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Mid carbon (C6+-C29+) in refractory black carbon aerosols is a potential tracer of open burning of rice straw: Insights from atmospheric observation and emission source studies
Atmospheric Environment ( IF 5 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.atmosenv.2020.117729
Yuji Fujitani , Akihiro Fushimi , Katsumi Saitoh , Kei Sato , Akinori Takami , Yoshinori Kondo , Kiyoshi Tanabe , Shinji Kobayashi

Abstract Biomass burning (BB) is one of the major sources of atmospheric aerosols. These aerosols cause global and regional issues such as adverse effects on human health, as well as contributing to climate change. Refractory black carbon (rBC) is a major chemical component emitted from BB. Recent studies indicate that the mass spectra distribution of carbon cluster ions (Cn+) in rBC reflects burning materials and combustion temperatures. Therefore, emissions apportionment is in principle possible with respect to rBC. However, emissions data for Cn+ are still not widely available. In this study, a combustion experiment using rice straw was conducted to identify the mass spectra of Cn+ measured using a soot particle aerosol mass spectrometer. Further, to confirm the results, atmospheric observations were conducted during the harvest season in a suburban area surrounded by rice fields because intentional open burning of rice straw (agricultural residues) is often conducted. Source apportionment of airborne ambient rBC generated from open burning of rice straw is carried out by a positive matrix factorization (PMF) method applied to hourly mean data for rBC and Cn+, as well as organic aerosols. During the atmospheric observation period, there was an intensive day of open burning of rice straw, confirmed by visual inspections around the observation site. Concentrations of PM2.5 reached up to 100 μg/m3 and the contribution of BB to rBC increased by up to 50% on that day. Tracers of BB (C2H4O2+ which is a fragment ion of levoglucosan and refractory K+ ions) were also enriched at that time. This study also found that the signal fraction of mid carbon (C6+-C29+) to total carbon (C1+-C35+) in rBC was 0.21 for dry rice straw burning. Correspondingly, the BB-related PMF factor in atmospheric observations (0.18) was close to the emission source data. Rice straw burning tends to occur at a low combustion temperature and generates amorphous (immature) soot, which was further confirmed by a larger value (0.41) for wet rice straw burning with low modification combustion efficiency. On the other hand, the mid carbon fraction of rBC from automobile exhausts (diesel and gasoline) was low (below 0.02) in laboratory studies and the automobile exhaust-related PMF factor was also relatively low (0.08) in atmospheric observations. These emissions are likely generated as graphite (mature) soot under a high combustion temperature. This study also quantifies and explores the atmospheric aging of BB aerosols. Decay rates of mid carbon were about 2 times slower than that of levoglucosan estimated from atmospheric observations. In sum, the mid carbon fraction can be used to differentiate sources of rBC emissions between open burning of rice straw at low combustion temperatures and other rBC sources at high combustion temperatures (such as internal combustion engines) on the one hand and estimate the decay rates of aerosols emitted from BB on the other hand.

中文翻译:

耐火黑碳气溶胶中的中碳 (C6+-C29+) 是稻草露天燃烧的潜在示踪剂:来自大气观测和排放源研究的见解

摘要 生物质燃烧(BB)是大气气溶胶的主要来源之一。这些气溶胶会导致全球和区域问题,例如对人类健康产生不利影响,以及导致气候变化。难熔黑碳 (rBC) 是 BB 排放的主要化学成分。最近的研究表明,rBC 中碳簇离子 (Cn+) 的质谱分布反映了燃烧材料和燃烧温度。因此,原则上可以对 rBC 进行排放分配。然而,Cn+ 的排放数据仍未广泛获得。在本研究中,使用稻草进行了燃烧实验,以确定使用烟尘颗粒气溶胶质谱仪测量的 Cn+ 的质谱图。此外,为了确认结果,大气观测是在收获季节期间在一个被稻田包围的郊区进行的,因为经常有意识地露天焚烧稻草(农业残留物)。通过对 rBC 和 Cn+ 以及有机气溶胶的每小时平均数据应用正矩阵分解 (PMF) 方法,对稻草露天燃烧产生的空气中 rBC 进行源解析。大气观测期间,观测现场周围目视检查证实,稻草露天焚烧密集的一天。当天PM2.5浓度高达100微克/立方米,BB对rBC的贡献增加了50%。BB 的示踪剂(C2H4O2+ 是左旋葡聚糖的碎片离子和难降解的 K+ 离子)也在当时富集。该研究还发现,干稻草燃烧时,rBC 中碳 (C6+-C29+) 与总碳 (C1+-C35+) 的信号分数为 0.21。相应地,大气观测中BB相关的PMF因子(0.18)与排放源数据接近。稻草燃烧往往发生在低燃烧温度下并产生无定形(未成熟)烟灰,这进一步被湿稻草燃烧的较大值(0.41)证实,而改性燃烧效率低。另一方面,在实验室研究中,来自汽车尾气(柴油和汽油)的 rBC 的中碳分数较低(低于 0.02),而在大气观测中与汽车尾气相关的 PMF 因子也相对较低(0.08)。这些排放物很可能在高燃烧温度下以石墨(成熟)烟灰的形式产生。本研究还量化和探索了 BB 气溶胶的大气老化。中碳的衰变率比从大气观测估计的左旋葡聚糖慢约 2 倍。总之,一方面,中碳部分可用于区分低燃烧温度下稻草露天燃烧和高燃烧温度下其他 rBC 源(如内燃机)的 rBC 排放源,并估计衰减率另一方面,从 BB 发出的气溶胶。
更新日期:2020-10-01
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