Molecular composition and photochemical lifetimes of brown carbon chromophores in biomass burning organic aerosol
Molecular composition and photochemical lifetimes of brown carbon chromophores in biomass burning organic aerosol
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DOI：
10.5194/acp-20-1105-2020
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发表时间：
2020-01-28
影响因子：
6.3
通讯作者：
Nizkorodov, Sergey A.
中科院分区：
文献类型：
--
作者：
Fleming, Lauren T.;Lin, Peng;Nizkorodov, Sergey A.
为了更好地了解野火对空气质量和气候的影响，重要的是要评估烟雾中发色化合物的出现并表征其光学特性。这项研究探讨了光吸收有机气溶胶或棕色碳（BrC）的分子组成，该分子组成在米苏拉消防科学实验室采样，作为FIREX 2016年秋季实验室密集的一部分。共12个生物质燃料从不同的植物类型进行了测试，包括裸子植物（针叶）和被子植物（开花）植物和不同的生态系统组成部分，如达夫，凋落物，和冠层。排放的生物质燃烧有机气溶胶（BBOA）颗粒收集到聚四氟乙烯过滤器和离线分析使用高效液相色谱耦合到光电二极管阵列分光光度计和高分辨率质谱仪（HPLC-PDA-HRMS）。分离的BrC发色团根据其保留时间、吸收光谱、近紫外和可见光谱范围（300-700 nm）内的积分吸光度以及精确m/z测量的化学式进行分类。BrC发色团分为以下类别和亚类：木质素衍生产物，其包括木质素热解产物;蒸馏产物，其包括香豆素和类黄酮;硝基芳烃;和多环芳烃（PAH）。所观察到的类别和子类在大多数燃料类型中是常见的，尽管具体的BrC发色团根据植物类型（裸子植物或被子植物）和燃烧的生态系统成分而变化。为了研究所观察到的BrC化合物在光降解方面的稳定性，将BBOA颗粒样品直接照射在具有近UV（300-400 nm）辐射的过滤器上，然后进行提取和HPLC-PDA-HRMS分析。单个BrC发色团的寿命取决于燃料类型和相应的燃烧条件。木质素衍生类和类黄酮类的BrC一般具有最长的寿命，相对于紫外光降解。此外，相同类型的BrC发色团的寿命根据生物质燃料和燃烧条件而变化。虽然个别BrC发色团消失了几天的时间尺度，样品的整体光吸收持续时间较长，大概是因为凝聚相光化学过程转换成另一组发色团没有完全光漂白或未检测到的BrC发色团，光漂白更慢。为了模拟BrC对气候的影响，重要的是要了解总吸收系数随时间的变化。我们测量了整个BrC吸收系数的等效大气寿命，范围从10到41天，亚高山冷杉的寿命最短，针叶树树冠，即，杜松，寿命最长。从生物质燃料负荷，包括多个生态系统组成部分（凋落物，灌木，冠层）的BrC排放的吸收寿命的范围的低端。这些结果表明，通过凝聚相光化学的BBOA的光漂白是相对缓慢的。竞争的化学老化机制，如由OH的非均相氧化，可能是更重要的控制率的BrC光漂白BBOA。
To better understand the effects of wildfires on air quality and climate, it is important to assess the occurrence of chromophoric compounds in smoke and characterize their optical properties. This study explores the molecular composition of light-absorbing organic aerosol, or brown carbon (BrC), sampled at the Missoula Fire Sciences laboratory as a part of the FIREX Fall 2016 lab intensive. A total of 12 biomass fuels from different plant types were tested, including gymnosperm (coniferous) and angiosperm (flowering) plants and different ecosystem components such as duff, litter, and canopy. Emitted biomass burning organic aerosol (BBOA) particles were collected onto Teflon filters and analyzed offline using high-performance liquid chromatography coupled to a photodiode array spectrophotometer and a high-resolution mass spectrometer (HPLC-PDA-HRMS). Separated BrC chromophores were classified by their retention times, absorption spectra, integrated absorbance in the nearUV and visible spectral range (300-700 nm), and chemical formulas from the accurate m/z measurements. BrC chromophores were grouped into the following classes and subclasses: lignin-derived products, which include lignin pyrolysis products; distillation products, which include coumarins and flavonoids; nitroaromatics; and polycyclic aromatic hydrocarbons (PAHs). The observed classes and subclasses were common across most fuel types, although specific BrC chromophores varied based on plant type (gymnosperm or angiosperm) and ecosystem component(s) burned. To study the stability of the observed BrC compounds with respect to photodegradation, BBOA particle samples were irradiated directly on filters with near UV (300-400 nm) radiation, followed by extraction and HPLC-PDA-HRMS analysis. Lifetimes of individual BrC chromophores depended on the fuel type and the corresponding combustion condition. Lignin-derived and flavonoid classes of BrC generally had the longest lifetimes with respect to UV photodegradation. Moreover, lifetimes for the same type of BrC chromophores varied depending on biomass fuel and combustion conditions. While individual BrC chromophores disappeared on a timescale of several days, the overall light absorption by the sample persisted longer, presumably because the condensed-phase photochemical processes converted one set of chromophores into another without complete photobleaching or from undetected BrC chromophores that photobleached more slowly. To model the effect of BrC on climate, it is important to understand the change in the overall absorption coefficient with time. We measured the equivalent atmospheric lifetimes of the overall BrC absorption coefficient, which ranged from 10 to 41 d, with subalpine fir having the shortest lifetime and conifer canopies, i.e., juniper, having the longest lifetime. BrC emitted from biomass fuel loads encompassing multiple ecosystem components (litter, shrub, canopy) had absorption lifetimes on the lower end of the range. These results indicate that photobleaching of BBOA by condensed-phase photochemistry is relatively slow. Competing chemical aging mechanisms, such as heterogeneous oxidation by OH, may be more important for controlling the rate of BrC photobleaching in BBOA.