The Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) has been flying aboard the NASA ER‐2 high‐altitude aircraft since October 2010. In step‐and‐stare operation mode, AirMSPI acquires ...radiance and polarization data in bands centered at 355, 380, 445, 470*, 555, 660*, 865*, and 935 nm (* denotes polarimetric bands). The imaged area covers about 10 km by 11 km and is typically observed from nine viewing angles between ±66° off nadir. For a simultaneous retrieval of aerosol properties and surface reflection using AirMSPI, an efficient and flexible retrieval algorithm has been developed. It imposes multiple types of physical constraints on spectral and spatial variations of aerosol properties as well as spectral and temporal variations of surface reflection. Retrieval uncertainty is formulated by accounting for both instrumental errors and physical constraints. A hybrid Markov‐chain/adding‐doubling radiative transfer (RT) model is developed to combine the computational strengths of these two methods in modeling polarized RT in vertically inhomogeneous and homogeneous media, respectively. Our retrieval approach is tested using 27 AirMSPI data sets with low to moderately high aerosol loadings, acquired during four NASA field campaigns plus one AirMSPI preengineering test flight. The retrieval results including aerosol optical depth, single‐scattering albedo, aerosol size and refractive index are compared with Aerosol Robotic Network reference data. We identify the best angular combinations for 2, 3, 5, and 7 angle observations from the retrieval quality assessment of various angular combinations. We also explore the benefits of polarimetric and multiangular measurements and target revisits in constraining aerosol property and surface reflection retrieval.
Key Points
An reliable retrieval algorithm has been developed for simultaneous retrieval of aerosol properties and surface reflection using AirMSPI
We identify the best angular combinations for multiangle observations from the retrieval quality assessment
We explore the benefits of polarimetric and multiangular measurements and target revisits in constraining aerosol and surface retrieval
To assess the impact of atmospheric aerosols on health, climate, and air traffic, aerosol properties must be measured with fine spatial and temporal sampling. This can be achieved by actively ...involving citizens and the technology they own to form an atmospheric measurement network. We establish this new measurement strategy by developing and deploying iSPEX, a low‐cost, mass‐producible optical add‐on for smartphones with a corresponding app. The aerosol optical thickness (AOT) maps derived from iSPEX spectropolarimetric measurements of the daytime cloud‐free sky by thousands of citizen scientists throughout the Netherlands are in good agreement with the spatial AOT structure derived from satellite imagery and temporal AOT variations derived from ground‐based precision photometry. These maps show structures at scales of kilometers that are typical for urban air pollution, indicating the potential of iSPEX to provide information about aerosol properties at locations and at times that are not covered by current monitoring efforts.
Key Points
The iSPEX add‐on turns smartphones into aerosol measurement devices
Thousands of iSPEX measurements across the Netherlands form AOT maps
The iSPEX AOT data match MODIS and AERONET data and have 2 km resolution
Characterization of aerosol vertical distribution in the planetary boundary layer (PBL) using passive remote sensing requires advances in the current state of the art. To quantify the performance of ...various passive sensor designs within a common framework we developed an aerosol climatology of the Los Angeles basin and applied observing system simulation experiments (OSSEs) to estimate the information content retrievable from a variety of sensors measuring reflected near-infrared solar radiation. In addition to simulating current and planned satellite sensors, we also characterize the sensitivity of the California Laboratory for Atmospheric Remote Sensing – Fourier Transform Spectrometer (CLARS-FTS), located at Mt. Wilson (1.67 km above sea level), which is utilized in this work as a testbed for aerosol profiling remote sensing. We estimate the impacts of spectral coverage, radiance and polarization, spectral resolution, signal to noise ratio (SNR), and number of viewing angles on the information content and retrieval uncertainties of aerosol profiles in the PBL. We found that by adding high spectral resolution (full-width half-maximum of 3 cm−1 or better), polarimetric measurements with a SNR of at least 212 to radiance measurements with SNR of 300 for both O2 A and 1∆ bands, the degrees of freedom for signal (DOFS) of a single CLARS-FTS measurement is raised from 2.1 to 2.8. This improvement is sufficient to simultaneously quantify three key parameters: aerosol optical depth, aerosol peak height, and aerosol layer thickness in the PBL. Current satellite-borne instruments (OCO-2, OCO-3, TEMPO, TROPOMI, and EPIC) and planned instruments (TEMPO, MicroCarb, SPEXone, and MAIA), individually provide a DOFS ≤ 2.25, which is insufficient to simultaneously quantify all three aerosol profiling parameters in the PBL. Joint radiometric and polarimetric measurements of the O2 A and B bands with 3 cm−1 spectral resolution, SNR of 500 for radiance and 353 for polarization, acquired at three viewing angles, can provide sufficient sensitivity to retrieve the three aerosol parameters simultaneously. The inclusion of high spectral resolution radiometric and polarimetric measurements reduces the required number of viewing angles, which is advantageous when the multiangular data are acquired with a pointable instrument. In this case a larger number of viewing angles reduces the spatial coverage that can be achieved for a given target.
•Aerosol profiling in the planetary boundary layer (PBL) using O2 bands are studied.•Current satellite sensors' sensitivity is insufficient for PBL's aerosol profiling.•Combining multiple angles and polarization with high resolution achieves the goal.
The total effect of aerosols, both directly and on cloud properties, remains the biggest source of uncertainty in anthropogenic radiative forcing on the climate. Correct characterization of intensive ...aerosol optical properties, particularly in conditions where absorbing aerosol is present, is a crucial factor in quantifying these effects. The southeast Atlantic Ocean (SEA), with seasonal biomass burning smoke plumes overlying and mixing with a persistent stratocumulus cloud deck, offers an excellent natural laboratory to make the observations necessary to understand the complexities of aerosol-cloud-radiation interactions. The first field deployment of the NASA ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) campaign was conducted in September of 2016 out of Walvis Bay, Namibia.
A broadband, high efficiency polarized beam splitter (PBS) metagrating based on integrated resonant units (IRUs) to enable simultaneous polarization analysis, spectral dispersion, and spatial imaging ...in the near infrared (NIR) is developed. A PBS metagrating with a diameter of 60 mm is the key technology component of the high‐resolution multiple‐species atmospheric profiler in the NIR (HiMAP‐NIR), which is a spaceborne instrument concept crafted to be a core payload of NASA's new generation Earth System Observatory. HiMAP‐NIR will enable the aerosol profiling in Earth's planetary boundary layer (from surface to2 km altitude) by simultaneously measuring four spatial‐spectral‐polarimetric images from 680 to 780 nm. Through detailed optimization of hybridized resonant modes in IRUs, the PBS metagrating shows a diffraction efficiency of 70% (or better) for all four linear‐polarized incident light, and polarization contrasts between orthogonal states are 0.996 (or better) from 680 to 780 nm. It meets the stringent performance required by the HiMAP‐NIR exploiting a new paradigm for the broad applications of metasurfaces.
An polarized beam splitter, enabling to simultaneously measure four spatial‐spectral‐polarimetric images with an efficiency ≈70% in the entire near‐infrared oxygen bands and a polarization contrast greater than 0.996, are realized by hybridized gap‐plasmonic metasurfaces. This meets the standard of NASA's high‐resolution multiple‐species atmospheric profiler instrument for aerosol profiling in the planetary boundary layer.
An optimization approach has been developed for simultaneous retrieval of aerosol properties and normalized water-leaving radiance (nLw) from multispectral, multiangular, and polarimetric ...observations over ocean. The main features of the method are (1) use of a simplified bio-optical model to estimate nLw, followed by an empirical refinement within a specified range to improve its accuracy; (2) improved algorithm convergence and stability by applying constraints on the spatial smoothness of aerosol loading and Chlorophyll a (Chl a) concentration across neighboring image patches and spectral constraints on aerosol optical properties and nLw across relevant bands; and (3) enhanced Jacobian calculation by modeling and storing the radiative transfer (RT) in aerosol/Rayleigh mixed layer, pure Rayleigh-scattering layers, and ocean medium separately, then coupling them to calculate the field at the sensor. This approach avoids unnecessary and time-consuming recalculations of RT in unperturbed layers in Jacobian evaluations. The Markov chain method is used to model RT in the aerosol/Rayleigh mixed layer and the doubling method is used for the uniform layers of the atmosphere–ocean system. Our optimization approach has been tested using radiance and polarization measurements acquired by the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) over the AERONET USC_SeaPRISM ocean site (6 February 2013) and near the AERONET La Jolla site (14 January 2013), which, respectively, reported relatively high and low aerosol loadings. Validation of the results is achieved through comparisons to AERONET aerosol and ocean color products. For comparison, the USC_SeaPRISM retrieval is also performed by use of the Generalized Retrieval of Aerosol and Surface Properties algorithm (Dubovik et al., 2011). Uncertainties of aerosol and nLw retrievals due to random and systematic instrument errors are analyzed by truth-in/truth-out tests with three Chl a concentrations, five aerosol loadings, three different types of aerosols, and nine combinations of solar incidence and viewing geometries.
An optimization algorithm is developed to retrieve liquid water cloud properties including cloud optical depth (COD), droplet size distribution and cloud top height (CTH), and above‐cloud aerosol ...properties including aerosol optical depth (AOD), single‐scattering albedo, and microphysical properties from sweep‐mode observations by Jet Propulsion Laboratory's Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) instrument. The retrieval is composed of three major steps: (1) initial estimate of the mean droplet size distribution across the entire image of 80–100 km along track by 10–25 km across track from polarimetric cloudbow observations, (2) coupled retrieval of image‐scale cloud and above‐cloud aerosol properties by fitting the polarimetric data at all observation angles, and (3) iterative retrieval of 1‐D radiative transfer‐based COD and droplet size distribution at pixel scale (25 m) by establishing relationships between COD and droplet size and fitting the total radiance measurements. Our retrieval is tested using 134 AirMSPI data sets acquired during the National Aeronautics and Space Administration (NASA) field campaign ObseRvations of Aerosols above CLouds and their intEractionS. The retrieved above‐cloud AOD and CTH are compared to coincident HSRL‐2 (HSRL‐2, NASA Langley Research Center) data, and COD and droplet size distribution parameters (effective radius reff and effective variance veff) are compared to coincident Research Scanning Polarimeter (RSP) (NASA Goddard Institute for Space Studies) data. Mean absolute differences between AirMSPI and HSRL‐2 retrievals of above‐cloud AOD at 532 nm and CTH are 0.03 and <0.5 km, respectively. At RSP's footprint scale (~ 323 m), mean absolute differences between RSP and AirMSPI retrievals of COD, reff, and veff in the cloudbow area are 2.33, 0.69 μm, and 0.020, respectively. Neglect of smoke aerosols above cloud leads to an underestimate of image‐averaged COD by ~15%.
Key Points
Above‐cloud aerosol and cloud property retrievals are coupled
Empirical relations are found between cloud optical depth and droplet size parameters
Pixel‐scale cloud optical depth and droplet size distributions are retrieved
Comparison of observations of radio recombination lines in the interstellar medium with theoretical models can be used to constrain electron temperature and density of the gas. An important component ...of the models is spontaneous transition rates between bound levels. Calculating these rates relies on accurate bound–bound oscillator strengths, which can be cast in terms of the Gaunt factor. The Gaunt factor contains terminating hypergeometric functions that cannot be calculated with sufficient accuracy for high quantum levels (n ≳ 50) by standard machine-precision methods. Methods to overcome the accuracy problem have been developed, which include asymptotic expansions and recursion relations. These methods, used in astrophysical models to calculate oscillator strengths, can introduce errors, sometimes up to as much as ∼8 per cent. Detections of radio recombination lines with the new Low Frequency Array have prompted an examination of theoretical models of the interstellar medium. We revisit the calculation of the Gaunt factor, employing modern arbitrary-precision computational methods to tabulate the Gaunt factor for transitions up to quantum level n = 2000, sufficient to model low-frequency carbon radio recombination lines. The calculations provide a relative error of ∼3 × 10−4 when compared to more detailed calculations including relativistic corrections. Our values for the Gaunt factor are provided for download in a tabular format to be used for a wide range of applications.
PDF
