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. In recent years, intense haze events in megacities such as Beijinghave received significant attention. Although secondary organic aerosol (SOA)has been identified as a major contributor to such events, knowledge of itssources and formation mechanisms remains uncertain. We investigate thisquestion through the first field deployment of the extractive electrosprayionisation time-of-flight mass spectrometer (EESI-TOF) in Beijing, togetherwith an Aerodyne long-time-of-flight aerosol mass spectrometer (L-TOF AMS).Measurements were performed during autumn and winter 2017, capturing thetransition from non-heating to heating seasons. Source apportionmentresolved four factors related to primary organic aerosols (traffic, cooking,biomass burning, and coal combustion), as well as four related to SOA. Of the SOA factors, two were related to solid fuelcombustion (SFC), one to SOA generated from aqueous chemistry, and one tomixed/indeterminate sources. The SFC factors were identified from spectralsignatures corresponding to aromatic oxidation products, while the aqueousSOA factor was characterised by signatures of small organic acids anddiacids and unusually low CO+/CO2+ fragment ratios measuredby the AMS. Solid fuel combustion was the dominant source of SOA during theheating season. However, a comparably intense haze event was also observedin the non-heating season and was dominated by the aqueous SOA factor.During this event, aqueous chemistry was promoted by the combination of highrelative humidity and air masses passing over high-NOx regions to thesouth and east of Beijing, leading to high particulate nitrate. Theresulting high liquid water content was highly correlated with theconcentration of the aqueous SOA factor. These results highlight the strongcompositional variability between different haze events, indicating the needto consider multiple formation pathways and precursor sources to describeSOA during intense haze events in Beijing.

Quantification of solid fuel combustion and aqueous chemistry contributions to secondary organic aerosol during wintertime haze events in Beijing