We present a sensitive, compact detector that measures total reactive nitrogen (NOy), as well as NO2, NO, and O3. In all channels, NO2 is directly detected by laser diode based cavity ring-down spectroscopy (CRDS) at 405 nm. Ambient O3 is converted to NO2 in excess NO for the O3 measurement channel. Likewise, ambient NO is converted to NO2 in excess O3. Ambient NOy is thermally dissociated at ∼700° to form NO2 or NO in a heated quartz inlet. Any NO present in ambient air or formed from thermal dissociation of other reactive nitrogen compounds is converted to NO2 in excess O3 after the thermal converter. We measured thermal dissociation profiles for six of the major NOy components, and compared ambient measurements with other instruments during field campaigns in Utah and Alabama. Alabama measurements were made in a rural location with high biogenic emissions, and Utah measurements were made in the wintertime in unusual conditions that form high ozone from emissions related to oil and gas production. The NOy comparison in Alabama, to an accepted standard measurement method (a molybdenum catalytic converter/chemiluminescence instrument), agreed to within 12%, which we define as an upper limit to the accuracy of the NOy channel. The 1σ precision is <30 pptv at 1 second and <4 pptv at 1 minute time resolution for all measurement channels. The accuracy is 3% for the NO2 and O3 channels, and 5% for the NO channel. The precision and accuracy of this instrument make it a versatile alternative to standard chemiluminescence-based NOy instruments.