The optical properties, physical properties and direct radiative forcing of urban columnar aerosols in the Yangtze River Delta, China
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 observation...
| Main Authors: | , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-02-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/1419/2018/acp-18-1419-2018.pdf |
| Summary: | 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 + 0.0084<i>i</i> 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<sup>−2</sup> 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<sup>−2</sup>) 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. |
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| ISSN: | 1680-7316 1680-7324 |