Comment on acp-2022-710

Abstract

Methane (CH4) is a potent greenhouse gas and anthropogenic CH4 emissions contribute significantly to global warming. In this study the CH4 emissions of the city of Hamburg, Germany were quantified with measurements from four solar-viewing Fourier Transform Infrared Spectrometers (FTIR), mobile in-situ measurements and an inversion framework. For source type attribution an Isotope Ratio Mass Spectrometer was deployed in the city. The urban district hosts an extensive industrial and port area in the south, as well as a large conglomerate of residential areas north of the Elbe river. For emission modeling the TNO GHGco inventory was used as a basis. In order to improve the inventory, two approaches were followed: Firstly the addition of a large natural CH4 source, the Elbe river, which was previously not in the inventory. Secondly mobile measurements were carried out to update the spatial distribution of emissions in the TNO GHGco gridded inventory and derive two updated versions of the inventory. The addition of the river emissions improved model performance, while the correction of the spatial distribution with mobile measurements did not have a significant effect on the total emission estimates for the campaign period. A comparison of the updated inventories with emission estimates from a Gaussian Plume Model (GPM) showed that the updated versions of the inventory in several cases match the GPM emissions estimates well, highlighting the potential of mobile measurements to derive up-to-date emission inventories. The mobile measurement survey also revealed a large, so far unknown point source of fossil origin with a magnitude of 0.069 ± 0.047 Gg/yr. The isotopic measurements show strong indications that there is a large biogenic CH4 source in Hamburg which produced repeated enhancements of over 1ppm. The CH4 emissions (anthropogenic and natural) of the city of Hamburg were quantified as 14 ± Gg/yr of which 7.9 ± 4.4 Gg/yr are of anthropogenic origin. This study reveals that mobile measurements at street-level may miss the majority of total methane emissions, potentially due to sources within buildings including stoves and boilers operating on fossil gas (also referred to as "natural gas"), as well as due to large area sources like for instance the Alster lake.