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Centre for Atmospheric Science

Modelling halogen cycling in the marine boundary layer

In the marine boundary layer (MBL) halogen (chlorine, bromine and iodine) species have two major influences on the atmosphere: destruction of ozone through catalytic cycles, which do not deplete the concentrations of active halogen compounds; and the formation
of new aerosol particles in the coastal boundary layer, which may go on to form cloud condensation nuclei.

The primary source of reactive inorganic chlorine and bromine in the MBL are sea-salt aerosols produced by waves breaking on the ocean surface. Biogenic halo-carbon compounds and molecular iodine are also emitted at the ocean surface, and from macroalgae (seaweed) exposed at low tide. These are the major sources of iodine in the MBL. Sea-salt aerosols are believed to act as a sink, rather than a source, for reactive iodine because iodine concentrations in sea water, and so in fresh sea-salt aerosols too,
are very low compared to chlorine and bromine concentrations.

The reactive halogen atoms (X) destroy ozone (O3) in the reaction:

X + O3 -> XO + O2

The halogen oxide (XO) produced by this reaction is recycled back to reactive halogen by one of the following three cycles:

XO + HO2 -> HOX + O2
HOX + hv -> X + OH
net: O3 + HO2 -> 2O2 + OH

XO + NO2 -> XONO2
XONO2 + hv -> X + NO3
NO3 + hv -> NO + O2
net: O3 + NO2 -> 2O2 + NO

XO + XO -> 2X + O2 (via photolysis of X2O2, OXO, or X2)
net: 2O3 -> 3O2

These cycles do not just destroy ozone, they also can influence the oxidative capacity of the local atmosphere (by changing the HO2:OH ratio) and interact with atmospheric pollutants such as nitrogen oxides. These cycles depend on photolysis of HOX and XONO2, meaning that they will only occur during the day; however HOX and XONO2 can also be recycled through reactions within aerosol particles, leading to night-time ozone loss. The rates of these recycling processes are strongly controlled by the
composition of the aerosol particles involved.

At the University of Manchester we have developed MANIC (Microphysical Aerosol Numerical model Incorporating Chemistry) for modelling the chemistry both within, and between, the gas-phase and liquid aerosol particles. The model also simulates particle growth due via condensation from the gas-phase, and particle production, through processes such as bubble bursting, and loss, via sedimentation.

MANIC has been used for studying mixed-phase halogen chemistry in the remote marine atmosphere for the MAP (Marine Aerosol Production) project. We have also used it to study halogen chemistry in a polluted coastal atmosphere for the RHaMBLe (Reactive Halogens in the Marine Boundary Layer) project.