What term describes fragments of dust, sand, pollen, and smoke in the air?

The correct term that describes fragments of dust, sand, pollen, and smoke in the air is 'particulates.' Particulates refer to tiny solid or liquid particles suspended in the atmosphere. These particles can vary in size and composition and are significant in various environmental and health contexts, contributing to air quality issues and climate effects.

While hygroscopic nuclei do play a role in cloud formation by attracting water vapor, they are specifically a subset of particulates that have water-attracting properties. Pollutants is a broader term that encompasses various harmful substances in the environment, not just these small particles. Aerosols refer specifically to suspensions of fine solid particles or liquid droplets in the air, which can include particulates but also includes substances specifically used in sprays or mist. Therefore, 'particulates' is the term that best encapsulates the fragments mentioned in the question.