Ozone-initiated chemistry in indoor environment

Rai, Aakash C. Ph.D., Purdue University, May 2014. Ozone-initiated Chemistry in Indoor Environment. Major Professor: Qingyan Chen, School of Mechanical Engineering. Ozone is a major indoor air pollutant, and exposure to it is associated with increased morbidity and mortality risks for humans. Ozone is also a major driver of indoor air chemistry, and it reacts with various indoor materials such as fragrances, cleaners, paints, carpets, etc. to generate a myriad of byproducts. These ozone-initiated byproducts include numerous volatile and semi-volatile organic compounds as well as particles, which can be even more harmful to human health than ozone itself. Therefore, it is imperative to characterize the indoor exposure of humans to ozone and ozone-initiated byproducts. A large number of investigations have measured ozone reactions with terpene-based consumer products and found these reactions to be sources of volatile organic compounds (VOCs) and particles in buildings. Ozone chemistry in airliner cabins has also received considerable attention because the ozone concentration is typically much higher in aircraft cabins than in buildings, which translates to a high risk of exposure to ozone and its byproducts for passengers and crew. These investigations have shown that the occupants themselves provide significant sites for ozone consumption and VOC emissions through ozone reactions with their skin, hair, and clothing. In contrast to the large number of experimental studies devoted to ozone reaction chemistry, only a few modeling studies can be found in the literature. It is crucial to develop modeling tools because they can provide cheap and quick estimates of human exposure to ozone and the byproducts of its reactive chemistry and can also aid in the development of possible mitigation strategies. Therefore, the aim of this investigation was to develop modeling tools for studying exposure to ozone and its initiated byproducts in indoor environments with an emphasis on airliner cabins, which have been identified as high-risk environments. To generate the experimental data needed for developing the models, we first measured the ozone consumption by, and ozone-initiated VOC emissions from, reactions with a T-shirt soiled with human skin-oils. The measurements were conducted in an environmental chamber, and they were designed to identify the impact of various factors on ozone reactions. These factors included ozone concentration, relative humidity, the degree to which the T-shirt was soiled by human skin-oils, and ventilation rate. It was found that ozone reactions with the soiled T-shirt consumed ozone and generated VOCs. The ozone deposition velocity, which was used to quantify the ozone removal rate by the soiled T-shirt, ranged from 0.15 to 0.29 cm/s. The rate of ozone removal by the T-shirt increased with increasing soiling level and ventilation rate, decreased at high ozone concentrations, and was relatively unaffected by the humidity level. The ozone-initiated VOC emissions included C6-C10 straight-chain saturated aldehydes, acetone, and 4-oxopentanal (4-OPA). The VOC emissions were generally higher at higher ozone concentrations, humidity level, soiling degree of the T-shirt, and ventilation rate. The total molar yield was approximately 0.5 in most of the experiments, which means that for every two moles of ozone removed by the T-shirt surface, one mole of VOCs was produced. In addition to measurements of ozone consumption and ozone-initiated VOC emissions, we explored the possibility of particle generation from ozone reactions with the soiled T-shirt by using the same environmental chamber setup. Our measurements showed that the ozone/T-shirt reactions did indeed generate sub-micron particles, and this generation was enhanced by the soiling of the T-shirt with skin-oils. In these reactions, a burst of ultrafine particles (UFPs) was observed approximately one hour after the injection of ozone into the chamber, and then the particles grew to larger sizes. Particle generation from the ozone/T-shirt reactions was also significantly affected by the various factors that were studied, and these reactions were identified as another potential source of indoor UFPs. On the basis of the aforementioned measurements, we subsequently developed modeling techniques for studying ozone reactions in realistic indoor settings. We used computational fluid dynamics (CFD) to study ozone consumption by, and ozone-initiated VOC emissions from, different surfaces in an aircraft cabin. The CFD model for ozone was based on existing models in the literature, and we developed new empirical models for computing the emissions of several major VOCs generated from ozone/clothing reactions, such as acetone, 4-OPA, nonanal, and decanal. Good agreement was generally observed between the CFD results and the available experimental data for ozone consumption by different surfaces in the aircraft cabin. For the VOC concentrations, the model predictions showed accurate trends qualitatively, but there were quantitative discrepancies between the predictions and the corresponding measurements, which were mainly due to the small experimental dataset used for model development. Nevertheless, the models are promising and can be improved with the use of additional data. Our CFD analysis also obtained detailed concentration distributions of ozone and its initiated VOCs in the aircraft cabin. The analysis predicted that in the breathing zone of the passengers, the concentration of ozone would typically be 10% lower, and that of ozone-initiated VOCs 10-50% higher, than their respective bulk air concentrations. Therefore, to accurately assess passengers\u27 exposure to ozone and its initiated VOCs, their breathing zone concentrations should be used. Finally, this investigation developed a numerical model to probe particle generation from ozone reactions with human-worn clothing under realistic indoor conditions. The model was based on the particle generation measured in our environmental chamber as well as physical models of particle nucleation, condensational growth, and deposition. In five out of the six test cases, the model was able to predict particle size distributions reasonably well. The failure in the remaining case demonstrated the fundamental limitations of the nucleation models that were employed. The numerical model that had been developed was then used to predict particle generation under various building and airliner-cabin conditions. These predictions indicate that ozone/clothing reactions could be an important source of UFPs in densely occupied indoor spaces such as classrooms and airliner cabins. Such reactions could account for about 40% of the total UFPs measured on a Boeing 737-700 flight. However, the model predictions at this stage are indicative and should be improved further. In summary, this investigation studied ozone reactions with human-worn clothing through chamber measurements and numerical simulations and came to the following conclusions: • Our measurements showed that the ozone/clothing reactions consumed ozone and generated several VOCs such as C6–C10 straight-chain saturated aldehydes, acetone, and 4-OPA, together with sub-micron particles. • Ozone consumption and ozone-initiated VOCs and particle generation were generally found to be significantly affected by the different factors that were studied, such as ozone concentration, relative humidity, soiling degree of clothing, and ventilation rate. • Our CFD simulations showed that the concentration of ozone decreased, and that of ozone-initiated VOCs increased, in the breathing zones of aircraft cabin passengers as compared with the corresponding bulk air concentrations. Thus, it is recommended that the breathing zone concentrations be used for estimating the exposure of passengers to ozone and ozone-initiated VOCs. • The numerical model for particle generation extended our chamber measurements to realistic indoor e..