Atmospheric observation and source analysis of CFCs, HCFCs and HFCs at the Medog background site on the southern slope of the Himalayas, China

Abstract

The ozone-depleting substances chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and their major substitutes hydrofluorocarbons (HFCs) are all potent greenhouse gases, and their atmospheric concentrations and emission sources have received international attention. The relevant studies have been carried out at multiple background sites around the world, but there is no report on the southern slope of the Himalayas. For the first time, this study set up the Medog background observation site 30 km away from Medog County on the southern slope of the Himalayas (95.5262ºE, 29.5314ºN, 1298.8 m above sea level) in China. From August 10 to September 19 and November 3 to November 30 in 2021, 114 atmospheric samples were collected instantaneously using evacuated electro-polished stainless-steel canisters and manual pressurization equipment and 1-3 samples were collected per day. Nine substances (CFC-11, 12, 113; HCFC-22, 141b, 142b; HFC-23, 125, 134a) were analyzed using a high-precision measurement system of halogenated gases (ODS5-pro). For each substance, all the atmospheric samples were divided into two categories: background samples and polluted samples. The background samples are those whose concentrations were within the range of the background concentrations (measured at the Ragged Point site at the same latitude in 2021) ± 3σ (instrument precision), and the others were classified as polluted samples. Our results showed that the number of the polluted samples of each substance accounted for about 40%-90%. In addition, we found that the mixing ratios of CFC-113, HFC-23, and HCFC-22 in the polluted samples were 93, 44 and 311 pptv, respectively, with higher mixing ratios (31.6%, 24.5%, and 22.5%) above the background levels than other substances (4.3%~10.2%). Furthermore, CFC-113 and HFC-23 were significantly correlated (R = 0.429, P < 0.01), suggesting that they may have similar sources. Both Backward Trajectory and Potential Source Contribution Function of the polluted samples of CFC-113, HFC-23, and HCFC-22 found that the polluted air masses mainly came from the northeast of India and other regions southwest of the sampling site. In contrast, the background air masses mainly came from the local areas. The concentrations of CFC-113, HFC-23, and HCFC-22 in the polluted samples observed in this study, likely from the northeast of India, were about 20-30% higher than the results of aircraft samples conducted by Say et al. (2019) over Northern and Southern India in 2016. In the future, we will continue to carry out atmospheric observations at the Medog background site and try to use the suitable atmospheric transport model to inverse the emissions in the surrounding areas.