AN IMPROVED REPRESENTATION OF MODE DYNAMICS AND SIZE-RESOLVED COMPOSITION IN MODAL AEROSOL MODELS

摘要

The modal method for modeling atmospheric aerosol size distributions takes advantage of their observed lognormal characteristics to formulate computationally efficient algorithms for many of the dynamical processes governing their evolution in time (Whitby et al., 1991). Thus this approach has been particularly attractive in large regional-to-global-scale simulations that must be run over long periods of time, as in climate change studies. In previous applications of the modal method as employed in the MCNC Multiscale Air Quality Simulation Platform (MAQSIP) and the Ford Research Center Aachen's Modal Aerosol Dynamics model for Europe (MADE), the fine aerosol distribution is simulated using two lognormal modes throughout the simulation. A drawback of this approach in some applications has been that the smaller of the fine modes, used to represent Aitken particles, tends to grow to sizes typical of the larger (accumulation) mode and thus cause mode overlap. This has necessitated the redistribution of aerosol moments between the modes to maintain mode distinguishability. This redistribution is performed at arbitrary intervals of time for an arbitrarily defined maximum degree of overlap allowable between the modes in size space. Recently, under funding from the National Science Foundation, more generalized techniques have been developed to monitor and "manage" mode dynamics for a variable number of active modes, so that the appearance and disappearance of modes due to aerosol dynamical processes can be handled without causing numerical instabilities. In addition, the Multicomponent Aerosol Dynamics Modeling - Modal Approach System version I (MADMAcs I) of Wilck and Stratmann (1996) has been explored as a way to treat aerosol composition variations among the active modes. These techniques have been implemented in the MAQSIP and the MADE to investigate their efforts on the aerosol size range important for radiative transfer. Comparisons with previous results from these models of aerosol optical properties and tropospheric heating rates are presented over the eastern United States and western Europe for two summer episodes. Comparisons of aerosol optical properties with available observational data are also presented.