The detection of NO2 at sub part per billion by volume (ppbv) concentrations has been demonstrated using a cavity ring-down spectrometer with a blue diode laser operating at wavelengths close to 410 nm. Measurements at peaks in the absorption spectrum at wavelengths of 410.4955 and 410.3533 nm demonstrate a minimum detectable number density of NO2 of 9.8 x 10(9) molecules cm(-3), which would correspond to 0.4 ppbv under atmospheric conditions in the lower troposphere. Additional experiments performed with a pulsed, tuneable dye laser system provide absorption cross-sections for NO2 over the restricted wavelength ranges of 408.5-410.5 and 435.0-435.5 nm, but at a resolution of similar to0.06 cm(-1) that is higher than previously reported and close to the Doppler broadening limit at 295 K. These spectra show considerably more structure than is evident in published Fourier transform (FT) spectra obtained at lower resolution, but yield cross-sections that are in quantitative agreement with the FT measurements. (C) 2002 Elsevier Science B.V. All rights reserved.