Derivation of OH concentration from satellite infrared measurements of NO₂ and HNO₃

Abstract

The hydroxyl radical is a very important radical in the chemistry of the atmosphere. In the stratosphere it is involved not only in the catalytic destruction of ozone but also in the interaction of the nitrogen, chlorine and hydrogen families and in the partitioning of individual species within these families¹. A thorough knowledge of the distribution and behaviour of OH is therefore very useful. However, OH is difficult to measure and only a few published profiles^2–4 exist, confined to 32 °N. The ability to infer global OH fields from direct measurements of NO₂ and HNO₃ would therefore be a major advance in the study of the stratosphere. Geophysical quantities (for example, geostrophic winds, eddy heat and momentum fluxes) have been derived indirectly from satellite data and have proved useful in understanding the behaviour of the stratosphere and mesosphere. The advent of satellite measurements of various trace gases by the stratospheric and mesospheric sounder (SAMS) (CH₄, N₂O, H₂O, NO and CO) and the limb infrared monitor of the stratosphere (LIMS) (O₃, NO₂, HNO₃ and H₂O) now allows us to derive further chemical quantities. Here we present for the first time a cross-section of OH in the middle stratosphere derived from satellite measurements of NO₂ and HNO₃ by the LIMS instrument flown on Nimbus 7. The values obtained agree well with the limited number of previous direct measurements and the variation of derived OH with height and latitude agrees well with model predictions. Further calculations are being made to derive the seasonal behaviour of a radical of central importance in the chemistry of the middle atmosphere. The present calculations nevertheless demonstrate the great power of coordinated satellite measurements.