Aircraft-based assessment of relationships between CCN concentration and visible light extinction

Some estimates of the aerosol indirect effects rely on satellite observations. They infer aerosol loading from the retrieved optical properties of atmospheric particles. One of the fundamental limitations in the satellite-based estimate is that the optical properties are not directly relevant to the physics that govern the aerosol effects on cloud properties. Therefore, the satellite-based inference of CCN can be justified only if the aerosol optical properties are related to the number concentration of CCN in the actual atmosphere and if the relationships are accurately represented in the model. Whether these conditions are met can be investigated using airborne observations. This paper reports on the relationship in local air masses between CCN concentration, aerosol size distribution and light extinction observed during ARCTAS, INTEX-B and other airborne experiments. We demonstrate that a logarithm of the CCN number to light extinction ratio shows an approximately linear correlation with Angstrom exponent. We compare this observation with previous expressions by Nakajima et al. [2001] and Andreae [2009]. We discuss the observed natural variability around our fit and its implications on the use of visible and near-infrared aerosol optical depths from nadir-view passive satellite sensors.