On morning 12 march it was about
Looking at the data from earth.nullschool.net at a few hours before that morning I saw surface temperature of 5.1°C and 5.8°C at 1000hPa.
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.