Geoscience Reference
In-Depth Information
there is a decoupling between nucleation and growth (Kulmala et al. 2000 ), which
allows formation of new aerosol particles even during high dust loadings.
Ozone
Trace amounts of TiO 2 may be responsible for ozone loss. Doping of SiO 2 with TiO 2
increases ozone decomposition (Nicolas et al. 2009 ), and TiO 2 -containing Saharan
dust shows irreversible ozone uptake (Hanisch and Crowley 2003 ). Ozone decay
rate is significantly higher under irradiation compared to dark conditions, indicating
that this reaction is also photoinduced.
Heterogeneous photochemistry of O 3 depends on the surface properties of
oxides. For example, Fe 2 O 3 shows high uptake capacity and catalytic decomposition
towards O 3 ,butAl 2 O 3 is deactivated by O 3 exposure. The photoenhancement of O 3
decomposition on Fe 2 O 3 is explained by its semiconductor properties. Irradiation
of Fe 2 O 3 with UV photons having energies higher than 2.2 eV creates electron-
hole pairs, which initiate photochemical reactions. Since mineral dust aerosol can
contain a variety of semiconductor metal oxides, such as TiO 2 ,ZnOandFe 2 O 3 ,
heterogeneous photodecomposition of O 3 may be an important path of ozone loss
in the troposphere.
Other Inorganic and Organic Species
Adsorption of hydrogen peroxide on SiO 2 and CaCO 3 is mainly reversible. Metal
oxides exhibit catalytic behaviour towards the decomposition of hydrogen peroxide,
while SiO 2 does not (Wang et al. 2011 ). The uptake of hydrogen peroxide by
TiO 2 was found to decrease at higher RH, while irradiation enhances H 2 O 2
decomposition with formation of HO 2 intermediate (Yi et al. 2012 ).
Adsorption of acidic gases on calcium carbonate under dry conditions shows the
formation of carbonic acid on the calcite surface (Al-Hosney and Grassian 2004 ).
The acid is stable over long periods at low temperatures even in the presence of
water vapour (Bernard et al. 2012 ). Thus, in the high troposphere, H 2 CO 3 may be
adsorbed on mineral dust particles.
Organic compounds are often associated with dust particles (Lee et al. 2002 ;
Russell et al. 2002 ) and adsorption of organics on minerals is an important process.
Adsorption of carbonyls on SiO 2 particles is nonreactive and reversible (Carlos-
Cuellar et al. 2003 ), but it is irreversible on other oxides ('-Al 2 O 3 ,TiO 2 , '-Fe 2 O 3 ,
SiO 2 and CaO) (Xu et al. 2006 ). Carbonyl compounds can undergo photolysis and
react with OH radicals, yielding peroxy radicals and peroxyacetyl nitrates (PAN)
(Li et al. 2001 ).
Multiple studies show that tropospheric particles are often associated with
organic acids (Lee et al. 2002 ). Dissociation of formic acid yields mono- and
bidentate formate species. The adsorption of acetic acid produces both molecularly
adsorbed acetic acid and acetate groups (Takeuchi et al. 2010 ). The extent of the
Search WWH ::




Custom Search