Optical properties of carbonaceous aerosols and source impacts in the Lanzhou valley: In-situ observations by means of topographical relief

This study conducted a one-year observation of the optical properties of carbonaceous aerosols at three observation sites (urban areas, Mountains Gaolan and Maxian) in the Lanzhou River Valley, based on the principle of increasing altitude. The light absorption coefficients of elemental carbon (EC) and brown carbon (BrC) were the highest at the mountainside of Gaolan Mountain, followed by the urban areas and the summit of Maxian Mountain. The annually average contributions of EC to the light absorption coefficient at 405 nm were 54.54 % for urban areas, 55.07 % for mountainside of Gaolan Mountain, and 63.15 % for the summit of Maxian Mountain. The mass absorption cross-sections of EC (BrC) at the mountainside of Gaolan Mountain was 2.23 (2.42), 2.44 (6.92), 2.87 (1.33), and 1.94 (4.87) times that of the urban site in summer, autumn, winter, and spring, respectively. The integrated results of EC at the mountaintop observation site over the 300-950 nm wavelength range were significantly higher than those at other sites, while the results for brown carbon (BrC) are the highest at the the mountainside of Gaolan Mountain. At 405 nm, the pollution sources with the highest contributions to the light absorption coefficients of carbonaceous aerosols at the urban areas, Mountains Gaolan and Maxian observation sites were industry, motor vehicles, and coal combustion, respectively. The study will provide some helps for mitigating global warming by modulating the specific sources.