Abstract
Abstract. The optical and physical properties as well as the direct radiative forcings (DRFs) of fractionated aerosols in the urban area of the western Yangtze River Delta (YRD) are investigated with measurements from a Cimel sun photometer combined with a radiation transfer model. Ground-based observations of aerosols have much higher temporal resolutions than satellite retrievals. An initial analysis reveals the characteristics of the optical properties of different types of fractionated aerosols in the western YRD. The total aerosols, mostly composed of scattering components (93.8 %), have mean optical depths of 0.65 at 550 nm and refractive index of 1.44 + 0.0084i at 440 nm. The fine aerosols are approximately four times more abundant and have very different compositions from coarse aerosols. The absorbing components account for only ∼ 4.6 % of fine aerosols and 15.5 % of coarse aerosols and have smaller sizes than the scattering aerosols within the same mode. Therefore, fine particles have stronger scattering than coarse ones, simultaneously reflecting the different size distributions between the absorbing and scattering aerosols. The relationships among the optical properties quantify the aerosol mixing and imply that approximately 15 and 27.5 % of the total occurrences result in dust- and black-carbon-dominating mixing aerosols, respectively, in the western YRD. Unlike the optical properties, the size distributions of aerosols in the western YRD are similar to those found at other sites over eastern China on a climatological scale, peaking at radii of 0.148 and 2.94 µm. However, further analysis reveals that the coarse-dominated particles can also lead to severe haze pollution over the YRD. Observation-based estimations indicate that both fine and coarse aerosols in the western YRD exert negative DRFs, and this is especially true for fine aerosols (−11.17 W m−2 at the top of atmosphere, TOA). A higher absorption fraction leads directly to the negative DRF being further offset for coarse aerosols (−0.33 W m−2) at the TOA. Similarly, the coarse-mode DRF contributes to only 13.3 % of the total scattering aerosols but > 33.7 % to the total absorbing aerosols. A sensitivity analysis states that aerosol DRFs are not highly sensitive to their profiles in clear-sky conditions. Most of the aerosol properties and DRFs have substantial seasonality in the western YRD. The results further reveal the contributions of each component of the different size particles to the total aerosol optical depths (AODs) and DRFs. Additionally, these results can be used to improve aerosol modelling performance and the modelling of aerosol effects in the eastern regions of China.
Highlights
Atmospheric aerosols have a significant influence on air quality, human health and regional/global climate changes
The largest aerosol optical depths (AODs) appear in the spring for the coarse aerosols, whereas they appear in the summer for the fine aerosols in urban area of Nanjing (urNJ)
A combined profile of the averaged Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and polarization Raman lidar (PRL) values is included and indicates that aerosols account for approximately 75 % of the total below 4 km and approximately 60 % in the boundary layer for the combined profile, which is somewhat similar to the default profile of TUV (Palancar and Toselli, 2004)
Summary
Atmospheric aerosols have a significant influence on air quality, human health and regional/global climate changes. In addition to the aerosol optical properties, the observation-based aerosol DRFs have been estimated around the world (such as those found in Markowicz et al, 2008; Khatri et al, 2009; Kuhlmann and Quaas, 2010; Alam et al, 2011; Zhuang et al, 2014a; Xia et al, 2016) Almost all these investigations focused on the total aerosol forcing. In the YRD or eastern China, most of the investigations of the aerosol optical properties have focused on the coastal, lake and rural regions (Pan et al, 2010; Yu et al, 2011; Che et al, 2015a; Qi et al, 2016) of the central to eastern YRD Most of these studies address only the total aerosol optical properties (independent of modes and compositions), except for the work of Qi et al (2016).
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