A zenith sky spectrometer has been deployed at a number of remote locations in the Northern and Southern Hemispheres. The technique of differential optical absorption spectroscopy was used to determine the column density of the nitrate radical (NO3) as a function of solar zenith angle (SZA) during sunrise. The tropospheric column densities show a large degree of day-to-day variability and indicate that a significant fraction of tropospheric NO3 exists in the free troposphere (FT). The change in the NO3 column density with SZA was then used to retrieve the vertical concentration profile of NO3, by applying an optimal estimation method Coe et al., 2002>. This analysis indicates that there is often substantially more NO3 in the upper boundary layer and lower FT than at the surface. Back-trajectory calculations and a simple box model are used to account for the observations.