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

Abstract

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. Regression applied to a variety of airborne and ground-based measurements identifies the CCN (cm-3) at 0.4±0.1% supersaturation with 100.3α+1.3σ0.75 where σ (Mm-1) is the 500 nm extinction coefficient by dried particles and α is the Angstrom exponent. The deviation of one kilometer horizontal average data from this approximation is typically within a factor of 2.0. ∂logCCN/∂logσ is less than unity because, among other explanations, aerosol growth processes generally make particles scatter more light without increasing their number. This, barring extensive data aggregation and special meteorology-aerosol connections, associates a doubling of aerosol optical depth with less than a doubling of CCN, contrary to common assumptions in satellite-based analysis of aerosol-cloud interactions.