For some time the government has been spraying something into the air from what look like large concrete mixer trucks during and before the rainy season. Does anyone know what exactly they are spraying and what chemicals are contained in the fine mist they produce?

Just water to reduce the particle based airpolution.

It doesn't smell like water and when it hits my helmet visor it leaves

a residue of some kind. Maybe just the quality of the water......

It's polluted water of course. When I was living in a complex in the North, they use to use water that smelled like shit to water the grass. Turns out it was sewage water. Yuk!

Indeed for the watering of vegetation and the spraying of streets any water including rather polluted surface water is used. For these vapor canons they have to use rather clean water otherwise the spray nozzles would block. Most probably just Kunming tap water.

I really doubt if this really works. The street spaying seams to work on the principle that by spraying the streets the dirt attaches to the cars on that street.

I notised as well that now several building sites have water sprayers on their fences in order to stop dust from escaping. Now it rains down on the pedestrians and cyclers passing the place. I noticed as well water sprinklers used at demolition sites.

yep all the crap on your vizor has been washed out of the air. prove it works dont it

Back in the day, tanker trucks would pump water out of the sewers to be used for watering roadside trees and plants. Maybe even washing down streets. You will notice Chinese are desperate to avoid contact with water sprayed by trucks.

If it looks like a misting machine - it's usually a mosquito insecticide. Is it toxic - probably - but how would you know. As it's disbursed into the open air - just try to NOT follow behind and OD on the vapors/mist.

Above reactions now show a mix up of insecticide spay trucks and water vapor trucks. Insecticide is sprayed early in the morning when there is limited traffic. These water vapor air cleaning trucks are driving up and down some of the main streets like DongFeng xi-zhong-dong street. These are open boulevards and the water falls on the ground (If it not hits pedestrians, cyclist and cars). They are really driving up and down which is not useful for insecticide spraying and during the middle of the day.
Insecticide is spayed in a different way and is specifically targeted at trees etc because these are the resting places of the insects.
Indeed people try to avoid being hit by the water jets coming from the street cleaning trucks; this very simple in order to stay dry and clean.
Conspiracy theorist will have a field day on this topic again but stay calm; indeed Kunming sprays against insects. No the water vapor canons are not insecticide but just water. If both things are effective is an other thing however.

The big fat mister cannons are spraying for particulate pollution.
On the other sprayers, I have seen the road surface sprayers fill up from local watercourses, which were often stagnant and stinky. I say were, as the watercourses have become much cleaner in the last two years. I have also seen these same trucks used to spray roadside vegetation, and that would be the same water.