OMI UV spectral irradiance:
comparison with ground
based measurements in an
urban environment
Stelios Kazadzis
A. Bais, A. Arola
OMI science team meeting
Helsinki, June 2008
Finnish Meteorological Institute
Laboratory of Atmospheric Physics, Thessaloniki, Greece
31.7.2017
Outline
• OMI – ground based UV spectral irradiance
comparison – statistics
• Aerosol absorption – post correction approaches
• Campaign: Spacial and temporal UV variability within
an OMI grid
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
2
• The problem - absorbing aerosols
OMI – GB UV comparison – statistics
Current OMI UV algorithm does not account for absorbing aerosols
(e.g. organic carbon, smoke, dust )
Tokyo: +32%
Tanskannen et al., JGR 2007
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
3
OMI – GB UV comparison – statistics
Thessaloniki Area
-High aerosol load
Sahara - Dream model
- Aerosol transport
- Very high PM10 conc.
agriculture
wetlands
grasslands
Latitude (degrees)
Sahara dust intrusions
Biomass burning from NE
forest
other
Thessaloniki
Fire hot spots - summer
Longitude (degrees)
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
4
OMI – GB UV comparison – statistics
Instrumentation - comparison
Thessaloniki September 2004-December 2007
Brewer instrument
(spectral-calibrated, wavelength shift corrected)
UV irradiance at 305, 324, 380 nm and CIE
Total column ozone
spectral AOD at UV wavelengths
CIMEL
AOD (340nm) , SSA(440nm), ..
NILU-UV
305nm, 324nm, 340nm, 380nm
Rooftop of the School of
Natural Sciences
Cloud – cloudless case separation
Pyranometer, observations, sky camera pix
Daily OMI overpass time (mean over ±15 minutes)
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
5
OMI – GB UV comparison – statistics
Results
80
500
305 nm
324 nm
400
300
Brewer
Brewer
60
40
200
20
100
0
0
0
20
40
60
OMI
305nm OMI +30%
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
80
0
100
200
300
400
500
OMI
324nm OMI +17%
31.7.2017
6
OMI – GB UV comparison – statistics
Results
0.3
0.3
Days
cloud free
Days
cloud free
380 nm
cloudy
CIED
cloudy
0.2
ΔΝ/Ν
ΔΝ/Ν
0.2
0.1
0.1
0.0
0.0
0
1
2
OMI - GB ratio
380nm OMI +11%
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
3
0
1
2
3
OMI - GB ratio
CIED OMI +20%
31.7.2017
7
OMI – GB UV comparison – statistics
Results - statistics
All data
Wavel.
(nm)
cloudless
m
R2
W10
W20
m
R2
W10
W20
305
1.30
0.94
43.3
68.2
1.27
0.95
64.8
87.6
324
1.17
0.89
51.3
73.1
1.15
0.91
76.9
92.4
380
1.13
0.89
47.2
70.7
1.11
0.91
71.8
89.7
CIED
1.20
0.93
51.5
75.4
1.19
0.95
75.0
91.9
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
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OMI – GB UV comparison – statistics
Results - statistics
All data
Wavel.
(nm)
cloudless
m
R2
W10
W20
m
R2
W10
W20
305
1.30
0.94
43.3
68.2
1.27
0.95
64.8
87.6
324
1.17
0.89
51.3
73.1
1.15
0.91
76.9
92.4
380
1.13
0.89
47.2
70.7
1.11
0.91
71.8
89.7
CIED
1.20
0.93
51.5
75.4
1.19
0.95
75.0
91.9
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
9
Post correction methods
Post correction methods - TOMS experience
Cloudless cases:
E ( )
Eac ( ) 
1  b abs ( )
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
Ta(λ) = AOD(λ) * [1 - SSA(λ)]
Arola et al., JGR 2005
Krotkov et al., OE 2004
31.7.2017
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Post correction methods
UV attenuation – Thessaloniki, cloudless cases
UV attenuation
OMI-Ground based % bias
% UV attenuation due to aerosol at (324 nm)
0
-5
-10
77%
of cases
-15
-20
-25
-30
-35
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Months
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
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Post correction methods
Post correction : method 1
Ta (λ) = AOD(λ) * [1 - SSA(440nm)]
Aerosol absorption CF(λ) = 1.1 + 1.5 * Ta(λ)
Irradiance ratio OMI / Brewer at 324 nm
2.0
ssa >0.98
1.8
ssa= [0.94, 0.98]
ssa= [0.88, 0.94]
1.6
ssa < 0.88
1.4
1.2
•no sza dependence
•SSA @ UV ?
•need of GB data
1.0
0.8
0.6
0.4
Slope = 1.5 per unit of absorption optical depth
Intercept = 1.10
0.2
0.0
0.00
0.02
0.04
0.06
0.08
Absorption optical depth
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
0.10
0.12
31.7.2017
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Post correction methods
Post correction: method 2
Tas =Ta / cos(sza)
Aerosol absorption CF(λ) = 1.07 + 1.8 * Tas(λ)
Irradiance ratio OMI / Brewer at 324 nm
2.0
ssa >0.98
1.8
ssa= [0.94, 0.98]
ssa= [0.88, 0.94]
1.6
ssa < 0.88
1.4
•SSA @ UV ?
•need of GB data
1.2
1.0
0.8
0.6
0.4
Slope = 1.8 per unit of slant absorption optical depth
Intercept = 1.07
0.2
0.0
0.00
0.04
0.08
0.12
slant column absorption optical depth
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
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Post correction methods
Post correction: use of RT model
Abs + scat
sza=40
0.8
0.82
0.84
0.86
0.88
0.9
0.92
0.94
0.96
0.98
1
1.6
1.5
1.4
1.3
1.2
1.1
1
0
0.2
0.4
0.6
0.8
1
aerosol optical depth at 340 nm
1.2
1.4
sza=60
1.2
correction for irradiance at 324nm
correction for irradiance at 324nm
1.7
0.8
1.18
1.16
1.14
total correction
0.82
ssa=0.88
0.84
0.86
scat
0.88
0.9
0.92
1.12
0.94
0.96
1.1
0.98
1
1.08
1.06
correction for scattering
ssa=1,
no absorption
1.04
0.4
0.45
0.5
0.55
aerosol optical depth at 340 nm
0.6
S1: AOD and SSA synchronous measurements
S2: AOD and SSA@440 = const
S3: AOD= const and SSA@340 = const
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
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Post correction methods
Overview of post corrections
6th Approach: CF(λ) = 1 + 3 * Ta(λ)
Table with all the results of the 6 approaches:
Method
305 nm
Mean (1σ)
324 nm
Mean (1σ)
380 nm
Mean (1σ)
Obs
Original
No correction
1.27 (0.15)
1.15 (0.10)
1.11 (0.12)
267
S1
Apply Ta slope
1.17 (0.13)
1.07 (0.09)
1.05 (0.13)
135
S2
Apply Tas slope
1.18 (0.13)
1.09 (0.10)
1.05 (0.13)
135
M1
Model
1.13 (0.12)
1.04 (0.08)
1.01 (0.11)
135
M2
Model constant SSA
1.14 (0.13)
1.03 (0.09)
0.99 (0.10)
267
M3
Model const. SSA-AOD
1.12 (0.13)
1.02 (0.09)
0.98 (0.10)
267
S3
1 + 3 * Ta (λ)
1.11 (0.13)
1.03 (0.10)
1.01 (0.11)
135
OMI/Brewer
ratio [R]
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
15
Post correction methods
Overview of post corrections
6th Approach: CF = 1 + 3 * Ta(λ)
Table with all the results of the 6 approaches:
Method
305 nm
Mean (1σ)
324 nm
Mean (1σ)
380 nm
Mean (1σ)
Obs
Original
No correction
1.27 (0.15)
1.15 (0.10)
1.11 (0.12)
267
S1
Apply Ta slope
1.17 (0.13)
1.07 (0.09)
1.05 (0.13)
135
S2
Apply Tas slope
1.18 (0.13)
1.09 (0.10)
1.05 (0.13)
135
M1
Model
1.13 (0.12)
1.04 (0.08)
1.01 (0.11)
135
M2
Model constant SSA
1.14 (0.13)
1.03 (0.09)
0.99 (0.10)
267
M3
Model const. SSA-AOD
1.12 (0.13)
1.02 (0.09)
0.98 (0.10)
267
S3
1 + 3 * Ta (λ)
1.11 (0.13)
1.03 (0.10)
1.01 (0.11)
135
OMI/Brewer
ratio [R]
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
16
Post correction methods
Overview of post corrections
6th Approach: CF = 1 + 3 * Ta(λ)
Table with all the results of the 6 approaches:
Method
305 nm
Mean (1σ)
324 nm
Mean (1σ)
380 nm
Mean (1σ)
Obs
Original
No correction
1.27 (0.15)
1.15 (0.10)
1.11 (0.12)
267
S1
Apply Ta slope
1.17 (0.13)
1.07 (0.09)
1.05 (0.13)
135
S2
Apply Tas slope
1.18 (0.13)
1.09 (0.10)
1.05 (0.13)
135
M1
Model
1.13 (0.12)
1.04 (0.08)
1.01 (0.11)
135
M2
Model constant SSA
1.14 (0.13)
1.03 (0.09)
0.99 (0.10)
267
M3
Model const. SSA-AOD
1.12 (0.13)
1.02 (0.09)
0.98 (0.10)
267
S3
1 + 3 * Ta (λ)
1.11 (0.13)
1.03 (0.10)
1.01 (0.11)
135
OMI/Brewer
ratio [R]
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
17
Post correction methods
Overview of post corrections
6th Approach: CF = 1 + 3 * Ta(λ)
Table with all the results of the 6 approaches:
Method
305 nm
Mean (1σ)
324 nm
Mean (1σ)
380 nm
Mean (1σ)
Obs
Original
No correction
1.27 (0.15)
1.15 (0.10)
1.11 (0.12)
267
S1
Apply Ta slope
1.17 (0.13)
1.07 (0.09)
1.05 (0.13)
135
S2
Apply Tas slope
1.18 (0.13)
1.09 (0.10)
1.05 (0.13)
135
M1
Model
1.13 (0.12)
1.04 (0.08)
1.01 (0.11)
135
M2
Model constant SSA
1.14 (0.13)
1.03 (0.09)
0.99 (0.10)
267
M3
Model const. SSA-AOD
1.12 (0.13)
1.02 (0.09)
0.98 (0.10)
267
S3
1 + 3 * Ta (λ)
1.11 (0.13)
1.03 (0.10)
1.01 (0.11)
135
OMI/Brewer
ratio [R]
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
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Post correction methods
Correction results
324 nm
305 nm
500
80
OMI irradiance mW/m2nm
OMI irradiance mW/m2nm
70
60
50
40
30
305nm +11%
20
10
0
0
10
20
30
40
50
60
2
Brewer irradiance mW/m nm
70
400
300
200
100
324nm +2%
80
0
0
380 nm
100
200
300
2
Brewer irradiance mW/m nm
400
500
OMI irradiance mW/m2nm
1000
800
600
400
380nm +0%
200
0
0
200
400
600
800
2
Brewer irradiance mW/m nm
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
1000
31.7.2017
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Post correction methods
Effects of sza, AOD, SSA, ozone, time on ratios
1.8
1.8
1.8
original
corrected-mod
corrected mod-c
corrected 1+3*Tabs
OMI//Brewer
Brewerratio
ratio305nm
at 324 nm
OMI
OMI / Brewer ratio 380nm
1.6
1.6
1.6
1.4
1.4
1.4
1.2
1.2
1.2
1
11
0.8
0.8
0.8
0.6
0.6
0.6
0.4
0.4
0.4
0.2
10
0.2
10
0.2
0
original original
1.15 (0.10)
corr mod-c
corrected-mod1.03 (0.09)
corr 1+1.5*Tabs
1.07 (0.09)
mod-c
corrected
corr 1+3*Tabs
1.03 (0.10)
corrected 1+3*Tabs
20
30
20
0.02
40
30
50
60
50
Sza 40
(deg)
0.04
0.06angle (deg)
0.08
solar zenith
70
60
80
0.1
70
80
0.12
Tabs
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
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Spatial and temporal UV variability within an OMI grid
Campaign: 1 to 30 October, 2007
•3 sites
Each:
NILU UV at 305, 324, 380nm
CIMEL (AOD, SSA, ..)
Pyranometer, sky camera
Main site
+ Brewers
Spectral UV, ozone
CCD (spectral AOD)
2 Lidars (City – Rural)
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
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Spatial and temporal UV variability within an OMI grid
UV Measurements at the three sites
Irradiance at 324 nm (mW m -2 nm -1)
350
NILU-UV AUTH
NILU-UV EPANOMI
NILU-UV SINDOS
OMI
Brewer cloudless
Brewer all
300
250
200
150
100
50
0
02-Oct
07-Oct
12-Oct
17-Oct
22-Oct
27-Oct
01-Nov
Day of 2007
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
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Spatial and temporal UV variability within an OMI grid
AOD variability in an OMI grid
1.4
Location
Epanomi / rural site
Sindos / industrial site
1.2
AOD at 340 nm
AUTH / urban site
1.0
0.8
0.6
RAIN
0.4
0.2
0.0
30-Sep 3-Oct
6-Oct
9-Oct
12-Oct 15-Oct 18-Oct 21-Oct 24-Oct 27-Oct 30-Oct
Day of year - 2007
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
23
Spatial and temporal UV variability within an OMI grid
UV differences in an OMI grid
AUTH
BREWER AUTH
40
EPANOMI
20
0
% UV Irradiance difference
from AUTH (380 nm)
+20%
SINDOS
-20
AOD at 340 nm
-20%
0.6
0.5
0.4
0.3
0.2
0.1
0.0
14-Oct
15-Oct
16-Oct
17-Oct
18-Oct
19-Oct
Day of year 2007
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
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Spatial and temporal UV variability within an OMI grid
Spatial UV variability at 3 stations
(2 * sigma / mean)*100
Integration Time
UV Variability at 324 nm
120
60
100
50
100*[2*SD /mean]
80
40
60
30
40
20
80% of cloudy cases
20
90% of cloudless cases
0
270
275
10
280
285
290
295
300
305
Days of 2007
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
25
Spatial and temporal UV variability within an OMI grid
Temporal UV variability
Integration Time
UV Variability
at 3242007
nm for OMi overpass
Average UV variability
of October
time
12030
60
29
100
324 nm
28
50
100* [ 2*SD / mean]
100*[2*SD /mean]
8027
40
26
60
25
30
24
40
20
80% of23
cloudy cases
2022
90% of cloudless cases
10
(2 * sigma / mean)*100
21
0
270
20
0
275
280
10
285
290
20Days of 2007
30
295
40
300
305
50
60
Integration Time (min)
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
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Conclusions
•3.5 years of OMI and ground based at Thessaloniki, Greece: measurement comparison
showed an OMI overestimation of UV irradiances.
Cloudless cases: Main reason is the aerosol absorption. Higher deviations at lower wavelengths
•Possible methods to correct this effect: AOD and SSA measurements or/and
absorption climatology needed in a global scale
an aerosol
•SSA in the UV: while mean SSA at 440 nm is 0.90 (Thessaloniki) an SSA of 0.82 is needed for
eliminating GB and OMI UV differences at 305nm. SSA at UV-B wavelengths needs further
investigation.
•Simple public information (e.g. UVINDEX) retrieved from OMI at such populated-urban areas
are affected from this bias. +20% on cloudless day.
•Aerosol variation within an OMI satellite pixel can cause UV differences equal to a percentage
(~18%) that 90% of cloudless comparison cases lie within. Statistical analysis limitations ?
•Spatial and temporal UV variability has to be taken into account when comparing GB and
satellite UV, especially at city areas.
•Comparison under cloudy conditions requires more investigation as absolute differences are
large and spatial and temporal UV variability plays a very important role on single station –
satellite, comparison.
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
27
Thank you
Campaign acknowledgments:
D. Balis, N. Kouremeti, V. Amiridis, M. Zebila,
E. Giannakaki, J. Herman, AERONET
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
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Spatial and temporal UV variability within an OMI grid
OMI – GB normalized biases 3 stations
30
305 nm
380 nm
20
normalized OMI - GB % difference
324 nm
10
0
-10
-20
-30
27-Sep
2-Oct
7-Oct
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
12-Oct
17-Oct
day of Year 2007
22-Oct
27-Oct
1-Nov
31.7.2017
29
Back up air masses 4 day back traj
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
30
Back up – Lidar 2 days
5
5
06.10.2007
16.10.2007
06.10.2007
16.10.2007
06.10.2007
16.10.2007
4
4
532 nm
HEIGHT, asl [km]
355 nm
355 - 532 nm
3
3
2
2
1
1
0
0
0
3
6
9
-1
-1
BACKSC. COEF. [Mm sr ]
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
0
1
2
3
-1
-1
BACKSC. COEF. [Mm sr ]
0
2
4
6
COLOR INDEX
31.7.2017
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Back up TOMS and UVA correction
TOMS - BREWER RATIO
original = 1.18 (0.13)
Use of additional UVA measurements
2.0
corrected = 1.01 (0.09)
1.8
Toms / Brewer at 324 nm
1.6
1.4
1.2
1.0
No UVA measurements
0.8
0.6
0.4
0.2
0.0
1996
1997
1998
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
1999
2000
Year
2001
2002
2003
2004
31.7.2017
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Back up Brewer –MODIS (2000-2007)
MODIS/Terra vs BREWER 30 minute coincidences
2
MODIS/Terra MEAN AOD v0.04
MODIS/Terra CORRECTED AOD@355nm
MODIS/Terra MEAN AOD v0.05
1.6
1.2
Equation Y = 0.806 * X + 0.2059
Number of data points used = 161
Average X = 0.353 & Y = 0.49
R-squared = 0.453
Equation Y = 0.8981 * X + 0.11322
Number of data points used = 311
Average X = 0.395 & Y = 0.468
R-squared = 0.576
0.8
0.4
0
0
0.4
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
0.8
1.2
BREWER AOD@355nm
1.6
2
31.7.2017
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Back up Brewer AOD (1996-2007)
Clusters
Mean AOD at 340 nm
0.80
0.60
#1
North West (Atlantic)
#2
North
#3
West
#4
East, North-East
#5
Western, Local and Saharan dust
Mean AOD
0.40
0.20
0.00
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Months
31.7.2017
Back up SSA Thessaloniki (1998-2005)
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
35
Brewer (350nm)
CIMEL (440nm)
DUTH (550nm, average)
DUTH (550nm, average 0-1 km)
Ground nephelometer (450nm)
Back up SSA scout
1
Single Scattering Albedo
0.95
0.9
0.85
0.8
0.75
195
196
197
198
199
200
201
202
203
204
205
206
Day of year
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
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Spatial and temporal UV variability within an OMI grid
Spectral measurements of direct and global UV irradiance at the surface
were made with two Brewer spectroradiometers. In addition, global (diffuse
plus direct) UV irradiance and photosynthetically active radiation (PAR) were
measured, on a minute basis, at each of the three sites with three NILU-UV
multi-channel radiometers.
In-situ measurements of aerosol vertical profiles were derived from two Lidar
systems operating at (AUTH) and the site of Epanomi.
Total ozone column was derived from the Brewers and cloud observations
and sky images at the AUTH site. Cloud observations were performed at all
sites at a half hour basis.
Sun and sky radiance measurements were conducted with three CIMEL
automatic sun tracking photometers, each installed at one of the three sites.
These data were used to derive aerosol optical properties such as the aerosol
optical depth (AOD), the Angstrom exponent a (AEa) and the single scattering
albedo (SSA).
Stelios Kazadzis, OMI science team meeting Helsinki, June 2008
31.7.2017
37