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1. Theoretical Uncertainty Analysis of Satellite Retrieved Aerosol Optical Depth Associated with Surface Albedo and Aerosol Optical Properties

   https://doi.org/10.3390/rs13030344  

   Huang, Jingting ; Arnott, William Patrick ; Barnard, James C. ; Holmes, Heather A. ( February 2021 , Remote Sensing)

   null (Ed.)

   Deriving aerosol optical depth (AOD) from space-borne observations is still challenging due to uncertainties associated with sensor calibration drift, cloud screening, aerosol type classification, and surface reflectance characterization. As an initial step to understanding the physical processes impacting these uncertainties in satellite AOD retrievals, this study outlines a theoretical approach to estimate biases in the satellite aerosol retrieval algorithm affected by surface albedo and prescribed aerosol optical properties using a simplified radiative transfer model with a traditional error propagation approach. We expand the critical surface reflectance concept to obtain the critical surface albedo (CSA), critical single scattering albedo (CSSA), and critical asymmetry parameter (CAP). The top-of-atmosphere (TOA) reflectance is not sensitive to significant variability in aerosol loading (AOD) at the critical value; thus, the AOD cannot be determined. Results show that 5% bias in surface albedo (A), single scattering albedo (SSA), or asymmetry parameter (g) lead to large retrieved AOD errors, especially high under conditions when A, SSA, or g are close to their critical values. The results can be useful for future research related to improvements of satellite aerosol retrieval algorithms and provide a preliminary framework to analytically quantify AOD uncertainties from satellite retrievals. 

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2. Radiative Influence of Horizontally Oriented Ice Crystals over Summit, Greenland

   https://doi.org/10.1029/2018JD028963  

   Stillwell, Robert A. ; Neely, III, Ryan R. ; Thayer, Jeffrey P. ; Walden, Von P. ; Shupe, Matthew D. ; Miller, Nathaniel B. ( November 2019 , Journal of Geophysical Research: Atmospheres)

   Ice crystals commonly adopt a horizontal orientation under certain aerodynamic and electrodynamic conditions that occur in the atmosphere. While the radiative impact of horizontally oriented ice crystals (HOIC) has been theoretically studied with respect to their impact on shortwave cloud albedo, the longwave impact remains unexplored. This work analyzes the occurrence of HOIC at Summit, Greenland, from July 2015 to June 2017. Using polarization lidar and ancillary atmospheric sensors, ice crystal orientations are identified and used to interpret cloud radiative impact on the surface radiation budget. We find HOIC occur in at least 25.6% of all ice‐only column observations. We find that the shortwave impact of HOIC is to increase cloud radiative effect by approximately 22% for a given solar zenith angle. We also find that the longwave impact of HOIC compared to randomly oriented ice crystals are statistically different at the p < 0.01 significance level, increasing the surface radiative effect by approximately 8% for clouds with infrared optical depths < ~1. We suggest that the observed difference between the surface radiative effect for clouds containing randomly oriented ice crystals and HOIC may be due to enhanced scattering, but this hypothesis needs to be further explored with more detailed observations and modeling.

    

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3. AUTOMATIC EXTRACTION OF SOLAR AND SENSOR IMAGING GEOMETRY FROM UAV-BORNE PUSH-BROOM HYPERSPECTRAL CAMERA

   https://doi.org/10.5194/isprs-annals-V-3-2022-131-2022  

   Bhadra, S. ; Sagan, V. ; Nguyen, C. ; Braud, M. ; Eveland, A. L. ; Mockler, T. C. ( January 2022 , ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences)

   Abstract. Calculating solar-sensor zenith and azimuth angles for hyperspectral images collected by UAVs are important in terms of conducting bi-directional reflectance function (BRDF) correction or radiative transfer modeling-based applications in remote sensing. These applications are even more necessary to perform high-throughput phenotyping and precision agriculture tasks. This study demonstrates an automated Python framework that can calculate the solar-sensor zenith and azimuth angles for a push-broom hyperspectral camera equipped in a UAV. First, the hyperspectral images were radiometrically and geometrically corrected. Second, the high-precision Global Navigation Satellite System (GNSS) and Inertial Measurement Unit (IMU) data for the flight path was extracted and corresponding UAV points for each pixel were identified. Finally, the angles were calculated using spherical trigonometry and linear algebra. The results show that the solar zenith angle (SZA) and solar azimuth angle (SAA) calculated by our method provided higher precision angular values compared to other available tools. The viewing zenith angle (VZA) was lower near the flight path and higher near the edge of the images. The viewing azimuth angle (VAA) pattern showed higher values to the left and lower values to the right side of the flight line. The methods described in this study is easily reproducible to other study areas and applications. 

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4. Rapid evolution of aerosol particles and their optical properties downwind of wildfires in the western US

   https://doi.org/10.5194/acp-20-13319-2020  

   Kleinman, Lawrence I. ; Sedlacek III, Arthur J. ; Adachi, Kouji ; Buseck, Peter R. ; Collier, Sonya ; Dubey, Manvendra K. ; Hodshire, Anna L. ; Lewis, Ernie ; Onasch, Timothy B. ; Pierce, Jeffery R. ; et al ( January 2020 , Atmospheric Chemistry and Physics)

   null (Ed.)

   Abstract. During the first phase of the Biomass Burn Operational Project (BBOP) fieldcampaign, conducted in the Pacific Northwest, the DOE G-1 aircraft was usedto follow the time evolution of wildfire smoke from near the point ofemission to locations 2–3.5 h downwind. In nine flights we maderepeated transects of wildfire plumes at varying downwind distances andcould thereby follow the plume's time evolution. On average there was littlechange in dilution-normalized aerosol mass concentration as a function ofdownwind distance. This consistency hides a dynamic system in which primaryaerosol particles are evaporating and secondary ones condensing. Organicaerosol is oxidized as a result. On all transects more than 90 % ofaerosol is organic. In freshly emitted smoke aerosol, NH4+ isapproximately equivalent to NO3. After 2 h of daytime aging, NH4+ increased and is approximately equivalent tothe sum of Cl, SO42, and NO3. Particle size increased with downwind distance,causing particles to be more efficient scatters. Averaged over nine flights,mass scattering efficiency (MSE) increased in ∼ 2 h by 56 % and doubled in one flight. Mechanisms for redistributing mass from small to large particles are discussed. Coagulation is effective at movingaerosol from the Aitken to accumulation modes but yields only a minor increase in MSE. As absorption remained nearly constant with age, the timeevolution of single scatter albedo was controlled by age-dependentscattering. Near-fire aerosol had a single scatter albedo (SSA) of 0.8–0.9. After 1 to 2 h of aging SSAs were typically 0.9 and greater. Assuming global-average surface and atmospheric conditions, the observedage dependence in SSA would change the direct radiative effect of a wildfire plume from near zero near the fire to a cooling effect downwind. 

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5. Spectral Radiative Properties of Polydispersed SiO2 Particle Beds

   https://doi.org/10.2514/1.T6524  

   Chen, Chuyang ; Yang, Chiyu ; Ranjan, Devesh ; Loutzenhiser, Peter G. ; Zhang, Zhuomin M. ( October 2022 , Journal of thermophysics and heat transfer)

   The focus of this work is on the measurement and analysis of the radiative properties of polycrystalline SiO2 particle beds with various layer thicknesses. The particles are polydispersed with average diameters of 222, 150, and 40 μm. The spectral, directional–hemispherical reflectance and transmittance of the particle bed are measured at wavelengths from 0.4 to 1.8 μmusing a monochromator, and the reflectance measurement is extended to 15 μmusing a Fourier-transform infrared spectrometer. Particles are closely packed between two transparent windows for measuring the radiative properties. In the visible and near-infrared region up to 1.8 μm, the inverse adding–doubling method yields the effective absorption and scattering coefficients. The results suggest that short wavelength absorption needs to be included in modeling the behavior of particle beds due to multiple scattering. A discrete-scale Monte Carlo ray-tracing method is developed to model the radiative properties by assuming monodispersed spherical particles, and the simulated results compare well with measurements. The effective absorption and scattering coefficients of the particle beds obtained from the independent scattering theory are compared to those from the inverse method. The impact of dependent scattering on the packed beds is observed for smaller-sized particles. 

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