3.3.1. Ratio of Specific Species

The ratio of toluene to benzene is commonly used to determine the sources of traffic emissions, fuel combustion, and industrial and solvent use in the current regional atmospheric environment [37–40]. Isopentane and n-pentane have similar chemical reactions with free radicals, and their ratios can also be used to indicate different sources [38,41,42]. The ratio of m/p-xylene to ethylbenzene is often used to evaluate the degree of aging of gas clusters [42–44], and the length of photochemical age. Ethylbenzene is less reactive than m/p-xylene and the ratio decreases in atmospheric chemical reactions where m/p-xylene is consumed faster—the smaller the ratio, the higher the degree of aging of the gas cluster. Moreover, BTEX pollution may be affected by regional transmission [45]. Table 3 shows the degree of air mass aging for m- and p-xylene and ethylbenzene for the ranges of benzene to toluene and isopentane to n-pentane ratios, respectively, corresponding to the pollution sources. Benzene to toluene, isopentane to n-pentane, and m-p-xylene to ethylbenzene ratios were selected for the study; additionally, the emission share of isoprene was considered to study the emissions from natural sources, where a larger ratio indicates that the air mass is fresher, and a smaller ratio indicates that the air mass is aging.


**Table 3.** Ratio data of benzene to toluene and isopentane to n-pentane.

Jinghong is located at the southern tail end of the longitudinal valley of the Hengduan Mountains, which has a tropical and subtropical humid monsoon climate with high precipitation and dense vegetation covering a wide area. As shown in Table 4, the concentration of isoprene in the suburban area (59.80% in the dry season and 74.10% in the rainy season) was higher than that in the urban area (8.45% in the dry season and 12.38% in the rainy season), indicating that the release of large amounts of VOCs from dense vegetation in

the suburb had an important impact on VOCs in the ambient air, and isoprene emission from vegetation exponentially increased with sunlight and temperature on rainy days. The benzene/toluene ratios of 2.42 and 2.70 in the dry and rainy seasons in the suburban area, respectively, showed that fresh emissions or exhaust emissions and biomass burning were the primary source. The ratios of 0.09 and 0.10 in the dry and rainy seasons in the urban area, respectively, which were less than 0.2, showed that solvent use was the primary source. From the ratios of isopentane/n-pentane, it shows that the ratios of 3.24 and 2.10 for the dry and rainy seasons in suburban areas, respectively, were mainly influenced by gasoline fuel volatilization, and the ratios of 2.66 and 5.19 for the dry and rainy seasons in urban areas, respectively, were mainly influenced by motor vehicle exhaust and fuel volatilization. The ratio of M-p-xylene/Ethylbenzene was small in the urban and suburban areas in the rainy season, which means that the air mass was relatively aging. In addition to local emissions, some of the benzene series pollution came from regional transmission. In the dry season, the ratio between urban and suburban areas was bigger, which means that the air mass was relatively fresh, and the pollution was mainly transmitted locally.


**Table 4.** Species ratio in dry and rainy seasons in urban and suburban areas of Jinghong.

DU: Dry season—Urban; DS: Dry season—Suburb; RU: Rainy season—Urban; RS: Rainy season—Suburb; TVOCs: Total Volatile Organic Compounds.

#### 3.3.2. The Long-Range Transport

As shown in Figure 4, the incoming air masses during the year of the sampling period were resolved using the HYSPLIT model to track the trajectories of air masses arriving in the region in the past 72 h. The air masses from the Southwest were the most abundant in the region. Almost all air masses originated from Southeast Asian countries, with the most air masses coming from Myanmar, accounting for the most southwestern air mass transport (Cluster 1 air masses, 48.09%) and the lowest transport altitude (pressure above 870 hPa).

The trajectory air masses in different directions may contain different levels of pollutants. The backward trajectory clustering and pollutant concentration information were combined to analyze the influence of each trajectory air mass on pollutants in the study area. The results are shown in Table 5. The pollutant concentrations in different trajectory air masses varied widely, with the largest values of pollutants being SO2, NO2, CO, O3, PM2.5, and PM10 in cluster 2 and the second largest values in cluster 5, mainly from the central region of Myanmar, becoming the most important transport path affecting the atmospheric pollutant concentrations in Jinghong. The airflow near Thailand and Laos (clusters 1 and 4) had the lowest ozone concentration values, compared to the other airflow (from Myanmar) trajectories, which had higher ozone concentration values, probably related to the more frequent biomass burning activities in Myanmar. It can be speculated that the outbound transport of VOCs may be mainly influenced by the transport from Myanmar.
