Enhancement of the deposition of ultrafine secondary organic aerosols by the negative air ion and the effect of relative humidity

Deposition is an important process for the removal of aerosol particles. Negative air ion (NAI) generators can charge the ultrafine airborne particles and enhance their deposition rate. However, many NAI generators may also emit ozone and increase the concentration of particles in the presence of biogenic volatile organic compounds owing to the secondary organic aerosol (SOA) production. To validate the effectiveness of NAI generator, the authors investigated the enhancement effect of an NAI generator on the deposition of the ultrafine SOAs generated from the ozonolysis of d-limonene in a test chamber under controlled ventilation rate and relative humidity (RH). The experimental results demonstrated that compared with other effects, including the gravity, particle eddy diffusion, and the Brownian diffusion, the effect of NAIs is the most dominate one on the deposition of SOA particles onto the wall surface in the near-wall region (<1 cm away from the wall). According to these experiments, the tested NAI generator could efficiently enhance the deposition rate by an enhancement factor ranging from 8.17 ± 0.38 to 25.3 ± 1.1, with a low ozone production rate. This NAI generator had better performance on the deposition of the SOAs with smaller particle sizes and it performed even better under higher RH. The enhancement effect of the NAI generator was related to its high NAI production and electric field strength.

Implications: This study investigated a novel technique of negative air ion (NAI) generator that can enhance the precipitation of nano-scale secondary organic aerosol (SOA). The tested NAI generator can significantly improve the deposition rate of SOA with an enhancement factor of about 8.17 ± 0.38 to 25.3 ± 1.1. The enhancement factor rose when relative humidity increased.