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Optics & Laser Technology 40 (2008) 208–214

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A study of NOX production in air heated by laser discharges: Effect ofenergy, wavelength, multiple discharges and pressure

Mahbubur Rahman�, Vernon Cooray

Department of Engineering Sciences, The Angstrom Laboratory, Division for Electricity and Lightning Research,

Uppsala University, P.O. Box 534, SE-751 21 Uppsala, Sweden

Received 19 September 2005; accepted 31 January 2007

Available online 30 March 2007

Abstract

An experimental study on the production of NOX in air heated under the action of a concentrated laser beam is presented. In this

experiment laser induced plasma was produced in air in a closed Teflon chamber of inner volume 1600 cm3 by focusing a laser beam with

either the wavelength of 1064 or 532 nm from a Q-switched Nd:YAG laser. The NOX production was measured by chemiluminescence

method and the possible effect of wavelengths, multiple discharges, and pressure on the yield of NOX was studied. The results show that

within the studied plasma energy range of 26–253mJ for 532 nm beam and 16–610mJ for 1064 nm beam, the NOX production scales

linearly with the dissipated plasma energy. For a given energy, 532 nm beam produces more NOX in air at atmospheric pressure than the

1064 nm beam. In an attempt to see the possible influence of multiple discharges on the production of NOX, discharges were created using

2–8 pulses with a repetition rate of 10 pulses per second in stationary air at atmospheric pressure. The results indicate that a certain

amount of the NOX created by a given pulse is destroyed by the subsequent pulses. In order to study the pressure dependence of the NOX

production, the pressure was varied from 16 to 100 kPa in the chamber and it was found that the NOX production efficiency scales

linearly with pressure.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Nitrogen oxides; Laser induced plasma; Lightning

1. Introduction

The oxides of nitrogen, NO and NO2 (NOX ¼

NO+NO2) are very important in atmospheric chemistry,especially because of their role in the radical chemistry andthe production and destruction of tropospheric andstratospheric ozone [1]. The quantification of theirproduction rates on a global scale has therefore been amajor subject of research interest together with theidentification of the mechanisms responsible for their netproduction. Atmospheric electrical discharges, particularlylightning, has long been known as a source of fixednitrogen in atmosphere [2,3]. Lightning has also beenidentified as a very important source of NOX in the freetroposphere and in remote marine areas, where no other insitu sources are known to be important [4]. It has also been

e front matter r 2007 Elsevier Ltd. All rights reserved.

tlastec.2007.01.007

ing author. Tel.: +4618 471 5805; fax: +46 18 471 5810.

ess: Mahbubur.Rahman@angstrom.uu.se (M. Rahman).

speculated that NOX-emission from electrical dischargesover the oceans could be a suitable in-situ source which canaccount for the amount of nitrates measured in the remotepacific [5].In order to quantify and understand the mechanism of

the net production of NOX from atmospheric electricaldischarges several laboratory experiments [6–16] have beencarried out by using electrical sparks which are consideredto be a good ‘model’ of lightning discharge with the majordifference being the small size and the short duration of thelaboratory electric sparks. However, in addition to thequestion of the validity of extrapolating the laboratoryspark data to the case of natural lightning, which indeed isa very complex event containing different phases with eachphase probably having a different NOX productionefficiency [17,18], two other difficulties faced by scientistsworking with laboratory discharges is the difficulty ofquantifying the exact amount of energy dissipated in theelectric discharge and the difficulty of quantifying the

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Table 1

Estimates of NOX yields form laboratory sparks and LIP

References Energy (J) NOX molecules� 1016 J�1

[7] 0.036 671

1350 874

[9] 12,000 572

[10] Unknown 1.6–4.2

[11] 0.005770.0015 1.470.7

[13] 31–113 1

[14] 128–288 10–40

[15] (98,00077000) 1.170.2

[16] 0.5�2.5 3

[33](LIP) 0.013�0.099 6.770.5

M. Rahman, V. Cooray / Optics & Laser Technology 40 (2008) 208–214 209

influence of metal electrodes (heat loss and contaminationof the air with metal vapour) on the NOX production. Forexample, in the case of laboratory sparks the dissipatedenergy measurement is difficult due to the disturbances inthe measuring system caused by the rapid change of currentduring the inception of the spark and the variation of thevoltage across the gap over several orders of magnitudeduring the formation of the spark. On the other hand,creating hot air using a laser beam, Laser-Induced Plasma(LIP), is free from both these issues. By bringing a laserbeam of sufficiently high intensity to a tight focusbreakdown of air could be achieved without electrodesand the dissipated energy in the discharge could bemeasured more accurately with a minimum amount ofdisturbance of the system.

Borucki et al.’s [19] pioneering work (Q-switchedNd:YAG laser, 5-ns, 180mJ/pulse, 1.06m, 10Hz) showedthat the LIP-emission spectrum in air and the terrestriallightning spectrum in the wavelength range of 300–900 nm[20] are nearly indistinguishable. Note that emissionspectrum is indicative of different physical processesimportant for chemical changes. In a later study Boruckiet al. [21,22] found that the optical efficiency of LIP in theatmospheric air is also close to the value derived for that ofa 4-m long spark in air [23], but is only about half thatderived for terrestrial lightning [24]. Encouraged by thesestudies LIP has been used by a number of researchers tosimulate the production of chemical species by lightningdischarges on earth and possibly on other planets [25–30].In the literature, the trace gas production rate by lightningis determined under the assumptions that the trace gasproduction rate by lightning scales linearly with the rate ofenergy dissipated by lightning activity and the same rate ofproduction with respect to energy is applicable to the LIP.On the other hand, Jebens et al. [31] analysed the timeresolved spectra of LIP and determined the electrontemperature and densities and compared them with thoseobtained from natural lightning [32]. The results showedthat the initial temperature of both discharges wascomparable under local thermodynamic equilibrium con-ditions for optically thin plasmas while the density of LIPwas several times higher than that of lightning at the sametemperature. Moreover, comparison showed that thecooling time-scale, which is important for chemical yield,of the LIP is significantly shorter than that of lightning.These experimental findings give some indication that theNOX production efficiency of lightning could be differentthan that of LIP or small laboratory sparks. Thus cautionshould be applied in extrapolating LIP or laboratory sparkdata to lightning.

In a recent study, LIP was used by Rahman and Cooray[33] in an experimental study on the production of NOX as afunction of dissipated energy in air at atmospheric pressure.In Table 1 the results obtained in that study are comparedwith the results from laboratory sparks available in theliterature. Note that the NOX production efficiency in the LIPis not far from the values obtained from spark discharges.

In the present study we analysed the effect of wave-length, pressure, and multiple discharges on the NOX

production by LIP. The wavelength dependence has to bestudied before one can extrapolate the data to lightning.This is the case since if the NOX production in LIP is astrong function of the laser wavelength then one has toanswer the question as to the wavelength which is suitablefor the simulation of the NOX production in electricaldischarges and lightning. The pressure dependence isimportant because the atmospheric pressure varies alongthe lightning channel with the pressure at the cloud end ofthe channel being about a factor two less than the pressureat the channel sections close to the ground. Moreover,cloud flashes take place completely in an environmentwhere the pressure is about half the atmospheric pressureat sea level. The effects of multiple discharges areimportant to understand the effects of different dischargeprocesses that propagate in succession along the samelightning channel. In order to quantify the effect of thelaser wavelength on NOX production we have repeated theexperiment described in [33] with two laser wavelengths,namely, 532 and 1060 nm, with the maximum energy closeto 360mJ for 532 nm and 850mJ for 1060 nm. In order tostudy the effect of pressure on the NOX production themeasurements were conducted at pressures varying fromatmospheric pressure down to a pressure of about 16 kPa.The effects of multiple discharges were studies by creating2–8 different plasmas in stationary atmospheric air with100ms time interval between pulses and measuring thenumber of NOX molecules created.

2. Experimental

A plasma in air was obtained by focusing a beam from aQ-switched Nd:YAG laser source (Quantel BrilliantB)operated either in single shot mode with remote controlor at a pulse repetition rate of 10Hz into the centreof a closed Teflon cylinder of inner volume 1600 cm3

fitted with a plano-convex optical glass lens of 15 cmnominal focal length, 2.54 cm of diameter with anti-reflec-tion coating. In this experiment both the fundamental(l ¼ 1064 nm, �0.8 cm beam diameter, �5.2 ns pulse-width

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Laser

Chamber

Probe

Probe

Filtered air in

Plasma

BeamSplitter

OscilloscopeNOxAnalyser Pump

Pump

Computer

Airfilter

Needlevalves

Pressuregauge

Needlevalves1 2

3

4

Lens Lens

Fig. 1. Experimental arrangement used in the measurements of NOX

production in air heated by laser discharges.

M. Rahman, V. Cooray / Optics & Laser Technology 40 (2008) 208–214210

at half-maximum, energy per pulse up to 0.85 J) and thesecond harmonic (l ¼ 532 nm, �0.6 cm beam diameter,�5 ns pulse-width at half-maximum, energy per pulse up to0.36 J) beams of the laser were used. The experimental set-up is schematically shown in Fig. 1. The laser pulse energywas varied from 70 to 360mJ for 532 nm beam and from110 up to 850mJ for 1064 nm beam. The transmitted partof the beam was allowed to exit through another lens of thesame type from the Teflon chamber. The incident laserenergy before entering the chamber was measured by usinga pulsed power energy detector (Ophir, PE50BB) incombination with a calibrated beam splitter with anti-reflection coating (1:99). The energy of the laser beamexiting the chamber was measured with an additionalpulsed power energy detector (Ophir, PE50BB-DIF). Theenergy absorbed in the plasma was obtained by thedifference between these two measured energies aftercorrecting the results for the losses in the optical system.Total error in the energy measurement is estimated to beless than 10% in the worst case.

Air coming into the chamber was filtered by passing itthrough a 5-mm particle filter. The air from the chamberwas sucked along a Teflon tube into a calibrated NOX

analyser of model 9841B (Monitor Labs). The analysermeasures the concentrations of NO and NOX usingchemiluminescence and can detect NO and NOX concen-trations as low as about 1 ppb. Measured NO and NOX

concentrations are available as output voltage signals,which are proportional to the concentrations and wererecorded by an oscilloscope of model LeCroy 9310L. Theinaccuracy in the NOX concentration measurement isestimated to be about 5–10%.

The following experimental procedure is used during theexperiment. For studying the dependence of NOX produc-tion on laser wavelength, after choosing the laser wave-length and installing the necessary optics, the filteredlaboratory air at atmospheric pressure was allowed to enterthe Teflon chamber through needle valve 2 and was suckedcontinuously through the needle valve 3. Needle valves 1and 4 were closed all the time during this measurement.

NO and NOX were measured continuously to determinethe background concentration of these gases. When asingle shot plasma with a chosen laser energy is created inthe chamber the concentrations of NO and NOX increasedinitially to a peak value which depends on the laser energyand subsequently decreased exponentially due to thedilution of the NOX inside the chamber by the freshincoming air. The number of NOX molecules created in thechamber was calculated by integrating the whole concen-tration curve after subtracting the background level. Fordifferent incident laser energies the procedure wasrepeated. In the case of multiple discharges, prior to thecreation of the plasma valves 2 and 3 were closed and airwas sampled through the needle valve 4. Needle valve 1 wasclosed all the time during this measurement. Two differentplasmas were created at the same place in stationary air byusing the repetition rate of the laser, 10Hz (i.e. 100ms timedifference between two pulses). Right after the creation ofthe plasma the sampling procedure is started by openingthe valves 2 and 3 (valves 1 and 4 closed) as describedabove. This experiment was repeated for different energiesand for different number of laser pulses (2, 4, 5 and 8shots). To study the pressure dependence of the NOX

production the air was evacuated from the chamber bypump 1 while the valves 2 and 3 were closed. Once thedesired pressure was established in the chamber the valve 1was closed and a single laser shot of a particular energy ledto the creation of plasma in the chamber. Immediately afterthis event, the sampling of the air inside the chamber wasstarted by opening the valves 2 and 3 as described above.This experiment was repeated for the same laser energywith different pressures. A Balzers manometer which hasan accuracy of 10% was connected to the chamber tomonitor the pressure inside the chamber. The temperature,and relative humidity of the air were measured using aVaisala temperature and humidity indicator of typeHMI31. Their values were 20–27 1C, and 17–49% RH,respectively, during the experiments.

3. Results and discussions

3.1. Energy dependence

The experiment with 532 nm laser beam was repeated asmentioned above and the data was obtained from 34 newmeasurements where the dissipated energy varied from 26to 253mJ. Recall that in the experiment conducted byRahman and Cooray [33] the maximum energy availablewas 99mJ. Thus, these measurements extend the maximumenergy from 99 to 253mJ for 532 nm beam. In Fig. 2 thenumber of NOX molecules produced are plotted as afunction of dissipated laser energy for both 532 and1064 nm laser beams. The results again show that the NOX

production is linearly correlated to the dissipated plasmaenergy within the studied energy range. From the results, itcan be seen that the laser plasma generates 6.7� 1016NOX

molecules J�1. But note that in Fig. 2 with increased laser

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0

7

14

21

28

35

0 0.1 0.2 0.3 0.4 0.5 0.6

Plasma Energy [J]

NO

x m

ole

cule

s[∗ 1

015]

Fig. 2. NOX production as a function of dissipated energy for 532

(triangle) and 1064 (circle) nm laser beams.

M. Rahman, V. Cooray / Optics & Laser Technology 40 (2008) 208–214 211

energy and consequently with increased dissipated energyin the plasma the scattering of data points increases. Thisscattering is rather high when the dissipated plasma energyis at around 200mJ. This spread in the data points iscaused by the self-focusing of the laser beam which is anonlinear optical effect that becomes more significant whenthe electromagnetic field strength of the optical field issufficiently high. The self focusing causes the laser beam tobreak up into plasma bars of different lengths and theyoccur at random locations from one shot to another [34].This will cause differences in the input energy density andin the transmitted/scattered energy from shot to shot givingrise to the spread in NOX yield. The experiment with1064 nm laser beam was carried out in the same way asmentioned above and the data was obtained from 74measurements where the dissipated energy varied from 16to 610mJ. The results show again that the NOX productionrates are linearly correlated to the dissipated plasma energywithin the studied energy range. From the results it can beseen that the laser plasma generates 5.1� 1016NOX

molecules J�1. The effect of the self focusing of the laserbeam is more pronounced in the case of 1064 nm beam withmost prominent scattering appearing somewhere close tothe dissipated plasma energy around 200mJ.

3.2. Wavelength dependence

As mentioned previously, two wavelengths (1064 and532 nm) were studied in an attempt to see the impact ofdifferent wavelengths on the production of NOX. Eventhough the production efficiencies given above for thesetwo wavelengths show that the efficiencies are different,one has to apply caution before making such a conclusion.For example, the reason for this difference could be thelarge scatter in the data associated with the 1064 nm beam.In order to rule out this possibility we compared the datafor both beams only up to 200mJ, the point beyond whichthe scattering becomes important. This will not change theefficiency of the 532 nm beam but the efficiency of 1064 nmbeam was increased to 5.9� 1016NOX molecules J�1 whichis still 13% less than that of the 532 nm beam. Even after

considering the experimental errors associated with theexperiment, the difference falls just above the error marginindicating that the effect is due to the wavelength. Thepossible reason for any wavelength dependence could bethat the air is heated by different physical processes fordifferent wavelengths. For example, in the visible wave-length region heating will be occurring more efficiently dueto the electrical excitation in contrast to the infraredregion, where the heating will be occurring due tovibrations and rotational excitation. As a consequenceNOX yield will be more in visible wavelength region than inthe infrared region. In a study conducted by Zipf andPrasad [35], synthetic air or O2/N2 mixtures was irradiatedwith either ultraviolet and vacuum ultraviolet continuumradiation from a deuterium arc lamp (210–400 nm) or byHg resonance line radiation (185 nm). The results showedthat the production of NO2 was higher for 185 nm than for210–400 nm.

3.3. Dependence multiple discharges

A typical ground flash may contain about 3–5 returnstrokes and the time interval between successive returnstrokes in a flash is usually several tens of milliseconds,although it can be as large as many hundreds ofmilliseconds or as small as one milliseconds or less[36,37]. In an attempt to see the possible influence of suchmultiple discharges on the production of NOX, either 2, 4,5 or 8 laser plasmas were created in the Teflon chamber atthe same place in stationary air by using the pulserepetition capabilities of the laser. The 532 nm beam isused in this experiment. The laser is capable of generatingthe pulses at a rate of 10 pulses/s making the separationbetween successive pulses equal to 100ms. This value fallswithin the range of observed time intervals betweensubsequent return strokes although it does not representthe most typical values. On the other hand, the number ofpulses chosen in the experiment, however, represents thetypical number of subsequent strokes in lightning flashes.The number of pulses to be applied in a given experiment iscontrolled by a computer attached to the electronics of thelaser.The result obtained in the experiment is plotted in Fig. 3

where the produced NOX is shown as a function ofdissipated energy. Noting that each pulse in a givensequence has the same energy, the results at a first glanceappear to show that the NOX production by multiplepulses is almost equal to the sum of the productions ofNOX by individual pulses. For example, two pulses withsame energies separated by 100ms time interval producedouble amount of NOX molecules as one pulse does withthe same energy. Sobral et al. [38] studied this question bytaking the temporal evolution of two synchronised LIPs inair using a shadowgraphy and interferometry method andcame to the conclusion that the amount of NO freezed-outby the double return stroke flash is more or less equal tothe sum of NO freezed-out by individual pulses. But, a

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0

2

4

6

8

0 30 60 90 120

Pressure [kPa]

NO

x m

ole

cule

s(∗ 1

016)/

J

Fig. 4. NOX production per Joule as a function of air pressure.

0

2E+16

4E+16

6E+16

0 40 80 120

Plasma Energy [mJ]

NO

x m

ole

cule

s

Fig. 3. NOX production as a function of dissipated energy for 2, 4, 5 and 8

repetitive pulses. NOX yields for 1 pulse is also shown. diamond, 1 pulse;

square, 2 pulses; triangle, 4 pulses, circle, 5 pulses; cross, 8 pulses.

M. Rahman, V. Cooray / Optics & Laser Technology 40 (2008) 208–214212

closer examination of our results show that the productionefficiency decreases from the sum of individual productionby 0.8% for 2 pulses, by 1.2% for 4 pulses, by 1.2% for 5pulses and by 3.2% for 8 pulses. This indicates that acertain amount of NOX produced by a given pulse isdestroyed by the subsequent pulses. The effect could bemore significant if the separation between the pulses isreduced from the 100ms value used in the experiment. Inthe case of return strokes the average separation betweenthe return strokes is about 30–40ms indicating a significantdestruction by subsequent strokes of NOX produced byfirst return stroke.

3.4. Pressure dependence

The pressure along a lightning flash channel varies fromatmospheric pressure to about 10 kPa and therefore, beforethe laboratory data could be extrapolated to evaluate theNOX production in lightning flashes one has to investigatethe effect of pressure on the NOX production. In thisexperiment the effect of pressure on NOX production inLIP is investigated. The 532 nm laser is used in thisexperiment and the pressure in the chamber was variedfrom about 16 to 100 kPa. In order to be able to create ameasurable amount of NOX at low pressures at each event,8 pulses were applied irrespective of the pressure. Theresults obtained in the experiment is shown in Fig. 4 wherethe NOX molecules produced per Joule of energy is plottedas a function of pressure. The result shows that the NOX

production efficiency scales linearly with pressure. Wang[14] in a study with electrical sparks also found that NOproduced per metre of the discharge channel is approxi-mately linearly proportional to the initial laboratory airpressure. Pressure dependence of NO2 production was alsofound in [35] where the studied pressure range was betweenabout 100 and 1000Torr. Ammann and Timmins [39]showed that the net effect of decreasing the pressure is toincrease the rate of quenching while at the same timedecreasing the homogeneous reaction rates. The reactionrate of the bi-molecular reactions that are thought to beresponsible for NOX production is related to the square of

the pressure, while the quenching rate (rate of cooling) isinversely proportional to pressure leading to a lineardependence of NOX production with pressure. Drost [40]also found a similar pressure dependence.

3.5. Theoretical calculation of NOX yield

Navarro-Gonzalez [41] evaluated the NOX yield in LIP(pulsed Nd:YAG laser, 1.06m, 6mm beam diameter, 7 nspulse width, 300mJ, 10Hz) by using the followingprocedure. First, the images of the temporal and spatialevolution of plasma, shock wave and hot air core wereobtained in air from the ns to the ms time scale byshadowgraphy and interferometry [42] to determine thevolume and the temperature of heated air. According to themeasurements, the volume in the hot core air reached thefreeze-out temperature of NO, about 2300–2700K, in75–80 ms after the plasma initiation. By knowing thevolume of air at that temperature they evaluated theNOX production efficiency of laser induced plasma to be1.5� 1017molecules J�1. This estimate is not far from thevalues measured here.In a previous publication Rahman and Cooray [33]

evaluated the volume of air in a laser plasma heated to agiven temperature by thermodynamic considerations. Intheir calculation they used the value of heat capacity of airat constant pressure, corresponding to the room tempera-ture, i.e. Cp ¼ 1010 J kg�1K�1. However, the value of Cp

for air varies as a function of temperature and in thetemperature range of 2500–10,000K it varies between3� 103 to 6� 103 J kg�1K�1 [43]. If we use an averagevalue of about 5� 103 J kg�1K�1 and repeat the calcula-tions of Rahman and Cooray [33] we find that the volume,V (in m3) of air heated to 2700K in a laser plasma with aninput energy, E (in J), is given by

V ¼ E�5:2� 10�7: (1)

Assuming that the NO freeze-out temperature is 2700Kand using the equations given by Borucki [44], we find thatthe number of NO molecules, NNO generated for a given

ARTICLE IN PRESSM. Rahman, V. Cooray / Optics & Laser Technology 40 (2008) 208–214 213

input energy E is given by

NNO ¼ E�4:1� 1016. (2)

This agrees reasonably well with the results presented inthis paper.

Acknowledgement

The research work reported here is funded in partby a Grant (Grant no. 621-2003-3465) from SwedishResearch Council. Thanks are due to Jan Isberg for lettingus use the laser and to Dr. Anders Larsson for fruitfuldiscussions.

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