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 estimate

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. For a variety of airborne and ground-based observations not dominated by dust, regression identifies the CCN (cm^-3) at 0.4±0.1% supersaturation with 10^0.3α+1.3σ^0.75 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, growth processes generally make aerosols scatter more light without increasing their number. This, barring special meteorology-aerosol connections, associates 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.