Particulate C2-3 Organosulfates: Method Development and Investigation of Environmental Factors Influencing Formation Processes

C_2-3 organosulfates (C_2-3OSs) are significant contributors to the overall abundance of OSs and secondary organic aerosols, featuring widespread occurrence and notable impact on aerosol properties. However, due to the lack of authentic standards and demanding techniques, accurate quantification of these C_2-3OSs remains scarce, causing bias in our understanding of their atmospheric chemistry. Existing data primarily rely on offline liquid chromatography/electrospray ionization-mass spectrometry (LC/ESI-MS) analysis. Neglecting matrix effects, as well as variations in instrument configurations across laboratories in implementing LC separation and electrospray ionization, introduce large uncertainties when comparing results. In this study, we first evaluated the efficacy of two previously adopted LC methods – reverse-phase liquid chromatography (RPLC) and hydrophilic interaction liquid chromatography (HILIC) – coupled with Orbitrap MS, in characterizing and quantifying PM_2.5-bound C_2-3OSs. Ambient aerosol samples were collected at four sites in southern China (two in Hong Kong and two in Guangzhou) during the summer (SM) and autumn+winter (AW) of 2020. We focused on three C_2-3OSs with available authentic standards: hydroxyacetone sulfate (HAS), glycolic acid sulfate (GAS), and lactic acid sulfate (LAS). Our results demonstrated that HILIC outperformed RPLC in retentive capacities and peak resolving abilities, generating more reliable quantitative measurements. RPLC had poor retention of small polar analytes. The RPLC/ESI-MS method significantly underestimated the concentrations of C_2-3OSs, which was attributed to the prevalent matrix effect that occurred in the gradient-front and the lack of adequate internal standards for compensation. This analytical work underscores the need for careful methodological considerations when studying small and polar OSs like C_2-3OSs.

Based on the HILIC/ESI-MS analysis, the sum concentration of C_2-3OSs across the four sites was dominated by GAS (avg. 37±56 ng/m³) and was in the range of 0.2-517 ng/m³, equivalent to 0.004-0.9% of PM_2.5 mass. The three C_2-3OSs exhibited strong correlations, suggesting their common or similar precursors and/or formation pathways. Despite geographical proximity, the Guangzhou sites recorded higher C_2-3OSs abundance than the Hong Kong sites (stricter air quality regulations). Contrary to the seasonality of biogenic emissions, C_2-3OSs concentrations were generally higher in AW than in SM for both regions, plausibly attributed to the mild seasonal contrasts in meteorological conditions (ΔT_SM-AW=8°C, ΔRH_SM-AW=15%) and the regional transport of polluted inland air by the northeasterly monsoon wind during AW. Key factors influencing C_2-3OSs formation included aerosol acidity, liquid water content, sulfate, O_x (O₃+NO₂), and trace metals (e.g., Fe). Notably, a typhoon-induced episode captured exceptionally high levels of C_2-3OSs, characterized by regional transport of air masses from polluted northwestern regions, stagnant air, aerosol pH at ~2.5, and high O_x & low NO conditions. These field-based observations uncover the intricate interplays between biogenic emissions, atmospheric chemistry, and meteorological parameters, especially highlighting the importance of atmospheric oxidation capacity, regional transport, and potential atmospheric aging processes in understanding C_2-3OSs formation.