The effect of environmental parameters on squalene-ozone particle formation

Particles formed from the squalene-ozone (SqOz) reaction have potential health concerns. Squalene, a component of skin oil, is ubiquitously found indoors on skin, surfaces, and clothing. Since people spend the majority of their time indoors, it is important to understand how indoor relative humidity (RH), ozone concentrations, and squalene loadings affect SqOz particle formation. Particle formation from the SqOz reaction within a reaction chamber was monitored using a condensation particle counter (CPC) while varying environmental conditions. The particle production curves over time were similar as RH was varied from 15% to 45%, but particles were not formed at RH 55% and 65%, probably due to a shift in primary formation pathway towards volatile products and/or potential losses of volatile compounds from particles produced as secondary reaction products. As ozone concentrations were increased from 25 ppb to 200 ppb, the time to maximum particle number concentration decreased, and the maximum particle number concentration and initial rate of particle generation increased non-linearly. This suggests that particle formation was dependent upon the ozone concentration but was limited by the amount of squalene remaining on the surface. As surface loadings were raised from 1.28 μg/cm2 to 270 μg/cm2 the maximum particle number concentration increased linearly. Further increases in squalene loading did not cause higher levels of particle production. This pattern is consistent with a shift from a monolayer to bi-layer formation or non-reactive products coating the squalene at the higher loadings. Our results indicate that SqOz particle formation is a function of environmental conditions.