Page 1MIPAS QWG#12 14/16 February 2007

Retrieval of 15N/14N isotopic ratio from MIPAS spectra

Feasibility of 15N/14N isotopic ratio profile retrieval from MIPAS spectra

G. Brizzi, M. Carlotti, M. Ridolfi, E. PapandreaBologna University

J.-M. FlaudLaboratoire Interuniversitaire des

Systèmes Atmosphériques

B.M. DinelliISAC-CNR

Page 2MIPAS QWG#12 14/16 February 2007

Retrieval of 15N/14N isotopic ratio from MIPAS spectra

Detection of H15NO3 in the atmosphere

Spectral interval 870 – 871.5 cm-1 (MIPAS band A)

G. Brizzi, M. Carlotti, J.-M. Flaud, A. Perrin, M. Ridolfi,

First observation of H15NO3 in atmospheric spectra,Geophys. Res. Lett., Vol. 34, No. 3, 2007.

Signal of H15NO3 evident in residual spectra !

Low S / N → use of average MIPAS spectra

T.H. range 15 - 23 km

Based on the comparisonof averaged residuals

Page 3MIPAS QWG#12 14/16 February 2007

Retrieval of 15N/14N isotopic ratio from MIPAS spectra

Retrieval strategy

1 - Using retrieval software based on UNIBO GMTR software, modified to retrieve isotopic ratio.

Subsequent scans: optimal estimation with a-priori profile = profile retrieved in the previous scan with its own error (full VCM).

H14NO3 → optimal estimation with a-priori profile retrieved at step 2.

15N/14N → First scan: optimal estimation with a-priori profile = double of natural isotopic ratio (error = 50%) and Tikhonov first derivative regularization (strength tuned on the basis of a suitable trade-off between noise error and vertical resolution).

2 - First retrieving MIPAS main targets with classical GMTR approach.

3 - Scan by scan (homogeneous atmosphere) joint retrieval of H14NO3 and 15N/14N.

Page 4MIPAS QWG#12 14/16 February 2007

Retrieval of 15N/14N isotopic ratio from MIPAS spectra

Tests with simulated observations

- MIPAS simulated observations for 72 scans from orbits 2081-2082 (HR 17 sweeps)

R=115N/14N natural isotopic ratio = 0.00365 Assumed True-value

Fitted profile

- 3 MWs: MW1 H15N0001 870.000 - 871.500 cm-1

MW2 HNO30001 876.375 - 879.375 cm-1

MW3 HNO30006 885.100 - 888.100 cm-1

- Tangent altitude range from 12 to 42 km (maximum HNO3 VMR around 20 km)

Used in OFLHNO3

retrieval

Retrieval assumptions

15 143 3H NO H NO

VMR VMR R

Page 5MIPAS QWG#12 14/16 February 2007

Retrieval of 15N/14N isotopic ratio from MIPAS spectra

A-priori profileRetrieved profileIn-guess and a-priori profile of scan 1: R = 2±1True value: R = 1

R profiles retrieved from simulated observations

Medium-strength regularization

15N/14N profile – sequence 115N/14N profile – sequence 215N/14N profile – sequence 315N/14N profile – sequence 415N/14N profile – sequence 515N/14N profile – sequence 2015N/14N profile – sequence 4015N/14N profile – sequence 7015N/14N profile – sequence 7115N/14N profile – sequence 72

Retrieved profile moves towards true valueESD reduction

Retrieved profile still on the initial guess (scan 1)

R

No information !

Page 6MIPAS QWG#12 14/16 February 2007

Retrieval of 15N/14N isotopic ratio from MIPAS spectra

R profiles retrieved from simulated observations (2)

Strong regularization

15N/14N profile – sequence 115N/14N profile – sequence 215N/14N profile – sequence 315N/14N profile – sequence 415N/14N profile – sequence 515N/14N profile – sequence 2015N/14N profile – sequence 4015N/14N profile – sequence 7015N/14N profile – sequence 7115N/14N profile – sequence 72

A-priori profileRetrieved profileIn-guess and a-priori profile of scan 1: R = 2±1True value: R = 1

R

Retrieved profile moves towards true valueESD reduction

Retrieved profile still on the initial guess (scan 1)

No information !

Page 7MIPAS QWG#12 14/16 February 2007

Retrieval of 15N/14N isotopic ratio from MIPAS spectra

How many spectra have to be processed ?

ESD of R decreases with increasing number of processed scans

Page 8MIPAS QWG#12 14/16 February 2007

Retrieval of 15N/14N isotopic ratio from MIPAS spectra

Error budget estimation

- Using MWMAKE (Oxford University)

1 MW (H15N0001)

H14NO3 H15NO3

Preliminary results !

Page 9MIPAS QWG#12 14/16 February 2007

Retrieval of 15N/14N isotopic ratio from MIPAS spectra

Error budget estimation (2)

Preliminary results !

3 MWs

H14NO3 H15NO3

Page 10MIPAS QWG#12 14/16 February 2007

Retrieval of 15N/14N isotopic ratio from MIPAS spectra

Future work

4 - Establish an acceptable trade-off between the noise error (number of scans to be processed for the retrieval of an individual R profile) and the strength of the regularization.

1 - Estimate the error budget of the isotopic ratio profile (R).

5 - Carry out the analysis using real measurements (HR 17 sweeps).

3 - Based on the resulting number of scans to be processed for the retrieval of an individual R profile, establish suitable latitudinal and time windows from which MIPAS measurement should be selected.

2 - Calculate the vertical resolution (AK).