Atmospheric Aerosols - Helsingin yliopisto · Atmospheric Aerosols Slides partly by Antti Lauri and...
Transcript of Atmospheric Aerosols - Helsingin yliopisto · Atmospheric Aerosols Slides partly by Antti Lauri and...
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Atmospheric Aerosols
Slides partly by Antti Lauri and Hannele Korhonen
Aerosol particles
Liquid or solid particles suspended in a carrier gasDescribed by their
SizeConcentration
- Number- Surface- Mass- Volume
Chemical compositionLifetime in troposphere typically hours – days
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The Earth’s energy balance
Kiehl and Trenberth, 1997
Climate effects of aerosols
Direct effectsScattering and absorbing short- and longwave radiationExamples: sulphate, organic carbon, black carbon,aerosols from biomass burning, mineral dust
Indirect effectsCloud formation: effectiveness of the aerosol acting ascloud condensation nuclei (CCN)Depend on: size, chemical composition, ambientenvironment
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Aerosols, clouds and climate
Polluted cloudmore dropletslarger albedolonger lifetime
Clean cloudless dropletssmaller albedolower lifetime
Indirect (cloud) effectDirect effect
(mostly) COOLING17501900 Figure by AriAsmi / EUCAARI
Indirect climate effects of aerosols
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Atmospheric Brown Cloud (Pollution)
MODIS3.5.2006 09:29http://www.sat.dundee.ac.uk/
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Effect on air quality (Helsinki, August 2006)
Photos:
Pia Anttila, FMI
Aerosols and Health
Current estimates showthat aerosol particleshave significant effect onboth life expectancy andlife quality
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Types of Aerosols
Based on their formation processes,aerosols are either primary or secondary:
Primary aerosols are directly emitted to theatmosphere.Secondary aerosols are formed in the atmosphereby gas-to-particle conversion processes:
Based on their sources, aerosols are eithernatural or anthropogenic:
Natural aerosols are emitted as a result ofprocesses in the nature (windblown dust, pollen,plant fragments, seasalt, seaspray, volcanicemissions)Anthropogenic aerosols are somehow related tohuman activities (fossil fuel burning, industrialprocesses, traffic, burning of biomass or biofuel,agricultural activities, etc.)
0.001 0.01 0.1 1 10 100 1000(1mm)
10 000(1 cm)
Hair
VirusesBacteria
Pollen
Cigarette smoke
Concrete dust
Gasmolecules Light
Fog Rain
Particle diameter m
Big particlesSmall particles
Nanoparticles
Sand Gravel
Energy production
Traffic Road dust
Figure byMikko Moisioand Ilona Riipinen
CCN
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Structures
In model calculations,the shape of anaerosol particle isassumed sphericalIn practice, this is notalways the caseThere are severalways ofcharacterizing realparticles with acertain diameter sothat some of theirfeatures correspondto the features of aspherical particle ofthe given size
Electrical mobilityDensity
Asbestos
Coal
Volcanoes
Wielding
Variation of Aerosol Concentrations
Concentration varies depending on location and timehigh concentrations are encountered when there arenearby sources.
1 – 20Stratosphere (background)
100 – 100 000Free troposphere
> 2020 – 100
10 – 505 – 20
0.02 – 1
> 100 00010 000 – 50 000
500 – 10 000200 – 5000
50 – 500
Lower troposphere:
-Urban traffic-Urban background
-Rural-Marine
-Remote
PM10 [ g/m3]PN [cm-3]
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Aerosol size distributions
The total concentration of atmospheric aerosolparticles can vary over 7 orders of magnitude(~101 – ~108 #/cm3)
The size range spans over 5 orders ofmagnitude (~1 nm – ~100 m)
Size affects both the lifetime and the physicaland chemical properties
How to describe the aerosol size andnumber/area/volume in a simple way?
Aerosol size distribution
10nm 100nm 1000nm
#/cm3
Diameter
MarineRemote continentalUrbanFree troposphere
nucleation Aitken accumulation coarse
~101 molec. ~109 -1012 molec.
Figure by Hanna Vehkamäki and Veli-Matti Kerminen
primary combustionFrom gasesDust, sea salt,
cloud droplets
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Lognormal distribution function
It has been observed that atmospheric aerosols can bedescribed rather well with a set of log-normal distributionfunctions (log-normal = normally distributed in logarithmic scale)
n
i i
pip
i
ipN
DDNDn1
2
2
2/1 log2loglog
explog2
log
nN(log Dp): number ofparticles of diameter Dp
Ni: number concentrationof particles in the mode
i: geometric standarddeviation
: median diameter ofthe mode
piD
Representations of Aerosol Concentrations
Aerosol particleconcentrations can beexpressed by Number,Surface area, Volume, orMass per unit volume:The number concentration is(in most cases) dominatedby the ultrafine aerosols.The mass or volumeconcentration is dominatedby the coarse andaccumulation aerosols. Representation of number and volume aerosol size distributions
Figure from Seinfeld & Pandis, 2006
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Urban aerosol
Mixture of primary emissionsfrom industry, transportation,power generation, and naturalsources and secondary aerosolsthrough gas-to-particleconversionNumber concentrationdominated by ultrafine particlesSurface area mostly in the 0.1-0.5 m sizesMass typically has twodominating modes:accumulation and coarseHuge variation depending on themeasurement site and currentmeteorological conditions
Typical urban aerosol size distributionFigure from Seinfeld & Pandis, 2006
Rural aerosol
Typical rural aerosol size distributionFigure from Seinfeld & Pandis, 2006
Mainly of natural origin, but withsome influence of anthropogenicsourcesNumber concentration typicallyhas two dominating modes inthe ultrafine size rangeSurface area mostly in the 0.1-0.5 m sizesMass dominated by coarsemode
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Remote continental aerosol
Typical remote continental aerosol sizedistribution. Figure from Seinfeld & Pandis, 2006
Mainly natural primary particlesincluding dust, pollen, plantwaxes and secondary oxidationproductsNumber concentration typicallyhas two dominating modes(nucleation mode, accumulationmode)Surface area mostly inaccumulation modeMass dominated byaccumulation mode
Marine aerosol
Marine aerosol size distributions from differentmeasurements and a model distribution.Figures from Seinfeld & Pandis, 2006
Mostly of marine origin:evaporation of seaspray,seasalt, secondary aerosolsformed after oxidation ofdimethyl sulfide emitted byphytoplanktonNumber concentration typicallyhas two dominating modesaround 60 nm and 200 nmMass dominated by coarsemode
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Free tropospheric aerosol
Typical free tropospheric aerosol size distribution.Figure from Seinfeld & Pandis, 2006
Mid- and upper troposphereabove cloudsModes around 10 nm and 250nmNumber concentration ofaccumulation mode particlestypically higher than in the lowertroposphere
No precipitation scavengingNucleation mode often present
Suitable conditions for newparticle formation
Polar aerosol
Typical polar aerosol size distribution. Figure fromSeinfeld & Pandis, 2006
Very low total concentrationsAccumulation mode dominates“Arctic haze” during the winterand early spring: anthropogenicsourcesComposition: agedcarbonaceous aerosolsoriginated from mid-latitudepollution sources, sulfate, sea-salt, mineral dust
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Desert aerosol
Typical desert aerosol size distribution.Figure from Seinfeld & Pandis, 2006
Three overlapping modes at 10nm, 50 nm, and 10 mSurface area and volumestrongly dominated bywindblown sandIndividual dust storms cantransfer desert aerosol over theocean
Parameters for model aerosol distributions
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Processes Modifying Atmospheric Aerosols
Processes affecting the concentration and otherproperties (size, chemical composition) ofatmospheric aerosols include:
Emissions (primary particles, emissions of aerosolprecursor gases)Atmospheric transportationDeposition 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 anairmass over a colder land surface or colder airmass
Adiabatic cooling (no heat exchange)Ascending air parcel – pressure decrease, volumeexpansion, temperature decrease
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Cloud/fog formation
Droplet formation without existing nuclei would requireconsiderable supersaturations
e.g. pure water: RH 300-500%Aerosol particles that can facilitate droplet formation atlow supersaturations are called cloud condensation nuclei(CCN)
Cloud droplet composition
LiquidsWater (solvent)Dissolved compounds, e.g. O3(aq), H2O2(aq)Ions, e.g. SO4
2-, NH4+
SolidsSoluble compounds, e.g. ammonium sulfateSlightly soluble compounds, e.g. organic acids, calciumsulfateInsoluble compounds, e.g. dust, elemental carbon
Condensing gasese.g. H2O, HNO3, NH3, SO2, H2O2
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Mixing state of the aerosol
Important when determining e.g. CCN solubility or aerosoloptical propertiesExternal mixture: each particle from only one sourceInternal mixture: all the particles of a certain size contain auniform mixture of components from each source
An example oftime evolutionof sizedistribution inHyytiälä
1.Apr 30.Apr
10
100
nm
A
B
New particles appearing inthe 3-25 nm size range (A)
Newborn particles growing,sometimes to sizes wherethey can act as cloudcondensation nuclei (B)
A new particle formationevent
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Vapor source(e.g. organics+ , sulphuric acid)
Vapor sink(condensation)
3 nm
Particle sink(coagulation)
Growth by condensation
Particle source(nucleation) Activation to
CCN
Pre-existing aerosol
Formation dynamics
Figure by Miikka Dal Maso and Ilona Riipinen
Condensation & Coagulation sinks
Condensation sink (CS)describes the aerosol population’s ability to remove vapor bycondensation
Coagulation sink (CoagS(Dp))describes the aerosol population’s ability to remove particlesof size Dp
Sinks sensitive to particle size changeshygroscopic growth must be accounted for
iiiM
0M NrDdrn(r)(r)rDCS
pDppppp dD)n(DT,...),'D,K(D)CoagS(D
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Aerosol Dynamics: What?
A way to try to understand nature, in this case thebehaviour of aerosol particles
Aerosol Dynamics describes formation, growth andtransportation of aerosol particles
Based on existing theories
Basic Aerosol Dynamics suitable for process-level modelstudies
For atmospheric models, parameterisations are needed
Aerosol Dynamics: Processes
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Aerosol dynamic processes
1. Primary emissions2. Nucleation3. Activation for Growth4. Condensational Growth5. Coagulation6. Cloud Processes7. Deposition8. Connection with atmospheric chemistry and
meteorology
Aerosol dynamic processes
1. Primary emissions2. Nucleation3. Activation for Growth4. Condensational Growth5. Coagulation6. Cloud Processes7. Deposition8. Connection with atmospheric chemistry and
meteorology
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Nucleation
Formation of a new phaseIn the atmosphere: vapour liquid, vapour solid (icenucleation)Types of nucleation
Homogeneous nucleation: No foreign nuclei or surfacesHeterogeneous nucleation: Nucleation on a foreignsubstance
- e.g. nucleation onto surface of aerosol particlesIon induced nucleation: Nucleation on charged particles
Number of speciesOne: homomolecular or unary nucleationTwo or more: heteromolecular or binary, ternary, …nucleation
Homogeneous vs. heterogeneous nucleation
Figure by Hanna Vehkamäki
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Homogeneous nucleation in the atmosphere
formation of new aerosol particles in the atmosphere fromgaseous precursors
the initial size of these particles is typically of the order ofone nanometer in diameter
potential nucleation pathways in the atmosphere:binary water-sulphuric acid nucleation (mainly freetroposphere)ternary water-sulphuric acid-ammonia nucleation (lowertroposphere)ion-induced nucleationnucleation of some organic compounds
there are large uncertainties in both calculating andmeasuring the atmospheric homogeneous nucleation rate
Nucleation treatment in atmospheric models
Each theoretical approach is way too heavy to be handledin a large-scale model
Parameterisations available for some substancesWater – sulphuric acidWater – sulphuric acid – ammonia
Parameterisations are based on calculations with one ormore of the theories available
Input: RH, T, concentrationsOutput: J
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Aerosol dynamic processes
1. Primary emissions2. Nucleation3. Activation for Growth4. Condensational Growth5. Coagulation6. Cloud Processes7. Deposition8. Connection with atmospheric chemistry and
meteorology
Heterogeneous nucleation
Nucleation on a pre-existingsurface (e.g. aerosol particle)Formation of a new phaseOccurs more likely thanhomogeneous nucleation
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Example: Freshly-nucleated thermodynamically stableclusters (TSCs), composed of ammonium bisulfateand water, are activated for condensational growth byan water-soluble organic vapour
Activation means that the organic vapor starts tocondense irreversibly into these particles
However, this does not occur before TSCs havereached a certain threshold size!
The nano-Köhler mechanism
Aerosol dynamic processes
1. Primary emissions2. Nucleation3. Activation for Growth4. Condensational Growth5. Coagulation6. Cloud Processes7. Deposition8. Connection with atmospheric chemistry and
meteorology
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Condensation
Condensation: aerosol particle grows in size by taking upvapours from the gas phase
The most important mechanism of particle growth in theatmosphere
Gas – liquid (or gas – solid) phase transition: alwaysaccompanied with characteristic energy released(condensation) or absorbed (evaporation)
mass and heat transfer to/from droplet coupled by latentheat of evaporation (also enthalpy of vaporization)Condensation processes can be modelled by solvingappropriate mass and heat transfer equations
Condensational growth of atmospheric aerosolparticles
condensational growth is most effective in the size range< 0.1 µm
in the atmosphere, gaseous compounds causing thecondensational growth include sulphuric and nitric acid,water, ammonia and numerous organic vapours
difficulties in estimating the particle condensational growthin the atmosphere:
many compounds responsible for atmosphericcondensational growth have not been identified yetthe saturation vapour pressures of many condensingcompounds are not known accurately (or not at all)
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Aerosol dynamic processes
1. Primary emissions2. Nucleation3. Activation for Growth4. Condensational Growth5. Coagulation6. Cloud Processes7. Deposition8. Connection with atmospheric chemistry and
meteorology
when two aerosol particles collide with each other in theair, they usually stick together (coagulation)
in the atmosphere, coagulation is usually caused by theparticle Brownian motion (Brownian coagulation)
the influence of Brownian coagulation on the aerosolparticle population is relatively easy to calculate(analytical equations)
in the atmosphere, the main role of coagulation is todeplete the smallest (Dp < 10 nm) aerosol particles (bycoagulation into larger particles)
Coagulation
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coagulation = collision + coalescence
agglomeration = collision + sticking (nocoalescence)
successive agglomeration events result in irregular structurescalled agglomerates
Definitions
Gravitation
Shear
Brownian motion
Turbulence
In coagulation/agglomeration, the number concentrationdecreases and the mean size increases
shear
What causes coagulation?
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Aerosol dynamic processes
1. Primary emissions2. Nucleation3. Activation for Growth4. Condensational Growth5. Coagulation6. Cloud Processes7. Deposition8. Connection with atmospheric chemistry and
meteorology
Cloud processing
when the ambient relative humidity exceeds 100%, afraction of aerosol particles (Dp > 50-100 nm) activates toform cloud/for droplets with diameters >10 µm
activation is a result of water vapour condensation ontothe aerosol particles and it occurs within a few minutes
in cloud droplets, many chemical reactions take place andnew compounds are formed
only about 10% of clouds form rain droplets andprecipitate; the rest evaporate and release cloud-processed aerosol particles
during cloud evaporation, most of the water and a fractionof other material present in the cloud droplets istransferred into the gas-phase
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Aerosol dynamic processes
1. Primary emissions2. Nucleation3. Activation for Growth4. Condensational Growth5. Coagulation6. Cloud Processes7. Deposition8. Connection with atmospheric chemistry and
meteorology
Deposition
aerosol particles are removed from the atmosphere bydeposition (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 forcoarse (2.5 µm < Dp < 10 µm) and ultrafine (Dp < 0.1 µm)particles
in wet deposition, particles are removed from theatmosphere by rain or fog
wet deposition is the most important removal pathway forfine particles, especially particles in the size range 0.1-1µm
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Aerosol dynamic processes
1. Primary emissions2. Nucleation3. Activation for Growth4. Condensational Growth5. Coagulation6. Cloud Processes7. Deposition8. 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 isstill very poor
extremely important for stratospheric chemistry (formationof ozone hole)
tropospheric importance largely unknownimportant for marine sulphur chemistry (SO2 oxidation in seasalt particles)seems to be important for ageing of secondary organicaerosolsindications that takes also place in recently-formed aerosolparticles
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Dynamical processes and their effects on sizedistributions
Decrease, DecreaseDecreaseDeposition
No effect, DecreaseDecreaseCoagulation
Increase, IncreaseNo effectCondensation
Increase, IncreaseIncreaseNucleation
Particle mass &surface
Particle number