Peroxyacetyl nitrate (PAN) and peroxyacetic acid (PAA) measurements by iodide chemical ionisation mass spectrometry: first analysis of results in the boreal forest and implications for the measurement of PAN fluxes

Abstract. We describe measurements of peroxyacetyl nitrate (CH₃C(O)O₂NO₂, PAN) and peroxyacetic acid (CH₃C(O)OOH, PAA) in the Boreal forest using iodide chemical ionization mass spectrometry (ICIMS). The measurements were made during the Hyytiälä United Measurement of Photochemistry and Particles – Comprehensive Organic Particle and Environmental Chemistry (HUMPPA-COPEC-2010) measurement intensive. Mixing ratios of PAN and PAA were determined by measuring the acetate ion signal (CH₃C(O)O⁻, m/z = 59) resulting from reaction of CH₃C(O)O₂ (from the thermal dissociation of PAN) or CH₃C(O)OOH with iodide ions using alternatively heated and ambient temperature inlet lines. During some periods of high temperature (~ 30 °C) and low NO_x (< 1 ppbv), PAA mixing ratios were similar to, or exceeded those of PAN and thus contributed a significant fraction of the total acetate signal. PAA is thus a potential interference for ICIMS measurements of PAN, and especially eddy covariance flux measurements in environments where the PAA flux is likely to be a significant proportion of the (short timescale) acetate ion variability. Within the range of mixing ratios of NO_x measured during HUMPPA-COPEC, the modelled ratio of PAA-to-PAN was found to be sensitive to temperature (through the thermal decomposition rate of PAN) and the HO₂ mixing ratio, thus providing some constraint to estimates of photochemical activity and oxidation rates in the Boreal environment.