The role of H₂SO₄ in stratospheric negative‐ion chemistry

Abstract

Gas‐phase ion‐molecule reactions involving H 2 SO 4 and a variety of positive and negative atmosphericions have been studied by using a flowing afterglow apparatus. These ions include NO 3 − · n HNO 3 ( n = 0, 1, 2) and H 3 O 4 + · n H 2 O ( n = 0, 1, 2, 3). All of the negative ions studied react rapidly with H 2 SO 4 to form HSO 4 − · n HNO 3 . These reactions establish a lower limit for the electron afiinity of HSO 4 :4.5 eV. They also indicate that the terminal negative ions in the upper stratosphere will be cluster ions of the type HSO 4 − · n H 2 SO 4 · m HNO 3 , which is in accord with recent in situ observations. On the basis of the present laboratory results and the reported in situ ion composition measurements, the H 2 SO 4 concentration at 35 km is estimated to be 6×10 5±1.6 cm −3 . It is found that H 2 SO 4 probably plays very little role in stratospheric positiveion chemistry, since it does not replace H 2 O in the H 3 O + · n H 2 O ion clusters.