| Black carbon (BC) particles in the atmosphere have important impacts on
climate. The amount of BC in the atmosphere must be carefully
quantified to allow evaluation of the climate effects of this type of
aerosol. In this study, we present the treatment of BC aerosol in the
developmental version of the 4th generation
Canadian Centre for Climate modelling and analysis (CCCma)
atmospheric general circulation
model (AGCM). The focus of this work is on the
conversion of insoluble BC to soluble/mixed BC by physical and
chemical ageing. Physical processes include the
condensation of sulphuric and nitric acid onto the BC aerosol, and
coagulation with more soluble aerosols such as sulphates and
nitrates. Chemical processes that may age the BC aerosol include the
oxidation of organic coatings by ozone.
Four separate parameterizations of the ageing process are compared to a control simulation
that assumes no ageing occurs. These simulations use 1) an exponential decay
with a fixed 24h half-life, 2) a condensation and coagulation
scheme, 3) an oxidative scheme, and 4) a linear
combination of the latter two ageing treatments. Global BC burdens
are 2.15, 0.15, 0.11, 0.21, and 0.11TgC for the control run, and four
ageing schemes, respectively. The BC lifetimes are 98.1, 6.6,
5.0, 9.5, and 4.9 days, respectively. The sensitivity of modelled BC
burdens, and concentrations to the factor of two uncertainty in the
emissions inventory is shown to be greater than the sensitivity to the
parameterization used to represent the BC ageing, except for the
oxidation based parameterization. A computationally efficient parameterization that represents the
processes of condensation, coagulation, and oxidation is shown to
simulate BC ageing well in the CCCma AGCM. As opposed to the globally
fixed ageing time scale, this treatment of BC ageing is responsive to
varying atmospheric composition. |