Using the sun as an infrared source, we determined the total atmospheric column absorbance of methane and carbon dioxide spectral lines in the 8‐ to 10‐µm infrared region. At our laboratory located in an urban region, these absorbances showed fluctuations larger than can be accounted for by known variabilities in the background atmosphere. We interpret these observations in terms of large changes in concentration of methane and carbon dioxide within the urban boundary layer. These increases could affect the radiative balance in urban locations and contribute to the urban heat is