Seasonality of Atmospheric CH4 and δ13CH4: New measurement at IMAU and analysis of Global long-term time series and model data

Abstract

In this project, we measured methane samples at the Institute for Marine and Atmospheric Research Utrecht (IMAU) and analyzed long-term time series observations from NOAA as well as model outputs to explore the seasonality of CH4 and δ13CH4. In the first part, we measured surface flask samples from two IMAU methane stable isotope measurement projects and compared them with NOAA/INSTAAR measurements. The results show that after quality control, data cleaning and offset correction, our δ13CH4 measurements agree well with the INSTAAR results and show consistent seasonality. In the second part, we analysed the NOAA long-term time series of CH4 and δ13CH4 at 18 stations. In the southern hemisphere,there is a negative correlation between the seasonality of CH4 and δ13CH4, their seasonality is mainly controlled by OH sink. In the Northern Hemisphere, the anti-correlation is not significant, the seasonality of δ13CH4 is more affected by emissions rather than OH sink. Using the Miller & Tans method, we use smooth fit and trend as two different backgrounds and obtain the corresponding source δ13CH4. We find that the regional source δ13CH4 in the Northern Hemisphere has a clear seasonality, which is mainly controlled by regional emissions, and has seasonal characteristics consistent with microbial emissions. CH4 and δ13CH4 ’s seasonal amplitudes and source δ13CH4 show latitudinal gradients, which are related to the distribution of emission and δ13CH4. High latitudes show stronger seasonality and more depleted isotopic signals, indicating that they are more related to microbial emissions. In contrast, the proportion of enriched emissions is higher in mid- and low-latitude regions. The high-latitude CH4 seasonal amplitudes show downward long-term trends, accompanied by a depletion trend of regional source δ13CH4, indicating an increase in the proportion of wetland emissions. The inter-annual trend in mid- and low-latitudes is less obvious, which may be masked by fossil or pyrogenic emissions. The inverse model can reproduce the long-term trend and seasonal cycle of CH4 well but underestimates the seasonality of δ13CH4. The discrepancy of seasonality of source δ13CH4 between the inverse model and observations highlights the potential underestimation of high-latitude microbial emissions and mid- and low-latitude fossil fuel emissions. More δ13CH4, δD − CH4 observations in emission regions are required, especially in tropical regions, to better constrain emission sources and sinks.