## 3 Results and Discussion
### Aerosol Optical Properties
#### 3.3.2 Aerosol Absorption Properties  
The aerosol absorption coefficient \(\sigma_{\text{ab}}\) can reflect the absorption ability of aerosols. It can be measured directly through observing the amount of light attenuation using an aethalometer or calculated indirectly using BC concentration. Previous studies have described how to calculate \(\sigma_{\text{ab}}\) directly at any wavelength [58, 59]; however, in this experiment, \(\sigma_{\text{ab}}\) was calculated indirectly based on the following formula:  
\[\sigma_{\text{ab}}=\alpha\cdot\left[\text{BC}\right] \tag{6}\]  
where \(\alpha\) is the BC absorption efficiency conversion factor, which can be calculated according to the Mie theory or through linear regression between BC concentrations and aerosol absorption coefficients [60, 61, 62]. In this study, the value of \(\alpha\) used was 8.28 m\({}^{2}\cdot\text{g}^{-1}\). This was obtained through linear regression (with the correlation coefficient R\({}^{2}\) = 0.92) of the aerosol absorption coefficients (units: Mm\({}^{-1}\)) measured by PAS at 532 nm and BC concentrations (units: \(\mu\)g\(\cdot\)m\({}^{-3}\)) measured by the aethalometer at 880 nm [63]. [PERSON] _et al._ used this value at the Shangdianzi (SDZ) Station [64] in northern China, and [PERSON] _et al._ also used this value in the NCP [3]. The value of 8.28 m\({}^{2}\cdot\)g\({}^{-1}\) is higher than the optimal estimated value provided by Bergin (8 m\({}^{2}\cdot\)g\({}^{-1}\)) [65] and Bond (7.5 m\({}^{2}\cdot\)g\({}^{-1}\)) [66], and lower than that obtained in Colorado (10 m\({}^{2}\cdot\)g\({}^{-1}\)) [67] and the Mexico City Metropolitan Area (8-10 m\({}^{2}\cdot\)g\({}^{-1}\)) [68]. [PERSON] _et al._ found that the \(\alpha\) value is low (7 m\({}^{2}\cdot\)g\({}^{-1}\)) for diesel soot [69]. Table 3 lists the BC absorption efficiency conversion factor \(\alpha\) for the different sites.  
\begin{table}
\begin{tabular}{c c c} \hline
**Site** & \(\boldsymbol{\alpha(\text{m}^{2}\cdot\text{g}^{-1})}\) & **Reference** \\ \hline Urban Guangzhou, southern China & 8.28 & [63] \\ BTH, China & 8.28 & [3] \\ Texas, USA & 8.5 & Big Bend Regional Aerosol and Visibility \\  & & Observation Study (BRAVO) [61] \\ SDZ, China & 8.28 & [64] \\ Urban Beijing, China & 8 & [65] \\ Urban, Illinois, USA & 7.5 & [66] \\ Colorado, USA & 10 & Often used for urban aerosols [67] \\ Mexico City, Mexico & \(\sim\)8–10 & [68] \\ Colorado, USA & 7 & Diesel soot [69] \\ \hline \end{tabular}
\end{table}
Table 3: BC absorption efficiency conversion factor \(\alpha\) at different sites.  
Figure 4a shows the variability of \(\sigma_{\rm ab\_532}\). It increased sharply during the haze period and reached a maximum value (0.15 km\({}^{-1}\)) in the afternoon of 11 June. The mean values during the haze period and non-haze period were 0.086 and 0.025 km\({}^{-1}\), respectively, _i.e._, the former was approximately 3.4 times the latter. The results of [PERSON] _et al._ reveal that the mean absorption coefficient during haze periods is 0.056 km\({}^{-1}\) in the NCP, which is 5.2 times that during non-haze periods [3]. Additionally, the study by [PERSON] _et al._[49] at Wuqing indicates that the mean absorption coefficients during air pollution events in summer and spring were 0.11 and 0.09 km\({}^{-1}\), respectively. The mean value of \(\sigma_{\rm ab\_532}\) in Wuhan was larger than that in the NCP and similar to that at Wuqing, which shows that summer haze pollution from transported biomass combustion is a serious problem in central China.