Molecular characterization of organic aerosol in the Himalayas: insight from ultra-high-resolution mass spectrometry

An increased trend in aerosol concentration has been observed in the Himalayas in recent years, but the understanding of the chemical composition and sources of aerosol remains poorly understood. In this study, molecular chemical composition of water-soluble organic matter (WSOM) from two filter samples collected during two high aerosol loading periods (denoted as P1 and P2) at a high-altitude station (Qomolangma Station, QOMS; 4276 m a.s.l.) in the northern Himalayas was identified using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR MS). More than 4000 molecular formulas were identified in each filter sample which were classified into two compound groups (CHO and CHON) based on their elemental composition, with both accounting for nearly equal contributions in number (45 %-55 %). The relative abundance weighted mole ratio of O/C-w for P1 and P2 was 0.43 and 0.39, respectively, and the weighted double bond equivalents (DBEw), an index for the saturation of organic molecules, were 7.12 and 7.87, respectively. Although the O/C-w mole ratio was comparable for CHO and CHON compounds, the DBEw was significantly higher in CHON compounds than CHO compounds. More than 50% molecular formulas in the Van Krevelen (VK) diagram (H/C vs. O/C) were located in 1-1.5 (H/C) and 0.2-0.6 (O/C) regions, suggesting potential lignin-like compounds. The distributions of CHO and CHON compounds in the VK diagram, DBE vs. number of C atoms, and other diagnostic diagrams showed high similarities among each other, suggesting their similar source and/or atmospheric processes. Many formulas formed from biogenic volatile organic compounds (e.g., ozonolysis of alpha-pinene products) and biomass-burning-emitted compounds (e.g., phenolic compounds) were found in the WSOM, suggesting the important contribution of these two sources in the Himalayas. The high DBE and high fraction of nitrogen-containing aerosol can potentially impact aerosol light absorption in this remote region. Further comprehensive study is needed due to the complexity of organic aerosol and limited molecular number identified in this study.

近年来，喜马拉雅山脉观测到气溶胶浓度呈上升趋势，但对气溶胶化学成分和来源的了解仍然知之甚少。在本研究中，利用电喷雾电离傅里叶变换离子回旋共振质谱（ESI - FTICR MS）对喜马拉雅山脉北部一个高海拔站点（珠穆朗玛峰站，QOMS；海拔4276米）在两个高气溶胶负载时期（分别记为P1和P2）采集的两个滤膜样本中水溶性有机物（WSOM）的分子化学成分进行了鉴定。在每个滤膜样本中鉴定出了4000多种分子式，根据其元素组成将它们分为两类化合物组（CHO和CHON），两者在数量上的贡献几乎相等（45% - 55%）。P1和P2的O/C - w相对丰度加权摩尔比分别为0.43和0.39，加权双键当量（DBEw）（有机分子饱和度的一个指标）分别为7.12和7.87。尽管CHO和CHON化合物的O/C - w摩尔比相当，但CHON化合物的DBEw明显高于CHO化合物。在范克雷维伦（VK）图（H/C与O/C）中，超过50%的分子式位于1 - 1.5（H/C）和0.2 - 0.6（O/C）区域，表明可能存在类木质素化合物。VK图中CHO和CHON化合物的分布、DBE与碳原子数以及其他诊断图之间彼此高度相似，表明它们具有相似的来源和/或大气过程。在WSOM中发现了许多由生物源挥发性有机化合物（例如，α - 蒎烯产物的臭氧分解）和生物质燃烧排放的化合物（例如，酚类化合物）形成的分子式，表明这两种来源在喜马拉雅山脉具有重要贡献。高DBE和高含氮气溶胶比例可能会影响这个偏远地区的气溶胶光吸收。由于有机气溶胶的复杂性以及本研究中鉴定出的分子数量有限，需要进一步开展综合研究。