[1] In this paper, the three-dimensional distribution of air pollutants in the Beijing region using aircraft measurements is reported, and Mountain Chimney Effect (MCE) on the distribution of air pollutants in this region is studied. A remarkable two-pollution-layer structure was observed by aircraft measurement in Beijing on 18 August 2007. Gaseous and particle pollutants were well mixed with high concentrations in the planetary boundary layer. There was an elevated pollution layer (EPL) at the altitude of 2500-3500 m, and the concentrations of pollutants were high and comparable with that in the planetary boundary layer. Analysis of aircraft measurement indicates that pollutants in the two pollution layers originated from the same source. On the basis of analysis of the Weather Research and Forecasting (WRF)-TRACER model and wind profile data, the formation of EPL is discussed. The wind flow of Beijing region was dominated by mountain-valley breeze, which has MCE on the distribution of pollutants in this region. Air pollutants were injected from the planetary boundary layer into the free troposphere due to this effect. These pollutants were subsequently transported back over the city by the elevated northerly wind. Thus the structure of two pollution layers over Beijing is formed. Modeling results show that the persistence of a polluted layer over the boundary layer from the previous day has significant contribution to the surface concentrations of pollutants. When the mixing depth increases, the elevated pollutants are recaptured into planetary boundary layer and mixed downward. The rapid increase of surface concentrations of pollutants may be attributed to the vertical down-mixing of pollutants.

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 (N CCN ) and calculated CCN number concentration based on the aerosol number size distribution and size-resolved activation properties.The observed CCN number concentration (N CCN-obs ) are higher than those observed in other locations than China, with average N CCN-obs of roughly 2000, 3000, 6000, 10 000 and 13 000 cm 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 particleCorrespondence to: C. S. Zhao (zcs@pku.edu.cn) 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 (N CCN-calc ) based on the size-resolved activation ratio and aerosol number size distribution correlate well with the N CCN-obs , and show an average overestimation of 19%. Sensitivity studies of the CCN closure show that the N CCN at each supersaturation is well predicted with the campaign average of sizeresolved 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.

Abstract. North China Plain (NCP) is one of the most populated and polluted regions in China. During the recent years, haze and fog occur frequently and cause severely low visibility in this region. In order to better understand the impact of aerosol particles on the formation of haze and fog, a long-term record of haze and fog occurrences in the past 56 yr (from 1954-2009) over NCP is analyzed. The results show that there are rapid changes in the occurrences of haze and fog over NCP. The occurrences of haze and fog were low during 1970-1980, and reached a maximum during 1981-1998. After 1999, the occurrences of haze and fog slightly decreased. There was a nonlinear relationship between the occurrences of haze and fog. When the occurrence of haze was lower than 40 days yr −1 , the occurrence of fog was strongly proportional to the occurrence of haze. However, when the occurrence of haze was high (larger than 75 days yr −1 ), the occurrence of fog was not sensitive to the occurrence of haze. In order to better understand the relationship between the occurrences of haze and fog as well as the effect of aerosol particles on the formation of haze and fog, an in-situ field experiment was conducted during a period with a mixed occurrence of haze and fog. The analysis of the experiment suggests that there were considerably high aerosol concentrations during the measurement period with an averaged aerosol number concentration of 24 000 cm −3 . The measurement also shows that a large amount of aerosol particles can act as condensation nuclei to enhance the formation of fog droplets. As a result, a large amount of fog droplets (>1000 cm −3 ) with small size (5-6 µm) were observed during the fog period, resulting in extremely low visibility (less than 100 m).

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[1] Using a large amount of aircraft measurements of cloud droplet size distributions, the relationship between cloud spectral relative dispersion (e) and cloud droplet number concentration (N c ) is studied. The results indicate that the value of e varies between 0.2 to 0.8 when the cloud droplet number concentration is low (about 50 cm À3 ), and converges toward a narrow range of 0.4 to 0.5 when the cloud number concentration is higher. Because the distribution of the cloud droplet size is an important parameter in estimating the first indirect radiative effect of aerosols on the climate system, the uncertainty in the corresponding radiative forcing can be reduced by 10-40% (depending on cloud droplet number density) under high aerosol loading.. This finding is important for improving climate change projections, especially for the regions where aerosol loading is high and continues to increase.Citation: Zhao, C., and et al. (2006), Aircraft measurements of cloud droplet spectral dispersion and implications for indirect aerosol radiative forcing, Geophys.