Atmospheric Aerosols - Helsingin yliopisto · Atmospheric Aerosols Slides partly by Antti Lauri and Hannele Korhonen Aerosol particles Liquid or solid particles suspended in a carrier

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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/

http://www.sat./

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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



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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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



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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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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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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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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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

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Atmospheric Aerosols - Helsingin yliopisto · Atmospheric Aerosols Slides partly by Antti Lauri and Hannele Korhonen Aerosol particles Liquid or solid particles suspended in a carrier