The chemical transport model Oslo CTM2 has been used to test two different parameterizations of the black carbon (BC) aerosol. Key uncertainties associated with the representation aerosols in modeling are size distribution, mixing state and removal processes. An important parameter for black carbon is the aging time. In the original aerosol parameterization, aging was represented by a constant transfer of 24% per day from hydrophobic to hydrophilic mode. A new aerosol parameterization called M7, which gives a more physical representation of BC aging by including particle interaction, was recently included in the CTM2 model. This module describes size distribution, mixing state and particle interaction for sulphur, dust, black and organic carbon and sea salt.
In this thesis, several simulations have been done to test the effect of using M7 on the modeled BC distribution, lifetime, deposition and regional contributions. Applying the M7 module results in regional and seasonal differences in BC aging. In high-latitudes, the aging is slower than with the original version, leading to an increased burden of mainly insoluble BC particles. Between 30 ◦N and 30 ◦S the burden is reduced, indicating a shorter lifetime caused by faster aging. Global mean BC lifetime and burden is only slightly changed; from 7.63 days and 0.17 Tg with the original parameterization to 7.3 days and 0.14 Tg.
Modeled BC content in Arctic snow and ice show high concentrations on the continents, andmaximumnear industrialized areas. Concentrations in the snow and ice in the Arctic Ocean are mostly less than 10 ng g−1 with both aerosol representations.
Regional experimentswith emissions in China and Europe separately, show that European emissions contributemost to total BC burden north of 60 ◦N in the lower atmosphere, while emissions from China are important above 6 km. European emissions also provide the largest contribution to accumulated BC in snow and ice north of 60 ◦N. These results are consistent with several other studies. The choice of aerosol parameterization strongly influences the regional impact. BC aging time in China is reduced from 4.16 days in the original version to 3.16 days with M7, while aging time in Europe is increased to 5.16 days. As a consenquence, the contribution from China to atmospheric BC burden and to accumulated BC in snow and ice is reduced, while the European contributions are strenghtened. There is potential for improvement in the M7 module. However, the regional and seasonal variation in BC aging is captured, and there does not appear to be any large errors in the model results compared to previous calculations and observations. M7 allow for a closer study of regional impact and interaction between co-emitted pollutants, and may improve the calculation of radiative forcing
