Aerosol optical and microphysical parameters retrieved from ground-based sun-sky photometer （ CE318） measurements are used to investigate aerosol properties during heavy haze days. The heavy haze episode in January 2013 is analyzed

and we find that: （ 1） Aerosol Optical Depth （ AOD） in haze days （ an average of 0. 87 at 440 nm）

is much higher than that of clean days

and can reach up to about 3 in some seriously polluted conditions. （ 2） Aerosol properties are well related to the haze pollution levels. For example

the averaged ngstrm exponent

imaginary part of refractive index

single scattering albedo and asymmetry factor under clean condition are 1. 3

0. 04

0. 73 and 0. 58

respectively

while the corresponding values in polluted days change to 0. 95

0 . 01

0. 92 and 0. 67

respectively. （ 3） The volume proportion of fine mode aerosol is higher than that of coarse mode during the haze episode. The average volume ratio of fine mode to total aerosol is 73%

with the maximum of 90. 5% . The fine mode radius peak increases with the increase of AOD

indicating clearly hygroscopic growth effects

while that of coarse mode aerosol decreases accordingly. Moreover

the peak radius of size distribution is 0. 43 μm when the heaviest haze pollution during January 2013 occurs.