*2.1. Sampling Site and Sample Collection*

Wenshan is a developing industrialized city with half a million inhabitants in an urban area of 75 km2. With a longitude of 103◦43 –104◦27 E and a latitude of 23◦06 –23◦44 N, Wenshan lies in southwest China (Figure 1), which is the transition zone of the Yunnan-Guizhou Plateau and Vietnam Basin.

Wenshan lies in a small basin valley on the plateau and is surrounded by mountains on three sides. The terrain inclines from northwest to southeast, and the mountain range runs almost from north to south. Therefore, a corridor topography consisting of high points on both sides and low points in the middle is formed. The relative altitude difference is 1751.2 m, with the highest altitude of 2991.2 m in Bozhu Mountain and the lowest of 1240 m in the urban area. Wenshan is dominated by a subtropical monsoon climate, which is characterized by a long spring and autumn, no bitter cold in winter, no brutal heat in summer and delightful weather in all seasons. Airflow near the ground can only enter the urban area from the southeast. The cold air in Siberia from the north is obstructed by mountains, and the monsoon of the Beibu Gulf and the Bay of Bengal traveling from the southeast flows right into the urban area, which forms comfortable temperatures, low wind speeds and high ultraviolet (UV) light conditions throughout the year. Strong ultraviolet light is conducive to the formation of photochemical atmospheric effects, and the conditions of low pressure and low oxygen can lead to the incomplete combustion of fuel and can increase the resulting pollution emissions. The conditions of low wind speed (<3 m/s) and the large diurnal temperature variation readily form an inversion layer, hindering the diffusion of pollutants.

**Figure 1.** Location of the sampling site in Wenshan, China.

Measurement campaigns of PM2.5 sampling at three sites at the Convenience Service Center of Wenshan (CSCW), Water Authority of Wenshan (WAW) and Environmental Monitoring Station of Wenshan (EMSW) in Wenshan city (Figure 1), were carried out in two periods in 2016, i.e., 19 April to 3 May and 12 October to 26 October. Daily measurements of 22 ± 1 h with an intelligent mid-volume atmospheric particulate sampler (TH-150F, Tianhong, China) were conducted at a 100 L/min sample flow. Teflon filters (China, 90 mm) were used to analyze IEs, and quartz filters (PALL Inc., USA, 90 mm) were used to analyze OC particles, EC components and WSIIs. A total of 178 PM2.5 samples and 12 blank samples were collected. After sampling, the filters were individually placed into plastic boxes and put into a freezer at −20◦C until transport and subsequent analysis.

#### *2.2. Chemical Analysis and Quality Control*

#### 2.2.1. WSII Analysis

The anion (i.e., F−, Cl−, SO4 <sup>2</sup><sup>−</sup> and NO3 <sup>−</sup>) and cation (i.e., K+, Ca2+, Mg2+ and NH4 +) concentrations were measured by ion chromatography (DX-600, Dionex, USA). This system was outfitted with a separation column (Dionex AS-14A for anions and CS-12A for cations) and a guard column (Dionex AG14A for anions and CG12A for cations). A gradient weak base eluent (3.5 mmol/L Na2CO3; 1 mmol/L NaHCO3) was used for anion detection, while a weak acid eluent (18 mmol/L methanesulfonic acid) was used for cation detection. The measurement error of each ion in a standard solution is within 10%, and the average relative standard deviations of anions and cations are 3.0% and 4.0%, respectively. For quality assurance, two blank spaces were detected in each batch of samples, and the test was carried out at 10%. At least six standard solution concentrations needed to be mixed for each ion component.
