Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China.
Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China; University of Chinese Academy of Sciences, Beijing 100049, China.
Environ Pollut. 2021 Feb 1;270:116089. doi: 10.1016/j.envpol.2020.116089. Epub 2020 Nov 19.
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) 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 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) 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 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 may have weakened the BC absorption and accelerated the decrease in light absorption resulting from the reduction in rBC loading.
黑碳（BC）是最重要的气溶胶吸光成分，其辐射强迫效应取决于其微物理特性。本研究调查了 2013 年至 2019 年北京城区难熔黑碳（rBC）的两个微物理参数，即粒径分布和混合状态，以了解近年来源变化的影响。rBC 的质量等效直径（D）在所有季节均呈双峰对数正态分布，主要模态占 rBC 质量的大部分（>85%）。冬季（209nm）的质量中值直径（MMD）明显大于夏季（167nm），可能是由于来自固体燃料燃烧的更大 D 的 rBC 以及冬季污染时 rBC 凝聚增强的贡献。冬季 rBC 颗粒的覆盖度较厚，厚覆盖 rBC 的数分数（f）在 29%-48%之间，而夏季在 12%-14%之间。然而，冬季并未观察到 BC 吸光能力的明显增加。这一发现可能与冬季较大 rBC 的吸收效率较低有关，这部分抵消了涂层引起的光增强。观察到 MMD 和 f 的两个阶段下降，同时 rBC 负荷持续下降，反映了源控制措施对 rBC 负荷和物理性质的不同影响。在 2016 年之前的早期阶段，控制措施更有效地降低了 rBC 负荷，但对 rBC 的微物理性质影响较小。自 2016 年以来，由于大气负荷低，rBC 浓度的降低速度放缓。然而，rBC 的核心尺寸明显减小，涂层减少。f 的降低可能削弱了 BC 的吸收并加速了 rBC 负荷降低导致的光吸收减少。
