The rapid rate at which the trace composition of Earth's atmosphere has changed over the past few decades has resulted in a dramatic increase in atmospheric research. In particular, a large effort has been directed towards measurement of atmospheric trace-gases. trace-gases are known to play a disproportionately large role in atmospheric chemistry and climate and yet, because of their sparse abundance, can be very difficult to measure. Knowledge of these trace-species is essential for understanding and quantifying these atmospheric changes. It is also important for the proper validation and improvement of chemical models which can, in turn, be used to predict the future state of the atmosphere. There are two main observation techniques in the measurement of trace-gases: in-situ and remote sensing. In-situ techniques make observation at the location of the instrument. In the stratosphere, measurements can only be made from mobile platforms which severely limits their ability to make long-term observations.
Remote sensing techniques, by measuring the intensity of light at many wavelengths, interpret the results of the interaction of electromagnetic waves with the atmosphere through absorption, emissions, or scattering. Remote measurements utilize the fine structure of the molecular absorption lines, either electronic, vibrational, or rotational. The spectral regions utilized include the UV, visible, infrared, and microwave. A larger number of techniques and instruments exist which are able to measure many of the different trace-species throughout the homosphere. Each technique offers advantages and disadvantages including complexity of the instrument, complexity of the data analysis, precision and accuracy, spatial and temporal resolution, and number of species measured. Some of the more common methods include Fourier Transform Infrared Spectroscopy (FTIR), Differential Optical Absorption Spectroscopy (DOAS), a variety of laser absorption and fluorescence techniques, lidar, Dobson and Brewer spectrophotometers, and many types of microwave sounding, to name a few. A survey of some of these spectroscopic methods has been made by Schiff (1992). Platforms used to house these instruments include ground-based, balloon, rocket, aircraft, and satellite platforms. These platforms and methods of measurement are illustrated in Figure 6.1 (Schmidt and Zander, 1996).
In this chapter, the DOAS technique is used to retrieve column densities of ozone, NO2, and BrO from CPFM measurements. Towards this end, a number of tools necessary for the retrieval process are introduced. The first is the air mass factor (AMF) which is used to convert apparent column density to vertical column density. The DOAS technique is then developed and a number of spectral fit examples are presented. Following this, the retrieval algorithm used to extract profile information is outlined and measurements of ozone, NO2, and BrO are presented from the most recent ER-2 campaign which took place throughout the Spring and Summer of 1997. The last section deals with the application of some of these measurements towards the polar sunrise ozone depletion phenomena.