  Rotating fans are the prevalent forced cooling method for heat generating
equipment and buildings. As the concentration of atmospheric pollutants has
increased, the accumulation of microscale and nanoscale particles on surfaces
due to advection-diffusion has led to adverse mechanical, chemical and
electrical effects that increase cooling demands and reduce the reliability of
electronic equipment. Here, we uncover the mechanisms leading to enhanced
deposition of particle matter (PM$_{10}$ and PM$_{2.5}$) on surfaces due to
turbulent axial fan flows operating at Reynolds numbers, $Re \sim 10^5$.
Qualitative observations of long-term particle deposition from the field were
combined with \textit{in situ} particle image velocimetry on a
telecommunications base station, revealing the dominant role of impingement
velocity and angle. Near-wall momentum transport for $10 < y^+ < 50$ were
explored using a quadrant analysis to uncover the contributions of turbulent
events that promote particle deposition through turbulent diffusion and eddy
impaction. By decomposing these events, the local transport behaviour of fine
particles from the bulk flow to the surface has been categorised. The
transition from deposition to clean surfaces was accompanied by a decrease in
shear velocity, turbulent stresses, and particle sweep motions with lower flux
in the wall-normal direction. Finally, using these insights, selective
filtering of coarse particles was found to promote the conditions that enhance
the deposition of fine particle matter.
