New Particle Formation Driven by 222 nm Germicidal Ultraviolet Light

MATTHEW GOSS, Jesse Kroll, MIT

     Abstract Number: 182
     Working Group: Indoor Air Purification Technologies, Best Practices, and their Health Impacts

Abstract
Germicidal ultraviolet lamps with a peak emission at 222 nm (GUV₂₂₂) are effective at reducing the airborne spread of pathogens, but recent studies have identified these lamps as a source of ozone and additional secondary pollutants such as secondary organic aerosol (SOA). While ozone production from 222 nm light is now relatively well-quantified, SOA formation under GUV₂₂₂ conditions has received less study. This study compares SOA formation from limonene under GUV₂₂₂ irradiation (which involves UV photons as well as secondary ozone), or in the presence of ozone only (with no UV radiation), to distinguish the role of photolytic processes versus purely ozone-driven chemistry in aerosol formation. This work takes place in a large chamber (7.5 m³, used for SOA yield measurements) and a small chamber (0.15 m³, used to study nucleation and compositional differences). Additional experiments investigate particle formation in outdoor air sampled into the small chamber, and explore the influence of NO₂ and HONO, both relevant indoor air pollutants. We consistently observe far more new particle formation under high-fluence-rate GUV₂₂₂ conditions (45 µW cm^-2) than when O₃-only conditions are used. Despite clear differences in nucleation, aerosol yield and composition differ only slightly between GUV₂₂₂ and O₃-only conditions. While most observations are well-explained by traditional ozone chemistry, new particle formation is clearly enhanced by direct or indirect photolysis processes, and represents an additional reason to employ GUV₂₂₂ only at the lowest effective levels.