3.2.3. OC and EC Levels

The mean concentrations of OC were 12.03 ± 2.24 μg/m3 and 9.32 ± 2.13 μg/m3 in spring and autumn, respectively (Figure 5). The mean EC concentrations were 3.66 ± 0.47 μg/m<sup>3</sup> and 2.88 ± 0.61 μg/m<sup>3</sup> in spring and autumn, respectively. Wenshan is located in the basin valley on the plateau, with wind speeds that are too low to be conducive to pollutant spreading during the two seasons. During the two sampling campaigns, the daily mean value of OC/EC was 2.64–4.17 in spring and 2.74–3.65 in autumn, all of which exceeded 2.0, which indicated that Wenshan experienced secondary organic carbon (SOC) pollution in both seasons. Moreover, OC and EC in Wenshan had a better correlation in autumn (R = 0.86) than in spring (R = 0.69), which showed that the measured OC and

EC were derived from similar sources during autumn and from complex sources during spring. The possible reason is that the spring is affected by the long-distance transmission of biomass combustion sources in Southeast Asia due to climatic conditions.

**Figure 5.** Concentrations of OC and EC, the value of OC/EC and the relevance of OC and EC in PM2.5 during the sampling period. (**a**) Concentrations of OC and EC in spring. (**b**) Concentrations of OC and EC in autumn. (**c**) The value of OC/EC and the relevance of OC and EC in PM2.5 during the spring sampling period. (**d**) The value of OC/EC and the relevance of OC and EC in PM2.5 during the autumn sampling period.

Since there is no simple and direct calculation method for SOC, this study estimates the content of SOC considering the lowest value of the OC/EC ratio in the two seasons [62–64]. The principle of this method is the use of the lowest value of OC/EC rather that of the OC/EC of the primary pollutant in every season. SOC=TOC, EC×(OC/EC)min, and TOC and EC are the concentrations of OC and EC in PM2.5, respectively [65,66]. The average values of SOC in PM2.5 are 2.36 ± 1.00 μg/m<sup>3</sup> and 1.41 ± 0.46 μg/m3 in spring and autumn, respectively. The values of SOC/OC in the two seasons are 19.65% and 15.14% in spring and autumn, respectively. The radiation and temperature in autumn were higher than those in spring, which represented more favorable photochemical conditions for the formation of SOC in autumn. However, there was more rain in autumn in Wenshan, which could limit the formation of SOC. The order of OC, SOC and SOC/EC was the same pattern as spring > autumn, which meant that the impact of radiation and temperature was less than that of rainfall, and rainfall was the main influencing factor and had a greater impact on SOC [67,68].
