20. A Global Emission Inventory of Primary Carbonaceous Aerosols Appropriate for Climate Modeling

Carbonaceous aerosols, including black carbon (BC) and organic carbon (OC), make up a large fraction of the atmospheric aerosols and affect the radiative balance of the earth either by directly scattering and absorbing solar radiation or through indirect influence on cloud optical properties and cloud lifetimes. Black carbon is thought to be one of the largest contributors to the absorption of visible light. The only sources of BC are combustion processes, mainly biomass and fossil fuel burning. Because different combustion practices play an important role in determining emission rate of BC and OC to the atmosphere, estimates of carbonaceous aerosol emissions from different researches are very uncertain both regionally and globally. OC is nearly always emitted with BC during combustion. Because OC scatters light and BC absorbs it, it is possible that OC can oppose the warming effect of BC, so that the net climatic effect of carbonaceous aerosols is not known.

Organics are the largest components of biomass burning aerosols and are also components of aerosols emitted from fossil fuel burning. They are widespread in the atmosphere and are possibly also important constituents of upper-tropospheric aerosols. Though there are hundreds of different organic species in aerosols, global climate models treat organics as one “compound” and the emissions of OC are often simply derived by multiplying BC emissions with OC/BC ratio. Organic compounds with different characteristics have different optical properties, which affects radiative forcing. This adds to the uncertainty in estimating the radiative forcing of carbonaceous aerosols.

We present a global emission inventory of primary carbonaceous aerosols that has been designed for global climate modeling purpose. The inventory considers emissions from fossil fuels, biofuels, and open biomass burning. Fuel type, combustion type, and emission controls, and their prevalence on a regional basis are combined together to determine emission factors for all types of carbonaceous aerosols.

Since one single organic “compound” is not sufficient to represent all the organics in aerosols, we propose a preliminary classification for organic aerosols based on structural and optical properties. We provide broad classes aiming at global models instead of very detailed classifications, which are not amenable for use in global-scale models due to the calculation cost. Organic matter (OM) which includes the hydrogen and oxygen bound to this carbon is broken down into several surrogate compounds with varied absorption and scattering capabilities. Because our inventory tabulates emissions from specific sources, we can make use of data available from source characterization.

The uncertainty in emissions is constructed and we discuss how this uncertainty translates to radiative forcing. The uncertainty of emissions results from factors including emission rates, speciation of particulate matter, prevalence of different technologies, and activity rates. Finally future development for carbonaceous aerosol global inventory will be discussed and some comments and recommendations will be presented as well.