Aerosols affect Earth’s energy level by scattering and absorbing radiation and by changing the properties of clouds. Such effects influence the precipitation patterns and lead to modifications of the global circulation systems that constitute Earth’s climate. The aerosol effects on our climate cannot be at full scale estimated due to the insufficient knowledge of their properties at a global scale. Achieving global measurement coverage requires an instrument with a large instantaneous field of view that can perform polarization measurements with high accuracy, typically better than 0.1%. Developing such an instrument can be considered as the most important challenge in polarimetric aerosol remote sensing.
Using a novel technique to measure polarization, we have designed an instrument for a low-Earth orbit, e.g. International Space Station, that can simultaneously characterize the intensity and state of linear polarization of scattered sunlight, from 400 to 800 nm and 1200 to 1600 nm, for 30 viewing directions, each with a 30° viewing angle. In this article we present the instrument’s optical design concept.
Global characterization of atmospheric aerosol in terms of the microphysical properties of the particles is essential for understanding the role aerosols in Earth climate . For more accurate predictions of future climate the uncertainties of the net radiative forcing of aerosols in the Earth’s atmosphere must be reduced . Essential parameters that are needed as input in climate models are not only the aerosol optical thickness (AOT), but also particle specific properties such as the aerosol mean size, the single scattering albedo (SSA) and the complex refractive index. The latter can be used to discriminate between absorbing and non-absorbing aerosol types, and between natural and anthropogenic aerosol. Classification of aerosol types is also very important for air-quality and health-related issues .
Remote sensing from an orbiting satellite platform is the only way to globally characterize atmospheric aerosol at a relevant timescale of ∼ 1 day . One of the few methods that can be employed for measuring the microphysical properties of aerosols is to observe both radiance and degree of linear polarization of sunlight scattered in the Earth atmosphere under different viewing directions . The requirement on the absolute accuracy of the degree of linear polarization PL is very stringent: the absolute error in PL must be smaller then 0.001+0.005⋅PL in order to retrieve aerosol parameters with sufficient accuracy to advance climate modelling and to enable discrimination of aerosol types based on their refractive index for air-quality studies .
In this paper we present the SPEX instrument, which is a multi-angle spectropolarimeter that can comply with the polarimetric accuracy needed for characterizing aerosols in the Earth’s atmosphere. We describe the implementation of spectral polarization modulation in a prototype instrument of SPEX and show results of ground based measurements from which aerosol microphysical properties are retrieved.
Highly accurate multi-angle polarimeters are essential for taking the next step in global characterization of atmospheric aerosol. Spectral polarization modulation enables highly accurate snapshot polarimetry and is very suitable for ground-, air- and space-based instrumentation. In this paper we present two instruments that employ this technology, the SPEX prototype and groundSPEX. We have performed ground-based measurements at the CESAR Observatory in the Netherlands with these two instruments. We compare the measured degree of linear polarization of co-located measurements, which show an rms difference of 0.005. Aerosol microphysical properties that have been retrieved from these measurements agree well with similar retrievals from AERONET measurements. Finally, we discuss the current efforts to upgrade the SPEX prototype to an autonomous instrument suitable for flying on NASA’s ER-2 high altitude aircraft.
We present the Spectropolarimeter for Planetary EXploration (SPEX), a high-accuracy linear spectropolarimeter
measuring from 400 to 800 nm (with 2 nm intensity resolution), that is compact (~ 1 liter), robust and
lightweight. This is achieved by employing the unconventional spectral polarization modulation technique, optimized
for linear polarimetry. The polarization modulator consists of an achromatic quarter-wave retarder and
a multiple-order retarder, followed by a polarizing beamsplitter, such that the incoming polarization state is
encoded as a sinusoidal modulation in the intensity spectrum, where the amplitude scales with the degree of
linear polarization, and the phase is determined by the angle of linear polarization. An optimized combination
of birefringent crystals creates an athermal multiple-order retarder, with a uniform retardance across the field
of view. Based on these specifications, SPEX is an ideal, passive remote sensing instrument for characterizing
planetary atmospheres from an orbiting, air-borne or ground-based platform. By measuring the intensity and
polarization spectra of sunlight that is scattered in the planetary atmosphere as a function of the single scattering
angle, aerosol microphysical properties (size, shape, composition), vertical distribution and optical thickness can
be derived. Such information is essential to fully understand the climate of a planet. A functional SPEX prototype
has been developed and calibrated, showing excellent agreement with end-to-end performance simulations.
Calibration tests show that the precision of the polarization measurements is at least 2 • 10-4. We performed
multi-angle spectropolarimetric measurements of the Earth's atmosphere from the ground in conjunction with
one of AERONET's sun photometers. Several applications exist for SPEX throughout the solar system, a.o. in
orbit around Mars, Jupiter and the Earth, and SPEX can also be part of a ground-based aerosol monitoring
Several organizations in the Netherlands are cooperating to develop user requirements and instrument concepts in the line of SCIAMACHY and OMI but with an increased focus on measuring tropospheric constituents from space. The concepts use passive spectroscopy in dedicated wavelength sections in the range of 300 to 2400 nm and wide angle, non-scanning, swath viewing.
To be able to penetrate into the troposphere small ground pixels are used to obtain a fair fraction of cloud-free pixels and to allow precise detection of the sources of polluting gases.
The trace gas products aimed for are O3, NO2, HCHO, H2O, SO2, Aerosol (optical depth, type and absorption index), CO and CH4, covering science issues on air quality and climate.
The main challenge in the instrument design is to obtain a good signal-to-noise for cloud free pixels and for low ground albedo and light levels. Also the retrieval of separated tropospheric and stratospheric column amounts from a nadir looking instrument is challenging.
The paper discusses the user requirements and compares alternative measurement strategies. It explains the selection of passive UV-Visible-NIR spectroscopy and comes with an instrument concept which provides the current best realisation of the user requirements.
We present an analysis of the aerosol retrieval capabilities of
different types of satellite measurements. Here, we consider single-
and multiple-viewing-angle measurements of intensity and of intensity
together with polarization. In particular, we investigated their
information content with respect to aerosol size distribution, optical
thickness, and refractive index. For our investigation we employed a
newly developed linearized vector radiative transfer model. This
radiative transfer model accurately simulates the intensity vector and
additionally calculates the derivatives with respect to the relevant
aerosol properties. The use of an accurate linearized radiative
transfer model in combination with an analytical inversion approach
allows a solid error analysis and quantification of the information
content of the different measurement types. In order to obtain optimal
aerosol information from satellite measurements, multiple-wavelength
multiple-viewing-angle measurements of intensity and polarization are
needed. Furthermore, multiple-wavelength multiple-viewing-angle
measurements of only intensity provide better aerosol information than
multiple-wavelength measurements of intensity and polarization in one
viewing direction. On the other hand, for single-viewing-angle instruments the inclusion of polarization leads to an improvement in accuracy in effective radius, refractive index, and optical thickness of a factor 10-100 compared to intensity measurements alone. Here, the inclusion of polarization has an even stronger effect than for