Abstract
Abstract. Clouds cool Earth's climate by reflecting 20% of the incoming solar energy, while also trapping part of the outgoing radiation. The effect of human activities on clouds is poorly understood, but the present-day anthropogenic cooling via changes of cloud albedo and lifetime could be of the same order as warming from anthropogenic addition in CO2. Soluble trace gases can increase water condensation to particles, possibly leading to activation of smaller aerosols and more numerous cloud droplets. We have studied the effect of nitric acid on the aerosol indirect effect with the global aerosol-climate model ECHAM5.5-HAM2. Including the nitric acid effect in the model increases cloud droplet number concentrations globally by 7%. The nitric acid contribution to the present-day cloud albedo effect was found to be −0.32 W m−2 and to the total indirect effect −0.46 W m−2. The contribution to the cloud albedo effect is shown to increase to −0.37 W m−2 by the year 2100, if considering only the reductions in available cloud condensation nuclei. Overall, the effect of nitric acid can play a large part in aerosol cooling during the following decades with decreasing SO2 emissions and increasing NOx and greenhouse gases.
Highlights
Throughout industrialization humankind has injected increasing amounts of greenhouse gases (GHGs) into the atmosphere and thereby induced anthropogenic global warming
We have studied the effect of nitric acid on the aerosol indirect effect with the global aerosol-climate model ECHAM5.5-HAM2
We have presented global model simulations with explicit inclusion of the effect of nitric acid condensation on cloud droplet activation
Summary
Throughout industrialization humankind has injected increasing amounts of greenhouse gases (GHGs) into the atmosphere and thereby induced anthropogenic global warming. There has been a simultaneous increase in emissions of counteracting agents: aerosols and their precursors (e.g. SO2). The present-day anthropogenic aerosol forcing (direct and cloud albedo effect) ranging from −0.5 to −2.2 W m−2 (Forster et al, 2007) acts to cool Earth’s climate, partly masking the warming from e.g. increased CO2 concentration. The indirect aerosol effects (−0.5 to −1.9 W m−2 for the cloud albedo effect, −0.3 to −1.4 W m−2 for the cloud lifetime effect (Lohmann and Feichter, 2005)) are dominating the anthropogenic aerosol forcing over the direct effect (−0.50 ± 0.40 W m−2 (Forster et al, 2007)). In China, SO2 emissions increased between 2000 and 2006 by 53 %, but have already
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