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Using a regional model, we show that iodine catalysed reactions cause large regional changes to the chemical composition in the northern Indian Ocean with peak changes of up to 25 % in O3, 50 % in nitrogen oxides (NO and NO2), 15 % in hydroxyl radicals (OH), 25 % in hydroperoxyl radicals (HO2), and up to a 50 % change in the nitrate radical (NO3). These results show the importance of including iodine chemistry in modelling the atmosphere in this region.
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https://doi.org/10.5194/acp-2020-1219
https://doi.org/10.5194/acp-2020-1219

  10 Dec 2020

10 Dec 2020

Review status: this preprint is currently under review for the journal ACP.

Modelling the Impacts of Iodine Chemistry on the Northern Indian Ocean Marine Boundary Layer

Anoop S. Mahajan1, Qinyi Li2, Swaleha Inamdar1,3, Kirpa Ram3, Alba Badia4, and Alfonso Saiz-Lopez2 Anoop S. Mahajan et al.
  • 1Indian Institute of Tropical Meteorology, Ministry of Earth Sciences, Pune, 411016, India
  • 2Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, 28006, Spain
  • 3Institute of Environment and Sustainable Development, Banaras Hindu University, Varanasi, 221 005, India
  • 4Institute of Environmental Science and Technology (ICTA), Universitat Autònoma de Barcelona (UAB), Barcelona, Spain

Abstract. Recent observations have shown the ubiquitous presence of iodine oxide (IO) in the Indian Ocean marine boundary layer (MBL). In this study, we use the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem version 3.7.1), including halogens (Br, Cl and I) sources and chemistry, to quantify the impacts of the observed levels of iodine on the chemical composition of the MBL. The model results show that emissions of inorganic iodine species resulting from the deposition of ozone (O3) on the sea surface are needed to reproduce the observed levels of IO, although the current parameterisations overestimate the atmospheric concentrations. After reducing the inorganic emissions by 40 %, a reasonable match with cruise-based observations is found. A strong seasonal variation is also observed, with lower iodine concentrations predicted during the monsoon period when clean oceanic air advects towards the Indian subcontinent, and higher iodine concentrations predicted during the winter period, when polluted air from the Indian subcontinent increases the ozone concentrations in the remote MBL. The results show that significant changes are caused by the inclusion of iodine chemistry, with iodine catalysed reactions leading to regional changes of up to 25 % in O3, 50 % in nitrogen oxides (NO and NO2), 15 % in hydroxyl radicals (OH), 25 % in hydroperoxyl radicals (HO2), and up to a 50 % change in the nitrate radical (NO3). Most of the large relative changes are observed in the open ocean MBL, although iodine chemistry also affects the chemical composition in the coastal environment and over the Indian subcontinent. These results show the importance of including iodine chemistry in modelling the atmosphere in this region.

Anoop S. Mahajan et al.

 
Status: open (until 15 Feb 2021)
Status: open (until 15 Feb 2021)
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Anoop S. Mahajan et al.

Anoop S. Mahajan et al.

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Short summary
Using a regional model, we show that iodine catalysed reactions cause large regional changes to the chemical composition in the northern Indian Ocean with peak changes of up to 25 % in O3, 50 % in nitrogen oxides (NO and NO2), 15 % in hydroxyl radicals (OH), 25 % in hydroperoxyl radicals (HO2), and up to a 50 % change in the nitrate radical (NO3). These results show the importance of including iodine chemistry in modelling the atmosphere in this region.
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