Atmospheric aerosol profiling with a bistatic imaging lidar system

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<ul><li><p>Atmospheric aerosol profiling with a bistatic imaging lidarsystem</p><p>John E. Barnes, N. C. Parikh Sharma, and Trevor B. Kaplan</p><p>Atmospheric aerosols have been profiled using a simple, imaging, bistatic lidar system.A vertical laser beamis imaged onto a charge-coupled-device camera from the ground to the zenith with a wide-angle lens(CLidar). The altitudes are derived geometrically from the position of the camera and laser with submeterresolution near the ground. The system requires no overlap correction needed in monostatic lidar systemsand needs a much smaller dynamic range. Nighttime measurements of both molecular and aerosol scat-tering were made at Mauna Loa Observatory. The CLidar aerosol total scatter compares very well with anephelometer measuring at 10 m above the ground. The results build on earlier work that compared purelymolecular scattered light to theory, and detail instrument improvements. 2007 Optical Society ofAmerica</p><p>OCIS codes: 010.1100, 010.1110, 010.1120, 010.3640, 040.1520, 010.1310.</p><p>1. Introduction</p><p>Mapping the aerosol structure of the atmosphere overboth time and altitude provides important informationfor a host of atmospheric science research fields. Aero-sols play a role in climate change1 both directly, byaffecting Earths radiation balance, and indirectly, byaffecting cloud properties. Understanding their contri-bution to climate change will require active aerosolprofiling information.2 Both natural and manmadeaerosols impact local and regional air quality, leadingto human health and public policy implications. Aero-sols may be used as tracers for air mass transport andatmospheric dynamics studies. Aerosols also play arole in local precipitation patterns. Many of the effectsof aerosols depend strongly on aerosol properties thatare highly variable with altitude including aerosoltypes, chemistry, and concentrations. Thus aerosolprofiling fills an important need in the atmosphericscience community.</p><p>Aerosol distribution in the atmosphere is nonuni-form, with most aerosols occurring in the first fewkilometers of the atmosphere. This low-altitudeaerosol-rich region is typically colocated with theboundary layer. Measurements of boundary layeraerosols, while highly important, are often technicallychallenging. In situ aerosol measurements may bemade at a variety of altitudes by plane or balloon-borne instruments. Such measurements are oftenable to provide more specific aerosol information(e.g., chemical composition) than remote-sensing tech-niques. They are, however, expensive and typicallyprovide only a few profiles. Thus theymay notmap thetemporal evolution of the aerosol structure. Passive,ground-based, remote-sensing aerosol measurementsare often conducted with radiometers, which provideintegrated aerosol optical depth over the entire atmo-sphere. Advantages of these types of measurementsinclude their relatively low cost and their ability to berun continuously. A primary disadvantage is the in-ability to generate the altitude-dependent aerosol pro-file.Lidar remote sensing of aerosol profiles offers capa-</p><p>bilities for altitude and time-series monitoring of aero-sol conditions. The most frequently used lidar systemsfor this purpose are elastic backscatter lidars, whichemploy pulsed laser transmitters and collect backscat-tered light with a colocated time-binned detector.Often elastic lidar is used in conjunction with otherinstruments3 (e.g., Sun photometers) as one parameteris measured and two parameters (extinction and back-scatter) are to be determined. An alternate method4</p><p>J. E. Barnes (John.E.Barnes@noaa.gov) and T. B. Kaplan arewith the NOAAEarth System Research LaboratoryGlobal Mon-itoring Division, Mauna Loa Observatory, Hawaii, 1437 KilaueaAvenue, Hilo, Hawaii 96720. N. C. Parikh Sharma is with theDepartment of Physics and Earth Sciences, Central ConnecticutState University, 1615 Stanley Street, New Britain, Connecticut06050, USA.Received 19 July 2006; revised 7 November 2006; accepted 11</p><p>December 2006; posted 22 January 2007 (Doc. ID 73181); pub-lished 1 May 2007.0003-6935/07/152922-08$15.00/0 2007 Optical Society of America</p><p>2922 APPLIED OPTICS Vol. 46, No. 15 20 May 2007</p></li><li><p>uses the Raman scattered signal to get extinction.Aerosol scatter has also been measured with multipleor movable receivers in conjunction with lidar andused to derive aerosol properties under well-mixedconditions.57 Challenges faced by typical lidar instru-ments include possible afterpulse effects8 (which couldlead to detector saturation at the lowest altitudes),large dynamic-range requirements,8 and field-of-viewoverlap effects9,10 (in which the illuminated volume isnot entirely within the detector field of view at near-instrument ranges). Frequently data from elasticbackscatter lidars are not used within the overlaprange, which is precisely the near-ground range ofinterest in many cases. There are monitoring pro-grams using 500 m towers11 that could benefit fromwell-calibrated aerosol measurements throughoutthis region.An alternative bistatic lidar instrument has been</p><p>developed and tested for profiling a boundary-layeraerosol.12 The instrument employs the bistatic con-figuration in which the laser transmitter and thedetector are spatially separated. The new instru-ment, calledCLidar, uses aCCD camera as its detectorand determines scattering altitudes from geometryrather than timing considerations. Compared withtraditional monostatic lidar, the new approach hassubstantially less stringent dynamic-range require-ments.12,13 Integration of the signal is performed onthe camera and data acquisition is the camera-supplied control electronics. Afterpulse saturation ef-fects are eliminated. The new instrument also providesuseful data from where the beam is first imaged withexcellent near-ground altitude resolution. As with allbistatic systems, the scattering angle changes withaltitude. Aerosol properties may be derived from bi-static signals given additionalmeasurements or appro-priate assumptions.</p><p>2. Technique</p><p>The CCD camera-based lidar (CLidar) system imagesa vertical pulsed laser beam onto a CCD using wide-angle optics. The choice of wide-angle optics eliminatesthe need for scanning to cover the entire altituderange. This system operates only at night. The first-generation CLidar system, its altitude resolution, anddynamic-range requirements, are described in detailin Barnes et al.12 A second systemhas been designed toenhance the signal-to-noise ratio. A schematic of thegeometry is shown in Fig. 1.The CCD camera and optics are located a distance</p><p>D away from the laser. The laser transmits light withenergy EL. Here EL represents the total laser energysummed over the image exposure time. The lightscatters off air molecules, aerosols andor cloud par-ticles, and the portion of the scatter, which is in thedirection of the scattering angle and is at the po-larization angle , is collected by the wide-angle op-tics and imaged onto a CCD. The polarization angle isdetermined by the plane of polarization of the laserand the location of the camera.12 For each pixel in theresulting image, the altitude may be calculated frommeasurements of camera location in relation to the</p><p>laser and from the field of view per pixel. The inten-sity of the scattering is given by the recorded counts.The CLidar signal, Er, is made up of the photonsmeasured by a pixel with field of view d, from ascattering event that took place at altitude z andrange R from the camera.12 The analysis here as-sumes that d is constant, which requires the opticsto accurately (linearly in angle) image the laser beam.The signal Er is given as</p><p>ErKELATatm zTatm R, , zdzR2, (1)</p><p>where dz is given by</p><p>dz R2Dtand21 zDtand2 R cosd2 zRDsind2DR sind2 1.</p><p>(2)</p><p>Here K is a calibration constant representing thesystem optical efficiency and A is the effective collect-ing area of the optics, Tatm z and Tatm R are the total(aerosol and molecular) atmospheric transmittancefrom the laser to altitude z and from altitude z alongthe slant path R to the CCD detector, respectively,and , , z is the scattering coefficient. In practice,dzR2 in Eq. (1) is very nearly equal to dD, whichmeans that Eq. (1) does not have the 1R2 depen-dence of the familiar lidar equation.12,13 The altitudeimaged in each pixel grows with height in such a wayas to almost cancel out the typical R2 dependence oftraditional backscatter lidar signals, resulting in lowdynamic-range requirements. This also leads to analtitude resolution, which is excellent in the near-ground region for which the instrument is designed,but which degrades at higher altitudes.It is often desirable to use the aerosol backscatter</p><p>signal from a lidar quantitatively, for example, tocalculate extinction (total scatter plus absorption) oraerosol optical depth. This requires a relation betweenthe light scattered at a single angle and total scatter(extinction-to-backscatter ratio in monostatic lidar),which must be either calculated from assumed aerosolparameters or related to additional measurements.</p><p>Fig. 1. CLidar system schematic.</p><p>20 May 2007 Vol. 46, No. 15 APPLIED OPTICS 2923</p></li><li><p>This can lead to large errors in the quantitativeparameters since the angular dependence of aero-sol scatter, the aerosol phase function, varies widely,especially in the 90180 range. In Fig. 4 of Ansmannet al.,2 for example, the extinction-to-backscatter ratiovaried by a factor of 3 in the boundary layer measure-ments of extinction. The same can be true for somesatellite retrievals of aerosols that rely on single-anglescatter. Kalashnikova and Sokolik14 used microscopydata and discrete-dipole approximation calculations toestimate the phase functions for mixtures of mineraldust characterized by particular combinations of com-position, shape, and size. This study showed that aero-sol optical depth retrievals would be overestimated byforward-looking, ground-based measurements by asmuch as 1.5 times and underestimated by techniquesbased on backscatter measurements (such as lidarsand satellites) by 2 times if spherical particle in-stead of irregular shapes are assumed in the re-trieval.The CLidar technique does require an assumption of</p><p>an extinction-to-single scatter ratio for a range of back-scatter angles (90180) when calculating quantitiessuch as aerosol optical depth, rather than just oneangle as in monostatic lidar. But given the alreadylarge uncertainty in this ratio, little additional un-certainty is added in the CLidar method. And sincescattered light is measured accurately at the groundlevel, in situ aerosol measurements of absorption andtotal scatter can be used to directly measure the ex-tinction to single-angle-scatter ratio for at least theone location.</p><p>3. Instruments</p><p>Two CLidar instruments have been constructed. Bothused a linearly polarized Spectra-Physics Nd:YAGmodel GCR-6 with 300600 mJpulse operating at awavelength of 532 nm, a pulse width of 8 ns, and apulse repetition rate of 30Hz. Both the first-generationand the second-generation CLidar instruments utilizea 10 nm full width at half-height (FWHH) laser linefilter between the camera and the lens to filter outbackground light.Detailed specifications on the first- and second-</p><p>generation systems are provided in Table 1. For sys-tem 1 the effective optical diameter is 1.984 0.50mmpixel. This diameter was misprinted in Barneset al.12 The second-generation CLidar instrumentemploys an Apogee Alta camera model E260. Thesensor employs transparent gate technology, whicheliminates interpixel dead space where image infor-mation may be lost and also many fewer high-noisepixels. An undesirable side effect of these gates, how-ever, is that the two gate materials do not have equaltransparencies, which gives rise to intensity oscilla-tions for objects focused to the size of a pixel or less.15Thus practical considerations require that the imagebe slightly defocused to eliminate this effect. This hasthe unfortunate consequence of introducing addi-tional noise as wider beam images includemore back-ground noise due to the larger number of pixelssummed. The Coastal Optics lens is expected to raise</p><p>the second-generation system signal several timesthat of the first-generation system.The second-generation CLidar system also incorpo-</p><p>rates a chopper and custom-designed holder to reducebackground light. Since background light is collectedfor the duration of the image exposure time (typicallya few minutes), while the laser pulses for only a smallfraction of that time, the sky background may be sig-nificantly reduced by blocking the CCD sensor duringtimes in between pulses. The optomechanical chopperis a Thor Labs model MC1000 with phase-adjustmentcapabilities. The standard 10 slot blade was modifiedto leave only two openings. A housing wasmachined toallow the chopper to be placed between the lens andthe interference filter. The chopper is synchronized toa transistortransistor logic (TTL) trigger from the la-ser flashlamp. A cable connection was used in thesemeasurements, but a wireless connection would bemore versatile. The phase of the chopper is adjustedrelative to the trigger to ensure the CCD chip is unob-structed for the duration of the pulse travel time (ap-proximately 1 ms). From a practical standpoint thismay be accomplished by adjusting the phase offsetuntil the laser-beam image brightness is constantwhen the beam is imaged with either edge of the lens.The chopper reduced the sky background by approx-imately a factor of 5. This is especially important formeasurements in urban areas where sky back-grounds may be large. Greater reductions may bepossible depending on the pulse-repetition fre-quency of the laser.</p><p>4. Image Analysis</p><p>In general, the image of the beam is not aligned per-fectly with a column or a row of the CCD. In Barneset al.12 the beam was aligned close enough to use asimple summation of beam pixels and subtraction ofbackground. In fact, it can be useful to align the beamdiagonally on the CCD, which allows a larger image.</p><p>Table 1. SBIG ST-237 and Apogee Alta Camera Specifications</p><p>Feature, Specification ST-237 (1) Apogee E260 (2)</p><p>Active area (mm) 4.86 3.66 10.2 10.2Pixels 657 495 512 512Pixel size (m) 7.4 7.4 20 20Read noise (countspixel) 14 19.5Gain 1.6Dark (electronsspixel) 5 at 0 C 0.72 at 22.3 CFull well capacity(electrons)</p><p>20,000 102,000</p><p>Analog-to-digital converter(bits)</p><p>16 16</p><p>Pixel digitization rate (kHz) 30 1300Bias (counts) 100 1578CCD sensor TI TC237 KAF-0261E-1Quantum efficiency at532 nm</p><p>47% 46%</p><p>532 nm interference filterFWHH (nm)</p><p>10 10</p><p>Wide-angle lens Kinoptic, f4 Coastal Optics DigitalSLR, f5.2</p><p>Lens focal length (mm) 1.98 4.88Effective optical diameter(mmpix)</p><p>0.50 0.94</p><p>Angle per pixel (degpix) 0.194 0.171</p><p>2924 APPLIED OPTICS Vol. 46, No. 15 20 May 2007</p></li><li><p>In this study, a more general, versatile technique isused.To analyze an image, the best fit to the beam is</p><p>calculated based on image intensities. A second-degreepolynomial was found to work well enough. Thebeam is then divided into equal, pixel length seg-me...</p></li></ul>

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