Abstract
Abstract. The dominant removal mechanism of soluble aerosol is wet deposition. The atmospheric lifetime of aerosol, relevant for aerosol radiative forcing, is therefore coupled to the atmospheric cycling time of water vapor. This study investigates the coupling between water vapor and aerosol lifetimes in a well-mixed atmosphere. Based on a steady-state study by Pruppacher and Jaenicke (1995) we describe the coupling in terms of the processing efficiency of air by clouds and the efficiencies of water vapor condensation, of aerosol activation, and of the transfer from cloud water to precipitation. We extend this to expressions for the temperature responses of the water vapor and aerosol lifetimes. Previous climate model results (Held and Soden, 2006) suggest a water vapor lifetime temperature response of +5.3 ± 2.0% K−1. This can be used as a first guess for the aerosol lifetime temperature response, but temperature sensitivities of the aerosol lifetime simulated in recent aerosol–climate model studies extend beyond this range and include negative values. This indicates that other influences probably have a larger impact on the computed aerosol lifetime than its temperature response, more specifically changes in the spatial distributions of aerosol (precursor) emissions and precipitation patterns, and changes in the activation efficiency of aerosol. These are not quantitatively evaluated in this study but we present suggestions for model experiments that may help to understand and quantify the different factors that determine the aerosol atmospheric lifetime.
Highlights
IntroductionAerosol influences the radiative budget through scattering and absorption of solar radiation (the direct and semi-direct ping and McFiggans, 2012)
Aerosol influences the radiative budget through scattering and absorption of solar radiation.Current uncertainties lead to relatively large intermodel differences in aerosol represenStaotiloind
Changing the surface temperature in climate models leads to changes in the computed atmospheric water vapor burden and lifetime, in evaporation and precipitation, and in the spatial and temporal distribution of clouds and precipitation
Summary
Aerosol influences the radiative budget through scattering and absorption of solar radiation (the direct and semi-direct ping and McFiggans, 2012). The radiative consequences of these changes are expressed in the so-called cloud radiative feedback and contribute relatively much to the intermodel variability in climate sensitivity (Dufresne and Bony, 2008; Andrews et al, 2012; Webb et al, 2013) These changes may affect the efficiency of aerosol wet deposition. A recent climate model study (Fang et al, 2011) indicated that the atmospheric lifetime of an ideal aerosol-like tracer changes between different climates, mainly due to shifting cloud and precipitation patterns and a decreased precipitation frequency We note that this climate-change-induced modification of the aerosol lifetime influences the atmospheric burden and induces an aerosol radiative forcing even when other properties (emissions, size distribution, chemical composition) remain the same.
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