Which of the following is NOT a process that causes pesticides to break down after application?

Atmospheric degradation refers to the breakdown of pesticides as they are exposed to environmental factors in the atmosphere, such as sunlight, moisture, and temperature changes. This process can lead to the transformation or degradation of pesticides, but it is commonly grouped under several degradation methods, particularly focusing on external atmospheric influences.

The other options—photo degradation, microbial degradation, and chemical degradation—are all recognized processes whereby pesticides can decompose after they have been applied. Photo degradation involves the breakdown of pesticides due to exposure to light, particularly ultraviolet light. Microbial degradation is the process by which microorganisms, such as bacteria and fungi, metabolize and break down chemical compounds in pesticides. Chemical degradation refers to the alteration of pesticides through chemical reactions, often involving the pesticide interacting with other chemicals in the environment.

Given this understanding, the option identified as not being a process that causes breakdown is indeed atmospheric degradation, as it encompasses a broader range of environmental influences rather than constituting a distinct process on its own like the others listed.