Measurements and Model Calculations of HNO4: Implications for HO(x)

Establishing a quantitative understanding of the abundance of peroxynitric acid (HNO4) is of interest because the reaction of OH with HNO4 is believed to be a significant sink for HO(x) in the lower stratosphere and upper troposphere. We present concentration profiles of KNO4 obtained by the MkIV Fourier transform infrared spectrometer using the solar occultation technique during two flights, one at 35 N on 25 September 1993 (930925) and the other near 66 N on 7 May 1997 (970507). The observations of HN04 present an apparent dilemma: the calculated profile for HNO4 found using standard kinetics is in relatively good agreement with the mid-latitude observations, but the calculated profile for HNO4 found using the same kinetics overestimates the high-latitude summer observations by nearly a factor of two. Resolution of this dilemma is the focus of our investigation. We show that introduction into our photochemical model of a speculative long-wave (lambda greater than 650 nm) photolytic pathway for HNO4 leads to good agreement between theory and observation of HN04 for both the mid-latitude and high-latitude regions. Implications for the budget of HO(x) are also discussed.