Folkard Wittrock, Hilke Oetjen, Andreas Richter, and John P. Burrows

1. Multi-axis differential absorption spectroscopy (MAX-DOAS) at the Andøya Rocket Range in February and March 2003 Folkard Wittrock, Hilke Oetjen, Andreas Richter, and John P. Burrows

2. Structure • Focus of DOAS observations • DOAS analysis • Introduction to MAX-DOAS • Selected results from Andøya campaign • BREDOM • Summary and outlook

3. DOAS observations: Focus • Determination of atmospheric trace gases: • Ozone, NO2, NO3, OClO, BrO, IO, HCHO • Validation of satellites (GOME, SAGE, SCIAMACHY) and of model calculations • standard data products • vertical columns of ozone and NO2 • slant columns of OClO, BrO, HCHO and IO • further data products • vertical columns of BrO, OClO, HCHO • tropospheric amounts e.g. for NO2, BrO

4. Differential Optical Absorption Spectroscopy (DOAS) • Only narrowband, i. e. differential structures of the spectra are used to detect the absorbers. • Broadband absorption and other broadband structures like instrumental features and features caused by Mie and Rayleigh scattering are removed by a polynomial. • Basically, all kind of light sources can be used. • Here: Light scattered in the zenith or at the horizon

5. DOAS – Equation: • Radiation in matter is attenuated according to the Lambert-Beer-Law • The current intensity I(l) is compared to a reference I0(l) • A polynomial is added to compensate for scattering and other broadband structures • Slant columns of several absorbers (with known absorption cross sections si’) can be retrieved simultaneously • The slant column is the density of the absorber along the photon path and depends on the SZA:

6. Multi Axis (MAX)-DOAS • Extension of the light path in the troposphere • High sensitivity for tropospheric absorbers, similar path through the stratosphere • 3 angles to describe the geometry: • 1. elevation angle 2. SZA 3. relative azimuth

7. Radiative Transfer and Vertical Columns • Light path is simulated by a radiative transfer model • The air mass Factor (AMF) weights the absorption due to the changing SZA, viewing angle, azimuth • Vertical column (V) is the vertical density of the absorber: • Here: SCIATRAN (CDIPI-Version) by A. Rozanov • Input: • viewing geometry • profile of absorbers • T and p profiles • wavelength region • albedo • aerosols

8. CCDs: • 1320x400 pixel • 1024x256 pixel • Wavelength • region: • 325 – 413 nm • 330 – 490 nm • FWHM: • 0.45 nm • 0.60 nm • Integration • time: • 1 min 2 Instruments: Asymmetric Czerny-Turner Spectrometer + CCD

9. Multi-Axis Telescopeon the roof of the old Lidar-Building • Commercial stainless steel box • Shadings to avoid direct sun • Pointing of telescope towards NNW

10. MAX-DOAS telescope Automated measurements in 4 directions: 3°, 7.5°, 12.5° and zenith • Different viewing directions given by moving mirror • Box is heated to ensure operation of the motor and ice free windows • Calibration unit with lamps: Tungsten and HgCd • Quartz fiber bundle adapter

11. MAX-DOAS measurements: data analysis Basic idea: • Use O4 to find correct model settings • derive slant columns of O4 for given directions • simulate O4 slant columns (vertical column is known) and vary aerosol scenario and surface albedo until closure for all lines of sight and also solar zenith angles is obtained • Use information from different lines of sight to derive profile information for the absorber of interest: • simulate absorber’s slant column using aerosol and albedo settings from O4 retrieval and vary profile until good agreement with retrieved slant columns is reached for all directions RTM: SCIATRAN – full spherical, multiple scattering, refraction

12. MAX-DOAS measurements: O4

13. MAX-DOAS measurements: NO2

14. MAX-DOAS measurements: BrO

15. Andøya MAX-DOAS: Summary and Outlook • Results: • Multi Axis DOAS measurements from 4 different groups are being performed during the campaign • O3, NO2, BrO columns can be retrieved • tropospheric amounts of BrO and NO2 have been estimated • Andoya measurements are very valuable to check the consistency of different MAX-DOAS setups – Analysis still ongoing • Problems: • clouds make the analysis very complex • automated profile retrieval still under development

16. Acknowledgments • We like to thank the whole ARI staff for there great support and especially Reidar, Michael, June, Petter Tusen takk!

17. Bremian DOAS network for atmospheric measurements (BREDOM) • Three tropical stations • Similar setup for all measurement sites • High-sensitivity DOAS-instruments for stand-alone operation • Multiple viewing directions (MAX-DOAS) • Retrieval of ozone and NO2 as well as minor absorbers (e.g. BrO, OClO, SO2, HCHO)

18. What is the rationale behind BREDOM? For the validation of satellites (e.g. SCIAMACHY on ENVISAT), we need • long term validation • of many trace species • on a global scale From recent validations (e.g. GOME) we learned, that • DOAS UV/visible instruments can provide such validation • that tropical stations are mandatory • that tropospheric products need extra validation • Build on the existing DOAS network, extend the range of species, add stations in the tropics, add capability to monitor troposphere

19. Bremian DOAS network for atmospheric measurements (BREDOM) • Three tropical stations • Similar setup for all measurement sites • High-sensitivity DOAS-instruments for stand-alone operation • Multiple viewing directions (MAX-DOAS) • Retrieval of ozone and NO2 as well as minor absorbers (e.g. BrO, OClO, SO2, HCHO) • Campaign instrument • Kaashidhoo (5° N, 73° W, 5m asl) February – March 1999 • Po area (46° N, 9° E, 400m asl) July–August 2002, • September–October 2003 • Andoya (69° N, 16° E, 20m asl) February–March 2003