Black carbon ageing in the Canadian Centre for Climate modelling and analysis atmospheric general circulation model
[摘要] Black carbon (BC) particles in the atmosphere have important impacts onclimate. The amount of BC in the atmosphere must be carefullyquantified to allow evaluation of the climate effects of this type ofaerosol. In this study, we present the treatment of BC aerosol in thedevelopmental version of the 4th generationCanadian Centre for Climate modelling and analysis (CCCma)atmospheric general circulationmodel (AGCM). The focus of this work is on theconversion of insoluble BC to soluble/mixed BC by physical andchemical ageing. Physical processes include thecondensation of sulphuric and nitric acid onto the BC aerosol, andcoagulation with more soluble aerosols such as sulphates andnitrates. Chemical processes that may age the BC aerosol include theoxidation of organic coatings by ozone.
Four separate parameterizations of the ageing process are compared to a control simulationthat assumes no ageing occurs. These simulations use 1) an exponential decaywith a fixed 24h half-life, 2) a condensation and coagulationscheme, 3) an oxidative scheme, and 4) a linearcombination of the latter two ageing treatments. Global BC burdensare 2.15, 0.15, 0.11, 0.21, and 0.11TgC for the control run, and fourageing schemes, respectively. The BC lifetimes are 98.1, 6.6,5.0, 9.5, and 4.9 days, respectively. The sensitivity of modelled BCburdens, and concentrations to the factor of two uncertainty in theemissions inventory is shown to be greater than the sensitivity to theparameterization used to represent the BC ageing, except for theoxidation based parameterization. A computationally efficient parameterization that represents theprocesses of condensation, coagulation, and oxidation is shown tosimulate BC ageing well in the CCCma AGCM. As opposed to the globallyfixed ageing time scale, this treatment of BC ageing is responsive tovarying atmospheric composition.
