Aerosol optical properties and instantaneous radiative forcing based on high temporospatial resolution CARSNET ground-based measurements over eastern China

Variations in the optical properties of aerosols and their radiative forcing were investigated based on long-term synchronous observations made at three-minute intervals from 2011 to 2015 over seven adjacent CARSNET (China Aerosol Remote Sensing NETwork) urban (Hangzhou), suburban (Xiaoshan, Fuyang, LinAn, Tonglu, Jiande) and rural (ChunAn) stations in the Yangtze River Delta region, eastern China. The aerosol optical depth (AOD) varied from 0.68 to 0.76, with two peaks in June and September, and decreased from the eastern coast to western inland areas. The ratio of the AOD of fine-mode particles to the total AOD was > 0.90 and the extinction Angström exponent was > 1.20 throughout the year at all seven sites. The Moderate Resolution Imaging Spectroradiometer (MODIS) C6 retrieval AOD was validated by comparison with ground-based observations. The correlation coefficients (R2) between the MODIS C6 AOD data and the values measured on the ground were ~ 0.73–0.89. The single-scattering albedo varied from 0.91 to 0.94, indicating that scattering aerosol particles are dominant in this region. The real parts of the refractive index were ~ 1.41–1.43, with no significant difference among the seven urban, suburban and rural sites. Large imaginary parts of the refractive index were seen in August at all urban, suburban and rural sites. The fine-mode radii in the Yangtze River Delta region were ~ 0.2–0.3 μm with a volume of 0.10–0.12 μm3 and the coarse-mode radii were ~ 2.0 μm with a volume close to 0.07 μm3. The fine-mode aerosols were obviously larger in June and September than in other months at almost the sites. The absorption AOD was low in the winter. The absorption Angström exponent and the extinction Angström exponent were used to classify the different types of aerosol and the components of mixtures. The aerosols caused negative radiative forcing both at the Earth's surface and at the top of the atmosphere all year round in the Yangtze River Delta region of eastern China.