Transcript Slide 1

Accumulation range: (0.08 to 2 μm)
Contribute up to 50% by mass but 5% by number
Sources:
Condensation of low volatility vapours
Self-coagulation of small particles within the nuclei range
Coagulation of nuclei range particles with particles in the
accumulation range
Typically have a higher organic content than coarse
particles
Also contain soluble inorganics: NH4+, NO3-, SO42A bimodal peak is often observed in this range
Particles are too small for much sedimentation
Dominant removal processes are therefore:
Rainout: incorporation into cloud droplets
Washout: removal during precipitation
Dry deposition: losses on surfaces
The long lifetime of particles in this mode makes them
important in atmospheric chemistry
Aitken nuclei: (0.01 to 0.08 μm)
Contribute the largest number of particles, but the overall mass
of particles in this range is small
Sources:
Gas-to-particle conversion at ambient temperatures
May be formed in combustion – hot, supersaturated vapours
form and condense
Aitken nuclei particles grow larger either by:
Acting as nuclei for condensation of low vapour
pressure gaseous species
Coagulation to form larger particles
The lifetime of particles in this range is short owing to
rapid coagulation
Ultrafine particles: (< 0.01 μm)
The 4th mode of the size distribution – sometimes referred to as
the nucleation mode
Particles in this size range are difficult to measure and are not
well understood
Can occur in significant numbers (e.g., 104 particles cm-3) but
have a very small mass
Formed via gas-to-particle conversion:
Characterising size distributions
A log-normal distribution provides a good empirical fit to
observed size distributions:
2


(ln
D

ln
D
)
dN
NT
gN

exp

2
d ln D
2(ln g )
2 ln  g


where:
σg is the geometric standard deviation
NT is the total number of particles
DgN is the geometric number mean diameter
This is analogous to a normal distribution:
y = A exp[-(x – x0)2 / 2σ2]
σg is a measure of the spread of a distribution
 (ln D  ln DgN ) 2 
dN
NT

exp

2
d ln D
2
(ln

)
2 ln  g


g
ln σg = ln D – ln DgN →
σg = D / DgN
68% of particles fall between σg = DgN / σg and DgN σg
The value of σg is the same for different
distributions and mean diameters:
geometric mass mean diameter, DgM
geometric surface mean diameter, DgS
geometric volume mean diameter, DgV
2.2 PARTICLE MOTION
Particle motion driven by:
Gravitational settling
Brownian motion
important for:
Gravitational settling  large particles D > 0.2 μm
Brownian motion  smaller particles D < 0.2 μm
Approximately equal contributions at D ~ 0.2 μm
Other contributions?
Moving air → turbulent motion
2.3 INTERACTION WITH RADIATION
Extinction in the atmosphere is the sum of absorption and
scattering of both particles and molecules:
I  I 0 exp[ ext L]
where  ext   ag   sg   ap   sp
and
σag = absorption of gas
σsg = scattering of gas (Rayleigh)
σap = absorption of particles
σsp = scattering of particles (Mie)
Scattering:
D << λ
D~λ
D >> λ
→
→
→
Rayleigh scattering
Mie scattering
Mie scattering
Mie scattering
is dominant
Impact of aerosols on
radiative balance:
From the IPCC report
on climate change: