Molecular Characterization of Organosulfates in Organic Aerosols from Shanghai and Los Angeles Urban Areas by Nanospray-Desorption Electrospray Ionization High-Resolution Mass Spectrometry

Fine aerosol particles in the urban areas of Shanghai and Los Angeles were collected on days that were characterized by their stagnant air and high organic aerosol concentrations. They were analyzed by nanospray-desorption electrospray ionization mass spectrometry with high mass resolution (m/Delta m = 100,000). Solvent mixtures of acetonitrile and water and acetonitrile and toluene were used to extract and ionize polar and nonpolar compounds, respectively. A diverse mixture of oxygenated hydrocarbons, organosulfates, organonitrates, and organics with reduced nitrogen were detected in the Los Angeles sample. A majority of the organics in the Shanghai sample were detected as organosulfates. The dominant organosulfates that were detected at two locations have distinctly different molecular characteristics. Specifically, the organosulfates in the Los Angeles sample were dominated by biogenic products, while the organosulfates of a yet unknown origin found in the Shanghai sample had distinctive characteristics of long aliphatic carbon chains and low degrees of oxidation and unsaturation. The use of the acetonitrile and toluene solvent facilitated the observation of this type of organosulfates, which suggests that they could have been missed in previous studies that relied on sample extraction using common polar solvents. The high molecular weight and low degree of unsaturation and oxidization of the uncommon organosulfates suggest that they may act as surfactants and plausibly affect the surface tension and hygroscopicity of atmospheric particles. We propose that direct esterification of carbonyl or hydroxyl compounds by sulfates or sulfuric acid in the liquid phase could be the formation pathway of these special organosulfates. Long-chain alkanes from vehicle emissions might be their precursors.

在空气停滞且有机气溶胶浓度较高的日子里，收集了上海和洛杉矶市区的细气溶胶颗粒。使用具有高分辨率（质荷比m/Δm = 100,000）的纳米喷雾解吸电喷雾电离质谱法对其进行分析。乙腈和水以及乙腈和甲苯的混合溶剂分别用于提取和电离极性和非极性化合物。在洛杉矶的样本中检测到多种含氧化合物、有机硫酸盐、有机硝酸盐以及含氮还原的有机物的混合物。上海样本中的大部分有机物被检测为有机硫酸盐。在两个地点检测到的主要有机硫酸盐具有明显不同的分子特征。具体而言，洛杉矶样本中的有机硫酸盐以生物源产物为主，而上海样本中发现的来源尚不清楚的有机硫酸盐具有长脂肪族碳链以及低氧化度和不饱和度的独特特征。乙腈和甲苯溶剂的使用有助于观察到这种类型的有机硫酸盐，这表明在以往依赖普通极性溶剂进行样品提取的研究中可能遗漏了它们。这种不常见的有机硫酸盐的高分子量以及低不饱和度和氧化度表明它们可能起到表面活性剂的作用，并可能影响大气颗粒的表面张力和吸湿性。我们提出，液相中硫酸盐或硫酸对羰基或羟基化合物的直接酯化可能是这些特殊有机硫酸盐的形成途径。机动车排放的长链烷烃可能是它们的前体。