Abstract
Abstract. 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 (ΣPN = ΣRO2NO2) and of total peroxyacyl nitrates (ΣPAN = ΣRC(O)O2NO2) is described. The instrument features multiple detection channels to monitor the NO2 background and the ROx ( = HO2 + RO2 + ΣRO2) radicals generated by TD of ΣPN and/or ΣPAN. 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 ΣPN and ΣPAN, 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 °C, respectively, PNA and ΣPAN fully dissociated, though their TD profiles partially overlap. Furthermore, interference from ozone (O3) was observed at temperatures above 150 °C, rationalized by its partial dissociation to O atoms which react with C2H6 to form C2H5 and OH radicals. Quantification of PNA and ΣPAN 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 °C range. The (1 s, 2σ) limit of detection (LOD) of TD-PERCA-CRDS is 6.8 pptv for PNA and 2.6 pptv for ΣPAN and significantly lower than TD-CRDS without chemical amplification. The feasibility of TD-PERCA-CRDS for ambient air measurements is discussed.
Highlights
The main goal of this work was to evaluate the feasibility of using ethane-based thermal dissociation (TD)-peroxy radical chemical amplification (PERCA) to quantify PN and peroxyacetic nitric anhydride (PAN) in ambient air
The ethane-based TD-PERCA-CRDS has demonstrated great limit of detection (LOD) (< 1 pptv for PAN∗ and peroxynitric acid (PNA)). This constitutes a considerable improvement compared to our previous generation TD-CRDS, whose LOD was in the hundreds of pptv (Paul and Osthoff, 2010), and represents the first optical absorption measurement of PNA at concentration levels of the same magnitude as found in ambient air (Table 1)
The measurement can tolerate a large NO2 background through selective amplification of the desired signal: in Fig. 12, for example, the NO2 background was > 30 ppbv, yet PNA and PAN were quantified with a 1 s σ precision of < 6 and < 40 pptv, respectively (Table 6)
Summary
The ROx (= OH + HO2 + RO2) radicals and the nitrogen oxides (NOx = NO + NO2) are important trace constituents of the atmosphere that drive diverse processes such as the photochemical production of ozone (O3) in the troposphere (Kirchner and Stockwell, 1996; Fleming et al, 2006), the catalytic destruction of O3 in the stratosphere (Bates and Nicolet, 1950; Stenke and Grewe, 2005; Solomon, 1999; Portmann et al, 1999), and the chemistry of organic aerosol formation (Ziemann and Atkinson, 2012; Ehn et al, 2014; Crounse et al, 2013). The concentrations of these species are frequently buffered by ROx and NOx reservoir species, of which peroxynitric acid (PNA, HO2NO2), alkyl peroxy nitrates such as methyl peroxynitrate (CH3O2NO2, MPN), and peroxyacyl nitrates (PANs, RC(O)O2NO2) are important examples (Singh et al, 1992; Roberts, 1990). Taha et al.: Quantification of peroxynitric acid and peroxyacyl nitrates
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