Thermal decomposition of peroxyacrylic nitric anhydride (APAN)†

Abstract

The peroxycarboxylic nitric anhydrides (PANs; RC(O)O₂NO₂ with R ≠ H) are important trace gas constituents of the troposphere. One of the lesser studied molecules of the PAN family is peroxyacrylic nitric anhydride (APAN; CH₂CHC(O)O₂NO₂) which is found in elevated concentration in biomass burning (BB) plumes and downwind from petrochemical plants. In this work, we conducted laboratory and field experiments to constrain the thermal decomposition (TD) rates of APAN in the atmosphere. The TD of APAN was studied in laboratory experiments using a Pyrex reaction coil at temperatures between 295.2 K and 320.7 K as a function of flow rate (i.e., residence time). Gas streams containing APAN were generated from a diffusion source containing a synthetic sample stored in tridecane at water-ice temperature. Nitric oxide (NO) was added to this gas stream to prevent recombination of the TD products. Concentrations of APAN were monitored by gas chromatography with electron capture detection (PAN-GC). The TD rate constant is best described by 10^{(17.88±0.80)} e^{−(121.2±4.8) kJ mol−1/(RT)} s⁻¹, where R is the universal gas constant, and T is the temperature in kelvin. We report ambient air mixing ratios of peroxyacetic nitric anhydride (PAN), peroxypropionic nitric anhydride (PPN), and APAN measured by PAN-GC at the Calgary Central (Inglewood) air quality station from April 17 to May 31, 2023. From May 16 to May 21, the measurement location was blanketed by a BB plume as judged from co-located observations of fine particulate matter (PM_2.5) and carbon monoxide (CO). During this time, mixing ratios as high as 3.4 ppbv (PAN), 455 pptv (PPN), and 220 pptv (APAN) were observed. After sunset, mixing ratios of the PANs decreased with pseudo-first order kinetics, rationalized by a combination of dry deposition and loss by TD.