Slides partly by Antti Lauri and Hannele Korhonen. Liquid or solid particles suspended in a carrier gas Described by their

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1 Atmospheric Aerosols Slides partly by Antti Lauri and Hannele Korhonen Aerosol particles Liquid or solid particles suspended in a carrier gas Described by their Size Concentration - Number - Surface - Mass - Volume Chemical composition Lifetime in troposphere typically hours days 1

2 The Earth s energy balance Kiehl and Trenberth, 1997 Climate effects of aerosols Direct effects Scattering and absorbing short- and longwave radiation Examples: sulphate, organic carbon, black carbon, aerosols from biomass burning, mineral dust Indirect effects Cloud formation: effectiveness of the aerosol acting as cloud condensation nuclei (CCN) Depend on: size, chemical composition, ambient environment 2

3 Aerosols, clouds and climate Direct effect Indirect (cloud) effect Clean cloud less droplets smaller albedo lower lifetime Polluted cloud more droplets larger albedo longer lifetime (mostly) COOLING Figure by Ari Asmi / EUCAARI Indirect climate effects of aerosols 3

4 Atmospheric Brown Cloud (Pollution) MODIS :29 dundee.ac.uk/ 4

5 Effect on air quality (Helsinki, August 2006) Photos: Pia Anttila, FMI Aerosols and Health Current estimates show that aerosol particles have significant effect on both life expectancy and life quality 5

6 Types of Aerosols Based on their formation processes, aerosols are either primary or secondary: Primary aerosols are directly emitted to the atmosphere. Secondary aerosols are formed in the atmosphere by gas-to-particle conversion processes: Based on their sources, aerosols are either natural or anthropogenic: Natural aerosols are emitted as a result of processes in the nature (windblown dust, pollen, plant fragments, seasalt, seaspray, volcanic emissions) Anthropogenic aerosols are somehow related to human activities (fossil fuel burning, industrial processes, traffic, burning of biomass or biofuel, agricultural activities, etc.) Nanoparticles Gas molecules Small particles CCN Light Cigarette smoke Big particles Fog Concrete dust Rain Sand Gravel Pollen Viruses Bacteria Hair Traffic Road dust Energy production Particle diameter m (1mm) (1 cm) Figure by Mikko Moisio and Ilona Riipinen 6

7 Structures Asbestos Volcanoes In model calculations, the shape of an aerosol particle is assumed spherical In practice, this is not always the case There are several ways of characterizing real particles with a certain diameter so that some of their features correspond to the features of a spherical particle of the given size Electrical mobility Density Coal Wielding Variation of Aerosol Concentrations Concentration varies depending on location and time high concentrations are encountered when there are nearby sources. PN [cm -3 ] PM 10 [g/m 3 ] Lower troposphere: -Urban traffic -Urban background -Rural -Marine -Remote Free troposphere Stratosphere (background) > >

8 Aerosol size distributions The total concentration of atmospheric aerosol particles can vary over 7 orders of magnitude (~10 1 ~10 8 #/cm 3) The size range spans over 5 orders of magnitude (~1 nm ~100 m) Size affects both the lifetime and the physical and chemical properties How to describe the aerosol size and number/area/volume in a simple way? Aerosol size distribution #/cm 3 nucleation Aitken accumulation coarse ~10 1 molec. ~ molec. Marine Remote continental Urban Free troposphere 10nm 100nm 1000nm Diameter From gases primary combustion Dust, sea salt, cloud droplets Figure by Hanna Vehkamäki and Veli-Matti Kerminen 8

9 Lognormal distribution function n N log D It has been observed that atmospheric aerosols can be p described rather well with a set of log-normal distribution functions (log-normal = normally distributed in logarithmic scale) n i1 2 N i exp 2 log i 1/ n N (log D p ): number of particles of diameter D p N i : number concentration of particles in the mode i : geometric standard deviation log D p log D 2log 2 i pi D pi : median diameter of the mode 2 Representations of Aerosol Concentrations Aerosol particle concentrations can be expressed by Number, Surface area, Volume, or Mass per unit volume: The number concentration is (in most cases) dominated by the ultrafine aerosols. The mass or volume concentration is dominated by the coarse and accumulation aerosols. Representation of number and volume aerosol size distributions Figure from Seinfeld & Pandis,

10 Urban aerosol Mixture of primary emissions from industry, transportation, power generation, and natural sources and secondary aerosols through gas-to-particle conversion Number concentration dominated by ultrafine particles Surface area mostly in the m sizes Mass typically has two dominating modes: accumulation and coarse Huge variation depending on the measurement site and current meteorological conditions Typical urban aerosol size distribution Figure from Seinfeld & Pandis, 2006 Rural aerosol Mainly of natural origin, but with some influence of anthropogenic sources Number concentration typically has two dominating modes in the ultrafine size range Surface area mostly in the m sizes Mass dominated by coarse mode Typical rural aerosol size distribution Figure from Seinfeld & Pandis,

11 Remote continental aerosol Mainly natural primary particles including dust, pollen, plant waxes and secondary oxidation products Number concentration typically has two dominating modes (nucleation mode, accumulation mode) Surface area mostly in accumulation mode Mass dominated by accumulation mode Typical remote continental aerosol size distribution. Figure from Seinfeld & Pandis, 2006 Marine aerosol Mostly of marine origin: evaporation of seaspray, seasalt, secondary aerosols formed after oxidation of dimethyl sulfide emitted by phytoplankton Number concentration typically has two dominating modes around 60 nm and 200 nm Mass dominated by coarse mode Marine aerosol size distributions from different measurements and a model distribution. Figures from Seinfeld & Pandis,

12 Free tropospheric aerosol Mid- and upper troposphere above clouds Modes around 10 nm and 250 nm Number concentration of accumulation mode particles typically higher than in the lower troposphere No precipitation scavenging Nucleation mode often present Suitable conditions for new particle formation Typical free tropospheric aerosol size distribution. Figure from Seinfeld & Pandis, 2006 Polar aerosol Very low total concentrations Accumulation mode dominates Arctic haze during the winter and early spring: anthropogenic sources Composition: aged carbonaceous aerosols originated from mid-latitude pollution sources, sulfate, seasalt, mineral dust Typical polar aerosol size distribution. Figure from Seinfeld & Pandis,

13 Desert aerosol Three overlapping modes at 10 nm, 50 nm, and 10 m Surface area and volume strongly dominated by windblown sand Individual dust storms can transfer desert aerosol over the ocean Typical desert aerosol size distribution. Figure from Seinfeld & Pandis, 2006 Parameters for model aerosol distributions 13

14 Processes Modifying Atmospheric Aerosols Processes affecting the concentration and other properties (size, chemical composition) of atmospheric aerosols include: Emissions (primary particles, emissions of aerosol precursor gases) Atmospheric transportation Deposition from the atmosphere to surfaces (ground, vegetation, water) Aerosol dynamics and chemistry Cloud/fog formation RH > 100% Due to cooling (isobaric/adiabatic) Isobaric cooling (pressure remains constant) Radiative losses of energy, horizontal movement of an airmass over a colder land surface or colder airmass Adiabatic cooling (no heat exchange) Ascending air parcel pressure decrease, volume expansion, temperature decrease 14

15 Cloud/fog formation Droplet formation without existing nuclei would require considerable supersaturations e.g. pure water: RH % Aerosol particles that can facilitate droplet formation at low supersaturations are called cloud condensation nuclei (CCN) Cloud droplet composition Liquids Water (solvent) Dissolved compounds, e.g. O 3 (aq), H 2 O 2 (aq) Ions, e.g. SO 4 2-, NH 4 + Solids Soluble compounds, e.g. ammonium sulfate Slightly soluble compounds, e.g. organic acids, calcium sulfate Insoluble compounds, e.g. dust, elemental carbon Condensing gases e.g. H 2 O, HNO 3, NH 3, SO 2, H 2 O 2 15

16 Mixing state of the aerosol Important when determining e.g. CCN solubility or aerosol optical properties External mixture: each particle from only one source Internal mixture: all the particles of a certain size contain a uniform mixture of components from each source nm An example of time evolution of size distribution in Hyytiälä Apr 30.Apr B A new particle formation event New particles appearing in the 3-25 nm size range (A) A Newborn particles growing, sometimes to sizes where they can act as cloud condensation nuclei (B) 16

17 Formation dynamics Pre-existing aerosol Vapor sink (condensation) Particle sink (coagulation) 3 nm Growth by condensation Particle source (nucleation) Vapor source (e.g. organics+, sulphuric acid) Activation to CCN Figure by Miikka Dal Maso and Ilona Riipinen Condensation & Coagulation sinks Condensation sink (CS) describes the aerosol population s ability to remove vapor by condensation 0 CS D r M (r) n(r) dr D i M r N i i Coagulation sink (CoagS(D p )) describes the aerosol population s ability to remove particles of size D p CoagS(D ) Sinks sensitive to particle size changes hygroscopic growth must be accounted for p D p K(D,D ',T,...) n(d ) dd p p p p 17

18 Aerosol Dynamics: What? A way to try to understand nature, in this case the behaviour of aerosol particles Aerosol Dynamics describes formation, growth and transportation of aerosol particles Based on existing theories Basic Aerosol Dynamics suitable for process-level model studies For atmospheric models, parameterisations are needed Aerosol Dynamics: Processes 18

19 Aerosol dynamic processes 1. Primary emissions 2. Nucleation 3. Activation for Growth 4. Condensational Growth 5. Coagulation 6. Cloud Processes 7. Deposition 8. Connection with atmospheric chemistry and meteorology Aerosol dynamic processes 1. Primary emissions 2. Nucleation 3. Activation for Growth 4. Condensational Growth 5. Coagulation 6. Cloud Processes 7. Deposition 8. Connection with atmospheric chemistry and meteorology 19

20 Nucleation Formation of a new phase In the atmosphere: vapour liquid, vapour solid (ice nucleation) Types of nucleation Homogeneous nucleation: No foreign nuclei or surfaces Heterogeneous nucleation: Nucleation on a foreign substance - e.g. nucleation onto surface of aerosol particles Ion induced nucleation: Nucleation on charged particles Number of species One: homomolecular or unary nucleation Two or more: heteromolecular or binary, ternary, nucleation Homogeneous vs. heterogeneous nucleation Figure by Hanna Vehkamäki 20

21 Homogeneous nucleation in the atmosphere formation of new aerosol particles in the atmosphere from gaseous precursors the initial size of these particles is typically of the order of one nanometer in diameter potential nucleation pathways in the atmosphere: binary water-sulphuric acid nucleation (mainly free troposphere) ternary water-sulphuric acid-ammonia nucleation (lower troposphere) ion-induced nucleation nucleation of some organic compounds there are large uncertainties in both calculating and measuring the atmospheric homogeneous nucleation rate Nucleation treatment in atmospheric models Each theoretical approach is way too heavy to be handled in a large-scale model Parameterisations available for some substances Water sulphuric acid Water sulphuric acid ammonia Parameterisations are based on calculations with one or more of the theories available Input: RH, T, concentrations Output: J 21

22 Aerosol dynamic processes 1. Primary emissions 2. Nucleation 3. Activation for Growth 4. Condensational Growth 5. Coagulation 6. Cloud Processes 7. Deposition 8. Connection with atmospheric chemistry and meteorology Heterogeneous nucleation Nucleation on a pre-existing surface (e.g. aerosol particle) Formation of a new phase Occurs more likely than homogeneous nucleation 22

23 The nano-köhler mechanism Example: Freshly-nucleated thermodynamically stable clusters (TSCs), composed of ammonium bisulfate and water, are activated for condensational growth by an water-soluble organic vapour Activation means that the organic vapor starts to condense irreversibly into these particles However, this does not occur before TSCs have reached a certain threshold size! Aerosol dynamic processes 1. Primary emissions 2. Nucleation 3. Activation for Growth 4. Condensational Growth 5. Coagulation 6. Cloud Processes 7. Deposition 8. Connection with atmospheric chemistry and meteorology 23

24 Condensation Condensation: aerosol particle grows in size by taking up vapours from the gas phase The most important mechanism of particle growth in the atmosphere Gas liquid (or gas solid) phase transition: always accompanied with characteristic energy released (condensation) or absorbed (evaporation) mass and heat transfer to/from droplet coupled by latent heat of evaporation (also enthalpy of vaporization) Condensation processes can be modelled by solving appropriate mass and heat transfer equations Condensational growth of atmospheric aerosol particles condensational growth is most effective in the size range < 0.1 µm in the atmosphere, gaseous compounds causing the condensational growth include sulphuric and nitric acid, water, ammonia and numerous organic vapours difficulties in estimating the particle condensational growth in the atmosphere: many compounds responsible for atmospheric condensational growth have not been identified yet the saturation vapour pressures of many condensing compounds are not known accurately (or not at all) 24

25 Aerosol dynamic processes 1. Primary emissions 2. Nucleation 3. Activation for Growth 4. Condensational Growth 5. Coagulation 6. Cloud Processes 7. Deposition 8. Connection with atmospheric chemistry and meteorology Coagulation when two aerosol particles collide with each other in the air, they usually stick together (coagulation) in the atmosphere, coagulation is usually caused by the particle Brownian motion (Brownian coagulation) the influence of Brownian coagulation on the aerosol particle population is relatively easy to calculate (analytical equations) in the atmosphere, the main role of coagulation is to deplete the smallest (D p < 10 nm) aerosol particles (by coagulation into larger particles) 25

26 Definitions coagulation = collision + coalescence agglomeration = collision + sticking (no coalescence) successive agglomeration events result in irregular structures called agglomerates What causes coagulation? Gravitation Shear Brownian motion Turbulence shear In coagulation/agglomeration, the number concentration decreases and the mean size increases 26

27 Aerosol dynamic processes 1. Primary emissions 2. Nucleation 3. Activation for Growth 4. Condensational Growth 5. Coagulation 6. Cloud Processes 7. Deposition 8. Connection with atmospheric chemistry and meteorology Cloud processing when the ambient relative humidity exceeds 100%, a fraction of aerosol particles (D p > nm) activates to form cloud/for droplets with diameters >10 µm activation is a result of water vapour condensation onto the aerosol particles and it occurs within a few minutes in cloud droplets, many chemical reactions take place and new compounds are formed only about 10% of clouds form rain droplets and precipitate; the rest evaporate and release cloudprocessed aerosol particles during cloud evaporation, most of the water and a fraction of other material present in the cloud droplets is transferred into the gas-phase 27

28 Aerosol dynamic processes 1. Primary emissions 2. Nucleation 3. Activation for Growth 4. Condensational Growth 5. Coagulation 6. Cloud Processes 7. Deposition 8. Connection with atmospheric chemistry and meteorology Deposition aerosol particles are removed from the atmosphere by deposition (dry and wet deposition) in dry deposition, particles touch/hit a surface (soil, ground, water) and remain there dry deposition the most important removal pathway for coarse (2.5 µm < D p < 10 µm) and ultrafine (D p < 0.1 µm) particles in wet deposition, particles are removed from the atmosphere by rain or fog wet deposition is the most important removal pathway for fine particles, especially particles in the size range µm 28

29 Aerosol dynamic processes 1. Primary emissions 2. Nucleation 3. Activation for Growth 4. Condensational Growth 5. Coagulation 6. Cloud Processes 7. Deposition 8. Connection with atmospheric chemistry and meteorology Aerosol-phase reactions take place at the surface or inside aerosol particles compared to other aerosol dynamical processes, mechanistic understanding of aerosol-phase reactions is still very poor extremely important for stratospheric chemistry (formation of ozone hole) tropospheric importance largely unknown important for marine sulphur chemistry (SO 2 oxidation in sea salt particles) seems to be important for ageing of secondary organic aerosols indications that takes also place in recently-formed aerosol particles 29

30 Dynamical processes and their effects on size distributions Particle number Particle mass & surface Nucleation Increase Increase, Increase Condensation No effect Increase, Increase Coagulation Decrease No effect, Decrease Deposition Decrease Decrease, Decrease 30