Peroxycarboxylic nitric anhydrides, or acyl peroxynitrates (APNs), have long been considered important atmospheric constituents (Roberts, 1990, 2007; Stephens, 1987). Often the most abundant odd nitrogen species, e.g., NOy (NO + NO2+ NO3+ 2N2O5+ HNO3+ HONO + HO2NO2+ APNs + organic nitrates + …), in the atmosphere, APNs play an integral part of ozone photochemistry in remote regions (Singh et al., 1992; Roberts, 1990, 2007) as byproducts of the reaction of NOx (NO + NO2) and volatile organic compounds (VOCs). Thermal decomposition of APNs, on a timescale of hours to months, is often the most dominant atmospheric sink and results in release of NOx. APNs, as a NOx reservoir, therefore serve as an important pathway for transport of NOx to remote regions (Moxim et al., 1996; Horowitz and Jacob, 1999).

The most atmospherically abundant and well-understood of the APNs is acetyl peroxynitrate (PAN, CH3C(O)O2NO2) formed from the oxidation of VOCs (Roberts, 2007; Fischer et al., 2014; Singh et al., 1992; LaFranchi et al., 2009; Moxim et al., 1996; Phillips et al., 2013; Roberts et al., 2004, 2007; Williams et al., 1997). Recent research has become increasingly focused on less-well-studied APNs due to their involvement in several key atmospheric processes. Of these, interest has been generated for acryloyl peroxynitrate (APAN, CH2CHC(O)O2NO2), a product of acrolein oxidation, where the dominant atmospheric source is understood to be from aged biomass burning emissions (Yokelson et al., 2009). Methacryloyl peroxynitrate (MPAN, CH2C(CH3)C(O)O2NO2) has received recent attention due to its involvement as an intermediate in secondary organic aerosol formation from isoprene (Surratt et al., 2010). propionyl peroxynitrate (PPN) – formed from precursors such as propanal, 1-butene, and alkanes (Williams et al., 1997; Roberts et al., 2001) – is understood to be a tracer for anthropogenic photochemistry, as biogenic precursors are expected to be a negligible source.

A multitude of techniques have been employed for routine measurement of atmospheric APNs since the middle of the 20th century. The oldest and most widely used of these techniques is gas chromatography (GC) coupled to a detection method such as (i) electron capture (Darley et al., 1963; Williams et al., 2000; Flocke et al., 2005; Schrimpf et al., 1995; Roberts et al., 2006; Roberts, 1990), (ii) chemiluminescence (Gaffney et al., 1998), or (iii) mass spectrometry (Tanimoto et al., 1999). Liquid chromatography has also been used for the measurement of PAN, with limited results (Grosjean et al., 1991). Optical techniques such as Fourier transform infrared spectroscopy (FTIR; Tuazon et al., 1978) and thermal-dissociation laser-induced fluorescence (TD-LIF; Wooldridge et al., 2010) have been employed with more recent developments, such as cavity ring-down spectroscopy (Paul et al., 2009), showing promise as fast-response measurement techniques.

Among the newer methods gaining traction are mass spectrometric techniques, such as chemical ionization mass spectrometry, CIMS (Hastie et al., 2010; Hansel and Wisthaler, 2000; Huey, 2007). An increasingly popular method is thermal-dissociation chemical ionization mass spectrometry (TD-CIMS) for the measurement of APNs (Slusher et al., 2004; Phillips et al., 2013; Zheng et al., 2011). Comparisons of the TD-CIMS method with other techniques, such as GC and TD-LIF, show reasonably good agreement (Wooldridge et al., 2010; Tyndall et al., 2005). However, there exists the potential for various interferences, such as peroxy acetic acid and carboxylic acids, requiring additional consideration (Phillips et al., 2013).

Detection sensitivity is not inherently deducible from the TD-CIMS method; rather it is a function of the thermal-dissociation efficiency, ion transmission, and inlet losses; therefore calibration is a necessary requirement in order to perform accurate, quantitative measurements. A multitude of APN calibrations procedures have been developed over the years, with the earliest examples focused exclusively on the synthesis and calibration to PAN, with later methods expanded to include additional APN species. Currently there exist two widely used synthesis methods, wet chemical synthesis (Kravetz et al., 1980; Nielsen et al., 1982) followed by purification (Gaffney et al., 1984; Holdren and Spicer, 1984; Ciccioli et al., 1992) and online photoproduction (Furgeson et al., 2011; Grosjean et al., 1984; Warneck and Zerbach, 1992a; Joos et al., 1986; Grosjean and Harrison, 1985). While both methods are effective, the amount of time, difficulty, and hazards involved in the wet chemical handling, storage, and disposal make it a less-desirable approach. Online photoproduction of APNs, however, offers a more rapid alternative with significantly reduced complexity in comparison to wet chemical synthesis methods.

In this work, we present a new, dynamic photochemical source for the production of PAN, PPN, APAN, MPAN, and CPAN in the laboratory. The method developed here relies on the photolysis of acyl chloride (RC(O)Cl) compounds or ketones (RC(=O)R) in the presence of O2 and NO2 resulting in the formation of various APNs. Results will be shown validating the production of APAN, MPAN, and CPAN from acyl chloride precursors through the use of preparatory-scale GC and subsequent detection with both a NOy detector and a TD-CIMS using I- primary ions (I- CIMS). Production of PAN and PPN, via photolysis of acetone and 3-pentanone, respectively (Furgeson et al., 2011; Warneck and Zerbach, 1992a; Volz-Thomas et al., 2002), will also be shown. A brief discussion on APN sensitivity as a function of inlet dissociation temperature will also be provided.

Figure 1 shows a simplified schematic of the photolysis source used in this work. This photolysis source is similar to that used in previous photo-based PAN calibration methods (Furgeson et al., 2011). In fact, the production of PAN and PPN used here has been described previously using the respective precursor compounds acetone and 3-pentanone (Furgeson et al., 2011; Warneck and Zerbach, 1992a; Volz-Thomas et al., 2002). In this work we show a new method for the online synthesis of APAN, MPAN, and CPAN using acryloyl chloride, methacryloyl chloride, and crotonyl chloride precursor compounds, respectively. Precursor compounds are diluted with tridecane, in the ratios reported in Table 1 according to their volatility, and placed into a glass diffusion cell with a headspace flow of 20 sccm zero air. The outflow of the diffusion cell is mixed with approximately 5 sccm of a 5 ppmv NO2 mixture. The mixture is photolyzed using a 254 nm Pen-Ray^® lamp in a fused silica cell, with a residence time of approximately 6 min. Photolysis of acyl chloride precursors in the presence of O2 and NO2 proceed via the following reactions: RC(O)Cl⟶ΔRC(O)⚫+Cl⚫,RC(O)⚫+O2⟶RC(O)O2⚫,RC(O)O2⚫+NO2⟶RC(O)O2NO2. Resulting APN products are subsequently separated using a prep-scale GC similar to that described by Flocke et al. (2005). Flow from the photolysis cell is injected onto an Rtx-200 gas chromatography column (15 m, 0.53 mm inside diameter, 1.5 µm) and held at a temperature of 30 ∘C with a helium carrier gas flow of 10 sccm. The column effluent is analyzed using both a NOy analyzer and I- CIMS.

NOy was quantified using a laboratory-built analyzer that converts PAN to NO on a molybdenum tube followed by chemiluminescence detection (Ridley and Howlett, 1974; Fehsenfeld et al., 1987). The conversion efficiency of the NOy instrument used in this study was approximately 96 % with a sensitivity of 4.07 ± 0.08 Hz pptv-1.

A detailed description of the I- CIMS is presented elsewhere (Slusher et al., 2004). Briefly, APNs are dissociated in a heated inlet, subsequently react with iodide ions (I-), and are detected as carboxylate anions using a quadrupole mass analyzer: RC(O)O2NO2⟶ΔRC(O)O2+NO2,RC(O)O2+I-⟶RC(O)O-+IO. Iodide ions are generated via the electron attachment reaction of CH3I in a 210Po ionizer. A critical orifice was used to maintain an inlet flow of 2.2 slpm at a flow tube pressure of 50 mbar. A switchable inlet was used for sampling through a heated dissociator to allow detection of APNs via Reactions (R1) and (R2), or a “cold” (30 ∘C) inlet, to serve as an instrumental background. Unless otherwise stated, the inlet dissociator temperature was set to 150 ∘C for the experiments presented in this work. In response to a well-known dependence of PAN detection on the water concentration in the flow tube (Slusher et al., 2004), a 100 % relative humidity, 10 sccm N2 flow was added directly to the flow tube. The instrument was tuned to minimize the detection of cluster ions such that the observed I⚫H2O- : I- ratio was 0.002.

In this work we have detailed a new method for the production and calibration of APNs. This method provides a simplified alternative to liquid-based synthesis of some of the less-well-studied APN species, e.g., APAN, MPAN, and CPAN. Due to the ease of APN production, it may be possible to improve on the methodology presented here to develop a field-deployable calibration system. Unfortunately, due to impurities formed in the photolysis of acyl chloride compounds this method necessitates the use of a prep-scale GC, which may be limiting. However, this ease of operation and ability to generate simultaneous APN calibrations makes this technique ideal for a laboratory setting. Although not shown here, this method should be compatible with GC–ECD (electron capture detector) systems provided that pre-separation is used to prevent exposure of the ECD system to any chlorinated starting materials and associated photoproducts.Edited by: G. Phillips