One issue of importance for future scenarios concerns the treatment of black carbon removal rates. Whereas most models projected changes for BC to be approximately linear, the ULAQ and GISS models had somewhat different treatments. In the ULAQ model, the reaction of O₃ with BC leads to additional losses for BC. This caused a change of -30% for BC in the A2 2100 scenario in the simulation that included consideration of chemistry feedbacks (SC6) relative to the scenario without changes in chemistry (SC3). The GISS model, on the other hand, did not consider any heterogeneous loss, but did assume that the loss of BC associated with wet scavenging depended on the interaction of BC with SO₄^2-. In this model, as SO₂ emissions decrease, the lifetime of BC increased. This model projects a 50% increase in BC lifetime (and thus in the concentrations relative to emissions) in 2100 associated with the A2 scenario. Thus, the uncertainty associated with the projection of BC concentrations is larger than that for the other aerosol types.

In addition to the studies outlined above, one additional study was performed. In this study, the A2 2100 emission scenario was used to simulate HNO₃ concentrations in 2100 using the Harvard University model (see Chapter 4) and the present day anthropogenic NH₃ emissions were scaled by the increase in N₂O emissions in 2100 from the draft SRES A2 scenario. Then the model described by Adams et al. (1999) was used to estimate direct forcing after condensation of the additional HNO₃ and NH₃ onto the calculated sulphate aerosol. The nitrate and ammonium burdens increased by about a factor of 4.7 and a factor of 1.9, respectively, in the A2 2100 scenario. The estimated forcing associated with anthropogenic SO₄^2-, nitrate, and ammonium aerosols increased from -1.78 Wm^�2 to -2.77 Wm^�2. Thus, the control of sulphate aerosol in future scenarios may not necessarily lead to decreases in forcing, if the levels of ammonium nitrate in aerosol increase.

Two final considerations include the possible impact of chemistry and climate changes on future concentrations. These were examined by the ULAQ model for the A2 2100 scenario. Future concentration changes were small for the simulation that included changes in chemistry only (scenario SC6). Climate feedbacks, however, were significant. Aerosol concentrations changed by as much as -20% relative to the model simulations that did not include climate change.

As we have seen, anthropogenic aerosols may have a substantial effect on the present day aerosol abundance, optical depth and thus forcing of climate. While we have made substantial progress in defining the role of anthropogenic aerosols on direct forcing, significant uncertainties remain, particularly with the role of anthropogenic organic and black carbon aerosols in detemining this forcing. Our ability to assess the indirect forcing by aerosols has a much larger uncertainty associated with it. The largest estimates of negative forcing due to the warm-cloud indirect effect may approach or exceed the positive forcing due to long-lived greenhouse gases. On the other hand, there is sufficient uncertainty in the calculation of indirect forcing to allow values that are substantially smaller than the positive forcing by greenhouse gases. Other factors which have not been assessed include possible anthropogenic perturbations to high level cirrus clouds as well as to clouds in the reservoir between 0°C and -35°C. As we have discussed, significant positive forcing is possible from aerosol-induced increased formation of cirrus and/or from increased glaciation of clouds in the region between 0°C and -35°C.

Concerns about aerosols derive from a number of other considerations. These include visibility, toxic effects and human health, interactions of aerosols with chemical processes in the troposphere and stratosphere, acid deposition, and air pollution. Of these concerns, those associated with toxic effects and visibility have led the industrialised countries to promulgate standards to reduce the concentrations of aerosols in urban and also more pristine locations. Also, concerns about the effects of acid rain have led to increased controls over the emissions of SO₂. As shown above, the SRES scenarios for the future (Nakic´enovic´ et al., 2000) have all assumed that emissions of SO₂ will eventually decrease, and the A1T and B2 scenarios predict that sulphur emissions start to decrease on a global average basis almost immediately.

Emissions of carbon aerosols in the future may not necessarily follow the scenarios outlined for SO₂ within the SRES scenarios. Furthermore, older scenarios, developed by IPCC (1992), do not presume that SO₂ emissions would necessarily decrease. We have evaluated a set of scenarios for future aerosols that account for a range of possible future emissions. We noted that one of the future effects that need to be evaluated includes the perturbation to the cycles of natural aerosols. Another includes the interaction of chemical cycles with aerosols and the prediction of how changes in other gas-phase emissions may be perturbing aerosols. We must also consider how changes in aerosols may perturb gas phase cycles. Our evaluation of these interactions has been necessarily limited. Both the interaction of atmospheric chemical cycles with aerosols and the perturbation to natural aerosol components through changing climate patterns need to be better understood.

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