The CAPS NO2 monitor provides a direct absorption measurement of nitrogen dioxide at 450 nm in the blue region of the electromagnetic spectrum. Unlike standard chemiluminescence-based monitors, these instruments require no conversion of NO2 to another species and thus are not sensitive to other nitro-containing species. Levels of detection(3σ noise levels) of less than 100 parts per trillion with a 10 second sampling period can readily be attained; a fast response version which offers 1 second time response at reduced levels of detection (< 2ppb) is also available. These instruments offer completely linear response at concentrations up to several parts-per-million of NO2; this limitation at high concentrations is purely one of optical opacity. This monitor can be rack mounted and requires only a source of NO2-free air for periodic (minutes to hours) baseline measurements. The standard gas flow is 0.85 lpm but lower flow rates with reduced time response can be chosen without loss of sensitivity. Twelve months of on-board data storage (at a 10 second sampling rate) is standard.