Including Coarse Mode Aerosol Microphysics in a Climate Model: Model Development and First Application

Recent climate model simulations indicated that sulfate (SO4) formed from ship emissions may be one of the major contributors to the negative anthropogenic aerosol radiative forcing. Due to increasingly stringent regulations on the maximum sulfur content of ship fuels this contribution is expected to decrease strongly in the future. Possibly, nitrate (NO3) formation will compensate for part of the reduction, but measurements indicate that it may be crucial to include coarse mode particle interactions with condensable trace gases in order to quantify this effect. However, none of the aerosol (sub)models previously used for such assessments accounted for the coarse mode particle effects. This provided the motivation to extend one of those submodels, namely MADE, in the present work. The new submodel, MADE3, is based on the second generation of MADE, called MADE-in. It includes nine lognormal modes to represent three size ranges with three types of aerosol particles each. The associated increase in complexity w.r. t. to MADE and MADE-in required a complete revision of the code and careful reexamination of the underlying physical assumptions, as only the fine modes had been considered in the gas–particle interactions in the predecessor submodels. The main new features of MADE3 are the ability of coarse mode particles to take up condensing vapors and to coagulate with fine mode particles, and the gas–particle partitioning of chlorine, which is mainly contained in sea spray (SS) particles. In order to test the algorithms used in the new submodel it was run in a box model setup and the results were compared to those obtained in an analogous setup with the much more detailed, particle-resolved aerosol model PartMC-MOSAIC. The comparison was performed for an idealized marine boundary layer test case and showed improved performance of MADE3 over MADE in the representation of coarse mode particles and total aerosol composition. Subsequently, MADE3 was implemented into the atmospheric chemistry general circulation model EMAC. Due to the new mode structure this required extensive adaptations to other submodels, specifically to the one used for cloud and precipitation processing of aerosol particles. EMAC does not track interstitial aerosol particles separately from those immersed in cloud droplets, ice crystals, or precipitation. Hence, a sophisticated scheme was devised and implemented for the assignment of the in-cloud or in-precipitation aerosol to one of four possible modes, instead of just one possible mode in the MADE case. The coupled model, EMAC with MADE3, was thoroughly evaluated by comparison of simulation output to station network measurements of near-surface aerosol component mass concentrations, to airborne measurements of vertical aerosol mass mixing ratio and number concentration profiles, to ground-based and airborne measurements of particle size distributions, and to station network and satellite measurements of aerosol optical depth. Satisfactory agreement with the observations was obtained and it was thus shown that MADE3 is ready for application within EMAC. The results from an identically designed simulation with the predecessor submodel MADE led to the conclusion that a fraction of the secondary aerosol species partitions to the coarse modes in MADE3 and is thus removed more quickly from the atmosphere. Furthermore, a new evaluation method was developed, which allows for comparison of model output to size-resolved electron microscopy measurements of particle composition. Both submodels, MADE3 and MADE, were finally used in EMAC simulations of the effect of ship emissions on the atmospheric aerosol. As in previous studies for year 2000 conditions, SO4 was found to be the dominant species in the fine modes in this context. In contrast to SO4, the major fraction of ship emissions-induced near-surface NO3 was found to partition to the coarse modes in the MADE3 simulations. A similar amount of fine mode NO3 as in the present and former MADE simulations was also formed. Hence, fine mode particle growth due to ship emissions was also similar, and was reduced in idealized simulations of a future low-sulfur fuel scenario. Particle volume concentration decreased by about 1 % due to ship emissions in the MADE3 simulations, but not in the MADE simulations. This finding was independent of the fuel sulfur content. In summary, the inclusion of coarse mode particle interactions and the gas–particle partitioning of chlorine could alter prior conclusions on the climate effect of ship emissions-induced aerosol perturbations, mainly due to the differences in NO3 formation. This climate effect will be re-quantified in a follow-up study by coupling the MADE3 aerosol to a two-moment cloud microphysics scheme. Further planned applications of the new submodel include the quantification of climate effects of aerosol perturbations via their influence on ice clouds as well as simulations with boundary conditions specific to measurement campaigns. Results from the latter may lead to further model improvements and can also provide guidance for the interpretation of measurement results.