Background Atmospheric Monitoring of Perfluorocarbons at the Medog Reference Site on the Southern Himalayan Slope of the Qinghai-Tibet Plateau

Perfluorocarbons (PFCs) are one of the seven types of greenhouse gases regulated under the climate convention, and their emissions have drawn international attention. Existing studies have conducted atmospheric observations and source analyses of these substances at multiple global background sites (such as those in the AGAGE network), and estimated global and regional emissions using inversion models and other methods based on observational data. China and India, as the world's top one and top four greenhouse gas emitters, respectively, and the top two aluminum producers, warrant particular attention regarding their PFCs emissions. However, while China has some atmospheric background monitoring stations such as Shanghai Dongtan, Shenzhen Xichong, and Zhejiang Shanghuang, there is still little research on background observations in the Qinghai-Tibet Plateau region, especially the southern Himalayas. Moreover, South Asian regions, like India, lack background monitoring stations and few observational studies have been developed in the past five years. This study conducted a three-year atmospheric background observation experiment for three perfluorocarbons (PFCs, including PFC-116, PFC-218, PFC-318) at the Medog background station, located 30 km outside Medog County on the southern Himalayas. From August to November 2021 and July to October 2022, 229 valid instantaneous atmospheric samples (1–3 per day) were collected using Summa canisters and manual pressurization equipment, analyzed with Medusa. From June to August 2024, continuous observations were conducted using the Tianji ODS system, 434 valid samples were in-situ collected, 12 atmospheric samples daily. Based on the 663 valid samples, this study employed the Robust Extraction of Baseline Signal algorithm to analyze background concentrations of each substance and compared them with simulated background concentrations base on the AGAGE12-BOX model by Rigby et al. (currently updated only to December 2023). The results show that the observed background concentration of PFC-116 deviates from the simulated values by less than ±5%; For PFC-218, observed background concentrations also remained within ±5% of simulated values for most periods, except in August 2021 and early September 2021 when they exceeded 12-BOX simulation results by 10%–15%. However, PFC-318 background concentrations were elevated by at least 40% compared to 12-BOX simulations throughout the sampling period. During the study period, polluted information were only caught for PFC-116 in November 2021, accounting for 72% of samples with 16.8% concentration enhancement. Back Trajectory analysis indicated pollution information primarily originated from regions southwest of the sampling site, including Northeast India etc., while clean samples mainly derived from local sources, Tibet in northern China, and Bangladesh in the south. This study fills a gap in atmospheric background PFC observations over the southern Himalayas of the Tibetan Plateau and provides preliminary insights into PFC pollution sources from South Asian regions like Northeast India. Future work will build on these findings to conduct emission inversion studies for South Asian regions including India, offering scientific support for clarifying global PFC emissions.