Miten kondensaatiohiukkaslaskureiden kehitys tarkentaa näkemystä ilmakehän hiukkasmuodostuksesta

Aerosol particles deteriorate air quality, atmospheric visibility and our health. They affect the Earth s climate by absorbing and scattering sunlight, forming clouds, and also via several feed-back mechanisms. The net effect on the radiative balance is negative, i.e. cooling, which means that particles counteract the effect of greenhouse gases. However, particles are one of the poorly known pieces in the climate puzzle. Some of the airborne particles are natural, some anthropogenic; some enter the atmosphere in particle form, while others form by gas-to-particle conversion. Unless the sources and dynamical processes shaping the particle population are quantified, they cannot be incorporated into climate models. The molecular level understanding of new particle formation is still inadequate, mainly due to the lack of suitable measurement techniques to detect the smallest particles and their precursors. This thesis has contributed to our ability to measure newly formed particles. Three new condensation particle counter applications for measuring the concentration of nano-particles were developed. The suitability of the methods for detecting both charged and electrically neutral particles and molecular clusters as small as 1 nm in diameter was thoroughly tested both in laboratory and field conditions. It was shown that condensation particle counting has reached the size scale of individual molecules, and besides measuring the concentration they can be used for getting size information. In addition to atmospheric research, the particle counters could have various applications in other fields, especially in nanotechnology. Using the new instruments, the first continuous time series of neutral sub-3 nm particle concentrations were measured at two field sites, which represent two different kinds of environments: the boreal forest and the Atlantic coastline, both of which are known to be hot-spots for new particle formation. The contribution of ions to the total concentrations in this size range was estimated, and it could be concluded that the fraction of ions was usually minor, especially in boreal forest conditions. Since the ionization rate is connected to the amount of cosmic rays entering the atmosphere, the relative contribution of neutral to charged nucleation mechanisms extends beyond academic interest, and links the research directly to current climate debate.Ilmakehän pienhiukkaset huonontavat ilmanlaatua ja näkyvyyttä. Aerosolihiukkasia, esimerkiksi katupölyä, päätyy hengitysilman mukana elimistöön, jossa niillä on kielteisiä terveysvaikutuksia. Hiukkaset vaikuttavat myös maapallon ilmastoon sirottamalla auringonsäteilyä ja muodostamalla pilvipisaroita, sekä monien palauteilmiöiden kautta. Hiukkasten kokonaisvaikutuksen uskotaan olevan ilmastoa viilentävä, eli ne tasapainottavat kasvihuonekaasujen lisääntymisen aiheuttamaa ilmastonmuutosta. Hiukkaset ovat ilmastonmuutoskeskustelun musta hevonen, sillä niiden lähteet ja vaikutukset tunnetaan vielä melko heikosti. Aerosolihiukkasilla on sekä luontaisia että ihmisperäisiä lähteitä, osa päätyy ilmaan hiukkasmuodossa, kun taas osa muodostuu vasta ilmakehässä kaasu-hiukkasmuuntuman seurauksena. Hiukkasten muodostuminen alkaa siitä, että höyrymolekyylejä tarttuu toisiinsa muodostaen pieniä ryppäitä, jotka sopivissa olosuhteissa kasvavat hiukkasiksi, kun niiden päälle tiivistyy lisää höyryä. Hiukkasten muodostumista ilmakehässä ei vielä täysin ymmärretä, sillä kaikista pienimpiä hiukkasia ja niitä muodostavia höyryjä ei ole tähän asti pystytty mittaamaan. Väitöstyön aikana kehitettiin kolme uutta laitteistoa, joilla voidaan mitata kaikista pienimpiä hiukkasia ja molekyyliryppäitä. Menetelmät perustuvat kondensaatiotekniikkaan, jossa hiukkasten päälle tiivistetään höyryä kunnes ne kasvavat optisesti havaittaviin kokoihin. Näin pystyttiin havaitsemaan jopa yhden nanometrin (nm), eli millimetrin miljoonasosan kokoisia hiukkasia, jotka voivat koostua vain muutamista molekyyleistä. Kehitettyjen laitteistojen toimivuus todennettiin laboratoriokokein ja niiden todettiin soveltuvan jatkuviin kenttämittauksin. Väitöstyössä osoitettiin, että pitoisuuden lisäksi kondensaatiohiukkaslaskureilla voidaan saada tietoa myös pienten hiukkasten kokojakaumasta. Ilmakehätutkimuksen lisäksi laitteistoilla voi olla mahdollisia sovelluskohteita esimerkiksi nanoteknologian alalla. Soveltaen em. kondensaatiohiukkaslaskureita, saatiin ensimmäistä kertaa tietoa alle 3 nm kokoisten hiukkasten pitoisuuden ajallisesta ja paikallisesta vaihtelusta. Pitoisuuksia mitattiin kahdella kenttäasemalla, josta toinen sijaitsee Hyytiälässä Pirkanmaalla ja toinen Irlannin länsirannikolla. Nanohiukkasia havaittiin esiintyvän ilmakehässä jatkuvasti, ja pitoisuudet havumetsäympäristössä olivat yleensä selvästi suurempia kuin rannikolla, todennäköisesti johtuen kasvillisuudesta vapautuvista orgaanisista höyryistä, jotka voivat tiivistyä hiukkasiksi. Ilmakehässä on tunnetusti myös pieniä ioneja, joita syntyy kosmisen säteilyn ja maaperän radioaktiivisten aineiden hajoamisen seurauksena. Väitöstyön tuloksena pystyttiin osoittamaan että alle kolmen nanometrin kokoluokassa on silti aina huomattavasti enemmän sähköisesti neutraaleja hiukkasia kuin näitä pieniä ioneja. Väitöstyö ja sen aikana kehitetyt laitteistot auttavat selvittämään ilmakehän hiukkasten syntymekanismeja. Kun hiukkasten muodostumiseen johtavat prosessit tunnetaan, ne voidaan sisällyttää ilmastomalleihin, ja näin saada selville miten pienhiukkaset säätelevät ilmastoa

Multicomponent new particle formation from sulfuric acid, ammonia, and biogenic vapors

A major fraction of atmospheric aerosol particles, which affect both air quality and climate, form from gaseous precursors in the atmosphere. Highly oxygenated organic molecules (HOMs), formed by oxidation of biogenic volatile organic compounds, are known to participate in particle formation and growth. However, it is not well understood how they interact with atmospheric pollutants, such as nitrogen oxides (NO_x) and sulfur oxides (SO_x) from fossil fuel combustion, as well as ammonia (NH_3) from livestock and fertilizers. Here, we show how NO_x suppresses particle formation, while HOMs, sulfuric acid, and NH3 have a synergistic enhancing effect on particle formation. We postulate a novel mechanism, involving HOMs, sulfuric acid, and ammonia, which is able to closely reproduce observations of particle formation and growth in daytime boreal forest and similar environments. The findings elucidate the complex interactions between biogenic and anthropogenic vapors in the atmospheric aerosol system

The effect of acid–base clustering and ions on the growth of atmospheric nano-particles

The growth of freshly formed aerosol particles can be the bottleneck in their survival to cloud condensation nuclei. It is therefore crucial to understand how particles grow in the atmosphere. Insufficient experimental data has impeded a profound understanding of nano-particle growth under atmospheric conditions. Here we study nano-particle growth in the CLOUD (Cosmics Leaving OUtdoors Droplets) chamber, starting from the formation of molecular clusters. We present measured growth rates at sub-3 nm sizes with different atmospherically relevant concentrations of sulphuric acid, water, ammonia and dimethylamine. We find that atmospheric ions and small acid-base clusters, which are not generally accounted for in the measurement of sulphuric acid vapour, can participate in the growth process, leading to enhanced growth rates. The availability of compounds capable of stabilizing sulphuric acid clusters governs the magnitude of these effects and thus the exact growth mechanism. We bring these observations into a coherent framework and discuss their significance in the atmosphere

Effects of different correction algorithms on absorption coefficient - a comparison of three optical absorption photometers at a boreal forest site

We present a comparison between three absorption photometers that measured the absorption coefficient (sigma(abs)) of ambient aerosol particles in 2012-2017 at SMEAR II (Station for Measuring Ecosystem-Atmosphere Relations II), a measurement station located in a boreal forest in southern Finland. The comparison included an Aethalometer (AE31), a multi-angle absorption photometer (MAAP), and a particle soot absorption photometer (PSAP). These optical instruments measured particles collected on a filter, which is a source of systematic errors, since in addition to the particles, the filter fibers also interact with light. To overcome this problem, several algorithms have been suggested to correct the AE31 and PSAP measurements. The aim of this study was to research how the different correction algorithms affected the derived optical properties. We applied the different correction algorithms to the AE31 and PSAP data and compared the results against the reference measurements conducted by the MAAP. The comparison between the MAAP and AE31 resulted in a multiple-scattering correction factor (C-ref) that is used in AE31 correction algorithms to compensate for the light scattering by filter fibers. C-ref varies between different environments, and our results are applicable to a boreal environment. We observed a clear seasonal cycle in C-ref, which was probably due to variations in aerosol optical properties, such as the backscatter fraction and single-scattering albedo, and also due to variations in the relative humidity (RH). The results showed that the filter-based absorption photometers seemed to be rather sensitive to the RH even if the RH was kept below the recommended value of 40 %. The instruments correlated well (R approximate to 0.98), but the slopes of the regression lines varied between the instruments and correction algorithms: compared to the MAAP, the AE31 underestimated sigma(abs) only slightly (the slopes varied between 0.96-1.00) and the PSAP overestimated sigma(abs) only a little (the slopes varied between 1.01-1.04 for a recommended filter transmittance >0.7). The instruments and correction algorithms had a notable influence on the absorption angstrom ngstrom exponent: the median absorption Angstrom exponent varied between 0.93-1.54 for the different algorithms and instruments.Peer reviewe

Molecular Steps of Neutral Sulfuric Acid and Dimethylamine Nucleation in CLOUD

We have run a set of experiments in the CLOUD chamber at CERN, Switzerland, studying the effect of dimethylamine (DMA) on sulfuric acid (SA)-water nucleation using a nitrate based Chemical Ionization Atmospheric Pressure ionization Time-Of-Flight Mass Spectrometer (CI-APi-TOF). Experiment was designed to produce neutral high m/z SA-DMA clusters in close to atmospherically relevant conditions to be detected and characterized by the CI-APi-TOF. We aimed in filling up the gap in measurement techniques from molecular level up to climatically relevant aerosol particles and thus improve our understanding of the role of sulfuric acid and DMA in atmospheric nucleation

The effect of acid–base clustering and ions on the growth of atmospheric nano-particles

The growth of freshly formed aerosol particles can be the bottleneck in their survival to cloud condensation nuclei. It is therefore crucial to understand how particles grow in the atmosphere. Insufficient experimental data has impeded a profound understanding of nano-particle growth under atmospheric conditions. Here we study nano-particle growth in the CLOUD (Cosmics Leaving OUtdoors Droplets) chamber, starting from the formation of molecular clusters. We present measured growth rates at sub-3 nm sizes with different atmospherically relevant concentrations of sulphuric acid, water, ammonia and dimethylamine. We find that atmospheric ions and small acid-base clusters, which are not generally accounted for in the measurement of sulphuric acid vapour, can participate in the growth process, leading to enhanced growth rates. The availability of compounds capable of stabilizing sulphuric acid clusters governs the magnitude of these effects and thus the exact growth mechanism. We bring these observations into a coherent framework and discuss their significance in the atmosphere

Multicomponent new particle formation from sulfuric acid, ammonia, and biogenic vapors

A major fraction of atmospheric aerosol particles, which affect both air quality and climate, form from gaseous precursors in the atmosphere. Highly oxygenated organic molecules (HOMs), formed by oxidation of biogenic volatile organic compounds, are known to participate in particle formation and growth. However, it is not well understood how they interact with atmospheric pollutants, such as nitrogen oxides (NO_x) and sulfur oxides (SO_x) from fossil fuel combustion, as well as ammonia (NH_3) from livestock and fertilizers. Here, we show how NO_x suppresses particle formation, while HOMs, sulfuric acid, and NH3 have a synergistic enhancing effect on particle formation. We postulate a novel mechanism, involving HOMs, sulfuric acid, and ammonia, which is able to closely reproduce observations of particle formation and growth in daytime boreal forest and similar environments. The findings elucidate the complex interactions between biogenic and anthropogenic vapors in the atmospheric aerosol system

New Particle Formation in the Atmosphere : From Molecular Clusters to Global Climate

New particle formation (NPF) represents the first step in the complex processes leading to formation of cloud condensation nuclei. Newly formed nanoparticles affect human health, air quality, weather, and climate. This review provides a brief history, synthesizes recent significant progresses, and outlines the challenges and future directions for research relevant to NPF. New developments include the emergence of state-of-the-art instruments that measure prenucleation clusters and newly nucleated nanoparticles down to about 1 nm; systematic laboratory studies of multicomponent nucleation systems, including collaborative experiments conducted in the Cosmics Leaving Outdoor Droplets chamber at CERN; observations of NPF in different types of forests, extremely polluted urban locations, coastal sites, polar regions, and high-elevation sites; and improved nucleation theories and parameterizations to account for NPF in atmospheric models. The challenges include the lack of understanding of the fundamental chemical mechanisms responsible for aerosol nucleation and growth under diverse environments, the effects of SO2 and NOx on NPF, and the contribution of anthropogenic organic compounds to NPF. It is also critical to develop instruments that can detect chemical composition of particles from 3 to 20 nm and improve parameterizations to represent NPF over a wide range of atmospheric conditions of chemical precursor, temperature, and humidity. Plain Language Summary In the atmosphere, invisible to the human eye, there are many microscopic particles, or nanoparticles, that affect human health, air quality, and climate. We do not fully understand the chemical processes that allow these fine particles to form and be suspended in the air nor how they influence heat flow in Earth's atmosphere. Laboratory experiments, field observations, and modeling simulations have all shown different results for how these particles behave. These inconsistencies make it difficult to accurately represent the processes of new particle formation in regional and global atmospheric models. Scientists still need to develop instruments that can measure the smallest range of nanoparticles and to find ways to describe particle formation that allow for differences in temperature, humidity, and level of pollution.Peer reviewe

Combining instrument inversions for sub-10 nm aerosol number size-distribution measurements

Resolving aerosol dynamical processes in the sub-10 nm range is crucial for our understanding of the contribution of new particle formation to the global cloud condensation nuclei budget or air pollution. Accurate measurements of the particle size distribution in this size-range are challenging due to high diffusional losses and low charging and/or detection efficiencies. Several instruments have been developed in recent years in order to access the sub-10 nm particle size distribution; however, no single instrument can provide high counting statistics, low systematic uncertainties and high size-resolution at the same time. Here we compare several data inversion approaches that allow combining data from different sizing instruments during the inversion and provide python/Julia packages for free usage of the methods. We find that Tikhonov regularization using the L-curve method for optimal regularization parameter estimation gives very reliable results over a wide range of tested data sets and clearly improves standard inversion approaches. Kalman Filtering or regularization using a Poisson likelihood can be powerful tools, especially for well-defined chamber experiments or data from mobility spectrometers only, respectively. Nullspace optimization and non-linear iterative regression are clearly inferior compared to the aforementioned methods. We show that with regularization we can reconstruct the size-distribution measured by up to 4 different mobility particle size spectrometer systems and several particle counters for datasets from Hyytiala and Helsinki, Finland, revealing the sub-10 nm aerosol dynamics in more detail compared to a single instrument assessment.Peer reviewe

What controls the observed size-dependency of the growth rates of sub-10 nm atmospheric particles?

The formation and growth of atmospheric particles involving sulfuric acid and organic vapors is estimated to have significant climate effects. To accurately represent this process in large-scale models, the correct interpretation of the observations on particle growth, especially below 10 nm, is essential. Here, we disentangle the factors governing the growth of sub-10 nm particles in the presence of sulfuric acid and organic vapors, using molecular-resolution cluster population simulations and chamber experiments. We find that observed particle growth rates are determined by the combined effects of (1) the concentrations and evaporation rates of the condensing vapors, (2) particle population dynamics, and (3) stochastic fluctuations, characteristic to initial nucleation. This leads to a different size-dependency of growth rate in the presence of sulfuric acid and/or organic vapors at different concentrations. Specifically, the activation type behavior, resulting in growth rate increasing with the particle size, is observed only at certain vapor concentrations. In our model simulations, cluster-cluster collisions enhance growth rate at high vapor concentrations and their importance is dictated by the cluster evaporation rates, which demonstrates the need for accurate evaporation rate data. Finally, we show that at sizes below similar to 2.5-3.5 nm, stochastic effects can importantly contribute to particle population growth. Overall, our results suggest that interpreting particle growth observations with approaches neglecting population dynamics and stochastics, such as with single particle growth models, can lead to the wrong conclusions on the properties of condensing vapors and particle growth mechanisms.Peer reviewe