Particle formation from squalene-ozone reactions in indoor environments
Creators: name: Coffaro, Breann
Publisher: name: No Publisher Supplied
Year: 2022

description: People spend the majority of their time indoors and are exposed to products of indoor air chemistry that includes particles generated by the squalene-ozone (SqOz) reaction. This reaction occurs when ambient ozone infiltrates indoors and reacts with the human skin oil component, squalene, directly on skin, hair, clothing, surfaces that have been touched, and house dust, and produces particles that affect indoor air quality. This dissertation begins by reviewing the current state of literature surrounding the squalene-ozone reaction and then investigates SqOz particle formation through three different studies: 1) Particle seeding experiments to understand formation mechanisms, 2) Experiments involving varying environmental parameters to understand their effect on particle formation, and 3) chemical analysis of laboratory and real-world air and surface wipe samples for SqOz reaction products. Secondary organic aerosols (SOA) seeding particles generated from the reaction of limonene and ozone were successful in facilitating the squalene-ozone reactions, while the inorganic seeds generated by heating a nichrome wire were not. The seeding particles shifted the particle formation mechanism from a mixture of homogenous nucleation and gas-to-particle partitioning to primarily a partitioning formation mechanism. This resulted in quicker, more consistent particle formation at higher particle counts than when the seed particles were not present. Indoor environments typically have SOA that can be seeds for SqOz reactions when ozone is present.The experimental studies performed by varying environmental parameters examined the slope of particle generation, the maximum particle number formed, and time to maximum particle formation to understand their effects on particle formation. Relative humidity (RH) conditions ranging from 15% to 45% RH resulted in very similar curves, but for 55% and 65% RH, particle formation was not seen due to a shift in the formation pathway toward volatile products or the loss of particle mass from secondary reactions producing volatile compounds that volatilize from the particles. As ozone concentrations were increased from 25 ppb to 200 ppb, the time to form the maximum number of particles decreased, the maximum particle number count increased, and the slope of particle generation increased, the latter two following a second order polynomial. As surface loadings increased, the max particle number increased linearly until it leveled off and no further increases were seen. This may be due to squalene molecules initially forming a monolayer on the surface to optimize the SqOz reaction, followed by bi- or multi-layers formed or reaction products coating the squalene decreasing the particle formation from the SqOz reaction.The SqOz reaction products were measured in surface film and air samples using gas chromatography- mass spectrometry after derivatization of the carboxylic functional group. For the surface samples, the compounds identified, in order from highest surface loading to lowest, were: geranylacetone (5.23-23.7 ng/cm²), geranyllinalool (4.47-9.27 ng/cm²), farnesylacetone (0.222-2.95 ng/cm²), geranylgeranylacetone (0.117-1.75 ng/cm²), levulinic acid (0.214-1.58 ng/cm²), and succinic acid (0.017-0.052 ng/cm²). Chamber air samples contained succinic acid (188 – 610 ng/m³) and several additional compounds above the blank values: hexanoic acid, decanoic acid, 3-methyl-2-butenoic acid, and 2,3-dimethyl-2,3-butanediol. Air samples collected from 37 homes in NJ, GA, and NC contained 2.35-75.64 ng/m³ levulinic acid and 5.78-54.43 ng/m³ succinic acid. To our knowledge, this is the first time these compounds, which are squalene-ozone reaction products, have been measured in particles collected from the indoor air of homes, though multiple sources of these compounds existed.