Methane emission mapping and quantification in two medium-sized cities in Germany: Heidelberg and Schwetzingen

Estimating the contribution of cities to regional and global methane (CH₄) budgets is challenging due to the complex infrastructures of cities. Mobile measurement devices are well suited to detect CH₄ sources via real-time ambient air measurements. Surveys involving mobile CH₄ measurements were conducted from May 2020 to July 2021 at the street level in two medium-sized cities in Germany. With coverage levels of 30% and 65% of the road networks in Heidelberg and Schwetzingen, respectively, Leak Indications (LIs) for CH₄ were observed with mole fractions of 100 to 9500 ppb above background. A minor portion of leaks (2 out of 70) was attributed to the sewer system, but most leaks originated from the gas distribution network with 0.48 LIs per km obtained in Heidelberg and 0.08 LIs per km determined in Schwetzingen. A method to assign an emission rate to all LIs developed by Weller et al. (2019) was assessed and adapted via controlled CH₄ release experiments in Heidelberg. The method was modified for cities with smaller street widths and smaller distances from the leak to the measurement device. The annual total street-level CH₄ emissions calculated for Heidelberg and Schwetzingen were 42 ± 17 and 1.5 ± 0.5 t CH₄ yr⁻¹ (1sigma), respectively, corresponding to 0.26 ± 0.11 and 0.03 ± 0.01 kg CH₄ yr⁻¹ per capita, respectively.