Nitrogen dioxide (NO2) is an important indicator of atmospheric pollution that is mainly derived from combustion processes. The gas is often present at undesirable levels in both open and closed environments worldwide, requiring monitoring under a variety of different conditions. This work describes the development of a sensitive, selective, and inexpensive method for the determination of NO2 in gaseous samples. The method is based on the processing of digital images of the product of the Griess-Saltzman (GS) colorimetric reaction. NO2 was collected and pre-concentrated using C-18 cartridges impregnated with triethanolamine, followed by elution with 5% methanol solution. The reaction for formation of the colored product only required 300 mu L volumes of sample containing reagent, minimizing the generation of chemical wastes. Calibrations using standard atmospheres showed that it was possible to measure NO2 in a concentration range from 5.1 to 100.0 ppb (9.4-188.0 mu g m(-3)), using a sampling flow rate of 0.50 L min(-1) and a collection time of 60 min. The limit of detection achieved with a solution volume of 300 mu L. was 5.0 ppb (9.6 mu g m(-3)), with a relative error of 2% and a coefficient of variation of 1.6%. ; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) ; Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)