Dynamics and origin of atmospheric CH4 in a Polish metropolitan area characterized by wetlands

The aim of this study was to assess the natural and anthropogenic input of methane (CH4) to the urban atmosphere of Wrocław (South West Poland). The study is based on the combined use of quantitative (measurement of concentrations) and qualitative analyses (analysis of stable carbon isotopic compositions). Air samples were collected every three weeks in 2011 from 15 sampling points distributed evenly over the city area. The methane concentration varied from 1859 ppb to 7691 ppb, with a mean annual value of 2096 ppb. Carbon isotopic composition of methane δ13C(CH4) varied from −61.5‰ to −37.8‰ with average of −48.1‰. The dominant CH4 source was biogenic, namely, the methane produced at sewage irrigation fields (man made biogenic emissions) and local wetlands (natural biogenic input). During the growing season emitted CH4 spread to considerable distances, depending on the prevailing meteorological conditions. During the cold period, biogenic input was negligible and other anthropogenic sources (biomass burning and city network gas) of methane could be observed.