What does the "BA/" stands for in this ATIS?
I think the BA/ references "Braking Action" in the ICAO D-ATIS sequence. Since the example report from LBTA (Istanbul Ataturk International Airport) does not suggest that a Braking Action value would be assigned (no rain, etc), it appears to be a placeholder for that information if it was provided.  

Below is an excerpt from Annex 11, to the publication - Convention on International Civil Aviation (Air Traffic Services), showing the sequencing (where it would normally appear in the D-ATIS report) of a Braking Action report (if provided/available).