the key factors affecting BC aging in various atmospheric environments. (3) Explore the role of BC particles in regional air pollution. (4) Conduct further studies on the interaction between BC and cloud formation.