Modeling of new particle formation and growth in the atmospheric boundary layer

Abstract: Atmospheric aerosols affect climate, but to what degree still remains one of the largest uncertainties in global climate models. To improve the representation of aerosols in future climate models we need a greater understanding of aerosol processes. In this thesis the process-based model ADCHEM has been used and further developed to study aerosol processes. Specifically; new particle formation and growth have been studied in boreal environments, but also over polluted marine environments.In the boreal forest, particles largely grow by condensation of oxidized organic compounds to form secondary organic aerosols (SOA). There are thousands of organic oxidation products and far from all are known, causing large uncertainties in the modeled aerosol population. Models that simulate the growth of particles must therefore make assumptions of which organic compounds, or group of compounds, that are potential condensation products and what properties they have. The modeled mass concentration of the resulting aerosol particles are dependent on the estimated saturation vapor pressures of the condensation products, but also on the further gas-phase aging of the organic oxidation products.Many models underestimate the SOA mass; one explanation could be missing gas-phase oxidation products. A newly proposed reaction pathway of monoterpenes containing endocyclic double bonds that form highly oxidized multifunctional organic molecules (HOMs) in the gas phase was implemented in the gas-phase chemistry module. The model was tested against observed HOM gas-phase composition and observed SOA formation during α-pinene ozonolysis experiments and field measurements. The model was able to reproduce the observed new particle formation events and particle growth if the HOM mechanism was included.ADCHEM was also used to study new particle formation in the marine boundary layer, to address whether particles formed over sea, or emitted anthropogenic gases over sea, have any importance on the cloud formation potential over land. If the air mass over the marine boundary layer is already polluted due to continental emissions, the importance of new particle formation over sea seems to be minor. The new particle formation and the further growth of particles are sensitive to the concentration of sulfuric acid. If strong new particle formation and rapid growth occurs close to the coast, the formed particles will act as a condensation sink for newly formed particles over land, with the potential to decrease the amount of particles that can act as cloud condensation nuclei over land.To achieve a greater understanding of aerosol processes and reduce the uncertainties in models, it is important that models are evaluated against observations at various locations and conditions. Much work remains to ensure that models give the right results for the right reasons.