Stratospheric aerosol profile retrieval from SCIAMACHY limb observations

• Introduction • Data and Methodology • Sensitivity Studies and Comparisons • Conclusions

Yang Jingmei Zong Xuemei

Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029, China

Introduction

high accuracy and high vertical resolution poor geographical coverage.

huge information of atmospheric trace gases and aerosols

1E-6 1E-5 1E-4 1E-3

10

15

20

25

30

35

40

Alti

tude

(km

)

Aerosol Extinction Coefficient (km-1)

50-60oN 40-50oN 30-40oN 20-30oN 10-20oN 0-10oN

The aim of this study: Retrieve aerosol extinction profiles from SCIAMACHY Limb measurements

A long term measurement of stratospheric aerosol distributions is necessary for a better understanding of the stratospheric processes.

Before the middle 2000s, the satellite aerosol profile measurements were mainly carried out by the instruments using the solar occultation technique .

The new-generation of satellite instruments perform limb measurements of the scattered solar radiation in the Earth’s atmosphere.

Annual averaged zonal mean SAGE II 1020 nm aerosol extinction profiles over 1998–2004. The observed heights of the tropopause are indicated by the vertical bars (one standard deviation.

DATA AND METHODOLOGY SCIAMACHY on board ENVISAT: Channels: 8 Spectral range: 240 – 2380 nm Spectral resolution: 0.2 – 0.5 nm Tangent heights: -3 – 100 km Vertical intervals: 3.3 km We use level 1 version 7.04 limb observation data

(Institute of Remote Sensing, University of Bremen). We use SCIATRAN for Limb scatter radiance simulation and weighting function calculations.

400 500 600 700 800 900 10000

1

2

3

4

5

6

Limb r

adian

ce (1

013ph

otons

s-1cm

-2nm

-1sr-1

)

Wavelength (nm)

12.0 km 15.2 km 18.6 km 21.8 km 25.1 km 28.4 km 31.7 km 35.0 km

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.1410

15

20

25

30

35

40 12.0 km 15.2 km 18.6 km 21.8 km 25.1 km 28.4 km 31.7 km 35.0 km

Altitu

de (k

m)dln(I)/dln(βa)

SCIAMACHY limb radiance spectra at 12.0, 15.2, 18.6, 21.8, 25.1, 28.4, 31.7 and 35.0 km tangent height on 21 January 2004, when the tangent point passed over western China (36.0°N, 97.3°E).

Sensitivity of the measured radiances at 750 nm to variations of the aerosol extinction coefficients for the above observing conditions.

Retrieval Method

The retrieval algorithm is established on the basis of the optimal estimation method described by Rodgers[4]. The calculation is performed in an iterative manner by employing the equation:

In order to reduce the sensitivity to the assumed surface albedo and to minimize the impact of instrument calibration errors, the limb radiance at each tangent height is normalized to the radiance at a reference tangent height:

The equation to be solved for the retrieval is a non-linear inverse problem :

yexFy += )(

)]()[()( 01111

001 nnnyTnny

Tnn xxKyySKKSKSxx −−−++= −−−−

+

),(),(),(

refTHTHTH

λλλ

III i

in =

))TH,(

)TH,(ln(

isn

iln

I

Iy

λλ

=To reduce the effect of the Rayleigh scattering, the ratio of two spectral channels is used in the retrieval. We choose the wavelength pare of λl = 750nm and λs = 470nm and construct the measurement vector y as :

F: the non-linear forward model; y: measurement vector; X: the state vector; ey :measurement errors of all kinds.

I (λ, THi ): the limb radiance at wavelength λ and tangent height THi, THref :reference tangent height (35 km).

In (λl,TH) and In (λs,TH): the normalized measured radiance at the wavelengths of λl and λs. λl = 750 nm and λs =470 nm.

S0 is the a priori covariance matrix of the solution; Sy is the error measurement covariace matrix, Kn is the matrix of weighting functions, x0 is the a priori aerosol profile, and yn is the calculated y value. Subscripts n and n+1 denote the number of the iteration.

(1)

(2)

(3)

(4)

Flow Chart for Retrieval Process

No

Output Retrieved Profile

Yes

Criterion for Iteration

End

Start

Input SCIAMACHY Observed Radiance

Input Latitude, Longitude, Solar Angles, Viewing Angles, Tangent Heights, and a Priori Aerosol Profile

SCIATRAN Simulated Radiance

Radiance Normalization

Forward Model

yexFy += )(

)]()[(

)(

01

101

11101

nnnyTnnn

nyTnn

xxKyySKSxx

KSKSS

−−−+=

+=−

++

−−−+

Aerosol Profile

Sensitivity Studies

Relative errors (Err) of the retrieved aerosol extinctions by using the assumed surface albedo in the retrieval when the actual surface albedo values are A = 0.1, 0.2, 0.4 and 0.5.

Sensitivity analyses are performed to investigate the impact of the bias in the assumed surface albedo on the accuracy of the aerosol extinction retrieval.

Height (km) A = 0.1 Err (%)

A = 0.2 Err (%)

A = 0.4 Err (%)

A = 0.5 Err (%)

15 −3.4 −1.2 0.8 1.5

20 −3.7 −1.8 1.2 2.5

25 −4.9 −2.0 1.6 3.1

30 −5.3 −2.7 1.8 3.5

35 −6.2 −3.6 2.2 4.2

The average surface albedo is used in the calculation:

COMPARISONS With SAGE II

The retrieved SCIAMACHY 1020 nm aerosol extinction profile (36.0 ºN, 97.3 ºE) on 21 January 2004 at 03:42 UT compared to SAGE II profile (35.2 ºN, 93.9 ºE) on the same day at 10:55 UT . Relative difference = (SCIAMACHY−SAHE II) /SAGE II×100%.

-40 -20 0 20 40

15

20

25

30

35

Altit

ude

(km

)

Relative difference (%)

1E-6 1E-5 1E-4

15

20

25

30

35

Altit

ude

(km

)

Aerosol extinction (km-1)

retrieved SAGE II

Same as the above Figure, but for the average values of 12 individual measurements. The measurements are within 35–40 ºN latitude band, and were performed on 21 January 2004.

1E-6 1E-5 1E-4

15

20

25

30

35

Altit

ude

(km

)

Aerosol extinction (km-1)

retrieved SAGE II

-150-100 -50 0 50 100 150

15

20

25

30

35

Altit

ude

(km

)

Relative difference (%)

CONCLUSIONS

Stratospheric aerosol extinction profiles are retrieved from SCIAMACHY Limb observations

Sensitivity analyses revealed that the errors caused by the bias of the assumed surface albedo in the retrieval are generally below 6% in the northern midlatitudes.

Comparisons with the SAGE II measurements showed that the retrieved SCIAMACHY limb aerosol extinction profiles have good agreement with the SAGE II profiles.

The validation will be extended in the near future to include additional retrieval profiles and satellite measurements.

Based on the present reported study, we can conclude that our retrievals are reliable and accurate.

[1] Bourassa, A. E., D. A. Degenstein, R. L. Gattinger, et al., 2007:

Stratospheric aerosol retrieval with optical spectrograph and infrared imaging system limb scatter measurements, J. Geophys. Res., 112, D10217, doi:10.1029/2006JD008079.

[2] Bovensmann, H., J. P. Burrows, M. Buchwitz, et al., 1999: SCIAMACHY: Mission objectives and measurement modes, J. Atmos. Sci., 56(2), 127–150.

[3] Ovigneur, B., Landgraf, J., Snel, R., et al., 2011: Retrieval of stratospheric aerosol density profiles from SCIAMACHY limb radiance measurements in the O2 A-band, Atmos. Meas. Tech., 4, 2359–2373, doi:10.5194/amt-4-2359-2011.

[4] Rodgers, C. D., 2000: Inverse Methods for Atmospheric Sounding: Theory and Practice, World Scientific, Singapore.

[5] Rozanov, V. V., M. Buchwitz, K.-U. Eichmann, et al., 2002: SCIATRAN—A new radiative transfer model for geophysical applications in the 240−2400nm spectral region: The pseudo-spherical version, Adv. Space Res., 11, 1831−1835.

[6] Rozanov, A., V. Rozanov, M. Buchwitz, et al., 2005: SCIATRAN 2.0—A new radiative transfer model for geophysical applications in the 175−2400nm spectral region, Adv. Space Res., 36, 1015−1019.

[7] Taha, G., D. F. Rault, R. P. Loughman, et al., 2011: SCIAMACHY stratospheric aerosol extinction profile retrieval using the OMPS/LP algorithm, Atmos. Meas. Tech., 4, 547–556, doi:10.5194/amt-4-547-2011.

We wish to acknowledge the European Space Agency for SCIAMACHY data. We also wish to acknowledge the NASA Langley Research Center for the SAGE II aerosol data. We are thankful to the Institute of Remote Sensing, University of Bremen for the SCIATRAN software package. The research was funded by the National Natural Science Foundation of China (Grant No. 41275047, and 41175029), and the National Basic Research Program of China (Grant No. 2011CB403401).