Lead-212 in the Urban Boundary Layer of New York City

Abstract

The radioactive emanation product lead-212 is useful in estimating rates of air exchange within the urban boundary layer. The concentration of lead-212 is negligible in air of oceanic origin as well as in air above continental areas under snow cover. On several days when conditions were such that one of these types of air mass approached New York City, measurements were made which show that the source strength of lead-212 within the city is relatively constant. On two such days vertical profiles of the concentration of lead-212 were measured from the Empire State Building, which served as a sampling tower. From the data of these profiles and a two-layer model of the urban boundary layer, we estimate the vertical eddy diffusivity to be of the order of tens of square meters per second and the residence time of air within the street layer to be of the order of S minutes. These results are consistent with the observed distribution of stable lead and with an independent estimate of the eddy viscosity from a wind profile. Under moderate wind conditions and with a mixing depth of hundreds of meters, virtually all the horizontal transport of lead-212 and other tracers with street-level sources takes place in the advective layer.