methane concentrations and source strengths in urban locations

Higher atmospheric concentrations of methane are found in air samples from urban locations than in contemporary samples at the same latitude in remote locations. Higher concentrations of several trace chlorocarbon gases are also found in the same urban samples than in the corresponding remote samples. The “urban excess”, i.e. urban concentration minus remote concentration, is generally 1000 to 2000 times larger on a molar basis for CH 4 than for CCl 3 F. Because almost all CCl 3 F is emitted in urban environments, the urban release of CH 4 is estimated from the observed molar ratios to be 30 to 60 megatons per year world‐wide. The fraction of world‐wide methane release occurring in the urban environment can be estimated from the concentration ratios, urban to remote, for CH 4 vs. CCl 3 F. About 8% to 15% of the atmospheric methane release is observed to occur in urban locations.