## 3 Results and Discussion
### Aerosol Optical Properties
#### 3.3.4 Aerosol Vertical Structure during Haze  
Ground-based observations could provide information about surface aerosol properties through field experiments. However, the vertical distribution of particles is also needed in order to obtain detailed information on the spatial structure of pollutants during haze events. Seasonal variations in the vertical distribution of aerosols have been studied employing Raman Lidar at Shangdianzi station. It has been established that a two-layer structure exists in haze events [75]. The present study made use of the CALIPSO 532 nm total attenuated backscatter coefficient \(\beta_{532,\mathrm{Total}}(\mathrm{z})\), volume depolarization ratio V(z), total attenuated color ratio X(z), and aerosol sub-layer classification product to explore the aerosol vertical structure. The variables can be calculated separately according to the following formulas ([[http://www-calipso.larc](http://www-calipso.larc)]([http://www-calipso.larc](http://www-calipso.larc)). nasa.gov/resources/calipso_users_guide/browse/index.php):  
\[\beta_{532,\mathrm{Total}}(\mathrm{z})=[\beta_{\mathrm{l}}(\mathrm{z})+\beta_{ \mathrm{\perp}}(\mathrm{z})]T_{532}^{2}(\mathrm{z}) \tag{7}\]  
Here, \(\beta_{\mathrm{l}}(\mathrm{z})\) and \(\beta_{\mathrm{\perp}}(\mathrm{z})\) are the backscatter coefficients of the perpendicular channel and the parallel channel at height z for wavelength 532 nm, respectively, and T\({}_{532}^{2}(\mathrm{z})\) is the atmospheric bidirectional transmission at the wavelength of 532 nm. It is computed using the following formula:  
\[T_{532}^{2}(\mathrm{z})=\exp\left\{-2\int\limits_{\mathrm{z}}^{\mathrm{z}_{ \mathrm{sat}}}\left[\sigma_{\mathrm{m}}(\mathrm{z}\cdot)+\sigma_{\mathrm{a}}( \mathrm{z}\cdot)+\sigma_{\mathrm{o}_{3}}(\mathrm{z}\cdot)\right]\mathrm{dz}\cdot\right\} \tag{8}\]  
where \(\sigma_{\mathrm{m}},\sigma_{\mathrm{a}}\), and \(\sigma_{\mathrm{o}_{3}}\) are the extinction coefficients of atmospheric molecules, aerosols, and ozone, respectively, and \(\mathrm{z}_{\mathrm{sat}}\) represents the height of the satellite. The volume depolarization ratio V(z) is defined as the ratio of the perpendicular channel to the parallel channel:  
\[V(\mathrm{z})=\frac{\beta_{532,\mathrm{\perp}}(\mathrm{z})}{\beta_{532, \mathrm{\parallel}}(\mathrm{z})} \tag{9}\]  
V(z) can reflect the irregularity and non-spherical features of the particles. The total attenuated color ratio X(z) is defined as the ratio of backscattering energy at 1064 nm to that at 532 nm, which can reflect particle size; the larger the size, the larger the value of X(z). It can be defined as follows:  
\[X(\mathrm{z})=\frac{\beta_{1064}(\mathrm{z})}{\beta_{532,\mathrm{Total}}( \mathrm{z})} \tag{10}\]  
As shown in Figure 1b, the CALIPSO satellite passed over Wuhan (29.00\({}^{\circ}\)-32.00\({}^{\circ}\)N, 113.91\({}^{\circ}\)E) at 02:19:48 (LZT) on 12 June 2012, which was the day with the most severe haze. Figure 5a shows the total attenuated backscatter coefficient for the 532-nm orbit height section of CALIPSO over Wuhan. For most cases, the green-yellow-orange colors represent the characteristics of the aerosols, whereas the red-gray-white colors represent the characteristics of the optically thick aerosol layer or optically thin clouds. Blue usually represents areas without signals [42]. Aerosols can be seen primarily at altitudes lower than 3 km, and their concentrations decrease considerably with height. The major components of the atmosphere above 5 km are air molecules. The CALIPSO Level 2 aerosol sub-layer classification product can be used to identify the types of aerosols (e.g., clean marine aerosols, dust aerosols, polluted continental aerosols, clean continental aerosols, polluted dust aerosols, and smoke aerosols). As shown in Figure 5b, several prominent stratifications can be seen below 3 km. During the haze episode, there were two main segments: a polluted dust aerosol layer and a polluted continental aerosol layer.