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We present a sensitive, compact detector that measures total reactive nitrogen (NO_y), 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 NO_y is thermally dissociated at ∼700 °C 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 NO_y 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 …