On the formation and growth of sub-3 nm atmospheric particles and molecular clusters

Air consists of different gas molecules but also liquid or solid aerosol particles. Despite their small size, aerosol particles significantly affect human life: they deteriorate air quality and influence the climate directly by scattering and absorbing solar radiation, and indirectly by modifying the properties of clouds. The majority of atmospheric aerosol particles are formed in the process called new particle formation. New particle formation proceeds by the formation of nanometer-sized clusters from low-volatile vapors and their following growth to larger particles. If the particles produced in this process reach large enough sizes they can finally act as cloud condensation nuclei and thus affect the climate. Due to the climate effects of atmospheric particle formation, it has been widely studied in recent decades. However, because of challenges in detecting nanometer-sized clusters and particles and their precursor vapors, many questions remain open. The aim of this thesis is to elucidate some of the unresolved issues related to the first steps of particle formation and growth by investigating the dynamics of sub-3 nm atmospheric particles and molecular clusters. The research in this thesis was conducted by analyzing measurements performed with novel instrumentation in different environments in the field and in the laboratory. The primary instrument was the Particle Size Magnifier (PSM), which is a recently developed condensation particle counter able to detect particles down to even 1 nm. The measurements with ion mobility spectrometers measuring atmospheric ions, and chemical ionization mass spectrometers detecting low-volatile gaseous compounds were also utilized. In addition, cluster population simulations were performed. The concentration of sub-3 nm particles was observed to vary strongly between different environments. The highest concentrations were detected in polluted environments, likely due to anthropogenic sources of precursor vapors. In boreal forest sub-3 nm particle concentration was higher in summer than in winter, suggesting the importance of biogenic precursor vapors. At all the study sites, sub-3 nm particle concentration was higher during daytime than at night. Electrically neutral particles were observed to dominate the sub-3 nm particle population in polluted environments and in boreal forest during spring and summer. The formation of sub-2 nm molecular clusters and their further growth were found to be two separate processes. Neutral particle formation mechanisms were observed to dominate over ion-mediated mechanisms. The results indicate that sulfuric acid is a key compound in particle formation but low-volatile organic compounds are likely also important, especially in accelerating particle growth. In the system containing only sulfuric acid, base compounds, and ions, small acid–base clusters and ions can also enhance the growth. The understanding of the first steps of particle formation and growth obtained in this thesis is essential when trying to reduce the uncertainties in the climate predictions related to the indirect effects of aerosol particles. In addition, the knowledge of aerosol formation mechanisms is needed to solve air quality problems faced in polluted environments.