GREENCYCLESII

BVOCs, climate change, and plant physiology: GREENCYCLESII T3.5

Biogenic volatile organic compounds (BVOCs) are now recognized to be of key importance in regulating tropospheric chemistry. Unlike methane, which is not usually classified as a BVOC, these higher molecular-weight compounds are produced by enzyme-catalysed reactions and emitted directly by plants. The total emission of BVOCs (of which the most abundant is the unsaturated hydrocarbon isoprene, C₅H₈) is so large as to be a significant "loss" term in the global carbon cycle, contributing to the discrepancy between the global total land biosphere uptake of CO₂ as measured geophysically (e.g. by combining O₂­ and CO­₂­ measurements) and the apparent sink of CO₂ as measured at a local scale by flux towers. Isoprene is also one of the most important consumers of atmospheric oxidizing capacity, and changes in isoprene emissions (associated with changes in forest cover and temperature) have been implicated, albeit controversially, in changes in the atmospheric lifetime (and therefore the concentration) of methane between glacial and interglacial periods.

Although the biological function of isoprene is not entirely resolved, the two leading hypotheses (thermoprotection of cell membranes, and protection against oxidative stress) are compatible with the observed strong increase in isoprene emission at temperatures above about 30°C. Isoprene emission also responds to drought (with increased emissions by plants under drought-stressed conditions), and to CO₂ concentration (with low ambient CO₂ stimulating isoprene synthesis and high ambient CO₂ inhibiting it). Monoterpenes, composed of two isoprene subunits, may fulfil some of the same functions as isoprene, and have also been implicated in defense mechanisms against insect herbivory. Many other BVOCs are emitted by plants, including oxygenated species such as methanol (CH₃OH) and acetone (CH₃COCH₃); however, little is known about their function or their environmental controls.

An additional Earth System process involving BVOCs is the production of secondary organic aerosol (SOA), another atmospheric constituent whose importance has only come to be appreciated during the past decade. SOA is produced because the atmospheric oxidation pathways for reduced VOCs include species that are liquid at ordinary atmospheric temperatures, and therefore condense as droplets.

Atmospheric chemistry and aerosol modelling (in so-called Chemistry-Transport models, CTMs, and Chemistry-Climate models, CCMs) has advanced to the point where the main reaction pathways of several BVOCs, and (in the most recent models) the production of SOA, are included explicitly. But in order to understand better the roles of these processes, including potential feedbacks to climate change (through e.g. effects of BVOC production on O₃ concentration and oxidizing capacity, as well as the production of SOA), it is essential to be able to model the emissions of the most important BVOCs by plants.

This field is much less well developed. Process-based representations of the environmental controls on BVOC production are in their infancy, and evaluation of modelled BVOC emission fields has been almost non-existent. Yet there is increasing information becoming available on the biochemistry of BVOC emission, environmental responses of emissions (across a large range of species), and—crucially—the spatial and temporal distribution of BVOC oxidation products, including formaldehyde (HCHO), from satellite observations, notably Sciamachy. When other sources such as industrial sources of HCHO, and pyrogenic sources of BVOCs, are duly taken into account, these observations could be used to test modelled responses of BVOC emission to spatial variations of plant community composition, and to the seasonal and interannual variability of climate.

Accordingly, the project to be undertaken by the ESR will: (a) develop a new synthesis of available plant-level information and understanding about sources of the most important BVOCs (isoprene and monoterpenes) and their environmental dependencies; (b) incorporate this information into a new generation of global BVOC emission model to be coupled into the versatile Land Processes and Exchanges (LPX) modelling framework; (c) perform model runs forced by historical to contemporary climates and CO₂ concentration; and (d) use the available remotely sensed information on HCHO, and potentially other species, in combination with LPX-modelled pyrogenic emissions and independent information on industrial sources, to evaluate the modelled fields for the years in which the satellite data are available. A final step (e) will entail using the information obtained in the model-data comparison to optimize the emissions model.

An ideal candidate for this ESR position would have a university background in Chemistry, and preferably have some knowledge of both biochemistry and atmospheric chemistry. (Training in the relevant aspects of plant physiology and Earth System science will be provided.) The work will involve a significant degree of collaboration and interaction: in particular with CREAF (where there is a strong experimental interest in BVOC emission) but also with other groups interested in BVOC modelling, including Lund University and the Met Office. The position will be based at Imperial College and will be one of the "founding" positions in Professor Colin Prentice's new research group, which will focus on different aspects of terrestrial biosphere modelling and its applications to Earth System and climate change impacts research. The work will benefit from further connections between LPX model development and the atmospheric chemistry modelling and remote-sensing observation communities (built up within the EU FP6 HYMN: Hydrogen, Methane, Nitrous oxide project), and Professor Prentice's additional connection to Macquarie University, Sydney, where his close collaborator Ian Wright is developing a comprehensive species-based data set of BVOC emissions measurements.

Colin Prentice

27 November 2009