Separation of brown carbon from black carbon for IMPROVE and Chemical Speciation Network PM_2.5 samples

The replacement of the Desert Research Institute (DRI) model 2001 with model 2015 thermal/optical analyzers (TOAs) results in continuity of the long-term organic carbon (OC) and elemental carbon (EC) database, and it adds optical information with no additional carbon analysis effort. The value of multiwavelength light attenuation is that light absorption due to black carbon (BC) can be separated from that of brown carbon (BrC), with subsequent attribution to known sources such as biomass burning and secondary organic aerosols. There is evidence of filter loading effects for the 25% of all samples with the highest EC concentrations based on the ratio of light attenuation to EC. Loading corrections similar to those used for the seven-wavelength aethalometer need to be investigated. On average, nonurban Interagency Monitoring of PROtected Visual Environments (IMPROVE) samples show higher BrC fractions of short-wavelength absorption than urban Chemical Speciation Network (CSN) samples, owing to greater influence from biomass burning and aged aerosols, as well as to higher primary BC contributions from engine exhaust at urban sites. Sequential samples taken during an Everglades National Park wildfire demonstrate the evolution from flaming to smoldering combustion, with the BrC fraction increasing as smoldering begins to dominate the fire event. <i>Implications</i>: The inclusion of seven wavelengths in thermal/optical carbon analysis of speciated PM_2.5 (particulate matter with an aerodynamic diameter ≤2.5 μm) samples allows contributions from biomass burning and secondary organic aerosols to be estimated. This separation is useful for evaluating control strategy effectiveness, identifying exceptional events, and determining natural visibility conditions.将沙漠研究所（Desert Research Institute，DRI）2001款模型替换为2015款热光分析仪（thermal/optical analyzers，TOAs），可实现长期有机碳（organic carbon，OC）与元素碳（elemental carbon，EC）数据库的连续性维持，且无需额外开展碳分析即可新增光学信息。多波长光衰减技术的核心优势在于，可将黑碳（black carbon，BC）与棕碳（brown carbon，BrC）引发的光吸收信号区分开来，进而将其归因于生物质燃烧、二次有机气溶胶等已知排放源。基于光衰减与EC的比值，有证据显示在EC浓度最高的25%样本中存在滤膜负载效应，需参考七波长黑碳仪的负载校正方法开展相关研究。平均而言，受生物质燃烧与老化气溶胶的影响更为显著，且城市站点的发动机排气会贡献更多原生黑碳，因此保护视觉环境跨机构监测网络（Interagency Monitoring of PROtected Visual Environments，IMPROVE）的非城市样本，其短波长吸收的棕碳占比高于化学组分监测网络（Chemical Speciation Network，CSN）的城市样本。大沼泽地国家公园野火期间采集的连续样本，展现了从明火燃烧到阴燃的火灾演化过程，随着阴燃逐渐主导火灾进程，棕碳占比随之升高。<i>研究启示</i>：对经组分表征的细颗粒物（PM₂.₅，空气动力学直径≤2.5μm的颗粒物）样品开展热光碳分析时，引入七波长检测技术，可估算生物质燃烧与二次有机气溶胶的贡献份额。该组分分离方法可用于评估大气管控策略的有效性、识别极端污染事件以及确定自然能见度条件。