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Remote sensing of angular scattering effect of aerosols in a North American megacity

Zeng, Zhao-Cheng and Xu, Feng and Natraj, Vijay and Pongetti, Thomas J. and Shia, Run-Lie and Zhang, Qiong and Sander, Stanley P. and Yung, Yuk L. (2020) Remote sensing of angular scattering effect of aerosols in a North American megacity. Remote Sensing of Environment, 242 . Art. No. 111760. ISSN 0034-4257. doi:10.1016/j.rse.2020.111760.

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The angle-dependent scattering effect of aerosols in the atmosphere not only influences climate through radiative forcing effects but also impacts trace gas remote sensing by modifying the path of radiation through the atmosphere. The aerosol phase function, which characterizes the angular signature of scattering, has been continuously monitored from ground-based and space-borne observations. However, the range of scattering angles these instruments can sample is very limited. Here, we report multi-year measurements from a mountain-top remote sensing instrument: the California Laboratory for Atmospheric Remote Sensing Fourier Transform Spectrometer (CLARS-FTS), which overlooks the Los Angeles megacity. The observational geometries of CLARS-FTS provide a wide range of scattering angles, from about 20° (forward) to about 140° (backward), which is larger than the range provided by any existing aerosol remote sensing instrument. We then quantify the aerosol angular scattering effect using the O₂ ratio, which is the ratio of retrieved O₂ Slant Column Density (SCD) to geometric O₂ SCD. The O₂ ratio quantifies the light path modification due to aerosol scattering, with a value of 1 representing an aerosol-free scenario. The lower the O₂ ratio value than 1, the stronger the aerosol loading. CLARS-FTS measurements are highly sensitive to the angular scattering effect of aerosols in the Los Angeles (LA) urban atmosphere, due to the long light path going through the boundary layer and the wide range of observational angles. The differences in aerosol scattering between different surface reflection points targeted by CLARS-FTS can be explained by differences in their angular scattering geometries. The correlation between measurements at different targets can be used to quantify the strength of the angular dependence of the aerosol phase function. Applying the correlation technique to CLARS-FTS measurements, we find that, from 2011 to 2018, there is no significant trend in the aerosol phase function in the LA megacity. Overall, this study provides a practical observing strategy for quantifying the angular dependence of aerosol scattering in urban atmospheres that could potentially contribute towards improved greenhouse gas remote sensing in megacities.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Zeng, Zhao-Cheng0000-0002-0008-6508
Xu, Feng0000-0001-5155-9478
Natraj, Vijay0000-0003-3154-9429
Pongetti, Thomas J.0000-0001-9465-0853
Shia, Run-Lie0000-0003-1911-3120
Zhang, Qiong0000-0002-8762-0557
Sander, Stanley P.0000-0003-1424-3620
Yung, Yuk L.0000-0002-4263-2562
Additional Information:© 2020 Elsevier Inc. Received 26 June 2019, Revised 27 February 2020, Accepted 9 March 2020, Available online 18 March 2020. We thank the reviewers for their constructive comments and suggestions that helped improve the manuscript. The CLARS project receives support from the California Air Resources Board and the NIST GHG and Climate Science Program. V. N. acknowledges support from the NASA Earth Science Division US Participating Investigator program (solicitation NNH16ZDA001N-ESUSPI). F. X. acknowledges support from the NASA Remote Sensing Theory program under grant 14-RST14-0100. We are also thankful for the support from the Jet Propulsion Laboratory Research and Technology Development Program. AERONET data for the Caltech site are available from We also thank Jochen Stutz from UCLA and his staff for their effort in establishing and maintaining the AERONET Caltech site. MERRAero is available from NASA GMAO at CLARS-FTS data are available from the authors upon request, and part of the data are available from the NASA Megacities Project at A portion of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Z. C. Zeng would like to dedicate this paper to his newborn daughter Judith Zeng. Credit author statement: Z.-C. Zeng: Conceptualization, Methodology, Software, Writing-Original draft preparation; F. Xu: Software, Validation, Methodology; V. Natraj: Software, Validation, Writing - Review & Editing; T. Pongetti: Data curation; R.-L. Shia: Software, Validation; Q. Zhang: Software, Validation; S. Sander: Supervision; Y. Yung: Supervision. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Group:Astronomy Department
Funding AgencyGrant Number
California Air Resources BoardUNSPECIFIED
National Institute of Standards and Technology (NIST)UNSPECIFIED
JPL Research and Technology Development FundUNSPECIFIED
Subject Keywords:Aerosol scattering; Angular dependence; Urban remote sensing; Megacity; CLARS
Record Number:CaltechAUTHORS:20200319-123810926
Persistent URL:
Official Citation:Zhao-Cheng Zeng, Feng Xu, Vijay Natraj, Thomas J. Pongetti, Run-Lie Shia, Qiong Zhang, Stanley P. Sander, Yuk L. Yung, Remote sensing of angular scattering effect of aerosols in a North American megacity, Remote Sensing of Environment, Volume 242, 2020, 111760, ISSN 0034-4257, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:101998
Deposited By: Tony Diaz
Deposited On:19 Mar 2020 19:53
Last Modified:16 Nov 2021 18:07

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