Abstract
In this work, the relative yields of aqueous secondary organic aerosols (aqSOAs) at the air–liquid (a–l) interface are investigated between photochemical and dark aging using in situ time-of-flight secondary ion mass spectrometry (ToF-SIMS). Our results show that dark aging is an important source of aqSOAs despite a lack of photochemical drivers. Photochemical reactions of glyoxal and hydroxyl radicals (•OH) produce oligomers and cluster ions at the aqueous surface. Interestingly, different oligomers and cluster ions form intensely in the dark at the a–l interface, contrary to the notion that oligomer formation mainly depends on light irradiation. Furthermore, cluster ions form readily during dark aging and have a higher water molecule adsorption ability. This finding is supported by the observation of more frequent organic water cluster ion formation. The relative yields of water clusters in the form of protonated and hydroxide ions are presented using van Krevelen diagrams to explore the underlying formation mechanisms of aqSOAs. Large protonated and hydroxide water clusters (e.g., (H2O)nH+, 17 < n ≤ 44) have reasonable yields during UV aging. In contrast, small protonated and hydroxide water clusters (e.g., (H2O)nH+, 1 ≤ n ≤ 17) form after several hours of dark aging. Moreover, cluster ions have higher yields in dark aging, indicating the overlooked influence of dark aging interfacial products on aerosol optical properties. Molecular dynamic simulation shows that cluster ions form stably in UV and dark aging. AqSOAs molecules produced from dark and photochemical aging can enhance UV absorption of the aqueous surface, promote cloud condensation nuclei (CCN) activities, and affect radiative forcing.
Highlights
Introduction iationsThe formation mechanisms of organic aerosols at the air–liquid (a–l) interface are poorly understood despite the speculation that this interface may be highly reactive due to the unique surface properties different from the bulk [1,2]
Our results show that water clusters play an important role in the aqueous secondary organic aerosols (aqSOAs) formation in UV or dark aging
Many cluster ions are formed from the glyoxal oxidation at the a–l interface consisting of glyoxal and H2 O2, whereas the control samples of only glyoxal or H2 O2 do not
Summary
The formation mechanisms of organic aerosols at the air–liquid (a–l) interface are poorly understood despite the speculation that this interface may be highly reactive due to the unique surface properties different from the bulk [1,2]. Earth can generate water-soluble secondary organic aerosols (SOA) under sunlight and significantly impact particle formation [6] and affect aerosol hygroscopicity [7]. Given the specific surface properties (i.e., surface tension, surface ionic strength, reactivity), the a–l interface becomes a different environment for interfacial chemical reactions forming aqueous. Glyoxal is a predominant source of global SOA, generating 3–13 TgC/year globally [12,13]. It mainly stems from the Licensee MDPI, Basel, Switzerland
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