Physical and Chemical Processes in the Formation of Biogenic secondary Organic Aerosols

Abstract: Vegetation emits a vast number of biogenic volatile organic compounds (BVOC). In the atmosphere they are oxidised, predominantly by O3, OH radicals or NO3 radicals, depending e.g. on chemical structure. When oxidised, these compounds can form products with low volatility that can be transferred from the gas phase into the condensed phase by gas-to-particle conversion, hence contributing to secondary organic aerosols (SOA). An important group of BVOC contributing to SOA is monoterpenes (C10H16). These compounds are light enough to be volatilised but at the same time heavy enough for producing multifunctional compounds with low volatility. The exact mechanisms by which these compounds are formed are not clear; there are still many questions to be answered. In this thesis, the impact of humidity, temperature and radical chemistry on aerosol formation from ozonolysis of limonene, ?3-carene and ?-pinene have been investigated. Additionally, the thermal characteristics of the particles were studied. A flow reactor G-FROST (Göteborg-Flow Reactor for Oxidation Studies at low Temperatures) was developed, enabling studies of aerosol formation. This set-up is a combination of a laminar flow reactor and a scanning mobility particle sizer (SMPS) system with a working temperature range of 243-323 K and reaction times between 50 and 500 s. The volatility of the particles was measured by using a volatility tandem differential mobility analyser (VTDMA) that was developed within this work. Experiments were also conducted in the large static reactor AIDA at Research Centre Karlsruhe. The results showed that limonene was the most efficient reactant in producing SOA and ? pinene the least, at all temperatures studied regarding number of particles formed. For mass, this was also true at high temperature, while at 273 and 243 K ?3-carene was the most efficient. The volatility properties of SOA changed with precursor, temperature and humidity. The amount of SOA was greatly reduced by the use of a scavenger, in these investigations 2-butanol or cyclohexane, reacting with the OH formed in the ozonolysis. The difference between experiments with and without an OH scavenger was decreasing with decreasing temperature. However, there is still a difference at 243 K indicating that the OH chemistry is still present at this temperature. This was also supported by OH yield measurements using cyclohexanone as OH formation indicator. The impact of increased water concentration on SOA formation varies with temperature, type of scavenger and concentrations. This work demonstrates that physical uptake can not be the only explanation; water must also affect the kinetics inducing changes in the chemical mechanism. Additionally, a strong water dependence of pinonaldehyde, a major product from ? pinene ozonolysis, was observed. The pinonaldehyde produced at low temperature was solely detected in the condensed phase. In this thesis the results are further discussed and put into an atmospheric context.

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