Temporal variation of atmospheric CH4 and the potential source regions at Waliguan, China

We used a revised dataset of the atmospheric CH4 mixing ratios that were continuously measured at Mount Waliguan Baseline Observatory of western China during 2002-2006 to study the temporal variations and the potential sources regions of ambient CH4. Approximately 58% observed values were filtered as background data by applying a robust local regression mathematic procedure. The median, 10% and 90% percentiles of CH4 background mixing ratios were 1831.8, 1820.7 and 1843.5 ppb (10(-9) mol mol(-1)), respectively; the atmospheric CH4 time series showed large variations with a fluctuation of 200 ppb (maximum minus minimum), but the background CH4 exhibited a much smaller variation (about 38 ppb), indicating strong impacts from local or regional sources and sinks. Ambient CH4 displayed significant diurnal variations in different seasons during 2002-2006, as was the combined result of changes of nearby sources/sinks and local meteorological circulation. The averaged CH4 seasonal cycle showed minimum in spring and winter, maximum in summer (from June to August), as is completely opposite to that observed at Mauna Loa and Niwot Ridge. The incongruous variation should be attributed to enhancing regional/local sources (e.g., herd) around the site as well as the dominant air flow from the polluted north/northeastern region (e.g., Xining and Lanzhou city) in summer. Another possible reason is, compared to Mauna Loa and Niwot Ridge, the photochemical reaction at Waliguan is weaker. An analysis of 5 d back-trajectory (500 hPa) showed that the episode of high CH4 mixing ratios was associated with advection from the heavily populated regions east or southeast (e.g., Xining and Lanzhou) of Waliguan and northwest of Qinghai via Ge'ermu urban area where growing industrial activities as well as crops residue burning provide large sources of CH4, suggesting a large source area; the low values were observed most frequently when air masses originated from the sparsely populated Tibet and south of Qinghai and Xinjiang Uygur Autonomous Region (XUAR). Additionally, atmospheric CH4 would be enhanced when air masses originated from Ningxia and northwest of Gansu areas (mainly growing rice) along the Yellow river region, especially in summer, suggesting agriculture source of CH4. Our study helps to accurately estimate sources and sinks of CH4, and thus is significant for understanding greenhouse effect over a regional scale and predicting global change in future.