Abstract
Abstract. Iodine monoxide (IO) differential slant column densities (DSCD) have been retrieved from a new multi-axis differential optical absorption spectroscopy (MAX-DOAS) instrument deployed at the Izaña subtropical observatory as part of the Network for the Detection of Atmospheric Composition Change (NDACC) programme. The station is located at 2370 m a.s.l., well above the trade wind inversion that limits the top of the marine boundary layer, and hence is representative of the free troposphere. We report daily observations from May to August 2010 at different viewing angles. During this period, the spectral signature of IO was unequivocally detected on every day of measurement. A mean IO DSCD of 1.52×1013 molecules cm−2 was observed at the 5° instrument elevation angle (IEA) on clear days using a single zenith reference for the reported period, with a day-to-day variability of 33% at one standard deviation. Based on the simulation of the DSCDs using radiative transfer calculations with five different hypothesized IO profiles, the IO mixing ratio is estimated to range between 0.2 and 0.4 pptv in the free troposphere. Episodes of Saharan dust outbreaks were also observed, with large increases in the DSCDs at higher IEA, suggesting an enhancement of IO inside the dust cloud.
Highlights
UV-visible spectroscopy has, for decades, been used for stratospheric chemistry studies
Results show that the differences in Aerosol Optical Depth (AOD) at 500 nm less than 0.05, are considered in the esticolumn density at different instrument elevation angle (IEA) are reduced or even disap- mation of iodine monoxide (IO) concentrations obtained at the observation site. pear in selected periods with high aerosol loading above the Daily means IO differential slant column densities (DSCD) for the complete period are shown in station
Three months of MAX-Differential Optical Absorption Spectroscopy (DOAS) measurements from the subtropical station of Izana starting in May 2010 show the presence of background levels of IO in the free troposphere
Summary
UV-visible spectroscopy has, for decades, been used for stratospheric chemistry studies. Improvements in the Differential Optical Absorption Spectroscopy (DOAS) technique and advances in profile retrievals from Multi Axis-DOAS (MAX-DOAS) observation modes (Platt and Stutz, 2008) have made possible the detection of trace gases of tropospheric interest at very low concentrations. The detection of iodine monoxide (IO) in the marine boundary layer (MBL) has been reported by numerous studies (Alicke et al, 1999; Allan et al, 2000; SaizLopez et al, 2007; Read et al, 2008). Observations performed during dedicated campaigns at Tenerife (Allan et al, 2000) and Cape Verde (Read et al, 2008) reported concentrations between 0.2 and 4 pptv.
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