A measurement of total reactive nitrogen, NO_y, together with NO₂, NO, and O₃ via cavity ring-down spectroscopy

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 NO ₂ 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 were made in the wintertime in unusual conditions that form high ozone levels from emissions related to oil and gas production. The NO_y 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 NO_y channel. The 1 precision is <30 pptv at 1 s and <4 pptv at 1 min time resolution for all measurement channels. The accuracy is 3% for the NO₂ and O ₃ channels and 5% for the NO channel. The precision and accuracy of this instrument make it a versatile alternative to standard chemiluminescence- based NO_y instruments.