An expert survey on chamber measurement techniques and data handling procedures for methane fluxes

Abstract

Methane is an important greenhouse gas, but the magnitude of global emissions from natural sources remains highly uncertain. To estimate methane emissions on large spatial scales, methane flux data sets from field measurements collected and processed by many different researchers must be combined. One common method for obtaining in situ methane flux measurements is flux chambers. We hypothesize that considerable uncertainty might be introduced into data synthesis products derived from chamber measurements due to the variety of measurement setups and data processing and quality control approaches used within the chamber flux community. Existing guidelines on chamber measurements promote more standardized measurement and data processing techniques, but, to our knowledge, so far, no study has investigated which methods are actually used within the chamber flux community. Therefore, we aimed to identify the key discrepancies between the measurement and data handling procedures implemented for chamber methane fluxes by different researchers. We conducted an expert survey to collect information on why, where, and how scientists conduct chamber-based methane flux measurements and how they handle the resulting data. We received 36 responses from researchers in North America, Europe, and Asia, which revealed that 80 % of respondents have adopted multi-gas analyzers to obtain high-frequency (< 1 Hz) methane concentration measurements over a total chamber closure time of, typically, between 2 and 5 min. Most but not all of the respondents use recommended chamber designs, including features such as airtight sealing, fans, and a pressure vent. We presented a standardized set of methane concentration time series recorded during chamber measurements and derived CH4 flux estimates based on the processing and quality control approaches suggested by the survey participants. The responses showed broad disagreement among the experts concerning the processes that they consider to be responsible for non-linear methane concentration increases. Furthermore, there was a tendency to discard low or negative CH4 fluxes. Based on the expert responses, we estimated a variability of 28 %, introduced by different researchers deciding differently on discarding vs. accepting a measurement when processing a representative data set of chamber measurements. Different researchers choosing different time periods within the same measurement for flux calculation caused an additional variability of 17 %. Our study highlights the importance of understanding the processes causing the patterns in CH4 concentrations visible from high-resolution analyzers, as well as the need for standardized data handling procedures in future chamber methane flux measurements. This is highly important to reliably quantify methane fluxes all over the world. The survey results, as well as the questionnaire, are publicly available at https://doi.org/10.1594/PANGAEA.971695 (Jentzsch et al., 2024b).