Presented At AMS Meeting, Long Beach, CA, 2003

Aerosol Phase Function And Size Distributions From Polar Nephelometer

Measurements During The SEAS Experiment

By John Porter, Barry Lienert and Shiv Sharma

Hawaii Institute of Geophysics and PlanetologyUniversity of Hawaii

Aerosol Phase Function Describes The Angular Distribution of Light Scattered By Aerosols.

Three Parameters Needed For Optical Calculations - Phase Functions - Absorption Coefficient - Scattering Coefficient

How To Calculate the Phase Function ?For Simple Aerosols Mie Theory Works WellFor Complex Aerosol Other Approaches Are Needed

(T-Matrix, Finite Difference, Discrete Dipole …)

But Often The Aerosol Composition, Shape And Index Of Refraction Are Not Known So MeasurementsOf The Aerosol Phase Function Are Useful

Particle Phase Function Are Fairly Complex But In General:

Aerosol Which Are Larger Than the Wavelength Tend To Scatter More Light In the Forward Direction

Aerosol Which Are Smaller Than The Wavelength Tend To Scatter More Light At Side Angles

0

5

10

15

20

25 dV

/d(log(D

))

0.1 1 10 100 DIAM (um)

13

5

Accumulation Mode Aerosols Coarse Mode Aerosol

1E-2

1E-1

1E0

1E1

1E2

1E3P

HA

SE

FU

NC

TIO

N

0 50 100 150 SCATTERING ANGLE

FREETROP SULF(15%)VOG SULF.(75%) VOG+SALT(75%)

SALT(75%) SALT+LOWSULF(75%)

Calculated From Porter and Clarke (JGR, 1997) Sea Salt Aerosol Models

0.01 0.1 1 10 100

0.0

1.0x10-5

2.0x10-5

3.0x10-5

4.0x10-5

Original model unimodal genetic

inversions

dA

/dlo

g er,

m-1

radius, m

Genetic Inversion Approach To Derive Aerosol Size Distribution

(Lienert et al., Applied Optics, 2001)

0.1 1 10 100

0.0

1.0x10-5

2.0x10-5

3.0x10-5

Original model Genetic Inversions

dA

/dlo

g er,

m-1

radius, m

When One Aerosol Mode Dominates Then It Is Not Possible Retrieve It From Genetic Inversion

Winchester Design Polar Nephelometer (Optical Engineering, 1983)

1E-12

1E-10

1E-08

1E-06

0.0001

0.01

SC

AN

ING

RA

DIO

ME

TE

R

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

SCATTERING ANGLE OF SCAN RADIOMETER

BELLOWS BEACH (8-5-98) FILES 3,4,5ESTIMATED WIND SPEED 15-20 MPH

DARK MEASUREMENTS

5E-10

1.5E-09

2.5E-09

3.5E-09

4.5E-09

FIX

ED

AN

GL

E R

AD

IOM

ET

ER

20.5 20.55 20.6

TIME (hour)

BELLOWS BEACH (8-5-98) FILE1.DATESTIMATED WIND SPEED 15-20 MPH

Ground Based Polar Nephelometer

0 20 40 60 80 100 120 140 160 180

0.1

1

10

100

measured data inverted unimodal models

ang

ular

sca

tter

, ar

b. u

nits

scattering angle, deg

Overall Good Fit But Problems In Backscatter RegionField of View (FWHM ~ 4 deg.)

Complex Volcanic Aerosols At This Time !Non-Spherical Aerosol ?

Instrument Error (Variable Attenuation Filter)

0 20 40 60 80 100 120 140 160 180

0.1

1

10

100

m=1.40+0.001i

m=1.45

m=1.40

m=1.36

measured data unimodal models

ang

ular

sca

tter

, ar

b. u

nits

scattering angle, deg

0.01 0.1 1 10 100

0.0

1.0x10-5

2.0x10-5

3.0x10-5

4.0x10-5

5.0x10-5

6.0x10-5

measured sizer data

unimodal genetic inversion models

dA

/dlo

g era

diu

s,

m-1

radius, m

Aerosol Size Distributions Inverted From Polar NephelometerAerodynamic Particle Sizer

Conclusion

External Aerosol Polar Nephelometer Measurements Offer An Exciting New Way To Measure Aerosol Optical Properties In The Atmosphere

Although Current Measurements Have Been Encouraging, A Dedicated Measurement and Modeling Effort Is Needed

New Fiber Optic, Logarithmic Amplifier, and Calibration Techniques Now Offer Exciting Opportunities For Ground and Aircraft

Measurements