Observations of iodine monoxide over three summers at the Indian Antarctic bases, Bharati and Maitri

 Iodine plays a vital role in oxidation chemistry over Antarctica, with past observations showing highly elevated levels of iodine oxide (IO) leading to severe depletion of boundary layer ozone in West Antarctica. However, observations across Antarctica are still rare, and have hitherto been mostly focused on the West Antarctic, which needs to be addressed in order for comprehensive model validation. Here, we present multi axis differential absorption spectroscopy (MAX-DOAS) based observations of IO over three summers (2015-2017) at the Indian Antarctic bases, Bharati and Maitri. IO was observed during all the campaigns, with mixing ratios below 2 pptv for the three summers, which are lower than the peak levels observed in West Antarctica. This suggests that sources in West Antarctica are different or stronger than sources of iodine compounds in East Antarctica. Vertical profiles estimated using a profile retrieval algorithm showed decreasing gradients, with a peak in the lower boundary layer. The ground-based instrument retrieved vertical column densities (VCDs) were approximately a factor of three-five higher than the VCDs reported using satellite-based instruments, which is most likely related to the sensitivities of the measurement techniques. Airmass back-trajectory analysis failed to highlight a source region, with most of the airmasses coming from coastal or continental regions. This study adds to the sparse observational database of iodine compounds in Antarctica and highlights the variation in iodine chemistry in different regions in Antarctica. It also shines light on the needs of more long-term datasets in different regions to validate models estimating the impacts of iodine chemistry across Antarctica.