Effect of source variation on the size and mixing state of black carbon aerosol in urban Beijing from 2013 to 2019: Implication on light absorption

Abstract

Black carbon (BC) is the most important aerosol light-absorbing component, and its effect on radiation forcing is determined by its microphysical properties. In this study, two microphysical parameters of refractory BC (rBC), namely, size distribution and mixing state, in urban Beijing from 2013 to 2019 were investigated to understand the effects of source changes over the past years. The mass equivalent diameter of rBC (D_c) exhibited bimodal lognormal distributions in all seasons, with the major modes accounting for most (>85%) of the rBC masses. The mass median diameter (MMD) was obviously larger in winter (209 nm) than in summer (167 nm) likely due to the contribution of more rBC with larger D_c from solid fuel combustion and enhanced coagulation of rBC in polluted winter. More rBC particles were thickly coated in winter, with the number fraction of thickly coated rBC (f_coatBC) ranging within 29%–48% compared with that of 12%–14% in summer. However, no evidential increase in BC light-absorption capability was observed in winter. This finding was likely related to the lower absorption efficiency of larger rBC in winter, which partly offset the coating-induced light enhancement. Two stage of decreases in MMD and f_coatBC were observed, accompanied with a persistent decrease in rBC loading, thereby reflecting the discrepant effects of source control measures on rBC loading and physical properties. The control measures in the earlier stage before 2016 was more efficient to reduce the rBC loading but slightly influenced the microphysical properties of rBC. As of 2016, the reduction in rBC concentration slowed down because of its low atmospheric loading. However, rBC showed a more obvious decrease in its core size and became less coated. The decrease in f_coatBC may have weakened the BC absorption and accelerated the decrease in light absorption resulting from the reduction in rBC loading.

Introduction

Black carbon (BC) plays a predominant role in aerosol light absorption, affecting the radiative forcing of atmospheric aerosols (Stocker et al., 2013; Bond et al., 2013). China is considered one of the most important source regions of BC emissions, which contribute to half of Asian emissions and approximately a quarter of global emissions (Qin and Xie 2012; Li et al., 2017). In response to the high BC loading in the atmosphere from intense emissions, the regional climate over China has been demonstrated to be greatly influenced and modulated by BC aerosols (Menon et al., 2002; Zhang et al., 2009). The high BC loading also strengthens the “dome effect” induced by solar-radiation absorption and atmosphere heating, resulting in deteriorated regional air quality (Ding et al., 2016).

The optical absorption and radiative forcing of BC largely depends on its microphysical properties, including morphology, size distribution, and mixing state (Scarnato et al., 2013; Cheng et al., 2014; Liu et al., 2017). Although BC particles freshly emitted from traffic exhaust exhibit fractal-like aggregates comprising numerous primary spherules, their compactness increases and they are coated by other substances after aging in the atmosphere (Peng et al., 2016). BC particles from different sources (e.g., traffic exhaust, coal combustion, or biomass burning) also largely differ in their morphology, size, and mixing state (Schwarz et al., 2008; Liu et al., 2014; Wang et al., 2016a). These microphysical properties are also influenced by burning conditions (Pan et al., 2017; Wang et al., 2018a). Different mixing states of BC aerosols, i.e., externally mixed with other aerosols compared with well-coated scattering shell, could result in a difference in radiative forcing by a factor of two (Jacobson, 2001). Moreover, although BC is hydrophobic itself, coating by water-soluble substances could also increase the hydrophilicity of BC particles and then influence its interaction with cloud and lifetime in the atmosphere, resulting in alterations in its climate effect (Hodnebrog et al., 2014).

BC reduction is considered a practicable way to mitigate global warming (Ramanathan et al., 2007) and improve regional air quality (Ding et al., 2016). Over the past decades, particularly since 2013 when severe haze events frequently swept across China, the national government has exerted numerous efforts to reduce fine particulate matters (PM_2.5) to which most of BC belongs. Benefiting from strict control measures, BC has clearly decreased in most of China (Zhang et al., 2019), accompanied by a decrease in PM_2.5 (Wang et al., 2017; Zheng et al., 2017, 2018; Fan et al., 2020). As one of the most polluted regions (An et al., 2019), the North China Plain (NCP) exhibits a more effective improvement in air quality owing to the stricter control measures than other regions. The annual mean PM_2.5 concentration decreased by 35% from 2013 to 2017 in Beijing, a megacity in the NCP (Cheng et al., 2019). A more significant decrease in BC was observed in Beijing, with the annual mean concentration decreasing by 61% from 2005 to 2017, which was largely attributed to the reduction in source emissions (Xia et al., 2020). However, the variations in the microphysical properties of BC, which determine its light-absorption properties, in response to the control measures remain unclear. Short-term control measures proposed during the 2014 Asia-Pacific Economic Cooperation (APEC) meeting in Beijing have shown a weakened effect on BC light-absorption capability with decreased coating thickness (Zhang et al., 2018c). However, variations in BC mixing state at a longer time scale, e.g., year-by-year variation, are rarely studied. These variations are important to evaluate the climate and environment effects of BC. In addition, traffic-related BC has been demonstrated to have smaller size than BC originated from coal combustion and/or biomass burning (Schwarz et al., 2008; Liu et al., 2014; Wang et al., 2016a). Alterations in BC size are also likely to occur owing to variations in emission-source contribution to BC arising from the control measures, thereby further affecting the light-absorption properties (Gyawali et al., 2009).

To better understand the impacts of source alteration on the microphysical properties of BC, in the present study, we focused on the size distribution and mixing state of refractory BC (rBC) in urban Beijing based on a series of single-particle soot photometer (SP2) measurements from January 2013 to July 2019. Their seasonal and secular changes were also examined. The potential impact of variation in rBC size and mixing state on light-absorption capability was discussed. Compared with previous studies on rBC microphysical properties based on SP2 measurements, which were mostly conducted in one or several short periods (with each period not exceeding ∼1 month), this study provided comprehensive insights into the size distribution and mixing state of rBC in urban Beijing and its response to source-emission controls. Such insights are also essential to modeling studies related to the evaluation of the climate and environment effects of BC.