High-pressure flowing-afterglow setup validated by the study of the CO 3− + HNO 3 reaction

Abstract

A high-pressure flowing-afterglow apparatus has been built in order to study in the laboratory gas-phase reactions of ions with neutral molecules playing an important role in the stratospheric ozone chemistry. The instrument consists of a flow tube, a first intermediate pressure chamber located between two conical electrodes, a quadrupole guide in a second intermediate pressure chamber, and a quadrupole mass analyzer in a third chamber. A method of measurement of the residence time for the ions in the flow tube has been used to derive absolute reaction rate coefficients. The validation of this setup was performed at room temperature over a pressure range of 1–3 hPa by the study of the well-known reaction CO 3 − + HNO 3. The rate coefficient was measured as (1.2 ± 0.3) × 10 −9 cm 3 s −1, and CO 3 −(HNO 3), NO 3 −, and NO 3 −(OH) were detected as primary product ions, in agreement with results previously reported. For the first time, the study was extended up to 24 hPa and at lower temperature. The rate coefficient was found to be independent of pressure above 1.1 hPa, but increased to a value of (2.4 ± 0.7) × 10 −9 cm 3 s −1 at 212 K. The relative yield of CO 3 −(HNO 3) increased with pressure to a value ≥ 60% above 12 hPa at 298 K, and ≥ 85% above 6 hPa at 212 K. These results are fundamental to the derivation of a HNO 3 mixing ratio in the stratosphere with balloon-borne instruments using active chemical ionization mass spectrometry.