Why do CO and other parameters peak during the night?
In the CAMS Global data there is a clear diurnal pattern to many of the pollution parameters, such as CO, PM25, etc..
These parameters peak in concentration during the night, and conversely reach their minimum in the mid-afternoon.
Near the surface the reason for this diurnal pattern is that the vertical stability of the lowest layer (Planetary Boundary Layer) is higher during the night. This means that the emitted pollutants are trapped (not diluted), which leads to higher concentration values.
This effect is not visible when you plot total columns. But also for total columns there is diurnal variability because the horizontal winds tend to be weaker during the night. The pollutants stay closer to the emission hotspots during the night.
In the global model we currently do not consider a diurnal cycle for most species (i.e. the emissions are constant during the day).