Precursor Gas Measurements (Chapter 2)

Assessments of stratospheric ozone have been conducted for nearly two decades and have evolved from describing ozone morphology to estimating ozone trends, and then to attribution of those trends. Stratospheric aerosol has only been integrated in assessments in the context of their effects on ozone chemistry and has not been critically evaluated itself. As a result, the Assessment of Stratospheric Aerosol Properties (ASAP) has been carried out by the WCRP project on Stratospheric Process and their Role in Climate (SPARC). The objective of this report is to present a systematic analysis of the state of knowledge of stratospheric aerosols including their precursors. It includes an examination of precursor concentrations and trends, measurements of stratospheric aerosol properties, trends in those properties, and modeling of aerosol formation, transport, and distribution in both background and volcanic conditions. The scope of this report is extensive; however, some aspects of stratospheric aerosol science have been deliberately excluded. For instance, we have not attempted to include an examination of polar stratospheric clouds (PSCs) or other clouds (such as cirrus clouds) occurring at or above the tropopause except in as much as they influence aerosol observations. Polar stratospheric clouds are the subject of a separate SPARC activity. We have produced a gap-free aerosol data base for use beyond this report. This required some new analysis that has not previously appeared in the technical literature. Similarly, the trend analysis required the development of a new analysis technique that is the subject of an article published in the Journal of Geophysical Research. New work is clearly identified in the present report.