diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0e3a0de..7a32b13 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - update hsrl.py to use py3 integer divide. fixes date parsing.
+- units for backscatter coeffiecient to km-1 sr-1 in output files generated with aop.py
   
 ### Added
 - add optional explicit bin ordering to aop.py yaml. this allows for calculating AOP's for a single bin, or subset of bins
diff --git a/src/pyobs/aop.py b/src/pyobs/aop.py
index f579f5d..35db30c 100644
--- a/src/pyobs/aop.py
+++ b/src/pyobs/aop.py
@@ -585,7 +585,7 @@ def getAOPext(self,Species=None,wavelength=None,fixrh=None,doaback=True):
         # ----------
         A = dict (EXT = {'long_name':'Aerosol Extinction Coefficient', 'units':'km-1'},
                   SCA = {'long_name':'Aerosol Scattering Coefficient', 'units':'km-1'},
-                  BSC = {'long_name':'Aerosol Backscatter Coefficient', 'units':'km-1'},
+                  BSC = {'long_name':'Aerosol Backscatter Coefficient', 'units':'km-1 sr-1'},
                   DEPOL = {'long_name':'Depolarization Ratio', 'units':'1'}
                   )
         if doaback: