Abstract. A 26-year, long-term record of atmospheric methane (CH4) measured in situ at the Mount Waliguan (WLG) station, the only World Meteorological Organization (WMO) and Global Atmosphere Watch (GAW) global station in inland Eurasia, is presented. Overall, a nearly continuous increase in atmospheric CH4 was observed at WLG, with a yearly growth rate of 5.1±0.1 parts per billion (ppb) per year during 1994–2019, except for some particular periods with near-zero or negative values, e.g., 1999–2000 and 2004–2006. The average CH4 mole fraction was only 1799.0±0.4 ppb in 1994 but increased to about 133 ppb and reached a historic level of 1932.0±0.1 ppb in 2019. The case study in the Tibetan Plateau showed that the atmospheric CH4 increased rapidly. During some special periods, it is even larger than that of city regions (e.g., 6.7±0.2 ppb yr−1 in 2003–2007). Generally, the characteristics of CH4 varied in different observing periods as follows: (i) the diurnal cycle has become apparent and the amplitudes of the diurnal or seasonal cycles increased over time; (ii) the wind sectors with elevated CH4 mole fractions switched from ENE-E-ESE-SE-SSE sectors (wind directions) in early periods to NNE-NE-ENE-E sectors in later years; (iii) the area of source regions increased as the years progressed, and strong sources shifted from northeast (city regions) to southwest (northern India); and (iv) the annual growth rates in recent years (e.g., 2008–2019) were significantly larger than those in the early periods (e.g., 1994–2007).

The research of greenhouse gases on the Tibetan Plateau is of great importance since its unique topography as the third pole of our planet and profound response on the climate change. In this study, we compared the concurrent observations of atmospheric carbon dioxide (CO 2 ), methane (CH 4 ), and carbon monoxide (CO) during 2010-2016 from two stations located on the Tibetan Plateau, which are Mt. Waliguan station (WLG), the only World Meteorological Organization/Global Atmosphere Watch global station in the inland of Eurasia, and Shangri-La station, a Chinese national station (XGLL). Although both stations are located at remote area, the atmospheric CO 2 , CH 4 , and CO concentrations are frequently influenced by regional sources, especially for XGLL throughout the year and WLG in summer. Due to the unique topography and regional conditions, the atmospheric CH 4 and CO at both stations display different trends with other sites in China, with higher values in summer. The atmospheric CO 2 , CH 4 , and CO at the XGLL mainly represent the conditions in regional scale. As the only World Meteorological Organization/Global Atmosphere Watch global station in the inland of Eurasia, the observation results at WLG can be used to represent the conditions on the Tibetan Plateau, but some of them are frequently influenced by the emissions from the cities located on the east or north east, and some even can be affect by emissions from the Ganges basin in autumn and winter, which should be treated with caution. By subtracting the influences of the cities, we updated the growth rate of 2.45 ± 0.02 ppm yr −1 for CO 2 , 8.2 ± 0.1 ppb yr −1 for CH 4 , and −0.4 ± 0.1 ppb yr −1 for CO, compared to the prior estimation of 2.31 ± 0.02 ppm yr −1 for CO 2 , 8.1 ± 0.1 ppb yr −1 for CH 4 , and −0.6 ± 0.1 ppb yr −1 for CO on the Tibetan Plateau.

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.