Techniques for measuring pernitric acid, peroxyacyl nitrates, and dinitrogen pentoxide by chemical ionization mass spectrometry

Two techniques were developed for the measurement of a range of thermally labile nitrogen-containing compounds by chemical ionization mass spectrometry (CIMS). The first uses SF6- to detect pernitric acid, HO2NO2, in dry environments. The second technique makes possible fast measurements of a series of peroxyacyl nitrates and dinitrogen pentoxide utilizing thermal decomposition followed by reaction with I-.; The first in situ measurements of pernitric acid were made at the South Pole with the SF6- detection scheme during the second Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT 2000). Observed HO2NO2 concentrations were significant, averaging 25 pptv. Simple steady-state calculations constrained by measurements show that the lifetime of pernitric acid was largely controlled by dry deposition, with thermal decomposition becoming increasingly important at warmer temperatures. The HO2NO2 equilibrium constant was determined to be less uncertain over the observed temperature range than indicated in the literature. One consequence of pernitric acid deposition to the snow surface is that it is an important sink for both NOx (NO + NO2) and HOx (OH + HO2). Another is that the photochemistry of HO2NO2 in the Antarctic snowpack may be a NOx source in addition to nitrate photolysis. This might be one of the important differences in snow photochemistry between the South Pole and warmer polar sites.; Peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), peroxyisobutyryl nitrate (PiBN) and peroxymethacrylolyl nitrate (MPAN) were measured during an informal comparison of the thermal decomposition - chemical ionization mass spectrometry (TD-CIMS) technique with a gas chromatograph (GC) equipped with an electron capture detector (ECD) in Boulder, CO during October 2002. Good agreement was obtained between the two instruments for ambient PAN and PPN measurements, and the detection limit for a 1 s TD-CIMS measurement was comparable to a 2.5 min GC/ECD measurement. The fast time response of the TD-CIMS will allow for the observation of transient features and direct flux measurements of peroxyacyl nitrates by eddy covariance. Ambient measurements made with the TD-CIMS also demonstrate that I- chemistry can be used to simultaneously detect dinitrogen pentoxide with peroxyacyl nitrates.