The missing information of aliphatic-like & aromatic-like Unresolved Complex Mixtures (UCMs) in background oceanic atmosphere: occurrence, identification and deposition

Abstract

Oceanic air samples (gas + particle phases) were collected over the Shanghai-Arctic background oceanic region during the Chinese Arctic Research expedition 2021. Full scan mode (semi-quantitative) and selective ion monitoring mode (quantitative) were used to analyze separately 72 polycyclic aromatic compounds (PACs) & C11–C35 n-Alkanes (n-Alks) and unresolved complex mixtures (UCMs, including aromatic-like and aliphatic-like compounds) in these samples. The concentration of ΣPACs in the gas-phase and particle-phase were 0.40–8.5 ng m−3 (2.4 ± 1.7 ng m−3) and 0.059–1.1 ng m−3 (0.21 ± 0.23 ng m−3), respectively. While Σn-Alks in the gas-phase and particle-phase were 2.2–80 ng m−3 (mean = 18 ± 24 ng m−3) and 2.2–20 ng m−3 (mean = 8.6 ± 5.0 ng m−3), respectively. The semi-quantitative result indicated that aromatic-like UCMs were 14–1400 times higher than those of ΣPACs in these two phases, while aliphatic-like UCMs were 1.2–25 times higher than those of Σn-Alks. Through three-dimensional identification of the one-dimensional GC-MS chromatogram (with the x-axis representing GC retention time, the y-axis representing m/z, and the z-axis representing peak intensity), we found that aromatic-like UCMs were primarily composed of phenanthrene & anthracene similar PACs, while aliphatic-like UCMs were predominated by long-chain alkanes. The source identification shows that oil-gas leakage was the dominant source of PACs, followed by petroleum & coal combustion and biomass combustion; n-Alks might be mainly originated from anthropogenic sources, while nature sources at high latitudes cannot be ignored as well. Aromatic-like UCMs had a decreasing latitude trend and the presence of 5–6 ring aromatic-like UCMs enclosed within organic aerosols could pose a significant ecological risk, particularly in high latitude regions. Net uptake of atmospheric carbon caused by dry deposition of aromatic-like & aliphatic-like UCMs varied significantly geographically with mid- and low-latitude seas. The non-polar and weakly polar substances were not the major components of organic carbon in oceanic air of high latitude areas.