Laboratory and field measurements of enantiomeric and non-enantiomeric biogenic VOCs and anthropogenic BTEX compounds
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Abstract
This doctoral thesis was focused on the investigation of enantiomeric and
non-enantiomeric biogenic organic compound (BVOC) emissions from both leaf and
canopy scales in different environments. In addition, the anthropogenic compounds
benzene, toluene, ethylbenzene, and xylenes (BTEX) were studied. BVOCs are
emitted into the lower troposphere in large quantities (ca. 1150 Tg C ·yr-1),
approximately an order of magnitude greater than the anthropogenic VOCs. BVOCs
are particularly important in tropospheric chemistry because of their impact on ozone
production and secondary organic aerosol formation or growth. The BVOCs
examined in this study were: isoprene, (-)/ (+)-α-pinene, (-)/ (+)-ß-pinene, Δ-3-carene,
(-)/ (+)-limonene, myrcene, eucalyptol and camphor, as these were the
most abundant
BVOCs observed both in the leaf cuvette study and the ambient measurements.
In the laboratory cuvette studies, the sensitivity of enantiomeric enrichment change
from the leaf emission has been examined as a function of light (0-1600 PAR) and
temperature (20-45°C). Three typical Mediterranean plant species (Quercus ilex L.,
Rosmarinus officinalis L., Pinus halepensis Mill.) with more than three individuals of
each have been investigated using a dynamic enclosure cuvette. The terpenoid
compound emission rates were found to be directly linked to either light and
temperature (e.g. Quercus ilex L.) or mainly to temperature (e.g. Rosmarinus
officinalis L., Pinus halepensis Mill.). However, the enantiomeric signature showed
no clear trend in response to either the light or temperature; moreover a large
variation of enantiomeric enrichment was found during the experiment. This
enantiomeric signature was also used to distinguish chemotypes beyond the normal
achiral
chemical composition method. The results of nineteen Quercus ilex L.
individuals, screened under standard conditions (30°C and 1000 PAR) showed four
different chemotypes, whereas the traditional classification showed only two.
An enclosure branch cuvette set-up was applied in the natural boreal forest
environment from four chemotypes of Scots pine (Pinus sylvestris) and one
chemotype of Norway spruce (Picea abies) and the direct emissions compared with
ambient air measurements above the canopy during the HUMPPA-COPEC 2010
summer campaign. The chirality of a-pinene was dominated by (+)-enantiomers from
Scots pine while for Norway spruce the chirality was found to be opposite (i.e.
Abstract
II
(-)-enantiomer enriched) becoming increasingly enriched in the (-)-enantiomer with
light.
Field measurements over a Spanish stone pine forest were performed to examine the
extent of seasonal changes in enantiomeric enrichment (DOMINO 2008). These
showed clear differences in
chirality of monoterpene emissions. In wintertime the
monoterpene (-)-a-pinene was found to be in slight enantiomeric excess over
(+)-a-pinene at night but by day the measured ratio was closer to one i.e. racemic.
Samples taken the following summer in the same location showed much higher
monoterpene mixing ratios and revealed a strong enantiomeric excess of (-)-a-pinene.
This indicated a strong seasonal variance in the enantiomeric emission ratio which
was not manifested in the day/night temperature cycles in wintertime. A clear diurnal
cycle of enantiomeric enrichment in a-pinene was also found over a French oak
forest and the boreal forest. However, while in the boreal forest (-)-a-pinene
enrichment increased around the time of maximum light and temperature, the French
forest showed the opposite tendency with (+)-a-pinene being favored.
For the two field campaigns (DOMINO 2008 and HUMPPA-COPEC 2010), the
BTEX were also investigated. For the DOMINO campaign, mixing ratios
of the
xylene isomers (meta- and para-) and ethylbenzene, which are all well resolved on the
ß-cyclodextrin column, were exploited to estimate average OH radical exposures to
VOCs from the Huelva industrial area. These were compared to empirical estimates
of OH based on JNO2 measured at the site. The deficiencies of each estimation
method are discussed. For HUMPPA-COPEC campaign, benzene and toluene mixing
ratios can clearly define the air mass influenced by the biomass burning pollution
plume from Russia.