Aerosol optical properties in the North China Plain during HaChi campaign: an in-situ optical closure study
1Department of Atmosphere, School of Physics, Peking University, Beijing, China
2Leibniz Institute for Tropospheric research, Leipzig, Germany
3Center of Global and Regional Environment Research, University of Iowa, Iowa, USA
4Chinese Academy of Meteorological Sciences of CMA, Beijing, China
5Wuqing Meteorological Bureau of Tianjin, Tianjin, China
Abstract. The largest uncertainty in the estimation of radiative forcings on climate stems from atmospheric aerosols. In winter and summer of 2009, two periods of in-situ measurements on aerosol physical and chemical properties were conducted within the HaChi project at Wuqing, a town between Beijing and Tianjin in the North China Plain (NCP). Aerosol optical properties including scattering coefficient (σsp), hemispheric back scattering coefficient (σbsp), absorption coefficient (σap, as well as single scattering albedo (ω) are presented. The characteristics of diurnal and seasonal variations are analyzed together with the meteorological and satellite data. The mean values of σsp, 550 nm of the dry aerosol in winter and summer are 280 ± 253 and 379 ± 251 Mm−1, respectively. The average σap for the two periods are respectively 47 ± 38 and 43 ± 27 Mm−1. The mean values of ω are 0.83 ± 0.05 and 0.87 ± 0.05 for winter and summer, respectively. The relative high levels of σsp and σbsp are representative of the regional polluted aerosol of the North China Plain. Pronounced diurnal cycle of σsp, σap and ω are found, mainly influenced by the evolution of boundary layer and accumulation of local emissions during night-time. Regional transport of pollutants from southwest in the NCP is significant both in winter and summer, while high values of σsp and σap correlate with calm winds in winter, which indicating the significant contribution of local emissions. An optical closure experiment is conducted to better understand uncertainties of the measurements. Good correlations (R>0.98) are found between values measured by nephelometer and values calculated with a modified Mie model. Monte Carlo simulations show an uncertainty of about 30% for the calculations. Considering all possible uncertainties of measurements, calculated σsp and σbsp agree well with measured values, indicating a stable performance of instruments and thus a reliable aerosol optical data.