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Remote Sensing

Aerosol retrieval over cities using an airborne directional polarimetric camera

High-spatial-resolution measurements of the optical and microphysical properties of aerosols could improve air-quality monitoring in cities by enabling identification of sources and control measures.
3 September 2010, SPIE Newsroom. DOI: 10.1117/2.1201008.003160

Aerosol particles play an important role in air quality and health. They can also affect the radiation balance of the Earth-atmosphere system by reflecting and absorbing sunlight. Their optical and microphysical properties, which have gained the most attention in aerosol particle monitoring, show great spatial and temporal variability.1 Consequently, many difficulties arise in assessing the effect of emitted aerosol pollutants on the local air quality and global climate.

Air-quality monitoring systems in most cities have relied exclusively on ground-based station networks, which do not provide adequate spatial coverage. Remote sensing complements ground-based measurements by providing systematic observations of more spatially continuous aerosol properties.2 However, to extract the optical and microphysical properties of aerosols from remote-sensing data over urban areas, it is essential to have more accurate information on surface reflectance and better aerosol optical models.3 Many approaches have been developed, but they have been only partially successful. To monitor these properties of aerosols over cities, we have developed a new airborne directional polarimetric camera (DPC) with high spatial resolution.

The DPC instrument (see Figure 1) is a CCD camera that covers the spectral range of 400–900nm with up to three polarized spectral bands (490, 665, and 865nm). It is a POLDER-type polarized camera with a significantly improved spatial resolution (4×4m2 at 4000m) for monitoring aerosol emission and absorption sources in cities.4 The retrieval approach is based on a lookup table for the surface-polarized reflectance model, an aerosol optical model, and the satellite and solar geometries. The bidirectional polarized reflectance factor is used to simulate the polarized radiance of the surface. To resolve most of the ambiguity in retrieving aerosol optical and microphysical properties using the DPC alone, ground-based measurements are used to constrain the inversion.

Figure 1. Our directional polarization camera.

We present a case study of DPC observations performed at high levels of air pollution in the Pearl River Delta (PRD) region of China. We analyzed the spatial and temporal variability of the optical and microphysical properties of aerosols over this region using DPC measurements.

We used the experimental model of Nadal and Bréon5 to estimate the polarized reflectance of vegetation in the PRD. To accurately simulate the polarized reflectance of the surface over the region, the model was adjusted using DPC measurements of the polarization reflectance of vegetation cover at a low altitude to minimize atmospheric effects. To improve the accuracy of the retrieval algorithm, we used ground-based measurements6 to constrain the inversion in terms of the most crucial characteristics of local aerosol particles, including their complex refractive-index and size distribution and the vertical distribution of aerosol optical parameters.

Figure 2 shows the first results derived from DPC measurements, which focus on side-scattered polarized light in the region 80°<Θ<130° (Θ: Scattering angle), where the polarization is large, over vegetated surfaces. We can see that the aerosol optical depth (AOD) at 0.665μm is approximately 0.73 and that at 0.865μm is ~0.51, which agrees well with nearby ground-based Sun photometer measurements. The ångström exponent of this image indicates very small particles with a high ångström exponent.

Figure 2. Results of DPC measurement in the Pearl River Delta. (a) True-color map from 4 December 2009. (b) Aerosol optical depth (AOD) at 0.665μm. (c) AOD at 0.865μm. (d) Ångström exponent over vegetated surface in the region Θ < 130°(Θ: Scattering angle). Sun azimuth angle = 59.8°.

The DPC is a new instrument that provides directional and polarized reflectance measurements. The proposed algorithm for retrieving the optical and microphysical properties of aerosols using DPC measurements has demonstrated its potential for use over cities. However, the limited cases examined so far make comparison difficult, and further validations are needed. We plan to perform a series of case-study analyses in which regional aerosol retrieval using DPC measurements will be validated with acquired in situ aerosol and aerodynamic particle-size measurements.

This work was supported by the State Key Laboratory of Remote Sensing Science (China), jointly sponsored by the Institute of Remote Sensing Applications and Beijing Normal University.

Xingfa Gu
State Key Laboratory of Remote Sensing Science
Institute of Remote Sensing Applications (IRSA)
Chinese Academy of Sciences (CAS)
Beijing, China

Xingfa Gu received his PhD degree in physical methods in remote sensing from the University of Paris VII. He is an academician at the International Academy of Astronautics. His expertise is in quantitative remote sensing and radiometric calibration, as well as polarized measurements of aerosols.

Tianhai Cheng, Donghai Xie, Zhengqiang Li, Tao Yu, Donghui Li
Beijing, China

Tianhai Cheng is an assistant researcher who received his PhD from the IRSA. His research interests include the optical and microphysical properties of aerosols and cloud retrieval using multi-angle, polarized remote sensing.

Zhengqiang Li received his PhD from Anhui Institute of Optics and Fine Mechanics (CAS). His research activities involve aerosol inversion algorithms, including polarization, vicarious calibration methods for radiance, polarization measurements of Sun-sky radiometers, and testing and improvement of prototype instruments.

Yanli Qiao
Anhui Institute of Optics and Fine Mechanics
Hefei, China