Abstract
Abstract. In the most advanced aerosol-climate models it is common to represent the aerosol particle size distribution in terms of several log-normal modes. This approach, motivated by computational efficiency, makes assumptions about the shape of the particle distribution that may not always capture the properties of global aerosol. Here, 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 the current values for two size distribution parameter settings in the modal scheme (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. Annual mean surface-level mass of sulphate, sea-salt, black carbon (BC) and organic carbon (OC) are within 25% in the two schemes in nearly all regions. 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 25–60% higher in the modal model, 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 and 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.
Highlights
Aerosol particles affect the radiative budget of the Earth’s atmosphere by scattering and absorbing solar and terrestrial radiation and by modifying the albedo and lifetime of clouds, referred to as the direct and indirect aerosol radiative effects (e.g., Seinfeld and Pandis, 1998)
In-situ observations and process modelling have led to major advances in the understanding of key aerosol processes and how they determine the evolution of the particle size distribution, and concentrations of cloud condensation nuclei (CCN)
By examining particle size distributions simulated with the two schemes, we aim to provide constraints for the choices of parameter values in modal schemes to reduce any systematic biases in the parameterized modal approach
Summary
Aerosol particles affect the radiative budget of the Earth’s atmosphere by scattering and absorbing solar and terrestrial radiation and by modifying the albedo and lifetime of clouds, referred to as the direct and indirect aerosol radiative effects (e.g., Seinfeld and Pandis, 1998). 3-mode scheme, with both performing against observations These studies have demonstrated differences in certain conditions, the extent to which the use of modal schemes in 3-D global models leads to systematic biases in simulated aerosol properties has not yet been established. Trivitayanurak et al (2008) compared GLOMAP-bin against another two-moment sectional scheme (TOMAS, Adams and Seinfeld, 2002) in different 3-D global models, and found major inter-model differences and discrepancies to observations, but the schemes had different process representations, emission inventories, size assumptions, oxidant fields, clouds and transport. Bergman et al (2011) implemented a twomoment sectional aerosol scheme (SALSA, Kokkola et al, 2008) into a general circulation model and compared against the existing two-moment modal scheme (M7, Vignati et al, 2004) They found the sectional model to better reproduce observed size distributions at CCN sizes, with both performing over integral properties. By making aerosol properties simulated by the modal scheme compare better to the sectional scheme in the offline transport model, we aim to increase the robustness of simulated aerosol radiative forcings, making simulations in the composition-climate model more reliable
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