some impacts of atmospheric aerosols direct and indirect effects on climate directly scattering...
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Some Impacts of Atmospheric Aerosols
Direct and Indirect Effects on Climate•directly scattering solar radiation
•altering number and size distribution of cloud drops
N2O5
HNO3
Effects on Gas-Phase Composition•Surfaces for heterogeneous chemistry
•Multiphase reaction volumes [S(IV)S(VII)]
SO2 +OH (+H2O)H2SO4
RH + OHRCOOH
Secondary Mass Growth
Secondary Particles
Primary particles
Some Sources of Aerosol Particles
Aerosol Particle Size: Diameter vs. Effective Diameters
For many particles, spherical geometry good assumption.“Diameter” has physical meaning
Spherical?
Some Effective Diameters
Aerodynamic Diameter
rp
Same terminal falling speed in air as a particle with density 1g/cm3 and radius rp
rp
Electric Mobility DiameterSame trajectory in calibrated electric field as a spherical singly charged particle with radius rp
Relation to aerodynamic diameter and other physical properties of particle not well understood for fractal like soot particles.
0.001
0.005
0.02
0.05
0.1
0.3
0.6
1.0
10
Num
ber
Con
cent
rati
on (
cm-3)
0.5 1.00Diameter (m)
Problems1. Information lost at small
sizes due to large size range2. Comparing particle
concentrations in different bins marred by varying bin size
3. Area under curve is not proportional to total particle number concentration
Bean Counting: Aerosol Size Distributions
0 0.2 0.4 0.6 0.8 1 1.20
500
1000
1500
2000
2500
3000
3500
4000
4500
Diameter (microns)
Nu
mb
er
Co
nc
en
tra
tio
n (
mic
ron
s-1 c
m-3
)
10-3
10-2
10-1
100
101
0
5
10
15
20
25
30
Diameter (microns)
dN
/dlo
g(D
p)
(cm-3
)
Visual Representation of Particle Size Distributions
Ni/Dpi vs. Dp Ni/log(Dpi) vs. Log(Dp)
Area under both curves yields Ntotal But dN/dlogDp vs. logDp is more informative
Questions
10-2
10-1
100
0
50
100
150
200
250
300
350
400
450
500
Diameter (microns)
Nu
mb
er
Dis
trib
uti
on
dN/dDpdN/dlogDpdN/dlnDp
The figure shows various representations of the same aerosol size distribution. Under which curve(s) is the area equal to the total particle number concentration?
•dN/dDp (blue)•dN/dlogDp (green)•dN/dlnDp (red)dN/dlogDp only. Why?
Area, Volume-Mass Distributions
Heterogeneous and multiphase reaction rates depend on surface area or volume, respectively.
10-3
10-2
10-1
100
101
0
50
100
150
200
250
300
Diameter (microns)
Dis
trib
uti
on
Fu
nc
tio
n
dN/dlogDp
dS/dlogDp
Number
Area
Gravitational settling rates depend on mass and air quality standards are mass-based.
--Assuming spherical geometry and dDp0
dS(Dp) = Dp2n(Dp)dDp
dV(Dp) = (/6)Dp
3n(Dp)dDp
Questions
1. What are the units of Stot and Vtot?
2. How is the mass distribution function calculated?
3. What is the relevant property (area, volume, mass) for the following aerosol particle processes:
• Scavenging of HNO3 by mineral dust• Acidification of aerosols by gas-phase H2SO4
• Light scattering efficiency• Amount of Fe deposited to ocean by dust
Smoothed Vertical Profiles of Aerosol Number Concentrations—(highly variable)
Boundary layer: 10 – 105 cm-3 range in number concentrationFree Troposphere: ~100-300 cm-3 on average
Common Modes of Atmospheric Aerosol Distributions
Typical Number Distribution for Urban Aerosols
Solid line: what would be observed, composed of 3 modes
Dotted/Dashed lines: Two common parameterizations
•Junge Distribution (dashed line) is a power law. Has some useful properties but requires care.
•Log-Normal distribution (dotted line) is most often used
Continental and Marine Number Distributions
Lower numbers in these regions relative to urban aerosols, especially in the nucleation mode.
Giant aerosols over ocean dominated by sea salt.
Dominant accumulation mode indicative of “aged” particles.
The Log-Normal Distribution
The familiar normal (Gaussian) distribution•Bell-curve shape in linear space•68% of variance about mean ( ) captured by 2 (width)
2
x
The Log-normal distribution•Bell-curve shape in log space
x x and ?
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