= Solution
First, weakly absorbing <sulfate> scatters shortwave <electromagnetic radiation> and tends to cool the surface/planetary energy budget, with magnitude and sign influenced by <cloud> background and particle optics. Second, <sulfate> can supply <cloud condensation nuclei>: at fixed <cloud> <water>, more activated droplets give smaller droplets and greater <cloud albedo> through the <Twomey effect>; <atmospheric precipitation> and water-content adjustments can modify that idealized cooling. Third, <sulfate> particles change atmospheric composition through uptake, aqueous chemistry and heterogeneous surface reactions, with their acidity and available <reactants> controlling what is converted. Particle surfaces alone do not guarantee polar-style <stratospheric chlorine activation> in an ordinary tropospheric <cloud>.<The Influence of Pollution on the Shortwave Albedo of Clouds by S. Twomey>[1977, fixed-water-content relation between droplet number and shortwave cloud albedo]{ref}
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