validation and refinement of modis aerosol optical depth product over coastal urban areas

NOAA/NESDIS Cooperative Research Program Second Annual Science Symposium

SATELLITE CALIBRATION & VALIDATION July 13-14 2004

Barry Gross (CCNY)Brian Cairns (NASA-GISS)Bill Lawrence (Bowie State)

Michael Hagigeorgiou (Ugrad CCNY)Min Min Oo (Grad CCNY)

Istvan Laszlo (NOAA-NESDIS)Stephan Ungar (NASA-GSFC)Thomas Brakke (NASA-GSFC)

Validation and Refinement of Modis Aerosol Optical Depth

Product over Coastal Urban Areas



Motivation

• Air Pollution forcasting (Ozone, Aerosols, etc) has become a major NOAA responsibility in support of EPA

• MODIS AOT algorithms are global in nature.• Application to regional areas may be difficult due to local

ground albedo anomolies.• Colocated matchups show that MODIS optical depth

retrievals often overestimate optical depth measurements on the North East coast in urban areas

• Separate aerosol and land contributions using high spatial resolution data to help– Examine urban ground reflectance to tune MODIS – Retrieve aerosols in urban areas



Colocated matchup procedure

• Intercompare MODIS Optical Depth with CIMEL optical depth. (Need to use only spatially homogeneous datasets)

• CIMEL Optical Depth taken between NYC and Brookhaven (5 hour mean to agree within 10%)

• MODIS 10km products. 3 x 3 cells to have std < 20% mean

• Few Points satisfy these conditions

brookhaven

CCNY



Intercomparision betweenCIMEL Sky radiometer and Satellite

CART Site CCNY Site

CIMEL

MODIS

CIMEL

MODIS



MODIS Aerosol algorithms over land

25.660

25.470

2160,

660,

2160,

470,

c

c

g

g

g

g

Surface correlations



MODIS-MISR BRDF Productexamining VIS-MIR correlation

1)470(4~)470(/)2160( cgg Red pixels Consistent with MODISassumptions

As we go toYellow, MODISUnderestimatesVIS ground albedo



Path Radiance (Regression) Approach

• Many Surfaces have covarying spectral responses between the VIS and MIR

• If the spectral responses covary in a similar way (one component model), a strong ‘linear’ correlation between the VIS and MIR bands occurs

,,,,

,

Atmvis

TOAMIRud

TOAvis

SurfVIS

SurfMIR

RRTTMIRVIScR

RMIRVIScR

slopeY intercept

Single scattering+small Lanbertian surface albedo



Application and validation of ground reflectance correlations

(MODIS over vegetation)

Vis

ible

/ N

IR R

efle

ctan

ceAtmvis

TOAMIR

TOAvis m

Y intercept gives atmospheric reflection while slope (m) is proportional to the ground correlation

1) Correlation betweenground reflection for different channels will result in correlations at the TOA

2) This can be used to separate ground andatmosphere components byplotting the MIR and VIS reflectances anddetermining their regression coefficients

MIR Reflectance TOAMIR (2160nm)

47.~)660(

18.~)470(

C

C



Hyperion as model of Special Events Hyperspectral Imager

30 meter pixel resolution

S/N between 60 and 150 (oh well)from blue to red.



Probing aerosol retrieval and ground correlations over urban scales

Regions of interest include vegetation(central park – green), Urban areas (red-black) the river (orange), andlower manhattan

Scan Column

Sca

n Li

ne

New York observed through Hyperion (30 meter resolution)



Correlation Coefficient vs wavelength

Urban environmentshave many wavelength independent reflection(geometric) mechanisms that improve the correlations between the VIS and MIRchannels

Note a sharp differencefor lower Manhattandue to shadowing /urban effect

500 600 700 800 900 1000 1100 1200-0.2

0

0.2

0.4

0.6

0.8

1

1.2light urban vegatation middle urbannear wtc

Shadow/Urban effect

TOAMIR

TOAVISCorr ,

)(nm



Hyperion Retrieval of Aerosol Reflection over heavy urban zone

Larger Deviationsabove 700nm dueperhaps to partial Vegetated scenes but Still much smaller than for vegetated scenesthemselves

400 500 600 700 800 900 1000 1100 12000

2

4

6

8

10

12

14

Aerosol Reflection based on Aeronet estimates of AOT and Phase Function

Total Reflection Single Scattering

Rayleigh Scattering]94[.a

Hyperion Retrieval

)(nm

TO

A R

efle

ctan

ce %



430nm (Quite noisy)

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

5 10 15 20 25 30

5

10

15

20

25

30 9

10

11

12

13

14

5 10 15 20 25 30

5

10

15

20

25

30

50 100 150 200 250

100

200

300

400

500

600

700

800

900

1000

Hyperion Scene

Correlation between VIS and MIROnly keep 80.

Aerosol Reflection (bad pixelsMasked with navy blue) Increased loading to WTC observed

WTC



Ground reflection correlation frequency histogram

No angle dependence (Lambertian) assumption

Correlation larger than MODISassumption of 0.5

This has been Observed elsewhereas seen below

2160,660c

Fre

quen

cy

* Source MODIS ATBD Document

Light Urban

Central Park

Heavy Urban



Gnd ref 0.02

Gnd ref 0.05

Gnd ref 0.1

Gnd ref 0.2

Pe

rce

nta

ge

Of

De

lata

Ta

u

Aerosol optical thickness (Tau)

Percentage of Optical depthOverestimation

AOT overestimatesseem to be fairly consistent with ground albedo ~0.05

MIR albedo Urban



Conclusions• Spatial Regression over urban areas using high spatial

resolution sensors can isolate aerosol Path Radiance directly and help decouple ground albedo from atmosphere.

• MODIS correlation coefficients are too low for urban scenes and leads to an overestimate of optical depth from MODIS.

• Preliminary Radiative Transfer calculations show that the 30% overestimate of VIS ground reflectance can help explain observed AOD overestimates.

• More urban data including high aerosol optical depth events needed.



Additional Slides



Aeronet Optical Depth

Hyperion Fly By

From GISS112th St



Water leaving radiance using AVIRIS (subtracting ground decoupled atmosphere signal from total water signal)

400 500 600 700 800 900 1000-0.01

-0.005

0

0.005

0.01

0.015

0.02

Obtained using high shadowingBetter water leaving retrieval

Weak shadowingPoor water leaving retrieval

Wat

er le

avin

g R

adia

nce

(Ref

lect

ion

Uni

ts)

)(nm

100 200 300 400 500 600 700

50

100

150

200

250

300

350

400

450

500

shadowing

Littleshadowing

water

validation and refinement of modis aerosol optical depth product over coastal urban areas

Documents