## 2 Equipment and Method  
### Ground-Based Instruments  
A nephelometer can measure the scattering and backscattering coefficients of atmospheric aerosols. An integrated nephelometer (model 3563; TSI Inc., MN, USA) with triple wavelengths (450, 550, and 700 nm) was used in the present study. Automatic zero calibration was performed daily, standard CO\({}_{2}\) was used as the span gas during the half-yearly span checking [28], and the equipment was calibrated a week prior to the experiment. When in operation, a built-in turbo-blower transfers air samples into the measuring chamber through a large feeding port at a flow rate of 30 L\(\cdot\)min\({}^{-1}\). The average sampling time was set to 1 min. The sensitivity of the equipment when measuring the light scattering coefficient was 2.0 \(\times\) 10\({}^{-7}\) m\({}^{-1}\) and the response time was less than 10 s. The observation angle was 7\({}^{\circ}\)-170\({}^{\circ}\) with the truncation error limited to 10% or less [29].  
The truncation error increases when large aerosol particles (such as dust) exist. Its correction is usually achieved based on the Mie theory and statistical features [30]. In the present study, the correction was performed based on the latter. For the wavelengths 450, 550, and 700 nm, the detection limits of the total scattering coefficient are 0.44, 0.17, and 0.26 Mm\({}^{-1}\), respectively, and those for the backscattering coefficient are 0.29, 0.11, and 0.21 Mm\({}^{-1}\), respectively [31]. Values lower than the detection limits were excluded from the analysis. Observation errors were derived mainly from truncation errors, non-Lambert characteristics of the light source, non-idealized wavelengths, and uncertainties in calibration. These factors contributed to approximately 7% of the determined data error [28,30].  
An aethalometer (AE-31; Magee Scientific Corp., CA, USA) was employed to obtain the mass concentration of BC. It uses seven wavelengths: 370, 470, 520, 590, 660, 880, and 950 nm, and the aerosol absorption coefficient is retrieved based on the BC mass concentration. The aethalometer measures the real-time light attenuation caused by particles clustered on the quartz filtration membrane. It is assumed that the light attenuation is caused by BC absorption. Therefore, the BC mass concentration can be calculated continuously based on changes in the light attenuation [31]. On a unit area of the filtration membrane, light attenuation exhibits a linear relationship with the BC mass concentration as follows:  
\[\Delta\mathrm{ATN}_{\lambda}=\ln\left(\frac{I_{0}}{I}\right)=\delta_{\lambda} \cdot\mathrm{BC} \tag{1}\]  
Here, \(\Delta\mathrm{ATN}_{\lambda}\) is the light attenuation at wavelength \(\lambda\) within the sampling period; \(I_{0}\) and \(I\) are the light intensities that pass through the filtration membrane before and after sampling, respectively; and \(\delta_{\lambda}\) is the attenuation coefficient at wavelength \(\lambda\) (units = m\({}^{2}\cdot\)g\({}^{-1}\)). For this study, the aethalometer was installed on the top of the lab building in a position free of any obstruction. The flow rate was 5 L\(\cdot\)min\({}^{-1}\) with a 2-min sampling time. The instrument inlet was connected through a 2.5-m-long black tube, the interior of which was coated with Teflon; hence, sample loss (particles attaching to the tube wall) could be neglected. A 2.5-\(\upmu\)m sampling head was installed in the experiment. Quality assurance measures included checking the stability of the light source, periodic calibration of flow rate (half-yearly), and control over the aerosol load on the filtration membrane (proportion of light attenuation was controlled at 75% and below).  
A PH-1 automatic weather station was also set up to measure routine meteorological variables that comprised temperature, relative humidity, pressure, and wind speed and direction [32]. Mass concentration of PM\({}_{2.5}\) was measured using the GRIMM 180 (GRIMM Aerosol Technik GmbH & Co., Ainring, Germany) particle detector.