On observations of newly formed nanoparticles : their detection, characterization and abundance in various environments

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http://urn.fi/URN:ISBN:978-952-7276-29-7
Title: On observations of newly formed nanoparticles : their detection, characterization and abundance in various environments
Author: Ahonen, Lauri
Contributor: University of Helsinki, Faculty of Science, ilmakehätieteiden osasto
Doctoral Programme in Atmospheric Sciences
Department of Mechanical Engineering of the University of Minnesota
Publisher: Helsingin yliopisto
Date: 2019-11-08
Language: en
Belongs to series: Report series in aerosol science
URI: http://urn.fi/URN:ISBN:978-952-7276-29-7
http://hdl.handle.net/10138/306655
Thesis level: Doctoral dissertation (article-based)
Abstract: New particle formation (NPF) is a dominant source for atmospheric aerosol particles in terms of their number concentration, and a major contributor to the number of cloud con-densation nuclei globally. Atmospheric aerosol particles have impact on Earth’s climate via direct and indirect effects. In addition to climate, aerosol particles have impact on human health. In polluted environments, airborne pollutants, especially particulate matter, shorten the lifetime expectancy by several years. Understanding the processes of NPF is in a key role, for example, while identifying the most effective acts to improve the air quality in megacities or assessing the role of anthropogenic emissions in climate change. A NPF event consists of formation of molecular clusters and their subsequent growth into larger particle sizes by condensable vapors and/or coagulation In order to quantify NPF events, measurements of particle number size distribution close to the size where gas-to-particle conversion takes place are necessary. The gas-to-particle conversion takes place in the 1-2 nm size range, where there exist electrically charged and neutral molecular clusters. On one hand, in most of the environments such clusters are present also in the absence of NPF events. The growth of the small clusters to the 2-3 nm size range is, on the other hand, indicative of a NPF event. In this thesis, we gather knowledge on the concentration of sub-3 nm aerosol particles by conducting both long-term and campaign-like measure-ments with particle size magnifier (PSM; Airmodus Ltd.). Our results were compared with the other available PSM data, from sites around the world, and presented in compilation study. In all the sites the sub-3 nm particle concentration had a daytime maximum. Gener-ally, the highest concentrations were observed at the sites with the highest anthropogenic influence. In this thesis, we also conducted a campaign to observe particle formation in a cleanroom environment, where PSM was used for the first time to monitor concentration of nanoparticles in such an environment. The results showed that sub-2 nm clusters were observed to be always present in this clean room in relatively small concentrations. Short periods of high concentrations were observed during active manufacturing processes in the clean room. Instrumental development was one important aspect of this thesis. We experimented with the possibility of using two commercial condensation particle counters (CPCs), with nomi-nal lower limit close to 10 nm, for the detection of sub-3 nm particles. Optimized operating temperatures and flow rates were tested in laboratory conditions and by using simulation tools. We showed that commercially-available CPCs can be optimized down to sub-3 nm detection. In addition, a differential mobility particle sizer (DMPS) was specially built to measure particle number size distributions in the sub-10 nm size range using PSM and half-mini differential mobility analyzer (DMA). Due to the improved overall transmission of our system, the counting uncertainty compared to a harmonized DMPS was reduced to a half in the sub-10 nm size range. An ion mobility-mass spectrometry was utilized to investigate the structures and hydration of iodine pentoxide iodic acid clusters, similar to ones observed during coastal nucleation events. The number of water molecules in hydrated clusters was sufficient to convert io-dine pentoxide into iodic acid but the water sorption beyond this amount was limited.
Subject: Ilmakehätieteet/fysiikka
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