Exposure-Relevant Ozone Chemistry in Occupied Spaces

Ozone reacts with alkenes and other organic compounds that contain carbon-carbon double bonds to form oxidation byproducts such as aldehydes, ketones, carboxylic acids, radicals, and secondary organic aerosol. Ozone-reactive species such as terpenes and unsaturated fatty acids are ubiquitous in the air and on surfaces in indoor environments. The most prevalent source of ozone indoors is ventilation of the space with ozone-containing ambient air, although some appliances that are used indoors also produce ozone. Reactions with surfaces are the ever-present and dominant sink of ozone and source of oxidation byproducts. Gas-phase ozone reactions must compete with the ventilation rate; they can be intermittently important ozone consumers and byproduct generators. Ozone-alkene reactions are chain-initiating because radicals are formed that 2 continue to react. The hydroxyl radical (OH) is a major secondary byproduct of ozone-alkene chemistry, and it is a less-selective, faster reacting oxidizer than ozone. Thus, species that do not readily react with ozone, i.e. compounds that do not contain a carbon-carbon double bond, may also be oxidized by the OH radical in ozone-initiated chemistry.

In this dissertation, the chemical and physical factors that affect transformation of ozone into other airborne pollutants in occupied indoor environments are explored. Ozone-initiated reaction with, and byproduct formation from, reactive gas-phase and surface-phase species common to indoor settings were investigated in four studies. Byproduct types and formation rates were characterized in laboratory experiments. Byproduct concentrations and exposures were predicted in various indoor environments using experimental data and a model that predicts ozone transport and uptake and byproduct formation and fate.

When both ozone and terpenes are present in indoor settings, terpenes can be a strong sink of ozone and source of gas- and particle-phase byproducts. I investigated secondary organic aerosol formation from the reaction of ozone with terpene-containing consumer products under conditions relevant for residential and commercial buildings. Gas-phase consumer product emissions and then ozone were introduced into a continuously ventilated 198-L chamber. At the onset of ozone addition, a nucleation event occurred, and nucleation and growth continued to occur as long as the reagents were introduced into the chamber. The particle formation and growth behavior in these experiments mimicked SOA dynamics from ozone-terpene reactions measured in actual buildings. The full particle size distribution was continuously monitored using an optical 3 particle counter and scanning mobility particle sizer. The resulting ultra-fine and fine particle concentrations were in the range of 10 to >300 μg m-3. Particle nucleation and growth dynamics under indoor conditions were characterized using the methods commonly applied to atmospheric nucleation events.

Commercial passenger aircraft can encounter elevated stratospheric ozone levels at cruising altitude, and because aircraft cabins are continuously ventilated, significant ozone levels can be present in the aircraft cabin. Reactions with fixed cabin surfaces and surfaces associated with passengers consume ozone and generate byproducts. I conducted chamber experiments at flight-relevant conditions to determine ozone uptake and by product emissions from individual materials found in the aircraft cabin environment.The materials tested included new and used samples of carpet, seat fabric, and plastics,and laundered and worn clothing fabric. For all materials, emission rates were higher with ozone than without. Ozone deposition velocities and reaction probabilities, and byproduct emission rates and byproduct yields, were determined for each of the surface categories. The most commonly detected byproducts included C1–C10 saturated aldehydes and skin oil oxidation products. A model of mass transport and uptake was employed to extrapolate results from chamber experiments to the cabin environment. I estimated the distribution of total byproduct levels using a Monte Carlo simulation of the cabin environment with three model parameters: byproduct yield, ozone level, and retention ratio. Airborne oxidation byproduct levels are predicted to be similar to ozone levels in the cabin, which have been found to be tens to low hundreds of ppb in the absence of an ozone converter. I also used this model to predict the concentrations of certain by products in the cabin, and exposure to these byproducts were compared in three important4 environments – an aircraft cabin, a residential building, and outdoors – using inhalation intake rate as a metric. For the byproducts examined, intake in the aircraft cabin can be similar to intake in buildings for those who spend a significant amount of time flying, such as crew members, despite much more time being spent in buildings, owing to higher levels of byproducts in the cabin.

Surface materials may be inherently reactive with ozone or may have ozone reactive residues applied during manufacture or use of the surface. I developed a model of ozone uptake by, and byproduct emission from, residual chemicals on surfaces. The model predicts the time-dependent rate of ozone consumption, residue consumption, and byproduct formation with the following inputs: residue surface concentration, ozone concentration, reactivity of the residue and the surface, near-surface airflow conditions, and byproduct yield. The effects of model input parameters on ozone uptake and byproduct formation were explored. There is potential for this model to help elucidate the dynamic ozone uptake behavior such as "aging" and "regeneration"—the gradual reduction in reactivity of material over the course of ozone exposure, and the rebound in material reactivity exhibited after a material has been exposed to ozone-free air for a period of time in between ozone exposures, respectively.

Permethrin is a residual (surface-bound) insecticide commonly used in aircraft cabins. The possibility that ozone could react with permethrin to form phosgene was investigated. From the literature, it was determined that surface levels of permethrin and airborne levels of ozone were sufficient to potentially form phosgene at a level of concern based on established health standards for phosgene. A derivatization technique was developed to detect phosgene at low levels, and experiments were conducted in which 5 permethrin-coated glass plates and aircraft cabin materials were exposed to ozone under flight-relevant conditions. Phosgene was not detected in these experiments, and on the basis of the research conducted, it does not appear likely that ozone-initiated oxidation or OH-related oxidation of permethrin is a major route of degradation for permethrin in indoor spaces. Permethrin likely has a very low reactivity with ozone owing to the presence of chlorine atoms adjacent to the double bond in permethrin. A mathematical model of ozone transport and uptake was employed to estimate an upper bound on phosgene formation and levels in an aircraft cabin. The reaction probability of permethrin is estimated to be < 10-7 and the cabin concentration of phosgene to be < 1 μg m-3. It was determined that phosgene formation, if it occurs in the aircraft cabin, is not likely to exceed the relevant, health-based phosgene exposure guidelines.

The research presented here provides evidence that ozone, a ubiquitous ambient pollutant, is transformed into other airborne pollutants in the indoor environment where we spend the majority of our time. Ozone-initiated chemistry lowers the indoor ozone level but may generate oxidation byproducts that can be as harmful, or more so, than ozone itself. The type and amount of byproducts that result from ozone reactions with common indoor surfaces, surface residues, and vapors were determined, pollutant concentrations were related to occupant exposure, and frameworks were developed to predict byproduct concentrations under various indoor conditions. This work also helped to elucidate the role of occupants in indoor ozone chemistry. Human skin oil is highly reactive with ozone, and oxidation byproducts are potentially formed very near the breathing zone. Ozone-initiated reactions that occur on or very near occupants, and the6control of ozone to reduce exposure to oxidation byproducts in occupied spaces, are emerging issues in indoor ozone chemistry.