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 near-UV and visible spectral range (300–700nm), 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 (300–400nm) 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 41d, 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.
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机译：为了更好地了解野火对空气质量和气候的影响，重要的是评估烟雾中发色合物的发生并表征它们的光学性质。本研究探讨了光吸收有机气溶胶，或棕色碳（BRC）的分子组成，在Missoula Firecents Laperatory中取样，作为Firex Fall 2016实验室密集型的一部分。测试了来自不同植物类型的12个生物量燃料，包括裸子植物（针叶）和Agiosperm（开花）植物和不同的生态系统组件，如Duff，垃圾和树冠。将燃烧的生物质燃烧的有机气溶胶（BboA）颗粒收集到Teflon过滤器上，并使用高性能液相色谱法分析与光电二极管阵列分光光度计和高分辨率质谱仪（HPLC-PDA-HMS）的高效液相色谱分析。分离的BRC发色团通过它们的保留时间，吸收光谱，近UV和可见光光谱范围（300-700nm）和来自精确M / Z测量的化学式。 BRC发色团被分组为以下课程和亚类：木质素衍生的产品，包括木质素热解产品;蒸馏产品，包括香豆素和黄酮类化合物;硝基甲骨;和多环芳烃（PAH）。观察到的类和子类在大多数燃料类型中是常见的，尽管特定BRC发色团基于植物型（裸子植物或高管植物）和生态系统组分燃烧。为了研究观察到的BRC化合物相对于光降解的稳定性，BboA颗粒样品直接照射在接近UV（300-400nm）辐射的过滤器上，然后萃取和HPLC-PDA-HRMS分析。各种BRC发色团的寿命依赖于燃料类型和相应的燃烧条件。 Lignin衍生的和类黄酮类别的BRC通常具有相对于UV光降解的最长的寿命。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。虽然单个BRC发色团在几天的时间内消失，但样品的整体光吸收持续时间延长，可能是因为冷凝相光化学方法将一组发色团转化为另一组，而无需完全光博或从未被发的BRC发色团相去光漂白。为了模拟BRC对气候影响的影响，重要的是要了解整体吸收系数随时间的变化。我们测量了总体BRC吸收系数的等效大气寿命，其范围为10至41d，亚高山杉木具有最短的寿命和针叶树檐篷，即杜松，具有最长的寿命。从包含多个生态系统组件（垃圾，灌木，遮篷）的生物质燃料负载发射的BRC在该范围的下端具有吸收寿命。这些结果表明，通过冷凝的相光化学的BboA光漂白是相对较慢的。竞争化学老化机制，例如非均相氧化哦，对于控制BboA中的BRC光漂白速率可能更为重要。
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