Temporal Characteristics of CH₄ Vertical Profiles Observed in the West Siberian Lowland Over Surgut From 1993 to 2015 and Novosibirsk From 1997 to 2015

We have carried out monthly flask sampling using aircraft, in the altitude range of 0–7 km, over the boreal wetlands in Surgut (61°N, 73°E; since 1993) and a pine forest near Novosibirsk (55°N, 83°E; since 1997), both located in the West Siberian Lowland (WSL). The temporal variation of methane (CH₄) concentrations at all altitudes at both sites exhibited an increasing trend with stagnation during 2000–2006 as observed globally from ground-based networks. In addition to a winter maximum as seen at other remote sites in northern middle to high latitudes, another seasonal maximum was also observed in summer, particularly in the lower altitudes over the WSL, which could be attributed to emissions from the wetlands. Our measurements suggest that the vertical gradient at Surgut has been decreasing; the mean CH₄ difference between 5.5 km and 1.0 km changed from 64 ± 5 ppb during 1995–1999 to 37 ± 3 ppb during 2009–2013 (mean ± standard error). No clear decline in the CH₄ vertical gradient appeared at Novosibirsk. Simulations using an atmospheric chemistry-transport model captured the observed decrease in the vertical CH₄ gradient at Surgut when CH₄ emissions from Europe decreased but increased from the regions south of Siberia, for example, East and South Asia. At Novosibirsk, the influence of the European emissions was relatively small. Our results also suggest that the regional emissions around the WSL did not change significantly over the period of our observations.