A new CCN activation parameterization and its potential influences on aerosol indirect effects

Unactivated particles mixed in cloud condensation nuclei (CCN) measurements lead to inaccurate activation parameterization, thereby overestimating aerosol indirect effects in current models. Data from five CCN field campaigns in Eastern China are analyzed to identify unactivated particles. The lower temperature, lower aerosol hygroscopicity (kappa), and the aerosol particle size distribution closer to that in the coast (compared to that in the continent) lead to a higher proportion of unactivated particles in CCN measurements; the underlying physical mechanisms are discussed. A correction for the CCN activation parameterization is essential for regions with the above characteristics. After removing the unactivated particles, it is found that the widely used Twomey power law activation parameterization does not adequately fit the relationship between the corrected CCN concentration and the supersaturation; therefore, a new, better-fitting, two-parameter activation parameterization is proposed. A method is presented for estimating the parameters in the new parameterization according to the kappa-Kohler theory, that is, the two parameters can be estimated as 59% and 66% of aerosol number concentration for continental regions (represented by Nanjing, 32.21 degrees N, 118.70 degrees E) and 69% and 79% of that for coastal regions (represented by Shenzhen, 22.48 degrees N, 114.56 degrees E), respectively. The new CCN activation parameterization significantly reduces the overestimation of CCN concentration at low supersaturation (SS) conditions, e.g., at SS = 0.1% in the five CCN campaigns, the overestimation is 77% to 102% for the classical power-law parameterization and only 6% to 13% for the new parameterization. As a result, the new parameterization may reduce the overestimation of aerosol indirect effects in models, especially at low supersaturation conditions.