Why are atmospheric BrO and ClO important to measure by satellite?
BrO and ClO significantly deplete ozone from the atmosphere.

Researchers at Harvard University state:


  It is a remarkable fact that perhaps the most important observation coupling climate forcing to UV dosage levels at the surface at mid-latitudes is the observation of high (e.g. > 10 ppmv) water vapor and low temperatures (&lt; 210 K) with the simultaneous determination of BrO and ClO in the lower stratosphere at mid-latitudes.


Similarly, Science magazine states:


  Halogen-radical chemistry was responsible for approximately one-third of the photochemical removal of O3; reactions involving BrO account for one-half of this loss. 


Additonally,


  ... where the chlorine and bromine thus released participate in the catalytic destruction of ozone.


Kinetic studies of the BrO plus ClO cross-reaction over the range T=246-314 K