The level (concentration) of carbon dioxide in the atmosphere varies throughout the year depending on the amount of photosynthetic activity and hence the amount of carbon 'locked up' in vegetation.
In the northern hemisphere many plants begin to grow in the Spring (April to June) taking carbon dioxide out of the atmosphere, reach maturity in the Summer (July to September) and die or shed leaves in the Autumn (October to December).
In the Winter (January to March) dead plant material decomposes returning carbon dioxide to the atmosphere.