Why does pollution build up during the day?

Answer 1

If by pollution you mean "photochemical smog" the answer is in the definition, the pollution you see as smog is produced by the action of the sun on chemicals in the atmosphere. Once the sun goes down, these reactions cease and the smog goes away. Sometimes, when it is nice and sunny because your city is sitting under a high pressure ridge, an inversion layer forms and the chemicals that form the smog during the day don't get blown away at night. When these combine with the new chemicals released by people gassing their vehicles and operating leaf blowers and such, the smog formed gets worse and worse. Ozone is produced in this process, which is why you get high ozone levels during smog events.

Google terms like:
photochemical smog
tropospheric ozone
secondary organic aerosol
peroxyacetylnitrate
wanky surfing aardvark

That last one has nothing to do with smog, but I'm betting you will get some weird hits with it.

Answer 2

pollution is thicker than normal air, so it is heavier. you just can't see it at night

and more people drive during the day too, so its rate is increaed more around those high traffic times