Fengsheng Zhao

Aerosol optical properties and their radiative effects in northern China

[1] As a fast developing country covering a large territory, China is experiencing rapid environmental changes. High concentrations of aerosols with diverse properties are emitted in the region, providing a unique opportunity for understanding the impact of environmental changes on climate. Until very recently, few observational studies were conducted in the source regions. The East Asian Study of Tropospheric Aerosols: An International Regional Experiment (EAST-AIRE) attempts to characterize the physical, optical and chemical properties of the aerosols and their effects on climate over China. This study presents some preliminary results using continuous high-quality measurements of aerosol, cloud and radiative quantities made at the first EAST-AIRE baseline station at Xianghe, about 70 km east of Beijing over a period of one year (September 2004 to September 2005). It was found that the region is often covered by a thick layer of haze (with a yearly mean aerosol optical depth equal to 0.82 at 500 nm and maximum greater than 4) due primarily to anthropogenic emissions. An abrupt ‘‘cleanup’’ of the haze often took place in a matter of one day or less because of the passage of cold fronts. The mean single scattering albedo is approximately 0.9 but has strong day-to-day variations with maximum monthly averages occurring during the summer. Large aerosol loading and strong absorption lead to a very large aerosol radiative effect at the surface (the annual 24-hour mean values equals 24 W m � 2 ), but a much smaller aerosol radiative effect at the top of the atmosphere (one tenth of the surface value). The boundary atmosphere is thus heated dramatically during the daytime, which may affect atmospheric stability and cloud formation. In comparison, the cloud radiative effect at the surface is only moderately higher (� 41 W m � 2 ) than the aerosol radiative effect at the surface.

Preface to special section on East Asian Studies of Tropospheric Aerosols: An International Regional Experiment (EAST‐AIRE)

daily mean surface solar radiation by � 30–40 W m � 2 , but barely changed solar reflection at the top of the atmosphere. Aerosol loading, particle size and composition vary considerably with location and season. The MODIS AOD data from Collection 5 (C5) agree much better with ground data than earlier releases, but considerable discrepancies still exist because of treatments of aerosol SSA and surface albedo. Four methods are proposed/adopted to derive the SSA by means of remote sensing and in situ observation,

Estimation of aerosol single scattering albedo from solar direct spectral radiance and total broadband irradiances measured in China

[1] Aerosol single scattering albedo (ω o ) is a primary factor dictating aerosol radiative effect. Ground-based remote sensing of ω o has been employed most widely using spectral sky radiance measurements made from a scanning Sun photometer. Reliable results can be achieved for high aerosol loadings and for solar zenith angle >50°. This study presents an alternative method using spectral direct radiance measurements or aerosol optical depths together with total sky irradiance to retrieve ω o . The method does not require sky radiance data that can only be acquired by the expensive scanning Sun photometer. The method is evaluated using extensive measurements by a suite of instruments deployed in northern China under the East Asian Study of Tropospheric Aerosols: An International Regional Experiment (EAST-AIRE) project. The sensitivities of the retrieval to various uncertain factors were first examined by means of radiative transfer simulations. It was found the retrieval is most sensitive to cloud screening, total irradiance and the Angstrom Exponent (AE), but only weakly depends on surface albedo and the fine structure of aerosol size distribution. Using 1 year of rigorously screened clear-sky measurements made at the Xianghe site, the retrieved ω o values were found to agree with those retrieved from the Cimel Sun photometer by the AERONET method to within ∼0.03 (RMS), and ∼0.003 (mean bias). As part of the differences originate from different sky views seen by the Sun photometers and pyranometer under comparison, a further test was conducted by using total sky irradiances simulated with the retrieved aerosol properties from the AERONET. The resulting estimates of ω o agree to within 0.01-0.02 (RMS differences) and 0.002-0.003 (mean bias). These values are better measure of the true retrieval uncertainties, as they are free from any data mismatch. The characteristics of ω o retrievals were discussed.

Opposite effects of absorbing aerosols on the retrievals of cloud optical depth from spaceborne and ground‐based measurements

Absorbing aerosols above or within cloud layers have drawn much attention in recent years due to substantially enhanced absorption of solar radiation that may affect reflection at the top of the atmosphere. The retrieval of cloud properties is usually conducted without any regard to aerosols. This study illustrates that retrievals of cloud optical depth (τc) from spaceborne and ground-based sensors are both affected by such aerosols and lead to opposite biases. A ground-based retrieval algorithm is developed for the simultaneous retrieval of τc and cloud droplet effective radius using spectral irradiance measurements from a multifilter rotating spectroradiometer and liquid water path (LWP) data from a microwave radiometer deployed in China. The algorithm is applied to data acquired from 17 May 2008 to 12 May 2009 at a heavily polluted site in the heart of the Yangtze delta region in China. The ground-based retrieval of cloud droplet effective radius increases with increasing LWP. Moderate Resolution Imaging Spectroradiometer retrievals tend to overestimate (underestimate) LWP when cloud LWP is less (greater) than about 200 g/m2. Model tests show strong sensitivities to the retrieval of τc from ground and spaceborne sensors under varying absorption, loading, and vertical distribution conditions. For absorbing aerosol mixed with cloud, τc tends to be underestimated from space, but overestimated from the ground, leading to very poor agreement between ground-based and Moderate Resolution Imaging Spectroradiometer retrievals. Their differences increase with increasing τc. This finding suggests that in a turbid atmosphere with absorbing aerosols, the aerosol effect should be considered, or it would mislead any validation using satellite and ground-based retrievals.

Correction to “Aerosol optical properties and their radiative effects in northern China”

[1] As a fast developing country covering a large territory, China is experiencing rapid environmental changes. High concentrations of aerosols with diverse properties are emitted in the region, providing a unique opportunity for understanding the impact of environmental changes on climate. Until very recently, few observational studies were conducted in the source regions. The East Asian Study of Tropospheric Aerosols: An International Regional Experiment (EAST-AIRE) attempts to characterize the physical, optical and chemical properties of the aerosols and their effects on climate over China. This study presents some preliminary results using continuous high-quality measurements of aerosol, cloud and radiative quantities made at the first EAST-AIRE baseline station at Xianghe, about 70 km east of Beijing over a period of one year (September 2004 to September 2005). It was found that the region is often covered by a thick layer of haze (with a yearly mean aerosol optical depth equal to 0.82 at 500 nm and maximum greater than 4) due primarily to anthropogenic emissions. An abrupt ‘‘cleanup’’ of the haze often took place in a matter of one day or less because of the passage of cold fronts. The mean single scattering albedo is approximately 0.9 but has strong day-to-day variations with maximum monthly averages occurring during the summer. Large aerosol loading and strong absorption lead to a very large aerosol radiative effect at the surface (the annual 24-hour mean values equals 24 W m ), but a much smaller aerosol radiative effect at the top of the atmosphere (one tenth of the surface value). The boundary atmosphere is thus heated dramatically during the daytime, which may affect atmospheric stability and cloud formation. In comparison, the cloud radiative effect at the surface is only moderately higher ( 41 W m ) than the aerosol radiative effect at the surface.