Besides light pollution, what terrestrial atmospheric factor dims starlight even on a clear, moonless night?

Scattering caused by atmospheric gases, dust particles, and water vapor.

Even in the absence of artificial light sources, the Earth's atmosphere itself interferes with the transmission of light from distant stars. As starlight traverses the air above us, it interacts with the naturally occurring components, specifically gases, microscopic dust particles, and varying amounts of water vapor. These elements cause a minute degree of scattering, slightly diffusing and dimming the incoming light. While this effect is generally less severe than urban light pollution, it becomes a significant factor when attempting to observe the faintest stars that are already near the limit of human visual perception.