Regarding pollution, http://iopscience.iop.org/article/10.1088/1748-9326/11/6/064004/meta says,
"The lowered wind speeds and vertical mixing during daytime led to stagnation of air near the surface, potentially causing air quality issues."
Why not distribute a solar air heater that can be attached to roofs? The solar air heaters will shade roofs during the day and heat air, but also be cooled by the air passing through. This should cause cloud formation which will keep the city warm at night. Paint manufacturers could team up with solar air heater manufacturers to produce this air heating, roof cooling and rain enhancing solar air heater on roofs situation.
See http://www.builditsolar.com/Experimental/PopCanVsScreen/PopCanVsScreen.htm for information on solar air heaters.
Drought in coastal cities: I am wondering if it would not be better to make buildings in, say, Los Angeles a dark colour to heat up more and enhance chances of rain (or use solar air heaters on roofs as mentioned above). One hears about a 10% increase in rainfall due to urban heat island effect. I have done some calculations:
Assume the air from the sea has a relative humidity (RH) of 70% and is at 18 deg C and it blows onto land and heats up over the city, but remains at 18 deg C over the rest of the area (a simplification to make calculations easier). My calculations use an environmental lapse rate of 6.5 deg C per km rise and a dry adiabatic lapse rate of 9.8 deg C per km rise. The dew point of the sea and land air remains at 12.5 deg C whether heated or not, but the RH changes on the air being heated. My calculations show the number of degrees the air heats up over the city (1 deg means it heats up to 19 deg C from the 18 deg sea air temperature). Then they show the change in RH of the air after heating. Then they show the height to which the heated air can rise and the height to which it needs to rise for clouds to form: 0) Heats up 0 deg over city, RH remains 70%, the air can rise 0 m and it needs to rise 694 m for clouds to form. 1) Heats up 1 deg over city, RH is now 65.8%, the air can rise 303 m and it needs to rise 819 m for clouds to form 2) Heats up 2 deg over city, RH is now 61.8%, the air can rise 606 m and it needs to rise 944 m for clouds to form 3) Air heats up 3 deg over city RH is now 58.1%, the air can rise 909 m and it needs to rise 1069 m for clouds to form 4) Air heats up 4 deg over city RH is now 54.6%, the air can rise 1212 m and it needs to rise 1194 m for clouds to form 5) Air heats up 5 deg over city RH is now 51.4%, the air can rise 1515 m and it needs to rise 1319 m for clouds to form 6) Air heats up 6 deg over city RH is now 48.4%, the air can rise 1818 m and it needs to rise 1444 m for clouds to form After step 4 the air is heated enough for clouds to form - see the graph. Note that more cloud formation could cool Earth overall.
Here is another very good design to get air moving upwards:
One could increase convection by using solar air heaters made by placing a black piece of corrugated iron roof sheet a few centimetres above a silver sheet of corrugated iron. Another method is to make the soil dark with biochar - plow biochar into the soil. You can make your own biochar by heating wood in a barrel.
You can also paint rocks black: Rocks have a high heat capacity - they hold a lot of heat. If cool roofs reduce rainfall, rocks painted black can increase rainfall.
Just as cool colours can decrease rainfall, so can dark colours incease rainfall by increasing convection. See http://www.xcmag.com/2007/10/thermal-flying-part-2-thermal-generators-and-triggers/
When clouds develop, because of convection, they will help with global warming by cooling the Earth.