Scanning tropospheric ozone and aerosol lidar with double-gated photomultipliers

Scanning tropospheric ozone and aerosol lidar withdouble-gated photomultipliers

Janet L. Machol,1 Richard D. Marchbanks,1,* Christoph J. Senff,1 Brandi J. McCarty,1

Wynn L. Eberhard,2 William A. Brewer,2 Ronald A. Richter,2 Raul J. Alvarez, II,2

Daniel C. Law,2 Ann M. Weickmann,2 and Scott P. Sandberg2

1Cooperative Institute for Research in the Environmental Sciences, University of Colorado andNational Oceanic and Atmospheric Administration (NOAA) Earth System Research

Laboratory (ESRL), 325 Broadway, Boulder, Colorado 80305, USA2NOAA ESRL, 325 Broadway, Boulder, Colorado 80305, USA

*Corresponding author: [email protected]

Received 30 July 2008; revised 11 December 2008; accepted 15 December 2008;posted 15 December 2008 (Doc. ID 99627); published 14 January 2009

The Ozone Profiling Atmospheric Lidar is a scanning four-wavelength ultraviolet differential absorptionlidar that measures tropospheric ozone and aerosols. Derived profiles from the lidar data include ozoneconcentration, aerosol extinction, and calibrated aerosol backscatter. Aerosol calibrations assume a clearair region aloft. Other products include cloud base heights, aerosol layer heights, and scans of particulateplumes from aircraft. The aerosol data range from 280m to 12km with 5m range resolution, while theozone data ranges from 280m to about 1:2km with 100m resolution. In horizontally homogeneous atmo-spheres, data from multiple-elevation angles is combined to reduce the minimum altitude of the aerosoland ozone profiles to about 20m. The lidar design, the characterization of the photomultiplier tubes,ozone and aerosol analysis techniques, and sample data are described. Also discussed is a double-gatingtechnique to shorten the gated turn-on time of the photomultiplier tubes, and thereby reduce the detec-tion of background light and the outgoing laser pulse. © 2009 Optical Society of America

OCIS codes: 010.3640, 010.4950, 040.5250, 280.1100, 280.1120, 280.1910.

1. Introduction

An understanding of the vertical distribution ofozone as provided by a lidar can be used to improveair quality forecast model accuracies, help reveal thecauses of high ozone and ozone depletion events, andaid in the development of strategies to mitigateozone pollution. A number of differential absorptionlidars (DIALs) have been built to measure tropo-spheric ozone and aerosols (e.g., see [1–10]) includingseveral airborne systems [11–14]. The Ozone Profil-ing Atmospheric Lidar (OPAL), developed at theNational Oceanic and Atmospheric Administration(NOAA), is a ground-based DIAL that emits ultravio-let (uv) laser pulses at four wavelengths to remotely

sense tropospheric ozone and aerosols. Two uniqueaspects of OPAL are the use of multiple-elevation-angle data to obtain a lower effective minimumheight for ozone DIAL measurements, and the useof double-gated photomultiplier tubes (PMTs) to re-duce detection of system laser scatter by decreasingthe detectors’ rise times.

A low minimum range is needed both to study sur-face layer processes and to help link surface-basedin situ measurements to those at higher altitudes.When OPAL points at one angle, the minimum rangedue to the transmit/receive overlap function is about280m for each beam. However, when a horizontallyhomogeneous atmosphere can be assumed, the lidarreturns from several different elevation angles canbe combined to produce profiles that range in heightfrom about 20 to about 1200m for ozone, and 15m to12km in height for uncalibrated aerosol backscatter

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512 APPLIED OPTICS / Vol. 48, No. 3 / 20 January 2009

profiles at 355nm. The upper range limits are deter-mined by the magnitude of the atmospheric absorp-tion of the laser line for ozone data and the file sizefor the aerosol data.When OPAL is deployed at sea, the lidar window

frequently becomes coated with salt that backscat-ters some of the outgoing laser light. Both becausethe beams are scanned, and two of the beams are al-most coaxial, it is difficult to physically block thisbackscatter. The PMTs must be gated off duringtransmission of the laser pulses to prevent this back-scatter from damaging the detectors or distorting thePMT response to the atmospheric return. A techni-que to pulse the PMT gates on twice for each laserpulse decreases the turn-on times of the PMTs. Thus,the PMTs can be turned back on faster after the laserpulse, which thereby reduces the minimum ranges ofthe atmospheric lidar returns.OPAL’s uncalibrated aerosol backscatter profiles

provide cloud base heights, mixed-layer heights, anda qualitative picture of any aerosol layers in the at-mosphere. Processed aerosol data include aerosol ex-tinction and calibrated aerosol backscatter profiles.Such profiles are valuable for research on aerosol di-rect [15,16] and indirect [17] effects on climate. Mea-surements of the geometry of exhaust plumes behindcommercial jet aircraft during takeoff were con-ducted because earlier air quality models assumeda passive point source for the engine emission [18]and were overpredicting surface concentrations ofengine emissions. In conjunction with the multiangleretrievals, a new simple slant-path method for theretrieval of aerosol extinction profiles was implemen-ted for situations when the system calibration ischanging and unknown.Since OPAL was first built in 1993 [19–21], the li-

dar has been deployed in numerous air quality fieldmissions including three times aboard a ship. Pastfield campaigns include four pollution experimentsin California [22,23], the 1999 Southern OxidantsStudies in Nashville and Atlanta [24], the 2000Texas Air Quality Study (TexAQS) in Houston, threeaircraft plume studies [25–27], the 2002 NewEngland Air Quality Study (NEAQS 2002) [28–30],NEAQS 2004, and most recently, TexAQS II in 2006.This paper provides the first open-literature instru-ment description of OPAL. The following sections de-scribe the lidar design, photomultiplier tube (PMT)optimization, the ozone and aerosol analysis techni-ques, and several example applications.

2. System Design

OPAL’s laser transmitter emits simultaneous pulsesat four uv wavelengths: 266, 289, 299, and 355nm.Aerosol measurements utilize the 355nm wave-length which has negligible absorption by ozone,while ozone concentrations are determined by apply-ing the DIAL technique to lidar returns from a pair ofwavelengths. The optimum wavelength pair for par-ticular atmospheric conditions is that which pro-duces ozone measurements with the highest signal

to noise ratio (SNR) over the desired range, and de-pends on the power, stability, and absorption crosssection of each wavelength. The receiver uses a sepa-rate gated PMT for each wavelength. Typical lidaroperating parameters are shown in Table 1. OPALis housed in a modified ISO freight container.

A. Transmitter

The four OPAL wavelengths are produced by sumfrequency generation and Raman conversion of lightfrom a 10Hz Continuum Surelite II Nd:YAG laserwith a 10ns full width at half-maximum pulselength. Following doubling and quadrupling ele-ments, the laser pulse energy is typically 25mJ at266nm. A beam splitter separates the wavelengthsinto two paths (Fig. 1). Along one path, the 266nmlight passes through a Raman cell in which the 289and 299nm DIAL wavelengths are generated [31].Along the other path, the 532 and 1064nm wave-lengths are combined in a β-barium borate (BBO)crystal to produce the 355nm light used as the aero-sol channel. To avoid optics losses from broadbandreflective mirrors, the high power laser beams aresteered with uv-fused silica (Corning 7980) prismswith antireflection coatings; this high-purity glassis less likely to produce color centers than other fusedsilica glasses.

The Raman shifting from 266:04nm light is per-formed in a 1m long high-pressure cell where thefirst order Stokes shifts of hydrogen (H2) and deuter-ium (D2) are used to produce light at 299.1 and289:0nm [32,33]. Although other Raman-shiftinggases that produce other wavelength combinationscould have been used (e.g., see [10,31,34,35]), this de-sign was chosen for the following three reasons. The289=299nm wavelength pair is nearly ideal for DIALmeasurements in moderate to high ozone with theminimum aerosol sensitivity [36]. The use of the sin-gle cell with two Raman-active gases instead of twocells each with a different gas has the advantage thatthe light at both generated wavelengths and at

Table 1. Typical Operating Parameters for OPAL

Specification

Parameter Aerosol Ozone

Minimum range withoutscanning (m)

280 280

Minimum height withscanning (m)

15 20

Maximum range (km) 12 1.2Range resolution (m) 5 100Temporal resolution (min) 3 20O3 accuracy (ppbv) – 10Wavelengths (nm) 355 266, 289, 299, 355Pulse repetition frequency (Hz) 10 10Pulse energies (near/far) (mJ) 0:5=7:5 2:5=4:7a

Pulse energy for plumestudies (mJ)

8 –

Divergences of transmit beams 200 μrad 200 μradaSummed pulse energies for 266–299nm.

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266nm are all colinear and have the same polariza-tion. Finally, the conversion efficiency ofH2 andD2 isbetter than that of CO2 [10], permitting OPAL to usean available Nd:YAG pump laser with sixfold lowerenergy than would be required for CO2 conversion.The H2 and D2 are buffered in argon (Ar) [31] to

increase the cell pressure and thereby raise thethreshold for generating higher-order Stokes modes.The cross-Stokes, anti-Stokes and other higher-orderStokes lines are blocked by narrowband filters in thereceiver. Using the results of laboratory experiments,the gas pressures and ratios in the Raman cell had tobe carefully balanced since small deviations result inunstable power and/or changes in the energy ratiosfor the various wavelengths. The gas mixture in theRaman cell has partial pressures of 300psi for D2,80psi forH2, and 110psi for Ar. To simplify the setupfor field experiments, a premixed gas mix was pur-chased with a specified error for the partial pressureratios for D2 and H2 of <3%. The lidar is typicallyoperated with 15mJ pulses of 266nm light intothe Raman cell, and resulting output pulse energiesof 5.6, 2.4, and 2:2mJ at 266, 289 and 299nm, respec-tively. The 266nm beam is focused into the Ramancell for the wavelength conversion with a 1m focal-length lens. The three wavelengths exiting the cellpass through a 3× achromatic beam expander, whichconsists of three commercially-available uv lensesand which enlarges the beam diameter to 25mm.Along the other path after the first beam splitter,

the 532 and 1064nm light are combined in a type IIBBO crystal to produce 8mJ pulses of 355nm lightvia sum frequency generation. Following the crystal,residual 532 and 1064nm light is removed with di-chroic mirrors, and a commercial 3× beam expanderincreases the 355nm beam diameter to 21mm. The

transmitted energy of the 355nm beam is monitoredto help calibrate the aerosol backscatter measure-ments. For aerosol-only measurements at 355nm,all light is directed along this path.

Before transmission into the atmosphere, the266–299 and the 355nm beams are each split intonear- and far-field beams. The near- and far-fieldenergies are approximately 2.5 and 4:7mJ for the266–299nm beams, and 0.6 and 7:5mJ for the355nm beams. The 266–299 and 355nm far-fieldbeams are transmitted separately at distances of18 and 20 cm from the telescope axis, respectively,while both near-field beams are transmitted within1 cm of the telescope axis. These spacings result inthe lidar achieving full overlap of the transmit andreceiver fields of view at ranges of approximately280m for the near-field beam and 700m for thefar-field beam. During lidar operation, a shutter al-ternately blocks either the near- or the far-fieldbeams so that they are transmitted in sequentialblocks of laser pulses. The outgoing laser beams havedivergences of 200 μrad and beam pointing stabilityof approximately 25 μrad. The transmit laser beamsare linearly polarized; there is no polarization discri-mination in the receiver.

The initial alignment of the transmit beams to thereceiver is done with retro-reflectors placed abovethe telescope. During operation, alignment uses theatmospheric returns from a horizontal transmitbeam. The pointing of the transmit beams is adjustedto maximize the linearity and SNR of the logarithmof the range-corrected lidar returns as observed on areal-time display.

The lidar beams can be scanned in elevation from−2° to 22° and can also be pointed vertically. The li-dar does not scan azimuthally. The scanner resolu-tion is about 200 μrad. If the atmosphere can beassumed to be horizontally homogeneous, verticalozone or aerosol profiles can be extrapolated fromeach of several different angles (by projecting onto90°), and then pieced together to produce an ex-tended vertical profile. Typically, the combination ofthe profiles from four angles (2, 6, 20 and 90°) yields anet minimum height of about 15m above the surface.The range resolution is 5m for a beam at any angle.When the shallower angles are extrapolated to a ver-tical profile, the resolution is compressed. Followingthis compression, the data are averaged into 5m gridpoints, so that the final resolution is 5m for the en-tire vertical profile. This altitude blending has so faronly been applied to shipborne OPALmeasurements,where it is safe to assume horizontal homogeneity ina less than 2km range around the ship, except forsome measurements taken close to shore. Combinedmultiangle measurements have been reported foruse with Raman lidars [37,38]. As discussed in Sub-section 4.D, to measure aircraft plumes the lidar isalso operated in a continuous scan mode with justthe 355nm beam. The eye-safe distance from OPALdepends on the specifics of the scanning mode and

Fig. 1. Schematic of OPAL. The wavelengths of the beams areshown in nm. Elements include the laser doubler (2×) and quad-rupler (4×), the BBO crystal (BBO), beam expanders (BEs), and asolar blind filter (SB). The receiver uses dichroic and narrowbandfilters to separate the wavelengths in the return signal. Mirrorsare black bars, and filters and beam splitters are gray. The finaltransmit/receive mirror scans in elevation. Shutters to alternatelyblock the near- and far-field beams are not shown.


wavelengths used. For a 10 s exposure, the eye-safedistances are 30m at 355nm and 800m at 266nm.The angle limitations are due to the locations of

the windows and the scanning mirror. The lidar hasa side window with a vertical surface through whichpass the lower angle beams, and a separate upperwindow set at Brewster’s angle to the vertical beam.Both windows are made of 12mm thick uv-gradefused silica, and the inside face of the side windowhas a broadband antireflection coating with wide in-cident-angle coverage. The scanner mirror substrate(0:4m× 0:4m × 0:05m) was cast with a honeycombback (Wangsness Optics) and weighs only 10:6kg.It is coated with enhanced uv aluminum and is onlyused with the lower angle beams which pass throughthe side window. The scanner rotation axis is sub-stantially offset in the plane of the mirror, so thatwhen the mirror is rotated to the vertical plane, thevertical transmit/receive beams bypass it. The eye-safe distance from OPAL depends on the specifics ofthe scanning mode and wavelengths used. For a 20 sexposure, the length of time the lidar points at oneangle when scanning between discrete angles, theeye-safe distances are 75m at 355nm and 1450mat 266nm.The scanner is based on a LabVIEW-controlled

200 steps/revolution stepping motor with a 100∶1harmonic gear reduction unit. The mirror positionis registered with a 15 bit encoder. For shipboardmeasurements, a GPS-based motion compensationsystem is used to correct the transceiver pointingin real time with a resultant angular error of ap-proximately �0:5° relative to the Earth referenceframe [39]. For a typical case of data taken at 2, 6,20 and 90°, and combined at altitudes of 30, 120and 300m, the vertical errors due to a 0:5° pointingerror are 5, 10, and 7m, respectively.

B. Receiver

The telescope is an off-axis Newtonian with a 0:2mdiameter, a 0:75m focal length, and uv-enhanced alu-minum mirrors. Following a 1:13mm diameter aper-ture which sets the field of view of the lidar to1:5mrad, the receiver separates and filters the fourlidar wavelengths. The 355nm path has a separate1nm wide bandpass filter with 40% transmission. Inthe ultraviolet receiver path, a solar blind filterblocks the sky background at visible wavelengths buthas 70% transmission for the 266–299nm wave-lengths. Each of these three wavelengths is then ex-tracted in a cascading sievelike scheme whereby adichroic filter in front of each detector transmitsthe nonreflected light to the next detector. A seconddichroic or narrowband filter before each PMT en-hances the filtering.The OPAL receiver has four gated PMT detectors,

one for each wavelength. The PMTs are operated atnegative high voltage (1200–1800V). Since OPALhas a low duty cycle (10Hz pulse rate) and PMTsare average-current limited, the gating of the PMTsreduces the impact of background light. The PMT

gates are pulsed on twice for each laser pulse (doublegated) to decrease the PMT turn-on time and preventdamage due to backscatter off of optics from the out-going laser pulse (Subsection 3.C). As the lidar cyclesbetween angles and near and far beams, the PMTvoltages and the amplifier gains are switched by thecomputer to values preset by the operator. The char-acterization, optimization, and gating of the PMTsare described in more detail in Section 3.

Low-noise, voltage-controlled variable gain ampli-fiers (VGAs; Analog Devices model AD8337) aremounted close to the PMTs to minimize noise pickup,and are followed by 5× amplifiers that serve as out-put drivers. The VGAs permit rapid (<1 μs) adjust-ment of the gain (þ8 to þ32dB) while providing highlinearity, fast saturation recovery, and largesignal bandwidth (100MHz). Since the DC-coupledVGAs have a gain-dependent offset, a voltage offsetis added to the amplified signals to keep them at po-sitive voltages for all gain settings.

The lidar is typically run in one of two vertically-scanning modes: step stare or vertical sweep. In thestep-stare mode, the lidar cycles through severalpreset angles. At each angle the data-acquisition se-quence consists of 1 s of electromagnetic interference(EMI) data with the telescope blocked by a shutter,followed by 10 s of near-field data, another 1 s ofEMI data, and then 10 s of far-field data. The laser-produced EMI signal is typically about 2% of the at-mospheric signal but can be as large as 10%. ThePMT voltages and amplifier gains are adjusted eachtime the receiver switches from between near and farfields. For the vertical-sweep mode, used only forsingle-channel aerosol plume measurements, thescans are continuous and 2 s of EMI data are takenevery few minutes. In both cases, data bins have a5m range (33ns), and for each laser pulse, 102 binsof prepulse background data and 1200 bins of lidardata are recorded. For the 355nm channel, 2800 binsof lidar data are acquired. The amplified PMT sig-nals pass through 50 ohm, 22MHz, DC low pass fil-ters and are collected with 30MHz sampling rate,12 bit digitizer cards.

An alternate layout in OPAL is sometimes used foraerosol-only studies. In this design, all of the 355nmlight is transmitted in the coaxial beam, and it hasthe advantage that the optics and alignment are sim-pler. Near and far-field data are generated by alter-nating between low and high gain on the PMTamplifier. In the far-field/high gain mode, the digiti-zer may saturate at the lower range gates, butprovides a linear signal with more resolution atthe farther ranges. It is key to saturate the digitizerwith the amplifier rather than the PMT signal levelto not damage the PMT. In this mode it is also impor-tant to prevent strong EMI from impacting the signalprior to the amplifier.

3. Photomultiplier Tubes

Lidars often incorporate PMTs as detectors becauseof their large dynamic range, high sensitivity, and


commercial availability. Generally, though, the use ofPMTs in lidars requires careful choice of model, in-dividual tubes, and circuit design due to issues suchas noise characteristics and variation between indi-vidual tubes. In 2005, new PMTs were selected to up-grade OPAL and for use in NOAA’s new 1kHz pulserate airborne DIAL, the Tunable Optical Profiler forAerosol and oZone (TOPAZ) [14]. OPAL’s originalPMTs exhibited considerable signal induced bias(SIB), a signal-dependent offset with a decay lastinghundreds of microseconds [40], which caused signif-icant data processing challenges and reduced thedata accuracy.Based on their past performance and desirable

characteristics, Hamamatsu model R2076 andR3479 PMTs were selected for the NOAA ozoneDIALS. In the 1990s, R2076 PMTs were studied [41]for use in the Airborne Ozone DIAL (ABDIAL) [12].These PMTs performed well in ABDIAL for 15 yearswithout replacement. The R3479 tubes are identicalto the R2076 except that they use different windowmaterials and are sensitive over a slightly differentwavelength range. These detectors are eight-stage,head-on PMTs with bialkali photocathodes, an anodegain of about 1:7 × 106, and a 15mm diameter effec-tive area. Desirable features of the PMTs includedgood uv sensitivity, a fast turn-on time when gated,high linearity, low fluorescence, minimal afterpul-sing and SIB, relatively high gain, and a large dy-namic range. As bialkali detectors they have thehighest quantum efficiency (QE) at these wave-lengths, and the head-on design provides excellentspatial uniformity [42]. Also, few stages, high vol-tages, and a small photocathode provide these PMTswith fast response times. The PMTs are run in OPALand TOPAZ in a gated analog mode.

A. Gate Characteristics

There are several benefits to gating PMTs in lidars.Gating reduces the average signal current by only al-lowing current through the PMT during the fractionof time that the gate is on. Since PMTs are average-current limited, gating reduces the total backgroundlight detected, and therefore can increase the dy-namic range available for the lidar return signal(e.g., see [43] and references therein.) This is espe-cially useful in high repetition rate systems like TO-PAZ which have a reduced maximum peak voltagedue to the average-current limits. For 50 μs gatesand 50 ohm termination, the maximum allowed sig-nal voltage is more than 1V at 10Hz but only 100mVat 1kHz. Gating is also useful to block strong scatterfrom the outgoing laser beam.The PMT divider circuit is adapted from that used

in ABDIAL [12,41] and is designed to provide ahighly linear response over the large dynamic rangeof the lidar signals. A tapered resistor chain and acapacitor at each dynode increase the pulsed re-sponse capabilities of the PMT, and an adjustable re-sistor (Radj) in the final stage provides the ability tomaximize the linearity limits for each particular

PMT (Fig. 2). Gating two dynodes (dynodes 2and 5) rather than just one improves signal currentsuppression.

The gate voltage is a key parameter defining gatedPMT behavior. It affects the slope of the response toconstant (cw) light, the gain, and the turn-on time,and so must be chosen as a compromise betweenhow it affects these characteristics. Ideally, the gatedPMT response to cw illumination within the gate hasa fast turn-on time (<1 μs) and then is level. How-ever, the measured signal has a slope which depends

Fig. 2. Divider network optimized for eight-stage HamamatsuR2076 and R3479 PMTs. Gating is applied at dynodes 2 and 5.

Fig. 3. Gated PMT response to cw light as a function of incidentlight level. The PMT signal is flat for lower light levels, but exhi-bits increased slope for higher light levels. These measurementswere made with the high voltage at 1500V and the gate voltageat 85V.


on both the incident light intensity (Fig. 3) and thegate voltage. This slope degrades the DIAL calcula-tions both as an error in the differential signalbetween range gates and in the background determi-nation. The slope can be adjusted with the gate vol-tage, but there is no gate voltage that produces zeroslope for all signal levels. The optimum gate voltageminimizes the slope at the lowest light levels wherethe SNR is most affected by a slope. The optimumgate voltage increases with the PMT high voltage, oc-curs close to the gain maximum, and is unique toeach PMT. To maintain an error of <5% in the ozoneDIAL measurement and assuming that the opticaldepth per range gate is 0.05, the SNR must be >800for the raw signals averaged over the gate and thesampling period. For the error contribution due tothe slope to be below this limit for a particular PMT,the optimum gate voltage must be such that jΔVj=V < 1=SNR (i.e., jΔVj=V < 0:0013) over the durationof a range gate at all signal voltages, where ΔV isthe signal voltage change over a range gate and Vis the signal voltage.

B. PMT Characterization

NOAA obtained 20 new R2076 and R3479 PMTs thathad been specially selected by Hamamatsu for fastturn-on times (<5 μs) and low afterpulses. However,further evaluation of the PMTs at NOAA using high-er incident light levels and gate voltages, determinedthat only six of these tubes were “suitable” for use inthe ozone DIALs. The nonideal PMTs exhibited slowrise times, stair-step cw responses, nonmonotonic ris-ing edges, and/or exceptionally low pulse linearitylimits. The six “suitable” new PMTs, and two R2076PMTs from the ABDIAL system were individuallycharacterized to optimize them for use in ozoneDIALs. Characteristics studied included afterpulses,linearity, rise times, gain behavior, and window fluor-escence. As has been noted in previous PMT studies(e.g., see [41,43,44]), many of the effects of para-meters such as light intensity, PMT high voltage,and gate parameters, are interdependent.The PMT afterpulses all occurred within about

500ns of the initial laser pulse. The afterpulse ratiois defined here as the ratio of the peak height of thelargest afterpulse to the peak height from the laserpulse. At a high voltage setting of 1500V, typicalafterpulse ratios were 1.6% for the new R2076 PMTs,0.1% for the new R3479 PMTs, and 4 and 7% for thetwo older R2076 PMTs. The afterpulses are smallerin the R3479 PMTs either because the Hamamatsuselection process is better for the R3479 PMTs [45],or because atmospheric helium penetrates the R3479“uv glass” window glass more slowly than the R2076“synthetic silica” window glass and helium inside ofthe evacuated PMT tube increases afterpulsing [42].Higher PMT voltages increase the afterpulse ratiosand decrease the delay between the afterpulses,while incident light levels do not significantly affectthe timing or afterpulse ratios. The convolution of theafterpulses with the lidar signal has a negligible ef-

fect on lidar data averaged to 75m (500ns), and asmall smearing effect on lidar data averaged overshorter range gates.

The dynamic range of a PMT depends on its line-arity limit which is the signal voltage beyond whichthe PMT response is nonlinear. The linearity limit fora PMT is the lesser of the average and the peak lin-earity limits. The average linearity limit is definedby a signal current that is 5% of the divider current[45]; for the OPAL, the average linearity voltage limitis 2V for an output impedance of 50Ω. The peak lin-earity limit must be measured, and varies with thehigh voltage and Radj, but is independent of the gatevoltage. For the gated operation with short (100 μs)gates, the peak linearity limit was found to be essen-tially the same for either pulsed or cw illumination,and so only the pulse linearity limit was character-ized for each PMT. The pulse linearity limits werefound by comparing the PMT response to a simulta-neously measured fast photodiode response for vary-ing laser pulse intensities. The pulse linearity limitwas the PMT signal level at which the PMT responsebecame superlinear. (See [41] and references there-in.) For each PMT, Radj was optimized to producethe maximum pulse linearity limit; this optimal va-lue is independent of the high voltage.

The linearity limits for the eight PMTs were testedat three high voltage settings: 1200, 1500, and1800V. At 1200V, the measured linearity limits ran-ged from 0.5 to 1V and only one PMT achieved themaximum tested limit value of 1V. At 1500 and1800V, 50 and 100% of the PMTs achieved a 1V lin-earity limit, respectively. The PMTs were also testedto verify that they satisfied the criterion described inSubsection 3.A regarding the slope of the gain undercw illumination.

We also verified that fluorescence from PMT win-dows will not contaminate lidar data. Measurementswere performed on a sample of the uv glass, since forthe two types of window materials, uv glass is knownto fluoresce more than the synthetic silica. For fusedsilica glasses, the main cause of fluorescence is oxy-gen defect centers that have absorption and fluores-cence maxima near 250nm [46,47]; therefore, theexperiments were performed at 266nm, the OPALwavelength with the most fluorescence. The ratioof the fluorescence from the uv glass to an incident266nm laser pulse energy was 1:3 × 10−7 at 266nm.The uv glass fluorescence caused by atmosphericlaser backscatter is therefore a negligible part ofthe 12 bit (4:1 × 103 counts) dynamic range of the li-dar. Another light source which could cause fluores-cence is laser backscatter off of the lidar optics. InOPAL, gating the PMT off during the laser pulse at-tenuates the instrument backscatter by a factor of10−3 and thereby increases the effective ratio ofthe later fluorescence to the measured backscattersignal to about 10−4, but this fluorescence is stilltoo small to be detected.


C. Use of Double Gate Pulses to Shorten PMT RiseTimes

Short PMT turn-on times are often essential forgated PMTs in lidar receivers. For instance, in OPAL,the PMT must be gated off during the laser pulsetransmission so that light scatter from optics doesnot saturate the detector, and then the PMT must bequickly gated on again to catch the lidar atmosphericreturn. For OPAL, a desirable turn-on time is ≤1 μs,which corresponds to a minimum lidar range of≤150m. The turn-on times of the tested R2076 andR3479 PMTs ranged from<1 μs to over 10 μs. Accord-ing to Hamamatsu [45], slow response times aremost likely due to gain drift caused by excess alkalimaterial on the dynodes, or possibly voltage hyster-esis and interdynode capacitance differences.We found that a technique that could significantly

reduce the turn-on times of the slower PMTs is todouble pulse the PMT gate (Fig. 4). Here the PMTgate is turned on briefly, turned off briefly, and thenturned on a second time. When the gate is doublepulsed, the turn-on/off times for the first pulse arethe same as for a single pulse, but the second pulsehas a greatly reduced turn-on time (e.g., decreasedfrom 10 to <1 μs).Double pulsing the PMT gate is very useful in

OPAL to prevent damage to the PMT from the lightbackscattered off of optics from the outgoing coaxiallaser beam. At sea, the primary source of this back-scatter is salt on the window. Background data is ta-ken during the 14 μs long first gate pulse. The laser isfired 400ns before the end of the following gated-offperiod. The gated-off PMT response is reduced by afactor of 10−3, resulting in only a small (<100mV)spike from the outgoing laser beam. This spike de-fines the zero-range point and is useful for the dataanalysis. The lidar atmospheric return is receivedduring the 93 μs long second gate. The short turn-on time for the second gate precludes it from impact-ing the minimum range of the lidar data.

The gated-off period between the two pulses is keptshort to minimize any background signal offset be-tween the two gate periods, but it must be greaterthan 4 μs to allow the PMT to turn off completely.Any slope in the gain as a function of time (inducedby a nonoptimal gate voltage) continues into the sec-ond pulse. Results of PMT studies by Barrick [48]suggest that an extended first pulse may reduce theslope in the second pulse. Triple pulsing the gate pro-duces no further improvements in the turn-on time.The PMT turn-off time is about 1 μs.

4. Measurement Examples

This section discusses some of the aerosol and ozoneproducts that have been obtained from OPAL. Theaerosol analysis techniques are described first be-cause the discussion of the aerosol correction to theozone data processing depends on some of the samederived variables. Many of the example figures inthis section are for data from the same day from theshipboard TexAQS II deployment.

A. Cloud Base Heights and Mixed-Layer Depth

Cloud detection is usually performed on uncalibratedtotal backscatter data, because detection of clouds isa prerequisite for calibrating the data for aerosol re-trieval. Clouds cause strong backscatter and cloudbase is defined as the lowest range where the signalexceeds a threshold value. This processing providesan auxiliary product of cloud cover and cloud baseheight in campaigns studying ozone and aerosol.

For OPAL data, the mixed-layer depth, especiallyduring convective conditions, is defined as the lowestheight with a distinct gradient in aerosol backscatter[49], chosen as either the height of the maximum de-rivative in backscatter or the maximum responsefrom a Haar wavelet analysis [50]. Either uncali-brated total backscatter or calibrated particulatebackscatter can be used. Human editing is usuallyrequired to avoid incorrect results when multiplegradients are present. The mixed-layer depth is akey factor in air quality.

B. Extinction and Calibrated Backscatter

The derivations of particulate extinction coefficient(αp) and calibrated backscatter (βp) profiles from the355nm lidar data use a common processing method(e.g., see [51]) based on a “clean air” reference layerwhere the uncalibrated backscatter profile indicatesvery little particulate backscatter and an assumedratio, Rp=m, of particulate to molecular backscatterin the layer. Typically, the same reference layer ischosen for an entire measurement day. If that isnot possible due to inhomogeneities in the aerosolstructure different reference layers are chosen forseveral multihour portions of the day. Typical valuesfor the reference scattering ratio Rp=m are 0.05 to0.1. A two-component (molecular and particulate)retrieval [52] is performed from the bottom of the re-ference layer down toward the lidar [53] using an es-timated particulate extinction-to-backscatter ratio,

Fig. 4. PMT signal response to cw light with a double-pulsedPMT gate. The turn-on time from the first gate is the same asif the PMT had only a single gate, but the turn-on time fromthe second gate is decreased. Vertical scale is expanded.


S ¼ αp=βp. The molecular backscatter and extinctioncomponents are calculated from an air density pro-file, which is obtained from either a hydrostatic mod-el or from radiosonde measurements. When cloudsintervene between the lidar and the reference layer,the reference height is moved down to below thecloud, and Rp=m is assumed to retain the value thatwas at this height before the cloud arrived. Addi-tional data can permit determination of S for theprofile retrieval. For example, we have iteratively ad-justed S to bring lidar optical depth to agree withcolumn optical depth measured by a sunphotometerin a manner similar to that reported by Marencoet al. [54].The data from different angles are combined to

extend the βp and αp profiles to heights as smallas 12m above the surface, assuming a horizontallyhomogeneous atmosphere. The two-component re-trieval method is applied to the vertical angle profiledown to close to its minimum range. The lower partbecomes a reference layer for the next lower angle.This stepwise process continues to near the mini-mum range of the lowest angle. The calibrated ex-tinction and backscatter measurements are used toinfer the semiquantitative vertical and temporalvariation of aerosol concentration, to investigatetransport and mixing processes, and to place in situmeasurements of aerosol particles in context of thevertical profile. Cloud base height and mixed-layerdepth [49,50] are also determined from this data.Figure 5 shows an example of calibrated aerosol

backscatter taken during the TexAQS II experimentusing the two-component retrieval method with a ty-pical marine value of S of 30. The profiles show the

growth of the boundary layer up to 1500mduring theday due to solar heating, and the development ofaerosol layers in the evening and night due to thevariation of wind direction with altitude. Changes re-flect both temporal and spatial variations as the shiptraveled between Galveston Bay and the Gulf ofMexico.

C. Slant-Path Methods to Determine Aerosol ExtinctionProfiles

Slant-path methods to determine extinction do notrequire an assumed value for S. A number of multi-angle slant-pathmethods have been demonstrated toobtain extinction coefficient profiles (See Chapter 9of [36] and references. therein). Since the calibrationfactor for the 355nm channel for OPAL varied withboth elevation angle and time, we developed a simpleslant-path method for the retrieval of aerosol extinc-tion profiles that requires no calibration information.

The concept is to determine the angle-dependentdifference in extinction through a vertical layer bycomparing the signal in thinner, bounding layersabove and below the main layer, assuming the aero-sol is horizontally homogeneous. The average extinc-tion coefficient due to particles in the main layer isgiven by

αp ¼ 12ðz2 − z1Þ

ln�Bðθa; z2ÞBðθb; z1ÞBðθa; z1ÞBðθb; z2Þ

�=

�1

sin θb−

1sin θa

�− αm;

where z1 and z2 are the midpoints of the boundinglayers, θa and θb are the two elevation angles, andB is the total uncalibrated backscatter integratedover the vertical extent of each bounding narrowlayer. Molecular extinction, αm, at the midpoint be-tween z1 and z2 is subtracted to obtain the averageαp in the layer. The near-surface value of αp can simi-larly be determined from nearly horizontal staredata using the slope method (e.g., see [51]) when the

Fig. 5. Calibrated aerosol backscatter profiles for 12:00, 15 Au-gust–12:00, 16 August 2006, Coordinated Universal Time (UTC)taken aboard a ship near Houston, Texas during the TexAQS IIexperiment. Local standard time was six hours later than UTC.The white line at the top of the main aerosol layer indicates thetop of the mixed layer during the day and the top of the residuallayer at night. Vertical white stripes to the surface are gaps be-tween data files. Shorter white stripes mark regions where thebackscatter could not be calculated above clouds.

Fig. 6. Average aerosol extinction coefficient in two layers usingthe slant-path method determined from shipboard measurementsfrom the northwestern Atlantic during NEAQS 2004.Missing datamostly represent times of fog or low clouds.


atmosphere is horizontally homogeneous with mini-mal vertical aerosol gradients.Figure 6 shows an example of the average aerosol

extinction coefficient in two layers determined fromshipboard measurements from the northwesternAtlantic. The elevation angle pairs were 6 and 20°for the lower layer and 20 and 90° for the higherlayer. Strong changes in extinction, such as late onJuly 15, occurred as wind shifts and ship transectschanged the sampled air mass from relatively cleanocean air to continental air. The latter had varyingdegrees of aerosol loading, including pollution attimes from upwind urban areas such as Boston.

D. Jet Exhaust Plume Measurements

The cross section of the exhaust behind commercialjet aircraft has been probed by OPAL by modifyingthe system to perform continuous scans at low anglesin addition to the stepped-scan mode used for ozoneand aerosol profiling. The main motivation is to de-termine plume growth and rise for various aircraft,operations (e.g., taxiing or rolling at the start oftakeoff), and meteorological conditions. These plumecross sections have been used to improve air qualitymodels, which previously assumed a passive pointsource [18]. The lidar’s average value of plume heightand size after the initial dynamics of the plume havewaned has brought model predictions of surface con-centrations of exhaust pollutants significantly closerto those observed by the airport monitoring network.The plume cross sections are scanned with the

lidar in 4 s and contain returns from 40 to 50laser pulses. The data are partitioned into 1m −

horizontal × 0:4m − vertical grid points and thenprocessed to determine the enhancement in back-scatter in the plume, βplume, above the ambient back-scatter. The uncalibrated total backscatter for eachpulse is first corrected for ambient atmospheric ex-tinction, which is determined by the slope methodfrom occasional low-angle stares through plume-freeair. The average ambient backscatter is then sub-tracted from the signal in the plume region. The re-

sulting βplume is expressed either as a ratio to theambient uncalibrated backscatter or as an aerosolbackscatter coefficient (Fig. 7), where the calibrationfactor is determined from occasional vertical profilesprocessed as in Subsection 4.B.

Continuing studies have the goal to better deter-mine the dependence of plume dispersion on aircrafttype and meteorological conditions (atmospheric sta-bility and wind speed). We are also attempting todevelop a method to infer soot emission rates fromthe calibrated backscatter [55]. Initial results are en-couraging, but necessary refinements to the underly-ing optical scattering and jet plume dynamicalmodels are in progress. Improvements underway forfuture campaigns include upgrading to a higher en-ergy 20Hz laser and changing from a 5 to 1:5m rangeresolution by increasing the digitizer resolutionto 14 bits.

E. Ozone DIAL Measurements

The three shorter wavelengths emitted by OPAL liewithin the broad Hartley–Huggins ozone absorptionband in the near uv, and their absorption increaseswith decreasing wavelength. At 355nm there is neg-ligible light absorption by ozone. In principle, anypair of the four OPAL wavelengths can be used to re-trieve range-resolved ozone concentration by theDIAL method (e.g., [56]). For the DIAL ozone calcu-lation, the derivative with respect to range of thelogarithmic ratio of the uncalibrated total backscat-ter is taken at the two DIAL wavelengths. To com-pute the derivative, a linear least-squares fit is madeto 30 adjacent gates of the logarithmic signal ratioprofile and the slope assigned to the middle gate.Ozone values are calculated at each range gate bysliding the 30-gate window along the profile. The re-sulting ozone profiles are displayed at 5m resolution,but the ozone retrievals are truly independent onlyevery 150m. During field campaigns this analysisprocedure is performed in real time, and ozone pro-files from the zenith beam can be displayed at 90 sresolution. During postprocessing, data are averagedto 20 min and ozone profiles are obtained at each ofthe four elevation angles. These profiles are thencombined with an altitude resolution of 5m.

Because the DIAL wavelengths are separated by10nm or more, the differences in molecular and aero-sol backscatter cannot be neglected. To correct fordifferential backscatter and extinction we use the fol-lowing approach. Correction terms for differentialmolecular (Rayleigh) scattering and extinction forboth DIAL wavelengths are computed from an as-sumed air density profile (Subsection 4.B). To correctfor differential aerosol effects, the backscatterand extinction profiles retrieved at 355nm (Subsec-tion 4.B) are extrapolated to the other DIALwavelengths by making assumptions about thewavelength dependence of aerosol backscatter andextinction. For the TexAQS II field campaign, wave-length dependences were assumed of λ−0:5 for aerosolextinction and λ−1 for aerosol backscatter [57].

Fig. 7. Cross section of the enhanced particle backscatter coeffi-cients behind a twin-jet MD83 aircraft during the early part of thetakeoff roll. Horizontal distance is from the lidar, and the height(scale greatly expanded) is above the runway.


Uncertainties in Rp=m and S that are needed for theretrieval of the backscatter profile at 355nm (Sub-section 4.B) as well as in the assumptions aboutthe aerosol backscatter and extinction wavelengthdependences may lead to incorrect estimates of theaerosol correction term for the ozone calculations.To minimize these potential errors, DIAL wave-

length combinations that exhibit a low sensitivityto aerosol effects are preferred. Desirable wave-length pairs have small backscatter and extinctioncorrection terms which are approximately propor-tional to Δλ=Δσ, where Δλ is the difference betweenthe on- and off-line wavelengths, andΔσ is the differ-ence between the on- and off-line ozone absorptioncross sections [56]. Acceptable choices of DIAL wave-lengths for OPAL are 266nm paired with any of theother three wavelengths and the 289=299nm pair[36]. DIAL pairs that include the strongly absorbing266nm as the “online” wavelength are most suitablefor low to moderate ozone levels. For high ozoneconcentrations, the choice of 289nm as the “online”wavelength provides better range coverage due toits lower absorption cross section.During OPAL’s recent shipborne deployments, the

Raman-shifted light at 289 and 299nm suffered fromlow pulse energies and inconsistent overlap charac-teristics; therefore, primarily the 266=355nm wave-length pair was used for DIAL calculations. Thestrong absorption by ozone at 266nm limited themaximum retrieval range to between about 700and 1200m depending on the ozone concentration.Beyond these maximum ranges the signal levelsfor the 266nm channel are so low that the errorsin the ozone retrieval are greater than 10ppb. Theseerrors can be either due to either the low SNR or sig-nal biases due to insufficient correction of EMI orSIB. Simulations indicate that, using the criteriaoutlined above, retrieving ozone profiles with the289 and 299nm channels would have resulted inmaximum ranges of about 1900 to 2500m underthe conditions encountered during the TexAQS II ex-

periment. Figure 8 shows a 24h ozone cross sectionfrom the same location and time period at the Tex-AQS II field experiment as Fig. 5. During the day,the ozone concentration increased due to photochem-istry while at night a layer remained aloft.

The precision of the ozone measurements dependson the choice of DIAL wavelength pair, the ambientozone and aerosol concentrations, and the time andheight resolutions. Figure 9 shows the vertical profileof the ozone root mean square (rms) error due to in-strument noise (predominantly signal shot noise) fora 6h portion of the ozone data in Fig. 8. The ozonerms error profile was determined by using an auto-covariance time series analysis for each altitude [58].The rms error profile was calculated for 3 min ozonedata. The error profile was then scaled to a 20 mintime resolution by assuming that the rms error is in-versely proportional to the square root of the numberof averaged lidar signal shots. The precision of theOPAL ozone measurements for this particular caseis mostly <2ppb, but increases significantly at alti-tudes above 800m, a range by which the 266nm sig-nal has been greatly attenuated. The positive spikesin ozone rms error at altitudes near 50 and 120m oc-cur just below the altitude blending points of the twolowest angles, where data are from ranges also ex-ceeding 800m.

The accuracy of the OPAL ozone data is generallymore difficult to quantify than the precision. Sourcesfor systematic biases in the ozone data include uncer-tainties in the ozone absorption cross section, insuf-ficient correction of differential aerosol backscatterand extinction as well as of SIB effects, and incom-plete overlap of the lidar beam and the receiver fieldof view. The uncertainty in the ozone absorption crosssection at the OPAL on-line wavelengths is 2% or less[59], corresponding to a bias of a few ppb even underhigh ozone conditions. For the 266=355 wavelengthpair, errors due to insufficient correction of differen-tial aerosol backscatter and extinction effects are atmost 10ppb in regions with inhomogeneous aerosoldistributions and significantly less everywhere else.

Fig. 8. Aerosol-corrected ozone profiles for the same 24h periodas the aerosol profiles in Fig. 5 (15 and 16 August 2006). Verticalwhite bars are data gaps between files. Horizontal white bars oc-cur where the DIAL data from one angle does not extend far en-ough to blend with the next higher angle data.

Fig. 9. Vertical profile of ozone rms error due to instrument noisecalculated from data taken from 18:00 to 24:00 UTC on 15 August2006.


This estimate is based on simulations that assume avertical gradient in aerosol backscatter at 355nm of2 � 10−8 m−2 sr−1. A gradient of this magnitude mayoccur in the transition zone from a polluted boundaryto a clean lower free troposphere. Biases due to SIBeffects and incomplete overlap can be significantlylarger and may vary from case to case. To minimizethese biases, a careful analysis tailored to individualcases is needed. Figure 10 shows a typical compari-son between a lidar ozone profile and ozonesonde andsurface ozone measurements. The lidar ozone mea-surements agree with the other instruments towithin approximately 10ppb in this example. Thisdiscrepancy could be due to systematic errors inthe lidar ozone retrieval caused by, for example, in-sufficient correction of signal biases, or due to the factthat the ozonesonde and OPAL were not probing ex-actly the same atmospheric volume.

5. Summary

OPAL has provided ozone, aerosol backscatter, andextinction profiles as well as cloud base and aerosollayer heights for a variety of air quality studies. Themultiangle technique has enabled extension ofprofiles to very near the surface, providing more com-plete observations of boundary-layer processes. Opti-mization of the double-gated PMTs has greatlyreduced artifacts in the signal and permitted a coax-ial transmitter design. Ozone profiles have been suc-cessfully measured, but further work is needed tostabilize the two Raman-shifted wavelengths to im-prove future ozone measurements. Jet exhaustplume measurements have been made using OPALin a continuous scanning mode.

The authors especially acknowledge YanzengZhao, who conceived the idea and built the original

version of OPAL. We also thank Hector Bravo,Michael Hardesty, James Howell, Keith Koenig,David Welsh, and James Wilson for discussionsand technical work. Development of OPAL wasfunded by the California Air Resources Board, theFederal Aviation Administration and partners, andNOAA’s Air Quality Program. The PMT studies werepartially funded by the Instrument IncubatorProgram of the NASA Earth Science TechnologyOffice as part of the Global Ozone Lidar Demon-strator Project. We thank Hamamatsu Photonicsfor the donation of the glass for the fluorescenceexperiments.

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