1. High-resolution spectroscopy of nitrous acid (HONO) and its deuterated species (DONO) in the far- and mid-IR spectral regions A. Dehayem-Kamadjeu, J. Orphal, A. Perrin, N. Ibrahim, I. Kleiner, and J.-M. Flaud Laboratoire Interuniversitaire des Systèmes Atmosphériques CNRS UMR 7583, Universités Paris-7 et -12 Créteil / Paris, France

2. 1. Motivation Nitrous acid HONO is one of the most important sources of tropospheric OH radicals in the morning hours HONO is formed in the troposphere by heterogeneous reactions involving NO x However its sources are currently not well understood (huge differences wrt. models) Spectroscopic studies are useful both in the laboratory and in the field

3. 2. Previous IR studies Previous IR studies of HONO (trans / cis) –Deeley and Mills, 1983-1985 –Guilmot, Herman et al., 1991-1993 –most of the six IR fundamental bands have been analyzed for trans-HONO –for cis-HONO some bands are missing DONO has been studied at high resolution too but only a few IR bands No far-infrared rotational spectra (but MW)

4. 3. This work First far-infrared rotational spectra of trans- and cis-HONO (and -DONO) First high-resolution spectra of the 1 bands of trans- and cis-DONO Re-analysis of the Coriolis-interacting 5 / 6 bands of trans-HONO around 16  m Ab-initio calculations, semi-experimental and equilbrium structures of HONO (with J. Demaison, Lille, see Prague 2006)

5. 4. Experimental Bruker IFS 120 HR Fourier spectrometer Spectral resolution 0.002 cm -1 Far-infrared spectra: –White cell made of Pyrex glass, path 800 cm, windows made of polyethylene, free spectral range 20-700 cm -1, He cooled bolometer Mid-infrared spectra: –Pyrex cell, path 300 cm, CaF 2 windows Synthesis: H 2 O + NO + NO 2 mixtures

6. 5. Far-infrared spectra Overview of the far-IR part (rotational lines)

7. 5. Far-infrared spectra Expanded view (80-140 cm -1 ), H 2 O lines

8. 5. Far-infrared spectra Zoom (82-89 cm -1 ), H 2 O (HDO, D 2 O) lines

9. 5. Far-infrared spectra Rotational transitions (b-type) up to K a =22

10. 5. Far-infrared spectra The observed spectrum is well reproduced

11. 5. Far-infrared spectra The DONO spectrum is even more dense

12. 5. Far-infrared spectra The DONO spectrum is even more dense

13. 5. Far-infrared spectra Observed vs. calculated spectrum (all species)

14. 5. Far-infrared spectra Observed vs. calculated spectrum (all species)

15. 6. The 1 band of trans-DONO Many other lines (HDO), weak b-type transitions

16. 6. The 1 band of cis-DONO Much (!) weaker, only b-type transitions observed

17. 6. The 1 band of cis-DONO Weak and noisy spectrum, but « easy » analysis

18. 6. The 1 band of trans-DONO Only trans-DONO shows a weak perturbation

19. 7. The 5 / 6 bands of trans-HONO

20. 7. The 6 band of trans-HONO

21. 7. The 5 band of trans-HONO

22. Future studies Line intensities in the far-IR (in progress) Other bands of DONO ( 5 / 6 ) and HONO Field measurements in the IR ! Acknowledgements O. Pirali, P.-M. Flaud (LPPM Orsay) J. Demaison (PhLAM Lille)