Abstract. This study investigates the impact of the aerosol hygroscopic
growth effect on haze events in Xingtai, a heavily polluted city in the
central part of the North China Plain (NCP), using a large array of instruments
measuring aerosol optical, physical, and chemical properties. Key instruments
used and measurements made include the Raman lidar for atmospheric water
vapor content and aerosol optical profiles, the PC-3016A GrayWolf six-channel
handheld particle and mass meter for atmospheric total particulate matter (PM)
that has diameters less than 1 and 2.5 µm (PM1 and
PM2.5, respectively), the aerosol chemical speciation monitor (ACSM) for
chemical components in PM1, and the hygroscopic tandem differential
mobility analyzer (H-TDMA) for aerosol hygroscopicity. The changes in
PM1 and PM2.5 agreed well with that of the water vapor content due
to the aerosol hygroscopic growth effect. Two cases were selected to further
analyze the effects of aerosol hygroscopic growth on haze events. The
lidar-estimated hygroscopic enhancement factor for the aerosol backscattering
coefficient during a relatively clean period (Case I) was lower than that
during a pollution event (Case II) with similar relative humidity (RH) levels
of 80 %–91 %. The Kasten model was used to fit the aerosol optical
hygroscopic growth factor (GF) whose parameter b differed considerably between
the two cases, i.e., 0.1000 (Case I) versus 0.9346 (Case II). The aerosol
acidity value calculated from ACSM data for Case I (1.35) was less than that
for Case II (1.50) due to different amounts of inorganics such as
NH4NO3, NH4HSO4, and (NH4)2SO4. Model
results based on H-TDMA data showed that aerosol hygroscopic growth factors
in each size category (40, 80, 110, 150, and 200 nm) at different RH levels
(80 %–91 %) for Case I were lower than those for Case II. For similar
ambient RH levels, the high content of nitrate facilitates the hygroscopic
growth of aerosols, which may be a major factor contributing to heavy haze
episodes in Xingtai.