MAX-DOAS observations and their application to validations of satellite and model data in Wuxi, China

1) Satellite group, Max Planck institute for Chemistry, Mainz, Germany2) Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, China3) Belgian Institute for Space Aeronomy – BIRA-IASB, Brussels, Belgium4) Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, Greece

01.09.15

Yang Wang, Thomas Wagner, Pinhua Xie, Ang Li, Steffen Beirle, Nicolas Theys, Isabelle De Smedt, MariLiza Koukouli, Trissevgeni Stavrakou

01.09.2015 - 19th OMI Science Team Meeting, 2015, Yang Wang- 2 -- 2 -

Overview:

Where is Wuxi city?

Motivation, satellite data and MAX-DOAS measurements in Wuxi

MAX-DOAS results, profiles of aerosols and trace gases

Effects of Aerosol and shape factor of trace gases on box AMF and

AMF of satellite retrieval

Comparison of daily averaged OMI VCD with MAX-DOAS VCD

Annual variation of aerosol and trace gases from MAX-DOAS, OMI

and CTM

Conclusion

- 3 -

Where is Wuxi?

Wuxi city (circle) is about 130 km north-west of Shanghai (triangle) and by the Yangtze river. The population in this city is about six millions. It is located at the boundary of the area with high pollution adjoined to Shanghai.

NO2 DOMINO SO2 BIRA HCHO BIRA

2011-2014 mean OMI

Motivation

- 4 -

NO2, SO2, HCHO are important for environment and climate science

Satellite is valuable way to obverse the global distribution.

Some challenges for the retrieval of the Trop. VCD for satellite:

1) SO2, HCHO SCD retrievals are influence by ozone and low absorption signal.

2) Tropospheric AMF calculations :

Shape factor of trace gases from chemistry transfer model

Not including aerosols

cloud products sensitive to aerosols

MAX-DOAS => aerosol and trace gases profiles and Trop. VCD => validate satellite products

Satellite data

- 5 -

Ozone monitor instrument:

Resolution: 13 x 24 km, daily global coverage, overpass time 13:30

Data sets (2011-2014):

1) NO2, DOMINO 2.0 product, trace gas shape factor from TM4 model (KNMI)

2) SO2,

• BIRA-IASB (N. Theys et al. JGRD., 2015), trace gas shape factor from

IMAGESv2, horizontal resolution of 2° × 2.5° (Stavrakou, Atmos. Chem. Phys.

2013)

• NASA, Nickolay Krotkov, http://disc.sci.gsfc.nasa.gov/

3) HCHO, BIRA-IASB (I. De Smedt et al. ACPD., 2015), trace gas shape factor

from IMAGESv2

Our measurements in Wuxi station

- 6 -

• Spectral range: 290 – 425 nm (NO2, SO2, HCHO and O4).• Elevation angle: 5°, 10°, 20°, 30° and 90°• Azimuth angle: Exact north

MINI MAX-DOAS from 2011 to 2014:

• O4 and NO2, 350 nm – 391 nm; SO2, 307.8 – 330; HCHO, 324.6 nm – 359 nm• Filters: shift < 1 pixel, offset correction < 1%, RMS < 0.01, SZA < 75

SCD retrieval:

VCD and profile:

• Nonlinear optimal estimation method to retrieve profiles of aerosol extinction and trace gas VMR, then integrate profiles to acquire VCD.

• Filters: difference between measured and retrieved dSCD < a threshold; keep trace gas profiles with convincing aerosol profiles

• the sky conditions (cloud free, aerosol and clouds) identified by MAX-DOAS (Atmos. Meas. Tech. Discuss. 8, 4653–4709, 2015) help avoid the influence of clouds on aerosol and trace gases results of MAX-DOAS.

- 7 -

Normalized profiles of aerosol extinction and trace gas VMR from MAX-DOAS

- 7 -

The profile shapes:Aerosol: Gaussian NO2: exponentialSO2: exponential but higher layer HCHO: Largest value near surface,a box shape above 0.5km to 1kmthen fast decrease

Different seasons:Shapes similar

In cloud free sky

shape factor

0.0 0.2 0.4 0.6 0.8 1.00

1

2

3

4

aerosol shape factor

alti

tude

/ km

0.0 0.2 0.4 0.6 0.8 1.001234 priori winter spring summer autumn

0.0 0.5 1.0 1.50

1

2

3

4NO2 shape factor

alti

tude

/ km

0.0 0.5 1.0 1.50

1

2

3

4SO2 shape factor

alti

tude

/ km

0.0 0.5 1.0 1.50

1

2

3

4HCHO shape factor

alti

tude

/ km

- 8 -

Discrepancy of shape factors from MAX-DOAS and CTM, and its effect on AMF

Difference: HCHO>SO2>NO2

clear sky: SZA: 40 SAA:-140 VZA:30 VAA:40Surface albedo: 0.05

including MAX-DOAS shape factor => decrease AMF (HCHO >SO2 > NO2)

Totally mean 13:00 to 14:00

0.0 5.0x10-6 1.0x10-5 1.5x10-50

1

2

3

4

altit

ude

/ km

shape factor

NO2: CTM MAX-DOASHCHO: CTM MAX-DOASSO2: CTM MAX-DOAS

NO2 at 435nm HCHO at 337nm SO2 at 319nm0.0

0.5

1.0

1.5

2.0

2.5

12%15%

2%

CTM shape factor MAX-DOAS shape factor

AM

F

- 9 -

0 1 20

1

2

3

4

0 1 2 0 1 2

clear sky aerosol

0% 20% 40% 60%0

1

2

3

4 NO2 HCHO SO2

alti

tude

/ km

relative difference

Geometry: SZA: 40 SAA:-140 VZA:30 VAA:40

Effects of Aerosol on box AMF of satellite retrieval- compared with clear sky

NO2 at 435nm HCHO at 337nm SO2 at 319nm

0.0 0.2 0.4 0.6 0.8 1.00

1

2

3

altitu

de /

km

aerosol extinction / km-1

depended on wavelength, stronger at short wavelength (SO2)

Shading effect occurs below 1km, its magnitude up to 50%

Enhancing effect above 1km, up to 10%

 

AOD: 0.83SSA: 0.9g: 0.72

mean aerosol profile from MAX-DOAS

box AMF

alti

tud

e /

km

- 10 -

0 1 20

1

2

3

4

0 1 2 0 1 2

alt

itu

de /

km

clear sky aerosol fake low clouds fake high clouds

0% 100% 200% 300%0

1

2

3

4

alti

tude

/ km

relative difference

NO2: fake low clouds fake high cloudsHCHO : fake low clouds fake high clouds SO2: fake low clouds fake high clouds

fake low clouds => CTP=1040 hPa (surface) fake high clouds => CTP=900 hPa (1km)

Effects of Aerosol on box AMF of satellite retrieval- effect of fake clouds due to aerosols on box AMF

Lambertian cloudsCF=10% -> CRF=25%

NO2 at 435nm HCHO at 337nm SO2 at 319nm

1. Treating aerosol as clouds, especially low clouds can overestimate the boxAMF strongly, up to 300% near the ground.

2. This overestimation is stronger at short wavelength (SO2)

CF up to 15%, CTP >900 hPa for high anthropogenic aerosol load (AOD>0.4)Cloud and aerosol classification for 2 ½ years of MAX-DOAS observations in Wuxi (China) and comparison to independent data sets.Atmos. Meas. Tech. Discuss. 8, 4653–4709, 2015

box AMF

- 11 -

Effects of Aerosol on AMF of satellite retrieval

AMF in clear sky is larger by 6%-10% (SO2 > NO2 > HCHO)

Treating aerosol as low clouds increase AMF by up to 100%. As high clouds increase AMF by up to 30% (HCHO and SO2), but good for NO2

NO2 HCHO SO20.0

0.5

1.0

1.5

2.0

2.5

AM

F

clear sky aerosol profile fake low clouds fake high clouds

boxAMF:

Mean shape factors from MAX-DOAS

4 types of boxAMFs

4 AMFs for NO2, SO2,HCHO

Effect depended on CTP (large

uncertainty in cloud products for

aerosols)

Suggest: calculate AMF in clear

sky when CTP>900 hPa

Poster: Evaluation of the effect of strong aerosol loads on satellite retrievals of tropospheric NO2, SO2 and HCHO using MAX-DOAS observations in Wuxi, China.=> six cases with AOD from 0.6 to 1.7 in cloud free sky, combine MAX-DOAS, Aeronet, MODIS and OMI observations

0 20 40 60 800

20

40

60

80CTP>900hPa (1km)

R2=0.84slope=0.78

OM

I T

rop.

VC

D /

1015

mol

ecs/

cm2

MAX-DOAS VCD / 1015 molecs/cm2

200

360

520

680

840

1000

cloud top pressure / hPa

Comparison of OMI VCD with MAX-DOAS VCD- NO2

- 12 -

0 20 40 60 800

20

40

60

80

CF<10%

CF<20%R2=0.80slope=0.73

OM

I T

rop.

VC

D /

1015

mol

ecs/

cm2

MAX-DOAS VCD / 1015 molecs/cm2

0%

5.0%

10%

15%

20%

R2=0.70slope=0.63

effective cloud fraction

CF<10%

CF<20%

The high cloud shading effect underestimate NO2 VCD strongly, slopes improved by 15% by excluding high clouds

Aerosol effect underestimate NO2 VCD strongly, slopes improved by 17% by excluding high clouds, treating aerosol as clouds.

For clear sky, OMI VCD lower than MAX-DOAS VCD by 5%

Cloud fraction Cloud top height

0 20 40 60 800

20

40

60

80AOD<0.5

R2=0.92slope=0.95

OM

I T

rop.

VC

D/1

015 m

olec

s/cm

2

MAX-DOAS VCD/1015 moles/cm2

0.20

0.36

0.52

0.68

0.84

1.0

AOD:CF<10% CTP>900hPa

MAX-DOAS: +- 30 minutes around overpass timeOMI: distance from pixel center to station < 50 km

Coincident criteria:

High cloud shading effect: Using CTP, Improvement of linear regression by excluding the data with CTP<900Aerosol effect: Using AOD, Improvement of linear regression by excluding the data for AOD>0.5

Comparison of daily averaged OMI VCD with MAX-DOAS VCD- SO2

- 13 -

High clouds shading effect: excluding high clouds, slope is improved by 10%.

Aerosol effect: excluding large aerosols, slope is improved by 11%. In clear sky underestimation by 40%? Speculation: shape factor, residual

aerosol and cloud effect, gradient smoothing effect and SCD retrieval.

-30 0 30 60 90 120 150-30

0

30

60

90

120

150

BIR

A O

MI

VC

D [

1015

mol

ec/c

m2 ]

CTP>900hPa

R2=0.89slope=0.46

CF<30%

R2=0.75slope=0.36

MAX-DOAS VCD [1015 molec/cm2]

200

360

520

680

840

1000

CTP

BIRA

-30 0 30 60 90 120 150-30

0

30

60

90

120

150

BIR

A O

MI

VC

D [

1015

mol

ec/c

m2 ]

CF<10% AOD<0.5

R2=0.91slope=0.57

CF<30%

R2=0.89slope=0.46

CF < 30%, CTP >900:0

0.2

0.4

0.6

0.8

1

AOD:

MAX-DOAS VCD [1015 molec/cm2]

Comparison of daily averaged OMI VCD with MAX-DOAS VCD-HCHO

- 14 -

-20 0 20 40-20

0

20

40

error<8

R2=0.80slope=0.79

OMI VCDrandam error

1015molec/cm2

all data

R2=0.64slope=0.72

OM

I V

CD

/ 10

^15

mol

ec/c

m2

MAX-DOAS VCD / 10^15 molec/cm2

4.0

8.0

12

16

20

mean OMI VCD =11.2mean OMI VCD random error=9.3mean MAX-DOAS VCD=13.8

The random error from the DOAS fitting of SCD causes the large scattering points. Excluding most cloud and aerosol effects improve the slope by 13% In clear sky OMI underestimate HCHO by 8%.

Cloud properties

Random error

-20 0 20 40-20

0

20

40CF<10%

R2=0.63slope=0.71

CF<30%

R2=0.64slope=0.72

OM

I V

CD

/ 10

^15

mol

ec/c

m2

MAX-DOAS VCD / 10^15 molec/cm2

0%

10%

20%

30%

CF:

-20 0 20 40-20

0

20

40

OM

I V

CD

/ 10

^15

mol

ec/c

m2

MAX-DOAS VCD / 10^15 molec/cm2

CF<10%

R2=0.81slope=0.92

0%

10%

20%

30%

error<8 CF<30%

R2=0.80slope=0.79

CF:

- 15 -

Annual variation of aerosol and trace gases- Bimonthly mean AOD from MAX-DOAS and AERONET

MAX-DOAS AERONET

90% 75%meanMedian25%10%

Well agreement There is not regular variation

AERONET level 1.513:00 to 14:00

7 11 3 7 11 3 7 11 3 7 110

1

2

AO

D

Month

2011 2012 2013 2014

- 16 -

Annual variation of aerosol and trace gases- Bimonthly mean VCD of trace gases

Comparison:Variation trend agree wellMAXDOAS>OMI>CTM

Maximum:NO2 in winter; SO2 in winterHCHO in summer;

NO2:

SO2:

HCHO:

2011 2012 2013 2014

Tro

p. V

CD

[10

^15

mo

lecs

/cm

^2]

0

20

40

60

80 MAX-DOAS DOMINO TM4

-10

0

10

20

30 MAX-DOAS BIRA IMAGES

5 7 9 11 1 3 5 7 9 11 1 3 5 7 9 11 1 3 5 7 9

MAX-DOAS

-50

0

50

100

150 BIRA NASA

13:00 to 14:00

- 17 -

Conclusion:

1. Differences of the profile shapes from MAX-DOAS and CTM are larger for

HCHO>SO2>NO2 => effect on AMF (HCHO > SO2 > NO2) in clear sky

2. Treating aerosol as clouds, especially low clouds cause the boxAMF overestimated by up

to 300% and AMF by up to 100%. The effect is strongly depended on CTP.

3. We suggest to calculate AMF in clear sky when CTP>900 hPa to avoid the large error

from treating aerosol as low clouds.

4. Cloud shading effect and aerosol effect (treating aerosol as clouds) make OMI

underestimate NO2, SO2 and HCHO strongly. We can use CTP>900 hPa and AOD<0.5 to

exclude high clouds and strong aerosol to improve validation.

- 18 -- 18 -

Great thanks for your attention!

30.04.2014 - EGU General Assembly 2014, Yang Wang

Comparison of daily averaged OMI VCD with MAX-DOAS VCD-HCHO

- 19 -

-20 0 20 40-20

0

20

40

error<8

R2=0.80slope=0.79

OMI VCDrandam error

1015molec/cm2

all data

R2=0.64slope=0.72

OM

I V

CD

/ 10

^15

mol

ec/c

m2

MAX-DOAS VCD / 10^15 molec/cm2

4.0

8.0

12

16

20

mean OMI VCD =11.2mean OMI VCD random error=9.3mean MAX-DOAS VCD=13.8

The random error from the DOAS fitting of SCD causes the large scattering points. Aerosol effect underestimate VCD by 13%.In clear sky underestimation by 8%.

Cloud properties

Random error

-20 0 20 40-20

0

20

40CF<10%

R2=0.63slope=0.71

CF<30%

R2=0.64slope=0.72

OM

I V

CD

/ 10

^15

mol

ec/c

m2

MAX-DOAS VCD / 10^15 molec/cm2

0%

10%

20%

30%

CF:

-20 0 20 40-20

0

20

40

OM

I V

CD

/ 10

^15

mol

ec/c

m2

MAX-DOAS VCD / 10^15 molec/cm2

CTP:CTP>900

R2=0.72slope=0.77

error<8

R2=0.80slope=0.79

200.0

360.0

520.0

680.0

840.0

1000

-20 0 20 40-20

0

20

40AOD<0.5

R2=0.79slope=1.2

error<8

R2=0.80slope=0.79

AOD:

OM

I V

CD

/ 10

^15

mol

ec/c

m2

MAX-DOAS VCD / 10^15 molec/cm2

0

0.20

0.40

0.60

0.80

1.0

-20 0 20 40-20

0

20

40

OM

I V

CD

/ 10

^15

mol

ec/c

m2

MAX-DOAS VCD / 10^15 molec/cm2

CF<10%

R2=0.81slope=0.92

0%

10%

20%

30%

error<8

R2=0.80slope=0.79

CF:

0 200 400 600 800 10000

10

20

OM

I H

CH

O V

CD

err

or /

10^1

5 m

olec

s/cm

2

CTP hPa0 10 20 30 40

0

1

2

AO

D

HCHO VCD from MAX-DOAS

R2=0.85

Comparison with collocated independent techniques- Point to point, time difference <15 min, in cloud free sky

- 20 -

0 1 2 30

1

2

3

AE

fro

m M

AX

-DO

AS

/ km

-1

aerosol extinction from visibility meter/ km-1

R=0.67, slope=0.96, intercept=0.20

0 1 2 30

1

2

3R=0.72, slope=0.54, intercept=0.31

AOD from AERONET

AO

D f

rom

MA

X-D

OA

S

0 20 40 60 800

20

40

60

80R=0.67, slope=0.54, intercept=7.85

NO

2 V

MR

fro

m M

AX

-DO

AS

/ ppb

NO2 VMR from LP-DOAS / ppb0 20 40 60 80

0

20

40

60

80

SO2

VM

R f

rom

MA

X-D

OA

S / p

pb

SO2 VMR from LP-DOAS / ppb

R=0.79, slope=1.08, intercept=5.07

1. Well agreement with sunphotometer (20km away), visibilitymeter and LP-DOAS (nearby). R ≈0.7

AOD Near surface aerosol extinction

Near surface NO2 VMR Near surface SO2 VMR

2. Aerosol:Best agreement in summer => stronger wind cause homogeneous vertical and horizontal distribution in 0 to 200meters.worst in spring => dusk storm3. Trace gases:MAX-DOAS larger in summer and spring due to lifted high concentration in 0 to 200 meters (Meng, et al, 2008, Tower measurements in Beijing )