The influence of relative humidity and emission source location on the migration regularity of bioaerosols in a typical urban environment

In recent years, urban biosecurity concerns have garnered significant attention both nationally and among the general populace. The aerodynamic migration mechanism using bioaerosols as carriers is not yet fully understood, so precise prevention and control of this issue cannot be achieved. This study aims to investigate the influence of relative humidity (RH) and the release locations of bioaerosols (i.e., open areas and dense areas upstream of buildings) within urban settings on bioaerosol migration regularity in downstream locations, including high-density and low-density areas. The research results show that RH has a significant impact on the urban flow field. With the increase of RH, the flow field in the horizontal direction of pedestrian height and the vertical direction of the area where the building is located gradually decreases, along the direction of the incoming flow. Both RH and the bioaerosol release location exert dual effects on bioaerosol migration and deposition. (1) Under two distinct release positions, elevated RH will lead to an expanded lateral migration range for bioaerosols. The bioaerosol concentrations in both horizontal and vertical directions are higher in high-density areas compared to low-density areas. Within high-density areas, the horizontal migration range is more extensive. On the windward side of buildings, bioaerosol concentration initially increases and then decreases, whereas vertical bioaerosol concentration first rises and then falls. The concentration of bioaerosol in the horizontal direction increased slightly in the low-density areas. With the increase of RH, the vertical bioaerosol concentration in the low-density region first increases and then decreases under the release condition in the open region, while the low-density region first decreases and then increases under the release condition in the dense region. (2) Compared with those released in dense areas, bioaerosols released in open areas have a smaller range and longer migration distance along the direction of incoming flow. The concentrations of bioaerosols in the horizontal and vertical directions of the high-density areas and the horizontal directions of the low-density areas are higher than those in the dense areas. The concentration of vertical bioaerosol in low-density areas is lower than that in dense areas. (3) Under two distinct release positions, the total deposition ratio of the building surface increases slightly with the increase of RH in open areas release conditions. The total deposition ratio of the building surface decreases first and then increases under the release condition of the dense area. The total deposition ratio of the building surface under the release condition of open areas is higher than that under the release condition of dense areas. The findings of this study provide reasonable theoretical support for the prevention and control of urban biosafety with bioaerosol as the carrier.