Airborne sunphotometer, airborne in-situ, space-borne, and ground-based measurements of tropospheric - Geoscience and Remote Sensing Symposiun, 2000. Proceedings. IGARSS 2000. IEEE 2000 International

Airborne Sunphotometer, Airborne

in-situ,

Space-borne, and Ground-based Measurements

of

Tropospheric Aerosol

in ACE-2

B.

Schmid',

D.

Collins',

S.

Ga~s6~,

E.

&tr0m4,

D.

Powell',

E.

Welton',

P. Durkee',

J. Livingston',

P. Russell9,

R.

Flagan",

J.

Seinfeld",

D.

Hegg3,

K.

Noone",

K.

Voss'',

J.

Reagan',

J.

Spit~hirne'~,

D.M.

McIn~osh'~

'Bay

Area 

Environmental Research

Institute, 3430

Noriega

Street,

San Francisco CA 94122,

USA

P

(650)

604-5933,

F

(650)

604-3625,

e-mail:

bschmid@mail.arc.nas.gov

%exas

A&M

University, David

G.

Eller

O&M

Bldg,

College Station,

TX

77843,

USA

3University of Washington,

Box

351650, Seattle,

WA

98195,

USA

4Hadley

Centre

for

Climate

Prediction and Research, Meteorological Office, Bracknell, Berkshire

RG12 2SY,

UK

'University

of Arizona,

ECE Building,

Tucson,

AZ

85721,

USA

'Science

S

stems and

Application

Inc.,

NASNGSFC

Code

912,

Greenbelt,

MD 20771,

USA

Gaval

Postgraduate School,

589

Dyer Road,

Monterey, CA

93943,

USA

'SRI

International,

333

Ravenswood

Avenue, Menlo

Park,

CA 94025,

USA

%ASA

Ames 

Research Center,

MS

245-5, Moffett Field, CA 94035,

USA

"California

Institute

of Technology,

1200 East California

Boulevard, Pasadena, CA

91125,

USA

"Meteorological

Institute

Stockholm University, Arrhenius Lab, Stockholm,

10691

Sweden

''University

of

Miami, 

1320 Campo

San0

Dr.,

Coral Gables,

FL

33146,

USA

13NASA

Goddard

Space Flight

Centex,

Code

912,

Greenbelt,

MD

20771,

USA

"Symtech

Corporation,

2750

Eisenhower

Ave.,

Suite 104

Alexandria,

VA

22314,

USA

INTRODUCTION

The

North 

Atlantic Regional Aerosol Characterization

Experiment (ACE-2)

of the

International

Global Atmospheric

Chemistry Project (IGAC)

ran

from 

16

June

to

25 July

1997.

The

results

presented

in

this

study

are

part

of

the

"Clear-sky

column

closure

experiment"

(CLFARCOLUMN)

activity,

one

of

6 ACE-2

activities

[l].

Clear-sky column

closure

experiments call

for characterization

of

aerosol

layers

by

simultaneous measurements using

different

techniques

that

can

be

[2].

A

wide 

range

of

aerosol

types

was

encountered throughout

the

ACE-2

area,

including background Atlantic marine,

European pollution-derived

and

Mican

mineral dust. In

a

series

of

papers,

we

reported on

ACE-2 CLEARCOLUMN

results

obtained

by

combining

airborne

sunphotometer

and

in-situ measurements taken aboard

the

Pelican

aircraft,

space-

borne

NOMAVHRR

data 

and ground-based lidar

and

sunphotometer measurements

[3]-[

101.

Those

and

other

CLEARCOLUMN

results

have

been

summarized

in

[l

11.

In

this

paper

we

only

report

on

results

not

shown

in

this

form

in [3]-[ll].

METHODOLOGY

We

are

using several

different

techniques

to

determine

aerosol

optical

depth (AOD), extinction and

size distributions

of

aerosol layers:

1)

The

NASA

Ames

Airborne

Tracking

14-channel

Sunphotometer (AATS-14) can

determine the

AOD

above

the airplane

at

13

wavelengths

between

(380

to

1558

nm).

AOD

vertical

profiles obtained

during

narrow

up

or

down

spirals

can

be

differentiated

to

obtain extinction profiles

[9].

0-7803-6359-O/OO/$lO.OO 

0

2000

IEEE

1613

Layer

AODs

or extinction

spectra

can

be

inverted

to retrieve

size

distributions.

2)

Continuous

airborne

size

distribution

measurements

(D=Snm-8~)

corrected

to

ambient

RH

together

with

measured/assumed composition information

can

be

used

to

compute

extinction

and layer

AOD

using

Mie

theory [3].

3)

Three 

airborne

nephelometers measured

scattering

coefficients

at

different

RH

(allowing

correction

to

ambient

RH)

[5],[7].

Corrections

for

inletatoff,

light-source

and

angular

truncation

need

to

be

applied, using information from

2) [3], [9]. Absorption

coefficients have

been

measured

by

an

airborne particle

soot absorption photometer (PSAP)

[7].

Corrections

for

scattering contributions and

inlet-cutoff

need

to

be

applied

based

on

the results from

2)

and

the

nephelometers [3],

[9].

Extinction

is then

obtained

by

adding

absorption and

scattering coefficients.

4)

Ground

based

Micro-Pulse

Lidars

measure extinction

or

AOD

profiles

[8],

[lo].

RESULTS 

On

July

17,

1997,

a vertical profile

flown

near

Tenerife

(Canary

Islands) in a

cloud

fiee

air

mass

reveals

three 

distinctly different

layers:

a

somewhat polluted

marine

boundary layer (MBL),

an

elevated

dust

layer and

a very

clean

layer

between

the

MBL

and the dust

layer. Figure

1

shows

the

AOD

and

extinction

(M25

nm)

obtained using

the

four

different

techniques. Note

that the integrated

extinctions

for

techniques

2 and

3 yield layer

AODs

only

and

the

AATS-14

AOD

value obtained

at

the

top

of

the profile

was

added

to

facilitate

comparison.

The

lidar

data 

show

that

the

elevated

dust

layer

extended

above

the

Pelican's

maximum

flight

altitude.

ACE-?

17.7.1 

997

16A2-lIlO 

UT

0

0.05

0.1

0.15

0.2

Aerosol

Optical

Depth

Extinction

[lh]

Figure

1:

Aerosol

optical

depth and extinction profile off

Tenerife retrieved from

AATS-14,

fiom

size distribution

measurements, and

by

combining scattering and

absorption

measurements

of

3

nephelometers and the

PSAP

instrument

during

Pelican

flight

U20

on

July 17,

1997.

Also

shown

is the

profile obtained

with

a nearby

(24

km)

Micro

Pulse

Lidar. 

With the exception

of

a few

points,

the

Caltech

OPC

extinctions (technique

2)

agree

with

the

AATS-14

results

throughout

the entire profile

within

the

errm

bars

of

both

techniques.

The

agreement between nephelometeriPSAP

and

AATS-14 extincion

is outside

the

error

bars

[9]

in the

dust

layer,

but

within

error

bars for most

of

the

MBL:

Above

the

MBL

the

lidar 

AOD

and

extinctions

agree

well

with

the

AATS-14

and the Caltech

OK

results.

Near

the

top

of

the

MBL

the

lidar

shows

considerably

larger

extinction

than

the

other

techniques.

This might

be

caused

by

the

spatial

separation

(24

lan)

of

the

profiles.

The

layer

AOD

comparisons

for

the dust and

MBL

are

shown

in

Figure

2

and

Figure

3.

In

the

MBL,

the layer

AODs

obtained

with

the

four

techniques agree

within

the

combined

error 

bars.

The

layer

AODs

obtained

with

in-situ techniques

(Caltech

OK

and

NephelometeriPSAP)

tend

to

be

lower

than the

remote sensing results

(AATS-14

and

Lidar). 

In

the

dust

(Rgure

3),

the

layer

AODs

of

Caltech

and

AATS-14

agree

almost perfectly

at

all

wavelengths except

at

1558

nm.

The

lidar 

AOD

agrees

very

well

with

the

Caltech

and

AATS-

14

results.

The

nephelometerESAP

AODs

are

lower

by

20%-

38%.

Only

the result

at the

shortest nephelometer wavelength

agrees

within

the

error

bars.

The

nephelometedPSAP

layer

AOD

spectrum

is

also

much

steeper.

Instead

of

computing extinction

from the in-situ size-

distribution

data 

and c:)mparing

this

with

the

extinction

or

layer

AOD

obtained

from

AATS-14,

we

may

compare size

distributions

by

inverting the

AATS-14

extinction

or

layer

AOD

spectra.

ACE-2

17.7.1907

...

J

0.4

0.5

0.6

0.7

0.8

0.9

1

12

1.4

1.6

Wavelength

IWnl

Figure

2:

Spectral

aerosol

optical

depth

for

the

MBL

(64- 

1121m

a.s.1.)

shown

in Figure

1.

ACE-2 

17.7.1907

...............

j

...........

...............

0.26

c

$0.24

0"

0.2

H

Layer:

1.844-3.891

lon

0.4

Wavelengfh

bun]

Figure

3:

Spectral aerosol optical

depth for

part

of

the

dust

layer

(1844-3891

m a.s.1.)

shown

in

:Figure

1.

We

have

used two

different inversion

methods:

the

widely

used King

constrained

linear 

inversion

method

[12]

and

a

method

that varies

the

amplitudes

of

a

predeteamined

multimodal lognormal

size distribution

(mode

radii and

widths

remain fixed and

are

chosen according

to

an

aerosol

climatology

by

Remer

et

al.

[13]).

The

results

for

the July 17,

1997

MBL

are

shown

in

Rgure

4.,

in

terms

of

area 

size

distributions.

A

wavelength-independent

refractive

index

of

m=1.4-0.00353

has

been

used

with

both

inversion

methods.

The

agreement

between

the

in-situ

data 

and

the

King

inversion

is within

error

bars

for

two

thirds

of

the

size

bins.

MT8-14.17

July

97.

MBl

-

AATS-14

Remer

-

Caltechopc

601

0.02

0.05

0.1

0.2

0.5

I

2

4

8

12

D

I” 

Figure

4: Comparison

of MBL

area

size

distributions

from

in-

situ

measurements and from inverted AATS-14 spectral

extinction measurements (using

two

different

inversion

methods,

see

text) during

Pelican

flight

tfZ0

on

July 17, 1997.

Dashed

lines indicate

uncertainties

of the

Caltech results.

No

error

bars are available

yet

for the

fixed mode-radii

inversion.

But

especially

the

result

for

the

accumulation

mode

is

in very good

agreement

with

the

in-situ

size

distribution.

The

AATS-14

size distributions

obtained using

the

King

method

cover

only

the

size range

where

the

aerosol

particles are

optically active.

The

fixed mode-radii inversion

allows

a physically

sound

extrapolation

to

smaller and

larger

sizes.

CONCLUSIONS

In

most

cases

we

find closure

among extinction

or

AOD

measured using

AATS-14

and computations

based

on

continuous

size

distribution

measurements

on

the same

aircraft

[3],

[lo].

However, considerable

effort

is required

to

arrive

at ambient extinction

Grom

measured

size

distributions

of

a

partially

dried

aerosol.

The

fact

that

the

nephelometers

and the

PSAP

sampled the

aerosol

through an

inlet

with

an

aerdyynamic diameter

cut-off

of

2.5

pn

makes

those measurements

less

useful

for

the

closure

study

carried

out here.

Large corrections

(especially

in

the

dust)

had

to

be

applied. Therefore,

it

is

not

surprising

that closure

with AATS-

14

was

not

always achieved.

Closure

between AATS-14

and

lidar

was

achieved

in

the

dust

layer for

the

case

shown

here

and

another one

discussed

in

[9] and

[lo].

Aerosol

size-distribution

closure

based

on

in-situ

size

distributions

and inverted

AATS-14

extinction

spectra (using

the

Method

of

King

et

al.

[12]) has

been

achieved

in the

MBL.

The

results

for

a newly

developed

fixed 

mode-radii

inversion technique

are

promising

but need more

validation.

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F.

Raes, T. Bates,

F.

McGovern,

andM.

Vanliedekerke,

“The

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Tellus,

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2000.

P.K.

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[I

11

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