diff --git a/cara/monte_carlo/data.py b/cara/monte_carlo/data.py
index 3b9172d2..290f5920 100644
--- a/cara/monte_carlo/data.py
+++ b/cara/monte_carlo/data.py
@@ -161,7 +161,7 @@ def expiration_distribution(BLO_factors):
     """
     Returns an Expiration with an aerosol diameter distribution, defined
     by the BLO factors (a length-3 tuple).
-    The total concentration of aerosols is computed by integrating
+    The total concentration of aerosols, cn, is computed by integrating
     the distribution between 0.1 and 30 microns - these boundaries are
     an historical choice based on previous implementations of the model
     (it limits the influence of the O-mode).
@@ -169,7 +169,7 @@ def expiration_distribution(BLO_factors):
     dscan = np.linspace(0.1, 30. ,3000)
     return mc.Expiration(CustomKernel(dscan,
                 BLOmodel(BLO_factors).distribution(dscan),kernel_bandwidth=0.1),
-                BLOmodel(BLO_factors).integrate(0.1, 30.))
+                cn=BLOmodel(BLO_factors).integrate(0.1, 30.))
 
 
 expiration_BLO_factors = {