Estimating cloud condensation nuclei number concentrations using aerosol optical properties: role of particle number size distribution and parameterization

The concentration of cloud condensation nuclei (CCN) is an essential parameter affecting aerosol–cloud interactions within warm clouds. Long-term CCN number concentration (inline-formulaN_CCN) data are scarce; there are a lot more data on aerosol optical properties (AOPs). It is therefore valuable to derive parameterizations for estimating inline-formulaN_CCN from AOP measurements. Such parameterizations have already been made, and in the present work a new parameterization is presented. The relationships between inline-formulaN_CCN, AOPs, and size distributions were investigated based on in situ measurement data from six stations in very different environments around the world. The relationships were used for deriving a parameterization that depends on the scattering Ångström exponent (SAE), backscatter fraction (BSF), and total scattering coefficient (inline-formulaσ_sp) of PMinline-formula₁₀ particles. The analysis first showed that the dependence of inline-formulaN_CCN on supersaturation (SS) can be described by a logarithmic fit in the range SS inline-formula<1.1 %, without any theoretical reasoning. The relationship between inline-formulaN_CCN and AOPs was parameterized as inline-formula $M9inlinescrollmathml{N}_{\mathrm{normal\; CCN}}\approx \left(\right(\mathrm{normal\; 286}\pm \mathrm{normal\; 46})$ 93pt13ptsvg-formulamathimg1db18230eee7b12e7ed258c4199f0c7d acp-19-15483-2019-ie00001.svg93pt13ptacp-19-15483-2019-ie00001.png SAE ln(SS/(inline-formula0.093±0.006))(BSF inline-formula− BSFinline-formula_min) inline-formula+ (inline-formula5.2±3.3))inline-formulaσ_sp, where BSFinline-formula_min is the minimum BSF, in practice the 1st percentile of BSF data at a site to be analyzed. At the lowest supersaturations of each site (SS inline-formula≈0.1 %), the average bias, defined as the ratio of the AOP-derived and measured inline-formulaN_CCN, varied from inline-formula∼0.7 to inline-formula∼1.9 at most sites except at a Himalayan site where the bias was inline-formula>4. At SS inline-formula>0.4 % the average bias ranged from inline-formula∼0.7 to inline-formula∼1.3 at most sites. For the marine-aerosol-dominated site Ascension Island the bias was higher, inline-formula∼1.4–1.9. In other words, at SS inline-formula>0.4 % inline-formulaN_CCN was estimated with an average uncertainty of approximately 30 % by using nephelometer data. The biases were mainly due to the biases in the parameterization related to the scattering Ångström exponent (SAE). The squared correlation coefficients between the AOP-derived and measured inline-formulaN_CCN varied from inline-formula∼0.5 to inline-formula∼0.8. To study the physical explanation of the relationships between inline-formulaN_CCN and AOPs, lognormal unimodal particle size distributions were generated and inline-formulaN_CCN and AOPs were calculated. The simulation showed that the relationships of inline-formulaN_CCN and AOPs are affected by the geometric mean diameter and width of the size distribution and the activation diameter. The relationships of inline-formulaN_CCN and AOPs were similar to those of the observed ones.