I am working through this problem on my own as well, but am feeling that some perspective is in order. So, ignoring the obvious effects where precipitation will wash stuff out of the air, and blocking sunlight will prevent chemical reactions from occurring that depend on photon energy - in fact, we'll just assume a chemically inert atmosphere with no significant deposition and emission of pollutants. We'll also go ahead and use the most common height and shape-based cloud type categorization system (http://scied.ucar.edu/webweather/clouds/cloud-types). Also, to simplify, lets generally assume particulate pollution.
How does the presence of a cloud impact pollutant levels in the air underneath it and how does this vary by cloud type?
An ideal answer will consider at least one of the below:
- Synoptic scale determinants of cloud type such as pressure systems
- Thermodynamic changes in stability/mixing and/or boundary layer height
- Humidity - can include quibbling over whether the meaning of pollutant levels means measurement of dry mass, visibility reduction, or measurement under ground conditions (But, unless you want to get into this mess, lets assume dry mass).
- Cloud height (low, middle, high)
- Cloud shape (stratus, cumulus, nimbus, cirrus, etc.)