Measuring the Aerosol Asymmetry Parameter g Instrument

Description and Initial MeasurementsGuoxun Tian (Tian@dri.edu), Hans Moosmüller, and W.Patrick Arnott,

Desert Research Institute, Nevada System of Higher Education, Reno, NV.

Introduction

In addition to aerosol scattering and absorption coefficients, the angular distribution of light scattered by aerosol particles is needed to determine the aerosol contribution to radiative forcing. This angular distribution is commonly

parameterized into a single value, the asymmetry parameter g. It is defined as the intensity-weighted average cosine of the scattering angle with values ranging from –1 for pure backscattering to +1 for pure forward scattering.

No instruments for the direct measurement of aerosol g in the atmosphere are available. Commonly the asymmetry parameter g is determined from measurements of the aerosol size distribution and refractive index using Mie

calculations under the assumption of spherical particles. The problem is that size distribution and refractive index are often poorly known and the particles may not be spherical.

Here a g-meter which can measure aerosol g directly is introduced. It is made of a quadrant detector conjunction with a laser beam. An accurate g value can be achieved by the signals of four detectors. The initial measurement will be

discussed.

Schematic diagram

1) Concept of Asymmetry parameter g 2) 1-D Physical meaning of g

4) Schematic diagram and picture of

the g-Meter

3)Design of the g meter

The angular distribution of light scattered by aerosol is needed to determine the

aerosol contribution to radiative forcing. Phase function from Mie theory for

spherical particles (Needs size parameter x = πd/λ and complex refractive index)

00

2

04

cossin2

1cossin

4

1cos

4

1PddPdPdg

where is the angle between incident light and scattering direction and P() is the phase

function giving the angular distribution of the scattered light.

We can calculate back scattering percentage and forward

scattering percentage very easily if asymmetry parameter g

is known.

particle

Incoming photons

Forward scattered photons

back

scattered

photon

Here P↓↓=75%. P↓↑=25%.

P↓↓+ P↓↑ = 100%.

g≡ P↓↓ - P↓↑

g = P↓↓ - (1- P↓↓)

Solving, we have:

The g-meter needs to reproduce the (sinθcosθ) angular weighing

Laser beam propagating

through the center of a

quadrant torus detector.

2-dimensional cross-section

or simplified version

Laser is scattered by the aerosol, so four

detectors receive the scattered light and

produce four signals, S1, S2, S3 and S4

2

2

0

1 cos1cossin PdPdPCS laser

S2 C Plaser d P 1 cos 0

2

d P sin cos 2

S3 C Plaser d P 1 sin 2

S4 C Plaser d P 1 sin 0

2

4321

43214321

SSSS

SSSS

S

SSSSSg

Sg 1

2d P 1 sin cos

0

2

d P 1 sin cos 2

0

)(2

1wg gPdS

For gw cos sgn 2 1sin

sgn x

1 x 0

0 x 0

1 x 0

wg*165.1cossin

gw SgPdPdg *165.1)(2

1*165.1cossin

2

1

00

Multiple channel lock

in amplifier

Pow

er Mo

du

latio

n

Spatial

filter

Power

meter

Computer

Data

Black hole

Signals of four detectors

Aerosol outlet

Aerosol inlet

Laser

The laser is 532nm wavelength green laser. We use a virtual multiple channel

lock in amplifier to get the signal from the four detectors and power meter

and transfer the data to the computer. The spatial filter cleans the laser beam

and the black hole absorbs the incident laser. They greatly reduce the

background light.

5) Calibration and truncation angle analyze

Calibration(1): angular response

1. Mount the cylindrical detector on a rotation stage and rotate it around the cylinder axis.

2. Illuminate the detector with a planar wave (e.g., expanded laser beam) propagating

perpendicularly to the cylinder axis.

3. Record the angular response and normalize the angular response to sin(θ)cos(θ).

0 100 200 300

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Degree (C)

Sin(Q)Cos(Q)

Normalized Sg

Normalized Sg-Sin(Q)Cos(Q)

Calibration(2): sensitivity

Calibrate the reciprocal nephelometer with two gases ( Clean Air and CO2).

0.000010 0.000015 0.000020 0.000025 0.000030

0.14

0.15

0.16

0.17

0.18

0.19

0.20

gco2

=0.030223908

CO2

Clean Air

Scattering coefficient (1/Mm)

Vo

lta

ge

(v)

S1+S2-S3-S4

Y=0.13758+2092.8382*X

standard deviation of the voltage

Co2: 3.95302E-06 V

Air: 6.50221E-06 V

0 1000 2000 3000 4000 5000 6000 7000 8000

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

g

Diameter (nm)

gm (g from mie theory)

g'

fit line gf=0.19398+0.53284*g

'

truncation angle analyze (right fig)Since the tube is not infinite long, there are

different truncation angles for different detectors.

The fig on the right show the difference of g

caused by truncation angle.

Where black line is the g value from Mie theory,

red line shows the g value of g-meter without

truncation angle correction and green line is the

fit line of the black line and red line. It can be

seen easily that the green line does match the

black line.

6) Initial measurement

ultrasonic

Scanning Mobility

Particle Sizer (SMPS)

Aerosol

inlet

outlet

Aerosol

generator

(salt)

g meter

Aerosol

inlet

Aerosol

outlet

Aerosol

outlet

An SMPS is used to measure the size

distribution of salt particles from the aerosol

generator. We can calculate the g value with

Mie theory and measure it with g meter.

System diagram Initial measurement of salt particle

Size distribution 1

Measured g value: 0.82

Calculated g value: 0.74

Size distribution 2

Measured g value: 0.78

Calculated g value: 0.69

Initial measurement of latex sphereSMPS

0.6 V 0.7V 0.8V g-meter(theory) Mie

0.465um 0.874801 0.878528 0.887532 0.7508 0.7023

1.025um 0.643154 0.654732 0.653253 0.5249 0.3559

1.898um 0.896966 0.887687 0.890569 0.9125 0.6465

Diameter

Control voltage of four PMTg value