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Centre for Atmospheric Science

ACES Project - Scaling-up activities relating local scale measurements to regional and global scales


Clouds and aerosols play an important role in the global climate, but the magnitude of their effects and how these effects will change in a changing climate are consistently identified as areas of high uncertainty in the IPCC reports. This uncertainty is due to the large number of contributing processes which operate in different areas of the globe, many of which are not fully understood or quantified. The work in this part of the ACES project seeks to address this some of this uncertainty for a limited number of processes by using the results of the chamber studies, the Danum measurements, and the mechanism development efforts to assess the effect of BVOC fluxes on aerosol and cloud properties at a regional level. It will also seek to examine feedback effects based on temperature and radiative properties which may influence the magnitude of these fluxes. Further it will examine the potential effects of changing land use patterns. This will be done by a combination of modelling studies on various scales, and a limited set of long term measurements at Danum. This work will be carried out by The University of Edinburgh, with collaboration with The University of Manchester and The University of Reading.


Quantifying the impact and uncertainty of SOA and precursor BVOC on regional atmospheric composition

The magnitude and speciation of terrestrial BVOC fluxes vary with location and time. The impact of these variable fluxes will be assessed using a combination of three different models operating on different scales. An improved BVOC flux model will be developed to include the BSOA precursors identified in the chamber studies, and an imporved flux parameterisation. The resulting BVOC flux model constitutes part of the core modelling activity of the consortium. In addition to the BVOC flux model, BVOC fluxes will also be derived from satellite observations of formaldehyde. These fluxes will be used to provide the input for two further models of gas phase and aerosol processes and transport whcih will be used to study the regional impact of SOA and its biogenic precursor gases on atmospheric composition over Borneo over two complete growing seasons. Gas phase and aerosol chemical mechanisms developed during the ACES project will be represented in the models, allowing a further validation of these mechanisms.

These models will be used to investigate how the impact of SOA and precursors on a regional scale is sensitive to the vertical distributions of emitted biogenic material, which is determined in part by vertical mixing processes. Further we will investigate how local meteorology and ambient aerosol loading (condensation versus nucleation) affect the formation of SOA downwind of the Danum Valley measurement site during different seasons. The co ntribution of biogenic material to aerosol optical depth (AOD) will be estimated. Model runs will also be undertaken to simulate the effects of various land usage change scenarios.

Model output from current land usage scenarious will be evaluated using MODIS and MISR satellite measurements of aerosol and cloud properties. These instruments provide output which is directly comparable with the model output, repeat coverage within several days and have been extensively evaluated using ground based data.

Model error will be quantified using an offline weighted linear leastsquares fit of the model aerosol properties to the same observed quantities, accounting for uncertainties in model and observed quantities. Such a data assimilation approach to quantifying model error is well suited for interpreting geographical, seasonal and interannual error in terms of model processes. Where possible uncertainty analyses associated with the reduced chemical schemes and for land-use change scenarios will be carried out using a Monte Carlo ensemble of runs, where perturbations to the initial conditions, parameter set (e.g., reaction rate coefficients and emission rates), and model structure (e.g., reaction schemes) will be used to assess quantitatively the likelihood of large perturbations to atmospheric composition downwind of the selected forested sites.


Measuring the change in AOD, and the diffuse component of PAR associated with cloud radiative properties, due to BSOA

We will establish a long-term (approximately 2-years) measurement programme in collaboration with Malaysian researchers at the GAW tower at Danum Valley, to test the hypothesis that BSOA significantly affects cloud radiative properties and subsequently the direct:diffuse partitioning of PAR. This addresses one of the central APPRAISE requirements to investigate "The role of aerosols in feedback processes between land, the biosphere and climate."

Long-term measurements will include aerosol properties (number and size distribution using an aerosol counter, AOD using a sunphotometer) and above-canopy radiation (direct:diffuse PAR using a sunshine sensor), accounting for existing meteorological and chemical measurements at the GAW tower site that will be used to identify pollution episodes. Recent work has shown that CCN concentrations can be approximated by measurements of size distribution and using only a crude parameterization of the chemical effects on CCN activation. We will also collect intermittent measurements of plant canopy properties throughout this period, such as LAI. The sunphotometer, together with MEGAN and satellite observations of HCHO, will be used to assess how the biogenic contribution to the column aerosol burden changes throughout the growing season. Because of the time-dependent nature of the BSOA formation we assume that the GAW site is representative of the entire valley site. The sunphotometer also provides invaluable ground-truth for modelled and for coincident satellite measurements of AOD.


Assess the magnitude and uncertainty of the hypothesized climate feedback between BVOC and cloud radiative properties

We will use a 1-D model of transport, chemistry, and aerosol and cloud microphysics to interpret the long-term measurements at Danum Valley and Abisko, Sweden where longterm measurments of BVOC flux and aerosol properties are also available. At the Abisko site the size and number concentration of particles above 10nm are monitored all year round; BVOC fluxes from sub-arctic mire vegetation are measured on a campaign basis during the growing season. Access to these data provided by Lund University provides an opportunity to compare results from tropical and arctic biomes. At both Danum and Abisko we will identify times of elevated anthropogenic and pyrogenic influence using ancillary data (e.g., MAAP at Danum) and back trajectory analyses.

The 1-D model framework allows a more detailed treatment of the individual components of the chemistry, transport, and cloud microphysics, and also ensures that the long-term dataset can be readily evaluated. Within the 1-D framework, the model of BVOC fluxes will be linked with the Edinburgh coupled atmosphere biosphere model. This model describes the response of photosynthesis to changes in surface, canopy and radiative environment, and predicts biosphere-atmosphere fluxes of energy, water and CO2. Prognostic variables, such as leaf temperature, will then be used to constrain the BVOC flux model. It also describes vertical transport of CO2 and BVOC's within the planetary boundary layer (PBL) up to a height of 10Km and is capable of representing the transition from stable to unstable PBL conditions. Model output will be validated against field measurements, particularly vertial profiles measured during the aircraft campaign and CO2 measurements from the GAW program. Clouds are not represented in detail in these models, so updraft velocities and aerosol distributions simulated here will be provided for use in the modelling activities being undertaken at The University of Manchester as part of the OP3 project. Further the improved radiative transfer model, developed at the University of Reading, will be used to quantify the impact of BSOA on cloud microphysics and subsequently on the direct:diffuse partitioning of photosynthetically active radiation with the results fed back into the 1-D models.

Using the models described above we will conduct a series of experiments to a) investigate the chemical and dynamical processes that determine the variability of SOA within and above the canopy on different timescales and compare these for tropical and arctic biomes; b) interpret the long-term measurements at Danum Valley; and c) quantify the direct and indirect impact of SOA on photosynthesis and BVOC fluxes. We will investigate the opportunity of developing a quantitative parameterization of the direct and indirect impact (point c) for use in chemistry-climate models.