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

Shen, Yicheng; Virkkula, Aki; Ding, Aijun; Luoma, Krista; Keskinen, Helmi; Aalto, Pasi P.; Chi, Xuguang; Qi, Ximeng; Nie, Wei; Huang, Xin; Petäjä, Tuukka; Kulmala, Markku; Kerminen, Veli-Matti

The concentration of cloud condensation nuclei (CCN) is an essential parameter affecting aerosol–cloud interactions within warm clouds. Long-term CCN number concentration (inline-formulaNCCN) data are scarce; there are a lot more data on aerosol optical properties (AOPs). It is therefore valuable to derive parameterizations for estimating inline-formulaNCCN from AOP measurements. Such parameterizations have already been made, and in the present work a new parameterization is presented. The relationships between inline-formulaNCCN, 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-formulaσsp) of PMinline-formula10 particles. The analysis first showed that the dependence of inline-formulaNCCN 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-formulaNCCN and AOPs was parameterized as inline-formula M9inlinescrollmathml N normal CCN ( ( normal 286 ± 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-formulamin) inline-formula+ (inline-formula5.2±3.3))inline-formulaσsp, where BSFinline-formulamin 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-formulaNCCN, 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-formulaNCCN 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-formulaNCCN varied from inline-formula∼0.5 to inline-formula∼0.8. To study the physical explanation of the relationships between inline-formulaNCCN and AOPs, lognormal unimodal particle size distributions were generated and inline-formulaNCCN and AOPs were calculated. The simulation showed that the relationships of inline-formulaNCCN and AOPs are affected by the geometric mean diameter and width of the size distribution and the activation diameter. The relationships of inline-formulaNCCN and AOPs were similar to those of the observed ones.

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Shen, Yicheng / Virkkula, Aki / Ding, Aijun / et al: Estimating cloud condensation nuclei number concentrations using aerosol optical properties: role of particle number size distribution and parameterization. 2019. Copernicus Publications.

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