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

The potential of SF 6 − for the detection of pernitric acid in air by chemical ionization mass spectrometry was studied in the laboratory via its reactions with HO 2 NO 2 , O 3 , H 2 O, and NO 2 . Measurements of HO 2 NO 2 using SF 6 − ion chemistry were then performed at the South Pole. SF 6 − reacts at the gas kinetic rate with HO 2 NO 2 to form SF 5 − , NO 4 − (HF), NO 2 − (HF), and NO 3 − . The NO 4 − (HF) product provides a unique signature for HO 2 NO 2 . The rate coefficient for this channel is 2.9 × 10 −10 cm³ molecule −1 s −1 at 16.5 torr in N 2 and decreases as the total pressure increases. NO 4 − (HF) undergoes ligand switching with H 2 O and HO 2 NO 2 , implying the existence of a NO 4 − core ion in the gas phase. SF 6 − reacts with H 2 O to produce a large number of product ions that increase nonlinearly with [H 2 O]. This limits the use of SF 6 − to regions of the atmosphere with low absolute humidities. However, our lab studies indicate that HO 2 NO 2 , 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 HO 2 NO 2 is expected to be thermally stable. We present field data from the South Pole showing typical HO 2 NO 2 mixing ratios of 20 pptv.