Measurement report: Changing characteristics of atmospheric CH₄ in the Tibetan Plateau: records from 1994 to 2019 at the Mount Waliguan station

• Main Authors: S. Liu, S. Fang, P. Liu, M. Liang, M. Guo, Z. Feng
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-01-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/393/2021/acp-21-393-2021.pdf

<p>A 26-year, long-term record of atmospheric methane (<span class="inline-formula">CH₄</span>) 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 <span class="inline-formula">CH₄</span> was observed at WLG, with a yearly growth rate of <span class="inline-formula">5.1±0.1</span> 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 <span class="inline-formula">CH₄</span> mole fraction was only <span class="inline-formula">1799.0±0.4</span> ppb in 1994 but increased to about 133 ppb and reached a historic level of <span class="inline-formula">1932.0±0.1</span> ppb in 2019. The case study in the Tibetan Plateau showed that the atmospheric <span class="inline-formula">CH₄</span> increased rapidly. During some special periods, it is even larger than that of city regions (e.g., <span class="inline-formula">6.7±0.2</span> ppb yr<span class="inline-formula">⁻¹</span> in 2003–2007). Generally, the characteristics of <span class="inline-formula">CH₄</span> 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 <span class="inline-formula">CH₄</span> 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).</p>