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

Abstract. Correlations of nighttime atmospheric methane (CH4) and 222radon (222Rn) observations in Heidelberg, Germany, were evaluated with the radon tracer method (RTM) to estimate trend annual nocturnal CH4 emissions from 1996–2020 footprint station. After an initial 30 % decrease 1996 2004, there was no further systematic but small inter-annual variations observed thereafter. This is accordance total until 2010 reported by EDGARv6.0 inventory for surroundings Heidelberg provides a fully independent top-down verification bottom-up changes. We show that reliability emission estimates RTM critically depends on accuracy representativeness 222Rn exhalation rates estimated soils site. Simply using fluxes as Karstens et al. (2015) could lead biases greenhouse gas (GHG) large factor 2. RTM-based GHG flux also depend parameters chosen correlations 222Rn, such period regressions R2 cut-off value goodness fit. Quantitative comparison inventories requires representative high-resolution modelling, particularly polluted areas where heterogeneity. Even then, are likely biased low if point sources play significant role station their may not be captured method, example, stack injected above top inversion layer or point-source surface well mixed into measurement Long-term indispensable order apply reliable quantitative estimations observations.