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Centre for Atmospheric Science

Aerosol Coupling in the Earth System (ACES)


The oxidation of organic compounds in the troposphere plays a central role in the generation of ozone, and leads to the formation of secondary organic aerosol (SOA) and other secondary pollutants. Approximately 90% of organic material emitted globally is estimated to originate from biogenic sources, with almost half of all reactive biogenic volatile organic compounds (BVOC) being emitted from tropical and sub-tropical forests. It is becoming increasingly clear from observational studies that biogenic SOA (BSOA) is the dominant source of aerosol organic carbon concentrations in remote environments. However, current understanding of the chemical and physical processes associated with the formation of BSOA is extremely uncertain, and so only rudimentary descriptions are included in most models of atmospheric chemistry. Much of this uncertainty derives from the chemical complexity of the system.

The emitted speciation of BVOC includes contributions from isoprene, monoterpenes, sesqiterpenes and oxygenated VOCs. Owing to wide variations in reactivity, these species are oxidised on a variety of time and associated spatial scales in the atmosphere (lifetimes range from minutes to days). The chemical structure of these compounds also has implications for degradation pathways, which can differ dramatically between BVOC, with corresponding variability in their ability to generate SOA.

The emission rates of BVOC are sensitive to changes in weather, climate, and land cover. There have been considerable changes in land use, particularly in the last two centuries, which are likely to continue in the future, significantly impacting the atmospheric aerosol burden and its physical and chemical characteristics and hence providing a feedback on climate. BSOA production in large forested regions, in particular, is thought to affect precipitation, creating a link between the biological and hydrological cycling through the effects of aerosol and cloud. Changes in patterns of nutrient transport (e.g. dust) will induce further vegetation responses. In addition, deposition rates of aerosol to vegetation are known to change with surface roughness and leaf area, particle size and meteorological parameters, so that changes in climate and land cover may alter their atmospheric residence time. An improved process understanding of vegetation responses, BVOC emissions, SOA formation mechanisms and its coupling to the climate system and hydrological cycle (and representation in atmospheric models) is therefore required to quantify and predict past, present and future feedbacks.

The ACES project is an integrated research programme that aims to reduce uncertainties in our fundamental understanding of the formation of BSOA and the subsequent impact on atmospheric composition, through coordinated chamber studies, field studies, process model development, and application of atmospheric models of chemistry and transport to assess coupling and feedbacks in the Earth system. Work undertaken in the ACES project will focus on a single but important biome, that of tropical forests and will be closely linked to the OP3 project which also has a focus on tropical forests.

The ACES project is a consortium project consisting of researchers from The Centre for Environmental Policy at Imperial College London, The School of GeoSciences at The University of Edinburgh, The Department of Chemistry at The University of Leicester, The Centre for Atmospheric Science at The University of Manchester, The Department of Chemistry at The University of York, Centre for Ecology and Hydrology (CEH), and The Department of Environmental Science at The University of Lancaster.