Atmospheric Aerosols Slides partly by Antti Lauri and Hannele - - PDF document
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
Slides partly by Antti Lauri and Hannele Korhonen
Size Concentration
Chemical composition
Kiehl and Trenberth, 1997
Scattering and absorbing short- and longwave radiation Examples: sulphate, organic carbon, black carbon,
aerosols from biomass burning, mineral dust
Cloud formation: effectiveness of the aerosol acting as
cloud condensation nuclei (CCN)
Depend on: size, chemical composition, ambient
environment
Polluted cloud more droplets larger albedo longer lifetime Clean cloud less droplets smaller albedo lower lifetime
(mostly) COOLING
Figure by Ari Asmi / EUCAARI
MODIS 3.5.2006 09:29 http://www.sat. dundee.ac.uk/
Photos: Pia Anttila, FMI
Primary aerosols are directly emitted to the
atmosphere.
Secondary aerosols are formed in the atmosphere
by gas-to-particle conversion processes:
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.)
0.001 0.01 0.1 1 10 100 1000 (1mm) 10 000 (1 cm) Hair Viruses Bacteria Pollen Cigarette smoke Concrete dust Gas molecules Light Fog Rain Particle diameter m
Big particles Small particles Nanoparticles
Sand
Gravel Energy production Traffic
Road dust
Figure by Mikko Moisio and Ilona Riipinen
CCN
the shape of an aerosol particle is assumed spherical
always the case
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
Asbestos Coal Volcanoes Wielding
high concentrations are encountered when there are nearby sources.
1 – 20 Stratosphere (background) 100 – 100 000 Free troposphere > 20 20 – 100 10 – 50 5 – 20 0.02 – 1 > 100 000 10 000 – 50 000 500 – 10 000 200 – 5000 50 – 500 Lower troposphere:
PM10 [g/m3] PN [cm-3]
10nm 100nm 1000nm #/cm3 Diameter Marine Remote continental Urban Free troposphere
nucleation Aitken accumulation coarse
~10 1 molec. ~10 9 -10 12 molec.
Figure by Hanna Vehkamäki and Veli-Matti Kerminen
primary combustion From gases Dust, sea salt, cloud droplets
It has been observed that atmospheric aerosols can be
described rather well with a set of log-normal distribution functions (log-normal = normally distributed in logarithmic scale)
i i pi p i i p N
1 2 2 2 / 1
particles of diameter Dp Ni: number concentration
i: geometric standard deviation : median diameter of the mode
pi
D
concentrations can be expressed by Number, Surface area, Volume, or Mass per unit volume:
(in most cases) dominated by the ultrafine aerosols.
concentration is dominated by the coarse and accumulation aerosols.
Representation of number and volume aerosol size distributions Figure from Seinfeld & Pandis, 2006
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 0.1-
0.5 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
Typical rural aerosol size distribution Figure from Seinfeld & Pandis, 2006
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 0.1-
0.5 m sizes
Mass dominated by coarse
mode
Typical remote continental aerosol size
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
Marine aerosol size distributions from different measurements and a model distribution. Figures from Seinfeld & Pandis, 2006
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
Typical free tropospheric aerosol size distribution. Figure from Seinfeld & Pandis, 2006
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 polar aerosol size distribution. Figure from Seinfeld & Pandis, 2006
Very low total concentrations Accumulation mode dominates “Arctic haze” during the winter
and early spring: anthropogenic sources
Composition: aged
carbonaceous aerosols
pollution sources, sulfate, sea- salt, mineral dust
Typical desert aerosol size distribution. Figure from Seinfeld & Pandis, 2006
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
Emissions (primary particles, emissions of aerosol
precursor gases)
Atmospheric transportation Deposition from the atmosphere to surfaces (ground,
vegetation, water)
Aerosol dynamics and chemistry
Due to cooling (isobaric/adiabatic)
Radiative losses of energy, horizontal movement of an
airmass over a colder land surface or colder airmass
Ascending air parcel – pressure decrease, volume
expansion, temperature decrease
considerable supersaturations
e.g. pure water: RH 300-500%
low supersaturations are called cloud condensation nuclei (CCN)
Water (solvent) Dissolved compounds, e.g. O3(aq), H2O2(aq) Ions, e.g. SO42-, NH4+
Soluble compounds, e.g. ammonium sulfate Slightly soluble compounds, e.g. organic acids, calcium
sulfate
Insoluble compounds, e.g. dust, elemental carbon
e.g. H2O, HNO3, NH3, SO2, H2O2
uniform mixture of components from each source An example of time evolution
distribution in Hyytiälä 1.Apr 30.Apr
10 100 nm
A B New particles appearing in the 3-25 nm size range (A) Newborn particles growing, sometimes to sizes where they can act as cloud condensation nuclei (B) A new particle formation event
3 nm
Pre-existing aerosol
Figure by Miikka Dal Maso and Ilona Riipinen
describes the aerosol population’s ability to remove vapor by
condensation
describes the aerosol population’s ability to remove particles
hygroscopic growth must be accounted for
i i M M
N r
n(r) (r)
D
D p p p p p
dD ) n(D T,...) , ' D , K(D ) CoagS(D
behaviour of aerosol particles
transportation of aerosol particles
studies
nucleation)
Homogeneous nucleation: No foreign nuclei or surfaces Heterogeneous nucleation: Nucleation on a foreign
substance
Ion induced nucleation: Nucleation on charged particles
One: homomolecular or unary nucleation Two or more: heteromolecular or binary, ternary, …
nucleation
Figure by Hanna Vehkamäki
gaseous precursors
binary water-sulphuric acid nucleation (mainly free
troposphere)
ternary water-sulphuric acid-ammonia nucleation (lower
troposphere)
ion-induced nucleation nucleation of some organic compounds
measuring the atmospheric homogeneous nucleation rate
in a large-scale model
Water – sulphuric acid Water – sulphuric acid – ammonia
more of the theories available
Input: RH, T, concentrations Output: J
surface (e.g. aerosol particle)
homogeneous nucleation
clusters (TSCs), composed of ammonium bisulfate and water, are activated for condensational growth by an water-soluble organic vapour
condense irreversibly into these particles
reached a certain threshold size!
vapours from the gas phase
atmosphere
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
< 0.1 µm
condensational growth include sulphuric and nitric acid, water, ammonia and numerous organic vapours
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)
air, they usually stick together (coagulation)
particle Brownian motion (Brownian coagulation)
particle population is relatively easy to calculate (analytical equations)
deplete the smallest (Dp < 10 nm) aerosol particles (by coagulation into larger particles)
coagulation = collision + coalescence agglomeration = collision + sticking (no coalescence) successive agglomeration events result in irregular structures called agglomerates
decreases and the mean size increases shear
fraction of aerosol particles (Dp > 50-100 nm) activates to form cloud/for droplets with diameters >10 µm
the aerosol particles and it occurs within a few minutes
new compounds are formed
precipitate; the rest evaporate and release cloud- processed aerosol particles
transferred into the gas-phase
deposition (dry and wet deposition)
ground, water) and remain there
coarse (2.5 µm < Dp < 10 µm) and ultrafine (Dp < 0.1 µm) particles
atmosphere by rain or fog
fine particles, especially particles in the size range 0.1-1 µm
mechanistic understanding of aerosol-phase reactions is still very poor
important for marine sulphur chemistry (SO2 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
Decrease, Decrease Decrease Deposition No effect, Decrease Decrease Coagulation Increase, Increase No effect Condensation
Increase, Increase Increase
Nucleation Particle mass & surface Particle number