Rapid economic development along with urbanization and industrialization has exacerbated air pollution in China

particularly haze pollution

in recent years. The Beijing-Tianjin-Hebei area

the Yangtze River Delta

and the Pearl River Delta region are the three major focus areas of haze pollution in China.In January 2013

a high-intensity continuous atmospheric haze pollution swept across East Central China. According to the AirQuality Index

this phenomenon occurred five times in Beijing

and the score reached beyond 200（ severe pollution）

occurring on January 6—8

9—15

17—19

22—23

and 25—31. At present

studies on haze pollution focus on physical and optical properties

influencing factors

haze source apportionment

aerosol chemistry and other aspects. Few studies investigated the vertical properties of large-scale

high-intensity haze aerosols. Vertical distribution of aerosols is one of the most critical and uncertain factors of radiative forcing and climate impact.The vertical distribution of aerosol optical properties during haze days in North China Plain（ NCP） in January 2013 are analyzed by using Cloud-Aerosol LiD AR with Orthogonal Polarization（ CALIOP） data

Aerosol Robotic Network（ AERONET）data

ground meteorological observation data

and Hybrid Single-Particle Lagrangian Integrated Trajectory（ HYSPLIT） model.CALIOP laser radar provided the vertical distribution of aerosol and cloud features with high vertical resolution（ 30 m）. The Level1 B aerosol layer products of CALIOP that overpass NCP（ 34° N to 41° N

114° E to 120° E） in January 2013

including attenuated backscatter coefficient

volume depolarization ratio

and total attenuated color ratio

were used in the study. The AERONET Aerosol Optical Thickness（ AOT） at 500 nm and Angstrom exponent of 440—870 nm at the Beijing（ 39. 97° N

116. 38° E） and Xianghe（ 39. 75° N

116. 96° E） sites were used to investigate the aerosol properties in hazy days. In addition

HYSPLIT model was used to simulate the backward trajectory of heavy haze process of NCP（ 34° N to 41° N

114° E to 120° E） on January29

2013.Our results are listed as below:（ 1） The meteorological elements of three haze events（ January 9—15

22—23

and 25—31） in January 2013 were compared with the 30-year（ 1980—2010） mean of meteorological elements at the Beijing site. The results showed that the average relative humidity during the haze events is about 47% higher than the 30-year mean and that the average wind speed during the haze events is about 35% lower than the 30-year mean. These findings indicate that the increased relative humidity and decreased wind speed in January 2013 are unfavorable for cleaning aerosols

thereby resulting in severe haze.（ 2） The lowest troposphere（ < 2 km） was polluted most severely during the hazy days in NCP in January 2013. The total attenuated backscatter coefficient increased to 0. 0045 km- 1sr- 1

volume depolarization ratios were greater than 20%

and the maximum value of color ratio was greater than 0. 8. These findings suggest that aerosol particles aggregate in the lowest troposphere

and a large number of irregular coarse particles coexist during the haze events in NCP.（ 3） The color ratios below 2 km and 6—8 km heights increased during the severe haze on January 29. This finding indicates an increased proportion of large particles. The 6—8 km height may be affected by coarse particle aerosol delivery and cloud particles. The lower atmosphere below 2 km is influenced by local construction dust

which resulted in increased proportion of coarse particles.（ 4） AOT（ 500 nm） increased from 0. 2 to 2. 1

and the Angstrom index decreased from 1. 4 to 0. 9 during the severe haze on January 29

which indicated that the proportion of coarse particles mixed in the atmospheric aerosol increased. The simulation results of HYSPLIT backward trajectory showed that the airflow at heights of 500 m

1000 m

and 1500 m comes from Mongolia

Inner Mongolia

China

and Northwest China

and finally affects the NCP.Three severe haze events in NCP in January 2013 were analyzed by using CALIOP data

AERONET data

ground meteorological observation data

and HYSPLIT model. The three-dimensional structure of the haze is provided when the size distribution and possible sources of atmospheric aerosol during haze events are analyzed in depth. Results show that heavy haze pollution in NCP is not only composed of fine aerosol particles from human activities but also accompanied by coarse dust aerosols. Dust aerosols from remote region could affect the haze composition in NCP.