Formation of secondary aerosol by 222 nm Far-UVC irradiation on SO₂

222 nm UV indoor disinfection using KrCl* excimer lamps has been gaining popularity due to claims of minimal ocular and skin damage from direct irradiation. However, the secondary aerosol formation under irradiation of KrCl* excimer lamps, which could be an inhalation hazard, is less explored. SO₂, a well-known precursor of outdoor sulfate aerosol, is also ubiquitous in indoor environments in urban cities in northern China where coal is used for domestic heating and cooking. In this work, we studied secondary aerosol formation by 222 nm irradiation on SO₂, using a Go: PAM flow reactor, a scanning mobility particle sizer (SMPS), and a time-of-flight aerosol chemical composition monitor (ToF-ACSM). The formation of sulfate nanoparticles was found much more effective at 222 nm than at 254 nm and under fluorescent lamp (FL) irradiation at the same UV doses and RH, likely due to different SO₂ oxidation mechanisms. We have also found that NH₃ and cooking volatile organic compounds (CVOC), as other indoor-relevant gases, promoted the formation of secondary aerosols by 222 nm radiation on SO₂. Overall, 222 nm disinfection can generate secondary pollutants in indoor environments. Caution should be taken during its indoor applications, especially in areas with high SO₂ concentrations such as coal-fueled households.