Size-resolved and bulk activation properties of aerosols in the North China plain: the importance of aerosol size distribution in the prediction of CCN number concentration

Abstract

Abstract. Size-resolved and bulk activation properties of aerosols were measured at a regional/suburban site in the North China Plain (NCP), which is occasionally heavily polluted by anthropogenic aerosol particles and gases. A CCN (Cloud Condensation Nuclei) closure study is conducted with bulk CCN number concentration (NCCN) and calculated NCCN based on the aerosol number size distribution and size-resolved activation properties. The observed NCCN are higher than those observed in other locations than China, with average NCCN of roughly 2000, 3000, 6000, 10 000 and 13 000 cm−3 at supersaturations of 0.056, 0.083, 0.17, 0.35 and 0.70%, respectively. An inferred critical dry diameter (Dm) is calculated based on the measured NCCN and aerosol number size distribution assuming homogeneous chemical composition. This inferred cut off diameter varies in a wide range, indicating that it is impossible to predict NCCN with a fixed critical diameter. Size-resolved activation measurements show that most of the 300 nm particles are activated at the investigated supersaturations, while almost no particles of 30 nm are activated even at the highest supersaturation of 0.72%. The activation ratio increases with increasing supersaturation and particle size. The slopes of the activation curves for ambient aerosols are not as steep as those observed in calibrations with ammonium sulfate suggesting that the observed aerosols is an external mixture of more hygroscopic and hydrophobic particles. This conclusion is confirmed by hygroscopicity measurements performed during two intensive field studies in 2009. The calculated NCCN based on the size-resolved activation ratio and aerosol number size distribution correlate well with the measured NCCN, and show an average overestimation of 19%. Sensitivity studies of the CCN closure show that the NCCN for each supersaturation is well predicted with the campaign average of size-resolved activation curves. These results indicate that the aerosol number size distribution is critical in the prediction of possible CCN. The NCCN can be estimated with average activation curve, along with a well described aerosol number size distribution.