Black-carbon absorption enhancement in the atmosphere determined by particle mixing state

Atmospheric black carbon makes an important but poorly quantified contribution to the warming of the global atmosphere. Laboratory and modelling studies have shown that the addition of non-black-carbon materials to black-carbon particles may enhance the particles’ light absorption by 50 to 60% by refracting and reflecting light. Real-world experimental evidence for this ‘lensing’ effect is scant and conflicting, showing that absorption enhancements can be less than 5% or as large as 140%. Here we present simultaneous quantifications of the composition and optical properties of individual atmospheric black-carbon particles. We show that particles with a mass ratio of non-black carbon to black carbon of less than 1.5, which is typical of fresh traffic sources, are best represented as having no absorption enhancement. In contrast, black-carbon particles with a ratio greater than 3, which is typical of biomass-burning emissions, are best described assuming optical lensing leading to an absorption enhancement. We introduce a generalized hybrid model approach for estimating scattering and absorption enhancements based on laboratory and atmospheric observations. We conclude that the occurrence of the absorption enhancement of black-carbon particles is determined by the particles’ mass ratio of non-black carbon to black carbon

Cooperative contractility: the role of stress fibres in the regulation of cell-cell junctions

We present simulations of cell-cell adhesion as reported in a recent study [Liu et al., 2010, PNAS, 107(22), 9944-9] for two cells seeded on an array of micro-posts. The micro-post array allows for the measurement of forces exerted by the cell and these show that the cell-cell tugging stress is a constant and independent of the cell-cell junction area. In the current study, we demonstrate that a material model which includes the underlying cellular processes of stress fibre contractility and adhesion formation can capture these results. The simulations explain the experimentally observed phenomena whereby the cell-cell junction forces increase with junction size but the tractions exerted by the cell on the micro-post array are independent of the junction size. Further simulations on different types of micro-post arrays and cell phenotypes are presented as a guide to future experiments. (C) 2014 Elsevier Ltd. All rights reserved.Science Foundation Ireland grant 10/RFP/ENM2960 and Short Term Travel Fellowship (STTF 11

Cellular contractility and substrate elasticity: a numerical investigation of the actin cytoskeleton and cell adhesion

Numerous experimental studies have established that cells can sense the stiffness of underlying substrates and have quantified the effect of substrate stiffness on stress fibre formation, focal adhesion area, cell traction, and cell shape. In order to capture such behaviour, the current study couples a mixed mode thermodynamic and mechanical framework that predicts focal adhesion formation and growth with a material model that predicts stress fibre formation, contractility, and dissociation in a fully 3D implementation. Simulations reveal that SF contractility plays a critical role in the substrate-dependent response of cells. Compliant substrates do not provide sufficient tension for stress fibre persistence, causing dissociation of stress fibres and lower focal adhesion formation. In contrast, cells on stiffer substrates are predicted to contain large amounts of dominant stress fibres. Different levels of cellular contractility representative of different cell phenotypes are found to alter the range of substrate stiffness that cause the most significant changes in stress fibre and focal adhesion formation. Furthermore, stress fibre and focal adhesion formation evolve as a cell spreads on a substrate and leading to the formation of bands of fibres leading from the cell periphery over the nucleus. Inhibiting the formation of FAs during cell spreading is found to limit stress fibre formation. The predictions of this mutually dependent material-interface framework are strongly supported by experimental observations of cells adhered to elastic substrates and offer insight into the inter-dependent biomechanical processes regulating stress fibre and focal adhesion formation.Irish Research Council for Science, Engineering and Technology (IRCSET) postgraduate scholarship under the EMBARK initiative and by the Science Foundation Ireland Research Frontiers Programme (SFI-RFP/ENM1726

On the role of the actin cytoskeleton and nucleus in the biomechanical response of spread cells

Micropipette aspiration (MA) has been used extensively in biomechanical investigations of un-adhered cells suspended in media. In the current study, a custom MA system is developed to aspirate substrate adhered spread cells. Additionally, the system facilitates immuno-fluorescent staining of aspirated cells to investigate stress fibre redistribution and nucleus deformation during MA. In response to an applied pressure, significantly lower aspiration length is observed for untreated contractile cells compared to cells in which actin polymerisation is chemically inhibited, demonstrating the important contribution of stress fibres in the biomechanical behaviour of spread cells. Additional experiments are performed in which untreated contractile cells are subjected to a range of applied pressures. Computational finite element simulations reveal that a viscoelastic material model for the cell cytoplasm is incapable of accurately predicting the observed aspiration length over the range of applied pressures. It is demonstrated that an active computational framework that incorporates stress fibre remodelling and contractility must be used in order to accurately simulate MA of untreated spread cells. Additionally, the stress fibre distribution observed in immuno-fluorescent experimental images of aspirated cells is accurately predicted using the active stress fibre modelling framework. Finally, a detailed experimental computational investigation of the nucleus mechanical behaviour demonstrates that the nucleus is highly deformable in cyto, reaching strain levels in excess of 100% during MA. The characterisation of stress fibres and nucleus biomechanics in spread cells presented in the current study can potentially be used to guide tissue engineering strategies to control cell behaviour and gene expression. (c) 2014 Elsevier Ltd. All rights reserved.Science Foundation Ireland – Research Frontiers Programme (10/RFP/ENM2960)

Measuring aerosol distribution and transport in London using a high density solar radiation measurement network

International audienceUrban aerosols are important to human health and also cause a local direct radiative effect at the surface and the TOA. Regions surrounding urban areas are also sensitive to these effects through the aerosol transportation within the planetary boundary layer. We present a novel technique for studying the distribution of aerosols within and out of London. The technique uses a new high density network of continuous solar radiation measurements across London which form part of the OPAL (Open Air Laboratories) weather station network. We perform Langley extrapolations on the measured irradiance to infer the columnar amount of aerosol, and hence aerosol optical depth. This allows a map of aerosol optical depth across greater London to be produced. We evaluate results from this method by comparing results to measurements from the EM25 field campaign which took place during June 2009. During EM25 measurements of aerosols were made firstly by the UK FAAM (Facility for Airborne Atmospheric Measurements) BAe-146 aircraft, performing in-situ measurements, and secondly by a truck that was driven around London and equipped with a 355nm wavelength backscatter lidar, showing the vertical distribution of aerosol. The solar radiation measurements are also compared to data from London's PM2.5 and PM10 ground-based network, and to satellite aerosol optical depth data from MODIS at 550nm. The solar radiation network results show aerosol being transported to the southwest of London under the effect of a north-easterly prevailing wind. This shows good agreement with data from MODIS, PM2.5 and the EM25 field campaign measurements. This novel technique of using a high density network of solar radiation measurements is therefore able to monitor the distribution of aerosol across and out of London, and is presented as a useful way to infer aerosol distribution across urban areas

Measuring aerosol distribution and transport in London using a high density solar radiation measurement network

International audienceUrban aerosols are important to human health and also cause a local direct radiative effect at the surface and the TOA. Regions surrounding urban areas are also sensitive to these effects through the aerosol transportation within the planetary boundary layer. We present a novel technique for studying the distribution of aerosols within and out of London. The technique uses a new high density network of continuous solar radiation measurements across London which form part of the OPAL (Open Air Laboratories) weather station network. We perform Langley extrapolations on the measured irradiance to infer the columnar amount of aerosol, and hence aerosol optical depth. This allows a map of aerosol optical depth across greater London to be produced. We evaluate results from this method by comparing results to measurements from the EM25 field campaign which took place during June 2009. During EM25 measurements of aerosols were made firstly by the UK FAAM (Facility for Airborne Atmospheric Measurements) BAe-146 aircraft, performing in-situ measurements, and secondly by a truck that was driven around London and equipped with a 355nm wavelength backscatter lidar, showing the vertical distribution of aerosol. The solar radiation measurements are also compared to data from London's PM2.5 and PM10 ground-based network, and to satellite aerosol optical depth data from MODIS at 550nm. The solar radiation network results show aerosol being transported to the southwest of London under the effect of a north-easterly prevailing wind. This shows good agreement with data from MODIS, PM2.5 and the EM25 field campaign measurements. This novel technique of using a high density network of solar radiation measurements is therefore able to monitor the distribution of aerosol across and out of London, and is presented as a useful way to infer aerosol distribution across urban areas