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 ?