Brightening of the global cloud field by nitric acid and the associated radiative forcing
Clouds cool Earth's climate by reflecting 20% of the incoming solar\nenergy, while also trapping part of the outgoing radiation. The effect\nof human activities on clouds is poorly understood, but the present-day\nanthropogenic cooling via changes of cloud albedo and lifetime could be\nof the same order as warming from anthropogenic addition in CO2. Soluble\ntrace gases can increase water condensation to particles, possibly\nleading to activation of smaller aerosols and more numerous cloud\ndroplets. We have studied the effect of nitric acid on the aerosol\nindirect effect with the global aerosol-climate model ECHAM5.5-HAM2.\nIncluding the nitric acid effect in the model increases cloud droplet\nnumber concentrations globally by 7%. The nitric acid contribution to\nthe present-day cloud albedo effect was found to be -0.32 W m(-2) and to\nthe total indirect effect -0.46 W m(-2). The contribution to the cloud\nalbedo effect is shown to increase to -0.37 W m(-2) by the year 2100, if\nconsidering only the reductions in available cloud condensation nuclei.\nOverall, the effect of nitric acid can play a large part in aerosol\ncooling during the following decades with decreasing SO2 emissions and\nincreasing NOx and greenhouse gases.