Aerosol optical properties, black carbon and their spatio-temporal variation

The amount and properties of atmospheric aerosol particles vary both in time and space depending on the proximity of the sources, atmospheric chemistry, and meteorological con-ditions. Atmospheric particulate matter worsens air quality and therefore affects human health. Aerosol particles have a notable effect also on the Earth’s climate by scattering and absorbing the solar radiation and via aerosol-cloud interactions. The absorbing fraction of particles warms the climate, but due to the aerosol-cloud interactions and the greater fraction of scattering particles, the total effect of aerosols on the climate is cooling. To determine the effect that particles have on the climate, it is crucial to know aerosol optical properties (AOPs) that describe the ability of atmospheric aerosol particles to scatter and absorb light at different wavelengths. The AOPs are determined by the size distribution, chemical composition, shape, and mixing state of the particles. This thesis aims to deepen the understanding of the AOPs and their relationships to the aerosol size distribution and chemical composition by combining comprehensive measurements of these parameters. The measurements were conducted at a rural boreal forest measurement site SMEAR II. This thesis also studies the spatial and temporal variation of aerosols, by utilizing long-term aerosol measurements from different environments that vary from background sites to urban locations. The study of the spatio-temporal variation focuses on the variation in equivalent black carbon (eBC), which stands for optically measured black carbon (BC). A majority of the aerosol absorption is caused by BC, and therefore it represents the aerosol particles that have a warming effect on the climate. Since BC is emitted mainly by anthropogenic activi-ties as a by-product of incomplete combustion, measurements of eBC give additional infor-mation on the health effects of aerosol particles since particles emitted from combustion sources are more harmful to health than aerosols from other sources. Studying the spatio-temporal variation in aerosol particles and especially in eBC concentration indicates the effect of anthropogenic activities on the aerosol concentration. The measurements of the AOPs are rather robust, cheap and easy to run, which is why the AOPs are commonly measured properties. However, challenges arise with absorption and eBC measurements, which are typically measured by filter-based methods. In optical filter measurements, also the filter interacts with the radiation causing nonlinearities and uncer-tainties in the measurements. In addition to understand better the AOPs and the spatio-tem-poral variation in the atmospheric particles, this thesis aims to improve the filter-based measurements and to understand better the effect of different instruments and filter loading correction algorithms on the measured AOPs.Aerosolihiukkaset ovat ilmassa leijuvia mikroskooppisen pieniä hiukkasia, jotka voivat olla joko nestemäisiä tai kiinteitä. Aerosolihiukkasia esiintyy kaikkialla maapallon ilmakehässä, mutta niiden määrä ja ominaisuudet vaihtelevat sekä ajallisesti että paikallisesti. Vaihtelu riippuu aerosolihiukkasten päästölähteistä, päästölähteiden etäisyydestä, ilmakehän kemiallisista prosesseista sekä meteorologisesta tilanteesta. Ilmakehän pienhiukkasilla on huomattava vaikutus ilmanlaatuun sekä maapallon ilmastoon. Aerosolihiukkaset vaikuttavat ilmastoon vuorovaikuttamalla sekä auringon säteilyn että pilvien kanssa. Hiukkaset vuorovaikuttavat säteilyn kanssa sirottamalla ja absorboimalla säteilyä. Tätä vuorovaikutusta kutsutaan suoraksi ilmastovaikutukseksi. Absorboiva osuus hiukkasista muuttaa säteilyn energian lämmöksi, mikä lämmittää ilmastoa. Kokonaisuudessa hiukkasten suora vaikutus ilmastoon on kuitenkin viilentävä, sillä suurin osa hiukkasista sirottaa auringon säteilyä, jolloin osa säteilystä heijastuu takaisin avaruuteen. Aerosolihiukkasten suora vaikutus ilmastoon riippuu hiukkasten optisista ominaisuuksista, jotka kuvaavat hiukkasten kykyä sirottaa ja absorboida säteilyä eri aallonpituuksilla. Optiset ominaisuudet riippuvat monesta eri tekijästä, joista tärkeimmät ovat hiukkasten kokojakauma sekä kemiallinen koostumus. Yksi tämän tutkielma tavoitteista onkin ymmärtää paremmin aerosolihiukkasten optisten ominaisuuksien, kokojakauman sekä kemiallisen koostumuksen välisiä riippuvuuksia. Näiden tekijöiden välisiä suhteita on tutkittu hyödyntämällä SMEAR II –asemalla suoritettuja pitkäaikaisia ja kattavia hiukkasmittauksia. Toinen tutkimuksen tavoitteista on selvittää kuinka hiukkasten määrä ja ominaisuudet vaihtelevat ajallisesti sekä paikallisesti. Ajallista ja paikallista vaihtelua tutkittiin hyödyntämällä pitkäaikaismittauksia, jotka on suoritettu erilaisissa ympäristöissä. Tutkitut ympäristöt vaihtelivat havumetsävyöhykkeellä sijaitsevasta tausta-asemasta (SMEAR II) kaupungissa sijaitseviin liikenneasemiin. Hiukkasten ajallisen ja paikallisen vaihtelun tutkiminen keskittyi etenkin mustan hiileen pitoisuuteen. Mustaa hiiltä päätyy ilmakehään polttoprosessien sivutuotteena, minkä vuoksi sen pitoisuus kuvaa hyvin ihmisperäisten hiukkasten lähteitä. Koska musta hiili absorboi säteilyä tehokkaasti, sillä on voimakas ilmastoa lämmittävä vaikutus. Etenkin polttoprosessien päästöt ovat haitallisia ihmisten terveydelle. Koska mustan hiilen pitoisuus edustaa hyvin polttoprosessien päästöjä, se antaa lisätietoa myös ilmanlaadusta. Aerosolihiukkasten säteilyn absorptiota sekä mustan hiilen pitoisuutta mitataan tyypillisesti samoja optisia mittalaitteita hyödyntäen. Nämä mittaukset ovat kuitenkin osoittautuneet haastaviksi, koska mittaukset perustuvat yleensä suodatinpohjaisiin menetelmiin. Suodatin aiheuttaa mittauksiin epävarmuuksia sekä virhelähteitä, koska suodattimelle kerättyjen hiukkasten lisäksi suodattimen kuidut vuorovaikuttavat säteilyn kanssa. Kolmas tämän tutkimuksen tavoitteista liittyykin suodatinpohjaisten mittausmenetelmien epävarmuuksiin, joita on tutkittu vertailemalla erilaisia SMEAR II –asemalla mitanneita laitteita sekä niihin liittyviä korjausalgoritmeja keskenään

Aerosolihiukkasten optiset ominaisuudet ja in-situ optinen sulkeuma Hyytiälässä

Maapallon ilmakehä sisältää mikroskooppisen pieniä aerosolihiukkasia, joilla on vaikutus ihmisten terveyteen sekä maapallon ilmastoon. Aerosolihiukkaset vaikuttavat ilmastoon vuorovaikuttamalla auringon säteilyn kanssa sekä osallistumalla pilvien muodostumisprosessiin. Tässä tutkielmassa keskitytään aerosolihiukkasten optisiin ominaisuuksiin, joilla tarkoitetaan niiden kykyä sirottaa sekä absorboida säteilyä eri aallonpituuksilla. Tutkielmassa käsitellään SMEAR II -asemalla suoritettuja in-situ mittauksia aerosolihiukkasten sironnasta, takaisinsironnasta ja absorptiosta, joita on saatavilla jo vuodesta 2006 alkaen. Mitattujen sironta-, takaisinsironta- sekä absorptiokertoimilla aerosolihiukkasille voidaan määrittää niiden kokojakaumaa ja koostumusta kuvaavia intensiivisiä suureita. Pitkäaikaisten mittausten avulla optisille ominaisuuksille nähtiin trendejä sekä selkeää vuodenaikaisvaihtelua. Vertaamalla optisia mittauksia kokojakauman mittauksiin, aerosolihiukkasille voitiin määrittää kompleksinen taitekerroin, jota käytetään mallinnettaessa Mie-sirontaa ja -absorptiota. Absorptiokertoimen mittauksia verrattiin lisäksi alkuainehiilen mittauksiin, jolloin voitiin määrittää aerosolihiukkasten massa-absorptioala. Massa-absorptioalan avulla voidaan määrittää mustan hiilen pitoisuus absorptiokertoimen mittauksista. SMEAR II -asemalla absorptiokerrointa on mitattu kolmella eri mittalaitteella (etalometri, Particle Soot Absorption Photometer (PSAP) sekä Multi-Angle Absorption Photometer (MAAP)), joita vertailtiin keskenään. Vertaamalla etalometrin sekä MAAPin mittaustuloksia toisiinsa määritettiin etalometrimittauksiin liittyville korjausalgoritmeille suodattimen moninkertaissirontaa kuvaavat parametrit SMEAR II -aseman olosuhteisiin. Tutkielmassa suoritettiin myös optinen sulkeuma ekstinktio-, sironta- sekä absorptiokertoimien mittaustuloksia vertailemalla. Tulosten perusteella ekstinktiota mittaavan Cavity Attenuated Phase Shift -ekstinktiomonitorin (CAPS) sekä sirontaa mittaavan integroivan nefelometrin mittaustarkkuudet eivät riitä mittaamaan aerosolifaasissa olevien hiukkasten absorptiota

Over a 10-year record of aerosol optical properties at SMEAR II

Aerosol optical properties (AOPs) describe the ability of aerosols to scatter and absorb radiation at different wavelengths. Since aerosol particles interact with the sun's radiation, they impact the climate. Our study focuses on the long-term trends and seasonal variations of different AOPs measured at a rural boreal forest site in northern Europe. To explain the observed variations in the AOPs, we also analyzed changes in the aerosol size distribution. AOPs of particles smaller than 10 mu m (PM10) and 1 mu m (PM1) have been measured at SMEAR II, in southern Finland, since 2006 and 2010, respectively. For PM10 particles, the median values of the scattering and absorption coefficients, single-scattering albedo, and backscatter fraction at lambda = 550 nm were 9.8 Mm(-1), 1.3 Mm(-1), 0.88, and 0.14. The median values of scattering and absorption angstrom ngstrom exponents at the wavelength ranges 450-700 and 370-950 nm were 1.88 and 0.99, respectively. We found statistically significant trends for the PM10 scattering and absorption coefficients, single-scattering albedo, and backscatter fraction, and the slopes of these trends were -0.32 Mm(-1), -0.086 Mm(-1), 2.2 x 10(-3), and 1.3 x 10(-3) per year. The tendency for the extensive AOPs to decrease correlated well with the decrease in aerosol number and volume concentrations. The tendency for the backscattering fraction and single-scattering albedo to increase indicates that the aerosol size distribution consists of fewer larger particles and that aerosols absorb less light than at the beginning of the measurements. The trends of the single-scattering albedo and backscattering fraction influenced the aerosol radiative forcing efficiency, indicating that the aerosol particles are scattering the radiation more effectively back into space.Peer reviewe

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

On the mode-segregated aerosol particle number concentration load : contributions of primary and secondary particles in Hyytiälä and Nanjing

Aerosol particle concentrations in the atmosphere are governed by their sources and sinks. Sources include directly-emitted (primary) and secondary aerosol particles formed from gas-phase precursor compounds. The relative importance of primary and secondary aerosol particles varies regionally and with time. In this work, we investigated primary and secondary contributions to mode-segregated particle number concentrations by using black carbon as a tracer for the primary aerosol number concentration. We studied separately nucleation, Aitken and accumulation mode concentrations at a rural boreal forest site (Hyytiala, Finland) and in a rather polluted megacity environment (Nanjing, China) using observational data from 2011 to 2014. In both places and in all the modes, the majority of particles were estimated to be of secondary origin. Even in Nanjing, only about half of the accumulation mode particles were estimated to be of primary origin. Secondary particles dominated particularly in the nucleation and Aitken modes.Peer reviewe

Spatiotemporal variation and trends in equivalent black carbon in the Helsinki metropolitan area in Finland

In this study, we present results from 12 years of black carbon (BC) measurements at 14 sites around the Helsinki metropolitan area (HMA) and at one background site outside the HMA. The main local sources of BC in the HMA are traffic and residential wood combustion in fire-places and sauna stoves. All BC measurements were conducted optically, and therefore we refer to the measured BC as equivalent BC (eBC). Measurement stations were located in different environments that represented traffic environment, detached housing area, urban background, and regional background. The measurements of eBC were conducted from 2007 through 2018; however, the times and the lengths of the time series varied at each site. The largest annual mean eBC concentrations were measured at the traffic sites (from 0.67 to 2.64 mu g m(-3)) and the lowest at the regional background sites (from 0.16 to 0.48 mu g m(-3)). The annual mean eBC concentrations at the detached housing and urban background sites varied from 0.64 to 0.80 mu g m(-3) and from 0.42 to 0.68 mu g m(-3), respectively. The clearest seasonal variation was observed at the detached housing sites where residential wood combustion increased the eBC concentrations during the cold season. Diurnal variation in eBC concentration in different urban environments depended clearly on the local sources that were traffic and residential wood combustion. The dependency was not as clear for the typically measured air quality parameters, which were here NOx concentration and mass concentration of particles smaller that 2.5 mu m in diameter (PM2.5). At four sites which had at least a 4-year-long time series available, the eBC concentrations had statistically significant decreasing trends that varied from -10.4 % yr(-1) to -5.9 % yr(-1). Compared to trends determined at urban and regional background sites, the absolute trends decreased fastest at traffic sites, especially during the morning rush hour. Relative long-term trends in eBC and NOx were similar, and their concentrations decreased more rapidly than that of PM2.5. The results indicated that especially emissions from traffic have decreased in the HMA during the last decade. This shows that air pollution control, new emission standards, and a newer fleet of vehicles had an effect on air quality.Peer reviewe

The effect of clouds and precipitation on the aerosol concentrations and composition in a boreal forest environment

Atmospheric aerosol particle concentrations are strongly affected by various wet processes, including below and in-cloud wet scavenging and in-cloud aqueous-phase oxidation. We studied how wet scavenging and cloud processes affect particle concentrations and composition during transport to a rural boreal forest site in northern Europe. For this investigation, we employed air mass history analysis and observational data. Long-term particle number size distribution (similar to 15 years) and composition measurements (similar to 8 years) were combined with air mass trajectories with relevant variables from reanalysis data. Some such variables were rainfall rate, relative humidity, and mixing layer height. Additional observational datasets, such as temperature and trace gases, helped further evaluate wet processes along trajectories with mixed effects models. All chemical species investigated (sulfate, black carbon, and organics) exponentially decreased in particle mass concentration as a function of accumulated precipitation along the air mass route. In sulfate (SO4) aerosols, clear seasonal differences in wet removal emerged, whereas organics (Org) and equivalent black carbon (eBC) exhibited only minor differences. The removal efficiency varied slightly among the different reanalysis datasets (ERA-Interim and Global Data Assimilation System; GDAS) used for the trajectory calculations due to the difference in the average occurrence of precipitation events along the air mass trajectories between the reanalysis datasets. Aqueous-phase processes were investigated by using a proxy for air masses travelling inside clouds. We compared air masses with no experience of approximated in-cloud conditions or precipitation during the past 24 h to air masses recently inside non-precipitating clouds before they entered SMEAR II (Station for Measuring Ecosystem-Atmosphere Relations). Significant increases in SO4 mass concentration were observed for the latter air masses (recently experienced non-precipitating clouds). Our mixed effects model considered other contributing factors affecting particle mass concentrations in SMEAR II: examples were trace gases, local meteorology, and diurnal variation. This model also indicated in-cloud SO4 production. Despite the reanalysis dataset used in the trajectory calculations, aqueous-phase SO4 formation was observed. Particle number size distribution measurements revealed that most of the in-cloud SO4 formed can be attributed to particle sizes larger than 200 nm (electrical mobility diameter). Aqueous-phase secondary organic aerosol (aqSOA) formation was non-significant.Peer reviewe

Estimating cloud condensation nuclei number concentrations using aerosol optical properties : role of particle number size distribution and parameterization

The concentration of cloud condensation nuclei (CCN) is an essential parameter affecting aerosol–cloud interactions within warm clouds. Long-term CCN number concentration (NCCN) data are scarce; there are a lot more data on aerosol optical properties (AOPs). It is therefore valuable to derive parameterizations for estimating NCCN from AOP measurements. Such parameterizations have already been made, and in the present work a new parameterization is presented. The relationships between NCCN, AOPs, and size distributions were investigated based on in situ measurement data from six stations in very different environments around the world. The relationships were used for deriving a parameterization that depends on the scattering Ångström exponent (SAE), backscatter fraction (BSF), and total scattering coefficient (σsp) of PM10 particles. The analysis first showed that the dependence of NCCN on supersaturation (SS) can be described by a logarithmic fit in the range SS 4. At SS >0.4 % the average bias ranged from ∼0.7 to ∼1.3 at most sites. For the marine-aerosol-dominated site Ascension Island the bias was higher, ∼1.4–1.9. In other words, at SS >0.4 % NCCN was estimated with an average uncertainty of approximately 30 % by using nephelometer data. The biases were mainly due to the biases in the parameterization related to the scattering Ångström exponent (SAE). The squared correlation coefficients between the AOP-derived and measured NCCN varied from ∼0.5 to ∼0.8. To study the physical explanation of the relationships between NCCN and AOPs, lognormal unimodal particle size distributions were generated and NCCN and AOPs were calculated. The simulation showed that the relationships of NCCN and AOPs are affected by the geometric mean diameter and width of the size distribution and the activation diameter. The relationships of NCCN and AOPs were similar to those of the observed ones.The concentration of cloud condensation nuclei (CCN) is an essential parameter affecting aerosol-cloud interactions within warm clouds. Long-term CCN number concentration (N-CCN) data are scarce; there are a lot more data on aerosol optical properties (AOPs). It is therefore valuable to derive parameterizations for estimating N-CCN from AOP measurements. Such parameterizations have already been made, and in the present work a new parameterization is presented. The relationships between N-CCN, AOPs, and size distributions were investigated based on in situ measurement data from six stations in very different environments around the world. The relationships were used for deriving a parameterization that depends on the scattering Angstrom exponent (SAE), backscatter fraction (BSF), and total scattering coefficient (sigma(sp)) of PM10 particles. The analysis first showed that the dependence of N-CCN on supersaturation (SS) can be described by a logarithmic fit in the range SS 4. At SS > 0 :4% the average bias ranged from similar to 0.7 to similar to 1.3 at most sites. For the marine-aerosol-dominated site Ascension Island the bias was higher, similar to 1.4-1.9. In other words, at SS > 0:4% N-CCN was estimated with an average uncertainty of approximately 30% by using nephelometer data. The biases were mainly due to the biases in the parameterization related to the scattering Angstrom exponent (SAE). The squared correlation coefficients between the AOP-derived and measured N-CCN varied from similar to 0.5 to similar to 0.8. To study the physical explanation of the relationships between N-CCN and AOPs, lognormal unimodal particle size distributions were generated and N-CCN and AOPs were calculated. The simulation showed that the relationships of N-CCN and AOPs are affected by the geometric mean diameter and width of the size distribution and the activation diameter. The relationships of N-CCN and AOPs were similar to those of the observed ones.Peer reviewe

Input-Adaptive Proxy for Black Carbon as a Virtual Sensor

Missing data has been a challenge in air quality measurement. In this study, we develop an input-adaptive proxy, which selects input variables of other air quality variables based on their correlation coefficients with the output variable. The proxy uses ordinary least squares regression model with robust optimization and limits the input variables to a maximum of three to avoid overfitting. The adaptive proxy learns from the data set and generates the best model evaluated by adjusted coefficient of determination (adjR2). In case of missing data in the input variables, the proposed adaptive proxy then uses the second-best model until all the missing data gaps are filled up. We estimated black carbon (BC) concentration by using the input-adaptive proxy in two sites in Helsinki, which respectively represent street canyon and urban background scenario, as a case study. Accumulation mode, traffic counts, nitrogen dioxide and lung deposited surface area are found as input variables in models with the top rank. In contrast to traditional proxy, which gives 20–80% of data, the input-adaptive proxy manages to give full continuous BC estimation. The newly developed adaptive proxy also gives generally accurate BC (street canyon: adjR2 = 0.86–0.94; urban background: adjR2 = 0.74–0.91) depending on different seasons and day of the week. Due to its flexibility and reliability, the adaptive proxy can be further extend to estimate other air quality parameters. It can also act as an air quality virtual sensor in support with on-site measurements in the future

Condensation/immersion mode ice-nucleating particles in a boreal environment

Ice-nucleating particle (INP) measurements were performed in the boreal environment of southern Finland at the Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II) in the winter-spring of 2018. Measurements with the Portable Ice Nucleation Chamber (PINC) were conducted at 242 K and 105 % relative humidity with respect to water. The median INP number concentration [INP] during a 6-week measurement period was 13 L-1. The [INP] spanned 3 orders of magnitude and showed a general increase from mid-February until early April. No single dominant local or regional sources of INPs in the boreal environment of southern Finland could be identified. Rather, it is hypothesised that the INPs detected at SMEAR II are a result of long-range transport and dilution of INPs sourced far from the measurement site. Despite high variability, the measured [INP] values fall within the range expected for the [INP] measured elsewhere under similar thermodynamic conditions. The [INP] did not correlate with any of the examined parameters during the entire field campaign, indicating that no one single parameter can be used to predict the [INP] at the measurement location during the examined time period. The absence of a correlation across the entire field campaign also suggests that a variety of particles act as INPs at different times, although it was indirectly determined that ambient INPs are most likely within the size range of 0.1-0.5 mu m in diameter on average. On shorter timescales, several particle species correlated well with the [INP]. Depending on the meteorological conditions, black carbon (BC), supermicron biological particles and sub-0.1 mu m particles, most likely nanoscale biological fragments such as ice-nucleating macromolecules (INMs), correlated with the INP signal. However, an increase in the concentration of any of these particle species may not necessarily lead to the increase in the [INP]; the reasons for this remain unknown. Limitations of the instrumental set-up and the necessity for future field INP studies are addressed.Peer reviewe