弄清大气细粒子混合状态（含黑碳气溶胶（BC）的比例）及其老化对光吸收增强（E_abs）的影响对准确评估其对空气质量和气候的影响至关重要。本研究使用挥发性串联差分电迁移率分析仪（VTDMA）于2019年冬季和夏季在北京进行了分粒径大气细颗粒物挥发性的外场观测，以测量的挥发性塌缩因子（VSF），反演和量化了大气中不含BC气溶胶（Non-BC）、内混BC气溶胶（In-BC）和外混BC气溶胶（Ex-BC）的数浓度和质量浓度；进一步研究了观测期间因BC老化引起的包裹层厚度，结合Mie散射理论计算了其潜在的吸收增强E_abs；并与2016年北京冬季的黑碳气溶胶质谱仪（SP-AMS）观测资料所获得的含BC气溶胶的E_abs进行对比，探究了E_abs和BC包裹厚度及包裹化学组分的关系。取得的主要结论如下：

（1）冬季和夏季，VSF的概率密度函数（VSF-PDF）呈双峰分布，具有高挥发模态和低或不挥发模态。平均而言，大气细粒子（40-300 nm）夏季比冬季的挥发性更大。夏季，VSF的昼夜变化更为明显，高挥发模态在中午和午后突出，而低挥发模态在夜间突出，说明了日间成核或新粒子生成过程易产生更多挥发性粒子。在冬季观测中没有出现类似的昼夜特征。

（2）夏季，由成核过程产生的Non-BC颗粒占主导，对气溶胶总数浓度的贡献为52-69%。反演得到的颗粒物混合状态的昼夜变化特征进一步表明，迅速的光化学过程（如成核）是Non-BC颗粒的主要来源。In-BC数浓度和数分数的日变化不显著，表明在北京，BC的老化速率可能一天中都比较恒定。除局地车辆排放和化石燃料不完全燃烧使Ex-BC在早晚高峰占比较大之外，Ex-BC的数浓度日变化主要受行星边界层（PBL）高度变化影响。

（3）冬季，含BC颗粒的数浓度占环境细颗粒物的60-78%，In-BC和Ex-BC分别占51-64%和9-23%。仅在成核过程为主导的清洁天，Non-BC数浓度才占主导（~54%）。 进一步结合BC质量浓度的反演结果表明，大气中大量细粒子均为含BC颗粒，占了总细颗粒质量分数的32-52%，显示了因BC老化过程使得BC对污染城市区域颗粒物的质量浓度贡献不可忽视。此外，北京冬季BC颗粒包裹层较厚，D_p/D_c（300℃加热前后BC颗粒粒径之比）为1.6-2.2，说明北京冬季大气BC老化程度较高。

（4）进一步与2016年北京冬季的SP-AMS观测资料进行对比，证实了北京冬季BC的包裹层较厚，R_BC（包裹层与BC核质量之比）平均值达到4.7。并且发现含BC气溶胶中有机物（Org）质量占比最高（~60%），其次是硝酸盐（NO₃^-，~9%），硫酸盐（SO₄^2-，~6%）和铵盐（NH₄⁺，~5%）。 观测期间，BC光吸收增强E_abs变化波动较大；E_abs在一天中的平均变化趋势与NO₃^-/BC较一致，说明NO₃^-对E_abs增加的贡献可能作用显著。随R_BC增加，E_abs逐渐增加，说明BC老化被包裹之后吸光逐渐增加，这基本与基于Mie散射理论估算的E_abs一致。E_abs与NO₃^-/BC呈正相关关系，说明NO₃^-包裹显著增强了BC核的光吸收，证明了在相同包裹厚度下化学组成的差异会对E_abs造成影响。实验室和外场观测结果均显示，含BC粒子的E_abs高度依赖于D_p/D_c的变化。不同化学成分构成的BC包裹层可能会带来光吸收增强的巨大差异。以上研究结果有助于理解BC在区域雾霾形成和气候强迫中的作用，为模式模拟改进提供理论依据。

Understanding the mixing state of atmospheric fine particles (the proportion of black carbon (BC)-containing aerosols) and the effect of aging on the light absorption enhancement (E_abs) is critical to accurately assessing its impact on air quality and climate. In this study, the volatile tandem differential mobility analyzer (VTDMA) was used in the winter and summer of 2019 to conduct field campaigns of size-resolved particles volatility in Beijing. The measured volatility shrink factor (VSF) is used to retrieve and quantify the number and mass concentrations of non-BC-containing particles (Non-BC), internally mixed BC particles (In-BC) and externally mixed BC particles (Ex-BC) in the atmosphere. The coating thickness due to the aging of BC was further studied, and the potential absorption enhancement E_abs was calculated based on the Mie scattering theory, which was compared with the E_abs obtained from the soot particle aerosol mass spectrometer (SP-AMS) in the winter of 2016 in Beijing, and the relationship between E_abs and BC coating thickness and the chemical compositions of the coating was explored. The main conclusions obtained are as follows:

(1) In the winter and summer, the probability density function of VSF presents a bimodal distribution, with a high volatile mode and a less or non-volatile mode. On average, atmospheric fine particles (40-300 nm) are more volatile in the summer than in the winter. In the summer, the diurnal variation of VSF is more obvious. The high-volatile mode is prominent at noon, while the less-volatile mode is prominent at night, indicating that daytime nucleation or new particle formation processes are prone to produce more volatile particles. No similar diurnal features appeared in the winter observations.

(2) In the summer, Non-BC produced by the nucleation process dominate and contribute 52-69% to the total number concentrations of aerosols. The diurnal characteristics of the retrieved mixing state of particles further indicate that rapid photochemical processes (such as nucleation) are the main source of Non-BC. The diurnal variations of the number concentrations and fractions of In-BC are not significant, indicating that in Beijing, the aging rate of BC may be relatively constant throughout the day. Except for local vehicle emissions and incomplete combustion of fossil fuels, which make Ex-BC a larger proportion in the morning and evening rush hours, the diurnal variation of Ex-BC number concentration is mainly affected by changes in the height of the planetary boundary layer (PBL).

(3) In the winter, the number concentration of BC particles accounted for 60-78% of the environmental fine particles, with In-BC and Ex-BC accounting for 51-64% and 9-23%, respectively. Only on clean days when the nucleation process is dominant, the Non-BC dominates (~54%). The retrieved results combined with the BC mass concentrations show that a large number of fine particles in the atmosphere are BC-containing particles, with mass fraction of 32-52%, suggesting the dominant role of BC in elevating mass concentration of particulate matter (PM) in a polluted urban area. In addition, the BC particles are thickly coated with coating thickness (characterized by D_p/D_c, ratio of the BC diameter before and after heating at 300°C) of 1.6-2.2, implying efficient aging of BC particles in polluted urban area.

(4) Further comparison with the observation data from the SP-AMS in the winter of 2016 in Beijing confirmed that, the BC in the winter of Beijing was thickly coated, with an average R_BC (the mass ratio of coatings to BC core) of 4.7. The organics (Org) component in BC-containing aerosols was the highest (~60%), followed by nitrate (NO₃^-, ~9%), sulfate (SO₄^2-, ~6%), and ammonium salt (NH₄⁺, ~5%). During the observation period, the BC E_abs fluctuates greatly; the average trend of E_abs in daytime is more consistent with that of NO₃^-/BC, indicating the contribution of nitrate to the increase of E_abs may be significant. With the increase of R_BC, E_abs gradually increased, indicating that the light absorption gradually increased after BC was aging and coated, which is basically consistent with the E_abs estimated based on the Mie scattering theory. E_abs has a positive correlation with NO₃^-/BC, indicating that the nitrate coating significantly enhances the BC core absorption. The calculated light absorption proves that the difference in chemical compositions under the same coating thickness will affect E_abs. Both laboratory and field observation results show that E_abs of BC-containing particles is highly dependent on the change of D_p/D_c. The BC coating composed of different chemical components may bring a huge difference in light absorption enhancement. The results of this study are helpful to understand the role of BC in the formation of regional haze and climate forcing, and provide theoretical basis for model simulation improvement.