Characterizing the sources and atmospheric chemistry of light-absorbing carbonaceous aerosols using resonance Raman spectroscopy

这项拟议的研究将利用紫外线共振拉曼光谱(UVRR)来研究大气气溶胶中吸收光的有机化合物的化学成分。尽管研究表明，棕色碳对气候的辐射强迫有重要影响，但人们对这一被称为棕色碳的化合物家族知之甚少。此外，碳质气溶胶--包括有机和反碳部分--目前是密集科学研究的主题，因为它们对气候和人类健康有重要影响。拟议的研究计划的目标是表征棕色碳的组成，识别棕色碳的不同来源，并了解这些吸收光的有机化合物在大气生命周期中的化学过程。 UVRR光谱是研究有机气溶胶的独特工具，因为它可以提供专门针对发色团的振动光谱，其吸收带与探测激光波长相匹配，即使这些发色团与非吸收有机物混合在一个颗粒中也是如此。UVRR研究将调查实验室中进行的有机反应的产物，这些反应模仿水颗粒或云滴中发生的化学物质，这些颗粒或云滴与棕色碳的产生有关。合成的棕色碳的UVRR光谱将在多个波长下进行测量，以确定负责光吸收的生色团。将探索不同的前体化合物，并确定由此产生的褐碳成分的变化。在类似的实验中，将收集现场的有机气溶胶样本，并使用氨气进行人工老化，这已被证明可以产生吸光的含氮有机物。在实验室研究中测量的光谱将与从现场样品收集的光谱进行比较，以评估水滴和云滴化学对观测到的棕色碳质量浓度的贡献。 还将对现场样品进行分析。除了量化棕碳官能团的含量外，还将使用正矩阵因式分解(PMF)等统计方法来识别棕碳的不同成分。然后，将利用棕色碳光谱特征和PMF组分与发射示踪剂和光化学年龄的相关性，将不同的来源分配给棕色碳组分。将利用现场数据探索组成和光吸收之间的关系，以找出可用于限制棕色碳组成的光吸收光谱中的特征。 许多技术已被应用于有机气溶胶成分的研究，包括质谱学、傅里叶变换红外光谱和拉曼光谱。这些技术提供了有价值的信息，但没有一种技术能够选择性地分析棕色碳，或者充其量它们只能表征一小部分棕色碳的质量。预计拟议的研究将提供有关棕色碳源和大气化学的新的、更完整的信息。最终，这项工作的结果可以用来约束和测试大气气溶胶模型，这对预测未来的气候至关重要。

The proposed research will utilize ultraviolet resonance Raman (UVRR) spectroscopy to study the chemical composition of light-absorbing organic compounds in atmospheric aerosols. This family of compounds, known as brown carbon, is poorly understood even though research indicates that brown carbon has an important impact on radiative forcing of climate that varies widely depending on location. Moreover, carbonaceous aerosols in general – both organic and back carbon fractions – are currently the subject of intense scientific research because of their important impact on climate and human health. The goal of the proposed research program is to characterize the composition of brown carbon, identify the different sources of brown carbon, and understand the chemical processing of these light-absorbing organic compounds during their atmospheric lifetime. UVRR spectroscopy is a unique tool for studying organic aerosol because it can provide vibrational spectra specifically for chromophores with an absorption band that matches the probing laser wavelength even when those chromophores are mixed with non-absorbing organics in a particle. The UVRR studies will investigate the products of organic reactions carried out in the laboratory that mimic the chemistry occurring in water-laden particles or cloud droplets, which have been linked to brown carbon production. The UVRR spectra of the ‘synthetic’ brown carbon will be measured at multiple wavelengths to identify the chromophores responsible for light absorption. Different precursor compounds will be explored, and the resulting changes in brown carbon composition will be identified. In similar experiments, field samples of organic aerosol will be collected and artificially aged using ammonia gas, which has been show to produce light-absorbing nitrogen-containing organics. The spectra measured in the lab studies will be compared to those collected from field samples to evaluate the contribution of aqueous and cloud droplet chemistry to the observed brown carbon mass concentrations. Analyses on field samples will be performed as well. In addition to quantifying the brown carbon functional group content, statistical methods such as positive matrix factorization (PMF) will be used to identify the different components of brown carbon. The correlations of the brown carbon spectral features and PMF components with emission tracers and photochemical age will then be utilized in the process of assigning different sources to the brown carbon components. The relationship between composition and light absorption will be explored using field data to find characteristics in the light absorption spectra that can be used to constrain brown carbon composition. Many techniques have been applied to the study of organic aerosol composition including mass spectrometry, FTIR spectroscopy, and Raman Spectroscopy. These techniques provide valuable information, but none are capable of selectively analyzing brown carbon, or, at best, they characterize only a small fraction of the brown carbon mass. It is expected that the proposed research will provide new and more complete information regarding brown carbon sources and atmospheric chemistry. Ultimately, the results of this work can be used to constrain and test models of atmospheric aerosols, which are critical for predicting future climate.