Table 3. Mechanisms associating physical decay of microorganisms and dust.

Mechanism	Definition[a]	Reference
Advection	The mean transport of a particle by the mean motion of the atmosphere, and occurs when the spatial gradient is nonzero and the particle is transported along the mean wind	(Baldocchi et al., 1988)
Brownian diffusion	The process of mass transfer of particles brought about by a random molecular motion (Brownian motion) and associated with a concentration gradient	(Vaithiyalingam et al., 2002)
Thermophoresis	The motion of a particle under the influence of a temperature gradient	(Langer and Holcombe, 1999)
Gravitational sedimentation	The separation of dispersed particles from gaseous phase under action of gravity	(Wunsch, 1994)
Impaction	The deposition of particles due to their momentum causing them to deviate from airflow streamlines and impacting at bifurcations	(Katz et al., 2001)
Electrostatic precipitation	The use of an electrostatic field for precipitating or removing charged particles from a gas flow in which the particles are carried	(Shen and Pereira, 1979)

^[a]Some definitions in this table that were originally for gases or molecules in nonaerial environments have been modified to make them suitable for describing the associating mechanisms of microorganisms and dust in the air.