Coupling of HOx, NOx and halogen chemistry in the antarctic boundary layer
Other literature type
Bloss, W. J.
Lee, J. D.
Heard, D. E.
Plane, J. M. C.
Bauguitte, S. J.-B.
Salmon, R. A.
Jones, A. E.
A modelling study of radical chemistry in the coastal Antarctic boundary
layer, based upon observations performed in the course of the CHABLIS
(Chemistry of the Antarctic Boundary Layer and the Interface with Snow)
campaign at Halley Research Station in coastal Antarctica during the austral
summer 2004/2005, is described: a detailed zero-dimensional photochemical box
model was used, employing inorganic and organic reaction schemes drawn from
the Master Chemical Mechanism, with additional halogen (iodine and bromine)
reactions added. The model was constrained to observations of long-lived
chemical species, measured photolysis frequencies and meteorological
parameters, and the simulated levels of HO<sub>x</sub>, NO<sub>x</sub> and XO
compared with those observed. The model was able to replicate the mean levels
and diurnal variation in the halogen oxides IO and BrO, and to reproduce
NO<sub>x</sub> levels and speciation very well. The NO<sub>x</sub> source term
implemented compared well with that directly measured in the course of the
CHABLIS experiments. The model systematically overestimated OH and HO<sub>2</sub>
levels, likely a consequence of the combined effects of (a) estimated physical
parameters and (b) uncertainties within the halogen, particularly iodine,
chemical scheme. The principal sources of HO<sub>x</sub> radicals were the
photolysis and bromine-initiated oxidation of HCHO, together with
O(<sup>1</sup>D) + H<sub>2</sub>O. The main sinks for HO<sub>x</sub> were peroxy
radical self- and cross-reactions, with the sum of all
halogen-mediated HO<sub>x</sub> loss processes accounting for 40% of the total
sink. Reactions with the halogen monoxides dominated
CH<sub>3</sub>O<sub>2</sub>-HO<sub>2</sub>-OH interconversion, with associated local chemical
ozone destruction in place of the ozone production which is associated with
radical cycling driven by the analogous NO reactions. The analysis highlights
the need for observations of physical parameters such as aerosol surface area
and boundary layer structure to constrain such calculations, and the
dependence of simulated radical levels and ozone loss rates upon a number of
uncertain kinetic and photochemical parameters for iodine species.