## 3 Results and Discussion
### Aerosol Optical Properties
#### 3.3.1 Aerosol Scattering Properties
It can show that a decreasing trend with increasing aerosol size [23] is related to the spectral dependence of the aerosol scattering coefficient, and can be measured using a multi-wavelength nephelometer. The value of \(\alpha_{\text{s}}\) is determined by the aerosol size distribution and refractive index, and  
Figure 4: Time series variations of aerosol optical properties during haze at LIESMARS site during 6–14 June 2012. (**a**) Aerosol total scattering coefficient \(\sigma_{\text{s}}\_550\) (\(\text{km}^{-1}\)) at wavelength of 550 nm and absorption coefficient \(\sigma_{\text{ab}}\_532\) (\(\text{km}^{-1}\)) at 532 nm; (**b**) Scattering Angstrom exponents \(\alpha_{\text{s}}\) and backscatter ratio b 550 at wavelength of 550 nm; (**c**) Up-scatter fraction \(\beta\_550\) and asymmetry parameter g\_550 at wavelength of 550 nm; and (**d**) SSA \(\omega_{0}\) at wavelength of 532 nm. exhibits little relationship with aerosol concentration. Generally, it can be described using an empirical equation in the range of the visible spectrum:  
\[\sigma_{\mathrm{s}}(\lambda)=\beta_{\mathrm{s}}\cdot\lambda^{-\alpha_{\mathrm{s}}} \tag{2}\]  
Here, \(\sigma_{\mathrm{s}}(\lambda)\) is the aerosol scattering coefficient at wavelength \(\lambda\), \(\beta_{\mathrm{s}}\) is the conversion constant, and \(\alpha_{\mathrm{s}}\) is the dimensionless scattering Angstrom exponent. In order to cover a wide wavelength range, \(\alpha_{\mathrm{s}}\) was calculated for wavelengths of 450 and 700 nm using the following formula:  
\[\alpha_{\mathrm{s}}=-\ln\left[\frac{\sigma_{\mathrm{s}}(\lambda_{1})}{\sigma_ {\mathrm{s}}(\lambda_{2})}\right]\cdot\left[\ln\left(\frac{\lambda_{1}}{ \lambda_{2}}\right)\right]^{-1} \tag{3}\]  
Figure 4b shows the variation of \(\alpha_{\mathrm{s}}\) during this study. As shown, \(\alpha_{\mathrm{s}}\) decreased rapidly from 1.7 in the morning to around 1.3 at noon on 11 June, and remained consistently at about 1.4 until noon on June 12. This result indicates that particle size increased during the haze period. In the study by [PERSON] _et al._, the mean value of \(\alpha_{\mathrm{s}}\) during the haze episode was 1.33, indicating that larger particles dominated in the haze [23]. In the current study in Wuhan, the \(\alpha_{\mathrm{s}}\) value of 1.4 indicates the growth trend of particle size, but that the particle size was not large and that the majority of aerosols were still fine particles. [PERSON] _et al._ reported that haze episodes caused by plumes transported over long distances often have higher values of \(\alpha_{\mathrm{s}}\) than haze caused by local emissions, and that long-distance transport appears to assist the formation of haze episodes [23]. In the present study, the relatively high value of \(\alpha_{\mathrm{s}}\) is consistent with the conclusions of [PERSON] _et al._; however, the results of [PERSON] _et al._ from a study in Guangzhou also demonstrated that fine particles can play a dominant role in causing haze [11]. Particles emitted from biomass combustion can increase in size during long-distance transport, leading to a decreasing trend of \(\alpha_{\mathrm{s}}\)[50], because particles can experience processes of solidification and condensation during transportation [51]. In such cases, aerosols also undergo an aging process over time, which also acts to increase particle size. Previous studies by [PERSON] _et al._[52, 53] and [PERSON] _et al._[54] have indicated that \(\alpha_{\mathrm{s}}\) can be reduced by \(>\)50% during the aging process of particles. Moreover, during long-distance transport, water-soluble secondary aerosols could be formed in haze. Owing to the comparatively higher RH during the haze period, such hydrophilic aerosols would have easier access to water, which would also result in increasing particle sizes [55]. (3) Aerosol Asymmetry Parameter and Backscatter Ratio  
The aerosol asymmetry parameter (g) reflects the forward scattering capability of particles and is typically dependent on the scattering phase function.