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

拟议的研究将利用紫外共振拉曼光谱法研究大气气溶胶中吸光有机化合物的化学组成。这个被称为棕色碳的化合物家族，尽管研究表明棕色碳对气候的辐射强迫具有重要影响，但人们对棕色碳的了解很少。此外，一般的含碳气溶胶-包括有机碳和回碳部分-由于其对气候和人类健康的重要影响，目前是密集的科学研究的主题。拟议研究计划的目标是表征棕色碳的组成，确定棕色碳的不同来源，并了解这些吸光有机化合物在其大气寿命期间的化学过程。 UVRR光谱是研究有机气溶胶的独特工具，因为它可以提供特定于发色团的振动光谱，其吸收带与探测激光波长相匹配，即使这些发色团与颗粒中的非吸收性有机物混合。UVRR研究将调查在实验室中进行的有机反应的产物，这些反应模拟了含水颗粒或云滴中发生的化学反应，这些反应与棕色碳的产生有关。将在多个波长下测量“合成”棕色碳的UVRR光谱，以确定负责光吸收的发色团。将探索不同的前体化合物，并确定棕色碳成分的变化。在类似的实验中，将收集有机气溶胶的实地样本，并使用氨气进行人工老化，这已被证明会产生吸光的含氮有机物。实验室研究中测量的光谱将与从现场样品中收集的光谱进行比较，以评估水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含量外，还将使用统计方法（如正矩阵因子分解（PMF））来识别棕碳的不同组分。棕色碳的光谱特征和PMF组分与排放示踪剂和光化学年龄的相关性，然后将在分配不同来源的棕色碳组分的过程中使用。将使用现场数据探索组合物和光吸收之间的关系，以找到可用于约束棕碳组合物的光吸收光谱中的特征。 有机气溶胶成分的研究已被广泛应用于质谱、傅里叶变换红外光谱和拉曼光谱等技术。这些技术提供了有价值的信息，但没有一个能够选择性地分析棕色碳，或者，充其量，它们只能表征棕色碳质量的一小部分。预计拟议的研究将提供有关棕色碳源和大气化学的新的和更完整的信息。最终，这项工作的结果可用于约束和测试大气气溶胶模型，这对预测未来气候至关重要。

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.