Modelling aerosol formation and precursor gases in the boundary layer

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Title: Modelling aerosol formation and precursor gases in the boundary layer
Author: Zhou, Luxi
Contributor: University of Helsinki, Faculty of Science, Department of Physics, Division of Atmospheric Sciences
Publisher: Helsingin yliopisto
Date: 2015-09-29
Thesis level: Doctoral dissertation (article-based)
Abstract: Aerosols are common air pollutants that pose serious threats to health. They also impact the Earth’s climate by interacting with solar radiation and altering cloud properties. One important phenomenon associated with the atmospheric aerosol system is new particle formation. This thesis is devoted to study the processes related to new particle formation as well as aerosol precursor gases in the boundary layer, where exchanges between the surface and the atmosphere take place. Model SOSAA, a one-dimensional transport model with detailed chemistry and aerosol microphysics, was constructed, tested and used as the main tool. Variations in the main biogenic volatile organic compounds (BVOCs), OH and H2SO4 across different environment were analysed. The simulated concentrations of main emitted organic compounds in two rural sites agree well with the observations. Due to missing OH reactivity in rural environment, the concentration of OH at Hyytiälä is overestimated. In contrast, the concentration of OH is likely underestimated in an urban environment due to missing production from HONO. H2SO4 production is underestimated at every site studied. New particle formation was studied in two phases: nucleation and growth. Large uncertainties are introduced to nucleation rates when calculating dependencies based on sulphuric acid alone. The oxidation products of monoterpenes by OH are shown not to be a suitable proxy for nucleation at either the urban site in Beijing or the rural site at Hyytiälä in summer. While they mainly contribute to the growth of particles at the rural site in Hyytiälä, the oxidation products of monoterpenes might only have major role in particle growth at Manitou during the evening. The rapid particle growth at Manitou in the daytime is possibly related to the oxidation products of 2-Methyl-3 Buten-2-Ol (MBO) by OH or any compound that has similar concentration and diurnal variation to the oxidation product of MBO by OH. The last model study assessed the climatic impact of aerosols in a hypothetic scenario with high atmospheric methane loading. The impact associated with aerosols and the change in atmospheric composition is shown to be as large as the impact due to methane itself. A NOx mitigation strategy was evaluated by SOSAA as effective to remove methane but the side product of hazardous tropospheric ozone makes the method unfavourable in real life. The process-based model SOSAA developed during this doctoral study is a useful tool for atmospheric research since it provides flexibility in time and space. The results call for further research into the ultimate mechanisms controlling nucleation and particle growth. This thesis also identifies knowledge gaps in the related precursor gases and encourages further studies in fields such as emission inventory of volatile organic compounds, sulphuric acid production, and atmospheric oxidation cycle.Aerosolit voivat heikentää ilmanlaatua, uhata terveyttä ja säädellä ilmastoa. Tässä työssä tarkasteltiin pienhiukkasten muodostumiseen liittyviä prosesseja ja muodostajakaasuja rajakerroksessa, jossa maanpinta ja ilmakehä vuorovaikuttavat. Tämä tehtiin tässä työssä kehitellyllä SOSAA-tietokonemallilla, joka on yksityiskohtaista kemiaa ja aerosolien mikrofysiikkaa sisältävä yksiulotteinen kuljetusmalli. Analyysi käsitteli luonnosta peräisin olevien haihtuvien orgaanisten yhdisteiden (BVOC), OH-radikaalin ja rikkihapon pitoisuusvaihteluita eri ympäristöissä sekä näiden vaikutusta pienhiukkasten muodostumiseen ja kasvuun.
Subject: physics
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