Environmental Engineering Reference
In-Depth Information
3.1 Introduction
Field data indicate that Biogenic Volatile Organic Compounds (BVOC) emitted
by vegetation represent a loss of biologically fixed carbon from the terrestrial bio-
sphere (Kesselmeier et al.
2002
). Since these losses account for 3-4 % of the Net
Ecosystem Production (NEP), they must be taken into account when assessing the
amount of CO
2
permanently fixed by forest ecosystems. In areas characterized by
high biodiversity, such as the Mediterranean basin, an accurate evaluation of the
BVOC emissions can be obtained only if the spatial resolution is high enough to
account for the difference in the emission mechanism and emission composition
existing between all plant species present in a forest ecosystem (Parra et al.
2004
;
Simon et al.
2006
). High temporal resolution data are also needed because iso-
prenoids emitted from terrestrial vegetation (mainly isoprene and monoterpenes)
react faster than many anthropogenic Volatile Organic Compounds (VOC) with
OH radicals and ozone to produce photochemical oxidants (Atkinson and Arey
2003
) and secondary organic aerosols (SOA; Arneth et al.
2010
; Carslaw et al.
2010
). The production of SOA from BVOC seems to significantly affect the earth
climate and contribute to its changes (Hoffmann et al.
1997
; Odum et al.
1997
). In
Italy, where conditions leading to photochemical smog episodes extend from the
end of March to the end of September (Ciccioli et al.
1989
) and levels of ozone
and peroxyacetyl nitrate (PAN) downwind large urban areas may reach values as
high as 200 and 50 ppbv respectively (Ciccioli et al.
1999
), BVOC emission needs
to be known at a minimum time resolution of 12 h to model the impact of veg-
etation on ozone and SOA production. According to Mill￡n-Mill￡n et al. (
1996
),
the extraordinary high ozone levels reached in the Mediterranean basin during the
summer are determined by the complex circulation of the air masses occurring
under high pressure conditions, when local sea-land breeze wind regimes prevail
over large-scale circulation. Under these conditions, the primary emission of NOx
and VOC, generated mostly along the Italian coasts during daytime hours, is rap-
idly transported inland, where forest areas are located. The interaction of anthro-
pogenically polluted air masses with BVOC increases the ozone levels further.
The presence of tall mountains (from 1,000 up to 3,000 m a.s.l.) near the coasts
(1-80 km) generates a return flow bringing photochemically polluted air masses
back to the sea. At night, when advection ceases, ozone stratifies over the sea and
is transported again towards the coasts at sunrise, when ozone layers are mixed
and the sea breeze is re-activated. In such high reactive conditions, the different
reactivity of isoprenoids is crucial for a correct prediction of ozone and SOA pro-
duction. Emission data expressed in terms of isoprene and total monoterpenes
might not always be sufficient for a correct prediction of secondary products. This
is particularly true for the Italian peninsula where the emission of monoterpe-
nes is comparable to that of isoprene (Steinbrecher et al.
2009
) and composed of
compounds displaying rather different potentials for ozone and SOA production
(Atkinson and Arey
2003
; Hoffmann et al.
1997
). The development of a specific
