Abstract
Organosulfur compounds are found to be ubiquitous in atmospheric aerosols — a majority of which are expected to be organosulfates (OSs). Given the atmospheric abundance of OSs, and their potential to form a variety of reaction products upon ageing, it is imperative to study the transformation kinetics and chemistry of OSs to better elucidate their atmospheric fates and impacts. In this work, we investigated the chemical transformation of an α-pinene derived organosulfate (C10H17O5SNa, αpOS-249) through heterogeneous OH oxidation at a relative humidity of 50 % in an oxidation flow reactor (OFR). The aerosol-phase reaction products were characterized using the high-performance liquid chromatography-electrospray ionization-high resolution mass spectrometry and the ion chromatography. By monitoring the decay rates of αpOS-249, the effective heterogeneous OH reaction rate was measured to be (6.72 ± 0.55) × 10−13 cm3 molecule−1 s−1. This infers an atmospheric lifetime of about two weeks at an average OH concentration of 1.5 × 106 molecules cm–3. Product analysis shows that OH oxidation of αpOS-249 can yield more oxygenated OSs having a nominal mass-to-charge ratio (m/z) at 247 (C10H15O5S−), 263 (C10H15O6S−), 265 (C10H17O6S−), 277 (C10H13O7S−), 279 (C10H15O7S−), and 281 (C10H17O7S−). The formation of fragmentation products, including both small OSs (C < 10) and inorganic sulfates, is found to be insignificant. These observations suggest that functionalization reactions are likely the dominant processes and that multigenerational oxidation possibly leads to formation of products with one or two hydroxyl and carbonyl functional groups adding to αpOS-249. Furthermore, all product ions except m/z = 277 have been detected in laboratory generated α-pinene derived secondary organic aerosols as well as in atmospheric aerosols. Our results reveal that OSs freshly formed from the photochemical oxidation of α-pinene could react further to form OSs commonly detected in atmospheric aerosols through heterogeneous OH oxidation. Overall, this study provides more insights into the sources, transformation, and fate of atmospheric OSs.
Highlights
Sulfur-containing aerosols are of particular significance for human health because of their high abundance and significant impacts on regional air quality and global climate
The formation of fragmentation products, including both small OSs (C < 10) and inorganic sulfates, is found to be insignificant. These observations suggest that 5 functionalization reactions are likely the dominant processes and that multigenerational oxidation possibly leads to formation of products with one or two hydroxyl and carbonyl functional groups adding to αpOS-249
Our results reveal that OSs freshly formed from the 10 photochemical oxidation of α-pinene could react further to form OSs commonly detected in atmospheric aerosols through heterogeneous OH oxidation
Summary
Sulfur-containing aerosols are of particular significance for human health because of their high abundance and significant impacts on regional air quality and global climate. 30 that the abundance of OSs reported in field studies may have been underestimated if their removal processes have not been properly accounted for Another significant implication is the urgency to obtain better understanding of the transformation of OSs, which can allow a better assessment of the sources and environmental impacts of atmospheric OSs. α-pinene is an atmospherically important biogenic VOC, which can undergo photochemical oxidation to form secondary organic aerosols (SOA) (Kanakidou et al, 2005; Pye et al, 2010; Guenther et al, 2012). The OH exposure, a product of gas-phase OH radical concentration and the residence time, ranged from 0–17.4 × 1011 molecule cm−3 20 s and was determined by measuring the decay of sulfur dioxide (SO2) in independent calibrating experiments in the presence or absence of αpOS-249 aerosols (Kang et al, 2007). Filters were immediately stored at -20 °C in the dark and analyzed within 3 months
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