Observations of iodine monoxide in the remote marine boundary layer

We report measurements of the iodine monoxide (IO) radical in the marine bo undary layer at three remote sites: Mace Head (Ireland), Tenerife (Canary I slands), and Cape Grim (Tasmania). IO was observed by long-path differentia l optical absorption spectroscopy using the A(2)Pi(3/2)-X(2)Pi(3/2) electro nic transition between 415 and 450 nm. The daytime IO concentration at thes e three locations was found to vary from below the detection limit (less th an or equal to 0.2 parts per trillion (ppt)) to a maximum of 4 ppt, with an average of about 1 ppt, Of particular note is that the IO observed off the north coast of Tenerife, which is probably typical of the open ocean sub-t ropical North Atlantic, exhibited a distinct diurnal cycle which correlated strongly with the solar actinic flux in the near UV. IO was also observed at Cape Grim to be present at much lower levels (approximate to 0.3 ppt) in westerly air from the Southern Ocean. As is shown in the companion paper ( McFiggans et al., this issue), these measurements of IO are satisfactorily reproduced by a photochemical box model incorporating the recycling of iodi ne through marine aerosol. This model indicates that the direct iodine-cata lyzed destruction of ozone in the boundary layer may well be similar to the losses caused by odd-hydrogen photochemistry and dry deposition. The signi ficance of this work is that IO is probably present in much of the open oce an boundary layer, at levels where it may cause significant depletion of oz one.