CHANGES IN-CLOUD PROPERTIES DUE TO NOX EMISSIONS

Climate forcing by anthropogenic aerosols has recently excited growing interest [Charlson et al., 1987; Charlson et al., 1990; Wigley, 1991; Charlson et al., 1992]. It has earlier been shown that increased clou d droplet concentrations can increase cloud albedos [e.g. Twomey et at , 1984]. Wigley [1991] and Charlson et al. [1990] have pointed out tha t the Northern Hemisphere may be warming more slowly than the Southern Hemisphere. According to them this is mainly due to the direct and in direct effects of sulphate aerosols. In this report we discuss an alte rnative mechanism which could enhance indirect climate forcing by aero sols. Increased concentrations of condensable vapours (such as HNO3 an d HCl) in the atmosphere could activate an increased number of pre-exi sting aerosol particles to act as cloud condensation nuclei [Kulmala e t al., 1993]. This would cause similar effects on optical properties o f clouds as an increase of aerosol particles in the atmosphere.