Validation of Aura Microwave Limb Sounder BrO observations in the stratosphere

Validation of stratospheric BrO vertical profiles obtained by the Microwave Limb Sounder (MLS) on the Aura satellite is discussed. MLS BrO measurements are compared with expectations of its latitudinal and seasonal dependence, as well as with more localized balloon-borne measurements of BrO. We describe the expected precision and systematic errors of the version 2.2 retrieval and show that scientific studies using MLS BrO vertical profiles require extensive averaging to increase the signal-to-noise ratio to useful values. A monthly zonal mean over a 10 degrees latitude bin ( about 3,000 individual profiles) results in a precision of approximately +/- 4 ppt (similar to 25% of a typical daytime signal). Moreover, it is necessary to take day/night differences to remove large biases. The pressure range over which the data are considered useful is 10 to 3.2 hPa. Over this range, the estimated accuracy in the day/night difference is about +/- 20%. The vertical resolution is 5.5 km for 10 to 3.2 hPa. Day/night differences are a good measure of daytime BrO from 10 to 4.6 hPa; for 3.2 hPa the nonnegligible nighttime BrO needs to be accounted for. We infer total inorganic bromine (Br-y) to be 22.1 +/- 5.5 ppt on the basis of analysis of MLS measurements of BrO, which implies a contribution of 6.5 +/- 5.5 ppt to stratospheric bromine from sources other than long-lived CH3Br and halons.