A sensitive, small detector was developed for atmospheric NO2 and NOx concentration measurements. NO2 is directly detected by laser diode based cavity ring-down spectroscopy (CRDS) at 404 nm. The sum of NO and NO2 (=NOx) is simultaneously measured in a second cavity by quantitative conversion of ambient NO to NO2 in excess ozone. Interferences due to absorption by other trace gases at 404 nm, such as ozone and water vapor, are either negligible or small and are easily quantified. The limit of detection is 22 pptv (2sigma precision) for NO2 at 1 s time resolution. The conversion efficiency of NO to NO2 is 99% in excess O3. The accuracy of the NO2 measurement is mainly limited by the NO2 absorption cross section to +/-3%. Because of the formation of undetectable higher nitrogen oxides in subsequent reactions of NO2 with ozone in the NOx channel, the (1sigma) accuracy of the NOx measurement is increased to approximately +/-5% depending on the level of NOx. The new instrument was designed to be easily deployed in the field with respect to size, weight and consumables. Measurements were validated against a photolysis/chemiluminescence detector during six days of sampling ambient air with colocated inlets. The data sets for NO2, NO and NOx exhibit high correlation and good agreement within the combined accuracies of both methods. Linear fits for all three species give similar slopes of 0.99 in ambient air.