Detection of IO in the MBL using an open-path CRDS June 25th, 2009 64th International Symposium on Molecular Spectroscopy Detection of IO in the MBL using an open-path CRDS *Ryuichi Wada, Joe M. Beames and Andrew J. Orr-Ewing University of Bristol, UK * Present address: Nagoya University, Japan

Overview Introduction Experimental set-up Experiments in the laboratory Field observation Conclusions

Iodine monoxide (IO) I IO OIO Aerosol Ocean I2 CH2I2 CH3I Source Atmospheric Iodine cycle O3 n IO h I2 CH2I2 CH3I I IO OIO Aerosol NO2 HO2 IONO2 HOI Aerosol Aerosol Deposition Ocean IO is important radical to understand the chemistry in the marine boundary layer (MBL).

Current observation of IO First observation of [IO] (Mace Head, Ireland) Observation technique [IO] = 0.5 - 7 ppt (1ppt=1/1000000000000) ~ 20 km Differential Optical Absorption Spectroscopy (DOAS) U. Platt and co-workers, Nature, 397, 572, 1999 DOAS is the only technique for observation of IO. All observed [IO] were up to ~ 10 ppt.

There is no technique to measure inhomogeneous IO. Disagreement with model and observation A model for MBL prediction of [IO] = ~100 pptv A. Ravishankara et al. Atmos. Chem. Phys., 4, 19, 2004 Possible interpretation Inhomo- geneous IO spot DOAS observed average concentration in light path (~ 10 ppt) Inhomogeneous sources of IO possibly exist (~ 100 ppt) Light path There is no technique to measure inhomogeneous IO.

Cavity ring down spectroscopy It =e-kt Time /ms Intensity /arb. units It =e-k0t ∆k = k–k0 With absorbers ∆k=c[X]σX Without absorbers k: Ring-down decay rate c: Speed of light σX: Cross-section of X High sensitivity Optical path length ~ 10 km Compactness Point measurement CRDS has a possibly to detect inhomogenious IO radicals.

Experimental set-up Reaction N2O + hn(193nm)→ N2 + O(1D) YAG-Dye laser (~435 nm) Excimer laser (193 nm) N2O/CF3I/Ar Reaction N2O + hn(193nm)→ N2 + O(1D) O(1D) + Ar → O(3P) CF3I + O(3P) → IO + CF3 Digitiser / PC

IO spectra IO A2P3/2 - X2P3/2 (3,0) Simulation Rotational structure Experiment P R Simulation spectrum: A.Orr-Ewing et al., J. Chem. Soc. Faraday Trans., 94, 2681, 1998

Absorption cross-section of IO (3,0) Experimental Model Calculation Numerical integration method [CF3I]int and [N2O]int k [IO] Reaction kinetics of IO N2O + hn = N2 + O(1D) O(1D) + M = O(3P) + M CF3I + O(3P) = IO + CF3 IO + IO = Products Wavelength σ=5.9×10-17 cm2 molecule-1 M.Kawasaki et al., J. Phys. Chem. A, 107, 6381, 2003 • • Time-dependent concentration of IO radicals

Result : time-dependent concentration of IO radicals ▪ Experimental Model calculation IO + IO → Product [IO] is good agreement between experiment and model.

Simulate the absorption cross-section IO A2P3/2 - X2P3/2 (3,0) Resolution 0.2 cm-1 24 cm-1 Kawasaki et al., J. Phys. Chem. A, 2003 5.9 ± 0.6 3.1±0.4 Harwood et al., J. Phys. Chem. A, 1997 - 2.7±0.4 convolution Unit: 10-17 cm2 molecule-1 Simulated spectrum Resolution: - 0.2 cm-1 - 24 cm-1

Open-path CRD spectrometer Detector YAG-Dye laser system HR mirror Optical table Sampling Air Electronics Wheel & Vibration isolator

Field observation in Roscoff High tide Low tide Extensive Seaweed beds are exposed.

Measurements method Total accumulation time for one data point: 30 s Bandhead IO 6 Total accumulation time for one data point: 30 s 4 / 10-17 cm2 molecule-1 Cross-section of IO 2 Baseline 22944 22948 22952 22956 Wavenumber /cm-1 NO2 NO2 has absorption in the measurement wavenumbers. 6 /10-19 cm2 molecule-1 Cross-section of NO2 / 10-19 cm2 molecule-1 4 Same value of cross-sections 2 No interferences of NO2 22944 22948 22952 22956 Wavenumber /cm-1

Observation results 25th September 2006 Fine & cloudy day ● Mixing ratio of IO ○ Minimum detectable limit Wind High [IO] radicals were successfully detected. The concentrations have inverse-trend with tidal heights.

Conclusions An open-path CRD spectrometer was designed and constructed for atmospheric IO radicals. The cross-section of IO was tested by both of kinetic measurement and model simulation. 3. The open-path CRD spectrometer had been deployed in the field campaign and successfully detected high concentration of IO radicals.

Acknowledgements Professor Andrew J. Orr-Ewing Dr Gordon McFiggans and observation team in RHaMBLe projects Professor Yutaka Matsumi Funding: EPSRC STE research lab in Nagoya University