Geosciences Division, Physical Research Laboratory, Ahmedabad-380 009, India.
Environ Sci Process Impacts. 2019 Jun 19;21(6):970-987. doi: 10.1039/c9em00089e.
In this paper, we synthesize the size distribution and optical properties of the atmospheric water-soluble fraction of light-absorbing organic carbon (brown carbon; BrC) in the continental outflow from the Indo-Gangetic Plain (IGP) in South Asia to the North Indian Ocean. A comparison of the mass absorption coefficient of water-soluble BrC (babs-WSBrC-365nm) in PM2.5 with that in PM10 sampled over the Bay of Bengal reveals the dominance of BrC in fine mode. Furthermore, the babs-BrC-365nm shows a significant linear relationship with mass concentrations of airborne particulate matter, water-soluble organic carbon and non-sea-salt-K+ in the continental outflow from the IGP. This observation emphasizes the ubiquitous nature and significant contribution of water-soluble BrC from biomass burning emissions (BBEs). Comparing the absorption properties from this study with global datasets, it is discernible that BBEs dominate BrC absorption. Furthermore, the imaginary refractive index of water-soluble BrC (kWSBrC-365nm) in marine aerosols sampled over the North Indian Ocean during November is significantly higher than during December to January. Thus, significant temporal variability is associated with crop-residue burning emissions in the IGP on the composition of BrC over the North Indian Ocean. Our estimates show that the babs-WSBrC-365nm and kWSBrC-365nm from post-harvest crop-residue burning emissions in the IGP are much higher than the BBEs from the southeastern United States and Amazonian forest fires. Another major finding of this study is the lack of significant relationship between kWSBrC-365nm and the mass ratio of elemental carbon to particulate organic matter, as previously suggested by chamber experiments to model varying BrC absorption properties in ambient aerosols. Therefore, considerable spatio-temporal variability prevails among emission sources (wood burning vs. crop-residue burning), which needs to be considered when assessing the regional radiative forcing of BrC relative to major absorbing elemental carbon.
本文综合了源自南亚印度-恒河平原（IGP）的大陆传输气流中水溶性吸光有机碳（棕色碳；BrC）的粒径分布和光学特性。将孟加拉湾地区 PM2.5 中水溶性 BrC 的质量吸光系数（babs-WSBrC-365nm）与 PM10 进行比较，结果表明 BrC 主要存在于细颗粒物中。此外，babs-BrC-365nm 与 IGP 大陆传输气流中的大气悬浮颗粒物、水溶性有机碳和非海相钾+的质量浓度呈显著线性关系。这一观察结果强调了源自生物质燃烧排放（BBE）的水溶性 BrC 的普遍存在和重要贡献。将本研究的吸收特性与全球数据集进行比较后，可以发现 BBE 主导了 BrC 的吸收。此外，在北印度洋采集的海洋气溶胶中，水溶性 BrC 的虚光折射率（kWSBrC-365nm）在 11 月显著高于 12 月至 1 月。因此，IGP 地区的作物残茬燃烧排放会导致 BrC 的组成在北印度洋上发生显著的时间变化。我们的估算表明，IGP 收获后作物残茬燃烧排放的 babs-WSBrC-365nm 和 kWSBrC-365nm 远高于美国东南部和亚马逊森林火灾的 BBE。本研究的另一个主要发现是，kWSBrC-365nm 与元素碳与颗粒有机物的质量比之间没有显著关系，而之前的腔室实验曾提出过这一关系，用于模拟环境气溶胶中不断变化的 BrC 吸收特性。因此，在评估 BrC 相对于主要吸光元素碳的区域辐射强迫时，需要考虑来自不同排放源（木材燃烧与作物残茬燃烧）的显著时空变化。
