A GCSE is a very basic qualification (!), so no, I can't explain much - but yes, CO2 trapping is greatly reduced in the winter (apart from evergreens, of course).

But all these websites put it much better than I can. Note particularly the 'sawtooth' pattern in the graph in my first LINK, which traces the rising CO2 levels from 1974 - 2006 - yes, the levels differ summer to winter each year, but the overall pattern is a decidedly upward trend.

THIS SITE also offers a clear layman's explanation of why CO2 peaks in May each year.

Although your figure of 0.04% carbon dioxide in earth's atmosphere suggests CO2's effect on life must be negligible, it really is not. See HERE.

We might profitably continue our discussion HERE if you like?