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Abstract. The implementation of the Air Pollution Prevention andControl Action Plan in China since 2013 has profoundly altered the ambientpollutants in the Beijing–Tianjin–Hebei (BTH) region. Here we showobservations of substantially increased O3 concentrations (about30 %) and a remarkable increase in the ratio of organic carbon (OC) toelemental carbon (EC) in BTH during the autumn from 2013 to 2015, revealingan enhancement in atmospheric oxidizing capacity (AOC) and secondary organicaerosol (SOA) formation. To explore the impacts of increasing AOC on the SOAformation, a severe air pollution episode from 3 to 8 October 2015 with high O3 and PM 2.5 concentrations is simulated using the WRF-Chem model.The model performs reasonably well in simulating the spatial distributionsof PM 2.5 and O3 concentrations over BTH and the temporalvariations in PM 2.5 , O3 , NO2 , OC, and EC concentrations inBeijing compared to measurements. Sensitivity studies show that the changein AOC substantially influences the SOA formation in BTH. A sensitivity casecharacterized by a 31 % O3 decrease (or 36 % OH decrease) reducesthe SOA level by about 30 % and the SOA fraction in total organic aerosolby 17 % (from 0.52 to 0.43, dimensionless). Spatially, the SOA decreasecaused by reduced AOC is ubiquitous in BTH, but the spatial relationshipbetween SOA concentrations and the AOC is dependent on the SOA precursordistribution. Studies on SOA formation pathways further show that when theAOC is reduced, the SOA from oxidation and partitioning of semivolatile primary organic aerosol(POA)and co-emitted intermediate volatile organic compounds (IVOCs) decreasesremarkably, followed by those from anthropogenic and biogenic volatile organic compounds (VOCs).Meanwhile, the SOA decrease in the irreversible uptake of glyoxal andmethylglyoxal on the aerosol surfaces is negligible.

Secondary organic aerosol enhanced by increasing atmospheric oxidizing capacity in Beijing–Tianjin–Hebei (BTH), China