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 Cloud Condensation Nuclei (CCN) closure study is conducted with bulk CCN number concentration (NCCN) and calculated CCN number concentration based on the aerosol number size distribution and size-resolved activation properties. The observed CCN number concentration (NCCN-obs) are higher than those observed in other locations than China, with average NCCN-obs 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 NCCN-obs and aerosol number size distribution assuming homogeneous chemical composition. The inferred cut-off diameters are in the ranges of 190–280, 160–260, 95–180, 65–120 and 50–100 nm at supersaturations of 0.056, 0.083, 0.17, 0.35 and 0.7%, with their mean values 230.1, 198.4, 128.4, 86.4 and 69.2 nm, respectively. 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. The calculated CCN number concentrations (NCCN-calc) based on the size-resolved activation ratio and aerosol number size distribution correlate well with the NCCN-obs, and show an average overestimation of 19%. Sensitivity studies of the CCN closure show that the NCCN at 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 CCN number concentration can be reliably estimated using time-averaged, size-resolved activation efficiencies without accounting for the temporal variations.
Highlights
The impact of aerosol on cloud and the corresponding feedbacks of cloud are currently considered as the largest uncertainty in climate system (IPCC, 2007)
Bulk chemical composition of aerosol particles with diameter smaller than 10 or 1 μm from filter sampling is often used in closure studies (Bougiatioti et al, 2009), while the knowledge of a size-resolved composition could possibly improve the prediction of cloud condensation nuclei (CCN) number concentration (Medina et al, 2007)
Under meteorological situation with strong winds, the number concentrations (NCCN)-obs can be as low as around 100 cm−3 at supersaturations of 0.056 and 0.083%, and less than 2000 cm−3 at 0.70%
Summary
The impact of aerosol on cloud and the corresponding feedbacks of cloud are currently considered as the largest uncertainty in climate system (IPCC, 2007). Bulk chemical composition of aerosol particles with diameter smaller than 10 or 1 μm from filter sampling is often used in closure studies (Bougiatioti et al, 2009), while the knowledge of a size-resolved composition could possibly improve the prediction of CCN number concentration (Medina et al, 2007). Size-resolved activation ratios (the fraction of the activated particles in the total aerosol number) are frequently determined in laboratory and field studies Such methods, described in detail by Frank et al (2006), are used in instrument calibrations in laboratory and applied in measurements of aerosol activation properties. The aerosol activation properties in the NCP are investigated, using the aerosol number size distributions, the size-resolved activation ratios, and the bulk CCN number concentrations. A closure study is performed between the measured (NCCN-obs) and calculated CCN number concentrations involving aerosol size distribution and size-resolved activation ratio
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