1 FINNISH METEOROLOGICAL INSTITUTE CONTRIBUTIONS No. 26 FIELD AND LABORATORY STUDIES ON THE PHYSICAL AND CHEMICAL PROPERTIES OF NATURAL AND ANTHROPOGENIC TROPOSPHERIC AEROSOL Aki Virkkula DISSERTATION To be presented, with the permission of the Faculty of Science of the University of Helsinki for public criticism in main Auditorium of the Department of Physics on October 20, 1999, at 12 o'clock noon Finnish Meteorological Institute Helsinki 1999
2 Authors Published by Finnish Meteorological Institute P.O. Box 503 FIN HELSINKI Finland Series title, number and report code of publication Contributions No. 26, FMI-CONT-26 Date October 1999 Name of project Aki Virkkula Commissioned by Title Field and laboratory studies on the physical and chemical properties of natural and anthropogenic tropospheric aerosol Abstract Physical and chemical properties of natural and anthropogenic tropospheric aerosol were determined experimentally at various sites. Most field measurements were carried out at Sevettijärvi in Finnish Lapland. The Sevettijärvi data consist of aerosol number concentrations, scattering coefficients, major water-soluble ion, trace metal, and black carbon concentrations. Air masses were classified into three main types using backtrajectories: clean marine air, continental air, and very polluted air arriving from Kola peninsula. Comparison of the aerosol concentrations with the concentrations of elements in snow showed that the deposition was proportional to the aerosol exposure. The contribution of the various source regions to the dry deposition was estimated. Other field measurements were done over the Atlantic Ocean and on Tenerife, thus describing the aerosol both in the clean marine environment and in pollution plumes arriving from continental sources in North America and Europe. The North Atlantic data consists of aerosol number size distributions, inorganic and organic ion, dust, and organic and black carbon concentrations. The role of entrainment from free troposphere in explaining the aerosol size distribution and chemical composition in the boundary layer was discussed. The contribution of major compounds to the aerosol were calculated. A comparison showed that the contribution of nss-sulfate was almost the same in Lapland and over North Atlantic, approximately 30% in clean air and 50% in polluted air. Laboratory measurements were conducted to study formation and hygroscopic properties of biogenic secondary organic aerosol, and the mixing of inorganic and organic aerosol. It was shown that the organic and inorganic part of an internally mixed aerosol take up water independently. Publishing unit Air Quality Classification (UDC) , , , , Key words aerosols, Arctic aerosols, marine aerosols, organic aerosols, Kola Peninsula, long-range transport ISSN and series title Finnish Meteorological Institute Contributions Language ISBN English Sold by Pages Price Finnish Meteorological Institute, Library 178 FIM 150 P.O. Box 503 Note FIN Helsinki, Finland
3 Tekijä(t) Julkaisija Ilmatieteen laitos Vuorikatu 24 PL HELSINKI Julkaisun sarja, numero ja raporttikoodi Contributions No 26, FMI-CONT-26 Julkaisuaika Lokakuu 1999 Projektin nimi Aki Virkkula Toimeksiantaja Nimeke Troposfäärin luonnollisten ja antropogeenisten aerosolien fysikaalisten ja kemiallisten ominaisuuksien kenttä- ja laboratoriotutkimuksia Tiivistelmä Troposfäärin luonnollisten ja antropogeenisten aerosolien fysikaalisia ja kemiallisia ominaisuuksia on tutkittu kokeellisesti sekä kenttä- että laboratorio-olosuhteissa. Suurin osa kenttämittauksista tehtiin Sevettijärvellä Suomen Lapissa. Sevettijärvellä mitattiin aerosolien lukumääräpitoisuuksia ja sirontakertoimia, tärkeimpien vesiliukoisten ionien, raskasmetallien ja mustan hiilen pitoisuuksia. Ilmanäytteiden alkuperä jaettiin trajektorien mukaan kolmeen pääluokkaan: puhtaaseen merelliseen ilmaan, mannerilmaan ja Kuolan niemimaalta saapuvaan erittäin saastuneeseen ilmaan. Aerosolien raskasmetallipitoisuuksia verrattiin lumesta havaittuihin pitoisuuksiin, ja lähdealueiden osuutta laskeumaan arvioitiin. Muut kenttämittaukset tehtiin Atlantin Valtamerellä ja Teneriffalla. Näiden mittausten aineisto jaettiin Pohjois-Amerikan ja Euroopan mantereilta saapuvaan saastuneeseen ilmaan ja puhtaaseen merelliseen ilmaan. Näissä mittauksissa tutkittiin aerosolien lukumääräkokojakaumia, tärkeimpien vesiliukoisten ionien, maaperäpölyn, orgaanisen ja mustan hiilen pitoisuuksia. Vapaan troposfäärin merkitystä rajakerroksen aerosolikokojakauman ja kemiallisen koostumuksen selittäjänä käsiteltiin. Tärkeimpien yhdisteiden osuudet hiukkasmassaan laskettiin. Vertailu osoitti, että eimerisuolasulfaatin osuus pienten hiukkasten massasta oli lähes sama Lapissa ja Pohjois-Atlantilla, noin 30% puhtaassa ja noin 50% saastuneessa ilmassa. Laboratoriomittauksissa tutkittiin biogeenisten sekundääristen aerosolien muodostumista, hygroskooppisia ominaisuuksia, sekä niiden sekoittumista epäorgaanisten hiukkasten kanssa. Aineiston analyysi osoitti, että hiukkasten epäorgaaninen ja orgaaninen osuus absorboivat vettä itsenäisesti. Julkaisijayksikkö Ilmanlaatu Luokitus (UDK) , , , , ISSN ja avainnimeke Kieli Asiasanat aerosolit, arktiset aerosolit, merelliset aerosolit, orgaaniset aerosolit, Kuolan niemimaa, kaukokulkeutuminen Finnish Meteorological Institute Contributions ISBN englanti Myynti Sivumäärä Hinta Ilmatieteen laitos, kirjasto 178 FIM 150 PL 503, Helsinki Lisätietoja
4 7 Preface Experimental atmospheric science is often done in measurement campaigns. Such a campaign consists of numerous phases, for instance planning what, where, when, and how to measure, installing sampling instrumentation, preparing filters, sampling, keeping various monitors running, chemical analyses of samples, calculating the origin of the air that was sampled, data analysis, and finally publishing the results. It is clear that all this work cannot be done by one person alone, it is done by groups of various sizes. I've been a member of such teams and I want to thank everyone I've been working with in these projects, although it is not possible to mention all their names in a short preface. Most of the papers of this thesis were written on the measurements carried out by the aerosol research group of the Finnish Meteorological Institute (FMI) at the Sevettijärvi measurement station. First, I want to thank my supervisor at the FMI, Dr. Risto Hillamo, the head of the aerosol research group, for providing me a very interesting job and for introducing me to the field of atmospheric aerosol science. I thank my colleagues Mr. Timo Mäkelä, Dr. Veli-Matti Kerminen, Dr. Tuomo Pakkanen, and Mr. Jukka Kiiski for sharing their experience in their own special fields. Mrs. Minna Aurela of FMI deserves a special acknowledgment for doing most of the chemical analyses and filter handling. I want to thank Mr. Mika Salonoja and Mr. Ilkka Valkama for teaching me to use their trajectory model. It was essential in all the source analyses of the Sevettijärvi measurements. As far as the source analyses are concerned, I am grateful for the cooperation with Dr. Andreas Stohl from University of Munich, Germany. I want to thank Eero-Pekka Aikio, Toini Sanila, and her daughters for taking good care of the measurement station at Sevettijärvi and also for their hospitality and delicious reindeer meals. Once I was carrying a pile of publications I had planned to read at their home during some measurement mission. Eero-Pekka looked at the pile and said: "So much paper made of air". That extraordinary process, making paper out of air, continues in this thesis. The other papers of this thesis were written on the measurements of the aerosol research group of the Environment Institute of Joint Research Institute (EI- JRC), Ispra, Italy. I am greatful to Prof. Markku Kulmala of University of Helsinki and Dr. Yrjö Viisanen of FMI who provided me information of the free PhD position at EI-JRC in spring I want to thank especially warmly doctors Frank Raes and Rita van Dingenen, my supervisors at EI-JRC, for taking me to their group and letting me participate in various very interesting, educational, and unique air research projects in Spain, Italy, and France. And I want to thank just as warmly doctors Jean-
5 8 Philippe Putaud and Monica Mangoni, and the whole aerosol research group and the people I worked with in the EI-JRC Atmospheric Processes unit for providing me a very stimulating and fruitful environment. My work that involved measurements at Sevettijärvi was financed by the Finnish Ministry of Agriculture and Forestry as part of the Lapland Forest Damage Project. My work at Joint Research Centre was financed by PhD grants from the European Commission and from the Vilho, Yrjö, and Kalle Väisälä Foundation, Helsinki, Finland. The financial support of the Academy of Finland during the last year of this work is also acknowledged. The data that is discussed in the papers of this thesis required several months of field work. During this time my wife Anita alone took care of our two children, Pyry and Roosa, and her own work, without any possibilities to adventurous missions like mine. Alone it was tough even to deal with the most essential tasks. Therefore I am forever deeply indebted to her for her patience and endurance. Varjele Luoja ihmisen lasta, ilmamiehen vaimoksi joutumasta. October 1999 Aki Virkkula
6 9 CONTENTS List of publications Introduction Important physical and chemical properties of atmospheric aerosol Size distributions Fine and coarse particles Nucleation mode particles Aitken mode particles Accumulation mode particles and CCN Chemical composition Optical properties Hygroscopic properties Earlier studies in fields relevant to the thesis Arctic aerosol Aerosol over North Atlantic Aerosol in forests: biogenic secondary organic aerosol Hygroscopic properties of organic aerosol Work conducted in the thesis Overview Measurement sites Measurement methods Physical properties Chemical composition Data analysis Results and discussion Summaries of the papers Comparison of aerosol composition in Lapland and North Atlantic Summary References 42 Original publications
7 10 List of publications The thesis consists of an introductory review part, followed by the following seven original research publications. 1. Virkkula A., Mäkinen M., Hillamo R., and Stohl A. (1995) Atmospheric aerosol in the Finnish Arctic: particle number concentrations, chemical characteristics, and source analysis, Water, Air, and Soil Pollution 85, Virkkula A., Hillamo R. E., Kerminen V.-M., and Stohl A. (1997) The influence of Kola Peninsula, continental European and marine sources on the number concentrations and scattering coefficients of the atmospheric aerosol in Finnish Lapland, Boreal Env. Res. 2(4), Kerminen V.-M., Aurela M., Hillamo R. and Virkkula A. (1997) Formation of particulate MSA - deductions from size distribution measurements in the Finnish Arctic, Tellus 49B, Virkkula A, Aurela M., Hillamo R., Mäkelä T., Kerminen V.-M., Maenhaut W., Francois F, and Cafmayer J. (1999) Chemical composition of atmospheric aerosol in the European sub-arctic: Contribution of the Kola Peninsula smelter areas, Central Europe and the Arctic Ocean, J. Geophys Res., in print. 5. Van Dingenen R., Raes F., Putaud J-P., Virkkula A., Mangoni M. (1999) Processes determining the relationship between aerosol number and non-sea-salt sulfate mass concentration in the clean and perturbed marine boundary layer. J. Geophys Res. 104, Putaud J.-P., Van Dingenen R., Mangoni M., Virkkula A., Raes F., Maring H., Prospero J.M., Swietlicki E., Berg O.H., Hillamo R., and Mäkelä T. (1999) Chemical mass closure and assessment of the origin of the submicron aerosol in the marine boundary layer and the free troposphere at Tenerife during ACE-2, Tellus, in print. 7. Virkkula A., Van Dingenen R., Raes F., Hjorth J. (1999) Hygroscopic properties of aerosol formed by oxidation of limonene, -pinene and -pinene, J.Geophys.Res. 104,
8 11 1. Introduction Aerosols are suspensions of solid or liquid particles in gas, in this work in air. There are a number of reasons why it is important to characterize aerosols physically and chemically. Atmospheric aerosols are a small but an important part of the atmosphere so knowledge of the formation, processes, composition, and properties of the aerosols increases our basic understanding of the physics and chemistry of the atmosphere. Aerosols have various effects that vary geographically from local to global. For instance, aerosol effects on human health are highest close to the emission sources, e.g., traffic. The health effects depend on the size distribution, concentration and chemical composition of the aerosols (Dockery et al., 1993). Aerosol deposition onto the ground and subsequent detrimental effects on the vegetation are generally significant up to some tens of kilometers away from the sources, for instance industry. Also deposition depends on the size and other properties of the particles, for instance their hygroscopic properties. The most wide-ranging are the effects of aerosols to the atmospheric chemistry and global climate. Aerosols affect the heat balance of the earth both directly by reflecting and absorbing solar radiation and by absorbing and emitting some terrestrial infrared radiation and indirectly by influencing the properties and processes of clouds and, possibly, by changing the heterogeneous chemistry of reactive greenhouse gases, e.g., ozone (Charlson and Heintzenberg, 1994). Observations and modelling the climatic effect of aerosols have increased after the recognition of their role in the climate change (e.g., Charlson et al., 1991 and 1992). Aerosol effects on atmospheric radiation are a leading source of uncertainty in predicting climate change (IPCC 1994 and 1995). Evaluation of the direct and indirect climatic effects of aerosols requires knowledge of the spatial distribution of the particles, their optical and hygroscopic properties, radiative transfer, cloud physics, chemical process, physical transformation, and transport models (Ogren, 1994). Validating these various models requires observations of the aerosol particles. The observations can be conducted either remotely, e.g. using laser remote sensing or satellites, or as point measurements, e.g., filter sampling and size distribution measurements. Large international projects have been conducted lately, e.g., ACE-1 and ACE-2 (Aerosol Characterization Experiment), AOE-96 (Arctic Ocean Expedition), TARFOX (Tropospheric Aerosol Radiative Frocing Observational Experiment), and BIOFOR (Biogenic aerosol formation in the boreal forest), all of which have produced large amounts of data that help to reduce the uncertainty of the climatic effects of atmospheric aerosols.
9 12 If the atmospheric aerosol is emitted directly as particles it is called primary, and if it is formed in the atmosphere after gas-to-particle conversion processes it is called secondary. The vast majority of aerosols found in the global atmosphere are secondary (Clement and Ford, 1999). Examples of gas-to-particle conversion are the formation of sulfate aerosol via the oxidation of SO 2, formation of methanesulfonic acid (MSA) aerosol via the oxidation of dimethylsulfide (DMS), formation of ammonium nitrate aerosol through a reaction of gaseous ammonia and nitric acid, and formation of organic aerosol by oxidation of some volatile organic compounds. In the atmosphere gas and aerosol phases are in continuous interaction, including heterogeneous nucleation, condensation and heterogeneous chemical reactions, such as the release of chlorine from sea-salt particles when they react with gas-phase nitric acid or sulfuric acid and the heterogeneous reactions on particles of polar stratospheric clouds which lead to ozone depletion. Particles injected into or formed in the atmosphere are grown and modified by condensation, coagulation, and cloud processes. After being transported by winds for a few days they finally get removed from the atmosphere by either dry or wet deposition. Figure 1 outlines the main stages of the life cycle and the climatic role of tropospheric aerosol. It also shows the papers of the thesis where the respective subjects are discussed. Most of the aerosol life cycle phases were touched in some degree of depth in the papers. Figure 1. The major sources, life phases and a description of the climatic role of tropospheric aerosol. The numbers in parentheses refer to the papers of the thesis where the respective subjects are discussed. (SOA = secondary organic aerosol, PBAP = primary biological aerosol particles)
10 13 The atmosphere can be divided into various layers. The troposphere is the layer where most of the weather phenomena take place, and it extends from the Earth's surface to about 10 to 15 km altitude. The troposphere can further be divided in boundary layer (BL) and the free troposphere (FT). The BL is that portion of the atmosphere in which the flow field is strongly influenced directly by interaction by interaction with the surface of the earth (Holton, 1992). The height of the BL may range from 30 to 3000 m, depending on the stability conditions, but for average midlatitude conditions it is roughly 1000 m (Holton, 1992). Over the sea the BL is called the marine boundary layer (MBL). The majority of this work, 6 papers, is based on field measurements in the BL, two of them also contains measurements in the FT. One paper is based on a series of laboratory experiments. They were designed for studying particles formed naturally in forests, so also the last paper discusses tropospheric aerosol. The sources of tropospheric aerosols are both natural and anthropogenic. The natural aerosol originates from four major sources: the sea (sea salt particles, methanesulphonic acid (MSA)), vegetation (primary biological aerosol particles (PBAP), secondary organic aerosol (SOA), biomass burning), soil dust (e.g., Saharan dust), and volcanic aerosol. Marine particles, soil dust, and biogenic SOA has been sampled in all field campaigns; SOA was studied in the laboratory experiments as well. The main sources of anthropogenic primary particles and gaseous precursors of aerosols are industrial emissions (sulfate, soot, trace metals), traffic (soot, sulfate, nitrate), agriculture (ammonium), and biomass burning (organic and elemental carbon). Anthropogenic aerosol was sampled in all the field campaigns. The aim of this work is to characterize the chemical and physical properties of tropospheric aerosols, when they are observed reasonably far from anthropogenic emissions, and to assess the contribution of natural and anthropogenic sources. The work proceeds from regional problematics, the influence of Kola peninsula industrial emissions on the aerosols in northernmost Europe, to the influence of anthropogenic sources on the aerosols over North Atlantic. Most of the field measurements published in the papers of this thesis were conducted at a site in Finnish Lapland. Although measured at one point, the coverage of the measurements can be considered larger than that because all the measurements were analyzed for geographical source areas using back-trajectories and statistical analyses based on these. In two papers the measurements were done over the Atlantic Ocean on locations exposed to various degrees of anthropogenic influence. This expands the geographic range of the measurements further than the back-trajectories for the site in
11 14 Lapland. The laboratory work discusses aerosol formed in forests, so the geographical extent and applicability of these measurements is also wide. 2. Important physical and chemical properties of atmospheric aerosol For a full description of an atmospheric aerosol system, information is required on various chemical, physical, and optical properties, as well as on various aerosol processes, such as aerosol formation, growth, and removal. Aerosol cycle parameters, e.g. emission rates, residence times, and removal rates by deposition, describe the atmospheric cycle of a species, including sources, atmospheric transport and transformations, and removal processes (Ogren, 1994). The parameters describing the microphysical, optical, and chemical properties of an aerosol are called state parameters. They can be divided into two groups, extensive and intensive state parameters (Ogren, 1994). Extensive state parameters depend upon aerosol concentration, for instance number and mass concentration, number and mass size distribution, and scattering coefficient. Intensive state parameters are independent of the amount of aerosol present, for instance geometric mean diameter and standard deviation. In general the production source reflects the chemical composition, while the production mechanism determines the size distribution and the shape of the particles (d'almeida et al., 1991). However, chemical and physical properties are closely linked to each other. 2.1 Size distributions Fine and coarse particles Typical number and mass size distributions of atmospheric aerosol are shown in Figure 2. Also cloud droplet distributions are included in the figure to illustrate the size range of aerosols. The atmospheric aerosol is typically classified into two size categories, fine and coarse particles, according to the particle diameter (D p ). There is usually a minimum in the size distribution in the range from 1 to 3 m. However, the shape of the distribution is not constant so setting a limiting diameter is essentially a matter of conventions. For instance, 2.5 m is often used as the division between fine and coarse particles as far as health effects are concerned (Dockery et al. 1993; Friedlander and Lippman, 1994). In the climate-related aerosol research the division between fine and coarse particles is often at 1 m. Coarse particles are generally primary and include soil dust produced by wind, bubble bursting from oceans, and primary biological particles such as algae, spores of
12 15 lichen, mosses, ferns and fungi and pollen. The origin of fine particles is mainly secondary, both natural and anthropogenic. The atmospheric lifetime of coarse and fine particles ranges from some hours to days and from days to weeks, respectively (Seinfeld and Pandis, 1998). In this time the coarse particles generally travel less than some tens of kilometers whereas the fine particles may travel from hundreds to thousands of kilometers (Seinfeld and Pandis, 1998). Occasionally, however, even coarse particles may travel thousands of kilometers: Saharan dust has often been observed on the west side of the Atlantic Ocean (e.g., Schütz et al., 1981; Arimoto et al., 1995). Cloud droplets log n (arbitrary units) log m (arbitrary units) Aitken mode Nucleation mode Accumulation mode Coarse particle mode Mass size distribution Number size distribution Cloud droplets D p (µm) Figure 2. Illustrative aerosol size distributions and associated terminology. Cloud droplets are not considered aerosols, they are included in the figure for illustrative reasons. The submicron aerosol is often classified into three modes: a nucleation mode below 10 nm, a peak called the Aitken mode around nm, and an accumulation mode around nm, in number size distributions. All these modes are usually not present simultaneously (Mäkelä et al., 1997). In mass size distributions the corresponding positions of the modes are towards larger particle diameters, the accumulation mode peak being typically around 300 to 500 nm. The origin of the various submicron modes is a complicated issue involving gas, solid, and liquid-phase processes. Here a short and simplified review is given, for a more thorough discussion see, e.g., Friedlander (1977), Raes et al. (1994) or Seinfeld and Pandis (1998).
13 Nucleation mode particles The nucleation-mode particles originate from homogeneous nucleation of precursor gases. Homogeneous nucleation is the formation of new thermodynamically stable particles from condensation of gas-phase species (Weber et al., 1999). The majority of particles found in the atmosphere are produced by nucleation from the vapour phase (Clement and Ford, 1999), such as gas phase sulfuric acid and water or some organics formed by oxidation of monoterpenes. In the atmosphere nucleation mode aerosol has been observed in various regions, for instance by Covert et al. (1996) in the Arctic marine boundary layer, Mäkelä et al. (1997) in a forest, and Weber et al. (1997) at a remote continental site in the Rocky Mountains. Weber et al. (1999) observed new particle formation by homogenous nucleation over a wide range of conditions, although exclusively in regions of enchanced sulfuric acid vapor concentrations, in the remote troposphere Aitken mode particles Part of the Aitken mode particles are primary (e.g. Hildemann et al., 1991; Kerminen et al., 1997), part of them secondary. The secondary Aitken mode particles are formed from nucleation mode particles by condensation and coagulation. The relative importance of these processes depends on conditions: in polluted air coagulation is the major process (Raes et al., 1994) whereas in background air condensational growth is dominant (Kerminen and Wexler, 1995 and 1997). Mäkelä et al. (1997) and Kulmala et al. (1998) observed that freshly formed nucleation mode particles grew to Aitken mode in 10 hours at a forest site. Using aerosol dynamical model simulations, Kulmala et al. (1998) explained this growth by diffusion/kinetically limited condensational growth Accumulation mode particles and CCN The accumulation mode particles are the most important ones in terms of radiative forcing (Raes et al, 1994). Their lifetime is longer than that of both smaller and larger particles and they can travel very long distances, thousands of kilometers. Part of the accumulation mode particles are primary (e.g. Hildemann et al., 1991; Kerminen et al., 1997), part of them secondary. The latter have grown from smaller particles mainly by condensation and cloud processing. Especially in the clean marine boundary layer the origin of the accumulation mode and the bimodality of the size distribution in the nm range is explained by cloud processing (Hoppel et al., 1990). During cloud formation RH rises above
14 17 100%, and part of the particles start growing very rapidly and become cloud droplets. This process is called activation of particles. The humidity around 100% can be expressed as supersaturation, s = RH - 100% (Rogers and Yau, 1989). The particles that become activated at a supersaturation s are called cloud condensation nuclei (CCN(s)) for this supersaturation (Seinfeld and Pandis, 1998). In the cloud, soluble gases, e.g. nitric acid, ammonia, and sulfur dioxide, dissolve into the droplets and react in the water: for instance, SO 2 oxidizes into SO 2-4. If the cloud is a nonprecipitating one, water in the droplets ultimately evaporates but the formed sulfate remains. The remaining particles are larger than the original CCN, now forming the accumulation mode of the size distribution. The above discussion applies mainly to clean regions, generally in the marine boundary layer far from pollution sources. In polluted conditions, various sources can sustain high concentrations of particles, and coagulation plays a more important role in particle growth (Raes et al., 1994). Cloud processing also occurs in polluted regions; however, the existence of particles with various chemical compositions leads to various critical sizes which obscures the separation between the Aitken and the accumulation modes (Raes et al., 1994). Therefore in polluted conditions often only one wide accumulation mode is present. To estimate the indirect radiative forcing by aerosols it is necessary to have information of the number of particles that can be activated in clouds. The amount of CCN depends on supersaturation, and on the size and chemical composition of the particles. The size and chemical composition of the particles determine their ability to take up water. The number of CCN can be measured with a CCN counter by exposing air to known supersaturations and then counting the concentration of droplets that are formed (Hudson, 1993). However, even without a CCN counter the number of CCN can be estimated from the aerosol number size distribution and chemical composition. Hoppel et al. (1990) suggested that particles larger than 80nm in diameter are potential CCN at the supersaturations prevailing usually in marine boudary layer stratus clouds. This counting method is used in paper 6 of this thesis Chemical composition The main consituents of atmospheric aerosol are sea salt, sulfate, ammonium, nitrate, organic and black carbon, crustal components, and water. In addition to these, small amounts of various metals are found in the particles. The chemical composition of the aerosol depends on the source. Therefore various elements can be used as tracers for defining the source of the air masses.
15 18 The major components of natural marine aerosols are sea-salt and biogenic sulfur compounds, mainly sulfuric and methanesulfonic acid (MSA). In the sea-salt aerosol the main constituents Cl -, Na +, SO 2-4, Mg 2+, Ca 2+, K +, and Br -, are mainly found in the same ratios as in the sea water. In atmospheric aerosol Na is mainly coming from sea salt, so the sea-salt concentration of aerosol is usually calculated from Na measured (1+ R ss i ) 3.256Na measured, where R ss i = X i /Na is the mass ratio of concentration of element X i to Na concentration in the average seawater (e.g., Riley and Chester, 1971; Seinfeld and Pandis, 1998). The contribution of non-sea-salt compounds to the sampled aerosol is usually calculated using the ratios R ss i. For instance, the contribution of non-sea-salt sulfur (nss S) is S measured Na measured. The elements found in Earth's crust often have ratios that don't vary significantly around the world. Therefore, ratios of elements to some reference element, e.g., Al, have been proposed as a method to determine the contribution of sources other than crustal material in aerosol (e.g., Rahn, 1976 and 1999; Hoff et al., 1983). Anthropogenic emissions consist of mainly of sulfates, nitrates, ammonium, carbonaceous particles, and trace elements. The carbonaceous fraction of the aerosol consists of elemental carbon (EC), also called black carbon (BC), and organic carbon (OC). BC is an indicator of combustion and thus a tracer for anthropogenic pollution. The ratios of trace elements are often characteristic for certain regions, and these ratios can be used for recognizing the sources and their contributions to the aerosol at a measurement point Optical properties An example of the links between the chemical and physical properties are the aerosol optical properties. They depend on the particle size and chemical composition. The latter usually varies with size and so extinction coefficient is calculated from 2 ext ( ) ) DpQEXT (, DP, m) n( Dp, m ddpdm 4 (1) where D p is the diameter of the particle, Q EXT is the extinction efficiency of the particle, is the wavelength of light, m is the complex index of refraction, and n is the size distribution of the aerosol. Q EXT is calculated from Mie theory and it is a sum of scattering efficiency and absorption efficiency. The refractive index m depends on the chemical composition of the particles, including the amount of water in them. On the other hand, the size and chemical composition of a particle determine water uptake as a function of relative humidity (RH). Generally the size of a particle increases with increasing humidity either in a monotonic or in a step-function manner even at relative humidities below 100%. There has been a lot of research on the relationship between
16 19 RH and aerosol light scattering, both experimental (e.g. Covert et al, 1972; Charlson et al, 1984; Kotchenruther et al., 1999) and theoretical (Khvorostyanov and Curry, 1999a and 1999b and references therein). The aerosol extinction cross section may increase by 2-10 times over that for the "dry" aerosol over the relative humidity range 30-95%, depending on aerosol composition (e.g., Fitzgerald et al. 1982; Khvorostyanov and Curry, 1999b). 2.4 Hygroscopic properties The above discussions on CCN and light scattering by aerosol show that hygroscopicity is one of the key properties of atmospheric aerosol. The response of the particle size to changes in ambient relative humidity influences the role of atmospheric aerosol in climate change, air quality, acid deposition, biogeochemical cycles, visibility reduction, and cloud and precipitation processes (Khvorostyanov and Curry, 1999a). Kulmala et al. (1996a) demonstrated by model simulations that there is a clear effect of hygroscopicity on the activated fraction of aerosol particles. Thus, hygroscopicity of pre-existing aerosol particles influences the optical thickness and the reflectance of clouds. Pilinis et al. (1995) stated that the single most important parameter in determining direct aerosol forcing is relative humidity, and that the most important process is the increase of the aerosol mass as a result of water uptake. Uncertainties in the knowledge of aerosol hygroscopic growth is also one of the main uncertainties in estimating the aerosol direct radiative forcing (Khvorostyanov and Curry, 1999b). 3. Earlier studies in fields relevant to the thesis 3.1 Arctic aerosol The sensitivity of the Arctic to perturbations from outside and the delicate balance between its physical, chemical, and ecological components makes it an 'early warning system' for global change, in which the signs of climate change are expected to occur first (Roederer, 1991). Pacyna (1995) reviewed various studies on the sources, transport routes, physical characteristics, and on the chemical composition of Arctic air pollution. The Arctic air pollution originates from various sources in Europe, Russia, and North America. On the European Arctic the Kola Peninsula industrial areas Nikel, Zapolyarnyj, and Monchegorsk are by far the largest source of sulphur and heavy metal aerosols (Tuovinen et al., 1993; Pacyna, 1995). The Arctic air pollution has a clear seasonal cycle, with the highest aerosol concentrations measured in
17 20 winter and spring and the lowest concentrations in summer (e.g. Barrie, 1986 and Bodhaine, 1989). The reasons for this seasonal cycle are a result of a combination of a seasonal variability in the long-range transport of air, in the atmospheric blocking phenomenon, in the pollutant removal processes, and in the thickness of surface temperature inversions (Pacyna, 1995 and references therein). In summer the Arctic is separated from anthropogenic air pollution (Leck and Persson, 1996). In the Arctic, ground-based measurements of atmospheric aerosols have been conducted in many locations including, eg., Alert and Barrow in Alaska (e.g., Raatz and Shaw, 1984; Barrie et al., 1989; Bodhaine, 1989; Djupström et al., 1993), Greenland (e.g., Hillamo et al., 1993; Jaffrezo and Davidson, 1993), and Ny Ålesund in Spitsbergen (e.g., Heintzenberg et al., 1981; Pacyna et al., 1985; Heintzenberg and Leck, 1994; Beine et al., 1996). Airborne aerosol measurements over European Arctic have been conducted, e.g., by Ottar et al. (1986) and Heintzenberg et al. (1991). Large campaigns have been conducted on cruises over the Arctic Ocean. The aim of the atmospheric chemistry programs of the International Arctic Ocean Expedition 1991 (IAOE-91) and the Arctic Ocean Expedition 1996 (AOE-96) was to study the sulfur aerosol and its effect on clouds and regional climate (Leck et al., 1996 and 1998). Maenhaut et al. (1996a) determined the elemental composition and sources of aerosol during IAOE-91. To apportion the measured aerosol concentrations to their sources, receptor models are often used (Hopke 1991 and references therein). For instance, chemical mass balance (CMB), principal component analysis (PCA), positive matrix factorization (PMF) (Paatero and Tapper, 1994), and Multilinear Engine (ME) (Paatero, 1998) have been used for source analysis of Arctic aerosol. Maenhaut et al. (1989) investigated trace element composition of aerosol in the Norwegian Arctic and applied absolute principal component analyses (APCA) for source determination. PMF and ME were applied to data from Alert (Xie et al, 1999a and 1999b). Air-mass back-trajectories have been used to identify source areas of air pollutants using various methods. A comprehensive bibliography of various statistical analyses of trajectories is given by Stohl (1998). One method is the potential source contribution function (PSCF). The PSCF can be interpreted as a conditional probability describing the spatial distribution of probable geographical source locations inferred by using trajectories arriving at the sampling site (e.g., Xie et al., 1999a). PSCF analysis has been used, e.g., by Cheng et al. (1993), Hopke et al. (1995), and Xie et al. (1999a) for the identification of possible sources and preferred pathways of various aerosol species to the high Arctic. For instance, Hopke et al. (1995) found that in winter the source areas of anthropogenic sulfate aerosol measured
18 21 at Alert, Canada, are in Europe, eastern Canada, east coast of North America and Urals in Russia. On the European Arctic the Kola Peninsula industrial areas are by far the largest source of sulphur aerosols. Most sulphur emissions north of the Arctic Circle originate from two regions: the Norilsk area in Siberia (2.2 Tg yr -1 of SO 2 ) and the Kola Peninsula (0.6 Tg yr -1 of SO 2 ) (Tuovinen et al., 1993). The sources in the Kola Peninsula contribute almost 20 % to the global anthropogenic sulphur emissions north of 60 N. The SO 2 emissions from the copper-nickel smelters in Nikel and Zapolyarnyj in the Kola Peninsula, close to the Norwegian border, amount to 0.26 Tg yr -1, which is twice the amount emitted from the whole of Finland (Tuovinen et al., 1993). The Norwegian Institute for Air Research (NILU) has conducted an extensive campaign on measuring the dispersion of air pollutants from Nikel and Zapolyarnyj (Sivertsen et al., 1992) and as a result of this campaign Hagen et al. (1996) presented an exhaustive data set of heavy metal concentrations in atmospheric aerosol in the border areas of Norway and Russia in Deposition of heavy metals around the Kola has been investigated, e.g., by Jaffe et al. (1995), Äyräs et al. (1996), De Caritat et al. (1997), and Chekushin et al. (1998). On the European mainland, the Arctic includes the northern parts of Norway, Sweden and Finland, and the northwestern parts of Russia. In these regions, continuous ground-based aerosol measurements have been carried out as part of existing monitoring programs (e.g., Cooperative Programme for Monitoring and Evaluation of the Long-range Transmission of Air Pollutants in Europe (EMEP)), and consist mainly of filter sampling and subsequent chemical analyses. Thus, especially data on particle number concentrations and scattering coefficients from the continental European Arctic were scarce before 1990's, when two measurement stations, dedicated for aerosol measurements, were founded in eastern Finnish Lapland, one at Värriö, another at Sevettijärvi. The Värriö environmental measurement station (Hari et al., 1994) is located in eastern Lapland, approximately 200 km south of Sevettijärvi. Kulmala et al. (1996b) examined the formation, growth and properties of atmospheric aerosol particles and cloud droplets in the Finnish Arctic, both by conducting field experiments at Värriö and Sevettijärvi and by modelling. The total aerosol number concentration (D p > 14 nm) showed a clear maximum during summer and spring, and a minimum during the winter (Ahonen et al., 1997). Low aerosol concentrations combined with high SO 2 concentration typically occurred during winter or autumn. Pirjola et al. (1998) presented aerosol size distributions measured using a diffusion battery and a CPC at Värriö. Numerous nucleation bursts were observed, some of which were associated with clean marine air and others with SO 2
19 22 pollution plumes from the Kola peninsula smelters. Pirjola et al. (1998) observed high SO 2 concentrations without nucleation mode particles during the darkest winter months. High nucleation mode concentrations were associated with high SO 2 concentrations when there was enough sunlight. The events of high concentrations of nucleation mode particles with SO 2 concentrations below detection limit were associated with marine air masses (Pirjola et al., 1998). 3.2 Aerosol over North Atlantic The oceans cover approximately two thirds of the surface of the earth. Therefore determining the physical and chemical characteristics, and the geographical distribution of aerosols over the oceans is essential for estimating both the direct and indirect effect of aerosols on climate. Another important reason for studying marine aerosols is that they play an important role in the sulfur cycle. The primary source of biogenic sulfur are the oceanic emissions of dimethylsulfide (DMS) to the atmosphere (Andreae and Raemdonck, 1983). Atmospheric DMS is oxidized rapidly to other compounds, including SO 2, sulfuric acid, methanesulfonic acid (MSA), dimethylsulfoxide (DMSO), and dimethylsulfone (DMSO 2 ). For MSA the only known source is the oxidation of DMS and it has therefore been used for estimating the contribution of biogenic sources to the total sulfur observed in aerosols (Bates et al., 1992; Li and Barrie, 1993; Heintzenberg and Leck, 1994). Fitzgerald (1991) published a comprehensive review paper of the research conducted on marine aerosol up to then. It had been found, e.g. that in the clean background areas particle number concentrations vary in the range of cm -3, that there is a source of small particles which allows particle concentrations recover to their background levels after removal by precipitation, that non-sea-salt sulfate (nsssulfate) is formed by gas-to-particle conversion of the oxidation products of organosulfur gases (mainly DMS), that the submicron particle size distribution is typically bimodal with peaks around 60 nm and 200 nm diameter, and that supermicron particles are composed primarily of sea-salt. Hoppel et al. (1990) suggested that the submicron portion of the aerosols in the remote marine areas maintain a bimodal size distribution by an aqueous phase conversion followed by droplet evaporation in non-precipitating cloud cycling. Such bimodal size distributions in clean marine environments have been observed also in various other measurement campaigns, e.g. by Hoppel et al. (1994), Van Dingenen et al. (1995), and Raes et al. (1997). One of the most influential papers on marine aerosols was written by Charlson et al. (1987). They suggested that cloud condensation nuclei (CCN) produced from
20 23 the oxidation of DMS generated by phytoplankton in seawater are responsible for climate modification in a cycle in which the biota may provide climate-regulating feedback. This is sometimes called the CLAW hypothesis (Charlson, Lovelock, Andreae, Warren) and it has led to increased experimental and theoretical research of production of DMS and formation of aerosols and CCN. The link between DMS and CCN has been confirmed by observations, e.g. by Ayers and Gras (1991) and Hegg et al. (1991). A very important issue remaines unresolved: it is not clear, where the actual particle formation takes place: in the marine boundary layer (MBL), as modelled, e.g., by Kerminen and Wexler (1995), or in the free troposphere (FT) as suggested by Raes (1995). Air over North Atlantic (NA) can be classified roughly into three categories: clean marine background air, polluted continental air from Europe and North America, and dusty air from Sahara. Dust over NA has been a subject of scientific reports dating from the late 1700s (e.g., Dobson, 1781). Various measurement campaigns and cruises have been conducted over NA, a selection of them is discussed here. Hoppel et al (1990) investigated marine aerosol during a transatlantic cruise from South Carolina to Scotland via Canary Islands in March and April The major result of this cruise was the explanation of the bimodal shape of the submicron aerosol size distribution, as explained above. Van Dingenen et al. (1995) measured aerosol chemical composition and size distributions during a cruise over Northern Atlantic from Halifax, Canada to the Moroccan coast and back in September - October The non-sea-salt sulfate concentration was found to be highly correlated with black carbon and accumulation mode particle concentrations. Van Dingenen et al. (1996) conducted shipborne aerosol measurements also in the northsouth direction during a cruise in October-November 1994, crossing the Atlantic between 53 N (Bremerhaven, Germany) and 53 S (Punta Arenas, Chile). The data of Van Dingenen et al. (1995 and 1996) are also included in the data analysis of paper 5 (Van Dingenen et al. 1999) of this thesis. In June 1992 marine stratocumulus clouds, trace gases, and aerosols were examined in the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE) experiment in the vicinity of the Azores islands (Huebert et al, 1996). The experiment had several goals, such as to gain understanding of the various chemical, biological, and physical mechanisms that control the exchange of trace gases and aerosols between the atmosphere and the ocean surface, and to develop formulations of ocean exchange processes for inclusion in global-scale climate and air chemistry models (Huebert et al., 1996). Most of the
21 24 results have been published in a special issue of Journal of Geophysical Research (Vol 101, D2, 1996). TARFOX (Tropospheric Aerosol Radiative Frocing Observational Experiment, Russell et al., 1999) was conducted on the US eastern seaboard, where one of the world major plumes of anthropogenic haze moves from the continent over the Atlantic Ocean. It was found in TARFOX that the carbonaceous compounds and water condensed on aerosol are unexpectedly important, that the mass of carbon in the aerosol was, on average, 50% of the total dry aerosol mass, and that the mass fraction of carbon in the aerosol increased with altitude (Hobbs, 1999 and references therein). Aerosol optical depths were due to three main chemical species which, in order of descending average contributions, were condensed water, carbonaceous materials, and sulfate (Hobbs, 1999 and references therein). A lot of the TARFOX results have been published in a special issue of Journal of Geophysical Research (Vol 104, D2, 1999). Arimoto et al. (1995) determined the concentrations of trace elements in atmospheric aerosol over the North Atlantic as part of a program designed to characterize the chemical climatology of the region. The measurements were part of the Atmosphere-Ocean Chemistry Experiment (AEROCE). The samples were collected at Bermuda, Barbados, Mace Head (Ireland), and Izaña observatory (Tenerife, Canary islands) in One major component of the aerosol was atmospheric dust, represented by aluminium, transported from Sahara. The impact of pollution sources on trace element concentrations was evident at all sites but varied with season and location (Arimoto et al., 1995). Raes et al. (1997) presented observations of aerosols the FT and MBL at two sites on Tenerife in July 1994 and discussed the processes determining the size distribution. It was found that in the FT the submicron aerosol distribution was predominantly monomodal with a geometric mean diameter of 120 nm and 55 nm during dusty and clean conditions, respectively. The FT data suggested that during clean conditions the origin of the aerosol is in the upper troposphere. The MBL aerosol size distributions were bimodal as in other MBL measurements. Raes et al. (1997) did not observe any nucleation bursts in the MBL. 3.3 Aerosol in forests: biogenic secondary organic aerosol Natural organic aerosols in the forests are either primary biological aerosol particles (PBAP): algae, spores of lichen, mosses, ferns and fungi (D >~ 1 µm) and pollen (D >~ 10 µm) (Matthias-Maser et al, 1996), or secondary organic aerosols (SOA), formed by the oxidation of biogenic volatile organic compounds (BVOC). Volatile organic compounds (VOCs) denote the entire set, several hundreds of vapor-
22 25 phase atmospheric organics excluding CO and CO 2 (Seinfeld and Pandis, 1998). VOCs are emitted by both anthropogenic and biogenic sources. Globally, the biogenic VOC emissions are much higher than the anthropogenic emissions: emissions of terpenes from vegetation have been estimated to be in the range between 127 and 480 Tg C per year (Guenther et al., 1995), compared to an estimated anthropogenic emission of non-methane hydrocarbons of 90 Tg per year (Hough and Johnson, 1991). SOA formed by the oxidation of VOCs probably accounts for a significant fraction of the aerosol mass in continental air. Already for four decades ago Went (1960) suggested that BVOCs might be important in the formation of tropospheric aerosols. Lately Andreae and Crutzen (1997) estimated that the production rate of secondary organic aerosols is comparable to that of biogenic and anthropogenic sulfate aerosols. However, the total amount of secondary organic aerosol formed from biogenic sources is uncertain: the Intergovernmental Panel on Climate Change (IPCC, 1995) has estimated that the biogenic sources will contribute with between 44 and 225 Tg/year with a best estimate of 65 Tg/year. Formation of SOA has been studied extensively during the past 20 years, both in the laboratory and in the field. Heisler and Friedlander (1977) measured the photochemical particle formation of some organics that contribute to urban smog. McMurry and Grosjean (1985) conducted smog chamber experiments on organics, oxides of nitrogen, and ammonia, and concluded that gas-phase chemical reactions, rather than reactions on or within the particles, were responsible for the formation of aerosol. Hatakeyama et al. (1989) observed 18.3% and 13.8% aerosol yields in the ozonolysis experiments of -pinene and -pinene, respectively. The measured yields of condensed material formed by the atmospheric photo-oxidation of terpenes appear to depend on the amount of reacted hydrocarbon so that higher product concentrations give rise to higher yields of aerosols, as shown e.g. by Odum et al. (1996), Hoffmann et al. (1997), and Griffin et al. (1999) but also the [VOC]/[NOx] ratio seems to be important for the aerosol yields (e.g., Hatakeyama et al. 1991). The chemical nature of the organic particulate material formed from the oxidation of biogenic VOCs is not fully known; complex mixtures of products have been found by laboratory studies (e.g., Palen et al. 1992). Christoffersen et al. (1998) found that cis-pinic acid is a possible precursor for organic aerosols formation from the ozonolysis of -pinene. Griffin et al. (1999) conducted a series chamber experiments to establish and characterize the significant atmospheric aerosol-forming potentials of the most important biogenic VOCs. Exhaustive reference lists of the laboratory work done on SOA are given by, e.g., Seinfeld and Pandis (1998), and Griffin et al. (1999).