Influence of ambient NO and NO₂ on the quantification of total peroxy nitrates (∑PNs) and total alkyl nitrates (∑ANs) by thermal dissociation cavity ring-down spectroscopy (TD-CRDS)

Abstract. Measurement of total peroxy nitrates (∑PNs) and alkyl nitrates (∑ANs) by instruments that use thermal dissociation (TD) inlets to convert the organic nitrate to detectable NO₂ may suffer from systematic bias (both positive and negative) resulting from unwanted secondary chemistry in the heated inlets. Here we review the sources of the bias and the methods used to reduce it and/or correct for it and report new experiments using (for the first time) atmospherically relevant, unsaturated, biogenic alkyl nitrates as well as two different peroxyacetyl nitrate (PAN) sources. We show that the commonly used commercial C3-alkyl-nitrate (isopropyl nitrate, IPN) inlet for characterising the chemistry of ANs is not appropriate for real-air samples that contain longer chain nitrates. ANs generated in the NO₃-induced oxidation of limonene are strongly positively biased in the presence of NO. By detecting NO_X rather than NO₂, we provide a simple solution to avoid the bias caused by the conversion of NO to NO₂ by primary and secondary peroxy radicals resulting from the complex chemistry in the thermal degradation of long-chain, alkyl nitrates in air at TD-temperatures. We also show that using a photochemical source of PAN to characterise the TD-inlets can result in a much stronger apparent bias from NO to NO₂ conversion than for a diffusion source of synthesised (“pure”) PAN at similar mixing ratios. This is explained by the presence of thermally labile trace gases such as peracetic acid (CH₃C(O)OOH) and hydrogen peroxide (H₂O₂).