We describe a thermal dissociation cavity ring-down spectrometer (TD-CRDS)for measurement of ambient NO, total peroxy nitrates (ΣPNs)and total alkyl nitrates (ΣANs). The spectrometer has two separatecavities operating at ∼ 405.2 and 408.5 nm. One cavity(reference) samples NO continuously from an inlet at ambienttemperature, the other samples sequentially from an inlet at 473 K in whichPNs are converted to NO or from an inlet at 723 K in which both PNsand ANs are converted to NO, difference signals being used to derivemixing ratios of ΣPNs and ΣANs. We describe an extensive setof laboratory experiments and numerical simulations to characterise the fateof organic radicals in the hot inlets and cavity and derive correctionfactors to account for the bias resulting from the interaction of peroxyradicals with ambient NO and NO. Finally, we present the firstmeasurements and comparison with other instruments during a field campaign,outline the limitations of the present instrument and provide an outlook forfuture improvements.
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