Abstract.A model of iodine chemistry in the marine boundary layer (MBL) has been used to investigate the impact of daytime coastal emissions of molecular iodine (I 2 ). The model contains a full treatment of gas-phase iodine chemistry, combined with a description of the nucleation and growth, by condensation and coagulation, of iodine oxide nano-particles. In-situ measurements of coastal emissions of I 2 made by the broadband cavity ring-down spectroscopy (BBCRDS) and inductively coupled plasmamass spectrometry (ICP/MS) techniques are presented and compared to long path differential optical absorption spectroscopy (DOAS) observations of I 2 at Mace Head, Ireland. Simultaneous measurements of enhanced I 2 emissions and particle bursts show that I 2 is almost certainly the main precursor of new particles at this coastal location. The ratio of IO to I 2 predicted by the model indicates that the iodine species observed by the DOAS are concentrated over a short distance (about 8% of the 4.2 km light path) consistent with the intertidal zone, bringing them into good agreement with the I 2 measurements made by the two in-situ techniques. The model is then used to investigate the effect of iodine emission on ozone depletion, and the production of new particles and their evolution to form stable cloud condensation nuclei (CCN).

Abstract. This paper presents a summary of the measurements made during the heavily-instrumented Reactive Halogens in the Marine Boundary Layer (RHaMBLe) coastal study in Roscoff on the North West coast of France throughout September 2006. It was clearly demonstrated that iodinemediated coastal particle formation occurs, driven by daytime low tide emission of molecular iodine, I 2 , by macroalgal species fully or partially exposed by the receding waterline. Ultrafine particle concentrations strongly correlate with the rapidly recycled reactive iodine species, IO, produced at high concentrations following photolysis of I 2 . The heterogeneous macroalgal I 2 sources lead to variable relative concentrations of iodine species observed by path-integrated and in situ measurement techniques.

Abstract:IO and BrO radicals are intermediates in the atmospheric photo-oxidation of iodo-and bromocarbons and can act as catalysts for ozone loss. We have studied the kinetics and mechanisms of the reactions of IO with itself and with BrO to establish their role in the atmospheric chemistry of iodine. We have found that iodine dioxide, OIO, is produced in these reactions. The results of these and other experimental observations together with a recent computational study suggest an unexpectedly high photochemical stability for OIO. It is shown that OIO formation and its attachment to particles could account for the high enrichment of iodine in the small size fraction of marine aerosol, which is important for the transport of iodine from the sea to the continents. OIO may be a route to the formation of iodate, which is present in atmospheric precipitation. OIO formation also implies a reduced efficiency for iodine catalysed ozone loss.

The flash photolysis/UV absorption technique has been used to study the self-reaction of BrO radicals over the temperature range 222-298 K and the pressure range 100-760 Torr of N 2 or O 2 . Two chemical sources of BrO radicals were used: photolysis of Br 2 in the presence of excess ozone and photolysis of O 2 in the presence of excess Br 2 . The overall rate constant, k 1 , for the BrO self-reaction (defined by -d[BrO]/dt ) 2k 1 [BrO] 2 ) was found to be temperature and pressure independent at T g 250 K, with k 1 ) (2.88 ( 0.20) × 10 -12 cm 3 molecule -1 s -1 . At temperatures below 250 K, k 1 was found to be pressure dependent, due to the emergence of a new termolecular channel of the BrO self-reaction 1c, -1c forming the BrO dimer, Br 2 O 2 (BrO + BrO + M h Br 2 O 2 + M). Channel-specific rate constants were determined for the two bimolecular channels of the BrO self-reaction above 250 K, giving for (1a) (BrO + BrO f 2Br + O 2 ) k 1a ) (5.31 ( 1.17) × 10 -12 exp{(-211 ( 59)/T} cm 3 molecule -1 s -1 and for (1b) (BrO + BrO f Br 2 + O 2 ) k 1b ) (1.13 ( 0.47) × 10 -14 exp{(983 ( 111)/T} cm 3 molecule -1 s -1 . Below 250 K, the overall rate coefficient of the two bimolecular channels is reduced as the dimer forming channel emerges. At 235 and 222 K, rate constants for the formation (k 1c ) and decomposition (k -1c ) of Br 2 O 2 were recorded. Using the values for K 1c , ∆H r for reaction 1c was estimated as -58.6 ( 0.1 kJ mol -1 . A UV absorption spectrum attributed to Br 2 O 2 was also recorded over the wavelength range 300-390 nm. The cross section of the smooth Br 2 O 2 spectrum was found to be 1.2 × 10 -17 cm 2 molecule -1 at 320 nm. These results are rationalized in terms of a mechanism of the BrO self-reaction that shows competition, at low temperatures, between collisional quenching and unimolecular dissociation of an excited BrOOBr* intermediate. The rate constant for the reaction of oxygen atoms with molecular bromine was also determined in the course of these experiments [O + Br 2 f BrO + Br (5)], giving k 5 ) (5.12 ( 1.86) × 10 -13 exp{(989 ( 91)/T} cm 3 molecule -1 s -1 . All errors are 1σ. The atmospheric implications of these results are discussed.