Abstract/Summary

This thesis describes experiments carried out to understand the formation of particles in the reactions of ozone with terpenes, processes that are important in atmospheric chemistry and have an impact on climate change. The main instrument used to study the reactions was a Scanning Mobility Particle Sizer (SMPS), with some experiments using an Electrical Low Pressure Impactor (ELPI+). The SMPS instrument was used first to study the evolution of aerosol particles formed from the ozonolysis of α-pinene. A strong dependence on OH scavenger, and the [RO2] / [HO2] ratios produced by each, was observed. The effect of relative humidity on the formed aerosol was investigated, and was found to influence both number and size of the particles formed. The relationship between aerosol mass concentration and mass yield was extended beyond the range reported in the literature. A rate constant for the α-pinene ozonolysis was calculated from the aerosol mass evolution, k = 1.05 ± 0.11 × 10-16 cm3 molecule-1 s -1 , which agrees well with the literature. Preliminary studies using the ELPI+ instrument were undertaken, and some successful experiments suggested that physical state of the aerosol is independent of relative humidity across the range studied. An enone derivative of α-pinene was synthesised to allow for study of one of the αpinene Criegee intermediates (CIs) in isolation. A rate constant for its reaction with ozone of k = 4.3 ± 0.9 × 10-17 cm3 molecule-1 s -1 was determined. Enone mass yields were 5–7 % lower than those of α-pinene, suggesting that products of CI 2 (enal) contribute more to the total aerosol mass than those of CI 1 (enone). The data suggest that CI 2 is primarily responsible for particle nucleation, and both CIs contribute significantly to their growth. The ozonolysis of α-terpinene was investigated using both static chamber experiments and an atmospheric pressure flow tube. This allowed for calculation of upper and lower bounds for the rate constant. The upper bound, k = 1.6 ± 0.29 × 10-14 cm3 molecule-1 s -1 , is in agreement with the literature. Increasing ozone concentration was noted to have a particularly large impact on the mass of aerosol produced. Mass yields were found to be much higher than for α-pinene, suggesting an array of very low volatility products. The effect of relative humidity on the initial stages of aerosol formation was studied using the flow tube, and a small effect on the particle size distribution was noted.