Dust deposition in a courtyard as an indicator of the effect of buildings on coarse aeolian particles

The effect of urban morphology on the behavior of coarse airborne particles of natural origin was studied in a prototypical courtyard at Sde Boqer, in the Negev desert. Dust deposition was sampled concurrently on clear glass slides at 27 points inside a courtyard and on the roof of the adjacent building. Analysis of the slides by optical microscope using a proprietary software tool provided a record of the dust by particle number and grain size distribution. The analysis shows that as a general rule, more dust was deposited on the leeward side than on the upwind part of the courtyard. However, the overall effect of the courtyard also appears to vary substantially with wind conditions: in strong winds, the courtyard gives partial protection from airborne dust, while in moderate winds it may act as a dust trap and it appears to have little effect at all in light winds. Furthermore, the dust deposition patterns inside the courtyard were changed: in strong wind there was more dust deposition in the upper level of the courtyard, while in moderate winds the dust deposited homogenously in all the levels of the courtyard. The variations in dust deposition are explained by different airflow patterns in the courtyard, which were monitored during the experiment and are illustrated by means of computer simulation using FLUENT. This suggests that the flow pattern varies in response to external wind speed: In moderate wind conditions, a lee vortex is formed, whereas in stronger winds, skimming flow dominates, and the bottom of the courtyard experiences proportionately lower air speeds.