You can see this post for a more detailed discussion, but the typical answer to your question is air stagnation. When the atmosphere is stable, there is little mixing, and the air can stagnate. Typically this coincides with low wind speeds and a low boundary layer. The boundary layer holds air near the surface like a blanket. The boundary layer collapses overnight, becoming shallow, and grows when the sun heats the surface of the Earth. So, in the morning, the shallow boundary layer allows less vertical mixing to occur.
You might be noticing that on cloudy days, when clouds are low, there is little wind and no solar heating. Air pollution has no place to go, so it sticks around.
Another aspect is that vehicles emit more emissions under cold-start conditions. This is because vehicles get cold overnight and take a while to get up to normal temperature when they startup in the morning. The cold vehicle will have a low combustion efficiency and catalytic converters won't work as well.