Research Collection Doctoral Thesis Investigation of new aerosol particle formation and growth at the CERN CLOUD chamber and at the high Alpine Research Station Jungfraujoch Author(s): Tröstl, Jasmin Publication Date: 2015 Permanent Link: https://doi.org/10.3929/ethz-a-010651976 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library Diss.-No. ETH 22983 INVESTIGATION OF NEW AEROSOL PARTICLE FORMATION AND GROWTH AT THE CERN CLOUD CHAMBER AND AT THE HIGH ALPINE RESEARCH STATION JUNGFRAUJOCH JASMIN TRÖSTL DISS. ETH NO. 22983 INVESTIGATION OF NEW AEROSOL PARTICLE FORMATION AND GROWTH AT THE CERN CLOUD CHAMBER AND AT THE HIGH ALPINE RESEARCH STATION JUNGFRAUJOCH A thesis submitted to attain the degree of DOCTOR OF SCIENCES of ETH ZURICH (Dr. sc. ETH Zurich) presented by JASMIN TROSTL¨ MSc, Faculty of Physics, Vienna University of Technology born on 12.06.1988 citizen of Austria accepted on the recommendation of Prof. Dr. Urs Baltensperger (examiner) Prof. Dr. Thomas Peter (co-examiner) Dr. George Biskos (co-examiner) Dr. Martin Gysel (co-examiner) 2015 Summary Aerosols, liquid or solid particles suspended in a gas, are ubiquitous in the Earth's atmosphere. They have an impact on the Earth's climate by changing the radiation balance of incident sun light. They scatter and absorb light, directly influencing this balance. Aerosols can also form clouds and in this way influence the radiation balance indirectly by modifying cloud properties. Aerosols are counteracting the positive radiative forcing mainly coming from emitted greenhouse gases, as they are one of the most important contributors to negative radiative forcing. However, they also yield high uncertainties in climate prediction scenarios, mainly coming from the unknown conditions in the pre-industrial period. It is important to reduce this uncertainty in order to improve current climate models. Aerosols can be directly emitted into the atmosphere (primary aerosols) or they are formed in the atmosphere (secondary aerosols). A recent model predicts that up to 45% of the cloud condensation nuclei (CCN) for low level clouds are secondary aerosols formed in the atmosphere via aerosol nucleation, a process that forms new particles via gas-to- particle conversion. Freshly nucleated particles range from 1 to 2 nm diameter. The newly formed particles can then grow to CCN size (50 - 100 nm). Alternatively, they can get scavenged by pre-existing larger particles before growing to CCN size via coagulation, which decreases the contribution to the total number of possible CCN. These coagulation losses are especially high for the smallest particle sizes. With increasing particle size the coagulation losses decline, increasing the survival probability of the particle and thus increasing the chance of the particle to grow to CCN size. At the size of freshly nucleated particles, the Kelvin effect can additionally hinder the condensation of vapour species, lowering the growth rate and decreasing the survival probability. Thus, the first steps of growth are decisive for the fate of the new particle. Therefore, early nanoparticle growth is - besides the aerosol nucleation rate - the second important mechanism that determines the fraction of newly formed particles to be able to grow to CCN size. It is thought that sulfuric acid (H2SO4) is necessary for aerosol nucleation to take place and is, therefore, the most important precursor. Oxidized organics can enhance the nucleation rates. However, it was so far unclear, if oxidized organics can nucleate in the absence of sulfuric acid. SO2 - the main precursor for sulfuric acid - is mainly emitted in industrial and energy processes and thus was not as abundant in the pre-industrial period, suggesting less importance of new particle formation events in the pre-industrial period. Biogenic emissions are expected to be similar in the pre-industrial period and today. Pure biogenic nucleation (without participation of sulfuric acid) would alter our view on the pre-industrial aerosol number concentration and subsequently change the estimated aerosol forcing. This thesis focuses in the first part on the investigation of pure biogenic aerosol nucleation and early growth during laboratory experiments at CERN within the CLOUD (Cosmics Leaving OUtdoor Droplets) project. The role of α-pinene - the most abundant biogenic monoterpene - in the formation of CCN was investigated. Ozonolysis of α-pinene rapidly forms highly oxygenated molecules (HOMs). The results show that these HOMs nucleate in the absence of sulfuric acid. This indicates that global climate models underestimate the pre-industrial aerosol number concentration, as nucleation was neglected so far. This would lead to a weaker net negative radiative forcing implying a reduction of the estimated climate ix Summary sensitivity. Further, it was investigated how the Kelvin effect could influence the condensational growth of HOMs from the freshly nucleated particle to CCN size. The observed particle growth in the CERN CLOUD chamber was compared to a dynamic volatility basis set model. For this, the volatility of HOMs was considered showing that these compounds have highly variable volatilities, ranging over several orders of magnitudes. It could be shown that a large fraction of the HOMs are extremely low-volatility organic compounds (ELVOC), which condense close to the kinetic limit already during the nucleation. Other compounds are low-volatility organic compounds (LVOC). Due to their higher volatility, these compounds are hindered by the Kelvin effect at the initial sizes and can only condense at a later stage. ELVOC are therefore important for nucleation forming thermodynamically stable particles. Only then LVOC can contribute to grow the particles to larger sizes. The observed growth rates were parameterized and implemented into GLOMAP, a global climate model, showing that the CCN concentration is sensitive to the initial growth. These results are an important step towards the understanding and quantification of aerosol growth. In the second part of the thesis, the focus is on field experiments in the free troposphere. There, only a limited number of studies are available and the knowledge of the contributing species to new particle formation is very limited due to the challenges and lack of instru- mentation at these high altitudes. So far, even highly advanced climate models, such as GLOMAP, solely assume binary nucleation of sulfuric acid and water in the free troposphere. The Sphinx building of the high alpine research station at the Jungfraujoch is located in the Swiss Alps at 3'580 m a.s.l. and is most time of the year in the free troposphere. There, two intensive measurement campaigns (January-February in 2013 and 2014) and one long- term measurement campaign (July 2013 - June 2014) were conducted to assess new particle formation and early growth. For the first time the chemical composition of the aerosol pre- cursor during nucleation events at such a high altitude were investigated. Sulfuric acid was observed to nucleate together with either ammonia and/or HOMs, which were entrained from the planetary boundary layer before. Often the nucleation was driven by HOMs alone. Once volatile organic compounds from the planetary boundary layer are entrained, oxidation can take place lowering their volatility until nucleation is possible. This is only possible in a short window of opportunity (24 - 48 h), before the low volatility vapours get lost due to the condensation onto pre-existing aerosols or dilution. Further, the contribution of new particle formation to the total particle number concen- tration and the CCN number concentration at the Jungfraujoch was considered. The impact on the number concentration in the size range 10 to 50 nm is high, yielding on average ∼1'000 additional particles per cm−3 per nucleation event compared to a background number con- centration of a few 100 cm−3. However, the freshly nucleated particles do not grow to CCN size within one day; they only grow up to ∼30 - 50 nm, in dependence on the season. Thus, a multi-step growth over several days is necessary. Also a first empirical equation describing the observed nucleation rates and involving the most important factors for nucleation at the Jungfraujoch was derived. These new findings were only possible due to the recent development in aerosol instru- mentation. The detection limits, time and size resolution of particle and molecule/cluster detectors have been significantly improved during the last decade. This was especially im- portant for nucleation studies, where highly dynamic aerosol distributions down to 1 nm can x now be investigated. Here, one particle instrument - the nano scanning mobility analyzer - was characterized in the third part of the thesis which allows to detect the particle size distribution down to a size of 2 nm. Scan times of 3 s are possible, compared to older models that had typical scan times of 60 s or more. The results obtained in this thesis shed new light on the underlying mechanisms of atmo- spheric new particle formation. Oxidation products from biogenic precursors are not only a vital link between freshly formed particles and CCN, they can also nucleate without sulfuric acid, significantly changing the current view on atmospheric aerosol nucleation and especially in the pre-industrial period. In addition, precursor gases of free tropospheric