Evaluation of the sectional aerosol microphysics module SALSA implementation in ECHAM5-HAM aerosol-climate model

Abstract

Abstract. We present the implementation and evaluation of a sectional aerosol microphysics model SALSA within the aerosol-climate model ECHAM5-HAM. This aerosol microphysics module has been designed to be flexible and computationally efficient so that it can be implemented in regional or global scale models. The computational efficiency has been achieved by keeping the number of variables needed to describe the size and composition distribution to the minimum. The aerosol size distribution is described using 20 size sections with 10 size sections in size space which cover diameters ranging from 3 nm to 10 μm divided to three subranges each having distinct optimised process and compound selection. The ability of the module to describe the global aerosol properties was evaluated by comparison against (1) measured continental and marine size distributions, (2) observed variability of continental modal number concentrations, (3) measured sulphate, organic carbon, black carbon and sea salt mass concentrations, (4) observations of AOD and other aerosol optical properties from satellites and AERONET network, (5) global aerosol budgets and concentrations from previous model studies, and (6) model results using M7 which is the default aerosol microphysics module in ECHAM5-HAM. The evaluation shows that the global aerosol properties can be reproduced reasonably well using the coarse resolution of 10 size sections in size space. The simulated global aerosol budgets are within the range of previous studies. Surface concentrations of sea salt, sulphate and carbonaceous species have an annual mean within a factor of five of the observations, while the simulated sea salt concentrations reproduce the observations less accurately and show high variability. Regionally, AOD is in relatively good agreement with the observations (within a factor of two). At mid-latitudes the observed AOD is captured well, while at high-latitudes as well as in some polluted and dust regions the modeled AOD is significantly lower than the observed. Regarding the most investigated aerosol properties, the performances of SALSA and the modal aerosol module M7 against observations are comparable. However, SALSA reproduces the observed number concentrations and the size distributions of CCN sized particles much more accurately than M7, and is therefore a good choice for aerosol-cloud interaction studies in global models. Our study also shows that when including activation type nucleation process in the boundary layer, the modeled concentrations of particles under 50 nm in diameter are reproduced much better compared to when only binary nucleation is assumed.