Atmospheric measurements of the nitrate radical generally detect its absorption of visible radiation in the band near 662 nm. This band is negatively correlated with strong absorptions due to tropospheric water vapour which must therefore be fitted in the spectral analysis to reduce the spectral residual to the level at which sufficient sensitivity to NO3 can be obtained. Previously this had been fitted using a cross-section typically derived from spectra obtained just before sunset or just after sunrise which took no account of the diurnal variation in temperature and water vapour column amounts. An improved method of accounting for water vapour absorptions is presented here. When fitted together with their temperature dependence, water vapour cross-sections calculated using a line-by-line approach gave a more accurate fitting of water vapour absorptions, thus improving the analysis for NO3. ¿ American Geophysical Union 1996