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2.5. MARHAL - MODELING OF MARINE AND HALOGEN CHEMISTRY 79
2.5.4 Importance of the surface reaction OH + Cl − on sea salt aerosol for
the chemistry of the marine boundary layer - a model study
Roland von Glasow
Abstract The implications for the chemistry of the marine boundary layer (MBL) of the reaction
of OH with Cl − on the surface of sea salt aerosol producing gas phase Cl2 and particulate OH − have
been investigated with a numerical model. They were found to be very minor in contradiction to
previous suggestions in the literature.
Figure 2.42: Temporal evolution of the most important gas phase compounds for scenario “remote
MBL”. The difference between the “cases” is how the reaction rate of the surface reaction is calculated
as explained in paranthesis. Case 1 (no surface reaction) - black, solid line; case 2 (“best guess”) -
red, dashed line; case 3 (no gas phase diffusion limitation) - blue, dotted line; case 4 (γ = 1, with
gas phase diffusion limitation) - blue, solid line; case 5 (γ = 1, no gas phase diffusion limitation) -
green, dash-dotted line. Note, that most lines except for Cl2 and Cl overlap. The abscissa is time
since model start in minutes.
Background The reaction OH + Cl − −→
OH − + Cl2 had been suggested by Laskin et
al. (2003) to play a major role in the sulfur
cycle in the marine boundary layer. The aqueous
phase oxidation of SO2 is strongly pH dependent,
and relevant mainly in fresh, non-acidified
sea salt aerosol particles. This pathway for sulfur
oxidation would gain in importance if alkalinity
would be produced in the particles as is the case
in the aforementioned reaction. Furthermore, it
was suggested that the gas phase product, Cl2,
might be relevant for the photochemistry of the
MBL.
Methods and results Based on literature data
a new “best estimate” for the rate coefficient of the
surface reaction was deduced and applied in the
box-model vesrion of MISTRA. Its importance for
the chemistry of the MBL has been investigated
under conditions typical for the pristine MBL of
the Southern Ocean, the remote MBL, and marine
regions influenced by polluted outflow from
the continent.
The results showed that the additional sulfate
production by this reaction is less than 1%, therefore
having only a minor impact on sulfate production.
Even though the gas phase concentration
of Cl2 increased strongly in the model (see Figure),
the concentration of Cl radicals increased by
less than 5% for the “best estimate” case and the
impact on O3 and other compounds is negligible.
Therefore it was concluded that - at least under
the investigated conditions - this reactions is of
minor importance for the chemistry of the MBL
and the marine sulfur cycle.
A very interesting feature of the acidification
of large sea salt particles that was predicted with
the model is a two-stage acidification of large fresh
sea salt aerosol. This effect is caused by the rapid
change in particle pH due to uptake of acids and
production of acidity during the production of sulfate
and the related change in pH-dependent reactions
rates.
Funding DFG: Emmy Noether Junior Research
Group MarHal GL 353/1-2
Main publication von Glasow [2006b]