Sensitivity of aerosol concentrations and cloud properties to nucleation and secondary organic distribution in ECHAM5-HAM global circulation model

R Makkonen,H Järvinen,H Korhonen,R Bennartz,A Asmi,J Feichter,H Kokkola,U Lohmann,S Järvenoja,P Räisänen,A Laaksonen,K E J Lehtinen,M Kulmala,V.-M Kerminen

doi:10.5194/acp-9-1747-2009

Abstract

Abstract. The global aerosol-climate model ECHAM5-HAM was modified to improve the representation of new particle formation in the boundary layer. Activation-type nucleation mechanism was introduced to produce observed nucleation rates in the lower troposphere. A simple and computationally efficient model for biogenic secondary organic aerosol (BSOA) formation was implemented. Here we study the sensitivity of the aerosol and cloud droplet number concentrations (CDNC) to these additions. Activation-type nucleation significantly increases aerosol number concentrations in the boundary layer. Increased particle number concentrations have a significant effect also on cloud droplet number concentrations and therefore on cloud properties. We performed calculations with activation nucleation coefficient values of 2×10−7s−1, 2×10−6s−1 and 2×10−5s−1 to evaluate the sensitivity to this parameter. For BSOA we have used yields of 0.025, 0.07 and 0.15 to estimate the amount of monoterpene oxidation products available for condensation. The hybrid BSOA formation scheme induces large regional changes to size distribution of organic carbon, and therefore affects particle optical properties and cloud droplet number concentrations locally. Although activation-type nucleation improves modeled aerosol number concentrations in the boundary layer, the use of a global activation coefficient generally leads to overestimation of aerosol number. Overestimation can also arise from underestimation of primary emissions.

Highlights

Atmospheric aerosols are an important, yet poorly understood, part of the climate system, with largest uncertainties being associated with aerosol-cloud interactions (Lohmann and Feichter, 2005; Penner et al, 2006; Forster et al, 2007; Baker and Peter, 2008)
The simulations performed in this study show clearly that aerosol particle number concentrations and, perhaps more importantly, cloud droplet number concentrations in the ECHAM5-HAM model are sensitive to the aerosol nucleation mechanism used
In the boundary layer and upper troposphere, aerosol number concentrations differ greatly depending on the choice of the nucleation mechanism

Summary

Introduction

Atmospheric aerosols are an important, yet poorly understood, part of the climate system, with largest uncertainties being associated with aerosol-cloud interactions (Lohmann and Feichter, 2005; Penner et al, 2006; Forster et al, 2007; Baker and Peter, 2008). Most current global climate models include the main aerosol types but have a rather simplistic treatment of the aerosol size distribution and associated microphysical processes Chen et al, 2007; Jones et al, 2007; Shindell et al, 2007) This is a serious shortcoming that needs, and probably will, be improved in next-generation climate models (Ghan and Schwartz, 2007; Textor et al, 2007). The aerosol climate model ECHAM5-HAM (Stier et al, 2005) has a relatively detailed description of aerosol microphysics, making it a promising tool for studying. In this study we will investigate how simulated aerosol and cloud droplet number concentrations depend on the implementation of these two processes in ECHAM5-HAM

Methods

Results

Discussion

Conclusion