diff --git a/da/stilt/obspack.py b/da/stilt/obspack.py
index 5f4ce7c57952b375d8c9e34700aead2b890355af..6f803e8a2becf3118d4486a53edd565cc12267e5 100755
--- a/da/stilt/obspack.py
+++ b/da/stilt/obspack.py
@@ -338,7 +338,7 @@ class ObsPackObservations(Observations):
 
 
 
-    def add_model_data_mismatch(self, filename, filename_cofilter):
+    def add_model_data_mismatch(self, filename):
         """
             Get the model-data mismatch values for this cycle.
 
@@ -383,6 +383,7 @@ class ObsPackObservations(Observations):
         site_hourly = {}   # option added to include only certain hours of the day (for e.g. PAL) IvdL
         site_foot = {} # option added to check for available footprints per observation
         site_incalt = {} # option to increase sampling altitude for sites specified in sites and weights file
+        co_filter = {}
         for key, value in sites_weights.iteritems():
             if 'co2_' in key or 'sf6' in key:  # to be fixed later, do not yet know how to parse valid keys from rc-files yet.... WP
                 sitename, sitecategory = key, value
@@ -401,13 +402,16 @@ class ObsPackObservations(Observations):
             if 'site.incalt' in key:
                 identifier, incalt = value.split(';')
                 site_incalt[identifier.strip()] = (int(incalt))
+            if 'co.filter' in key:
+                identifier_cofilter, cofilter = value.split(';')
+                co_filter[identifier_cofilter.strip()] = (str(cofilter))
 
         #for obs in self.datalist:
         do_not_simulate=[]
         do_simulate=[]
 
         eventids=[]
-        for line in open(filename_cofilter):
+        for line in open(str(cofilter)):
             columns = line.split()
             if columns[0] == "eventid":
                 pass