Radiative forcing due to BC on snow and the direct aerosol effect of BC in the Arctic Gunnar Myhre CICERO Center for International Climate and Environmental Research Oslo Tore F. Berglen, Terje Berntsen, Sebastian Gerland, Chris Hoyle, Ivar Isaksen, Mona Johnsrud, Christina Pedersen, Ragnhild Skeie Global mean RF estimates Highlights A stronger RF of direct aerosol effect of BC A weaker RF of BC on snow and sea ice Outline Model description Model evaluation Aerosol optical depth, BC, single scattering albedo Description of the modelling of BC in snow Results for BC concentration in snow and change in surface albedo Radiative forcing Tools A snow module implemented for simulations of BC concentration in snow and the impact on the albedo We use a 3D global chemistry transport model with all main aerosol components included Mineral dust, sea salt, sulphate, nitrate, BC and OC from fossil fuel and biomass burning, as well as secondary OC Spatial resolution of 1x1 degree and 40 vertical layers Most simulations for the period Annual mean aerosol optical depth (AOD). Model data included for days with available satellite data. oppdatert Aerosol optical depth (AOD) Arctic stations Comparisons at AERONET stations Air concentrations of BC near the surface (g m -3 ), annual mean values Single scattering albedo (scattering AOD/total AOD) x External mixtures Internal mixture Snow layers (max 10 layers) Layer 1: max 1e-3 meter of water equivalent BC in snow Snow data: Snow depth Snow fall Snow evaporation Snow melt Oslo CTM2 model is expanded with a routine which generates snow layers and Black Carbon contents in these layers Snow data from the ECMWF are used to build up and remove snow layers in each grid box. Deposition BC WetDry Melting: BC in the removed snow is assumed to remain at the surface of the snow pack. BC content in the snow column are conserved until the whole snow column has melted. In each time step BC are dry deposited to the surface layer. Svalbard: Observations and model BC concentrations in snow (ng /g) Year 2007 meteorology, T42 resolution, emission : FFC BC 1996, Bio BC 2006, spin up 6 mnd. Modelling of BC impact on surface albedo In the albedo calculations reduce from 10 to 2 snow layers First layer 1 mm water equivalent thickness Second layer rest of the snow layers Standard way to calculate change in snow albedo Use of Mie theory of snow and BC to calculate optical properties and the DISORT radiative transfer scheme to calculate the snow albedo change Surface albedo modelled based on surface scene (vegetation type) and snow cover and vegetation dependence on snow BC IN SNOW SNOW ALBEDO SURFACE ALBEDO BC concentration in snow (ng BC per g of water) Layer 1 Layer 2 Change in albedo as a function of BC concentration for various snow grain sizes In general in accordance with earlier studies BC impact is largest for large snow grain sizes (r e ) Layer 2 Change in albedo for variation in the BC concentration for various snow grain sizes In Layer 1 the results differ from Layer 2; snow grain size dependence is smaller Percentage change in snow albedo (below 850 nm) due to BC Less than 0.5% change in Arctic NH land many regions with 1% change Some regions close to industrialized areas with 10% change Greenland and Antarctica extremely small changes Radiative forcing due to BC on snow and ice (W m -2 ) Grain size 3000 m RF =0.03 W m -2 Grain size 50 m RF =0.025 W m -2 Radiative forcing due to the direct aerosol effect of BC (W m -2 ) Global mean 0.26 W m -2 Internal and external mixing of BC a)External mixture b)Internal mixture, homogeneous c)Internal mixture, coated sphere Bond et al. (2006) suggest that hydrophilic BC absorption is enhanced by 50% compared to external mixture RF of direct aerosol effect of BC External mixture 0.26 W m -2 (case a) Coated sphere 0.33 W m -2 (case c) Scattering aerosols enhance the RF of BC Total direct aerosol effect Global mean W m -2 Sulphate W m -2 Nitrate W m -2 BC W m -2 OC W m -2 Bio W m -2 Latitudinal variation in the RF Global Arctic Summary Radiative forcing Direct aerosol effect of BC ~0.3 W m -2 BC on snow ~0.03 W m -2 Further investigations Impact of several snow layers for the albedo BC in air and snow Absorption by BC and external vs internal mixing Snow grain size Climate effects of reducing Black Carbon emissions Project funded by Norwegian Research Council Participants: CICERO, NPI and UiO Activities: Measurements of BC in snow implications for albedo (NPI) Modelling of dispersion and deposition of BC (CICERO) Modelling of radiative and climate effects (UiO) Emission inventories and mitigation options (CICERO) Climate impacts of mitigation options (UiO/CICERO) Political and economical analysis of mitigation (CICERO) oppdatert Fig 5 OSLO-CTM2 Deposition BC WetDry In each time step BC are dry deposited to the surface layer. For wet-deposition of BC, the BC is included in both the surface layer and in the layer underneath, giving equal concentration of BC in both layers. Melting: BC in the removed snow is assumed to remain at the surface of the snow pack. BC content in the snow column are conserved until the whole snow column has melted. Ett hakk lengre st som kanskje passer bedre med Vagabond og Lomonosovfonna Snow cover