The relationship between cloud condensation nuclei (CCN) concentration and light extinction of dried particles: indications of underlying aerosol processes and implications for satellite-based CCN estimates

We examine the relationship between the number concentration of boundary-layer cloud condensation nuclei (CCN) and light extinction to investigate underlying aerosol processes and satellite-based CCN estimates. For a variety of airborne and ground-based observations not dominated by dust, regression identifies the CCN (cm 3 / at 0.4 0.1 % su- persaturation with 10 0:3 C1:3 0:75 where (Mm 1 / is the 500 nm extinction coefficient by dried particles and is the Angstrom exponent. The deviation of 1 km horizontal average data from this approximation is typically within a factor of 2.0. @logCCN=@log is less than unity because, among other explanations, growth processes generally make aerosols scatter more light without increasing their number. This, barring special meteorology-aerosol connections, as- sociates a doubling of aerosol optical depth with less than a doubling of CCN, contrary to previous studies based on heavily averaged measurements or a satellite algorithm.