Aircraft measurements of aerosol and cloud over Beijing areas

The rapid urbanization of Beijing as one of the largest megacities in China provides a good opportunity to investigate the role of anthropogenic aerosols on atmospheric optics and cloud physics. Here we present a summary of our recent research on aircraft observations for aerosol-cloud interactions from the research group of Cloud Physics and Atmospheric Chemistry, Department of Atmospheric Sciences, School of Physics, Peking University. A huge aircraft measurements dataset of aerosol and cloud over Beijing areas for the past 5 years is analyzed to understand the regional distribution of aerosols, cloud microphysics and aerosol-cloud interactions in this area.

Analysis of observed precipitation, MODIS data and meteorological sounding data over eastern central China shows that the precipitation in this region is significantly reduced during the last 40 years and this reduction of precipitation is strongly correlated to the high concentrations of aerosols. The vertical profiles of aerosol number concentration and size distribution with diameter ranging from 0.12 to 3.0ìm observed by the airborne optical spectrometer probe in Beijing are discussed. Comparison between aircraft-derived aerosol optical depth (AOD) and Moderate Resolution Imaging Spectroradiometer-derived AOD shows good agreement. The calculations suggest that the surface level number concentration, the boundary layer height and the structure of boundary layer can influence the AOD significantly. The impacts of local circulations like Mountain Valley Breeze of Beijing on the three-dimensional distribution of aerosols are also studied. Mountain Chimney Effect on the distribution of air pollutants in this region is investigated using aircraft measurements.

Using a large amount of aircraft measurements of cloud droplet size distributions, the relationship between cloud spectral relative dispersion and cloud droplet number concentration is studied. The results indicate that the value of cloud spectral relative dispersion varies between 0.2 to 0.8 when the cloud droplet number concentration is low (about 50 cm-3), and converges towards 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% under high aerosol loading. Warm cloud microphysical parameters including cloud droplet number concentration, liquid water content and effective radius (Re) from 75 flights around the Beijing area during 2005 and 2006 are summarized. Many records of high Nc above 1000 cm-3 are observed in this area. Various Re parameterization schemes for warm clouds in Beijing are also discussed.