The column abundance of NO3 has been measured through sunrise at the midlatitude site of Cambridge, England. (52.2 ¿N, 0.1 ¿E) using zenith sky visible absorption spectroscopy. This technique allows distinction between the tropospheric and stratospheric components of the total NO3 vertical column during sunrise. Measurements of the tropospheric component during July to October 1995 are presented and show an average concentration of 89 ppt if it is assumed that all tropospheric NO3 is present in a 1 km thick atmospheric boundary layer. The day to day variability is considerable. The NO3 measurements together with point measurements of O3, NO2, NO and temperature show that the NO3 lifetime is significantly longer than reported in other studies. The very short lifetime expected as a result of the measured levels of NO suggests that the ground level measurements of O3, NO2 and NO are not representative of the whole boundary layer and therefore that the NO3 lifetime at the surface is not representative of the average boundary layer lifetime. Idealized model calculations confirm that a gradient in NO3 concentration would be expected in the boundary layer as a result of the different timescales for mixing and reaction with NO in a stable nocturnal situation. Any other rapid loss reaction near the surface, for example, with dimethyl sulphide in the marine environment or isoprene at a wooded rural site, may have a similar effect. In addition, such high concentrations of NO3 may alter the balance toward significant nighttime oxidation of hydrocarbons either directly or by production of OH. ¿ 1998 American Geophysical Union