Emissions of volatile organic compounds from tropical savanna vegetation and biomass burning [Elektronische Ressource] / vorgelegt von Betina Kleiss

Betina Kleiss

Informations

• Publié par: johannes_gutenberg-universitat_mainz
• Publié le: 01 janvier 2005
• Nombre de lectures: 20
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

EMISSIONS OF VOLATILE ORGANIC COMPOUNDS FROM
TROPICAL SAVANNA VEGETATION AND BIOMASS
BURNING



Dissertation
zur Erlangung des Grades
Doktor der Naturwissenschaften

am Fachbereich Chemie und Pharmazie
der Johannes Gutenberg-Universität in Mainz



vorgelegt von

Betina Kleiss
geboren in München


Mainz, 2004













Tag der mündlichen Prüfung: 21. Dezember 2004
D77 – Mainzer Dissertation





ABSTRACT
Emissions of volatile organic compounds from tropical savanna
vegetation and biomass burning
This dissertation focuses on characterizing the emissions of volatile organic
compounds (VOCs) from tropical savanna vegetation and biomass fires. The measurements
were performed with a proton-transfer-reaction mass spectrometer (PTR-MS), which enabled
the online detection of a large number of VOCs.
The biogenic emissions of tropical savanna vegetation were studied in a woodland
savanna in Venezuela. Two field campaigns were carried out, the first during the wet season
in September/October 1999, and the second in March/April 2000 during the dry season. Three
of the most important grass and tree species of the Venezuelan savanna were studied, namely
the grasses Trachypogon plumosus, Hyparrhenia rufa and Axonopus canescens, and the tree
species Byrsonima crassifolia, Curatella americana and Cochlospermum vitifolium. The
emission rates and the controlling variables were determined with a dynamic plant enclosure
system. Most biogenic emissions showed a diurnal variation, with highest values at noon and
early afternoon, and low or no emissions during the night. In general, the emissions increased
exponentially with increasing temperature and solar radiation, but correlated better with
temperature. The emission rates of VOCs showed high variability caused, primarily, by the
natural fluctuations of meteorological conditions during field measurements. The emission
data were therefore normalized to a standard temperature of 30°C, and standard emission
rates thus determined allowed for interspecific and seasonal comparisons, as well as with data
from the literature.
The range of average daytime (10:00-16:00) emission rates of total VOCs measured
from green (mature and young) grasses was between 510-960 ngC/g/h. Methanol (detected at
protonated mass 33) was the primary emission (140-360 ngC/g/h), followed by acetaldehyde
(mass 45), butene and butanol (mass 57) and acetone (mass 59) with emission rates between
70-200 ngC/g/h. The emissions of propene (mass 43) and methyl ethyl ketone (MEK, mass
73) were <80 ngC/g/h, and the emission of isoprene and C -alcohols (mass 69) was between 5
10-130 ngC/g/h. The normalized average emissions at 30°C (standard emissions) of total
VOCs was in the range 200-400 ngC/g/h. The oxygenated species methanol, acetaldehyde,
acetone and MEK, represented 70-75% of the total. The remaining 30% consisted of olefins
(propene; butene) and other unidentified species. The total daytime VOC emissions from dry
grasses were in the range 120-470 ngC/g/h, and the standard emissions were between 24-44
ngC/g/h. Methanol accounted for about half of the emissions from dry grasses. The emission
of VOCs was found to vary by up to a factor of three between plants of the same species (for
both, green and dry grasses), and by up to a factor of two between the different species. The
annual source of methanol from savanna grasses worldwide estimated in this work was 3 to
4.4 TgC, which could represent up to 12% of the current estimated global emission from
terrestrial vegetation. For acetone, acetaldehyde and MEK, the savannas may contribute up to
~10%, 4% and 30% of the biogenic source, respectively, but these numbers are very uncertain
since particularly the biogenic sources of these species are poorly quantified.
Three of the most common tree species in the Venezuelan savanna were investigated.
Two of these species, Byrsonima and Cochlospermum, were isoprene emitters, and isoprene
was also their primary emission (which accounted for 70-94% of the total carbon emitted by
these species) followed by methanol and butene + butanol. The daytime average emission
rates of isoprene measured in the wet season were 27 µgC/g/h for Byrsonima and 123 µgC/g/h
for Cochlospermum. The daytime emission of methanol and butene + butanol was between
0.3 and 2 µgC/g/h. The total sum of VOCs emission measured during the day in the wet
season was between 30 and 130 µgC/g/h. In the dry season, in contrast, the methanol emission
from Cochlospermum saplings –whose leaves were still developing– was an order of
magnitude higher than in the wet season (15 µgC/g/h). The isoprene emission from
Byrsonima in the dry season was comparable to the emission in the wet season, whereas
isoprene emission from Cochlospermum was about a factor of three lower (~43 µgC/g/h). The
total sum of daytime VOCs emission in the dry season was between 2 and 26 µgC/g/h. The
ratio of carbon released as VOCs to the carbon assimilated by photosynthesis during daytime
varied from 0.2 to 15%. The higher ratios were observed for the specimens with higher
isoprene emission. The standard emission of total VOCs of all measured tree species ranged
between 0.6 and 27 µgC/g/h in the wet season, and 0.2 and 5 µgC/g/h in the dry season.
Biogenic emission inventories show that isoprenoids are the most prominent and best-studied
compounds. The standard emission rates of isoprene and monoterpenes of the young savanna
trees were in the lower end of the range found in the literature. The biogenic emission of other
VOCs has been sparsely investigated, but in general, the standard emissions reported here
were within the range observed in previous investigations.
The biomass burning study comprised the measurement of VOCs and other trace-gas
emissions of 44 fires from 15 different fuel types, primarily from Africa and Indonesia, in a
combustion laboratory. The average sum of emissions (excluding CO , CO and NO) from 2
African fuels was ~18 g(VOC)/kg. Six of the ten most important emissions were oxygenated
VOCs. Acetic acid was the major emission, followed by methanol and formaldehyde. The
emission of methane was of the same order as the methanol emission (~5 g/kg), and that of
nitrogen-containing compounds was ~1 g/kg. An estimate of the VOC source from biomass
burning of savannas and grasslands worldwide suggests that the sum of emissions is about 56
Tg/yr, of which 34 Tg correspond to oxygenated VOCs, 14 Tg to unsaturated and aromatic
compounds, 5 Tg to methane and 3 Tg to N-compounds. The estimated emissions of CO, CO 2
and NO are 216, 5117 and 9.4 Tg/yr, respectively.
The emission factors reported here for Indonesian fuels are the first results of
laboratory fires using Indonesian fuels. Acetic acid was the highest organic emission,
followed by acetol (mass 75) a compound not previously reported in smoke, methane, mass
97 (tentatively identified as a mixture of furfural, dimethylfuran, ethylfuran), and methanol.
The sum of total emissions of Indonesian fuels was 91 g/kg, which is 5 times higher than the
total emissions from African fuels. This difference was because Indonesian fuels burned
mostly by smoldering combustion, whereas African fuels were consumed primarily by
flaming combustion, during which mostly oxidized compounds (e.g. CO , H O and NO) are 2 2
produced.
The results of this study show that oxygenated compounds, especially methanol,
dominate the emissions of VOC from savanna grasses. For the measured tree saplings,
methanol was also an important emission, exceeded only by isoprene. The biomass burning
experiments also demonstrate the importance of oxygenated VOCs, which represent about
60% of the total trace-gas emission measured from the burning of African and Indonesian
fuels. Due to the vast area covered by tropical savannas worldwide, the biogenic and biomass
burning emission of methanol and other oxygenated compounds may be important for the
regional and even global tropospheric chemistry. This study represents a significant
improvement to the current biogenic and biomass-burning emission inventories.

Contents
1. INTRODUCTION............................................................................................................................... 1
1.1. TROPOSPHERIC CHEMISTRY OF VOCS ............................................................................................ 3
1.1.1. Formation of ozone, hydroxyl radicals and nitrate radicals in the troposphere ................... 3
1.1.2. Reaction mechanisms and products....................................................................................... 4
1.1.2.1. Reactions with OH and NO radicals..............................................................................................4 3
1.1.2.2. Reaction with O .............................................................................................................................6 3
1.1.3. Lifetimes of VOCs in the troposphere.................................................................................... 6