For a given time step, the photolysis module calculating these diagnostic variables is only called when the solar radiation in a given grid cell is greater than zero (i.e. daytime). During nighttime, there is no photolysis and no light extinction by aerosols. The best way to think about these zero nighttime values is “AOD / aerosol extinction not available”, analogous to how satellite products also do not provide any AOD during night. While “real” AOD / aerosol extinction values probably never go down to zero even in the cleanest environments, you could filter by solar radiation from METCRO2D (variable RGRND) to set all zero AOD values to missing when RGRND equals zero if this helps with your analysis.
Thanks for your reply.