Abstract. Water vapor has been proposed to amplify the severe haze
pollution in China by enhancing the aerosol–radiation feedback (ARF).
Observations have revealed that the near-surface PM2.5 concentrations
([PM2.5]) generally exhibit an increasing trend with relative humidity
(RH) in the North China Plain (NCP) during 2015 wintertime, indicating that the
aerosol liquid water (ALW) caused by hygroscopic growth could play an
important role in the PM2.5 formation and accumulation. Simulations
during a persistent and heavy haze pollution episode from 5 December 2015
to 4 January 2016 in the NCP were conducted using the WRF-Chem Model to
comprehensively quantify contributions of the ALW effect to near-surface
[PM2.5]. The WRF-Chem Model generally performs reasonably well in
simulating the temporal variations in RH against measurements in the NCP. The
factor separation approach (FSA) was used to evaluate the contribution of
the ALW effect on the ARF, photochemistry, and heterogeneous reactions to
[PM2.5]. The ALW not only augments particle sizes to enhance aerosol
backward scattering but also increases the effective radius to favor
aerosol forward scattering. The contribution of the ALW effect on the ARF
and photochemistry to near-surface [PM2.5] is not significant,
being generally within 1.0 µg m−3 on average in the NCP during the episode.
Serving as an excellent substrate for heterogeneous reactions, the ALW
substantially enhances the secondary aerosol (SA) formation, with an average
contribution of 71 %, 10 %, 26 %, and 48 % to near-surface sulfate,
nitrate, ammonium, and secondary organic aerosol concentrations.
Nevertheless, the SA enhancement due to the ALW decreases the aerosol
optical depth and increases the effective radius to weaken the ARF, reducing
near-surface primary aerosols. The contribution of the ALW total effect to
near-surface [PM2.5] is 17.5 % on average, which is overwhelmingly
dominated by enhanced SA. Model sensitivities also show that when the RH is
less than 80 %, the ALW progressively increases near-surface [PM2.5]
but commences to decrease when the RH exceeds 80 % due to the high
occurrence frequencies of precipitation.