Description and Evaluation of a Six-moment Aerosol Microphysical Module for Use in Atmospheric Chemical Transport Models

We describe and evaluate a six-moment aerosol microphysical module, 6M, designed for implementation in atmospheric chemical transport models (CTMs). 6M is based upon the Quadrature Method Of Moments (QMOM) [McGraw, 1997] and the Multiple Isomomental Distribution Aerosol Surrogate (MIDAS) method [Wright, 2000]. 6M evolves the lowest six radial moments of H2SO4-H2O aerosols for a comprehensive set of dynamical processes including the formation of new particles via binary H2SO4-H2O nucleation, condensational growth, coagulation, evolution due to cloud processing, size-resolved dry deposition, and water uptake and release with changing relative humidity. Performance of the moment-based aerosol evolution is examined and evaluated by comparison with results obtained using a high-resolution discrete model of the particle dynamics for a range of representative tropospheric conditions. Overall, the performance of 6M is good relative to uncertainties associated with other processes represented in CTMs for the 30 test cases evaluated. Differences between 6M and the discrete model in the mass/volume moment and in the partitioning of sulfur(VI) between the gas and aerosol phases remain under 1% whenever significant aerosol is present, and differences in particle number rarely exceed 15%. Estimates of cloud droplet number from 6M are on average within 16% of those of the discrete model, with a significant part of these differences attributable to limitations of the discrete dynamics. Multimodal lognormal (MIDAS) surrogates to the underlying size distributions derived from the 6M moments are in good agreement with the benchmark size distributions. To be submitted to The Journal of Geophysical Research.