145 research outputs found
Satellite remote sensing of cloud properties in support of troposheric trace gas retrievals
Aben, E.A.A. [Promotor
Photopolarimetric Retrievals of Snow Properties
Polarimetric observations of snow surfaces, obtained in the 410-2264 nm range with the Research Scanning Polarimeter onboard the NASA ER-2 high-altitude aircraft, are analyzed and presented. These novel measurements are of interest to the remote sensing community because the overwhelming brightness of snow plagues aerosol and cloud retrievals based on airborne and spaceborne total reflection measurements. The spectral signatures of the polarized reflectance of snow are therefore worthwhile investigating in order to provide guidance for the adaptation of algorithms currently employed for the retrieval of aerosol properties over soil and vegetated surfaces. At the same time, the increased information content of polarimetric measurements allows for a meaningful characterization of the snow medium. In our case, the grains are modeled as hexagonal prisms of variable aspect ratios and microscale roughness, yielding retrievals of the grains' scattering asymmetry parameter, shape and size. The results agree with our previous findings based on a more limited data set, with the majority of retrievals leading to moderately rough crystals of extreme aspect ratios, for each scene corresponding to a single value of the asymmetry parameter
Aerosol Retrieval from Multiangle Multispectral Photopolarimetric Measurements: Importance of Spectral Range and Angular Resolution
We investigated the importance of spectral range and angular resolution for aerosol retrieval from multiangle photopolarimetric measurements over land. For this purpose, we use an extensive set of simulated measurements for different spectral ranges and angular resolutions and subsets of real measurements of the airborne Research Scanning Polarimeter (RSP) carried out during the PODEX and SEAC4RS campaigns over the continental USA. Aerosol retrievals performed from RSP measurements show good agreement with ground-based AERONET measurements for aerosol optical depth (AOD), single scattering albedo (SSA) and refractive index. Furthermore, we found that inclusion of shortwave infrared bands (1590 and/or 2250 nm) significantly improves the retrieval of AOD, SSA and coarse mode microphysical properties. However, accuracies of the retrieved aerosol properties do not improve significantly when more than five viewing angles are used in the retrieval
Retrieval of liquid water cloud properties from POLDER-3 measurements using a neural network ensemble approach
This paper describes a neural network algorithm for the estimation of liquid
water cloud optical properties from the Polarization and Directionality of
Earth's Reflectances-3 (POLDER-3) instrument aboard the Polarization &
Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations
from a Lidar (PARASOL) satellite. The algorithm has been trained on synthetic
multi-angle, multi-wavelength measurements of reflectance and polarization
and has been applied to the processing of 1 year of POLDER-3 data.
Comparisons of the retrieved cloud properties with Moderate Resolution
Imaging Spectroradiometer (MODIS) products show that the neural network
algorithm has a low bias of around 2 in cloud optical thickness (COT) and
between 1 and 2 µm in the cloud effective radius. Comparisons with
existing POLDER-3 datasets suggest that the proposed scheme may have enhanced
capabilities for cloud effective radius retrieval, at least over land. An
additional feature of the presented algorithm is that it provides COT and
effective radius retrievals at the native POLDER-3 Level 1B pixel level.</p
Systematic Relationships Between Lidar Observables and Sizes And Mineral Composition Of Dust Aerosols
The physical and chemical properties of soil dust aerosol particles fundamentally affect their interaction with climate, including shortwave absorption and radiative forcing, nucleation of cloud droplets and ice crystals, heterogeneous formation of sulfates and nitrates on the surface of dust particles, and atmospheric processing of iron into bioavailable forms that increase the productivity of marine phytoplankton. Lidar measurements, such as extinction-to-backscatter, color and depolarization ratios, are frequently used to distinguish between aerosol types with different physical and chemical properties. The chemical composition of aerosol particles determines their complex refractive index, hence affecting their backscattering properties. Here we present a study on how dust aerosol backscattering and depolarization properties at wavelengths of 355, 532 and 1064 nm are related to size and complex refractive index, which varies with the mineral composition of the dust. Dust aerosols are represented by collections of spheroids with a range of prolate and oblate aspect ratios and their optical properties are obtained using T-matrix calculations. We find simple, systematic relationships between lidar observables and the dust size and complex refractive index that may aid the use of space-based or airborne lidars for direct retrieval of dust properties or for the evaluation of chemical transport models using forward simulated lidar variables. In addition, we present first results on the spatial variation of forward-simulated lidar variables based on a dust model that accounts for the atmospheric cycle of eight different mineral types plus internal mixtures of seven mineral types with iron oxides, which was recently implemented in the NASA GISS Earth System ModelE2
Intercomparison of Airborne Multi-Angle Polarimeter Observations from the Polarimeter Definition Experiment (PODEX)
In early 2013, three airborne polarimeters were flown on the high altitude NASA ER-2 aircraft in California for the Polarimeter Definition Experiment (PODEX). PODEX supported the pre-formulation NASA Aerosol-Cloud-Ecosystem (ACE) mission, which calls for an imaging polarimeter in polar orbit (among other instruments) for the remote sensing of aerosols, oceans and clouds. Several polarimeter concepts exist as airborne prototypes, some of which were deployed during PODEX as a capabilities test. Two of those instruments to date have successfully produced Level 1 (georegistered, calibrated radiance and polarization) data from that campaign: the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) and the Research Scanning Polarimeter (RSP). We compared georegistered observations of a variety of scene types by these instruments to test if Level 1 products agree within stated uncertainties. Initial comparisons found radiometric agreement, but polarimetric biases beyond measurement uncertainties. After subsequent updates to calibration, georegistration, and the measurement uncertainty models, observations from the instruments now largely agree within stated uncertainties. However, the 470nm reflectance channels have a roughly +6% bias of AirMSPI relative to RSP, beyond expected measurement uncertainties. We also find that observations of dark (ocean) scenes, where polarimetric uncertainty is expected to be largest, do not agree within stated polarimetric uncertainties. Otherwise, AirMSPI and RSP observations are consistent within measurement uncertainty expectations, providing credibility for subsequent creation of Level 2 (geophysical product) data from these instruments, and comparison thereof. The techniques used in this work can also form a methodological basis for other intercomparisons, such as of the data gathered during the recent Aerosol Characterization from Polarimeter and Lidar (ACEPOL) field campaign, carried out in October and November of 2017 with four polarimeters (including AirMSPI and RSP)
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Observations of Aerosol‐Cloud Interactions During the North Atlantic Aerosol and Marine Ecosystem Study
Clouds and their response to aerosols constitute the largest uncertainty in our understanding of 20th‐century climate change. We present an investigation that determines linkages between remotely sensed marine cloud properties with in situ measurements of cloud condensation nuclei (CCN) and meteorological properties obtained during the North Atlantic Aerosols and Marine Ecosystems Study. The first two deployments of this campaign have geographically similar domains but occur in different seasons allowing the response of clouds to a range of CCN concentrations and meteorological conditions to be investigated. Well‐defined connections between CCN and cloud microphysical properties consistent with the indirect effect are observed, as well as complex, nonlinear secondary effects that are partially supported by previously proposed mechanisms. Using the Research Scanning Polarimeter's remotely sensed effective variance parameter, correlation is found with liquid water path. In general, cloud macrophysical properties are found to better correlate with atmospheric state parameters than changes in CCN concentrations
Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 1: Methodology and evaluation with simulated measurements
We present a new remote sensing technique to infer the average asymmetry parameter of ice crystals near cloud top from multi-directional polarization measurements. The method is based on previous findings that (a) complex aggregates of hexagonal crystals generally have scattering phase matrices resembling those of their components; and (b) scattering phase matrices systematically vary with aspect ratios of crystals and their degree of microscale surface roughness. Ice cloud asymmetry parameters are inferred from multi-directional polarized reflectance measurements by searching for the closest fit in a look-up table of simulated polarized reflectances computed for cloud layers that contain individual, randomly oriented hexagonal columns and plates with varying aspect ratios and roughness values. The asymmetry parameter of the hexagonal particle that leads to the best fit with the measurements is considered the retrieved value. For clouds with optical thickness less than 5, the cloud optical thickness must be retrieved simultaneously with the asymmetry parameter, while for optically thicker clouds the asymmetry parameter retrieval is independent of cloud optical thickness. Evaluation of the technique using simulated measurements based on the optical properties of a number of complex particles and their mixtures shows that the ice crystal asymmetry parameters are generally retrieved to within 5%, or about 0.04 in absolute terms. The retrieval scheme is largely independent of calibration errors, range and sampling density of scattering angles and random noise in the measurements. The approach can be applied to measurements of past, current and future airborne and satellite instruments that measure multi-directional polarized reflectances of ice-topped clouds
The Infrared Spectra of Very Large Irregular Polycyclic Aromatic Hydrocarbons (PAHs): Observational Probes of Astronomical PAH Geometry, Size and Charge
The mid-IR spectra of six large, irregular PAHs with formulae (C84H24 -
C120H36) have been computed using Density Functional Theory (DFT). Trends in
the dominant band positions and intensities are compared to those of large,
compact PAHs as a function of geometry, size and charge. Irregular edge
moieties that are common in terrestrial PAHs, such as bay regions and rings
with quartet hydrogens, are shown to be uncommon in astronomical PAHs. As for
all PAHs comprised solely of C and H reported to date, mid-IR emission from
irregular PAHs fails to produce a strong CCstr band at 6.2 um, the position
characteristic of the important, class A astronomical PAH spectra. Earlier
studies showed inclusion of nitrogen within a PAH shifts this to 6.2 um for PAH
cations. Here we show this band shifts to 6.3 um in nitrogenated PAH anions,
close to the position of the CC stretch in class B astronomical PAH spectra.
Thus nitrogenated PAHs may be important in all sources and the peak position of
the CC stretch near 6.2 um appears to directly reflect the PAH cation to anion
ratio. Large irregular PAHs exhibit features at 7.8 um but lack them near 8.6
um. Hence, the 7.7 um astronomical feature is produced by a mixture of small
and large PAHs while the 8.6 um band can only be produced by large compact
PAHs. As with the CCstr, the position and profile of these bands reflect the
PAH cation to anion ratio.Comment: accepted by Ap
The 5.25 & 5.7 m Astronomical Polycyclic Aromatic Hydrocarbon Emission Features
Astronomical mid-IR spectra show two minor PAH features at 5.25 and 5.7
m (1905 and 1754 cm) that hitherto have been little studied,
but contain information about the astronomical PAH population that complements
that of the major emission bands. Here we report a study involving both
laboratory and theoretical analysis of the fundamentals of PAH spectroscopy
that produce features in this region and use these to analyze the astronomical
spectra. The ISO SWS spectra of fifteen objects showing these PAH features were
considered for this study, of which four have sufficient S/N between 5 and 6
m to allow for an in-depth analysis. All four astronomical spectra show
similar peak positions and profiles. The 5.25 m feature is peaked and
asymmetric, while the 5.7 m feature is broader and flatter. Detailed
analysis of the laboratory spectra and quantum chemical calculations show that
the astronomical 5.25 and 5.7 m bands are a blend of combination,
difference and overtone bands primarily involving CH stretching and CH in-plane
and CH out-of-plane bending fundamental vibrations. The experimental and
computational spectra show that, of all the hydrogen adjacency classes possible
on PAHs, solo and duo hydrogens consistently produce prominent bands at the
observed positions whereas quartet hydrogens do not. In all, this a study
supports the picture that astronomical PAHs are large with compact, regular
structures. From the coupling with primarily strong CH out-of-plane bending
modes one might surmise that the 5.25 and 5.7 m bands track the neutral
PAH population. However, theory suggests the role of charge in these
astronomical bands might also be important.Comment: Accepted ApJ, 40 pages in pre-print, 14 figures, two onlin
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