Effect of relative humidity on the deposition and coagulation of aerosolized SiO2 nanoparticles

- RH affects the size-resolved deposition and coagulation for airborne nanoparticles.
- The effect of RH on the deposition rate depends on the particle size.
- Both low and high RHs tend to enhance the deposition rate.
- The increased coagulation of the nanoparticles with RH is due to strong inter-particle adherence.

The temporal evolution of aerosolized SiO2 nanoparticles (NPs) released into an environmental test chamber has been investigated to interrogate the effect of relative humidity (RH) on the deposition and coagulation of the nanoparticles. The size-resolved deposition rate and Brownian coagulation coefficient for the particles at RH of ~ 10%, 27%, 40%, 54%, and 64% are estimated. The results show that the effect of RH on the deposition rate is size-dependent; for particle diameter (Dp) < 70 nm, the deposition rate reduces as the RH rises; while for Dp > 70 nm, it grows as the RH rises. Generally, both low and high RH tends to enhance the deposition rate, and the minimum rate appears at moderate RH (~ 54%). Electrostatic repulsion is probable for the inter-particles interaction at the low RH while the surface roughness due to water molecular adsorption is a main reason for the particle-wall interaction at higher RH. The increasing coagulation coefficient at high humidity correlates to the strong inter-particle adhesion, which may be caused by the water molecular adsorption on the hydrophilic surfaces of the SiO2 NPs due to the formation of nanometer-thick water film. This study suggests that air humidity plays unignorable roles in particle deposition and coagulation.