  Context. Recent observations of dust ejections from active asteroids,
including (3200) Phaethon, have drawn considerable interest from planetary
astronomers studying the generation and removal of small dust particles on
asteroids. Aims. In this work, we aim to investigate the importance of thermal
radiation pressure from asteroid regolith (AR) acting on small dust particles
over the surface of the AR. In particular, we aim to understand the role of
thermal radiation in the near-Sun environment. Methods. We describe the
acceleration of particles over the AR within the radiation fields (direct
solar, reflected (scattered) solar, and thermal radiation) in addition to the
asteroid's rotation and gravitational field. Mie theory is used because the
particles of interest have sizes comparable to thermal wavelengths (~1-100
{\mu}m), and thus the geometric approximation is not applicable. A new set of
formalisms is developed for the purpose. Results. We find that the acceleration
of particles with spherical radius < 1 {\mu}m to ~10 {\mu}m is dominated by the
thermal radiation from the AR when the asteroid is in the near-Sun environment
(heliocentric distance rh < 0.8 au). Under thermal radiation dominance, the net
acceleration is towards space, that is, outwards from the AR. This outward
acceleration is the strongest for particles of ~1 {\mu}m in radius, regardless
of other parameters. A preliminary trajectory integration using the
Phaethon-like model shows that such particles escape from the gravitational
field within about 10 minutes. Our results are consistent with the previous
observational studies on Phaethon in that the ejected dust particles have a
spherical radius of ~1 {\mu}m.
