Atmospheric Chemistry and Physics
www.atmos-chem-phys.net
1680-7316
1680-7324
7
21
2007
10.5194/acp-7-5675-2007
http://www.atmos-chem-phys.net/7/5675/2007/
http://www.atmos-chem-phys.net/7/5675/2007/acp-7-5675-2007.html
http://www.atmos-chem-phys.net/7/5675/2007/acp-7-5675-2007.pdf
5675
5694
2007-11-16
Secondary organic aerosol in the global aerosol – chemical transport model Oslo CTM2
C. R. Hoyle
c.r.hoyle@geo.uio.no
T. Berntsen
G. Myhre
I. S. A. Isaksen
Department of Geosciences, University of Oslo, Norway
The global chemical transport model Oslo CTM2 has been extended to include
the formation, transport and deposition of secondary organic aerosol (SOA).
Precursor hydrocarbons which are oxidised to form condensible species include
both biogenic species such as terpenes and isoprene, as well as species
emitted predominantly by anthropogenic activities (toluene, m-xylene,
methylbenzene and other aromatics). A model simulation for 2004 gives an
annual global SOA production of approximately 55 Tg. Of this total, 2.5 Tg
is found to consist of the oxidation products of anthropogenically emitted
hydrocarbons, and about 15 Tg is formed by the oxidation products of
isoprene. The global production of SOA is increased to about
69 Tg yr<sup>−1</sup> by allowing semi-volatile species to partition to ammonium
sulphate aerosol. This brings modelled organic aerosol values closer to those
observed, however observations in Europe remain significantly underestimated.
Allowing SOA to partition into ammonium sulphate aerosol increases the
contribution of anthropogenic SOA from about 4.5% to 9.4% of the total
production. Total modelled organic aerosol (OA) values are found to represent
a lower fraction of the measured values in winter (when primary organic
aerosol (POA) is the dominant OA component) than in summer, which may be an
indication that estimates of POA emissions are too low. Additionally, for
measurement stations where the summer OA values are higher than in winter,
the model generally underestimates the increase in summertime OA. In order to
correctly model the observed increase in OA in summer, additional SOA sources
or formation mechanisms may be necessary. The importance of NO<sub>3</sub> as an
oxidant of SOA precursors is found to vary regionally, causing up to
50%–60% of the total amount of SOA near the surface in polluted regions
and less than 25% in more remote areas, if the yield of condensible
oxidation products for β-pinene is used for NO<sub>3</sub> oxidation of all
terpenes. Reducing the yield for α-pinene and limonene oxidation in
line with recent measurements reduces the global fraction of SOA formed from
NO<sub>3</sub> oxidation products from 27% to about 21%. This study underscores the
need for SOA to be represented in a more realistic way in global aerosol
models in order to better reproduce observations of organic aerosol burdens
in industrialised and biomass burning regions.
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