Geoscience Reference
In-Depth Information
When oxide surfaces are exposed to NO
2
, it reacts with the surface to form
nitrite or nitrate at higher NO
2
pressure. In addition, gas-phase NO is produced.
Relative humidity and temperature influence the uptake of NO
2
by clays and soil
samples (Zhang et al.
2012
). Heterogeneous hydrolysis of NO
2
on silicate surfaces
produces HONO and NO, and it was proposed that this reaction does not depend
strongly on the nature of surface (Finlayson-Pitts et al.
2003
). This suggests that any
mineral dust can provide a surface for this reaction. Trace amounts of TiO
2
in the
authentic dust can play an important role in NO
2
uptake. When NO
3
adsorbed on
mineral dust surfaces containing TiO
2
is irradiated, NO and NO
2
(NO
x
) are detected;
however, SiO
2
does not show NO
x
formation. Consequently, TiO
2
is responsible for
NO
x
production (Ndour et al.
2009
). NO
2
uptake on TiO
2
as well as Saharan sand
samples shows a significant increase under irradiation.
NO adsorption on TiO
2
in the presence of molecular oxygen leads to the
formation of surface adsorbed nitrate and NO
C
(Hadjiivanov and Knozinger
2000
).
Heterogeneous reaction of NO with adsorbed nitric acid is a significant source
of HONO in polluted areas. Besides, this reaction also converts nitric acid to
NO
x
(Saliba et al.
2000
). The exposure of surface nitrate-coated aluminium oxide
to gaseous hydrogen chloride (HCl) yields several gas-phase products, including
NOCl, NO
2
and HNO
3
under dry or NO and N
2
O under humid conditions.
Daytime photochemistry of dust-adsorbed nitrogen species can be responsible
for formation of HONO, a precursor to the hydroxyl radical. Adsorbed molecular
HNO
3
undergoes photolysis to form HONO or HONO and H
2
O
2
in the presence
of water (Ramazan et al.
2006
). As the relative humidity decreases, the adsorbed
HONO reacts with adsorbed nitric acid to form NO
2
. Photolysis of surface nitrate
yields only NO
2
,NOandN
2
OonAl
2
O
3
(Rubasinghege and Grassian
2009
), but
HONO on SiO
2
(Ma et al.
2011
). The latter can account for the presence of
HONO in the dust-laden atmosphere. TiO
2
photocatalysis is another potential source
of daytime HONO, a process which shows a strong dependence on the relative
humidity (RH) (Gustafsson et al.
2006
). Another mechanism of HONO formation
involves heterogeneous photochemistry of NO
2
on soil particles and ice films coated
with humic substances (Bartels-Rausch et al.
2010
).
Sulphur Species
Sulphur dioxide adsorbs irreversibly on mineral oxides and authentic aerosol
samples to form surface sulphite, bisulphite and sulphate, and this process is
significantly enhanced in the presence of adsorbed water. Atmospheric sulphur
dioxide can be oxidised to sulphate or sulphuric acid on dust surfaces (Usher et al.
2002
;Wuetal.
2011
). Iron compounds are responsible for the removal of SO
2
from
gas mixtures with uptake limited to the outer layer of the particles. Adsorbed SO
2
oxidises on the surface of most iron oxides to form a bidentate or monodentate
surface sulphate species. The reaction of SO
2
on the surface of mixed particles of
Fe
2
O
3
and NaCl mainly produces sulphate/bisulphate species. Oxidation of sulphur
dioxide into sulphate readily occurs on TiO
2
yielding adsorbed sulphite in the dark
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