Abstract. The implementation of the Air Pollution Prevention and
Control Action Plan in China since 2013 has profoundly altered the ambient
pollutants in the Beijing–Tianjin–Hebei (BTH) region. Here we show
observations of substantially increased O3 concentrations (about
30 %) and a remarkable increase in the ratio of organic carbon (OC) to
elemental carbon (EC) in BTH during the autumn from 2013 to 2015, revealing
an enhancement in atmospheric oxidizing capacity (AOC) and secondary organic
aerosol (SOA) formation. To explore the impacts of increasing AOC on the SOA
formation, a severe air pollution episode from 3 to 8 October 2015 with high
O3 and PM2.5 concentrations is simulated using the WRF-Chem model.
The model performs reasonably well in simulating the spatial distributions
of PM2.5 and O3 concentrations over BTH and the temporal
variations in PM2.5, O3, NO2, OC, and EC concentrations in
Beijing compared to measurements. Sensitivity studies show that the change
in AOC substantially influences the SOA formation in BTH. A sensitivity case
characterized by a 31 % O3 decrease (or 36 % OH decrease) reduces
the SOA level by about 30 % and the SOA fraction in total organic aerosol
by 17 % (from 0.52 to 0.43, dimensionless). Spatially, the SOA decrease
caused by reduced AOC is ubiquitous in BTH, but the spatial relationship
between SOA concentrations and the AOC is dependent on the SOA precursor
distribution. Studies on SOA formation pathways further show that when the
AOC is reduced, the SOA from oxidation and partitioning of semivolatile primary organic aerosol
(POA)
and co-emitted intermediate volatile organic compounds (IVOCs) decreases
remarkably, followed by those from anthropogenic and biogenic volatile organic compounds (VOCs).
Meanwhile, the SOA decrease in the irreversible uptake of glyoxal and
methylglyoxal on the aerosol surfaces is negligible.
Aging of atmospheric aerosols and the role of iron in catalyzing brown carbon formation
