The question asks which radical dominates the tropospheric chemistry during the night. Let's analyze each option to determine which is the correct answer.

1. \(NO_{3}\) (Nitrate radical): During the night, the nitrate radical \(NO_{3}\) becomes the predominant reactive species in the troposphere. Sunlight decomposes \(NO_{3}\) rapidly, but at night, it accumulates in the atmosphere as it is stable without direct sunlight, making it a key player in nighttime chemistry.
2. OH (Hydroxyl radical): The hydroxyl radical, known as the "detergent" of the atmosphere, primarily dominates in the daytime chemistry due to its formation by the reaction of ozone with water vapor, which is driven by sunlight. It is not prevalent at night because its primary production mechanism depends on sunlight.
3. \(HO_{2}\) (Hydroperoxyl radical): This radical is also more common during daytime as it is a secondary product of the atmospheric reactions involving OH radicals and is less stable without continuous production driven by sunlight.
4. O (Oxygen atom): Atomic oxygen is rarely found as a free radical under typical tropospheric conditions. It mainly exists in other forms, such as part of ozone or dioxygen, and does not dominate nighttime chemistry.

Based on the examination of each option, it is clear that the nitrate radical \(NO_{3}\) is the primary reactive species dominating the nighttime tropospheric chemistry. Thus, the correct answer is:

Correct Answer: \(NO_{3}\)