Measurements of Iodine Monoxide Levels During the CAST Campaign Using Broadband Cavity Enhanced Absorption Spectroscopy

Abstract:

Iodine monoxide (IO) has been regarded as an important radical involved in the ozone destruction in the remote marine boundary layer. Here we presented the first in situ aircraft measurements of IO using broadband cavity enhanced absorption spectroscopy with 1s -sensitivity of ~1.5 ppt Hz^-1/2 on the surface level during the Coordinated Airborne Studies in the Tropics (CAST) campaign between January – February 2014. IO was retrieved from analysis of absorption spectrum recorded between 415 nm – 452.5 nm. Instrument baseline corresponding to the “zero” signal of IO was obtained by injection of ~20 ppb of nitric oxide (NO) into the sample air at chosen frequency and period. No clear absorption feature was observable from the spectra by eye with up to 100 seconds averaging, pointing to very low mixing ratios (<~0.5 ppt) of IO over the sampled area. A small positive bias (~0.3 ppt) of IO (against the baseline signal during NO titration) was obtained in the statistical histogram of retrieved IO from average of each straight and level run, but little altitude dependence was noted. In summary, our observation appears to support the existence of IO in the remote marine boundary above the Pacific Ocean at sub ppt levels, but the limited sensitivity precludes us from quantifying spatial gradients more accurately.