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. A time-of-flight chemical ionization mass spectrometer (CIMS)utilizing the Filter Inlet for Gas and Aerosol (FIGAERO) was deployed at aregional site 40 km north-west of Beijing and successfully identified andmeasured 17 sulfur-containing organics (SCOs are organo/nitrooxy organosulfates and sulfonates) with biogenic and anthropogenic precursors. The SCOswere quantified using laboratory-synthesized standards of lactic acid sulfateand nitrophenol organosulfate (NP OS). The variation in field observationswas confirmed by comparison to offline measurement techniques (orbitrap andhigh-performance liquid chromatography, HPLC) using daily averages. The meantotal (of the 17 identified by CIMS) SCO particle mass concentration was210  ±  110 ng m −3 and had a maximum of 540 ng m −3 ,although it contributed to only 2  ±  1 % of the organic aerosol (OA).The CIMS identified a persistent gas-phase presence of SCOs in the ambientair, which was further supported by separate vapour-pressure measurements ofNP OS by a Knudsen Effusion Mass Spectrometer (KEMS). An increase inrelative humidity (RH) promoted partitioning of SCO to the particle phase,whereas higher temperatures favoured higher gas-phase concentrations. Biogenic emissions contributed to only 19 % of total SCOs measured in thisstudy. Here, C10H16NSO7 , a monoterpene-derived SCO,represented the highest fraction (10 %) followed by an isoprene-derivedSCO. The anthropogenic SCOs with polycyclic aromatic hydrocarbon (PAH) andaromatic precursors dominated the SCO mass loading (51 %) with C11H11SO7 , derived from methyl naphthalene oxidation,contributing to 40 ng m −3 and 0.3 % of the OA mass. Anthropogenic-related SCOs correlated well with benzene, although their abundance dependedhighly on the photochemical age of the air mass, tracked using the ratiobetween pinonic acid and its oxidation product, acting as a qualitativephotochemical clock. In addition to typical anthropogenic and biogenicprecursors the biomass-burning precursor nitrophenol (NP) provided asignificant level of NP OS. It must be noted that the contribution analysishere is only representative of the detected SCOs. There are likely tobe many more SCOs present which the CIMS has not identified. Gas- and particle-phase measurements of glycolic acid suggest thatpartitioning towards the particle phase promotes glycolic acid sulfateproduction, contrary to the current formation mechanism suggested in theliterature. Furthermore, the HSO4 ⋅ H 2 SO 4 - cluster measured by theCIMS was utilized as a qualitative marker for acidity and indicates that theproduction of total SCOs is efficient in highly acidic aerosols with high SO 4 2 - and organic content. This dependency becomes more complexwhen observing individual SCOs due to variability of specific VOC precursors.

Online gas- and particle-phase measurements of organosulfates, organosulfonates and nitrooxy organosulfates in Beijing utilizing a FIGAERO ToF-CIMS