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Abstract

We describe a Thermal Dissociation Cavity-Ring-Down Spectrometer (TD-CRDS) for measurement of ambient NO2, total peroxy nitrates (ŒPNs) and total alkyl nitrates (ŒANs). The spectrometer has two separate cavities operating at 405.2 and 408.5 nm, one cavity (reference) samples NO2 continuously from an inlet at ambient 5 temperature, the other samples sequentially from an inlet at 473 K in which PNs areconverted to NO2 or from an inlet at 723 K in which both PNs and ANs are converted to NO2, dierence signals being used to derive mixing ratios of ŒPNs andŒANs. We describe an extensive set of laboratory experiments and numerical simulations to characterise the fate of organic radicals in the hot inlets and cavity and derive correction 10 factors to account for the bias resulting from interaction of peroxy radicals with ambient NO and NO2 . Finally, we present the first measurements and comparison with other instruments during a field campaign, outline the limitations of the present instrument and provide an outlook for future improvements.