Quantification of peroxynitric acid and peroxyacyl nitrates using an ethane-based thermal dissociation peroxy radical chemical amplification cavity ring-down spectrometer

Peroxy and peroxyacyl nitrates (PNs and PANs) are important trace gas constituents of the troposphere which are challenging to quantify by differential thermal dissociation with NO2 detection in polluted (i.e., high-NOx) environments. In this paper, a thermal dissociation peroxy radical chemical amplification cavity ring-down spectrometer (TD-PERCA-CRDS) for sensitive and selective quantification of total peroxynitrates (Sigma PN = Sigma RO2NO2) and of total peroxyacyl nitrates (Sigma PAN = Sigma RC(O)O2NO2) is described. The instrument features multiple detection channels to monitor the NO2 background and the ROx (=HO2 + RO2 + Sigma RO2) radicals generated by TD of Sigma PN and/ or Sigma PAN. Chemical amplification is achieved through the addition of 0.6 ppm NO and 1.6% C2H6 to the inlet. The instrument's performance was evaluated using peroxynitric acid (PNA) and peroxyacetic or peroxypropionic nitric anhydride (PAN or PPN) as representative examples of Sigma PN and Sigma PAN, respectively, whose abundances were verified by iodide chemical ionization mass spectrometry (CIMS). The amplification factor or chain length increases with temperature up to 69 +/- 5 and decreases with analyte concentration and relative humidity (RH). At inlet temperatures above 120 and 250 degrees C, respectively, PNA and Sigma PAN fully dissociated, though their TD profiles partially overlap. Furthermore, interference from ozone (O-3) was observed at temperatures above 150 degrees C, rationalized by its partial dissociation to O atoms which react with C2H6 to form C2H5 and OH radicals. Quantification of PNA and Sigma PAN in laboratory-generated mixtures containing O3 was achieved by simultaneously monitoring the TD-PERCA responses in multiple parallel CRDS channels set to different temperatures in the 60 to 130 degrees C range. The (1 s, 2 sigma) limit of detection (LOD) of TD-PERCA-CRDS is 6.8 pptv for PNA and 2.6 pptv for Sigma PAN and significantly lower than TD-CRDS without chemical amplification. The feasibility of TD-PERCA-CRDS for ambient air measurements is discussed.