The Aerosol Limb Imager (ALI) is an optical remote sensing
instrument designed to image scattered sunlight from the atmospheric
limb. These measurements are used to retrieve spatially resolved
information of the stratospheric aerosol distribution, including
spectral extinction coefficient and particle size. Here we present
the design, development and test results of an ALI prototype
instrument. The long-term goal of this work is the eventual
realization of ALI on a satellite platform in low earth orbit, where
it can provide high spatial resolution observations, both in the
vertical and cross-track. The instrument design uses a large-aperture acousto-optic tunable filter (AOTF) to image the sunlit
stratospheric limb in a selectable narrow wavelength band ranging
from the visible to the near infrared. The ALI prototype was tested
on a stratospheric balloon flight from the Canadian Space Agency (CSA) launch facility in Timmins, Canada, in
September 2014. Preliminary analysis of the hyperspectral images
indicates that the radiance measurements are of high quality, and we
have used these to retrieve vertical profiles of stratospheric
aerosol extinction coefficient from 650 to 1000
Stratospheric aerosol plays an important role in the global radiative forcing balance by scattering solar irradiation and causing an overall cooling effect that depends on the particle size distribution and the concentration (Kiehl and Briegleb, 1993; Stocker et al., 2013). These climate effects are an important and recent focus of research due to the potential contribution of stratospheric aerosol to the so-called global warming hiatus (Solomon et al., 2011; Haywood et al., 2014; Fyfe et al., 2013), and efforts to quantify the variability and trends in the global stratospheric aerosol load are underway with various ground-based and satellite data sets (Rieger et al., 2015; Ridley et al., 2014).
Since its discovery with stratospheric balloon observations (Junge et al., 1961), stratospheric aerosol has been measured with various techniques, although due to the variability of physical composition and particle size, no single measurement technique can determine the full range of aerosol properties unambiguously. In situ balloon observations continue to be used and have provided highly valuable data sets, including most notably the long time series of optical particle counter measurements from Laramie, WY (Deshler et al., 2003, 2006; Kovilakam and Deshler, 2015). Aircraft-borne nephelometers (Beuttell and Brewer, 1949; Charlson et al., 1969) acquire detailed in situ measurements, providing, for example, plume composition (Murphy et al., 2014), but are spatially limited to the aircraft track. Ground-based lidars have been used to do detailed studies of the extent of volcanic aerosol plumes (Chazette et al., 1995; Sawamura et al., 2012) and provide valuable insight into long-term local variability and trends in the aerosol layer. For example, lidar observations were used by Hofmann et al. (2009) to first report the observed increase in stratospheric aerosol over approximately the last decade. However, the global distribution, which can only really be obtained with satellite observations, provides invaluable insight into aerosol processes and variability. A good example of this is the use of satellite observations by Vernier et al. (2011b) to determine that the increased stratospheric aerosol load reported by Hofmann et al. (2009) was in fact due to a series of relatively minor, mostly tropical, volcanic eruptions.
Satellite instrumentation capable of remote sensing stratospheric
aerosol has been in use since the 1970s, beginning with limb-sounding
solar occultation measurements. These have provided a reliable,
accurate, and essentially continuous long-term record of vertically
resolved aerosol extinction coefficient measurements, mostly from the
series of Stratospheric Aerosol and Gas Experiment (SAGE) instruments
(Russell and McCormick, 1989; Thomason and Taha, 2003). These SAGE
measurements, which have a vertical resolution of approximately
1
More recently, limb-scattered sunlight measurements have been used for
stratospheric aerosol retrievals. Although this technique has the
advantage of being able to sample the atmosphere throughout the sunlit
hemisphere, it requires the use of a complex forward model of multiple
scattering processes along with at least some a priori knowledge of
the aerosol scattering cross section in order to retrieve the
extinction coefficient profile. The Optical Spectrograph and InfraRed
Imaging System (OSIRIS) instrument (Llewellyn et al., 2004), which was
launched in 2001 and is presently still operational, was the first
satellite limb scatter instrument to retrieve stratospheric aerosol
extinction (Bourassa et al., 2007). The current OSIRIS version 5.07
data product, which provides 750
The most recently launched limb scatter instrument is the Ozone Mapping Profiler Suite Limb Profiler (OMPS-LP) on the Suomi-NPP satellite. Although similar in spectral range and vertical resolution to OSIRIS, OMPS-LP is an imaging spectrometer that vertically images the limb in a single measurement. Both OSIRIS and SCIAMACHY are grating spectrometers with a narrow field of view, such that limb profiles are obtained by vertically scanning through a range of tangent altitudes. The imaging capability of OMPS provides a decrease in the time required to obtain a limb profile and so increases the along track sampling. Recent work on the feasibility of aerosol retrieval from OMPS-LP measurements show promising results (Rault and Loughman, 2013).
Several recent studies have highlighted the requirement for continued global stratospheric aerosol observations and especially the need to resolve, both vertically and horizontally, aerosol in the lowermost stratosphere and the upper troposphere. This is the case for tracking the evolution of aerosol from volcanic eruptions, which can have a substantial effect on the aerosol optical depth in the lowermost stratosphere (Ridley et al., 2014; Andersson et al., 2015). Furthering the understanding of the transport of aerosol near and across the tropopause would also benefit from higher spatial and temporal resolution observations. This is evident in the case of volcanic plumes, such as that from Nabro in 2011, the transport and origin of which has been studied extensively and the conclusions are somewhat controversial (Bourassa et al., 2012c, 2013; Vernier et al., 2013; Fromm et al., 2013, 2014; Fairlie et al., 2014; Clarisse et al., 2014). However, this is also the case for the formation of background-level aerosol, particularly in the region of the Asian and North American monsoons, which have been identified as a source of substantial, seasonal, and highly structured aerosol formation from precursor tropospheric source gases (Vernier et al., 2011a; Neely et al., 2014; Thomason and Vernier, 2013).
Many of the studies mentioned above have involved the use of Cloud
Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)
space-borne lidar measurements (Winker et al., 2007), which nominally
measure backscatter profiles approximately every 300
Continued stratospheric aerosol observations from space are drastically needed though few, if any, planned missions with such capability are underway. In this paper we present the design and test of a prototype instrument for potential future satellite-based stratospheric aerosol observation. The Aerosol Limb Imager (ALI) concept is a relatively small, low-cost, low-power passive instrument, suitable for microsatellite deployment, with the capability to provide high spatial resolution measurements, both vertically and horizontally, of the visible/near-infrared aerosol extinction coefficient. The basic idea is to leverage the clear advantages of the limb scatter technique as a passive, and therefore low mass and power, means to obtain daily global coverage, with a two-dimensional hyperspectral imager for filling cross-track observation.
The ALI instrument concept is built around the use of an acousto-optic
tunable filter (AOTF), which is a novel filtering technology that
provides the ability to rapidly select the central wavelength of an
image with no moving parts. These filters, which have recently been
developed as large-aperture, imaging quality devices, operate very
efficiently in the red and near-infrared spectral range, which is
a well-matched spectral range for limb scatter sensitivity to aerosol
and cloud (Rieger et al., 2014). Additionally, the spectral bandpass
of the AOTF has reasonable resolutions at these
wavelengths such as 3–6
It should be noted that the basic instrument design concept of ALI is very similar to that of the Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere (ALTIUS) (Dekemper et al., 2012), which is a Belgian instrument concept from the Belgian Institute for Space Aeronomy (BIRA). ALTIUS is designed to measure limb-scattered sunlight; however, it also has solar, stellar, and planetary occultation modes and is scientifically focused on trace gas measurements, particularly for ozone, whereas ALI is optimized for aerosol observation.
ALI is a simple optical system that images essentially a single wavelength at a time through the use of an AOTF. The AOTF is a unique device that allows for the filtering without any moving parts and relatively low power consumption. However, the AOTF operation requires important instrument design considerations to account for its optical operation. For example, the diffractive qualities of the AOTF depend on the angle that light enters the device. Additionally, in practice the AOTF output is limited to a single linear polarization, which reduces the system throughput and causes potential internal stray light in the system through the rejection of the other linear polarization. The following sections provide a brief introduction to the physical operation of the AOTF, considerations for implementation in a system designed specifically for aerosol, and an overview of the final ALI optical design.
The primary filtering device behind ALI and the technology that allows for the two-dimensional spatial imaging is the AOTF, which is typically made from a birefringent crystal. A radio frequency (RF) wave is propagated through the crystal and forms an acoustic shear wave that interacts with an incoming beam of light in an effect similar to the diffraction of a specific wavelength. The use of an AOTF for an imaging system has several distinct advantages due to its low mass, fast stabilization times of a few microseconds, and no moving parts. Although many applications use small, non-imaging AOTFs with various configurations, large-aperture, birefringent, non-collinear acousto-optic devices are typically used in imaging systems. A non-collinear device is one where the input light beam and the RF acoustic wave are not aligned. Thanks to recent advancements in non-collinear AOTF technology these devices now have relatively high efficiency and robust imaging quality (Georgiev et al., 2002; Voloshinov et al., 2007).
To create the diffraction of a specific wavelength, a momentum matching
criterion must be held where the wave vectors of the acoustic wave match the
difference of the incoming and diffracted light wave vectors as seen in
Fig. 1. This condition is known as the Bragg matching criterion and is given
by
The wave vectors generated by the AOTF experiment. From
Eq. (1), the incident wave vector,
For ALI prototyping purposes, a
The ALI prototype that we have developed has been designed
specifically for testing from a stratospheric balloon at a float
altitude of approximately 35
ALI in a stratospheric balloon geometry showing the complete
6
The use of the AOTF essentially limits the optical design to two possible basic layouts: the telecentric or the telescopic system. The telecentric system uses a layout that removes perspective from the image and object plane by creating a condition that requires the chief ray to be parallel to the optical axis in both object and image space. The telescopic system uses a simple two lens afocal system to resize and collimate the incoming rays of light into the AOTF. This limitation is mainly that the incoming light beams at the AOTF device must enter at less than the acceptance angle, which is defined by a threshold beyond which the diffraction efficiency falls off sharply. These AOTF layouts have been studied previously (Suhre et al., 2004); however, they are briefly explained here in the context of our intended purpose of limb imaging aerosol. The upshot is that the telescopic, or afocal, system causes a wavelength gradient to be formed across the image plane, whereas the telecentric design overcomes this problem but has a larger spectral point spread function and a slight change in focus with wavelength. The optical design software Code V was used to assist in designing and analysing the performance of both of the optical layouts.
A telecentric layout leads to focused light bundles passing through the AOTF. The filtered image then has a constant wavelength across the entire image with a larger spectral point spread function, since the diffracted wavelength is dependent on incident angle, as seen in Eq. (2). This layout has two inherent issues. First, it is sensitive to any surface defects of the crystal since the light path is focused very near the AOTF surfaces. Second, a shift in the location of the imaging focal plane occurs that is dependent on wavelength such that perfect focus can only be obtained for a single wavelength. Defocusing will occur at the image plane for all other wavelengths and in order to correct for this problem additional compensating optics would need to be added or the detector would need to be actively moved as the wavelengths are scanned.
In the telescopic layout, collimated light for each line of sight
passes through the AOTF. This results in a few fundamental differences
that both improve and degrade the imaging quality. First, the light
passing through the AOTF from a single line of sight enters the AOTF
at the same angle, so the image will have a narrower spectral point
spread function than the telecentric counterpart. However, each
line of sight will be diffracted with a different fundamental central
wavelength due to the angular dependence in the AOTF diffraction
(Eq. 2). The scanned spectrum then has better spectral resolution than
obtained with the telecentric system, but there will be a wavelength
gradient radiating out from the centre of the image. Second, since
light in this design passes through the AOTF collimated, the focal
point of the image no longer changes with wavelength. Instead,
a lateral displacement of each line of sight occurs based on the angle
of incidence and the diffracted wavelength, which causes a slight
change in magnification of the final image. The lateral displacement
that occurs is given by the following relation:
In light of the requirements for imaging aerosol, we have chosen a telescopic design for the ALI prototype. Since the wavelength gradient across the image is small compared to the slowly varying aerosol scattering cross section, the fixed image plane is preferable for the improvement it provides in spatial imaging, particularly as we desired to use as simple as possible an optical design.
Ray tracing diagram of the telescopic lens system for ALI
simulated by Code V optical design software. The elements in the
system are the following: (1) 150
We used a very simple three lens optical layout with commercial
off-the-shelf components. Two lenses before the AOTF form a simple
telescope for the front end optics (FEO), and a single focusing lens
behind the AOTF comprises the back end optics (BEO). The AOTF is
oriented such that the detected image is formed from the diffracted
beam of the vertically polarized, i.e. extraordinary, light (defined
at the entrance aperture). A linear polarizer with an extinction
ratio greater than 10
The extraordinary diffracted light is 2.7
ALI final system optical parameters.
The SASKTRAN-HR (Bourassa et al., 2008; Zawada et al., 2015) radiative
transfer model was used to assist in determining exposure times and
entrance pupil of ALI. This was performed by using ground-based sky
measurements during a cloudless day at an azimuth of 90
A long-standing concern in the design of limb scatter instruments is
the effective rejection of out-of-field stray light. This is due to
the bright surface very near to the targeted limb in combination with
the exponentially dropping limb signal with tangent altitude. For ALI
test observations from the stratospheric balloon, a front end baffle
was incorporated. This was designed to minimize the percentage of
out-of-field light that can reach the aperture without encountering at
least three baffle surfaces. To further reduce the unwanted signal,
each baffle maintains a height-to-pitch ratio greater than 0.5
(Fischer et al., 2008). The baffle is 300
An isometric view of the complete ALI system with the baffle
and 3
A SolidWorks rendition of the completed ALI prototype is shown in
Fig. 5. The base plate of the instrument is tilted at 3
Software and controlling hardware for the instrument was developed for
autonomous or commanded control during the balloon flight. A Debian
Linux operating system with C
It should be noted that our choice of a telescopic optical layout for ALI is
actually the opposite choice of that made for the ALTIUS design, which uses
a telecentric optical layout. For that instrument, the need for spectral
resolution for trace gas retrieval makes the decision to use telecentric
optics quite clear (Dekemper et al., 2012). Given that basic design
difference, the overall optical specifications are quite similar between the
ALI and ALITUS prototype instruments (again see Table 1 for ALI
specifications), although two key differences are noted. First, by using
a telescopic layout the maximum field of view for ALI is determined by
choosing lenses to ensure light enters ALI within the acceptance angle of the
AOTF. This allows for a larger possible field of view than with a telecentric
system where the field view is defined by the aperture of the AOTF. Second,
the F number for ALTIUS is 14.32 compared to 7.5 for ALI, which allows ALI to
increase light throughput at the cost of slightly higher aberrations in the
final image. Dekemper et al. (2012) report that the visible channel of
ALTIUS was breadboarded and tested by taking ground-based measurements of
a smoke stack plume. They used the measurements to retrieve
A series of pre-flight laboratory calibrations were performed in two
stages. First, the AOTF was characterized to calibrate it with
respect to wavelength registration and spectral point spread
function. Second, the instrument was characterized as a complete
system to provide calibrated radiance. The following calibration
measurements were performed on ALI:
AOTF wavelength calibration AOTF point spread function and diffraction efficiency stray light calibration flat-fielding correction.
The relationship between the applied acoustic wave frequency and the
diffracted wavelength, which is known as the tuning curve, defines the
wavelength registration to the RF wave of the collected images. This
was determined in the laboratory setting by filling the AOTF aperture
with collimated light and observing the diffracted, or filtered,
signal with a HORIBA iHR320 spectrometer and Synapse 354 308
These central wavelengths for the full set of spectra were empirically
found to follow a power function of the form
The spectral point spread function and diffraction efficiency of the
AOTF were also determined in a similar fashion. The same set of
experimental data that was used for the wavelength registration was
used to find the spectral point spread function by finding the full
width at half maximum for each obtained spectrum. These range from
2 to 5
An experiment was performed on several wavelengths to determine the RF power that yielded the highest throughput through the AOTF using an collimated light source. For the AOTF in ALI, the maximum throughput occurred when the RF power was at the limit of the AOTF, which was 2 W. Following this, the diffraction efficiency of the AOTF was determined by using two sets measurements. The first is the experimental data used to perform the wavelength calibration, and the second is measurements of the intensity of the incident collimated light beam. The light in both experiments was linearly polarized and aligned with the polarization axis of the AOTF; for the second set the AOTF was simply removed from the optical chain. It should be noted that the attenuation of the AOTF crystal itself was not determined independently and is combined with the diffraction efficiency. We are more concerned about signal throughput of the device so the combination of the effects is acceptable. The incident light source was then measured with the same iHR320 spectrometer and Synapse CCD. By taking the ratio of the intensity at the diffracted wavelength to the incident intensity the diffraction efficiency was determined. It was found to vary between 54 and 64 % across the measured spectral range. It should be noted that the diffraction efficiency changes also with respect to incoming angle and this experimental determination only measured the diffraction efficiency at normal incidence (Xu and Stroud, 1992).
A laboratory experiment to characterize the stray light in the ALI system was also performed. Two types of stray light exist: the first is out-of-field stray light, i.e. a signal that enters the optical path that originates outside of the field of view. The second is internal stray light, which is caused by scattering, reflections, or other imperfections in the optical elements. As mentioned above, stray light removal is quite critical for limb scatter measurements.
The use of the AOTF has the potential to increase the amount of internal
stray light due to the fact that the undiffracted beam and the
unmeasured polarization also propagate through the system. However,
the diffraction interaction only occurs when the acoustic wave signal
is applied, so without the acoustic wave the recorded measurement only
contains the stray light in the system. Using this characteristic, the
stray light of the system was measured in the
laboratory. A 250
A calibration image after stray light removal has been
performed where the measured wavelength is 750
The flat-field calibration corrects optical and detector level
differences in the system across the field of view such that
a calibrated image of a perfectly diffuse source yields a constant
value across the image. The experiment was set up using a 250 W halogen bulb that was collimated and
passed through a diffusing plate to yield a consistent even output for the source. The entrance aperture of
ALI was placed 100 mm from the diffusing plate and was completely illuminated. The diffusing plate was
imaged at a variety of wavelengths (from 600 to 1000 nm) and exposure times (ranging from 0.1 s to 2 min). Images from the diffuse source
described above were used to determine the flat-fielding corrections
for ALI. These were determined in two steps: spatial and
spectral. First, for the spatial correction, for each image at a given
wavelength, each pixel was scaled to the mean value of the centre
The Canadian Space Agency (CSA) balloon launch base is in Timmins,
Ontario (48.47
On 19 September 2014 at 05:35 UTC (01:35 LT) ALI was launched as
part of the Nimbus 7 mission from the CSA Timmins balloon launch
facility. During the launch, the sky was clear with light winds,
allowing for a safe and uneventful launch. The ascent of the gondola
occurred in darkness and reached its flight altitude of
36.5
During the mission, ALI operated in two primary acquisition modes:
a calibration mode and an aerosol imaging mode. The first mode, the
calibration mode, was primarily used during ascent when the gondola
was in the darkness and intermittently between the aerosol mode during
sunlit conditions. During this mode the filtering of the AOTF was not
enabled and the system imaged essentially only dark current during the
ascent in darkness and stray light during sunlit conditions. Eight
exposures are taken in the calibration mode with 0.05, 0.1, 0.5, 1, 2,
3, 5, and 10 s exposure times. The second operational mode, the aerosol
mode, recorded measurements in a cycle that contained 13 pairs of
images across the spectral range (650–950
Averaged ALI relative radiance vectors from 12 of the 13
wavelengths from the NIMBUS 7 flight. Each panel presents the
radiance vectors from a different wavelength measured which is
denoted in the top right corner. The dashed lines are radiance
profiles where the solar zenith angle is greater than 90
Level 1 relative radiances spectrally from 650 to
950
Left is the retrieved aerosol extinction profiles from the
last complete imaging cycle consisting of images 205 to 216 from the
0.0
After the successful post-flight recovery of ALI, 216 raw images were
obtained and calibrated as detailed in Sect. 3. An example of a calibrated
limb image is shown in Fig. 9a. This is image number 208 at 750
For ease of further analysis and to increase the precision of the
measurements to a minimum of 0.6 MTF, the images were averaged into
cells of 25 pixels horizontally and averaged vertically onto
a 1
A full cycle of 13 spectral images (numbers 204–216) were used in Fig. 11 to
show the spectrum of relative calibrated radiances at selected tangent
altitudes. The estimated uncertainty in the radiance is represented by the
shading. The uncertainty is approximately 5 % from 5 to
20
As a first application of the ALI measurements, we have applied
a slightly modified version of the standard OSIRIS stratospheric
aerosol extinction retrieval (Bourassa et al., 2012b) to the flight
measurements. This inversion algorithm, which is applied from the
tropopause to 30
The relative radiance measurements from ALI are used to create measurement
vectors,
Once a retrieval has been completed for a measured radiance profile,
the result is then used to estimate the error in the retrieved
extinction. For each altitude, a gain matrix,
Using the retrieved extinction profiles for the complete spectral range, we have attempted a determination of the Angström exponent using a method similar to that outlined by Rault and Loughman (2013) for the OMPS-LP analysis. In this method, the independently retrieved extinction profiles at each wavelength and altitude are fit with a straight line in log-wavelength, log-extinction space. The slope of this line corresponds to the Angström exponent. This is then used to find the best match to the spectral dependence of the Mie scattering cross section in order to update the particle size distribution. With only one piece of information, the mode width of the log-normal distribution is fixed to 1.6 and the mode radius is updated. The extinction retrievals are then performed again at each wavelength and the process is iterated until the Angström exponent, corresponding to the determined mode radius, converges.
Ideally, the ALI measurements would be used independently to also retrieve ozone in the Chappuis band. However, due to the spectral range of the prototype, only a small fraction of the long wavelength side of the absorption band was captured. For this analysis, we have not retrieved the ozone profile but have set the ozone profile in SASKTRAN-HR to an average of the five closest coincident ozone profiles measured by OSIRIS at the ALI location and time. The surface albedo used is also from the OSIRIS scans since the two instruments share a similar measurement method and should determine a similar albedo for the cloudy conditions. Preferably albedo would be determined from ALI following the method of Bourassa et al. (2012b); however, due to the lack of an absolute calibration this was not possible.
The above retrieval method was applied to a complete cycle of ALI
spectral images (numbers 204–216 of the balloon mission). The
retrieved aerosol extinction profiles can be seen in the left panel of
Fig. 12. Note the log scale. The difference
between the measurement and forward model vectors was less than
2 % for the majority of the retrieval region, approximately 13 to
28
The ALI 750
The top panel shows the convergence of two sample particle
size retrievals; blue and red represent an initial state of 0.08 and
0.12
The 750
The particle size method outlined above was also applied to this
measurement set. The retrieved extinction at a given altitude was
rejected from the straight line fit if the converged forward model
radiance at that altitude was not within 2 % of the measurement
vector. In the case shown in Fig. 13, at the 14.5
The ALI prototype, which is a telescopic acousto-optic imager, has been used to successfully measure two-dimensional spectral images of the atmospheric limb from stratospheric balloon. The observed radiances appear to be of high quality and show both vertical and horizontal features of the cloud and aerosol layers. Aerosol extinction coefficient profiles were retrieved from the ALI data that show reasonable agreement with OSIRIS satellite measurements.
No large-scale issues were found with the instrument performance; however, some future changes would be recommended. First, an absolute calibration of the instrument would allow ALI to determine the albedo directly, as is done with OSIRIS. This would remove some of the uncertainty in the model inputs and likely yield higher-quality results. This is simply a matter of having access to the calibration equipment. Also, even with the baffle and the robust method of removing stray light with the cycling of the AOTF, some stray light was still observed in the obtained images. Impact and mitigation of this should be tackled in future iterations of the instrument.
This work would have not been possible without funding from the CSA to design and build ALI through the FAST program as well as the CSA building and managing the launch facility in Timmins, Ontario. Also, thanks to CNES for funding and overseeing the launches at Timmins in 2014. The optical design analysis was performed in thanks to Synopsys for the use of a Code V software license. The CALIPSO data were obtained from the NASA Langley Research Center Atmospheric Science Data Center. Thanks as well to Nick Lloyd for help in development of the flight code; without his efforts, this work would have not been accomplished. Edited by: J. Joiner