comment why MDA8 output is specical case

pyaerocom/aeroval/experiment_output.py

@@ -21,7 +21,7 @@
 from pyaerocom.aeroval.setupclasses import EvalSetup
 from pyaerocom.aeroval.varinfo_web import VarinfoWeb
 from pyaerocom.exceptions import EntryNotAvailable, VariableDefinitionError
-from pyaerocom.stats.mda8.const import MDA8_OUTPUT_VARS
+from pyaerocom.stats.mda8.const import MDA8_INPUT_VARS, MDA8_OUTPUT_VARS
 from pyaerocom.stats.stats import _init_stats_dummy
 from pyaerocom.variable_helpers import get_aliases
 
@@ -699,8 +699,12 @@ def _is_part_of_experiment(self, obs_name, obs_var, mod_name, mod_var) -> bool:
             True if this combination is valid, else False.
 
         """
+
+        # MDA8 is computed on-the-fly ONLY if a MDA8_INPUT_VAR at hourly freq is detected.
+        # Consequently, it is not specified in a config but should be included as part of the experiment.
         if obs_var in MDA8_OUTPUT_VARS and mod_var in MDA8_OUTPUT_VARS:
             return True
+
         # get model entry for model name
         try:
             mcfg = self.cfg.model_cfg.get_entry(mod_name)