Thermal dissociation cavity-enhanced absorption spectrometer for measuring NO₂, RO₂NO₂, and RONO₂ in the atmosphere

<p>We developed thermal dissociation cavity-enhanced absorption spectroscopy (TD-CEAS) for the in situ measurement of NO<span class="inline-formula">₂</span>, total peroxy nitrates (PNs, RO<span class="inline-formula">₂</span>NO<span class="inline-formula">₂</span>), and total alkyl nitrates (ANs, RONO<span class="inline-formula">₂</span>) in the atmosphere. PNs and ANs were thermally converted to NO<span class="inline-formula">₂</span> at the corresponding pyrolytic temperatures and detected by CEAS at 435–455 nm. The instrument sampled sequentially from three channels at ambient temperature, 453 and 653 K, with a cycle of 3 min, to measure NO<span class="inline-formula">₂</span>, NO<span class="inline-formula">₂+</span> PNs, and NO<span class="inline-formula">₂+</span> PNs <span class="inline-formula">+</span> ANs. The absorptions between the three channels were used to derive the mixing ratios of PNs and ANs by spectral fitting. The detection limit (LOD, 1<span class="inline-formula"><i>σ</i></span>) for retrieving NO<span class="inline-formula">₂</span> was 97 parts per trillion by volume (pptv) in 6 s. The measurement uncertainty of NO<span class="inline-formula">₂</span> was 9 %, while the uncertainties of PN and AN detection were larger than those of NO<span class="inline-formula">₂</span> due to chemical interferences that occurred in the heated channels, such as the reaction of NO (or NO<span class="inline-formula">₂</span>) with the peroxy radicals produced by the thermal dissociation of organic nitrates. Based on laboratory experiments and numerical simulations, we created a lookup table method to correct these interferences in PN and AN channels under various ambient organic nitrates, NO, and NO<span class="inline-formula">₂</span>. Finally, we present the first field deployment and compare it with other instruments during a field campaign in China. The advantages and limitations of this instrument are outlined.</p>