Indoor secondary organic aerosol formation : influence of particle controls, mixtures, and surfaces
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Ozone (O₃) and terpenoids react to produce secondary organic aerosol (SOA). This work explored novel ways that these reactions form SOA indoors, with five investigations, in two categories: investigations of (i) the impacts of particle controls on indoor SOA formation, and (ii) two fundamental aspects of indoor SOA formation. For category (i), two investigations examined the particle control devices of ion generators, which are air purifiers that are ineffective at removing particles and emit ozone during operation. With a terpenoid source present (an air freshener), ion generators acted as steady-state SOA generators, both in a 15 m³ chamber and 27 m³ room. The final investigation in category (i) modeled how heating, ventilating, and air-conditioning (HVAC) systems influence SOA formation. Influential HVAC parameters were flow rates, particle filtration, and indoor temperature for residential and commercial models, as well as ozone removal by particle-laden filters for the commercial model. For category (ii), the first investigation measured SOA formation from ozone reactions with single terpenoids and terpenoid mixtures in a 90 L Teflon-film chamber, at low and high ozone concentrations. For low ozone, experiments with only d-limonene yielded the largest SOA number formation, relative to other mixtures, some of which had three times the effective amount of reactive terpenoids. This trend was not observed for high ozone experiments, and these results imply that ozone-limited reactions with d-limonene form byproducts with high nucleation potential. The second investigation in category (ii) explored SOA formation from ozone reactions with surface-adsorbed terpenoids. A model framework was developed to describe SOA formation due to ozone/terpenoid surface reactions, and experiments in a 283 L chamber determined the SOA yield for ozone/d-limonene surface reactions. The observed molar yields were 0.14–0.16 over a range of relative humidities, and lower relative humidity led to higher SOA number formation from surface reactions. Building materials on which ozone/d-limonene surface reactions are predicted to lead to substantial SOA formation are those with initially low surface reactivity, such as glass, sealed materials, or metals. The results from category (ii) suggest significant, previously unexplored mechanisms of SOA number formation indoors.