Can Chemical E®ects on Cloud Droplet Number Rival the First Indirect E®ect?

Abstract. An increase in cloud droplet number concentration resulting from an increase in ambient aerosol (and subsequent albedo increase) is typically identi¯ed as the ¯rst indirect (or \Twomey") climatic e®ect of aerosols [Twomey, 1974]. A key question is whether chemical e®ects (dissolution of soluble gases and slightly soluble substances, surface tension depression by organic substances and accommodation coe±cient changes) could potentially rival changes in droplet number from changes in aerosol number concentration. We assess the sensitivity of cloud droplet number concentration to such chemical factors, using a cloud parcel model. We ¯nd that numerous conditions exist, for which chemical in°uences on cloud droplet activation can indeed rival the Twomey e®ect. 1