Carbon-14, tree rings,and urban air pollution

Differences in natural ¹⁴C content of rings from trees from urban and rural locations have been compared on a year by year basis. The differences as a fraction of the urban tree's radioactivity reflect the local excess ¹⁴C-free carbon dioxide from fossil fuel combustion in the urban environment. New York City, Boston, and Washington, D.C., show different degrees of excess carbon dioxide. New York City averages about 6% excess carbon dioxide between 1950 and 1970. From this is infered an average carbon monoxide concentration during this period of 5 ppm.