Changing characteristics of atmospheric CH 4 on the Tibetan Plateau, records from 1994 to 2017 at Mount Waliguan station

Abstract. A 24-year long-term observation of atmospheric CH4 was presented at Mt. Waliguan (WLG) station, the only WMO/GAW global station in inland of Eurasia. Overall, during 1994–2017, continuously increase of atmospheric CH4 was observed at WLG with yearly growth rate of 5.1 ± 0.1 ppb yr−1, although near-zero and even negative growth appeared in some particular periods, e.g., 1999–2000, and 2004–2006. The average CH4 mole fraction was only 1805.8 ± 0.1 ppb in 1995, but unprecedented elevated ~ 100 ppb and reached a historic high of 1903.8 ± 0.1 ppb in 2016. The seasonal averages of atmospheric CH4 at WLG were ordered by summer, winter, autumn and spring, and the correlation slopes of ΔCO/ΔCH4 showed a maximum in summer and minimum in winter, which was almost opposite to other sites in the northern hemisphere, e.g., Mauna Loa, Jungfraujoch, and was caused by regional transport. Strong potential sources at WLG were predominately identified in northeast (cities, e.g., Xining, Lanzhou) and southwest (the Northern India), and air masses from west and northwest regions were accompanied with higher CH4 mole fractions than that from city regions. What is interesting is that obviously changes appeared in different observing periods. Generally, (i) the amplitudes of diurnal or seasonal cycles were continuously increasing over time, (ii) the wind sectors with elevated CH4 moved from ENE- ... -SSE sectors in early periods to NNE- ... -E sectors (city regions) in later years, (iii) the area of source regions was increasing along with the years, and strong sources gradually shifted from northeast to southwest, (iv) the annual growth rates in recent years (e.g., 2013–2016) were significantly larger than that in early periods (e.g., 1998–2012). We conclude that the site was more and more affected by regional sources along with the time. Northern India was possibly becoming the strongest source area to WLG rather than city regions before. The case study in the Tibetan Plateau showed that the atmospheric CH4 observed in Qinghai-Tibetan Plateau changed not as expected, the annual growth rate was even larger than that in city regions in some period (e.g., 7.3 ± 0.1 ppb yr−1 in 2013–2016). It is unambiguous that the anomalously fluctuations of atmospheric CH4 in this region are a warning to the world, its increasingly annual growth rate may be a dangerous signal to global climate change.