Infrared lidar observations of stratospheric aerosols H. N. Forrister, D. W. Roberts, A. J. Mercer, and G. G. Gimmestad* Electro-Optical Systems Laboratory, Georgia Tech Research Institute, 925 Dalney Street NW, Atlanta, Georgia 30318, USA *Corresponding author: [email protected] Received 21 January 2014; revised 25 April 2014; accepted 13 May 2014; posted 15 May 2014 (Doc. ID 204865); published 30 May 2014
We observed the stratospheric aerosol layer at 34° north latitude with a photon-counting 1574 nm lidar on three occasions in 2011. During all of the observations, we also operated a nearby 523.5 nm micropulse lidar and acquired National Weather Service upper air data. We analyzed the lidar data to find scattering ratio profiles and the integrated aerosol backscatter at both wavelengths and then calculated the color ratio and wavelength exponent for lidar backscattering from the stratospheric aerosols. The visible-light integrated backscatter values of the layer were in the range 2.8–3.5 × 10−4 sr−1 and the infrared integrated backscatter values ranged from 2.4 to 3.7 × 10−5 sr−1 . The wavelength exponent was determined to be 1.9 0.2. © 2014 Optical Society of America OCIS codes: (280.3640) Lidar; (290.1310) Atmospheric scattering; (280.1100) Aerosol detection. http://dx.doi.org/10.1364/AO.53.000D40
1. Introduction
The stratospheric aerosol layer was first discovered by balloon measurements, and Junge and Manson later determined it to be a persistent global layer of particles in the stratosphere that contain sulfur, by using impactors on aircraft followed by laboratory analyses [1]. The layer is now known to be predominantly composed of sulfuric acid droplets, and it generally starts at, or slightly below, the tropopause and it extends up into the stratosphere. In 1964, Fiocco and Grams [2] made the first observations of the layer using lidar, and in the decades since then, a number of other lidar researchers have been monitoring the layer and determining variations in its parameters over time [3–5]. A modern review on the stratospheric aerosol layer was presented by Deshler [6]. The Nabro volcano erupted in Eritrea on June 13, 2011, generating a layer of aerosols near 17 km altitude that was observed by ground-based lidars in 1559-128X/14/160D40-09$15.00/0 © 2014 Optical Society of America D40
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every continent in the Northern Hemisphere, as well as by the space borne CALIOP lidar [7]. Lidar systems that monitor and measure the stratospheric aerosol layer typically operate at 355, 532, 694, and 1064 nm. We report here stratospheric aerosol measurements made in Atlanta, Georgia, with a photon-counting 1574 nm lidar located at 34° N, 84° W as well as measurements with a nearby 523.5 nm micropulse lidar. By operating the two lidars simultaneously, we were able to acquire two-wavelength data and estimate the color ratio and wavelength exponent for lidar backscattering from the aerosol layer. The observations reported here overlapped in time with those reported in [7], but most of them were made a few months later. Researchers at the Georgia Tech Research Institute (GTRI) reported the first atmospheric lidar operating in the short-wave infrared spectrum in 1989 [8]. The GTRI lidar had an InGaAs PIN diode detector, which was sufficient for detection of clouds and aerosols. A high-performance volume-scanning 1540 nm lidar using an InGaAs APD detector was reported by Mayor and Spuler in 2004 [9]. Lidar researchers at institutes worldwide are currently
interested in the spectral region near 1600 nm for profiling CO2 and CH4 with DIAL systems, including space-based lidars. We report here stratospheric measurements with a short-wave infrared lidar that were made possible by photon-counting technology, which provides much greater sensitivity than PIN diodes and APDs with analog signal processing. 2. Instrumentation A.
Photon-Counting Infrared Lidar
The photon-counting infrared lidar (PC IR) was designed and developed by GTRI under an SBIR subcontract. Because of funding constraints, the lidar was mostly constructed from parts on hand. The major exception was the Hamamatsu IR photomultiplier tube (PMT), model H10330, which was purchased for the project. The parameters of the lidar system are listed in Table 1. The lidar transmitter system utilizes a Big Sky CFR400 Nd:YAG-pumped optical parametric oscillator to generate light at 1574 nm. The lidar transmitter uses an unobstructed catadioptric beam expander design, shown in Fig. 1, to expand the laser beam and reduce its divergence. The beam expander uses a spherical mirror to collimate the beam and a pair of commercial off-the-shelf negative lenses to diverge the beam for collimation. By using an unobstructed design, the number of photons transmitted into the atmosphere is maximized. Conventional catadioptric telescopes with a central obstruction block a substantial amount of the beam because its energy is heavily concentrated toward the center. The optical and mechanical designs of the receiver optical system and the mechanical layout of the transmitter system are shown in Fig. 2. The receiver was based on a Meade 10-in. (1 in: 2.54 cm) aperture Schmidt–Cassegrain telescope. We developed the layout of the receiver optical system with the ZEMAX lens design program, using a scaled Schmidt– Cassegrain design provided in the ZEBASE lens design database. This approach determined the locations of pupils and stops reasonably accurately, Table 1.
Fig. 1. Unobstructed beam expander.
which is required for determining the parameters of lens elements. The received light was focused into an optical assembly mounted on the front of the Hamamatsu IR photon-counting PMT. The telescope and its PMT assembly were mounted on a large dualaxis gimbal to enable alignment with the fixed transmitter beam. Note that the laser beam axis and the receiver field of view are both parallel to the table surface in Fig. 2; they are facing a large 45° turning mirror (not shown) under a roof hatch. The design of the receiver optical system is shown in Fig. 3. The aperture of the telescope (the entrance pupil) is imaged by a series of lenses onto the IR PMT photocathode in order to eliminate the effect of photocathode spatial sensitivity nonuniformities. Because the photocathode is only 1.6 mm in diameter, the 254 mm diameter telescope entrance aperture must be greatly de-magnified in order for its image (the exit pupil) to fit onto the photocathode. To aid optical alignment, we designed the optics to produce a 0.8 mm diameter exit pupil. The middle lens of the optical assembly was mounted on an X–Y–Z translation stage so that the exit pupil could be centered on the photocathode. A narrowband interference filter limited the sky light spectrum to a 14 nm wide bandpass centered at 1572 nm. An adjustable iris at the front of the optical assembly served as a field stop, and a shutter in front of the iris was used to block light completely when necessary to protect the PMT. Figure 4 shows the optomechanical design of the PMT region of the receiver. The alignment translation stages were equipped with micrometers so that the position of the alignment lens could be adjusted precisely and its position could be recorded. An opaque, loose-fitting bag made of rubber-coated nylon cloth formed a light-tight shield around the receiver optics.
Parameters of the PC IR Lidar
Transmitter Wavelength Pulse energy Pulse repetition frequency Beam diameter Beam divergence
1574 nm 80 mJ 20 Hz 15 cm 250 μrad
Telescope aperture Focal length Field of view Spectral width PMT (Hamamatsu)
25.4 cm 250 cm 800 μrad 14 nm H10330A-75
Hybrid analog/photon counting
Licel
Receiver
Data system Fig. 2. A SolidWorks model of the infrared lidar. 1 June 2014 / Vol. 53, No. 16 / APPLIED OPTICS
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Fig. 5. Photoelectrons versus photons incident on the PMT window. The slope is equal to the PMT’s quantum efficiency.
Fig. 3. Receiver optical design.
Hamamatsu representatives claimed a quantum efficiency of 8.7% for the photocathode of the PMT tube alone and stated that the PMT as a system (the PMT in its enclosure plus an amplifier/ discriminator) would be somewhat less efficient. For this reason, we measured the quantum efficiency of the system. A calibrated blackbody operating at 160 °C was used as the source. The temperature of the source was accurate to within 0.5°C, and its emissivity was greater than 0.99, allowing an accurate calculation of the number of photons that were incident on a 4.4 mm diameter aperture in front of the PMT. The spectral range of the light reaching the PMT was limited by a carefully characterized narrowband filter to a region centered at 1572 nm with an effective bandwidth of approximately 14 nm. This filter was later used in the lidar receiver, so the quantum efficiency measurements were directly applicable to it. The output current at the anode of the PMT was measured with a pico-ammeter for a number of blackbody distances relative to the PMT. Varying the blackbody-to-PMT separation allowed the photocathode irradiance to be varied in a precisely known way, avoiding the calibration errors that occur when
Fig. 4. Opto-mechanical portion of the PMT subassembly. D42
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using attenuation filters. A plot of the number of photoelectrons (calculated from the photocurrent) produced by the photocathode as a function of the number of photons incident on the photocathode, shown in Fig. 5, is linear and has a slope equal to the quantum efficiency. The average dark count rate was subtracted before plotting. The quantum efficiency at 1572 nm was found to be 8.1%. The dark count rate was approximately 2.7 × 105 counts per second, which is comparable to the rate specified by Hamamatsu. The pulse height distribution of the PMT was also investigated. Photomultipliers typically generate pulse heights produced by photons that are different from those produced by dynode dark current. The result is that photoelectron pulses at the anode are larger in amplitude than dynode electron pulses, which enables blocking of the latter by using a discriminator. The pulse height distribution was evaluated by counting photons as a function of discriminator voltage setting. Rather than exhibiting a typical pulse height distribution that is characteristic of visible light PMTs, the number of counts from the IR PMT increased nearly exponentially with decreasing discriminator voltage. This behavior was interpreted as being caused by the high dark current of the photocathode compared to the dynode dark current and by the large variation in pulse heights produced by the PMT when amplifying photoelectron pulses. This finding showed that the discriminator voltage setting is not a critical parameter when using the Hamamatsu model H10330A-75 infrared PMT. Hamamatsu representatives stated that the maximum DC anode current from the PMT is 1 μA, and they cautioned us not to exceed that value because of potential damage to the photocathode. DC anode current from the PMT is caused by sky background, but the translation from sky brightness to μA was not immediately obvious. For this reason, we undertook a careful study in order to discover the sky brightness limit without damaging the PMT. All of the data were acquired with the PMT bias that was used for all lidar data acquisition, −640 V, and they were acquired on several different occasions as a function of solar zenith angle. Neutral density (ND) filters were
Fig. 6. PMT anode current measured as a function of solar zenith angle.
inserted in the receiver as required to stay below the anode current limit, and then the effect of the ND filters was divided out to find the current that would have occurred without them. The results are shown by the data points in Fig. 6, along with a fitted smooth curve. Note that the 1 μA limit (1000 nA) is reached right at sunrise. For this reason, all lidar data were acquired before sunrise or after sunset. The data acquisition system was manufactured by Licel of Germany. It is a hybrid of photon-counting acquisition and analog waveform digitization; the photon-counting channel is of the nonparalyzable type. All of the stratospheric data reported here were acquired in the photon-counting regime. B.
Eye-Safe Atmospheric Research Lidar
the Agnes Scott students. The main parameters of the transmitter and receiver are reproduced here in Table 2, for convenience. The stratospheric aerosol layer is difficult to monitor with micropulse lidars, because it is high in the atmosphere and its backscatter coefficient is quite small, except during periods of a few years after major volcanic eruptions. However, EARL has a fairly large power-aperture product, with its 61 cm receiver and transmitted power of 35 mW, and it was designed with the intention that stratospheric aerosol layer measurements would be one application for it, although certainly the most challenging. Earlier experiments had demonstrated that EARL is just able to observe stratospheric aerosol backscatter under the following conditions: operation is during hours of darkness only; the integration time is at least 1 h, and the data are smoothed to 150 m vertical resolution (the data smoothing is mostly for visualization and fitting; the data products reported here are altitude-integrated quantities that are not improved by smoothing). The signal-to-noise ratio (SNR) in the data was experimentally found to be optimum with the PMT voltage set to −800 V. 3. Data Acquisition Procedure A. Geography
The PC IR lidar is on the Georgia Tech campus, colocated with an AERONET sun photometer. EARL is on the Agnes Scott College campus, 8.3 km due east. Both campuses are in Atlanta, Georgia. The National Weather Service launch site for the upper air data is 48.5 km SSW of Georgia Tech in Peachtree City, Georgia. Details are listed in Table 3. B. Timing
The micropulse lidar operating at 523.5 nm was developed by GTRI and Agnes Scott College under a grant from the National Science Foundation to develop an educational lidar [10]. The lidar was named the Eye-safe Atmospheric Research Lidar (EARL) by Table 2.
The lidars were operated during hours of darkness only, and data acquisition episodes lasted a minimum of two hours. The dates and times of our twowavelength stratospheric aerosol layer studies are listed in Table 4, along with the times of the upper
Parameters of EARL
Transmitter Wavelength Pulse energy Pulse repetition frequency Aperture diameter Beam divergence Fluence at aperture Receiver Aperture Diameter Percentage of light Effective focal length Field of view Bandwidth Optical efficiency Detector type Data system
523.5 nm 14 μJ 2500 Hz 200 mm 90 μrad 0.5 × 10−7 J∕cm2 Short range
Long range
610 mm 9% 2.57 m 1.2 mrad 1 nm 0.032 PMT
610 mm 91% 2.57 m 0.4 mrad 0.12 nm 0.23 PMT
2-channel, 16-bit, 10 M Sample/s A/D
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Table 3.
Instrument or Facility 1574 nm lidar AERONET 523.5 nm lidar NWS launch site
Instrument Locations
Name
Location
PC IR Georgia_Tech EARL 72215 FFC
Georgia Tech Georgia Tech Agnes Scott Peachtree City
air data launches (the dates correspond to those launch times). 4. Analysis A.
Upper Air Data
The air temperature and pressure profiles Th and Ph were used to calculate extinction and backscatter cross sections. Bucholtz [11] provided a table of these cross sections at discrete wavelengths from 0.2 to 4 μm for standard air (Ps 1013.25 mbar and T s 15°C), as well as a four-parameter fitted function for calculating them at intermediate wavelengths. The fit is said to be within 0.1% of the tabulated values for wavelengths >0.5 μm. Bucholz’s function was used to find the molecular cross section values listed in Table 5. The molecular number density N s in standard air is 2.54743 × 1025 mol:∕m3 , so (assuming that the atmospheric equation of state is represented by the ideal gas law) the extinction coefficient at altitude h is given by αh σN s
PhT s ; Ps Th
(1)
where σ is the extinction cross section taken from Table 5, and α is the extinction cross section in units of 1/m. Backscatter coefficient profiles were calculated in a similar manner. B.
Lidar Data
The first step in the lidar data analysis was to produce a time-height plot in order to find any time intervals during which clouds blocked the stratospheric lidar signal. All cloud-free profiles were then averaged together, the background level was calculated and subtracted, and the data values were multiplied by range squared. After the range Table 4.
Date (2011) August 4 September 11 December 1
Table 5.
Data Acquisition Episodes
Time Interval (UTC)
Upper Air (UTC)
0110–0320 2345–0200 1000–1200
0000 0000 1200
Molecular Cross Sections for Standard Air
Cross Section Extinction (m2 ∕mol:) Backscatter (m2 ∕mol: sr)
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523.5 nm
1574 nm
5.512 × 10−31 6.4878 × 10−32
6.488 × 10−33 7.6401 × 10−34
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Latitude, Longitude 33.78, 33.78, 33.77, 33.36,
Elevation (MSL)
−84.40 −84.40 −84.29 −84.56
290 295 315 245
m m m m
correction, the lidar elevations shown in Table 4 were added to the altitude scales to convert AGL to MSL, and the lidar profiles were smoothed to 150 m vertical resolution with a sliding boxcar filter. For each data acquisition episode and each wavelength, molecular attenuated backscatter profiles were generated from the relation βatten h βh exp −2
Z
h 0
αh dh ; 0
0
(2)
using the αh and βh profiles that were calculated from the corresponding upper air data as described above. The visible-light lidar signal is also attenuated by the stratospheric ozone layer, and archived ozone maps showed that the amount of ozone ranged from 250 to 300 Dobson units for our episodes [12]. Dobson units are column-integrated, and one unit corresponds to 2.69 × 1020 mol.∕m2 . The ozone cross section at 523.5 nm is 2.0 × 10−25 m2 ∕mol., yielding an optical depth per Dobson unit of 5.4 × 10−5 [13]. Because the vertical distribution of the ozone was not known, we adopted the ozone profile model given in the 1976 U.S. Standard Atmosphere [14], scaled to give the correct total value for each episode. The ozone optical depths were then added to the molecular optical depths in Eq. (2). They amount to a very small adjustment relative to other sources of uncertainty. There is also a potential source of molecular attenuation in the 1574 nm data: CO2 absorption lines, which occur with a spacing of about 1.5 cm−1 in the region of the laser line. Neither the laser line center wavelength nor its width is known with sufficient accuracy to model this effect, but on the other hand we see no evidence for it when scaling the lidar data to the attenuated molecular backscatter; an example is shown in Fig. 7, with good agreement at all altitudes above the crossover range. For this reason, we take any error caused by CO2 absorption to be negligible. The next step in the analysis was to examine the 1574 nm plots, such as that shown in Fig. 7, to determine the aerosol-free altitude ranges. The smoothed lidar signals at both wavelengths were then scaled to match the attenuated backscatter profiles in aerosolfree altitude regions. The results of scaling at both wavelengths for all three dates are shown in Fig. 8. The next step in the analysis was to plot scattering ratios, which are ratios of the total attenuated backscatter to the molecular attenuated backscatter, i.e., the values shown by the solid lines in Fig. 8 divided by the dotted lines. A ratio of 1.00 indicates pure
about 5% with EARL. For this reason, to obtain comparable IABS values for EARL and PC IR, we restricted the integration to those altitudes where EARL could clearly measure the stratospheric aerosol layer signal. Integrating from 15 to 19 km, we found IABS values and then calculated the color ratio and the wavelength exponent, which describes the wavelength dependence according to the relation R h2 h
βh; λ1 dh
h1
βh; λ2 dh
R h12
−a λ1 ; λ2
(4)
where λ is the wavelength of the lidar; β is the aerosol backscatter coefficient for that lidar; and a is the wavelength exponent. Results are shown in Table 7. Fig. 7. Range-corrected lidar signal X(R) (solid line) and scaled air density (dashed line). The lidar crossover range is ∼1500 m.
molecular backscatter, which is used to determine the altitudes that bound the stratospheric aerosol layer. The scattering ratios at both wavelengths for all three dates are shown in Fig. 9. 5. Comparisons A.
Integrated Aerosol Backscatter
The most common way of characterizing the stratospheric aerosol layer with lidar is to integrate the aerosol backscattering coefficient from some lower bounding altitude to an upper bound, to obtain the Integrated Aerosol Backscatter (IABS) in units of sr−1 . This procedure ignores extinction in the layer, but during times of low volcanic activity, the extinction is negligible even when integrating through layers that are several km thick [15]. The aerosol backscatter coefficient profiles were found from the scattering ratios and the molecular profiles according to βaer βmol SR − 1;
(3)
where SR is the scattering ratio. The vertical extent of the layer varies with time, as can be seen in Figs. 8 and 9, but it was always between 12 and 25 km during the three episodes reported here. The resulting IABS values for those integration boundaries are listed in Table 6 for both wavelengths. B.
Wavelength Dependence
Comparing our infrared and visible-light results is not straightforward because the PC IR lidar is capable of measuring the stratospheric aerosol layer clearly up to 27–28 km, while EARL can only measure the layer up to about 24 km, because its noise level is too high above that altitude to see the current very small levels of stratospheric aerosol layer backscatter. EARL is able to view the densest part of the stratospheric aerosol layer, but it misses a small percentage of the layer in the upper, more tenuous part. By inspection of Fig. 8, we estimated that we missed
6. Results
The characteristic exponential fall-off of molecular backscatter with altitude is clearly evident in all of the panels in Fig. 8, and the stratospheric aerosol layer backscatter can be seen in the 14–18 km altitude range. The prominent layer near 17 km is consistent with the observations reported in [7]. The results at 1574 nm illustrate the impressive performance obtained by deploying a photoncounting infrared lidar, and they show that the stratospheric aerosol layer extends up to about 28 km, which is higher than EARL can reach with reasonable SNR. The stratospheric aerosol layer backscatter had a different shape on each occasion, but the peak aerosol backscatter was always several times the molecular backscatter at 1574 nm. The shapes of the aerosol layer backscatter profiles were similar at the two wavelengths on each occasion (the small feature in the 523.5 nm data above 14 km on August 4, 2011, was due to a cirrus cloud that was not completely removed in the analysis). A shape comparison is shown in Fig. 10. At 1574 nm, the scattering ratio values range from 1 up to more than 6, and an increase in the statistical noise with altitude is apparent. At 532.5 nm, values range from 1 up to about 1.5. The relative difference between molecular and total backscatter is much smaller for EARL than for PC IR because the molecular component is 85 times larger at 523.5 nm than at 1574 nm, as shown in Table 5. The increase of noise with altitude is again apparent. The uncertainties in the measured 15–19 km IABS values at 1574 nm were estimated by standard statistical methods [16]. Using the 1574 nm case from December 1 as an example, the lidar data were scaled to fit the backscatter coefficients calculated from the corresponding upper air data in the 26.5–30 km altitude region, by making the mean value of the scattering ratio equal to unity. The scale factor is somewhat uncertain due to the statistical scatter in the scattering ratio values. The standard deviation of the data was 0.16, and there were 23 independent statistical samples in the 26.5–30 km region (because the data were smoothed to 150 m 1 June 2014 / Vol. 53, No. 16 / APPLIED OPTICS
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Fig. 8. Molecular attenuated backscatter coefficient profiles calculated from upper air data (dashed curves) and total backscatter coefficient profiles measured with lidars (solid curves) for the two lidar wavelengths and three measurement episodes. The lidar profiles were scaled to match the molecular profiles above and below the stratospheric aerosol layer.
resolution). Thepstandard deviation of the mean was therefore 0.16∕ 23 0.034. To find the effect of this uncertainty on IABS, the scale factor was changed by trial-and-error until the mean was equal to 1.034, and the new value of IABS was noted. It was 5% higher than the value listed in Table 7. The statistical scatter in the data on the other dates was similar, and the standard deviation of the 1574 nm D46
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IABS values was therefore taken to be 5% for all three episodes. The 523.5 nm data were much noisier, as shown in Figs. 8–10, and the standard deviation of 523.5 nm IABS results was estimated to be 30%. The values of IABS at 523.5 nm listed in Table 6 are comparable to those reported by other researchers for mid-visible lidars. For example,
Fig. 9. Scattering ratios (the ratio of the total backscatter coefficient to the molecular backscatter coefficient) for the two lidar wavelengths and three measurement episodes.
Trickl et al. reported baseline IABS values in the range 2 × 10−5 – 5 × 10−4 sr−1 from 2000 to 2012 at Garmisch-Partenkirchen [3]. Our results with EARL are also consistent with observations at the Mauna Loa observatory [4,5]. As shown in Table 7, the measured wavelength exponent ranged from 1.8 to 2.0. These values agree fairly well with Jäger’s finding that the wavelength dependence reaches “almost λ−2 ” during periods of Table 6.
Date August 4, 2011 September 11, 2011 December 1, 2011
Integrated Aerosol Backscatter
523.5 nm 10−4
sr−1
2.79 × 3.29 × 10−4 sr−1 3.54 × 10−4 sr−1
1574 nm 2.39 × 10−5 sr−1 3.69 × 10−5 sr−1 3.06 × 10−5 sr−1
low backscatter, which was based on results from four visible-light and near infrared lidar wavelengths [17]. Our estimate of the rms uncertainty in the color ratios due to IABS error is 30% (again using the methods in [16]), which causes a correspondingly large uncertainty in the wavelength exponent. For this reason, differences from one date to another are probably not significant, and so the average value of 1.9 should be used, with an uncertainty of 0.2. Other observers of the Nabro volcano aerosol reported their results in terms of aerosol optical depth (AOD) and suggested a lidar ratio of 50 sr in the mid-visible [7]. Multiplying our 523.5 nm IABS values in Table 6 by 50 sr, we find AODs ranging from 0.014 to 0.018. These values agree well with the average of 0.018 0.009 reported in [7]. 1 June 2014 / Vol. 53, No. 16 / APPLIED OPTICS
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Table 7.
IABS Values (15–19 km) and Wavelength Exponents
Date
IABS 523.5 nm
IABS 1574 nm
Color Ratio 532.5/1574
Wavelength Exponent
August 4, 2011 September 11, 2011 December 1, 2011
1.71 × 10−4 sr−1 1.96 × 10−4 sr−1 1.58 × 10−4 sr−1
1.94 × 10−5 sr−1 2.73 × 10−5 sr−1 1.89 × 10−5 sr−1
8.8 2.7 7.2 2.2 8.4 2.5
2.0 0.3 1.8 0.3 1.9 0.3
The 523.5 nm measurements were funded by NSF Grant No. DUE-0836997 “Enhancing Science Courses and Laboratories at a Women’s College using LIDAR,” and the 1574 nm lidar system development was funded under a subcontract from Bennett Aerospace, Inc. on NOAA SBIR project WC133R-09CN-0113. References
Fig. 10. Aerosol backscatter coefficient profile at 1574 nm (solid line) and the corresponding 523.5 nm profile (dashed line) scaled to match.
7. Conclusions
We observed the stratospheric aerosol layer at 34° north latitude on three occasions in 2011 with a photon-counting 1574 nm lidar and a nearby 523.5 nm micropulse lidar. The SNR in the resulting lidar profiles illustrated the impressive performance that can be obtained by employing photon-counting technology in the shortwave infrared region, and it also showed that the eye safe micropulse lidar known as EARL is marginally able to monitor the stratospheric aerosol layer during the current time of low-level volcanic activity and that EARL misses the uppermost part of the layer due to insufficient SNR at the higher altitudes. By using National Weather Service upper air data, we analyzed the lidar data to find scattering ratio profiles and the integrated aerosol backscatter at both wavelengths and then calculated the wavelength exponent for lidar backscattering from the stratospheric aerosols. The visible-light integrated backscatter values of the layer were 2.8–3.5 × 10−4 sr−1 and the AODs were 0.014–0.018, which are consistent with other reported values, and the average wavelength exponent for the three occasions was 1.9 0.2. To our knowledge, no other lidar observations of the stratospheric aerosols have been reported in the 1.6 μm wavelength region. D48
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1. C. E. Junge and J. E. Manson, “Stratospheric aerosol studies,” J. Geophys. Res. 66, 2163–2182 (1961). 2. G. Fiocco and G. Grams, “Observations of the aerosol layer at 20 km by optical radar,” J. Atmos. Sci. 21, 323–324 (1964). 3. T. H. Trickl, H. Giehl, H. Jäger, and H. Vogelmann, “33 years of stratospheric aerosol measurements at GarmischPartenkirchen: from Fuego to Eyjafjallajökull, and beyond,” Atmos. Chem. Phys. 13, 5205–5225 (2013). 4. J. E. Barnes and D. J. Hofmann, “Lidar measurements of stratospheric aerosol over Mauna Loa Observatory,” Geophys. Res. Lett. 24, 1923–1926 (1997). 5. J. E. Barnes and D. J. Hofmann, “Variability in the stratospheric background aerosol over Mauna Loa Observatory,” Geophys. Res. Lett. 28, 2895–2898 (2001). 6. T. Deshler, “A review of global stratospheric aerosol: measurements, importance, life cycle, and local stratospheric aerosol,” Atmos. Res. 90, 223–232 (2008). 7. P. Sawamura, J. P. Vernier, J. E. Barnes, T. A. Berkoff, E. J. Welton,L.Alados-Arboledas,F. Navas-Guzmán,G. Pappalardo, L. Mona, F. Madonna, D. Lange, M. Sicard, S. Godin-Beekmann, G. Payen, Z. Wang, S. Hu, S. N. Tripathi, C. Cordoba-Jabonero, and R. M. Hoff, “Stratospheric AOD after the 2011 eruption of the Nabro volcano measured by lidars over the Northern Hemisphere,” Environ. Res. Lett. 7, 034013 (2012). 8. E. M. Patterson, D. W. Roberts, and G. G. Gimmestad, “Initial measurements using a 1.54 micron eyesafe Raman-shifted lidar,” Appl. Opt. 28, 4978–4981 (1989). 9. S. D. Mayor and S. M. Spuler, “Raman-shifted eye-safe aerosol lidar,” Appl. Opt. 43, 3915–3924 (2004). 10. L. L. West, G. G. Gimmestad, D. W. Roberts, J. M. Stewart, J. W. Wood, and A. L. Bowling, “Atmospheric laser radar as an undergraduate educational experience,” Am. J. Phys. 74, 665–669 (2006). 11. A. Bucholtz, “Rayleigh-scattering calculations for the terrestrial atmosphere,” Appl. Opt. 34, 2765–2773 (1995). 12. Archived ozone maps were obtained from the Meteorological Service of Canada World Ozone and Ultraviolet Radiation Data Center at the website http://www.woudc.org/data/ OzoneMaps_e.html. 13. J. Brion, A. Chakir, J. Charbonnier, D. Daumont, C. Parisse, and J. Malicet, “Absorption spectra measurements for the ozone molecule in the 350–830 nm region,” J. Atmos. Chem. 30, 291–299 (1998). 14. National Center for Atmospheric Research, U.S. Standard Atmosphere, 1976 (U.S. Government Printing Office, 1976). 15. H. Jäger and D. Hofmann, “Midlatitude lidar backscatter to mass, area, and extinction conversion model based on in situ aerosol measurements from 1980 to 1987,” Appl. Opt. 30, 127–138 (1991). 16. H. D. Young, Statistical Treatment of Experimental Data (McGraw-Hill, 1962). 17. H. Jäger, “Long-term record of lidar observations of the stratospheric aerosol layer at Garmisch-Partenkirchen,” J. Geophys. Res. 110, D08106 (2005).
We observed the stratospheric aerosol layer at 34° north latitude with a photon-counting 1574 nm lidar on three occasions in 2011. During all of the observations, we also operated a nearby 523.5 nm micropulse lidar and acquired National Weather Service upper air data. We analyzed the lidar data to find scattering ratio profiles and the integrated aerosol backscatter at both wavelengths and then calculated the color ratio and wavelength exponent for lidar backscattering from the stratospheric aerosols. The visible-light integrated backscatter values of the layer were in the range 2.8–3.5 × 10−4 sr−1 and the infrared integrated backscatter values ranged from 2.4 to 3.7 × 10−5 sr−1 . The wavelength exponent was determined to be 1.9 0.2. © 2014 Optical Society of America OCIS codes: (280.3640) Lidar; (290.1310) Atmospheric scattering; (280.1100) Aerosol detection. http://dx.doi.org/10.1364/AO.53.000D40
1. Introduction
The stratospheric aerosol layer was first discovered by balloon measurements, and Junge and Manson later determined it to be a persistent global layer of particles in the stratosphere that contain sulfur, by using impactors on aircraft followed by laboratory analyses [1]. The layer is now known to be predominantly composed of sulfuric acid droplets, and it generally starts at, or slightly below, the tropopause and it extends up into the stratosphere. In 1964, Fiocco and Grams [2] made the first observations of the layer using lidar, and in the decades since then, a number of other lidar researchers have been monitoring the layer and determining variations in its parameters over time [3–5]. A modern review on the stratospheric aerosol layer was presented by Deshler [6]. The Nabro volcano erupted in Eritrea on June 13, 2011, generating a layer of aerosols near 17 km altitude that was observed by ground-based lidars in 1559-128X/14/160D40-09$15.00/0 © 2014 Optical Society of America D40
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every continent in the Northern Hemisphere, as well as by the space borne CALIOP lidar [7]. Lidar systems that monitor and measure the stratospheric aerosol layer typically operate at 355, 532, 694, and 1064 nm. We report here stratospheric aerosol measurements made in Atlanta, Georgia, with a photon-counting 1574 nm lidar located at 34° N, 84° W as well as measurements with a nearby 523.5 nm micropulse lidar. By operating the two lidars simultaneously, we were able to acquire two-wavelength data and estimate the color ratio and wavelength exponent for lidar backscattering from the aerosol layer. The observations reported here overlapped in time with those reported in [7], but most of them were made a few months later. Researchers at the Georgia Tech Research Institute (GTRI) reported the first atmospheric lidar operating in the short-wave infrared spectrum in 1989 [8]. The GTRI lidar had an InGaAs PIN diode detector, which was sufficient for detection of clouds and aerosols. A high-performance volume-scanning 1540 nm lidar using an InGaAs APD detector was reported by Mayor and Spuler in 2004 [9]. Lidar researchers at institutes worldwide are currently
interested in the spectral region near 1600 nm for profiling CO2 and CH4 with DIAL systems, including space-based lidars. We report here stratospheric measurements with a short-wave infrared lidar that were made possible by photon-counting technology, which provides much greater sensitivity than PIN diodes and APDs with analog signal processing. 2. Instrumentation A.
Photon-Counting Infrared Lidar
The photon-counting infrared lidar (PC IR) was designed and developed by GTRI under an SBIR subcontract. Because of funding constraints, the lidar was mostly constructed from parts on hand. The major exception was the Hamamatsu IR photomultiplier tube (PMT), model H10330, which was purchased for the project. The parameters of the lidar system are listed in Table 1. The lidar transmitter system utilizes a Big Sky CFR400 Nd:YAG-pumped optical parametric oscillator to generate light at 1574 nm. The lidar transmitter uses an unobstructed catadioptric beam expander design, shown in Fig. 1, to expand the laser beam and reduce its divergence. The beam expander uses a spherical mirror to collimate the beam and a pair of commercial off-the-shelf negative lenses to diverge the beam for collimation. By using an unobstructed design, the number of photons transmitted into the atmosphere is maximized. Conventional catadioptric telescopes with a central obstruction block a substantial amount of the beam because its energy is heavily concentrated toward the center. The optical and mechanical designs of the receiver optical system and the mechanical layout of the transmitter system are shown in Fig. 2. The receiver was based on a Meade 10-in. (1 in: 2.54 cm) aperture Schmidt–Cassegrain telescope. We developed the layout of the receiver optical system with the ZEMAX lens design program, using a scaled Schmidt– Cassegrain design provided in the ZEBASE lens design database. This approach determined the locations of pupils and stops reasonably accurately, Table 1.
Fig. 1. Unobstructed beam expander.
which is required for determining the parameters of lens elements. The received light was focused into an optical assembly mounted on the front of the Hamamatsu IR photon-counting PMT. The telescope and its PMT assembly were mounted on a large dualaxis gimbal to enable alignment with the fixed transmitter beam. Note that the laser beam axis and the receiver field of view are both parallel to the table surface in Fig. 2; they are facing a large 45° turning mirror (not shown) under a roof hatch. The design of the receiver optical system is shown in Fig. 3. The aperture of the telescope (the entrance pupil) is imaged by a series of lenses onto the IR PMT photocathode in order to eliminate the effect of photocathode spatial sensitivity nonuniformities. Because the photocathode is only 1.6 mm in diameter, the 254 mm diameter telescope entrance aperture must be greatly de-magnified in order for its image (the exit pupil) to fit onto the photocathode. To aid optical alignment, we designed the optics to produce a 0.8 mm diameter exit pupil. The middle lens of the optical assembly was mounted on an X–Y–Z translation stage so that the exit pupil could be centered on the photocathode. A narrowband interference filter limited the sky light spectrum to a 14 nm wide bandpass centered at 1572 nm. An adjustable iris at the front of the optical assembly served as a field stop, and a shutter in front of the iris was used to block light completely when necessary to protect the PMT. Figure 4 shows the optomechanical design of the PMT region of the receiver. The alignment translation stages were equipped with micrometers so that the position of the alignment lens could be adjusted precisely and its position could be recorded. An opaque, loose-fitting bag made of rubber-coated nylon cloth formed a light-tight shield around the receiver optics.
Parameters of the PC IR Lidar
Transmitter Wavelength Pulse energy Pulse repetition frequency Beam diameter Beam divergence
1574 nm 80 mJ 20 Hz 15 cm 250 μrad
Telescope aperture Focal length Field of view Spectral width PMT (Hamamatsu)
25.4 cm 250 cm 800 μrad 14 nm H10330A-75
Hybrid analog/photon counting
Licel
Receiver
Data system Fig. 2. A SolidWorks model of the infrared lidar. 1 June 2014 / Vol. 53, No. 16 / APPLIED OPTICS
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Fig. 5. Photoelectrons versus photons incident on the PMT window. The slope is equal to the PMT’s quantum efficiency.
Fig. 3. Receiver optical design.
Hamamatsu representatives claimed a quantum efficiency of 8.7% for the photocathode of the PMT tube alone and stated that the PMT as a system (the PMT in its enclosure plus an amplifier/ discriminator) would be somewhat less efficient. For this reason, we measured the quantum efficiency of the system. A calibrated blackbody operating at 160 °C was used as the source. The temperature of the source was accurate to within 0.5°C, and its emissivity was greater than 0.99, allowing an accurate calculation of the number of photons that were incident on a 4.4 mm diameter aperture in front of the PMT. The spectral range of the light reaching the PMT was limited by a carefully characterized narrowband filter to a region centered at 1572 nm with an effective bandwidth of approximately 14 nm. This filter was later used in the lidar receiver, so the quantum efficiency measurements were directly applicable to it. The output current at the anode of the PMT was measured with a pico-ammeter for a number of blackbody distances relative to the PMT. Varying the blackbody-to-PMT separation allowed the photocathode irradiance to be varied in a precisely known way, avoiding the calibration errors that occur when
Fig. 4. Opto-mechanical portion of the PMT subassembly. D42
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using attenuation filters. A plot of the number of photoelectrons (calculated from the photocurrent) produced by the photocathode as a function of the number of photons incident on the photocathode, shown in Fig. 5, is linear and has a slope equal to the quantum efficiency. The average dark count rate was subtracted before plotting. The quantum efficiency at 1572 nm was found to be 8.1%. The dark count rate was approximately 2.7 × 105 counts per second, which is comparable to the rate specified by Hamamatsu. The pulse height distribution of the PMT was also investigated. Photomultipliers typically generate pulse heights produced by photons that are different from those produced by dynode dark current. The result is that photoelectron pulses at the anode are larger in amplitude than dynode electron pulses, which enables blocking of the latter by using a discriminator. The pulse height distribution was evaluated by counting photons as a function of discriminator voltage setting. Rather than exhibiting a typical pulse height distribution that is characteristic of visible light PMTs, the number of counts from the IR PMT increased nearly exponentially with decreasing discriminator voltage. This behavior was interpreted as being caused by the high dark current of the photocathode compared to the dynode dark current and by the large variation in pulse heights produced by the PMT when amplifying photoelectron pulses. This finding showed that the discriminator voltage setting is not a critical parameter when using the Hamamatsu model H10330A-75 infrared PMT. Hamamatsu representatives stated that the maximum DC anode current from the PMT is 1 μA, and they cautioned us not to exceed that value because of potential damage to the photocathode. DC anode current from the PMT is caused by sky background, but the translation from sky brightness to μA was not immediately obvious. For this reason, we undertook a careful study in order to discover the sky brightness limit without damaging the PMT. All of the data were acquired with the PMT bias that was used for all lidar data acquisition, −640 V, and they were acquired on several different occasions as a function of solar zenith angle. Neutral density (ND) filters were
Fig. 6. PMT anode current measured as a function of solar zenith angle.
inserted in the receiver as required to stay below the anode current limit, and then the effect of the ND filters was divided out to find the current that would have occurred without them. The results are shown by the data points in Fig. 6, along with a fitted smooth curve. Note that the 1 μA limit (1000 nA) is reached right at sunrise. For this reason, all lidar data were acquired before sunrise or after sunset. The data acquisition system was manufactured by Licel of Germany. It is a hybrid of photon-counting acquisition and analog waveform digitization; the photon-counting channel is of the nonparalyzable type. All of the stratospheric data reported here were acquired in the photon-counting regime. B.
Eye-Safe Atmospheric Research Lidar
the Agnes Scott students. The main parameters of the transmitter and receiver are reproduced here in Table 2, for convenience. The stratospheric aerosol layer is difficult to monitor with micropulse lidars, because it is high in the atmosphere and its backscatter coefficient is quite small, except during periods of a few years after major volcanic eruptions. However, EARL has a fairly large power-aperture product, with its 61 cm receiver and transmitted power of 35 mW, and it was designed with the intention that stratospheric aerosol layer measurements would be one application for it, although certainly the most challenging. Earlier experiments had demonstrated that EARL is just able to observe stratospheric aerosol backscatter under the following conditions: operation is during hours of darkness only; the integration time is at least 1 h, and the data are smoothed to 150 m vertical resolution (the data smoothing is mostly for visualization and fitting; the data products reported here are altitude-integrated quantities that are not improved by smoothing). The signal-to-noise ratio (SNR) in the data was experimentally found to be optimum with the PMT voltage set to −800 V. 3. Data Acquisition Procedure A. Geography
The PC IR lidar is on the Georgia Tech campus, colocated with an AERONET sun photometer. EARL is on the Agnes Scott College campus, 8.3 km due east. Both campuses are in Atlanta, Georgia. The National Weather Service launch site for the upper air data is 48.5 km SSW of Georgia Tech in Peachtree City, Georgia. Details are listed in Table 3. B. Timing
The micropulse lidar operating at 523.5 nm was developed by GTRI and Agnes Scott College under a grant from the National Science Foundation to develop an educational lidar [10]. The lidar was named the Eye-safe Atmospheric Research Lidar (EARL) by Table 2.
The lidars were operated during hours of darkness only, and data acquisition episodes lasted a minimum of two hours. The dates and times of our twowavelength stratospheric aerosol layer studies are listed in Table 4, along with the times of the upper
Parameters of EARL
Transmitter Wavelength Pulse energy Pulse repetition frequency Aperture diameter Beam divergence Fluence at aperture Receiver Aperture Diameter Percentage of light Effective focal length Field of view Bandwidth Optical efficiency Detector type Data system
523.5 nm 14 μJ 2500 Hz 200 mm 90 μrad 0.5 × 10−7 J∕cm2 Short range
Long range
610 mm 9% 2.57 m 1.2 mrad 1 nm 0.032 PMT
610 mm 91% 2.57 m 0.4 mrad 0.12 nm 0.23 PMT
2-channel, 16-bit, 10 M Sample/s A/D
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Table 3.
Instrument or Facility 1574 nm lidar AERONET 523.5 nm lidar NWS launch site
Instrument Locations
Name
Location
PC IR Georgia_Tech EARL 72215 FFC
Georgia Tech Georgia Tech Agnes Scott Peachtree City
air data launches (the dates correspond to those launch times). 4. Analysis A.
Upper Air Data
The air temperature and pressure profiles Th and Ph were used to calculate extinction and backscatter cross sections. Bucholtz [11] provided a table of these cross sections at discrete wavelengths from 0.2 to 4 μm for standard air (Ps 1013.25 mbar and T s 15°C), as well as a four-parameter fitted function for calculating them at intermediate wavelengths. The fit is said to be within 0.1% of the tabulated values for wavelengths >0.5 μm. Bucholz’s function was used to find the molecular cross section values listed in Table 5. The molecular number density N s in standard air is 2.54743 × 1025 mol:∕m3 , so (assuming that the atmospheric equation of state is represented by the ideal gas law) the extinction coefficient at altitude h is given by αh σN s
PhT s ; Ps Th
(1)
where σ is the extinction cross section taken from Table 5, and α is the extinction cross section in units of 1/m. Backscatter coefficient profiles were calculated in a similar manner. B.
Lidar Data
The first step in the lidar data analysis was to produce a time-height plot in order to find any time intervals during which clouds blocked the stratospheric lidar signal. All cloud-free profiles were then averaged together, the background level was calculated and subtracted, and the data values were multiplied by range squared. After the range Table 4.
Date (2011) August 4 September 11 December 1
Table 5.
Data Acquisition Episodes
Time Interval (UTC)
Upper Air (UTC)
0110–0320 2345–0200 1000–1200
0000 0000 1200
Molecular Cross Sections for Standard Air
Cross Section Extinction (m2 ∕mol:) Backscatter (m2 ∕mol: sr)
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523.5 nm
1574 nm
5.512 × 10−31 6.4878 × 10−32
6.488 × 10−33 7.6401 × 10−34
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Latitude, Longitude 33.78, 33.78, 33.77, 33.36,
Elevation (MSL)
−84.40 −84.40 −84.29 −84.56
290 295 315 245
m m m m
correction, the lidar elevations shown in Table 4 were added to the altitude scales to convert AGL to MSL, and the lidar profiles were smoothed to 150 m vertical resolution with a sliding boxcar filter. For each data acquisition episode and each wavelength, molecular attenuated backscatter profiles were generated from the relation βatten h βh exp −2
Z
h 0
αh dh ; 0
0
(2)
using the αh and βh profiles that were calculated from the corresponding upper air data as described above. The visible-light lidar signal is also attenuated by the stratospheric ozone layer, and archived ozone maps showed that the amount of ozone ranged from 250 to 300 Dobson units for our episodes [12]. Dobson units are column-integrated, and one unit corresponds to 2.69 × 1020 mol.∕m2 . The ozone cross section at 523.5 nm is 2.0 × 10−25 m2 ∕mol., yielding an optical depth per Dobson unit of 5.4 × 10−5 [13]. Because the vertical distribution of the ozone was not known, we adopted the ozone profile model given in the 1976 U.S. Standard Atmosphere [14], scaled to give the correct total value for each episode. The ozone optical depths were then added to the molecular optical depths in Eq. (2). They amount to a very small adjustment relative to other sources of uncertainty. There is also a potential source of molecular attenuation in the 1574 nm data: CO2 absorption lines, which occur with a spacing of about 1.5 cm−1 in the region of the laser line. Neither the laser line center wavelength nor its width is known with sufficient accuracy to model this effect, but on the other hand we see no evidence for it when scaling the lidar data to the attenuated molecular backscatter; an example is shown in Fig. 7, with good agreement at all altitudes above the crossover range. For this reason, we take any error caused by CO2 absorption to be negligible. The next step in the analysis was to examine the 1574 nm plots, such as that shown in Fig. 7, to determine the aerosol-free altitude ranges. The smoothed lidar signals at both wavelengths were then scaled to match the attenuated backscatter profiles in aerosolfree altitude regions. The results of scaling at both wavelengths for all three dates are shown in Fig. 8. The next step in the analysis was to plot scattering ratios, which are ratios of the total attenuated backscatter to the molecular attenuated backscatter, i.e., the values shown by the solid lines in Fig. 8 divided by the dotted lines. A ratio of 1.00 indicates pure
about 5% with EARL. For this reason, to obtain comparable IABS values for EARL and PC IR, we restricted the integration to those altitudes where EARL could clearly measure the stratospheric aerosol layer signal. Integrating from 15 to 19 km, we found IABS values and then calculated the color ratio and the wavelength exponent, which describes the wavelength dependence according to the relation R h2 h
βh; λ1 dh
h1
βh; λ2 dh
R h12
−a λ1 ; λ2
(4)
where λ is the wavelength of the lidar; β is the aerosol backscatter coefficient for that lidar; and a is the wavelength exponent. Results are shown in Table 7. Fig. 7. Range-corrected lidar signal X(R) (solid line) and scaled air density (dashed line). The lidar crossover range is ∼1500 m.
molecular backscatter, which is used to determine the altitudes that bound the stratospheric aerosol layer. The scattering ratios at both wavelengths for all three dates are shown in Fig. 9. 5. Comparisons A.
Integrated Aerosol Backscatter
The most common way of characterizing the stratospheric aerosol layer with lidar is to integrate the aerosol backscattering coefficient from some lower bounding altitude to an upper bound, to obtain the Integrated Aerosol Backscatter (IABS) in units of sr−1 . This procedure ignores extinction in the layer, but during times of low volcanic activity, the extinction is negligible even when integrating through layers that are several km thick [15]. The aerosol backscatter coefficient profiles were found from the scattering ratios and the molecular profiles according to βaer βmol SR − 1;
(3)
where SR is the scattering ratio. The vertical extent of the layer varies with time, as can be seen in Figs. 8 and 9, but it was always between 12 and 25 km during the three episodes reported here. The resulting IABS values for those integration boundaries are listed in Table 6 for both wavelengths. B.
Wavelength Dependence
Comparing our infrared and visible-light results is not straightforward because the PC IR lidar is capable of measuring the stratospheric aerosol layer clearly up to 27–28 km, while EARL can only measure the layer up to about 24 km, because its noise level is too high above that altitude to see the current very small levels of stratospheric aerosol layer backscatter. EARL is able to view the densest part of the stratospheric aerosol layer, but it misses a small percentage of the layer in the upper, more tenuous part. By inspection of Fig. 8, we estimated that we missed
6. Results
The characteristic exponential fall-off of molecular backscatter with altitude is clearly evident in all of the panels in Fig. 8, and the stratospheric aerosol layer backscatter can be seen in the 14–18 km altitude range. The prominent layer near 17 km is consistent with the observations reported in [7]. The results at 1574 nm illustrate the impressive performance obtained by deploying a photoncounting infrared lidar, and they show that the stratospheric aerosol layer extends up to about 28 km, which is higher than EARL can reach with reasonable SNR. The stratospheric aerosol layer backscatter had a different shape on each occasion, but the peak aerosol backscatter was always several times the molecular backscatter at 1574 nm. The shapes of the aerosol layer backscatter profiles were similar at the two wavelengths on each occasion (the small feature in the 523.5 nm data above 14 km on August 4, 2011, was due to a cirrus cloud that was not completely removed in the analysis). A shape comparison is shown in Fig. 10. At 1574 nm, the scattering ratio values range from 1 up to more than 6, and an increase in the statistical noise with altitude is apparent. At 532.5 nm, values range from 1 up to about 1.5. The relative difference between molecular and total backscatter is much smaller for EARL than for PC IR because the molecular component is 85 times larger at 523.5 nm than at 1574 nm, as shown in Table 5. The increase of noise with altitude is again apparent. The uncertainties in the measured 15–19 km IABS values at 1574 nm were estimated by standard statistical methods [16]. Using the 1574 nm case from December 1 as an example, the lidar data were scaled to fit the backscatter coefficients calculated from the corresponding upper air data in the 26.5–30 km altitude region, by making the mean value of the scattering ratio equal to unity. The scale factor is somewhat uncertain due to the statistical scatter in the scattering ratio values. The standard deviation of the data was 0.16, and there were 23 independent statistical samples in the 26.5–30 km region (because the data were smoothed to 150 m 1 June 2014 / Vol. 53, No. 16 / APPLIED OPTICS
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Fig. 8. Molecular attenuated backscatter coefficient profiles calculated from upper air data (dashed curves) and total backscatter coefficient profiles measured with lidars (solid curves) for the two lidar wavelengths and three measurement episodes. The lidar profiles were scaled to match the molecular profiles above and below the stratospheric aerosol layer.
resolution). Thepstandard deviation of the mean was therefore 0.16∕ 23 0.034. To find the effect of this uncertainty on IABS, the scale factor was changed by trial-and-error until the mean was equal to 1.034, and the new value of IABS was noted. It was 5% higher than the value listed in Table 7. The statistical scatter in the data on the other dates was similar, and the standard deviation of the 1574 nm D46
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IABS values was therefore taken to be 5% for all three episodes. The 523.5 nm data were much noisier, as shown in Figs. 8–10, and the standard deviation of 523.5 nm IABS results was estimated to be 30%. The values of IABS at 523.5 nm listed in Table 6 are comparable to those reported by other researchers for mid-visible lidars. For example,
Fig. 9. Scattering ratios (the ratio of the total backscatter coefficient to the molecular backscatter coefficient) for the two lidar wavelengths and three measurement episodes.
Trickl et al. reported baseline IABS values in the range 2 × 10−5 – 5 × 10−4 sr−1 from 2000 to 2012 at Garmisch-Partenkirchen [3]. Our results with EARL are also consistent with observations at the Mauna Loa observatory [4,5]. As shown in Table 7, the measured wavelength exponent ranged from 1.8 to 2.0. These values agree fairly well with Jäger’s finding that the wavelength dependence reaches “almost λ−2 ” during periods of Table 6.
Date August 4, 2011 September 11, 2011 December 1, 2011
Integrated Aerosol Backscatter
523.5 nm 10−4
sr−1
2.79 × 3.29 × 10−4 sr−1 3.54 × 10−4 sr−1
1574 nm 2.39 × 10−5 sr−1 3.69 × 10−5 sr−1 3.06 × 10−5 sr−1
low backscatter, which was based on results from four visible-light and near infrared lidar wavelengths [17]. Our estimate of the rms uncertainty in the color ratios due to IABS error is 30% (again using the methods in [16]), which causes a correspondingly large uncertainty in the wavelength exponent. For this reason, differences from one date to another are probably not significant, and so the average value of 1.9 should be used, with an uncertainty of 0.2. Other observers of the Nabro volcano aerosol reported their results in terms of aerosol optical depth (AOD) and suggested a lidar ratio of 50 sr in the mid-visible [7]. Multiplying our 523.5 nm IABS values in Table 6 by 50 sr, we find AODs ranging from 0.014 to 0.018. These values agree well with the average of 0.018 0.009 reported in [7]. 1 June 2014 / Vol. 53, No. 16 / APPLIED OPTICS
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Table 7.
IABS Values (15–19 km) and Wavelength Exponents
Date
IABS 523.5 nm
IABS 1574 nm
Color Ratio 532.5/1574
Wavelength Exponent
August 4, 2011 September 11, 2011 December 1, 2011
1.71 × 10−4 sr−1 1.96 × 10−4 sr−1 1.58 × 10−4 sr−1
1.94 × 10−5 sr−1 2.73 × 10−5 sr−1 1.89 × 10−5 sr−1
8.8 2.7 7.2 2.2 8.4 2.5
2.0 0.3 1.8 0.3 1.9 0.3
The 523.5 nm measurements were funded by NSF Grant No. DUE-0836997 “Enhancing Science Courses and Laboratories at a Women’s College using LIDAR,” and the 1574 nm lidar system development was funded under a subcontract from Bennett Aerospace, Inc. on NOAA SBIR project WC133R-09CN-0113. References
Fig. 10. Aerosol backscatter coefficient profile at 1574 nm (solid line) and the corresponding 523.5 nm profile (dashed line) scaled to match.
7. Conclusions
We observed the stratospheric aerosol layer at 34° north latitude on three occasions in 2011 with a photon-counting 1574 nm lidar and a nearby 523.5 nm micropulse lidar. The SNR in the resulting lidar profiles illustrated the impressive performance that can be obtained by employing photon-counting technology in the shortwave infrared region, and it also showed that the eye safe micropulse lidar known as EARL is marginally able to monitor the stratospheric aerosol layer during the current time of low-level volcanic activity and that EARL misses the uppermost part of the layer due to insufficient SNR at the higher altitudes. By using National Weather Service upper air data, we analyzed the lidar data to find scattering ratio profiles and the integrated aerosol backscatter at both wavelengths and then calculated the wavelength exponent for lidar backscattering from the stratospheric aerosols. The visible-light integrated backscatter values of the layer were 2.8–3.5 × 10−4 sr−1 and the AODs were 0.014–0.018, which are consistent with other reported values, and the average wavelength exponent for the three occasions was 1.9 0.2. To our knowledge, no other lidar observations of the stratospheric aerosols have been reported in the 1.6 μm wavelength region. D48
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