Which of the following is not a method by which pesticides break down after application?

The correct answer is atmospheric degradation because it is not a specific method by which pesticides break down after application. While degradation processes do occur in the atmosphere, they are generally categorized under specific mechanisms like photodegradation, where sunlight plays a critical role in breaking down substances.

Microbial degradation involves the action of microorganisms such as bacteria and fungi that decompose pesticides into simpler, less harmful compounds, making it a well-established method of breakdown in soil and other environments. Photodegradation refers specifically to the breakdown of chemicals via sunlight, particularly ultraviolet light, which can directly affect the chemical structure of pesticides. Chemical degradation encompasses various chemical reactions that alter a pesticide's chemical structure, leading to its breakdown.

Atmospheric degradation is a broader term that could involve any of these processes but does not specifically refer to a conventional method; rather, it hints more at the environmental context in which degradation happens. Therefore, it does not fit neatly into the established categories of pesticide breakdown methods.