Atmospheric aging of monoterpene secondary organic aerosol

The behavior of monoterpene SOA with respect to tropospheric radiation is an important field of study because of the potential impacts on the chemical composition and energy balance of the Earth's atmosphere. Of concern is the lack of understanding of the absorption profiles of real atmospheric aerosol particles. Most models assume that the only aerosol absorbers of radiation are those particles that contain black carbon; all other particles are treated as scatterers and assigned negative radiative forcings. In Chapter 3 of this thesis, have described a method of measuring the absorption spectra of films of aerosol particles, the results of which cast doubt on this assumption. The new procedure allows us to measure spectra of the particles directly, without worrying about solvent effects. The results show that SOA formed from the ozonolysis of monoterpenes have absorption profiles that overlap significantly with the tropospheric actinic window (radiation with lambda > 295 nm). We have calculated the lifetime of these SOA with respect to photolysis, and found that it is often an order of magnitude shorter than the most important competing aging process, attack by OH.;The lifetime of the particles with respect to photolysis depends on the kind of reactions that absorbed radiation will be able to initiate. In Chapter 2 we investigated this question by detecting the gas phase products of limonene SOA photolysis in order to draw conclusions about the mechanisms of monoterpene SOA photolysis. Based on the product distribution and on the known products of limonene ozonolysis, we proposed that the Norrish type I and II photolysis of carbonyl-containing molecules will be an important mechanism of photochemical aging for monoterpene aerosols in the troposphere.;In Chapter 3, I have presented results from experiments on particle aging in the absence of UV radiation. These results imply that particles can be aged by non-photochemical processes, meaning that particles will be modified during their time in the troposphere even when conditions are unfavorable to photochemistry. The mechanism of the changes that alter the absorption profile of monoterpene SOA in the absence of radiation is unknown at this time.