Abstract：
Atmospheric brown carbon (BrC) aerosols were investigated at two urban sites in southern (Hefei) and northern (Shijiazhuang) China during summer and winter of 2019-2020 to explore regional variability in their compositional and optical properties. Organic matter in ambient PM2.5 samples were characterized at molecular level using ultrahigh performance liquid chromatography coupled with a diode array detector and an Orbitrap mass spectrometer. Although the molecular composition of organic aerosols varied substantially over different ambient environments, they were mainly composed by CHO and CHON species in positive ionization mode while CHO and CHOS species in negative mode. The mass absorption coefficients of BrC aerosols at wavelength range 250-450 nm were relatively higher for winter samples in both cities and for Shijiazhuang samples in both seasons, partly attributed to the higher concentration levels of anthropogenic air pollutants in these environments. The absorption Ångström exponents further revealed that BrC aerosols in winter seasons and in Shijiazhuang had a greater capacity of absorption at shorter wavelengths. A total of 26 BrC species with strong absorption were unambiguously identified from different environments, which mainly consisted of CHO, CHON, and CHN species and had higher degrees of unsaturation and lower degrees of oxidation. The presence and abundance of these BrC species varied dynamically across the seasons and cities, with a greater number of species presented in the winter of Shijiazhuang. The BrC species together contributed 12-26 % in the total absorbance of light-absorbing organic components at 250-450 nm. This study highlights the regional differences in BrC properties influenced by the sources and atmospheric processes, which should be taken into account to assess their climate impacts.
摘要：
在2019-2020年夏季和冬季，在中国南部（合肥）和北部（石家庄）的两个城市地区调查了大气褐碳（BrC）气溶胶，以探索其组成和光学特性的区域变化。使用超高效液相色谱结合二极管阵列检测器和Orbitrap质谱仪，在分子水平上表征了环境PM2.5样品中的有机物。尽管有机气溶胶的分子组成在不同的周围环境中变化很大，它们主要由正电离模式的CHO和CHON物种组成，而负电离模式的CHO和CHOS物种组成。两个城市的冬季样品和两个季节的石家庄样品在250-450nm波长范围内的BRC气溶胶的质量吸收系数相对较高。部分归因于这些环境中人为空气污染物的浓度较高。吸收系数进一步表明，冬季和石家庄的BRC气溶胶在较短波长下具有更大的吸收能力。从不同环境中明确鉴定出26种具有强吸收的BrC物种，主要由CHO组成，CHON,和CHN物种，并且具有较高的不饱和度和较低的氧化度。这些BRC物种的存在和丰富程度随季节和城市的变化而动态变化，石家庄冬季出现了更多的物种。BrC物种在250-450nm处的光吸收有机组分的总吸光度中贡献了12-26％。这项研究强调了受来源和大气过程影响的BrC性质的区域差异，评估它们对气候的影响时应该考虑这些因素。
