The optical properties of urban aerosol in northern China: A case study at Xi'an
Introduction
The light extinction of aerosol particles, including scattering and absorption, affects visibility and climate (e.g., Ackerman et al., 2000, Watson, 2002, Jacobson, 2006, Koren et al., 2008, Ramanathan and Feng, 2009, Cao et al., 2012a). The scattering aerosol species such as sulfate and nitrate contribute to atmospheric cooling, while light absorbing aerosol species such as black carbon (BC), brown carbon (BrC) and dust exert a positive radiative forcing and reinforce the atmospheric warming due to an increase in the greenhouse gases (Intergovernmental Panel on Climate Change (IPCC), 2007, Ramanathan and Carmichael, 2008).
The scattering coefficient (Bscat) and absorption coefficient (Babs) are two important optical parameters describing the scattering and absorption cross sections in a unit volume of air at a wavelength of λ. These two optical parameters are important for modeling atmospheric radiation transfer (Clarke et al., 1987). The sum of Bscat and Babs is the particle extinction coefficient (Bext) determining the attenuation of light in the atmosphere, while the particle single scattering albedo (SSA) is defined as the ratio of Bscat/Bext (Bodhaine, 1995):
The wavelength dependence of SSA is determined by the size, chemical composition, and mixing state of particles (Kokhanovsky, 2008, Moosmüller et al., 2009, Moosmüller et al., 2012). Though visibility and climate modeling should consider aerosol optical properties across the tropospheric solar spectrum (300–900 nm), SSA at mid-visible wavelength is often used to evaluate the aerosol radiative forcing. For example, Hansen et al. (1997) showed that a decrease of SSA at 550 nm from 0.9 to 0.8 may change the radiative forcing from negative (cooling) to positive (warming), depending on the surface albedo and aerosol optical depth. In general, the optical properties of atmospheric particles show a great spatial and temporal variability, due to the difference in concentration, particle size, chemical composition and mixing state (Kokhanovsky, 2008).
The urban areas in northern China are one of the most polluted regions in the world (e.g., Cao et al., 2012b, Zhang et al., 2013). However, direct measurements of aerosol optical properties in this region are very scarce to date, significantly hindering our understanding of the impact of aerosol on visibility and regional climate. Here, we present results of a 2-month field campaign carried out at Xi'an with a specific focus on aerosol scattering and absorption properties at both visible (532 nm) and near-infrared wavelengths (870 nm). The objectives of this study were: 1) investigating the variations of Bscat, Babs, and SSA, 2) establishing a conceptual equation for visibility degradation in Xi'an, and 3) estimating the main contributors to light extinction by using the Ångström coefficient.
Section snippets
Measurements
The measurements were conducted from mid-August to mid-October 2012 at the Institute of Earth Environment, Chinese Academy of Sciences in Xi'an, China (Fig. 1). Xi'an is located on the Guanzhong Plain at the south edge of the Loess Plateau 400 m above sea level. Samples were taken on the roof of a two-story building ~ 10 m above ground level (34.23°N, 108.88°E), ~ 50 m west of a moderately traveled 4-lane round and ~ 25 m north of a lightly traveled 2-lane road. The monitoring site was located in an
Variations of Bscat, Babs, and SSA
The hourly average values of Bscat, Babs and Bext show large variability during the study period (Fig. 3). Bscat vary over 60 folds, from 22 to 1255 Mm− 1 at 532 nm and over 100 folds from 3 to 332 Mm− 1 at 870 nm, while Babs range from 0.3 to 202 Mm− 1 at 532 nm and from 0.8 to 102 Mm− 1 at 870 nm. Average Bscat,532 nm value of 270 ± 200 Mm− 1 is higher than the average Bscat,870 nm value of 82 ± 64 Mm− 1. Average Babs values are comparable, i.e. 31 ± 28 Mm− 1 at 532 nm and 19 ± 14 Mm− 1 at 870 nm. The average values of Bext
Conclusions
Aerosol optical properties in the megacity of Xi'an, China are studied. The Bscat average is 272 Mm− 1 (ranging from 22 to 1255 Mm− 1) at 532 nm, but decreases to 82 Mm− 1 (ranging from 3 to 332 Mm− 1) at 870 nm. Compared to scattering, Babs is much lower with an average of 31 Mm− 1 at 532 nm and 19 Mm− 1 at 870 nm. The SSA at 532 nm (average 0.88) is higher than that at 870 nm (average 0.78), indicating that the light extinction is dominated by scattering. Diurnal variations of Bscat, Babs, and SSA are most
Acknowledgment
This study was supported by the National Natural Science Foundation of China (41230641, 41271481,40925009), and projects from “Strategic Priority Research Program” of the Chinese Academy of Science (XDA05100401), Ministry of Science & Technology (201209007), and Shaanxi Government (2012KTZB03-01-01).
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2020, Atmospheric ResearchCitation Excerpt :Hourly bscat,dry and babs values varied by 40 folds from 2.7 Mm−1 to 107.8 Mm−1 and 28 folds from 1.2 Mm−1 to 33.9 Mm−1, respectively, leading to SSA varying from 0.54 to 0.86. Compared with bscat,dry and babs observed in previous studies (Table S1), the average bscat,dry (32.5 ± 15.5 Mm−1) and babs (8.8 ± 4.7 Mm−1) at Sanya was well below the range of values (185.9–525.3 Mm−1 for bscat,dry and 23.9–119 Mm−1 for babs) measured at inland sites of China (Cao et al., 2012; Cui et al., 2016; Gong et al., 2015; He et al., 2009; Jing et al., 2015; Li et al., 2007a; Tao et al., 2014a; Wang et al., 2016b; Zhou et al., 2017; Zhu et al., 2015). Moreover, both bscat,dry and babs at Sanya were obviously lower than those reported for other coastal cities of China (Deng et al., 2016; Han et al., 2015; Tao et al., 2012; Tao et al., 2014b; Wang et al., 2017; Xu et al., 2012b), but they were comparable to those measured at coastal sites around the Mediterranean Sea such as Finokalia (44.2 ± 17.5 Mm−1 for bscat and 6.3 ± 2.7 for babs) and Corsica (30 ± 20 Mm−1 for bscat) (Bryant et al., 2006; Claeys et al., 2017).