Increasing diurnal and seasonal amplitude of atmospheric methane mole fraction in Central Siberia between 2010–2021

Abstract. Siberia's vast wetlands, permafrost, and boreal forests are significant, but their sources of methane (CH4) are poorly quantified. Using vertical CH4 profiles and meteorological data from the ZOtino Tall Tower Observatory (ZOTTO; 60°48′ N, 89°21′ E) in Central Siberia, we analyse long-term trends in CH4 growth rates, seasonal patterns, and diurnal cycles from 2010 to 2021. Our results show a persistent long-term trend in CH4 mole fractions and an insignificant increasing seasonal cycle amplitude, (2.12 ppb yr−1, p=0.12) along with a pronounced late-summer CH4 peak. Diurnal analysis reveals a growing late summer (July–October) CH4 amplitude over the analysed decade (5.29 ppb yr−1, p=0.001), driven by rising nighttime fluxes correlated with late summer soil temperature (R2=0.65, p<0.01), soil moisture (R2=0.36, p=0.032) and with preceding spring snow depth (R2=0.54, p=0.03). Notably high nighttime CH4 fluxes occurred in 2012 and 2019 mainly due to wildfires. These findings suggest that increasing late-summer CH4 emissions, primarily from wetlands to the west and southwest of ZOTTO, dominantly contribute to the overall CH4 rise. Our study underscores the importance of continuous, high-frequency greenhouse gas observations for accurately quantifying regional CH4 trends.