What seasons are co2 emissions highest in?

emissions are the highest during winter season.

Explanation:

Growth of plants require ample amount of sunlight so that the plants can effectively perform photosynthesis. It is during the summer season that sunlight is available in plenty. Therefore plants will concentrate more in carbon dioxide absorption.

The greenhouse gas carbon dioxide is an important input required for the process of photosynthesis. Thus the plants concentrate more n photosynthesis during the summer and it helps in lowering the levels in the atmosphere. During the winter months, sunlight availability is low and plants less on photosynthesis and more on respiration.  

Thus the levels of will be high during the winter compared to summer.