Modelled radiative forcing of the direct aerosol effect with multi-observation evaluation

Abstract

Abstract.A high-resolution global aerosol model (Oslo CTM2) driven by meteorological data and allowing a comparison with a variety of aerosol observations is used to simulate radiative forcing (RF) of the direct aerosol effect. The model simulates all main aerosol components, including several secondary components such as nitrate and secondary organic carbon. The model reproduces the main chemical composition and size features observed during large aerosol campaigns. Although the chemical composition compares best with ground-based measurement over land for modelled sulphate, no systematic differences are found for other compounds. The modelled aerosol optical depth (AOD) is compared to remote sensed data from AERONET ground and MODIS and MISR satellite retrievals. To gain confidence in the aerosol modelling, we have tested its ability to reproduce daily variability in the aerosol content, and this is performing well in many regions; however, we also identified some locations where model improvements are needed. The annual mean regional pattern of AOD from the aerosol model is broadly similar to the AERONET and the satellite retrievals (mostly within 10-20%). We notice a significant improvement from MODIS Collection 4 to Collection 5 compared to AERONET data. Satellite derived estimates of aerosol radiative effect over ocean for clear sky conditions differs significantly on regional scales (almost up to a facCorrespondence to: G. Myhre (gunnar.myhre@cicero.uio.no) tor two), but also in the global mean. The Oslo CTM2 has an aerosol radiative effect close to the mean of the satellite derived estimates. We derive a radiative forcing (RF) of the direct aerosol effect of −0.35 Wm −2 in our base case. Implementation of a simple approach to consider internal black carbon (BC) mixture results in a total RF of −0.28 Wm −2 . Our results highlight the importance of carbonaceous particles, producing stronger individual RF than considered in the recent IPCC estimate; however, net RF is less different. A significant RF from secondary organic aerosols (SOA) is estimated (close to −0.1 Wm −2 ). The SOA also contributes to a strong domination of secondary aerosol species for the aerosol composition over land. A combination of sensitivity simulations and model evaluation show that the RF is rather robust and unlikely to be much stronger than in our best estimate.

Quantifying immediate radiative forcing by black carbon and organic matter with the Specific Forcing Pulse

“…Fewer studies are available for regional variations in forcing by OM Naik et al, 2007;Koch et al, 2007). The studies broadly agree that SFP from South Asia, Africa and from biomass burning regions is of larger magnitude.…”

“…A recent model (Myhre et al, 2009) with an SFP of +0.99 GJ g −1 was not included in the ensemble because it included a lengthened aging time based on observed oxidation rates (Maria et al, 2004). This change increases lifetime and hence SFP, but tying aging to oxidation rates ignores the rapid aging by coating (Moffett and Prather, 2009).…”

Standard climate models radiation codes underestimate black carbon radiative forcing

“…The radiative transfer code was run either in a multi-stream mode (eight streams) or with two streams and the Delta-M method (Wiscombe, 1977). In the global simulations we used meteorological data from ECMWF, and specified aerosol optical properties (Myhre et al, 2009a) and aerosol distribution from the OsloCTM2 chemical transport model (Skeie et al, 2011). To study the impact of the radiation code on global mean RF of BC, input fields and results from OsloCTM2 part of AeroCom Phase II for several aerosol components were used.…”

“…This model has previously been run in idealized experiments with prescribed vertical profiles Published by Copernicus Publications on behalf of the European Geosciences Union. of aerosol extinction (Randles et al, 2013) and used for global climate simulations (Myhre et al, 2009a). The radiative transfer code was run either in a multi-stream mode (eight streams) or with two streams and the Delta-M method (Wiscombe, 1977).…”

Modeling the meteorological and chemical effects of secondary organic aerosols during an EUCAARI campaign

“…When mixed with inorganic components and soot, OOA increases aerosol scattering and contributes to its negative direct radiative forcing (Ming et al, 2005;Forster et al, 2007;Myhre et al, 2009;O'Donnell et al, 2011). Increases in aerosol extinction related to the presence of organic matter are mostly driven by large amounts of associated water, and enhanced mass due to condensation (Morgan et al, 2010a).…”