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青藏高原被誉为世界第三极,其冰雪含量仅次于南极和北极[1],在整个亚洲大陆的大气环流和水循环中起着重要作用[2 − 3]. 尽管青藏高原是全球最干净的地区之一[4],但其大气环境也受到来自远处的污染物输送和本地排放的影响[5 − 6],其中包括化石燃料和生物质燃烧产生的黑碳(black carbon,BC)[7 − 8]. BC在大气中能有效吸收几乎整个太阳光谱的太阳辐射,对大气层产生显著的辐射效应,从而影响气候变化[9]. 一旦BC沉降到冰雪表面,其强吸光性会显著降低雪冰表面反照率,导致雪冰表面更多地吸收太阳辐射[10]. 此外,BC经过老化后其吸光能力明显增强[11],进一步加速冰雪融化. 最新研究发现,青藏高原冰雪的快速退缩与冰雪中沉积的BC有关[12]. 由于青藏高原地处高寒缺能区域,牧区牧民经常使用牛羊粪作为生物质燃料取暖和烹饪. 该地区生物质能源消费在家庭能源消费中占比达41.4%,其中牦牛粪是重要来源之一[13].
目前,研究更多关注外来BC对青藏高原气候和生态环境的影响,对本地生物质燃料燃烧排放BC的研究则相对较少. 例如,Liu等[14]的研究表明,生物质燃烧对青藏高原东南缘BC质量浓度贡献占57%,主要来自印度东北部和缅甸北部输送;Yang等[15]使用区域大气化学模型WRF-Chem发现,南亚居民排放对青藏高原BC浓度贡献最大,传输过程中夏季为25.8%,冬季为44.8%. 尽管已有研究团队对当地居民燃烧牦牛粪排放的气溶胶进行了研究,但对青藏高原牦牛粪燃烧产生BC颗粒物的化学特性了解仍较少,特别是对其老化后的化学特征和混合状态的变化. 例如,Chen等[6]的研究表明,牦牛粪排放的气溶胶中有机碳(OC)和BC分别占55.2%和3.63%,所研究气溶胶的OC/BC比值为16.3±4.4,接近环境气溶胶OC/BC值(季风期为21.7,非季风期为23.1),进一步表明当地来源对青藏高原大气的影响.
研究当地居民燃烧牦牛粪排放BC颗粒物的化学特性对准确评估青藏高原的污染源具有重要作用. 本研究通过燃烧模拟实验,利用氧化流动反应器(potential aerosol mass,PAM)模拟牦牛粪燃烧产生的BC气溶胶在大气中的老化过程,然后采用单颗粒气溶胶质谱仪(single-particle aerosol mass spectrometer,SPAMS)对BC颗粒物的化学特征以及混合状态进行表征.
牦牛粪燃烧排放黑碳的化学老化过程
Chemical aging process of black carbon emissions from yak dung combustion
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摘要: 当前,对青藏高原牦牛粪燃烧排放的含黑碳(BC)颗粒物在老化过程中的演变机制了解还不足. 为深入研究这一问题,本文采用氧化流动反应器模拟牦牛粪燃烧排放BC颗粒物的不同老化程度,并利用单颗粒气溶胶质谱仪对老化前后BC颗粒物的化学组成、混合状态、老化机制等进行了表征. 结果显示,在不同程度的老化模拟过程中,牦牛粪燃烧排放的BC颗粒物均呈现显著的有机碳(OC)、硫酸盐(HSO4−)和硝酸盐(NO3−)离子峰特征. 老化前,有机碳(其特征峰主要为m/z = 27C2H3+/CNH+、37C3H+、43C2H3O+、50C4H2+、51C4H3+和63C5H3+)在正离子相对峰面积中占比为42.8%,而硫酸盐在负离子相对峰面积中占比高达72.4%. 基于自适应共振神经网络算法(ART-2a),将BC颗粒物分为4类:BC、BCN、BCOCS和Other. 硫酸盐与BCOCS颗粒类型高度混合(99%),而硝酸盐与BCN颗粒类型高度混合(98%). 此外,随着老化程度的增加,二次组分(如43C2H3O和62NO3−)在各颗粒类型中的混合程度也越高. 硝酸盐、草酸盐(89HC2O4−)和铵盐(18NH4+)的数量百分比与老化程度呈显著的正相关关系,表明在BC颗粒物老化的过程中它们的贡献会增加. 本研究有助于更加全面地理解青藏高原大气中BC颗粒物的行为,为该区域大气环境保护提供科学依据.Abstract: Currently, the evolution mechanism of black carbon (BC) particles emitted from yak dung combustion on the Qinghai-Tibet Plateau during the aging process remains poorly understood. In order to further study these issues, an oxidation flow reactor was used to simulate various aging degrees of BC particles emitted from yak dung combustion. The chemical compositions, mixing states, and aging mechanism of BC particles before and after aging were characterized by a single-particle aerosol mass spectrometer. The results showed that during the simulation of different aging degrees, the BC particles emitted from yak dung combustion exhibited distinctive peak characteristics of organic carbon (OC), sulfate (HSO4−), and nitrate (NO3−). Prior to aging, organic carbon (with characteristic peaks at m/z = 27C2H3+/CNH+37C3H+,43C2H3O+,50C4H2+,51C4H3+, and 63C5H3+) accounted for 42.8% of the relative peak area of positive ions, while sulfate accounted for 72.4% of the relative peak area of negative ions. Based on the adaptive resonance neural network algorithm (ART-2a), BC particles are divided into four categories: BC, BCN, BCOCS, and Other. Sulfates exhibited high mixing with the BCOCS particle type (99%), whereas nitrates showed high mixing with the BCN particle type (98%). Furthermore, as the aging degree increased, secondary components (such as 43C2H3O+ and 62NO3−) became more prevalent in each particle type. The percentages of nitrate, oxalate (89HC2O4−), and ammonium salt (18NH4+) positively correlated with the degree of aging, indicating an increased contribution during the aging of BC particles. This study contributes to a more comprehensive understanding of the behavior of BC particles in the atmosphere of the Qinghai-Tibet Plateau, providing a scientific basis for the protection of the atmospheric environment in this region.
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Key words:
- Qinghai-Tibet Plateau /
- black carbon /
- chemical composition /
- mixing state /
- aging simulation
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