Contrail radiative dependence on ice particle number concentration

Recent studies on low aromatic fuels have shown that lower soot number emissions may reduce contrail ice particle number concentrations (N ice). Here we implemented, in a sophisticated radiative transfer model, two ice particle size distribution schemes in order to estimate the contrail radiative forcing's (RFs) dependence on these prospective N ice reductions resulting from the introduction of sustainable aviation fuels. The results show that an 85% contrail N ice reduction produces a 35% smaller contrail RF, while neglecting all non-radiative effects. This estimate of an RF reduction only considers the effects of the N ice change assumed here, and neglects other potentially important microphysical mechanisms that may change the relationship between soot number emissions and N ice. A comparison of our results with previous published estimates from full climate model simulations, shows similar RF reductions to those which also take into account non-radiative mechanisms, evidencing the need for more studies in order to allocate the contribution from radiative and non-radiative changes, as this would guide possible mitigation implementations. Despite these modeled contrail RF reductions being largely independent of the assumed ice water content (IWC), it is only through simultaneous improvement of the IWC and N ice representation in models that contrail RF estimates can be better constrained. This is because our calculated RF varied by a factor of 3 when assuming a +/- 30% IWC range; and by a factor of 5 if a, still conservative, +/- 60% IWC range was prescribed, suggesting that the differences in the prescribed IWC and N ice values in different models may explain the large discrepancies amongst published RF estimates. Recent estimates of higher N ice values, and lower IWCs found in contrails even after several hours, compared to surrounding cirrus under similar atmospheric conditions, were assessed to conclude that it is mainly the differences in IWC that make young contrails have a smaller RF, and to reduce our previous estimate for linear contrail RF for year 2006 by 65%.