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. This study investigates the impact of the aerosol hygroscopicgrowth effect on haze events in Xingtai, a heavily polluted city in thecentral part of the North China Plain (NCP), using a large array of instrumentsmeasuring aerosol optical, physical, and chemical properties. Key instrumentsused and measurements made include the Raman lidar for atmospheric watervapor content and aerosol optical profiles, the PC-3016A GrayWolf six-channelhandheld particle and mass meter for atmospheric total particulate matter (PM)that has diameters less than 1 and 2.5  µ m (PM 1 andPM 2.5 , respectively), the aerosol chemical speciation monitor (ACSM) forchemical components in PM 1 , and the hygroscopic tandem differentialmobility analyzer (H-TDMA) for aerosol hygroscopicity. The changes inPM 1 and PM 2.5 agreed well with that of the water vapor content dueto the aerosol hygroscopic growth effect. Two cases were selected to furtheranalyze the effects of aerosol hygroscopic growth on haze events. Thelidar-estimated hygroscopic enhancement factor for the aerosol backscatteringcoefficient during a relatively clean period (Case I) was lower than thatduring a pollution event (Case II) with similar relative humidity (RH) levelsof 80 %–91 %. The Kasten model was used to fit the aerosol opticalhygroscopic growth factor (GF) whose parameter b differed considerably betweenthe two cases, i.e., 0.1000 (Case I) versus 0.9346 (Case II). The aerosolacidity value calculated from ACSM data for Case I (1.35) was less than thatfor Case II (1.50) due to different amounts of inorganics such as NH4NO3 , NH4HSO4 , and (NH4)2SO4 . Modelresults based on H-TDMA data showed that aerosol hygroscopic growth factorsin 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 similarambient RH levels, the high content of nitrate facilitates the hygroscopicgrowth of aerosols, which may be a major factor contributing to heavy hazeepisodes in Xingtai.

Aerosol hygroscopic growth, contributing factors, and impact on haze events in a severely polluted region in northern China