The present invention relates to an aqueous copolymer dispersion comprising an emulsion copolymer and two hydrophobic aromatic ketones, wherein the aromatic ketones are, based on the dry weight of the copolymer, from 0.1 to 3 wt % benzophenone and from 0.1 to 4 wt % a benzophenone derivative; wherein the dispersion comprises 0 or less than 0.1 wt % hydrophilic aromatic ketone. The copolymer dispersion is suitable for preparation of aqueous coating compositions which yields a relatively hard surface to provide not only short term but also long term dirt pick up resistance effects, after exposure to ultraviolet radiation.