Elsevier

Atmospheric Research

Volume 160, 15 June 2015, Pages 59-67
Atmospheric Research

The optical properties of urban aerosol in northern China: A case study at Xi'an

https://doi.org/10.1016/j.atmosres.2015.03.008Get rights and content

Highlights

  • Simultaneous measurements of particle scattering coefficient (Bscat) and absorption coefficient (Babs) were conducted at Xi'an.

  • Based on the Ångström coefficients, anthropogenic fine particles show dominant contribution during the sampling period, accompanied by occasional dust events.

  • A conceptual equation for visibility degradation in Xi'an was established.

Abstract

Simultaneous measurements of particle scattering coefficient (Bscat) and absorption coefficient (Babs) were conducted at Xi'an from mid-August to mid-October 2012 to estimate the particle single scattering albedo (SSA) and the Ångström coefficients in highly polluted urban air. The hourly averaged Bscat was 272 Mm 1 at 532 nm and 82 Mm 1 at 870 nm, while hourly averaged Babs was 31 Mm 1 at 532 nm and 19 Mm 1 at 870 nm. Similar diurnal variations for Bscat and Babs were observed between the two wavelengths. The averaged SSA was 0.88 at 532 nm and 0.78 at 870 nm. Based on the Ångström coefficients, anthropogenic fine particles show dominant contribution during the sampling period, accompanied by occasional dust events. Moreover, the major contributors to aerosol optical properties are attributed to the mixture of black carbon (BC) and brown carbon (BrC) with non-absorbing components over urban area in northern China. The findings provide useful insights into the factors affecting the visibility in northern Chinese cities and therefore essential knowledge for improving the air quality.

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):SSA=Bscat/Bext=Bscat/Bscat+Babs.

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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