Molecular and seasonal characteristics of organic vapors in urban Beijing: insights from Vocus-PTR measurements

Understanding the composition and evolution of atmospheric organic vapors is crucial for exploring their impact on air quality. However, the molecular and seasonal characteristics of organic vapors in urban areas, with complex anthropogenic emissions and high variability, remain inadequately understood. In this study, we conducted measurements in urban Beijing during 2021-2022, covering four seasons, using improved proton-transfer-reaction mass spectrometry (Vocus-PTR-MS). During the measurement period, a total of 895 peaks were observed, and 512 of them can be assigned to formulas. The contribution of C_xH_yO_z species is most significant, composing up to 54 % of the number and 74 % of the mixing ratio of total organics. With enhanced sensitivity and mass resolution, various species with multiple oxygens (≥ 3) or at a level of sub-parts per trillion (ppt) were observed, with 44 % of the number measured at sub-ppt level and 38 % of the number containing three-eight oxygen atoms. Organic vapors with multiple oxygens mainly consist of intermediate/semi-volatile compounds, and many formulae detected were reported to be the oxidation products of various volatile organic precursors. In summer, the fast photooxidation process generated organic vapors with multiple oxygens and led to an increase in both their mixing ratio and proportion, while, in other seasons, the variations in organic vapors with multiple oxygens were closely correlated with those of organic vapors with 1-2 oxygens, which could be substantially influenced by primary emissions. Organic vapors with low oxygen content (≤ 2 oxygens) are comparable to the results obtained by traditional PTR-MS measurements in both urban Beijing and neighboring regions.