Observations and modelings of aerosol and CCN in Northwestern China

Some observations of atmospheric aerosol particles and CCN (cloud condensation nuclei) were carried out at some cities (Xilinhaote, Zhurihe, Yinchuan and Yanchi) in northwestern China and Liaoning Province during 2005-2007. Many advanced instruments like WPS, M9300 integration nephelometer, DMT CCN (droplet measurement technologies (DMT) cloud condensation nuclei (CCN)) counter and PMS observation system were applied in these experiments and a great deal of valuable measurements were acquired. Based on the analysis of aerosol particle concentration and particle size distribution (PSD), we found that: 1) The mean concentration of aerosols within 1.00~2.500μm in dusty weather was 35 times that in the background weather. The daily change characteristics of particles between different diameters have large variations under variant weather conditions. 2) The PSDs were lognormal type within 0.01 to 0.1μm, lorentz type within 0.1 to 1.0μm and Junge type within 1.0~10μm in dust events. The mean particle size range of dusty and non-dusty days was between 0.07 to 0.20μm. 3)The particle transportation amount and distance were different at variant particle sizes in the dust storms. The optical characteristics of dust aerosol and black carbon (BC) were also analyzed. The results showed that in autumn and winter, the diurnal cycle of aerosol scattering coefficient (ASC) exhibited a clear bi-modal variation, and the ASC was indicative of the type of sandy weather. The ASC and BC mass concentration were influenced distinctly when the wind speed was lower than 3.5 ms-1, while little impact when that above 3.5 ms-1. Base on the time series of dust storm station-hour, the results indicated that the dust-storm events showed an apparent deceasing trend in China accompanied by obvious oscillations, and a turning point of the anomaly series was appeared in 1985—1986. Using a DMT CCN counter, distributions of CCN on the ground and in the air in Yanchi and Yinchuan of Ningxia Hui Autonomous Region were obtained. The study concluded that CCN had a distinctive diurnal cycle. The high concentration appears in the early morning and evening, affected by human activities. There was a positive correlation of CCN concentration with temperature during the day, and a negative correlation at the night. Heavy rainfall had a scavenging effect on CCN concentration, while dust aerosol did not increase CCN concentration effectively. Furthermore, simulations of cloud droplet size distribution (CDSD) showed that the CDSD for mineral aerosol coated with (NH4)2SO4 was wider than that for pure (NH4)2SO4 nuclei, and the former was in agreement better well with the observation than the latter. In addition, a dust intensity index was created after an analysis of relationship between dust events and visibility, wind speed and relative humidity. This index which was linear to the above three factors was proved consistent with the observations of dust concentrations.