Measuring light absorption by freshly emitted organic aerosols: optical artifacts in traditional solvent-extraction-based methods
Measuring light absorption by freshly emitted organic aerosols: optical artifacts in traditional solvent-extraction-based methods
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DOI：
10.5194/acp-19-8817-2019
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发表时间：
2019-07-11
影响因子：
6.3
通讯作者：
Chakrabarty, Rajan K.
中科院分区：
文献类型：
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作者：
Shetty, Nishit J.;Pandey, Apoorva;Chakrabarty, Rajan K.
最近的研究表明，有机气溶胶（OA）可能有一个不平凡的作用，在较短的可见光波长的大气光吸收。因此，有必要对OA光吸收进行良好的估计，以更好地估计气候模式中这些气溶胶引起的辐射强迫。用于测量OA光吸收的常用技术之一是过滤器样品的溶剂萃取技术，其涉及使用分光光度计测量颗粒物的溶剂可溶性有机部分的本体吸光度。随后使用比例因子将测量的溶剂相吸光度转换为粒子相吸收系数。传统观点是将校正因子2应用于基于Mie计算从溶剂萃取的OA获得的吸收系数。适当的比例因子是由于溶剂对有机碳（OC）的不完全提取和OA的尺寸依赖性吸收特性而引起的偏差的函数。这些偏差的范围沿着以及它们对烧伤条件的潜在依赖性是一个未探索的研究领域。在这里，我们进行了一个全面的实验室研究，涉及三种溶剂（水，甲醇和丙酮），以调查从溶剂萃取为基础的光度技术相比，在原位颗粒相吸收新鲜排放的OA从生物质燃烧的吸收系数的偏差。我们与OC/TC（总碳）的质量比和单次散射（SSA）的偏差，并观察到，水和甲醇提取的OA的常规使用的校正因子2可能无法扩展到所有系统，我们建议谨慎使用这样的校正因子来估计颗粒相OA吸收系数。此外，还建立了SSA与OC/TC比值之间的线性相关性。最后，从光谱数据中，我们分析了从溶液和颗粒相测量获得的吸收埃指数（A埃E）的差异。我们注意到，从溶剂相测量的AAE可能会偏离他们的OA同行显着。
Recent studies have shown that organic aerosol (OA) could have a nontrivial role in atmospheric light absorption at shorter visible wavelengths. Good estimates of OA light absorption are therefore necessary to better estimate radiative forcing due to these aerosols in climate models. One of the common techniques used to measure OA light absorption is the solvent extraction technique from filter samples which involves the use of a spectrophotometer to measure bulk absorbance by the solvent-soluble organic fraction of particulate matter. Measured solvent-phase absorbance is subsequently converted to particle-phase absorption coefficient using scaling factors. The conventional view is to apply a correction factor of 2 to absorption coefficients obtained from solvent-extracted OA based on Mie calculations. The appropriate scaling factors are a function of biases due to incomplete extraction of organic carbon (OC) by solvents and size-dependent absorption properties of OA. The range for these biases along with their potential dependence on burn conditions is an unexplored area of research. Here, we performed a comprehensive laboratory study involving three solvents (water, methanol, and acetone) to investigate the bias in absorption coefficients obtained from solvent-extraction-based photometry techniques as compared to in situ particle-phase absorption for freshly emitted OA from biomass burning. We correlated the bias with OC/TC (total carbon) mass ratio and single scattering albedo (SSA) and observed that the conventionally used correction factor of 2 for water and methanol-extracted OA might not be extensible to all systems, and we suggest caution while using such correction factors to estimate particle-phase OA absorption coefficients. Furthermore, a linear correlation between SSA and the OC/TC ratio was also established. Finally, from the spectroscopic data, we analyzed the differences in absorption Angstrom exponents (A angstrom E) obtained from solution-and particulate-phase measurements. We noted that AAE from solvent-phase measurements could deviate significantly from their OA counterparts.