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. Sulfate formation mechanisms have been discussedextensively but are still disputed. In this work, a year-long particulatematter (PM 2.5 ) sampling campaign was conducted together withmeasurements of gaseous pollutant concentrations and meteorologicalparameters in Beijing, China, from March 2012 to February 2013. The sulfuroxidation ratio (SOR), an indicator of secondary sulfate formation,displayed a clear summer peak and winter valley, even though no obviousseasonal variations in sulfate mass concentration were observed. A rapidrise in the SOR was found at a relative humidity (RH) threshold of ∼45  % or an O3 concentration threshold of ∼35  ppb, allowing us tofirst introduce the idea that RH and O3 concentrations are twoprerequisites for rapid sulfate formation via multiphase reactions. In thecase of the RH threshold, this is consistent with current understanding ofthe multiphase formation of sulfate, since it relates to thesemisolid-to-liquid phase transition of atmospheric aerosols. Correlationanalysis between SOR and aerosol water content (AWC) further backed this up. In the case of the O3 concentration threshold, this is consistent with the consumption ofliquid oxidants in multiphase sulfate formation. The thresholds introducedhere lead us to a better understanding of the sulfate formation mechanisms andsulfate formation variations. H2O2 might be the major oxidant ofsulfate formation, since another liquid-phase oxidant, O3 , haspreviously been shown to be unimportant. The seasonal variations in sulfateformation could be accounted for by variations in the RH and O3 prerequisites. For example, over the year-long study, the fastest SO2 -to-sulfate conversion occurred in summer, which was associated withthe highest values of O3 (and also H2O2 ) concentration andRH. The SOR also displayed variations with pollution levels; i.e. the SORincreased with PM 2.5 in all seasons. Such variations were primarilyassociated with a transition from the slow gas-phase formation of sulfate torapid multiphase reactions, since RH increased higher than its prerequisitevalue of around 45 % as pollution evolved. In addition, the self-catalyticnature of sulfate formation (i.e. the formation of hydrophilic sulfateaerosols under high RH conditions results in an increase in aerosol watercontent, which results in greater particle volume for further multiphasesulfate formation) also contributed to variations among the pollutionscenarios.

Relative humidity and O&lt;sub&gt;3&lt;/sub&gt; concentration as two prerequisites for sulfate formation

Relative humidity and O<sub>3</sub> concentration as two prerequisites for sulfate formation