I'm not sure if you can do so, simply because you are trying to compare different variables/or group of variables together.

Sun variable may include Solar flux, intensity, heat, duration ...etc.

The above-mentioned variables may increase the evapo-transpiration or photosynthesis or any physiological process.

In case of air pollution you will have first to identify your variable ( dust particles, chemicals  or type or air pollutions then to correlate but not to compare.

regards

we can compare between some pigments concentrations, protein content and anther materials  if take same factors such as temperature which is main factor effects in all synthesis.

The effects of air pollution is the highest when stomata of plants are open.

The maximum openess usually occurs near noon local time which is also the time of the day that radiation is the strongest. Although too much ultra-violet may damage plants, the damage to plant is more likely to occur by coincidence to highest sun radiation.

In fact, more radiation triggers opening of stomata which allows more pollution to enter into the plant and to make damage.

Hope it helps