Trace Constituents in the Mesosphere

Abstract

Recent observations of selected trace constituents in the mesosphere are reviewed. The review is divided into discussions of some long-lived constituents (CO, NO and H2O), and short-lived constituents (OH and O3) that are important for understanding the transport and photochemical processes in the mesosphere. CO and NO are produced in the lower thermosphere and lost in the lower mesosphere and stratosphere. There is now observational evidence that downward transport into the winter polar region, and subsequent "spillage" into the winter mid-latitude regions are important for CO and NO. In comparison with CO and NO the distribution of H2O is "upside down" in the sense that it is generated in the lower atmosphere and flows upward to its sink. The same transport mechanism may operate for H2O, but in the opposite sense of moving dry air downward in winter, and moist air upward in summer. The short-lived OH radical is important for understanding the HOx-Ox mechanism in the middle atmosphere. Unfortunately, there is an almost complete lack of data for its distribution above 50 km. Ozone and water vapor are out-of-phase in their seasonal behavior in the upper mesosphere; the variations in both species provide important clues for understanding the causes of eddy mixing in the mesosphere. The seasonal climatology of mesospheric ozone is now well documented as a result of five years of SME satellite observations.