OBSERVATIONAL INSIGHTS ON THE EVOLUTION OF ORGANIC AEROSOL IN THE BOREAL ENVIRONMENT

Aerosol particles, which are solid or liquid particles suspended in the air reduce air quality as well as influence Earth’s radiative balance through their direct and indirect interactions with solar radiation. The sensitivity of Earth’s climate to aerosol particles has remained elusive despite a wealth of studies conducted. Some of this uncertainty arises from the highly dynamic manner the physicochemical properties of aerosol particles evolve in the atmosphere. Recent advances in mass spectrometric measurement techniques have helped to assess the aerosol chemical (trans)formation altering many climate-relevant aerosol properties such as aerosol particle size, volatility and water affinity. The scope of my thesis is the formation and evolution of organic aerosol (OA) in the boreal environment. Boreal forests emit a wide variety of volatile organic compounds (VOCs), which can form secondary organic aerosol (SOA) after oxidation. Notably, the mechanisms behind the VOC to SOA conversion is not straightforward and depends highly on the type of VOC, oxidants and ambient conditions, such as relative humidity (RH) and temperature (T) among others. This process can therefore be sensitive to the changes projected to take place within the boreal biome along with the changing climate. Changes regarding tree species can alter the composition and relative abundances of the emitted VOCs, which can further influence boreal SOA formation. Anthropogenic emissions can also affect natural VOC to SOA conversion and further SOA processing (aging) even in the pristine regions of the boreal biome. Due to attempts in air quality improvements in several locations worldwide, many anthropogenic species have shown declining trends with potential consequences in SOA formation and aging. Therefore, both boreal vegetation changes and anthropogenic emission regulations subject possible perturbations in both regional and global climate through SOA. The studies conducted within this thesis can be divided into two categories: 1) examining the accumulated OA composition and its seasonal dynamics in the boreal forest to understand the present state of boreal OA (field studies), and 2) zooming in on the early stages of SOA formation and evolution and the potential impacts of anthropogenic emissions on them (laboratory studies). The measurements were conducted via online mass spectrometry. Obtaining results from the field required the retrieval of prevailing structures in the aerosol mass spectra recorded in real time (multidimensional data). This led to the development of an analysis framework utilising machine-learning tools for OA classification. The results highlight the importance of aged low-volatility oxygenated OA (LV-OOA) within the boreal environment throughout the year, with highly season-dependent sources. The LV-OOA production was found to be extremely sensitive to meteorological conditions, such as heat waves. In the laboratory, we examined the volatilities and fates of highly oxygenated organic molecules (HOMs), which form in boreal VOC oxidation. HOMs were shown to be primarily of low volatility and therefore good candidates as SOA precursors. In the presence of nitrogen oxides, which imply anthropogenic influence, HOMs of relatively higher volatilities were formed, potentially reducing HOM condensate formation. HOM condensate has been previously shown to be labile, and fragment quickly after formation. In our studies, this reaction was boosted in the presence of acidic aerosol particles. SOA formation was influenced by aerosol acidity also by significantly enhancing the SOA yield from semior intermediate volatility precursors and simultaneous oligomerisation reactions. The formation of high molecular weight oligomers significantly reduced SOA volatility. The suite of studies conducted in this thesis succeeded in both the characterisation of the accumulated ambient OA and gaining new insight on the initial SOA formation and processing with and without anthropogenic influence in the boreal forest. However, the difference observed between the ambient LVOOA and fresh laboratory SOA composition underlines the importance of photochemical aging needed for the formation of the highly oxidised ambient OA.