A chemical ionization technique for measurement of pernitric acid in the upper troposphere and the polar boundary layer

Abstract

The potential of SF6− for the detection of pernitric acid in air by chemical ionization mass spectrometry was studied in the laboratory via its reactions with HO2NO2, O3, H2O, and NO2. Measurements of HO2NO2 using SF6− ion chemistry were then performed at the South Pole. SF6− reacts at the gas kinetic rate with HO2NO2 to form SF5−, NO4−(HF), NO2−(HF), and NO3−. The NO4−(HF) product provides a unique signature for HO2NO2. The rate coefficient for this channel is 2.9 × 10−10 cm³ molecule−1s−1 at 16.5 torr in N2 and decreases as the total pressure increases. NO4−(HF) undergoes ligand switching with H2O and HO2NO2, implying the existence of a NO4− core ion in the gas phase. SF6− reacts with H2O to produce a large number of product ions that increase nonlinearly with [H2O]. This limits the use of SF6− to regions of the atmosphere with low absolute humidities. However, our lab studies indicate that HO2NO2, can be selectively detected in air with an ozone mixing ratio up to 550 ppbv and a dew point of −25°C or less, which corresponds to regions of the troposphere where HO2NO2 is expected to be thermally stable. We present field data from the South Pole showing typical HO2NO2 mixing ratios of 20 pptv.