Characterization of a thermal decomposition chemical ionization mass spectrometer for the measurement of peroxy acyl nitrates (PANs) in the atmosphere

Abstract

Abstract. This paper presents a detailed laboratory characterization of a thermal dissociation ionization mass spectrometer (TD-CIMS) for the atmospheric measurement of Peroxyacetyl nitrate (PAN) and its homologues. PANs are efficiently dissociated in a heated inlet tube and the resulting peroxy acyl radicals are reacted with I– ions in a flow tube. The CIMS detects the corresponding carboxylate ions to give a specific and quantitative measurement of each PAN species. PAN, peroxypropionyl nitrate (PPN), peroxyisobutyryl nitrate (PiBN), peroxy-n-butyryl nitrate (PnBN), peroxyacryloyl nitrate (APAN), peroxycrotonyl nitrates (CPAN) and peroxymethacryloyl nitrate (MPAN) were cross-calibrated with both a dual channel GC/ECD and a total odd-nitrogen (NOy) instrument for the NCAR TD-CIMS' typical aircraft operation conditions. In addition, the instrument sensitivity to a number of more exotic PAN homologues (peroxyhydroxyacetyl nitrate, methoxyformyl peroxynitrate, and peroxybenzoyl nitrate) was evaluated qualitatively by comparisons with a long-path FTIR instrument. The sensitivity for PPN is slightly higher than that of PAN. Larger aliphatic and olefinic PAN compounds generally showed lower sensitivities. These differences are owing to secondary reactions in the thermal decomposition region, which either reduce the yield of peroxy acyl radicals or cause losses of these radicals through intramolecular decomposition. The relative importance of these secondary reactions varies considerably between different PAN species. Results also indicate that the reaction of the peroxy acyl radicals with the ion-water cluster, I–(H2O)n proceeds about an order of magnitude faster than with I– alone. Variations among the individual PAN species at very low water vapor were observed. The results call for careful evaluation of each PAN species to be measured and for each desired operating condition of a TD-CIMS instrument.