*2.2. Sample Collection and Component Analysis*

According to the "Measuring Tank Sampling of Ambient Air Volatile Organic Compounds/Gas Chromatography-Mass Spectrometry (HJ759-2015)", the air samples were collected using a 3.2 L stainless-steel sampling tank produced by Entech Corporation of the United States. The inner wall of the SUMMA canister was cut and silanized, a flow valve controlled the sampling inlet flow, and a SUMMA canister sample was collected every 2 h. Sampling occurred in October 2016 and March, April, May, and June 2017, with 5 consecutive days of sampling at the beginning of each month. Meteorological conditions such as air temperature, air pressure, and wind speed during the sampling period were monitored and recorded online at a height of 3 m through the Vantage Pro2TM wireless weather station (Davis Instruments, Hayward, CA, USA). The rainfall in Jinghong varies significantly from month to month (abundant rainfall from June to October each year), so this paper defines the samples collected in October 2016 and June 2017 as the rainy season samples, and the samples collected in March, April, and May 2017 as the dry season samples.

This study analyzed 58 VOC species (alkanes, olefins, and aromatic hydrocarbons) by atmospheric preconcentration gas-conjugated methods. The main instruments were the atmospheric preconcentrator (Entech 7100, Entech Instruments, Simi Valley, CA, USA)-GC (Agilent 6890N, Agilent Technologies, Roseville, CA, USA)-MS (Agilent 5973N, Agilent Technologies, Roseville, CA, USA), and the column was the GS-GASPRO with a size of 60 m × 0.32 μm. The SUMMA canister was first connected to the autosampler, and 400 mL of the sample was pumped into the preconcentrator. The sample was concentrated by dewatering and using carbon dioxide interferences such as a 3-stage cold trap; then, the sample was transferred to GC–MS for detection. The carrier gas was high-purity helium, and the heating procedure was: initial temperature of 35 ◦C, held for 15 min, heated to 150 ◦C at 5 ◦C·min<sup>−</sup>1; held for 7 min; heated to 200 ◦C at 10 ◦C·min<sup>−</sup>1, maintained for 4 min. The inlet temperature was 140 ◦C, the carrier gas velocity was 1.0 mL·min<sup>−</sup>1, and the solvent delay time was 5.6 min. Mass spectrometry conditions: interface temperature, 250 ◦C; ion source temperature, 230 ◦C; mass spectrometry detector ion source type, electron bombardment ionization (EI), operated by full scan (SCAN), with scanning range of 35~300 u. Five standard curves of different concentration gradients were established using

a PAMS calibration gas mixture (Spectra Gases USA) with volume fractions of 0.25 × 10<sup>−</sup>9, 0.5 × 10<sup>−</sup>9, 1 × 10<sup>−</sup>9, 2 × 10<sup>−</sup>9, and 3 × 10<sup>−</sup>9.
