Atmos. Chem. Phys. Discuss., 12, 623-689, 2012
© Author(s) 2012. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Intercomparison of modal and sectional aerosol microphysics representations within the same 3-D global chemical transport model
G. W. Mann1,2, K. S. Carslaw2, D. A. Ridley3, D. V. Spracklen2, K. J. Pringle2, J. Merikanto4, H. Korhonen5, J. P. Schwarz6, L. A. Lee2, P. T. Manktelow2,*, M. T. Woodhouse2, A. Schmidt2, T. J. Breider7, K. M. Emmerson2,**, C. L. Reddington2, M. P. Chipperfield2, and S. J. Pickering2
1National Centre for Atmospheric Science, University of Leeds, Leeds, UK
2School of Earth and Environment, University of Leeds, Leeds, UK
3Dept of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA
4Department of Physics, University of Helsinki, Helsinki, Finland
5Finnish Meteorological Institute, Kuopio Unit, Kuopio, Finland
6Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA
7School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
*now at: Halcrow Group Ltd, Headingley, Leeds, UK
**now at: CSIRO Marine and Atmospheric Research, Aspendale, VIC 3195, Australia

Abstract. A global modal aerosol microphysics module (GLOMAP-mode) is evaluated and improved by comparing against a sectional version (GLOMAP-bin) and observations in the same 3-D global offline chemistry transport model. With both schemes, the model captures the main features of the global particle size distribution, with sub-micron aerosol approximately unimodal in continental regions and bi-modal in marine regions. Initial bin-mode comparisons showed that various size distribution parameter settings (mode widths and inter-modal separation sizes) resulted in clear biases compared to the sectional scheme. By adjusting these parameters in the modal scheme, much better agreement is achieved against the bin scheme and observations. Surface mass of sulphate, sea-salt, black carbon (BC) and organic carbon (OC) are, on the annual mean, within 25 % in the two schemes in nearly all regions. On the annual mean, surface level concentrations of condensation nuclei (CN), cloud condensation nuclei (CCN), surface area density and condensation sink also compare within 25 % in most regions. However, marine CCN concentrations between 30° N and 30° S are systematically higher in the modal scheme, by 25–60 %, which we attribute to differences in size-resolved particle growth or cloud-processing. Larger differences also exist in regions or seasons dominated by biomass burning and in free-troposphere and high-latitude regions. Indeed, in the free-troposphere, GLOMAP-mode BC is a factor 2–4 higher than GLOMAP-bin, likely due to differences in size-resolved scavenging. Nevertheless, in most parts of the atmosphere, we conclude that bin-mode differences are much less than model-observation differences, although some processes are missing in these runs which may pose a bigger challenge to modal schemes (e.g. boundary layer nucleation, ultra-fine sea-spray). The findings here underline the need for a spectrum of complexity in global models, with size-resolved aerosol properties predicted by modal schemes needing to be continually benchmarked and improved against freely evolving sectional schemes and observations.

Citation: Mann, G. W., Carslaw, K. S., Ridley, D. A., Spracklen, D. V., Pringle, K. J., Merikanto, J., Korhonen, H., Schwarz, J. P., Lee, L. A., Manktelow, P. T., Woodhouse, M. T., Schmidt, A., Breider, T. J., Emmerson, K. M., Reddington, C. L., Chipperfield, M. P., and Pickering, S. J.: Intercomparison of modal and sectional aerosol microphysics representations within the same 3-D global chemical transport model, Atmos. Chem. Phys. Discuss., 12, 623-689, doi:10.5194/acpd-12-623-2012, 2012.
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