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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:

  1. Synoptic scale determinants of cloud type such as pressure systems
  2. Thermodynamic changes in stability/mixing and/or boundary layer height
  3. 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).
  4. Cloud height (low, middle, high)
  5. Cloud shape (stratus, cumulus, nimbus, cirrus, etc.)
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I haven't found proper references yet, but this hasn't gotten an answer yet. To speculation!

Long-lived (non-precipitating) clouds can decrease solar insolation at the surface, reducing convection and mixing below the cloud and increasing pollutant concentrations. If clouds are just a symptom of a high-humidity environment, that can promote the condensation of semi-volatile species into particulates (nitrate, ammonia, some organics) if you don't count phase change as a reaction. Convective storms are associated with a lot of mixing, both within the boundary layer and between the boundary layer and the free troposphere, reducing concentrations.

Without the photolysis or rainout processes, I don't know enough about different cloud types and environments to speculate further. I'm hoping this at least sparks more discussion.

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  1. Typically low pressure systems bring the most amount of clouds. In high pressure systems, clouds are typically high cirrus or convectively driven cumulus (or cumuliform clouds, such as cumulonimbus), though there are exceptions.

  2. Convection, which is mostly what people think about when considering thermodynamics can lift surface pollution into the mixed layer and may return the next day. Boundary layer height is very important for air pollution, as it is represented in a concentration, i.e. mass per unit volume. Inversions, especially increase pollution.

  3. Humidity can cause small particles, mostly hygroscopic particles to swell to measurable levels. This changes the mass.

  4. Cloud height: This affects the boundary layer. If it increases the height of the boundary layer, all things being equal, will decrease the levels of pollution. If it decreases the boundary layer, all things being equal, it will decrease pollution.

  5. Cloud shape is only as important on how the cloud is formed, sustained, and other relative details.

You may also want to pay attention to air pollution advection, or mixing. Knowing the quality of the air from where the clouds and wind is blowing is important.

tl;dr If it decreases the boundary layer height, expect air pollution to increase.

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