Why is the atmospheric oxidation of biogenic organic compounds important?
Vegetation emits large amounts of biogenic volatile organic compounds (BVOC) into the atmosphere (~1 billion ton/year).
Their oxidation in the atmosphere has two main consequences.
- They react with important oxidizing chemical compounds like ozone and the hydroxyl radical, influencing their abundance.
This in turn affects the ability of the atmosphere to cleanse itself from pollutants, and the budget of greenhouse gases such as methane.
- Part of the oxidation products condense to fine particulate matter, also called aerosol. Aerosol of this origin is named secondary organic aerosol
(SOA) and it is an important part of the total aerosol. Aerosols affect both
human health (through inhalation) and climate (scattering/absorbing sun light and cloud formation).
Since BVOC emissions are expected to increase in response to climate warming, a better understanding of BVOC oxidation and biogenic SOA formation
is needed in order to assess the role of the biosphere in our changing environment.
The main objective of BIOSOA is to better understand and quantify the role of biogenic organic compounds in the atmosphere.
BIOSOA will focus on some of the major areas of uncertainties related to these compounds :
- The most important BVOC (~50% of the total emitted) is isoprene (C5H8).
Recently, it turned out it does not
lower the self-cleansing capacity of the atmosphere (by reaction with the hydroxyl radical), contrary to earlier expectations.
The combined effort of theoretical chemical mechanism development and global modelling was able to explain this, but
further work is needed as this new chemical mechanism is incomplete yet.
- Water-soluble isoprene oxidation products can be processed further in cloud droplets (oxidation, oligomerization) and stay in the
particulate phase after droplet evaporation, contributing to SOA. A detailed box model of
multiphase (gas/aqueous) oxidation chemistry will be developed.
Part of the kinetic rate constants will be determined in the laboratory.
- Isoprene and α-pinene have a similar contribution to SOA. Laboratory experiments and analytical measurements will be
performed to investigate the formation pathways of tracers, which can then be tested in a box model.
Tracers are molecules contributing significantly to SOA: e.g. 2-methyl tetrols (from isoprene) and 3-methyl-1,2,3-butane tricarboxylic acid
(from α-pinene). For other tracers, the structure itself must first be elucidated. This will provide insight in the oxidation mechanism of both BVOC.
Also, based on the occurrence of tracers a method will be developed to identify the portions that isoprene and α-pinene contribute to SOA.
- The impact of BVOC emissions on the budget and distribution of SOA will be investigated using a global model.
For computational reasons, the detailed box models (formation and ageing of SOA from α-pinene, aqueous phase
chemistry of isoprene oxidation products) will be simplified/parametrized to be incorporated in our global model.
The calculated relative contributions of different BVOC precursors will be compared with estimations based on observations of tracer compounds.
The calculated organic aerosol concentrations will be extensively compared with observations. Finally, the radiative effect of BVOC SOA will
be estimated, and compared with the corresponding effect of anthropogenic aerosols.