Aerosol optical properties in the North China Plain during HaChi campaign: an in-situ optical closure study

The largest uncertainty in the estimation of climate forcing stems from atmospheric aerosols. In early spring and summer of 2009, two periods of in-situ measurements on aerosol physical and chemical properties were conducted within the HaChi (Haze in China) project at Wuqing, a town between Beijing and Tianjin in the North China Plain (NCP). Aerosol optical properties, including the scattering coefficient (sigma(sp)), the hemispheric back scattering coefficient (sigma(bsp)), the absorption coefficient (sigma(ap)), as well as the single scattering albedo (omega), are presented. The diurnal and seasonal variations are analyzed together with meteorology and satellite data. The mean values of sigma(sp), 550nm of the dry aerosol in spring and summer are 280 +/- 253 and 379 +/- 251 Mm(-1), respectively. The average sigma(ap) for the two periods is respectively 47 +/- 38 and 43 +/- 27Mm(-1). The mean values of omega at the wavelength of 637 nm are 0.82 +/- 0.05 and 0.86 +/- 0.05 for spring and summer, respectively. The relative high levels of sigma(sp) and sigma(bsp) are representative of the regional aerosol pollution in the NCP. Pronounced diurnal cycle of sigma(sp), sigma(ap) and omega are found, mainly influenced by the evolution of boundary layer and the accumulation of local emissions during nighttime. The pollutants transported from the southwest of the NCP are more significant than that from the two megacities, Beijing and Tianjin, in both spring and summer. An optical closure experiment is conducted to better under-stand the uncertainties of the measurements. Good correlations (R>0.98) are found between the values measured by the nephelometer and the values calculated with a modified Mie model. The Monte Carlo simulation shows an uncertainty of about 30% for the calculations. Considering all possible uncertainties of measurements, calculated sigma(sp) and sigma(bsp) agree well with the measured values, indicating a stable performance of instruments and thus reliable aerosol optical data.