Organic aerosol formation from 222 nm germicidal light: ozone-initiated <i>vs.</i> non-ozone pathways

Germicidal ultraviolet lamps outputting 222 nm light (GUV₂₂₂) have the potential to reduce the airborne spread of disease through effective inactivation of pathogens, while remaining safe for direct human exposure. However, recent studies have identified these lamps as a source of ozone and other secondary pollutants such as secondary organic aerosol (SOA), and the health effects of these pollutants must be balanced against the benefits of pathogen inactivation. While ozone reactions are likely to account for much of this secondary indoor air pollution, 222 nm light may initiate additional non-ozone chemical processes, including the formation of other oxidants and direct photolytic reactions, which are not as well understood. This work examines the impacts of GUV₂₂₂ on SOA formation and composition by comparing limonene oxidation under GUV₂₂₂ and O₃-only control conditions in a laboratory chamber. Differences between these experiments enable us to distinguish patterns in aerosol formation driven by ozone chemistry from those driven by other photolytic processes. These experiments also examine the influence of the addition of NO₂ and nitrous acid (HONO), and investigate SOA formation in sampled outdoor air. SOA composition and yield vary only slightly with respect to GUV₂₂₂<i>vs.</i> ozone-only conditions; NO₂ and HONO photolysis do not appreciably affect the observed chemistry. In contrast, we observe consistent new particle formation under high-fluence 222 nm light (45 μW cm^-2) that differs substantially from ozone-only experiments. This observed new particle formation represents an additional reason to keep GUV₂₂₂ fluence rates to the lowest effective levels.