Question: Why do we use EESC as a proxy in trend analysis of stratospheric ozone data?

Answer: Depends on problem and on time period of data.

The problem that we are trying to solve is:

Is ozone in the stratosphere behaving as we would expect over the past several decades of increasing chlorine and the last 15-20 years of decreasing chlorine due to the provisions of the Montreal Protocol?

This is one of the primary questions for research into long-term trends in the ozone layer. Chlorine in the atmosphere increased as CFC production increased. The provisions of the Montreal Protocol removed most of the sources of stratospheric chlorine and the chlorine concentrations have peaked and begun to decrease. EESC is our measure of how chlorine has changed over time. It includes all of the known industrial sources of CFCs and is adjusted to include the chlorine equivalent of bromine sources. (This does not mean that we should not be looking for improved representations of EESC)