Quantification of natural CO2 emissions from mofettes using a low-cost sensor network at the Starzach site in South-West Germany

We present a top-down method to derive C O 2 emissions from mofettes, using only point measurement time series at irregular locations. Notably, no wind vector information is needed, as gas transport is derived from cross-correlations between sensor stations and subsequently integrated using Gauss’ divergence theorem. The method is applied to an existing low-cost sensor network at the Starzach site near the Black Forest in Germany, for which no comprehensive estimate of the total emissions exists yet. For validation, we use previous bottom-up measurements of individual mofette degassing and a Gaussian puff approach. Over a period of one and a half months around August 2022, we determine an average C O 2 emission rate of 3266   k g   d − 1 ± 42 % over a 400   m 2 area. This result is larger than expected and suggests that diffuse degassing plays a more important role at site than previously assumed. The method could also be applied for real-time monitoring of leaky CCS sites, for which the Starzach site is a natural analog.