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. Correlations of nighttime atmospheric methane (CH4) and222radon (222Rn) observations in Heidelberg, Germany, wereevaluated with the radon tracer method (RTM) to estimate the trend of annualnocturnal CH4 emissions from 1996–2020 in the footprint of thestation. After an initial 30 % decrease in emissions from 1996 to2004, there was no further systematic trend but small inter-annual variations wereobserved thereafter. This is in accordance with the trend of totalemissions until 2010 reported by the EDGARv6.0 inventory for the surroundingsof Heidelberg and provides a fully independent top-down verification of thebottom-up inventory changes. We show that the reliability of totalnocturnal CH4 emission estimates with the RTM critically depends onthe accuracy and representativeness of the 222Rn exhalation ratesestimated from soils in the footprint of the site. Simply using222Rn fluxes as estimated by Karstens et al. (2015) could lead tobiases in the estimated greenhouse gas (GHG) fluxes as large as a factor of 2. RTM-based GHG flux estimates also depend on the parameters chosen for thenighttime correlations of CH4 and 222Rn, such as thenighttime period for regressions and the R2 cut-off value for thegoodness of the fit. Quantitative comparison of total RTM-based top-down fluxestimates with bottom-up emission inventories requires representativehigh-resolution footprint modelling, particularly in polluted areas whereCH4 emissions show large heterogeneity. Even then, RTM-basedestimates are likely biased low if point sources play a significant role inthe station footprint as their emissions may not be fully capturedby the RTM method, for example, if stack emissions are injected above the topof the nocturnal inversion layer or if point-source emissions from thesurface are not well mixed into the footprint of the measurementsite. Long-term representative 222Rn flux observations in thefootprint of a station are indispensable in order to apply the RTM method forreliable quantitative flux estimations of GHG emissions fromatmospheric observations.

Limitations of the radon tracer method (RTM) to estimate regional greenhouse gas (GHG) emissions – a case study for methane in Heidelberg