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Direct measurement of NO2 in the marine atmosphere by laser-induced fluorescence technique
Atmospheric Environment 35 (2001) 2803}2814
Direct measurement of NO in the marine atmosphere by laser-induced #uorescence technique J. Matsumoto*, J. Hirokawa, H. Akimoto, Y. Kajii Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan Received 25 September 2000; received in revised form 12 December 2000; accepted 24 December 2000
Abstract A simple and compact instrument for NO measurement by laser-induced #uorescence (LIF) technique with a pulsed solid state laser and a multi-pass excitation system was developed and optimized for several conditions. As a result of laboratory experiment, the limit of detection (LOD) reached 94 pptv for 60 s integration. It was thought that an LIF instrument with this LOD value would be capable of quantifying sub-ppbv NO in unpolluted marine atmosphere. As the second step, a "eld test of the instrument was conducted in the marine atmosphere at Cape Hedo, Okinawa Island, Japan, in summer 1999. Intercomparison between the LIF instrument and a chemiluminescence detector with a photolytic converter (PLC-CL) was also made in this test. Consequently, the LIF instrument was shown to be of practical use for measuring NO in clean maritime air. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Nitrogen dioxide; Laser-induced #uorescence; Marine atmosphere; Intercomparison; Water vapor
1. Introduction Nitrogen oxides (NO "NO#NO ) are important V species in tropospheric photochemistry. NO plays a role V as a precursor of ozone. Ozone can be formed through the reactions of NO and peroxy radicals (HO , RO ). V V V Ozone formation is often limited by NO concentration. V NO can be "nally transformed to HNO which results V in acid rain. Thus, measurement of NO concentration is V essential to the understanding of chemical processes in the atmosphere. In this decade, interest in chemistry in the boundary layer of the marine atmosphere has increased. Nagao
* Corresponding author. Tel.: #81-3-5452-5144; fax: #813-5452-5143. Present address: School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Present address: Frontier Research System for Global Change, 3173 Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan. E-mail address: [email protected] (J. Matsumoto).
et al. (1999) discussed the possibility of ozone destruction by halogen species. Ozone variation and radical chemistry in the marine boundary layer (MBL) have been a focus (Carslaw et al., 1999). As for nitrogen species, reactions of NO with hydrocarbons, dimethyl sul"de (DMS), or sea-salt aerosols in the MBL can be important as nighttime radical sources or sinks (Allan et al., 1999; Rudich et al., 1998). Most of these phenomena in the MBL are related to NO variation. Thus, for the purpose V of studying these topics, &background NO ' in the clean V MBL is the most important factor. In order to quantify NO and NO concentration, several techniques have been utilized in previous work. NO has been widely measured by the ozone-chemiluminescence (CL) technique (Ridley and Howlett, 1974). This method has been developed su$ciently to detect NO directly at the pptv level for 60 s integration. In this paper, the limit of detection (LOD) is de"ned or calculated for 60 s integration and 1 standard deviation (1). NO measurements have been considered as more prob lematic than NO measurements. The luminol chemiluminescence technique (LC) can detect NO (Schi! et al., 1986); however, ozone and peroxyacetylnitrate (PAN) can in#uence the signal as interference (Fehsenfeld et al.,
1352-2310/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 0 7 8 - 4
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1990). Although the di!erential optical absorption spectroscopy (DOAS) technique can measure NO (Platt et al., 1979), it lacks mobility and spatial resolution due to a long optical path. The photofragmentation two-photon laser-induced #uorescence (PF-TP-LIF) technique detects NO after photolysis of NO (Bradshaw et al., 1999). Although the system is an indirect method and not so compact, it is very sensitive. The tunable diode laser absorption spectroscopy (TDLAS) technique has the capability to measure down to a level of 200 pptv, though care must be taken in the data analysis (Reid et al., 1978a, b; Fehsenfeld et al., 1990). The most widely used technique to measure NO simply is CL detection after conversion of NO to NO (Kley and McFarland, 1980). A photolytic converter (PLC) is thought to be the most reliable system to transform NO to NO, having higher selectivity than the metal-catalytic converter (Gao et al., 1994). However, this is an indirect method of NO detec tion involving the calculation of the di!erence of two signals detected as NO. Thus, some of its disadvantages are known as follows: (1) temporal variation of conversion e$ciency, (2) lower conversion e$ciency (e.g. 60}80% for Xe-arc lamp), (3) in#uence of the variation of NO concentration faster than one measurement cycle, (4) the possibility of artifacts from other nitrogen species, and (5) limit of the temporal resolution because of the necessity of two measurement modes for NO and NO . V As a direct, selective, in situ, and sensitive technique to detect NO with high temporal and spatial resolution, the laser-induced #uorescence (LIF) technique is desirable. A couple of workers have explored this technique. George and O'Brien (1991) developed a #uorescence assay by gas expansion (FAGE) instrument with a frequency-doubled Nd : YAG laser and the LOD was 500 pptv. Fong and Brune (1997) adopted a tunable dye laser and 44-pass repetition of the laser alignment, and the LOD was 300 pptv. Thornton et al. (2000) recently reported on a compact LIF system with a tunable dye laser and a 40-pass detection cell. The sophisticated con"guration of the parts of the instrument and the multipass excitation system results in an LOD of 3}17 pptv for 60 s averaging at S/N"1. Their instrument is excellent because it has high selectivity utilizing two wavelengths. When the e!ects of the interference like particles are thought to be little, the single wavelength excitation is acceptable. This technique is compact and simple because a stable and compact laser is utilized. Such simple instruments are desirable for "eld measurements. We have developed a compact LIF-NO instrument with a single wavelength excitation utilizing a diode-laserpumped Nd : YLF laser and a multi-pass excitation cell, for the purpose of simple, compact, e!ective and direct measurements of NO . A "eld measurement test includ ing an intercomparison with a PLC-CL detector was conducted at a subtropical observatory in Okinawa, in the northwest rim region of the Paci"c Ocean. In this
paper, a detailed description of our instrument, the data analysis concerning the variation of water vapor, and the intercomparison of the instrument with the PLC-CL are given in order to discuss the reasonability, stability, and capability of this instrument.
2. Principle of LIF-NO2 system The NO molecule absorbs photon in the visible re gion for transition between A and B states. When an NO molecule absorbs a photon in this region, the mol ecule is excited and emits a red-shifted #uorescence. This scheme is described below: NO #h PNOH, NOH PNO #h, NOH#MPNO #M.
(R1) (R2) (R3)
Reaction (R1) is the excitation of NO by a photon. NO* represents NO in the excited state. After excitation, NO* can emit #uorescence (R2) or be quenched by colli sion with the third molecule &M' in reaction (R3) to return to the ground state. In this scheme, the number of photons emitted in reaction (R2) is thought to be proportional to the initial concentration of NO . Thus, the NO concentration can be determined by detection of this #uorescence. The #uorescence signal, S, emitted from NO can be simply described as S"KI[NO ] , $
(1)
where K is the instrumental function which includes collection e$ciency of photon and sensitivity of the photon detector, I is the laser intensity (mW), is the absorption cross section of NO which at 523.5 nm is 1.36;10\ cm (Vandaele et al., 1997), [NO ] is the number density of NO in the ground state in the detec tion cell and is the quantum e$ciency of #uorescence. $ Eq. (1) can be rearranged for the volume mixing ratio of NO in the sample, m , as ,S"CKIm P , ,$
(1)
where P is the total pressure in the detection cell, C is the conversion coe$cient for transformation between [NO ] and m , independent of pressure and is ,$ de"ned as the branching ratio of radiation of #uorescence from the excited NO* molecule. In this paper, a time-gated photon counting method was utilized for improving the signal-to-noise ratio (SNR). Photon detection was temporally controlled after each laser pulse. In this case, can be shown as $ (t , t )" (t"R) $ $
R
R
e\R O dt,
(2)
J. Matsumoto et al. / Atmospheric Environment 35 (2001) 2803}2814
k k 1 (t"R), , , , $ k # k [M ] k 1# q P G G G G + G G (3) 1 ,1/k" , k 1# q P G G G +
(4)
q "k /k , (5) G G where t and t are, respectively, the initial and "nal times of the counting gate after the laser pulse, k is the radiative rate constant of the excited NO molecule (reported to be 2.3;10 s\) (Myers et al., 1966), k is the G rate constant of quenching by the third compound M , G [M ] is the number density of M , q is the quenching G G G factor for M (Torr\; 1 Torr"133.3 Pa) and P G is the G + partial pressure of M (Torr). G After normalization of the #uorescence signal (S; unit: cps"count s\) by NO mixing ratio (m , ppbv) and ,laser intensity (I, mW), the sensitivity of the LIF instrument, S (cps ppbv\ mW\), can be described as ,R P S "CK e\R O dt. (6) ,1# q P G G G + R Therefore, sensitivity is mainly in#uenced by instrumental function, cell pressure, quenching factor, and setup of the counting gate. In the case of #uorescence detection, LOD is thought to be limited by background signal (BG, cps). The LOD can be described as
1 (S/N) 1 # , (7) LOD+ % S I m n ,where (S/N) is the signal-to-noise ratio to de"ne the LOD, m and n represent the number of measurements of background and sample, respectively, and (cps) rep% resents the deviation of the background signal for a "xed integration time, t (s). In the photon counting method, the counted signal (cps) usually follows the Poisson distribution, and deviation of the signal closely corresponds to the square root of that signal as follows:
%
"
BG . t
%
.
Finally, Eq. (7) can be reformed with Eqs. (6)}(9) as
(S/N) 1 1 S % LOD+ . # S m n I t ,-
(10)
Therefore, in order to improve the LOD, it is necessary (a) to realize high sensitivity, (b) to increase laser intensity, and (c) to reduce background signal e!ectively. It should be also noted that this is the LOD estimated from laser-induced background consideration. For the strict estimation of LOD, #uctuation of zero air should be included. This #uctuation is shown in the following section.
3. Description of LIF-NO2 instrument Fig. 1 shows the schematic diagram of the LIF-NO instrument. As a light source, a frequency-doubled Nd : YLF pulse laser pumped by a solid state laser (523.5 nm, Spectra-Physics TFR laser) was utilized. The laser has the maximum output, 360 mW, at 3000 Hz of the pulse repetition rate. This laser passed through an optical lens for compensation of the beam divergence (5 mrad) and a half-wave plate for optimization of the beam polarization (not shown in the "gure), and then was introduced horizontally to the excitation cell. The length of the cell was 55 cm. The radius of its arm was 5 cm. Thus, the volume in the cell was 1100 cm. The material of the wall of the cell was aluminum in order to reduce the weight. The wall inside the cell was black-coated to reduce the number of scattered photons. Multi-pass alignment of the laser in this cell was utilized in order to
(8)
Smaller background signal can cause smaller deviation of signal and thus more precise measurement. Background signal is usually composed of scattered light of the laser in the cell and dark current noise in the photomultiplier tube. When the background signal of laser scattered light is still larger than that of dark current, BG can be regarded as proportional to laser intensity, I. In that case, &background sensitivity', S (cps mW\), can be de"ned % as background signal normalized by laser intensity: BG"IS
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(9)
Fig. 1. Schematic diagram of the LIF-NO instrument: Nd : YLF: laser instrument as a light source; M }M : mirrors for laser alignment; W: window with a slope of Brewstar angle; BP: ba%e plates to reduce scattered light; PM: photodiode for laser power monitor; LS: lens system; RF: red glass "lter ( "640 nm); PMT: photomultiplier; PCB: PMT cooler box. A solid line from Nd : YLF represents the pass of the laser beam.
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increase the e!ective volume for NO excitation and to acquire high sensitivity. The pass number was set to 18, considering the limit of the residual divergence of laser beam. Two re#ection mirrors were provided for the multi-pass. The deviation of laser output was monitored outside the cell by a photodiode detector. The variation was typically within 10%. Sensitivity of the instrument was corrected by laser output. In the cell, several ba%e plates with slits (5 mm high and 50 mm wide) for the laser beam to pass through were positioned to reduce background photons scattered at the windows, walls, and mirrors of the cell. Sample gas was drawn by an oil rotary pump with a capacity to draw a #ow of 350 l min\ (as a volume #ow rate); its weight and dimension were 30 kg and 45 cm;20 cm;25 cm, respectively. Sample gas was then introduced into the center of the cell vertically in the region where laser beam density was high. To reduce the cell pressure, the air sample passed through a critical ori"ce plate before entering the cell. A 10-Torr full-scale BARATRON pressure transducer (MKS Japan, Inc.) always monitored pressure in the cell. Photons emitted from NO were gathered on the sensitive surface of a dynode-gated photomultiplier (Hamamatsu, R943-02) through the optical lens system. The lens system consisted of 4 lenses as shown in Fig. 1. This was constructed through a calculation by the &ray trajectory analysis' in order to collect photons e!ectively at the center of the cell. A sharp-cuto! glass "lter was installed in front of the PMT to reduce background signal originating from scattered photons. The cuto! wavelength ( ) with trans mittance of 50% was 640 nm. In the "eld measurement, the PMT was kept at 03C by a simple cooling system to reduce dark current signal. The dark current signal was 3.7 cps on the average in this study. Photon signal was counted by a time-gated photon counting unit in order to improve (S/N) ratio. Trigger signal of the pulsed laser was used to decide the initial time in each cycle, and a digital delay/pulse generator (DG535, Stanford Research Systems) controlled the gate time for the detection. The background signal was monitored utilizing a zero air generator shown in Section 5.2. The uncertainty of zero air was estimated as 30 pptv. This value was acquired as a result of comparison with the synthetic air of a cylinder. This uncertainty should be included in the estimation of LOD. The laser head, optics, the excitation cell, and the detection system were installed on the aluminum board with a dimension of 80 cm;60 cm, and the height was 30 cm. This board and other accessories were put on a desk (140 cm;90 cm). Power supplies of high voltages for the PMT, a laptop PC, a control unit of the laser and a zero air generator were stacked in a 19 in lack (170 cm high). An oil rotary pump, a chiller for the laser, and a water tank for cooling the PMT were placed on the #oor. The whole instrument was set up within a space of
200 cm;130 cm and a height of 150 cm. The sum of their weights was 150 kg. Power consumption of the instrument is 2 kW.
4. Optimization of the instrument The instrument was optimized for several conditions to increase sensitivity and reduce background signal. In this section, the procedure of optimization is presented. 4.1. Estimation of sensitivity variation As shown in Section 2, the dominant factors which determine the capability of the LIF-NO instrument are pressure in the cell and timing of the counting gate. If the laser intensity is "xed to one constant value, sensitivity can be estimated based on these two factors. Here, optimization of the LIF instrument focuses on these two factors. From Eq. (6), dependence of sensitivity of the instrument on pressure and counting gate can be simulated. To estimate dependence on counting gate, the width of the gate was set to 3 s. At this stage, the quenching e!ect of water vapor was not of concern in order to determine the sensitivity variation of the instrument itself as a function of cell pressure and gate time. The results of simulations of sensitivity variation are shown in Fig. 2. The y-axis in this plot represents the initial gate time, t . Fig. 2(a) shows the relative variation of sensitivity under dry conditions. When the gate time is "xed, sensitivity has a peak at a pressure of 0.5}1.0 Torr. This trend is reasonable since too high a pressure increases the e!ect of quenching and too low a pressure reduces the number density of NO in the cell. On the other hand, if the pressure is "xed, sensitivity decreases as a counting gate is set later. This can be explained by the fact that NO* decreases as time passes after the laser pulse due to deactivation by radiation and quenching. To validate this estimate, the pressure dependence of sensitivity was observed for a "xed gate time and compared with the estimated trend. Standard NO was prepared by gas phase titration method (see Section 5.3). Fig. 2(b) shows the experimental results measured when the initial and "nal gate times, t and t , were set at 1.5 and 8.0 s, respectively. The experimental results agreed with the results of the simulation. This ensures the reasonability of the trend predicted in the simulation shown in Fig. 2(a). Fig. 3 shows the observed dependence of background and NO signals on the gate time of photon counting. The open circles and solid squares represent decays of signals for 0 and 227 ppbv of NO , respectively. Data are shown as counts for 1 s integration (0.7 Torr, 200 ns width of gate, 3 kHz, 360 mW). The decay of the signal for 227 ppbv resulted from the quenching e!ect by N and O . Because the background signal was independent of
J. Matsumoto et al. / Atmospheric Environment 35 (2001) 2803}2814
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Fig. 3. Dependence of signals on gate time for 200 ns gate width, 0 and 227 ppbv NO , 3 kHz repetition rate, and 360 mW laser intensity.
Table 1 Adopted conditions of LIF-NO in this work
Fig. 2. Dependence of sensitivity on the pressure in the excitation cell and gate time of the photon counting method. (a) Calculated sensitivity for a gate width of 3 s. The values on the contour map represent the relative sensitivity by an arbitrary unit. (b) Dependence of sensitivity on pressure. The gate time is "xed to (t , t )"(1.5 s, 8.0 s). Open squares with error bars are shown for experimental data and a solid line is for calculation.
the cell pressure, it was caused by laser photons scattered on the walls and the window in the cell. Sensitivity increased with the earlier start time of the gate. However, background signal also increased due to scattered laser. The degree of such an increase in the background signal was greater than the sensitivity as shown in Fig. 3. The maximum ratio of the sensitivity to the background in the "gure was acquired for gate times of 1.3 and 6.0 s. Thus, this gate time was adopted in order to reduce the background and acquire the best LOD value of Eq. (10). 4.2. Instrumental conxguration Other additional factors in the con"guration of the instrument were optimized as follows: (1) Hole size of the critical ori"ce before the excitation cell was selected to realize proper sample #ow rate and cell pressure. Here, ori"ces with a hole size of 0.1}0.8 mm were examined considering the power of the pump and the expected range of cell pressure. (2) LOD is expected to be better
Parameters
Adopted values
Sample #ow rate Cell pressure Counting gate t , t Laser repetition rate Laser intensity Number of passes
300 sccm 0.70 Torr 1.3, 6.0 s 3000 Hz 360 mW 18
t"0 is de"ned as the time of emission of laser pulse.
with higher laser intensity. The averaged power of the laser is highest at 3000 Hz of the pulse repetition rate. Typical power is 360 mW. We adopted the 3000 Hz repetition rate in order to realize the maximum output and the best LOD. (3) When the pass number of laser excitation increases, the e!ective volume of excitation also increases and higher sensitivity can be achieved. However, background signal can increase due to scattered laser photons from the mirrors, ba%e plates, and/or molecules of sample air. Thus, optimal pass number should be selected considering both sensitivity and background signal. Here, several conditions of pass number were compared considering the characteristics of the mirrors, the con"guration of the ba%e plates, and the divergence of the laser beam. (4) The PMT was cooled at as low a temperature as possible to reduce dark current. (5) A sharp-cuto! "lter was placed in front of the PMT, again to reduce background signal. Considering the dependence of sensitivity on pressure, counting gate and all factors of the instrument were determined for as low an LOD as possible. The measurement conditions adopted are summarized in Table 1. Sample #ow was automatically decided due to the
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Table 2 Comparison of speci"cations of the instrument with recent works
10 (cm) Laser intensity (mW) Sensitivity (cps ppbv\ mW\) Background (cps) LOD (pptv)
Fong and Brune (1997)
Thornton et al. (2000)
This work
0.6 250 0.12 2425 300
1 100 0.06 1.5 3.7
1.4 360 0.28 2400 94
LOD values are given for S/N"1, t"60 s, m"n"1.
combination of critical ori"ce, cell pressure, and the power of the pump. The dark current signal was 3.7 cps on the average when laser beam was not introduced into the cell. This signal was negligible in comparison with total background signal (typically 2400 cps). Thus, total background signal was thought to be proportional to laser intensity. The LOD estimated by Eq. (10) was 89 pptv. In addition, the #uctuation of zero air was typically 30 pptv. As a result, the "nal LOD was modi"ed to be 94 pptv (I"360 mW, S/N"1, m"1, n"1, t"60 s), as shown in Table 2. Table 2 shows the comparison of the instrument with recent studies of LIF-NO measurements. When this study is compared with Fong and Brune (1997), sensitivity of our instrument is larger than that of their instrument by a factor of 2.5. This advantage is appropriate to the di!erence in the absorption cross section, although other factors like the gate time and collection e$ciency of the lens system should be considered. Background signals are similar. Laser intensity is larger by a factor of 1.4. Consequently, the LOD is better by a factor of 3.2. In comparison with Thornton et al. (2000), the sensitivity of our instrument is only a half. Background signal of their instrument is su$ciently small beyond compare. As shown in Fig. 3, the background signal is high in the earlier gate time mainly due to laser scattered in the cell. As a result of the setting of gate time, the loss of NO signal for our instrument is roughly 50%. If the background signal could be reduced in the future, the gate time could be set at much earlier time and sensitivity could be increased by a factor of 2.
5. Ambient measurement in the marine atmosphere A "eld measurement using the LIF-NO instrument was conducted at a subtropical observatory in Okinawa, Japan, in the northwest rim region of the Paci"c Ocean, in August 1999. The goal of this measurement was to examine the performance of the instrument at the level of sub-ppbv of NO in the humid marine atmosphere.
A chemiluminescence detector with a photolytic converter (PLC-CL) was also utilized to carry out an intercomparison with the LIF instrument. 5.1. Site description and meteorological conditions This measurement was a part of the campaign, ORION'99, Observation at Remote Island in OkiNawa, from 1 to 15 August 1999 (Kanaya et al., in preparation). The speci"c location of the measurement site has been shown previously (Akimoto et al., 1996). Brie#y, Okinawa Island is located at the northwestern rim of the Paci"c Ocean, and 800 km away from the coastline of China along the East China Sea. Cape Hedo (26352N, 128315E) is the northernmost part of Okinawa Island, 100 km from Naha City, the most populated city on the island. Nago City, a large town in the northern Okinawa is 35 km southwest of the site; the population is 50,000. The measurement site was located 200 m from the western coast and 60 m above sea level. The eastern part of the site was covered mainly by sugar cane "elds and its height was about 1.5 m. There is a sightseeing spot at the northern end of the cape and an access route to it. However, anthropogenic activities around the site are rare. In summer, Okinawa Island is usually covered by the Paci"c high pressure system, and clean air masses come from the east region over the Paci"c Ocean. On sunny days, solar radiation is strong enough for the J value ,to be above 0.01 s\. In such a subtropical region surrounded by sea, the atmosphere near the surface has high humidity, 2.7}3.5% by volume. However, in the campaign period, the Paci"c high pressure system was weak and low pressure systems were active. So there were only three days without any rainfall: 6, 14, and 15 August 1999. In addition, several typhoons generated near the Philippine Islands passed over or near the island. Measurements were safely interrupted several times because of the possibility of electric power failure. After the typhoons passed over, the air mass around the island would be well mixed with the polluted air from the East China Sea originating from the Asian mainland or from the Japan Islands.
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Here, the e!ect of local wind systems should be noted. In the case of local wind from the north or west, the clean marine air mass could reach the site without the addition of pollutants from land. When an easterly wind was dominant around the observatory, measurement was mainly in#uenced by local emission of NO from the sugar cane "eld which covered the whole eastern region. Thus, NO concentration could reach a level of several V hundred pptv and soil emissions became the most important source of NO . Wind from the south or southwest V was thought to have passed over the island with anthropogenic pollutants added regionally. NO could be over V 1 ppbv in such cases.
Table 3 Typical speci"cations of LIF and PLC-CL instruments during the "eld campaign in this work
5.2. Measurement procedure
Calculated for laser intensity of 360 mW ("523.5 nm). Observed standard deviation for 60 s integration. Reduction of sensitivity by quenching e!ect of water vapor (3% by volume). LOD values are given for S/N"1, t"60 s, m"n"1.
Here, methods of measuring NO in Okinawa are shown, with an explanation of the intercomparison between LIF and PLC-CL techniques. At "rst, the measurement cycle of LIF-NO is described. Raw counts include both the LIF signal of NO and the background signal caused by scattered light and dark current. Thus, determination of background signal is important. Background signal can be monitored after NO is removed from the sample. A zero air supply (TECO Model 111) was utilized to generate &zero air' which includes little NO of less than 30 pptv. The measurement sequence here was: (1) zero air mode for background monitoring (60 s) and (2) ambient mode (120 s). A solenoid valve (SV) controlled automatically by a PC was utilized to change the mode between zero air and ambient sample. Between each mode, a 20 s #ush mode was allowed to ensure stability. Each cycle needed about 4 min. This cycle enabled monitoring of the short-term variation of the background signal. A PLC-CL instrument was also utilized. In this work, a chemiluminescence detector of NO (CLD770ALppt, ECO PHYSICS) with a photolytic converter (PLC760, ECO PHYSICS) was utilized as a PLC-CL instrument in order to conduct the intercomparison. The UV light from an Xe-arc lamp was irradiated to the sample air in the photolysis cell and NO was converted to NO. NO and NO were measured sequentially. Calibration of the PLC-CL instrument was conducted twice a day in the "eld measurement. A standard gas of NO (1 ppmv) in a cylinder was diluted by zero air, and the diluted gas was introduced into the NO detector (CLD). The conversion e$ciency of the photolytic converter (PLC) was acquired through the gas phase titration method (GPT). After the GPT calibration, the zero air was introduced into the PLC-CL instrument and the zero point was checked. Humidi"ed calibration is conducted in order to check the reduction of sensitivity by the quenching e!ect of water vapor. The GPT and humidi"ed calibrations are the same as for the LIF instrument, shown in Section 5.3. Speci"cation of the PLC-CL
Sensitivity Background Dark current SD of BG Conv. e!. Quench (H O) LOD
LIF-NO
PLC-CL NO, NO
100 cps ppbv\ 2400 cps 3.7 cps 6.4 cps * 16% 94 pptv
400 cps ppbv\ (NO) 300 cps (NO) 50 cps 6.0 cps (NO) 56% (NO to NO) 15% (NO) 21 pptv (NO), 54 pptv (NO )
instrument is summarized and compared with the LIF in Table 3. Measurement by PLC-CL consisted of four modes: (a) NO pre-chamber mode, (b) NO main-chamber mode, (c) NO pre-chamber mode, and (d) NO main-chamber V V mode. Pre-chamber mode determined the zero point of each measurement. Net signal of NO or NO was acV quired as the di!erence between main mode and temporally neighboring pre-mode. In order to reduce #ushing time between di!erent modes, the following sequence was adopted: (1) NO pre, (2) NO main, (3) NO V main, (4) NO pre, (5) NO main, and (6) NO main. As V V each mode required 15 s for the integration of the signal and 5 s for #ushing the lines in the instrument, one cycle (6 modes) lasted 2 min. Although frequency of premode was reduced in this sequence, more frequent measurement of the ambient sample (main-mode) could be realized. In addition, the sampling system for the two instruments is shown here. Fig. 4 shows the schematic diagram of the sampling system in calibration and in ambient measurement. In calibration, standard gas from the calibrator was introduced simultaneously into the two instruments. A solenoid valve controlled the #ow automatically to determine the zero of the LIF instrument. Zero point was de"ned here as the signal for zero air generated by a zero air supply. In ambient sampling, the same sampling tubing was used for the two instruments for the purpose of simplicity in the intercomparison. The sample inlet was placed on the top of a small tower, 5.5 m above the ground. In the observatory, the line branched out into two lines to the LIF and PLC-CL instruments. A PTFE "lter was installed in order to remove particles which can cause interference. Before introduction to the LIF cell, the sample passed through a critical ori"ce with
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J. Matsumoto et al. / Atmospheric Environment 35 (2001) 2803}2814
Fig. 4. Schematic diagram of #ow system for LIF-NO (left) and PLC-CL (right): AMB: ambient inlet at a height of 5.5 m; CAL: supply of standard gas for GPT calibration; ZG: zero air supply for periodical zero monitor; SV: solenoid valve automatically controlled by PC; MFC: thermal mass #ow controller; OR1: critical ori"ce plate (0.2); EC: excitation cell of LIF; P: pressure monitor; RP: oil rotary pump; DP2: diaphragm pump to reduce residence time of PLC-CL; NV: needle valve to set #ow to DP2; MFM: thermal mass #ow meter; PLC: photolytic converter; OR2: critical ori"ce plate (0.8); CLD: chemiluminescence detector; O2: oxygen cylinder to generate ozone for CLD; OS: ozone scrubber; DP1: diaphragm pump.
a pinhole (0.2 mm) to keep the desired low pressure in the cell. Then NO concentration in the sample was monitored as described in Section 3. This PLC-CL system required a #ow of 3 SLM (standard liter per minute as a mass #ow). Ozone for NO detection in the CL was generated by silent discharge of oxygen from an oxygen cylinder. In the ambient sampling, another diaphragm pump (DP2) was utilized to reduce residence time in the tubing, thus avoiding chemical interference, e.g. by PAN (Bradshaw et al., 1999) or NO#O (Fehsenfeld et al., 1990). Ambient temperature was typically 303C in daytime. Thus, the lifetime of PAN was small, and the concentration was expected to be negligible. The #ow was restricted to 10 SLM by a needle valve (NV), so that the pressure condition in the tubing for ambient sampling was the same as that for calibration. For the ambient sampling, residence time of LIF-NO in the tubing and the cell was 2.4 s. For the PLC-CL instrument, it took 2.2 s in NO measurement and additionally 8 s in the photolytic converter in NO measurement. These di!er ences in residence time were important in correcting for the reactions among NO, NO , and O , as shown in Section 5.3. As for the environmental constraints, room temperature at the observatory was kept at 28$53C by air conditioners. Room air was e!ectively circulated by three small fans. Heat emitted from the laser and the pump was blown o!. As a result of such heat exchange, the temperature of the instrument, especially at the cell and the detector, was kept at room temperature with a similar deviation.
5.3. Calibration and correction Routine calibration of the two instruments was conducted by &gas phase titration (GPT)' method. An NO/N cylinder (NIPPON SANSO Corp., 1 ppmv) was used as an NO standard source. The CL detector calib rated for NO was prepared. Ozone was added to the standard NO diluted by zero air in a dynamic dilution system (TECO, Model 146), and about half of the NO was converted to NO . The concentration of NO gener ated was known as the reduction of NO concentration monitored by the CL detector. This method is thought to provide a reliable standard NO sample. This calibration was carried out twice a day to monitor the stability of the instruments. As suggested in previous works such as Thornton et al. (2000), the quenching e!ect of water vapor is important. The concentration of water vapor in the marine atmosphere is expected to be high. Thus, the correction for the e!ect of water vapor should be considered in this study. Humidi"ed calibration to check the e!ect of water vapor was conducted for both the LIF and PLC-CL instruments. The humidi"ed standard was also prepared by the GPT method. Zero air was bubbled through a bottle "lled with distilled water, and then diluted by additional zero air #ow to vary the humidity. This &humidi"ed zero air' was utilized as NO/NO dilution air. After GPT with humidi"ed air, humidi"ed NO standard can be sup plied. The dew point of standard was monitored by a hygrometer to ensure the stability of the humidity. As a result, the reduction of the sensitivities of the LIF and the PLC-CL instrument was 16% and 15%, respectively,
J. Matsumoto et al. / Atmospheric Environment 35 (2001) 2803}2814
with water vapor of 3% by volume. The value for the LIF instrument is reasonable in comparison with that of previous works. These values were adopted for the correction of ambient data. In order to compare the results from the two instruments, other factors which cause di!erences need to be removed. Thus, observed data of LIF and PLC-CL should be selected and corrected considering the di!erences in measurement conditions. At "rst, comparison should be conducted for the data of the same ambient sample temporally. This synchronization is important because ambient NO itself can vary in a short time due to exchange of air mass or spike by local emission. For this purpose, data averaged for a similar duration in each cycle were selected for intercomparison. Additionally, ambient variation of NO can cause large di!erences V between the two instruments because of the in#uence of NO variation on the PLC-CL measurement. As a next step, the e!ect of ambient O should be corrected in order to cancel the contribution in the sampling lines. In this study, the residence times were 2.4 s (LIF), 2.2 s (PLC-CL, NO mode), and 10.2 s (PLC-CL, NO mode), respectively. Ambient NO has been thought to react with O and to be converted to NO in the sampling lines. Thus, it is possible that the measurement overestimates the concentration in the atmosphere. It has also been shown that NO converted from NO in PLC can be reconverted to NO due to the reaction with O from ambient air (Ridley et al., 1988). The measure ment by the PLC-CL instrument can underestimate NO concentration. Thus, it is necessary to correct NO and NO data using the ambient O data. The method of correction of the reaction in the tubing and PLC has been described elsewhere (Fehsenfeld et al., 1990). Brie#y, knowing the residence time in the tubing and PLC, the initial concentrations of ambient NO and NO are esti mated by simulation of the variations of concentrations due to the reaction of NO#O and photofragmentation of NO to NO in PLC. It was suggested in the previous study that NO by PLC-CL was totally corrected by ozone within a factor of #5%. Here, this factor for correction represents the degree of underestimate of the NO concentration in the measurement. 5.4. Acquired data of NO2 The LIF-NO instrument ran from 6 to 15 August. As a result of routine calibration with dry standard gas, sensitivity, background signal, conversion e$ciency for LIF and/or PLC-CL were found to be nearly constant during the campaign, with variations of these factors less than 5%. As a result of humidi"ed calibration in this measurement, the e!ects of water vapor for LIF and PLC-CL were 16% and 15%, respectively, for 3% H O by volume. In Okinawa, because water vapor varied from 2.7% to 3.5% by volume, the reduction of sensitivity was
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estimated from 15% to 19% for LIF and from 14% to 18% for PLC-CL. Thus, variations in sensitivity relative to typical values for LIF and PLC-CL were $2.4% and $2.6%, respectively. Ambient ozone varied from 10 to 45 ppbv in the campaign. Thus, correction factors for the e!ect of ambient ozone varied from 0 pptv (0% of NO ) to !40 pptv (!2.5%) for the LIF instrument and from #10 pptv (#4%) to #250 pptv (#16%) for the PLC-CL instrument, respectively. This di!erence between the correction factors caused by ozone is due to the di!erence of residence time in the sampling lines. The residence time in the PLC-CL instrument was 10.2 s. The large residence time is essential to the &indirect' method as in PLC-CL in order to acquire su$cient conversion e$ciency of the photolysis. Such a large residence time results in errors due to the e!ect of ambient ozone. However, the residence time in the LIF instrument was only 2.4 s. The e!ect of ambient ozone is negligible. This is the advantage of the direct LIF measurement. The "nal data from the two instruments as well as O are shown in Fig. 5. These are plotted as 12 min averages in order to simplify the intercomparison through smoothing of the variation. After data were selected considering temporal synchronization of the two instruments and the ambient variation of NO (see SecV tion 5.3), there were 371 measurements to compare as 12 min values. Among these data, there were 107 points below 500 pptv as the typical data for the marine boundary layer. There were a few interruptions of measurements due to calibration or required preparations for the typhoon. With these exceptions, the LIF-NO system continued to make stable measurements for almost 10 days. This instrument was also capable of running automatically without any operators even through the night. In this campaign, this compact LIF system was shown to be stable and reliable for "eld measurement. The concentration of NO was frequently over 1 ppbv when air masses came from the southwest, passing over the island. This high level of NO is reasonable concern ing the regional emission from anthropogenic sources in the populated area of the island. On the other hand, lower NO resulted from unpolluted air masses coming from the Paci"c Ocean. NO variation was caused by a combination of daytime photolysis of NO , reaction with O , and local emissions from the sugar cane "eld near the observatory. 5.5. Agreement between LIF and PLC-CL As shown in Fig. 5, the ambient data from the LIFNO instrument agreed with those of the PLC-CL instrument. A cross plot of all these data is shown in Fig. 6(a). The correlation between LIF and PLC-CL is represented by the unweighted linear regression line with a slope of 0.98$0.01 and an intercept of 88$25 pptv.
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J. Matsumoto et al. / Atmospheric Environment 35 (2001) 2803}2814 Table 4 The results of regression analyses for the LIF and PLC-CL data. Acquired values are shown for (A) all data and (B) less than 500 pptv of NO . Errors are given for 95% con"dential interval
Slope Intercept R
Fig. 5. 12 min averaged data observed in Okinawa, 1999: (a) LIF-NO ; (b) PLC-CL-NO ; (c) O for correction. Dotted lines represent midnight (00:00) by Japan Standard Time (JST).
Errors are shown for a 95% con"dence interval. The results of the regression analyses are summarized in Table 4. Because the correlation coe$cient, R, is 0.99, the excellent linearity between two instruments is shown in the range of observed NO concentration. The inter cept is not zero, but it is insigni"cant in this correlation because the full scale is still large (8000 pptv) and the intercept is in#uenced by the data of ppbv NO . The intercept &88 pptv' is su$ciently small considering the full scale (8000 pptv). In order to discuss the di!erence between two instruments at lower concentration level, data below 500 pptv are compared as shown in Fig. 6(b). The regression line with a slope of 1.17$0.20 and an inter-
(A)
(B)
0.98$0.01 88$25 0.99
1.17$0.20 19$63 0.57
cept of 19$63 pptv is acquired. R is small (0.57) and the correlation is not good. Data points are widely scattered in the plot. Such a large scatter may be caused by the LODs of the LIF (94 pptv) and the PLC-CL (54 pptv). Considering the deviation in the correlation, the slope is regarded as 1 and the intercept is 0 within errors. As a result, the agreement between two instruments is found in this measurement. This agreement suggests that the responses of LIF and PLC-CL are quite the same and reliable. In addition, the reasonability of correction for water vapor and ozone is also con"rmed. It is important that ozone can be highly in#uential especially for PLC-CL, due to large residence time. However, NO concentration in the marine boundary layer is generally expected to be below 100 pptv. Thus, improvement of the instrument is important in order to discuss the intercomparison more precisely. Although the LOD still needs improvement, the direct measurement by the LIF instrument has an advantage over the PLC-CL in its shorter residence time to diminish the reaction of NO#O . 5.6. Problems and future capability The LIF instrument here still has to be improved to quantify NO below 100 pptv. Now the LOD is deter mined mainly by a large background signal. Thus, it is
Fig. 6. Intercomparison between LIF-NO and PLC-CL-NO in Okinawa: (a) comparison of 12 min averaged data; (b) same as (a), except for below 500 pptv. Solid lines represent the regression lines.
J. Matsumoto et al. / Atmospheric Environment 35 (2001) 2803}2814
most important to reduce background signal. A multipass cell can potentially increase background due to laser beam scattering and emissions from the laser mirrors. In addition, the present multi-pass alignment is less e!ective because of the dependence of sensitivity on beam position in the cell. Only the central area of the cell is e!ective due to the characteristic of the lens system. Thus, it may be necessary to adopt and modify a single pass alignment to reduce background. Once this is achieved, a more powerful laser system can be utilized in order to increase sensitivity. If the improvement of the instrument could be explored, much more precise discussion of tropospheric photochemistry in the marine atmosphere could be possible. On the other hand, short-term phenomena such as radical chemistry can be conveniently studied after improvement of the LIF instrument to measure NO at the level of pptv with high temporal resolution.
6. Conclusions Laser-induced #uorescence technique was adopted to measure NO directly in the marine atmosphere. In order to increase sensitivity, a multi-pass excitation cell was utilized as the prototype instrument. Considering the mechanism of excitation and quenching, the cell pressure and the gate time of photon counting were determined, following calculation and validation experiments. After the optimization of other conditions, the LIF instrument's LOD was 94 pptv for 60 s integration. Field measurement was also conducted in Okinawa, in summer, in order to con"rm the reliability of the LIF instrument in the marine atmosphere. Continuous measurement for 10 days was conducted and the stability of the instrument was shown. Intercomparison with a commercial PLC-CL instrument resulted in no signi"cant di!erence between the two instruments in spite of a large correction for ozone e!ect. As a result, the LIF instrument can be seen as a simple, reliable, and promising method of measuring NO directly in the marine atmosphere. In future, although further improvement of the instrument is necessary, the LIF technique will be useful to investigate photochemistry in the marine atmosphere more precisely.
Acknowledgements We are grateful to Dr. Y. Kanaya, Dr. S. Kato (the University of Tokyo) and all other members of ORION'99 measurement team for their advice and practical help in the campaign. This work was supported by the Grant-in-Aid for Scienti"c Research (No. 09558066) from the Ministry of Education, Science, and Culture, and by Core Research for Evolutional Science and Technology from Japan Science and Technology Corporation.
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References Akimoto, H., Mukai, H., Nishikawa, M., Murano, K., Hatakeyama, S., Liu, C.-M., Buhr, M., Hsu, K.J., Ja!e, D.A., Zhang, L., Honrath, R., Merrill, J.T., Newell, R.E., 1996. Long-range transport of ozone in the East Asian Paci"c rim region. Journal of Geophysical Research 101, 1999}2010. Allan, B.J., Carslaw, N., Coe, H., Burgess, R.A., Plane, J.M.C., 1999. Observations of the nitrate radical in the marine boundary layer. Journal of Atmospheric Chemistry 33, 129}154. Bradshaw, J., Davis, D., Crawford, J., Chen, G., Shetter, R., Muller, M., Gregory, G., Sachse, G., Blake, D., Heikes, B., Singh, H., Mastromarino, J., Sandholm, S., 1999. Photofragmentation two-photon laser-induced #uorescence detection of NO and NO: comparison of measurements with model results based on airborne observations during PEM-Tropics A. Geophysical Research Letters 26, 471}474. Carslaw, N., Creasey, D.J., Heard, D.E., Lewis, A.C., McQuaid, J.B., Pilling, M.J., Monks, P.S., Bandy, B.J., Penkett, S.A., 1999. Modeling OH, HO , and RO radicals in the marine boundary layer 1. Model construction and comparison with "eld measurements. Journal of Geophysical Research 104, 30241}30255. Fehsenfeld, F.C., Drummond, J.W., Roychowdhury, U.K., Galvin, P.J., Williams, E.J., Buhr, M.P., Parrish, D.D., HuK bler, G., Langford, A.O., Calvert, J.G., Ridley, B.A., Grahek, F., Heikes, B.G., Kok, G.L., Shetter, J.D., Walega, J.G., Elsworth, C.M., Norton, R.B., Fahey, D.W., Murphy, P.C., Hovermale, C., Mohnen, V.A., Demerjian, K.L., Mackay, G.I., Schi!, H.I., 1990. Intercomparison of NO measurement tech niques. Journal of Geophysical Research 95, 3579}3597. Fong, C., Brune, W.H., 1997. A laser induced #uorescence instrument for measuring tropospheric NO . Review of Scient i"c Instrument 68, 4253}4262. Gao, R.S., Keim, E.R., Woodbridge, E.L., Ciciora, S.J., Pro$tt, M.H., Thompson, T.L., McLaughlin, R.J., Fahey, D.W., 1994. New photolysis system for NO measurements in the lower stratosphere. Journal of Geophysical Research 99, 20673}20681. George, L.A., O'Brien, R.J., 1991. Prototype FAGE determination of NO . Journal of Atmospheric Chemistry 12, 195}209. Kley, D., McFarland, M., 1980. Chemiluminescence detector for NO and NO . Atmospheric Technology 12, 63}69. Myers, G.H., Silver, D.M., Kaufman, F., 1966. Quenching of NO #uorescence. Journal of Chemical Physics 44, 718}723. Nagao, I., Matsumoto, K., Tanaka, H., 1999. Sunrise ozone destruction found in the sub-tropical marine boundary layer. Geophysical Research Letters 26, 3377}3380. Platt, U., Perner, D., Patz, H.W., 1979. Simultaneous measurement of atmospheric CH O, O , and NO by di!erential optical absorption. Journal of Geophysical Research 84, 6329}6335. Reid, J., Garside, B.K., Shewchun, J., El-Sherbiny, M., Ballik, E.A., 1978a. High sensitivity point monitoring of atmospheric gases employing tunable diode lasers. Applied Optics 17, 1806}1810. Reid, J., Shewchun, J., Garside, B.K., Ballik, B.A., 1978b. High sensitivity pollution detection employing tunable diode lasers. Applied Optics 17, 300}307. Ridley, B.A., Howlett, L.C., 1974. An instrument for nitric oxide measurements in the stratosphere. Review of Scienti"c Instrument 45, 742}746.
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Ridley, B.A., Carroll, M.A., Gregory, G.L., Sachse, G.W., 1988. NO and NO in the troposphere: technique and measure ments in regions of a folded tropopause. Journal of Geophysical Research 93, 15813}15830. Rudich, Y., Talukdar, R.K., Ravishankara, A.R., 1998. Multiphase chemistry of NO in the remote troposhere. Journal of Geophysical Research 103, 16133}16143. Schi!, H.I., Mackay, G.I., Castledine, C., Harris, G.W., Tran, Q., 1986. Atmospheric measurements of nitrogen dioxide with a sensitive luminol instrument. Water Air and Soil Pollution 30, 105}114.
Thornton, J.A., Wooldridge, P.J., Cohen, R.C., 2000. Atmospheric NO : in situ laser-induced #uorescence detection at parts per trillion mixing ratios. Analytical Chemistry 72, 528}539. Vandaele, A.C., Hermans, C., Simon, P.C., Carleer, M., Colin, R., Fally, S., Merienne, M.F., Jenouvrier, A., Coquart, B., 1997. Measurements of the NO absorption cross-section from 42,000 cm\ to 10,000 cm\ (238}1000 nm) at 220 K and 294 K. Journal of Quantitative Spectroscopy and Radiative Transfer 59, 171}184.
Direct measurement of NO in the marine atmosphere by laser-induced #uorescence technique J. Matsumoto*, J. Hirokawa, H. Akimoto, Y. Kajii Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan Received 25 September 2000; received in revised form 12 December 2000; accepted 24 December 2000
Abstract A simple and compact instrument for NO measurement by laser-induced #uorescence (LIF) technique with a pulsed solid state laser and a multi-pass excitation system was developed and optimized for several conditions. As a result of laboratory experiment, the limit of detection (LOD) reached 94 pptv for 60 s integration. It was thought that an LIF instrument with this LOD value would be capable of quantifying sub-ppbv NO in unpolluted marine atmosphere. As the second step, a "eld test of the instrument was conducted in the marine atmosphere at Cape Hedo, Okinawa Island, Japan, in summer 1999. Intercomparison between the LIF instrument and a chemiluminescence detector with a photolytic converter (PLC-CL) was also made in this test. Consequently, the LIF instrument was shown to be of practical use for measuring NO in clean maritime air. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Nitrogen dioxide; Laser-induced #uorescence; Marine atmosphere; Intercomparison; Water vapor
1. Introduction Nitrogen oxides (NO "NO#NO ) are important V species in tropospheric photochemistry. NO plays a role V as a precursor of ozone. Ozone can be formed through the reactions of NO and peroxy radicals (HO , RO ). V V V Ozone formation is often limited by NO concentration. V NO can be "nally transformed to HNO which results V in acid rain. Thus, measurement of NO concentration is V essential to the understanding of chemical processes in the atmosphere. In this decade, interest in chemistry in the boundary layer of the marine atmosphere has increased. Nagao
* Corresponding author. Tel.: #81-3-5452-5144; fax: #813-5452-5143. Present address: School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Present address: Frontier Research System for Global Change, 3173 Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan. E-mail address: [email protected] (J. Matsumoto).
et al. (1999) discussed the possibility of ozone destruction by halogen species. Ozone variation and radical chemistry in the marine boundary layer (MBL) have been a focus (Carslaw et al., 1999). As for nitrogen species, reactions of NO with hydrocarbons, dimethyl sul"de (DMS), or sea-salt aerosols in the MBL can be important as nighttime radical sources or sinks (Allan et al., 1999; Rudich et al., 1998). Most of these phenomena in the MBL are related to NO variation. Thus, for the purpose V of studying these topics, &background NO ' in the clean V MBL is the most important factor. In order to quantify NO and NO concentration, several techniques have been utilized in previous work. NO has been widely measured by the ozone-chemiluminescence (CL) technique (Ridley and Howlett, 1974). This method has been developed su$ciently to detect NO directly at the pptv level for 60 s integration. In this paper, the limit of detection (LOD) is de"ned or calculated for 60 s integration and 1 standard deviation (1). NO measurements have been considered as more prob lematic than NO measurements. The luminol chemiluminescence technique (LC) can detect NO (Schi! et al., 1986); however, ozone and peroxyacetylnitrate (PAN) can in#uence the signal as interference (Fehsenfeld et al.,
1352-2310/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 0 7 8 - 4
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1990). Although the di!erential optical absorption spectroscopy (DOAS) technique can measure NO (Platt et al., 1979), it lacks mobility and spatial resolution due to a long optical path. The photofragmentation two-photon laser-induced #uorescence (PF-TP-LIF) technique detects NO after photolysis of NO (Bradshaw et al., 1999). Although the system is an indirect method and not so compact, it is very sensitive. The tunable diode laser absorption spectroscopy (TDLAS) technique has the capability to measure down to a level of 200 pptv, though care must be taken in the data analysis (Reid et al., 1978a, b; Fehsenfeld et al., 1990). The most widely used technique to measure NO simply is CL detection after conversion of NO to NO (Kley and McFarland, 1980). A photolytic converter (PLC) is thought to be the most reliable system to transform NO to NO, having higher selectivity than the metal-catalytic converter (Gao et al., 1994). However, this is an indirect method of NO detec tion involving the calculation of the di!erence of two signals detected as NO. Thus, some of its disadvantages are known as follows: (1) temporal variation of conversion e$ciency, (2) lower conversion e$ciency (e.g. 60}80% for Xe-arc lamp), (3) in#uence of the variation of NO concentration faster than one measurement cycle, (4) the possibility of artifacts from other nitrogen species, and (5) limit of the temporal resolution because of the necessity of two measurement modes for NO and NO . V As a direct, selective, in situ, and sensitive technique to detect NO with high temporal and spatial resolution, the laser-induced #uorescence (LIF) technique is desirable. A couple of workers have explored this technique. George and O'Brien (1991) developed a #uorescence assay by gas expansion (FAGE) instrument with a frequency-doubled Nd : YAG laser and the LOD was 500 pptv. Fong and Brune (1997) adopted a tunable dye laser and 44-pass repetition of the laser alignment, and the LOD was 300 pptv. Thornton et al. (2000) recently reported on a compact LIF system with a tunable dye laser and a 40-pass detection cell. The sophisticated con"guration of the parts of the instrument and the multipass excitation system results in an LOD of 3}17 pptv for 60 s averaging at S/N"1. Their instrument is excellent because it has high selectivity utilizing two wavelengths. When the e!ects of the interference like particles are thought to be little, the single wavelength excitation is acceptable. This technique is compact and simple because a stable and compact laser is utilized. Such simple instruments are desirable for "eld measurements. We have developed a compact LIF-NO instrument with a single wavelength excitation utilizing a diode-laserpumped Nd : YLF laser and a multi-pass excitation cell, for the purpose of simple, compact, e!ective and direct measurements of NO . A "eld measurement test includ ing an intercomparison with a PLC-CL detector was conducted at a subtropical observatory in Okinawa, in the northwest rim region of the Paci"c Ocean. In this
paper, a detailed description of our instrument, the data analysis concerning the variation of water vapor, and the intercomparison of the instrument with the PLC-CL are given in order to discuss the reasonability, stability, and capability of this instrument.
2. Principle of LIF-NO2 system The NO molecule absorbs photon in the visible re gion for transition between A and B states. When an NO molecule absorbs a photon in this region, the mol ecule is excited and emits a red-shifted #uorescence. This scheme is described below: NO #h PNOH, NOH PNO #h, NOH#MPNO #M.
(R1) (R2) (R3)
Reaction (R1) is the excitation of NO by a photon. NO* represents NO in the excited state. After excitation, NO* can emit #uorescence (R2) or be quenched by colli sion with the third molecule &M' in reaction (R3) to return to the ground state. In this scheme, the number of photons emitted in reaction (R2) is thought to be proportional to the initial concentration of NO . Thus, the NO concentration can be determined by detection of this #uorescence. The #uorescence signal, S, emitted from NO can be simply described as S"KI[NO ] , $
(1)
where K is the instrumental function which includes collection e$ciency of photon and sensitivity of the photon detector, I is the laser intensity (mW), is the absorption cross section of NO which at 523.5 nm is 1.36;10\ cm (Vandaele et al., 1997), [NO ] is the number density of NO in the ground state in the detec tion cell and is the quantum e$ciency of #uorescence. $ Eq. (1) can be rearranged for the volume mixing ratio of NO in the sample, m , as ,S"CKIm P , ,$
(1)
where P is the total pressure in the detection cell, C is the conversion coe$cient for transformation between [NO ] and m , independent of pressure and is ,$ de"ned as the branching ratio of radiation of #uorescence from the excited NO* molecule. In this paper, a time-gated photon counting method was utilized for improving the signal-to-noise ratio (SNR). Photon detection was temporally controlled after each laser pulse. In this case, can be shown as $ (t , t )" (t"R) $ $
R
R
e\R O dt,
(2)
J. Matsumoto et al. / Atmospheric Environment 35 (2001) 2803}2814
k k 1 (t"R), , , , $ k # k [M ] k 1# q P G G G G + G G (3) 1 ,1/k" , k 1# q P G G G +
(4)
q "k /k , (5) G G where t and t are, respectively, the initial and "nal times of the counting gate after the laser pulse, k is the radiative rate constant of the excited NO molecule (reported to be 2.3;10 s\) (Myers et al., 1966), k is the G rate constant of quenching by the third compound M , G [M ] is the number density of M , q is the quenching G G G factor for M (Torr\; 1 Torr"133.3 Pa) and P G is the G + partial pressure of M (Torr). G After normalization of the #uorescence signal (S; unit: cps"count s\) by NO mixing ratio (m , ppbv) and ,laser intensity (I, mW), the sensitivity of the LIF instrument, S (cps ppbv\ mW\), can be described as ,R P S "CK e\R O dt. (6) ,1# q P G G G + R Therefore, sensitivity is mainly in#uenced by instrumental function, cell pressure, quenching factor, and setup of the counting gate. In the case of #uorescence detection, LOD is thought to be limited by background signal (BG, cps). The LOD can be described as
1 (S/N) 1 # , (7) LOD+ % S I m n ,where (S/N) is the signal-to-noise ratio to de"ne the LOD, m and n represent the number of measurements of background and sample, respectively, and (cps) rep% resents the deviation of the background signal for a "xed integration time, t (s). In the photon counting method, the counted signal (cps) usually follows the Poisson distribution, and deviation of the signal closely corresponds to the square root of that signal as follows:
%
"
BG . t
%
.
Finally, Eq. (7) can be reformed with Eqs. (6)}(9) as
(S/N) 1 1 S % LOD+ . # S m n I t ,-
(10)
Therefore, in order to improve the LOD, it is necessary (a) to realize high sensitivity, (b) to increase laser intensity, and (c) to reduce background signal e!ectively. It should be also noted that this is the LOD estimated from laser-induced background consideration. For the strict estimation of LOD, #uctuation of zero air should be included. This #uctuation is shown in the following section.
3. Description of LIF-NO2 instrument Fig. 1 shows the schematic diagram of the LIF-NO instrument. As a light source, a frequency-doubled Nd : YLF pulse laser pumped by a solid state laser (523.5 nm, Spectra-Physics TFR laser) was utilized. The laser has the maximum output, 360 mW, at 3000 Hz of the pulse repetition rate. This laser passed through an optical lens for compensation of the beam divergence (5 mrad) and a half-wave plate for optimization of the beam polarization (not shown in the "gure), and then was introduced horizontally to the excitation cell. The length of the cell was 55 cm. The radius of its arm was 5 cm. Thus, the volume in the cell was 1100 cm. The material of the wall of the cell was aluminum in order to reduce the weight. The wall inside the cell was black-coated to reduce the number of scattered photons. Multi-pass alignment of the laser in this cell was utilized in order to
(8)
Smaller background signal can cause smaller deviation of signal and thus more precise measurement. Background signal is usually composed of scattered light of the laser in the cell and dark current noise in the photomultiplier tube. When the background signal of laser scattered light is still larger than that of dark current, BG can be regarded as proportional to laser intensity, I. In that case, &background sensitivity', S (cps mW\), can be de"ned % as background signal normalized by laser intensity: BG"IS
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(9)
Fig. 1. Schematic diagram of the LIF-NO instrument: Nd : YLF: laser instrument as a light source; M }M : mirrors for laser alignment; W: window with a slope of Brewstar angle; BP: ba%e plates to reduce scattered light; PM: photodiode for laser power monitor; LS: lens system; RF: red glass "lter ( "640 nm); PMT: photomultiplier; PCB: PMT cooler box. A solid line from Nd : YLF represents the pass of the laser beam.
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increase the e!ective volume for NO excitation and to acquire high sensitivity. The pass number was set to 18, considering the limit of the residual divergence of laser beam. Two re#ection mirrors were provided for the multi-pass. The deviation of laser output was monitored outside the cell by a photodiode detector. The variation was typically within 10%. Sensitivity of the instrument was corrected by laser output. In the cell, several ba%e plates with slits (5 mm high and 50 mm wide) for the laser beam to pass through were positioned to reduce background photons scattered at the windows, walls, and mirrors of the cell. Sample gas was drawn by an oil rotary pump with a capacity to draw a #ow of 350 l min\ (as a volume #ow rate); its weight and dimension were 30 kg and 45 cm;20 cm;25 cm, respectively. Sample gas was then introduced into the center of the cell vertically in the region where laser beam density was high. To reduce the cell pressure, the air sample passed through a critical ori"ce plate before entering the cell. A 10-Torr full-scale BARATRON pressure transducer (MKS Japan, Inc.) always monitored pressure in the cell. Photons emitted from NO were gathered on the sensitive surface of a dynode-gated photomultiplier (Hamamatsu, R943-02) through the optical lens system. The lens system consisted of 4 lenses as shown in Fig. 1. This was constructed through a calculation by the &ray trajectory analysis' in order to collect photons e!ectively at the center of the cell. A sharp-cuto! glass "lter was installed in front of the PMT to reduce background signal originating from scattered photons. The cuto! wavelength ( ) with trans mittance of 50% was 640 nm. In the "eld measurement, the PMT was kept at 03C by a simple cooling system to reduce dark current signal. The dark current signal was 3.7 cps on the average in this study. Photon signal was counted by a time-gated photon counting unit in order to improve (S/N) ratio. Trigger signal of the pulsed laser was used to decide the initial time in each cycle, and a digital delay/pulse generator (DG535, Stanford Research Systems) controlled the gate time for the detection. The background signal was monitored utilizing a zero air generator shown in Section 5.2. The uncertainty of zero air was estimated as 30 pptv. This value was acquired as a result of comparison with the synthetic air of a cylinder. This uncertainty should be included in the estimation of LOD. The laser head, optics, the excitation cell, and the detection system were installed on the aluminum board with a dimension of 80 cm;60 cm, and the height was 30 cm. This board and other accessories were put on a desk (140 cm;90 cm). Power supplies of high voltages for the PMT, a laptop PC, a control unit of the laser and a zero air generator were stacked in a 19 in lack (170 cm high). An oil rotary pump, a chiller for the laser, and a water tank for cooling the PMT were placed on the #oor. The whole instrument was set up within a space of
200 cm;130 cm and a height of 150 cm. The sum of their weights was 150 kg. Power consumption of the instrument is 2 kW.
4. Optimization of the instrument The instrument was optimized for several conditions to increase sensitivity and reduce background signal. In this section, the procedure of optimization is presented. 4.1. Estimation of sensitivity variation As shown in Section 2, the dominant factors which determine the capability of the LIF-NO instrument are pressure in the cell and timing of the counting gate. If the laser intensity is "xed to one constant value, sensitivity can be estimated based on these two factors. Here, optimization of the LIF instrument focuses on these two factors. From Eq. (6), dependence of sensitivity of the instrument on pressure and counting gate can be simulated. To estimate dependence on counting gate, the width of the gate was set to 3 s. At this stage, the quenching e!ect of water vapor was not of concern in order to determine the sensitivity variation of the instrument itself as a function of cell pressure and gate time. The results of simulations of sensitivity variation are shown in Fig. 2. The y-axis in this plot represents the initial gate time, t . Fig. 2(a) shows the relative variation of sensitivity under dry conditions. When the gate time is "xed, sensitivity has a peak at a pressure of 0.5}1.0 Torr. This trend is reasonable since too high a pressure increases the e!ect of quenching and too low a pressure reduces the number density of NO in the cell. On the other hand, if the pressure is "xed, sensitivity decreases as a counting gate is set later. This can be explained by the fact that NO* decreases as time passes after the laser pulse due to deactivation by radiation and quenching. To validate this estimate, the pressure dependence of sensitivity was observed for a "xed gate time and compared with the estimated trend. Standard NO was prepared by gas phase titration method (see Section 5.3). Fig. 2(b) shows the experimental results measured when the initial and "nal gate times, t and t , were set at 1.5 and 8.0 s, respectively. The experimental results agreed with the results of the simulation. This ensures the reasonability of the trend predicted in the simulation shown in Fig. 2(a). Fig. 3 shows the observed dependence of background and NO signals on the gate time of photon counting. The open circles and solid squares represent decays of signals for 0 and 227 ppbv of NO , respectively. Data are shown as counts for 1 s integration (0.7 Torr, 200 ns width of gate, 3 kHz, 360 mW). The decay of the signal for 227 ppbv resulted from the quenching e!ect by N and O . Because the background signal was independent of
J. Matsumoto et al. / Atmospheric Environment 35 (2001) 2803}2814
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Fig. 3. Dependence of signals on gate time for 200 ns gate width, 0 and 227 ppbv NO , 3 kHz repetition rate, and 360 mW laser intensity.
Table 1 Adopted conditions of LIF-NO in this work
Fig. 2. Dependence of sensitivity on the pressure in the excitation cell and gate time of the photon counting method. (a) Calculated sensitivity for a gate width of 3 s. The values on the contour map represent the relative sensitivity by an arbitrary unit. (b) Dependence of sensitivity on pressure. The gate time is "xed to (t , t )"(1.5 s, 8.0 s). Open squares with error bars are shown for experimental data and a solid line is for calculation.
the cell pressure, it was caused by laser photons scattered on the walls and the window in the cell. Sensitivity increased with the earlier start time of the gate. However, background signal also increased due to scattered laser. The degree of such an increase in the background signal was greater than the sensitivity as shown in Fig. 3. The maximum ratio of the sensitivity to the background in the "gure was acquired for gate times of 1.3 and 6.0 s. Thus, this gate time was adopted in order to reduce the background and acquire the best LOD value of Eq. (10). 4.2. Instrumental conxguration Other additional factors in the con"guration of the instrument were optimized as follows: (1) Hole size of the critical ori"ce before the excitation cell was selected to realize proper sample #ow rate and cell pressure. Here, ori"ces with a hole size of 0.1}0.8 mm were examined considering the power of the pump and the expected range of cell pressure. (2) LOD is expected to be better
Parameters
Adopted values
Sample #ow rate Cell pressure Counting gate t , t Laser repetition rate Laser intensity Number of passes
300 sccm 0.70 Torr 1.3, 6.0 s 3000 Hz 360 mW 18
t"0 is de"ned as the time of emission of laser pulse.
with higher laser intensity. The averaged power of the laser is highest at 3000 Hz of the pulse repetition rate. Typical power is 360 mW. We adopted the 3000 Hz repetition rate in order to realize the maximum output and the best LOD. (3) When the pass number of laser excitation increases, the e!ective volume of excitation also increases and higher sensitivity can be achieved. However, background signal can increase due to scattered laser photons from the mirrors, ba%e plates, and/or molecules of sample air. Thus, optimal pass number should be selected considering both sensitivity and background signal. Here, several conditions of pass number were compared considering the characteristics of the mirrors, the con"guration of the ba%e plates, and the divergence of the laser beam. (4) The PMT was cooled at as low a temperature as possible to reduce dark current. (5) A sharp-cuto! "lter was placed in front of the PMT, again to reduce background signal. Considering the dependence of sensitivity on pressure, counting gate and all factors of the instrument were determined for as low an LOD as possible. The measurement conditions adopted are summarized in Table 1. Sample #ow was automatically decided due to the
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Table 2 Comparison of speci"cations of the instrument with recent works
10 (cm) Laser intensity (mW) Sensitivity (cps ppbv\ mW\) Background (cps) LOD (pptv)
Fong and Brune (1997)
Thornton et al. (2000)
This work
0.6 250 0.12 2425 300
1 100 0.06 1.5 3.7
1.4 360 0.28 2400 94
LOD values are given for S/N"1, t"60 s, m"n"1.
combination of critical ori"ce, cell pressure, and the power of the pump. The dark current signal was 3.7 cps on the average when laser beam was not introduced into the cell. This signal was negligible in comparison with total background signal (typically 2400 cps). Thus, total background signal was thought to be proportional to laser intensity. The LOD estimated by Eq. (10) was 89 pptv. In addition, the #uctuation of zero air was typically 30 pptv. As a result, the "nal LOD was modi"ed to be 94 pptv (I"360 mW, S/N"1, m"1, n"1, t"60 s), as shown in Table 2. Table 2 shows the comparison of the instrument with recent studies of LIF-NO measurements. When this study is compared with Fong and Brune (1997), sensitivity of our instrument is larger than that of their instrument by a factor of 2.5. This advantage is appropriate to the di!erence in the absorption cross section, although other factors like the gate time and collection e$ciency of the lens system should be considered. Background signals are similar. Laser intensity is larger by a factor of 1.4. Consequently, the LOD is better by a factor of 3.2. In comparison with Thornton et al. (2000), the sensitivity of our instrument is only a half. Background signal of their instrument is su$ciently small beyond compare. As shown in Fig. 3, the background signal is high in the earlier gate time mainly due to laser scattered in the cell. As a result of the setting of gate time, the loss of NO signal for our instrument is roughly 50%. If the background signal could be reduced in the future, the gate time could be set at much earlier time and sensitivity could be increased by a factor of 2.
5. Ambient measurement in the marine atmosphere A "eld measurement using the LIF-NO instrument was conducted at a subtropical observatory in Okinawa, Japan, in the northwest rim region of the Paci"c Ocean, in August 1999. The goal of this measurement was to examine the performance of the instrument at the level of sub-ppbv of NO in the humid marine atmosphere.
A chemiluminescence detector with a photolytic converter (PLC-CL) was also utilized to carry out an intercomparison with the LIF instrument. 5.1. Site description and meteorological conditions This measurement was a part of the campaign, ORION'99, Observation at Remote Island in OkiNawa, from 1 to 15 August 1999 (Kanaya et al., in preparation). The speci"c location of the measurement site has been shown previously (Akimoto et al., 1996). Brie#y, Okinawa Island is located at the northwestern rim of the Paci"c Ocean, and 800 km away from the coastline of China along the East China Sea. Cape Hedo (26352N, 128315E) is the northernmost part of Okinawa Island, 100 km from Naha City, the most populated city on the island. Nago City, a large town in the northern Okinawa is 35 km southwest of the site; the population is 50,000. The measurement site was located 200 m from the western coast and 60 m above sea level. The eastern part of the site was covered mainly by sugar cane "elds and its height was about 1.5 m. There is a sightseeing spot at the northern end of the cape and an access route to it. However, anthropogenic activities around the site are rare. In summer, Okinawa Island is usually covered by the Paci"c high pressure system, and clean air masses come from the east region over the Paci"c Ocean. On sunny days, solar radiation is strong enough for the J value ,to be above 0.01 s\. In such a subtropical region surrounded by sea, the atmosphere near the surface has high humidity, 2.7}3.5% by volume. However, in the campaign period, the Paci"c high pressure system was weak and low pressure systems were active. So there were only three days without any rainfall: 6, 14, and 15 August 1999. In addition, several typhoons generated near the Philippine Islands passed over or near the island. Measurements were safely interrupted several times because of the possibility of electric power failure. After the typhoons passed over, the air mass around the island would be well mixed with the polluted air from the East China Sea originating from the Asian mainland or from the Japan Islands.
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Here, the e!ect of local wind systems should be noted. In the case of local wind from the north or west, the clean marine air mass could reach the site without the addition of pollutants from land. When an easterly wind was dominant around the observatory, measurement was mainly in#uenced by local emission of NO from the sugar cane "eld which covered the whole eastern region. Thus, NO concentration could reach a level of several V hundred pptv and soil emissions became the most important source of NO . Wind from the south or southwest V was thought to have passed over the island with anthropogenic pollutants added regionally. NO could be over V 1 ppbv in such cases.
Table 3 Typical speci"cations of LIF and PLC-CL instruments during the "eld campaign in this work
5.2. Measurement procedure
Calculated for laser intensity of 360 mW ("523.5 nm). Observed standard deviation for 60 s integration. Reduction of sensitivity by quenching e!ect of water vapor (3% by volume). LOD values are given for S/N"1, t"60 s, m"n"1.
Here, methods of measuring NO in Okinawa are shown, with an explanation of the intercomparison between LIF and PLC-CL techniques. At "rst, the measurement cycle of LIF-NO is described. Raw counts include both the LIF signal of NO and the background signal caused by scattered light and dark current. Thus, determination of background signal is important. Background signal can be monitored after NO is removed from the sample. A zero air supply (TECO Model 111) was utilized to generate &zero air' which includes little NO of less than 30 pptv. The measurement sequence here was: (1) zero air mode for background monitoring (60 s) and (2) ambient mode (120 s). A solenoid valve (SV) controlled automatically by a PC was utilized to change the mode between zero air and ambient sample. Between each mode, a 20 s #ush mode was allowed to ensure stability. Each cycle needed about 4 min. This cycle enabled monitoring of the short-term variation of the background signal. A PLC-CL instrument was also utilized. In this work, a chemiluminescence detector of NO (CLD770ALppt, ECO PHYSICS) with a photolytic converter (PLC760, ECO PHYSICS) was utilized as a PLC-CL instrument in order to conduct the intercomparison. The UV light from an Xe-arc lamp was irradiated to the sample air in the photolysis cell and NO was converted to NO. NO and NO were measured sequentially. Calibration of the PLC-CL instrument was conducted twice a day in the "eld measurement. A standard gas of NO (1 ppmv) in a cylinder was diluted by zero air, and the diluted gas was introduced into the NO detector (CLD). The conversion e$ciency of the photolytic converter (PLC) was acquired through the gas phase titration method (GPT). After the GPT calibration, the zero air was introduced into the PLC-CL instrument and the zero point was checked. Humidi"ed calibration is conducted in order to check the reduction of sensitivity by the quenching e!ect of water vapor. The GPT and humidi"ed calibrations are the same as for the LIF instrument, shown in Section 5.3. Speci"cation of the PLC-CL
Sensitivity Background Dark current SD of BG Conv. e!. Quench (H O) LOD
LIF-NO
PLC-CL NO, NO
100 cps ppbv\ 2400 cps 3.7 cps 6.4 cps * 16% 94 pptv
400 cps ppbv\ (NO) 300 cps (NO) 50 cps 6.0 cps (NO) 56% (NO to NO) 15% (NO) 21 pptv (NO), 54 pptv (NO )
instrument is summarized and compared with the LIF in Table 3. Measurement by PLC-CL consisted of four modes: (a) NO pre-chamber mode, (b) NO main-chamber mode, (c) NO pre-chamber mode, and (d) NO main-chamber V V mode. Pre-chamber mode determined the zero point of each measurement. Net signal of NO or NO was acV quired as the di!erence between main mode and temporally neighboring pre-mode. In order to reduce #ushing time between di!erent modes, the following sequence was adopted: (1) NO pre, (2) NO main, (3) NO V main, (4) NO pre, (5) NO main, and (6) NO main. As V V each mode required 15 s for the integration of the signal and 5 s for #ushing the lines in the instrument, one cycle (6 modes) lasted 2 min. Although frequency of premode was reduced in this sequence, more frequent measurement of the ambient sample (main-mode) could be realized. In addition, the sampling system for the two instruments is shown here. Fig. 4 shows the schematic diagram of the sampling system in calibration and in ambient measurement. In calibration, standard gas from the calibrator was introduced simultaneously into the two instruments. A solenoid valve controlled the #ow automatically to determine the zero of the LIF instrument. Zero point was de"ned here as the signal for zero air generated by a zero air supply. In ambient sampling, the same sampling tubing was used for the two instruments for the purpose of simplicity in the intercomparison. The sample inlet was placed on the top of a small tower, 5.5 m above the ground. In the observatory, the line branched out into two lines to the LIF and PLC-CL instruments. A PTFE "lter was installed in order to remove particles which can cause interference. Before introduction to the LIF cell, the sample passed through a critical ori"ce with
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Fig. 4. Schematic diagram of #ow system for LIF-NO (left) and PLC-CL (right): AMB: ambient inlet at a height of 5.5 m; CAL: supply of standard gas for GPT calibration; ZG: zero air supply for periodical zero monitor; SV: solenoid valve automatically controlled by PC; MFC: thermal mass #ow controller; OR1: critical ori"ce plate (0.2); EC: excitation cell of LIF; P: pressure monitor; RP: oil rotary pump; DP2: diaphragm pump to reduce residence time of PLC-CL; NV: needle valve to set #ow to DP2; MFM: thermal mass #ow meter; PLC: photolytic converter; OR2: critical ori"ce plate (0.8); CLD: chemiluminescence detector; O2: oxygen cylinder to generate ozone for CLD; OS: ozone scrubber; DP1: diaphragm pump.
a pinhole (0.2 mm) to keep the desired low pressure in the cell. Then NO concentration in the sample was monitored as described in Section 3. This PLC-CL system required a #ow of 3 SLM (standard liter per minute as a mass #ow). Ozone for NO detection in the CL was generated by silent discharge of oxygen from an oxygen cylinder. In the ambient sampling, another diaphragm pump (DP2) was utilized to reduce residence time in the tubing, thus avoiding chemical interference, e.g. by PAN (Bradshaw et al., 1999) or NO#O (Fehsenfeld et al., 1990). Ambient temperature was typically 303C in daytime. Thus, the lifetime of PAN was small, and the concentration was expected to be negligible. The #ow was restricted to 10 SLM by a needle valve (NV), so that the pressure condition in the tubing for ambient sampling was the same as that for calibration. For the ambient sampling, residence time of LIF-NO in the tubing and the cell was 2.4 s. For the PLC-CL instrument, it took 2.2 s in NO measurement and additionally 8 s in the photolytic converter in NO measurement. These di!er ences in residence time were important in correcting for the reactions among NO, NO , and O , as shown in Section 5.3. As for the environmental constraints, room temperature at the observatory was kept at 28$53C by air conditioners. Room air was e!ectively circulated by three small fans. Heat emitted from the laser and the pump was blown o!. As a result of such heat exchange, the temperature of the instrument, especially at the cell and the detector, was kept at room temperature with a similar deviation.
5.3. Calibration and correction Routine calibration of the two instruments was conducted by &gas phase titration (GPT)' method. An NO/N cylinder (NIPPON SANSO Corp., 1 ppmv) was used as an NO standard source. The CL detector calib rated for NO was prepared. Ozone was added to the standard NO diluted by zero air in a dynamic dilution system (TECO, Model 146), and about half of the NO was converted to NO . The concentration of NO gener ated was known as the reduction of NO concentration monitored by the CL detector. This method is thought to provide a reliable standard NO sample. This calibration was carried out twice a day to monitor the stability of the instruments. As suggested in previous works such as Thornton et al. (2000), the quenching e!ect of water vapor is important. The concentration of water vapor in the marine atmosphere is expected to be high. Thus, the correction for the e!ect of water vapor should be considered in this study. Humidi"ed calibration to check the e!ect of water vapor was conducted for both the LIF and PLC-CL instruments. The humidi"ed standard was also prepared by the GPT method. Zero air was bubbled through a bottle "lled with distilled water, and then diluted by additional zero air #ow to vary the humidity. This &humidi"ed zero air' was utilized as NO/NO dilution air. After GPT with humidi"ed air, humidi"ed NO standard can be sup plied. The dew point of standard was monitored by a hygrometer to ensure the stability of the humidity. As a result, the reduction of the sensitivities of the LIF and the PLC-CL instrument was 16% and 15%, respectively,
J. Matsumoto et al. / Atmospheric Environment 35 (2001) 2803}2814
with water vapor of 3% by volume. The value for the LIF instrument is reasonable in comparison with that of previous works. These values were adopted for the correction of ambient data. In order to compare the results from the two instruments, other factors which cause di!erences need to be removed. Thus, observed data of LIF and PLC-CL should be selected and corrected considering the di!erences in measurement conditions. At "rst, comparison should be conducted for the data of the same ambient sample temporally. This synchronization is important because ambient NO itself can vary in a short time due to exchange of air mass or spike by local emission. For this purpose, data averaged for a similar duration in each cycle were selected for intercomparison. Additionally, ambient variation of NO can cause large di!erences V between the two instruments because of the in#uence of NO variation on the PLC-CL measurement. As a next step, the e!ect of ambient O should be corrected in order to cancel the contribution in the sampling lines. In this study, the residence times were 2.4 s (LIF), 2.2 s (PLC-CL, NO mode), and 10.2 s (PLC-CL, NO mode), respectively. Ambient NO has been thought to react with O and to be converted to NO in the sampling lines. Thus, it is possible that the measurement overestimates the concentration in the atmosphere. It has also been shown that NO converted from NO in PLC can be reconverted to NO due to the reaction with O from ambient air (Ridley et al., 1988). The measure ment by the PLC-CL instrument can underestimate NO concentration. Thus, it is necessary to correct NO and NO data using the ambient O data. The method of correction of the reaction in the tubing and PLC has been described elsewhere (Fehsenfeld et al., 1990). Brie#y, knowing the residence time in the tubing and PLC, the initial concentrations of ambient NO and NO are esti mated by simulation of the variations of concentrations due to the reaction of NO#O and photofragmentation of NO to NO in PLC. It was suggested in the previous study that NO by PLC-CL was totally corrected by ozone within a factor of #5%. Here, this factor for correction represents the degree of underestimate of the NO concentration in the measurement. 5.4. Acquired data of NO2 The LIF-NO instrument ran from 6 to 15 August. As a result of routine calibration with dry standard gas, sensitivity, background signal, conversion e$ciency for LIF and/or PLC-CL were found to be nearly constant during the campaign, with variations of these factors less than 5%. As a result of humidi"ed calibration in this measurement, the e!ects of water vapor for LIF and PLC-CL were 16% and 15%, respectively, for 3% H O by volume. In Okinawa, because water vapor varied from 2.7% to 3.5% by volume, the reduction of sensitivity was
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estimated from 15% to 19% for LIF and from 14% to 18% for PLC-CL. Thus, variations in sensitivity relative to typical values for LIF and PLC-CL were $2.4% and $2.6%, respectively. Ambient ozone varied from 10 to 45 ppbv in the campaign. Thus, correction factors for the e!ect of ambient ozone varied from 0 pptv (0% of NO ) to !40 pptv (!2.5%) for the LIF instrument and from #10 pptv (#4%) to #250 pptv (#16%) for the PLC-CL instrument, respectively. This di!erence between the correction factors caused by ozone is due to the di!erence of residence time in the sampling lines. The residence time in the PLC-CL instrument was 10.2 s. The large residence time is essential to the &indirect' method as in PLC-CL in order to acquire su$cient conversion e$ciency of the photolysis. Such a large residence time results in errors due to the e!ect of ambient ozone. However, the residence time in the LIF instrument was only 2.4 s. The e!ect of ambient ozone is negligible. This is the advantage of the direct LIF measurement. The "nal data from the two instruments as well as O are shown in Fig. 5. These are plotted as 12 min averages in order to simplify the intercomparison through smoothing of the variation. After data were selected considering temporal synchronization of the two instruments and the ambient variation of NO (see SecV tion 5.3), there were 371 measurements to compare as 12 min values. Among these data, there were 107 points below 500 pptv as the typical data for the marine boundary layer. There were a few interruptions of measurements due to calibration or required preparations for the typhoon. With these exceptions, the LIF-NO system continued to make stable measurements for almost 10 days. This instrument was also capable of running automatically without any operators even through the night. In this campaign, this compact LIF system was shown to be stable and reliable for "eld measurement. The concentration of NO was frequently over 1 ppbv when air masses came from the southwest, passing over the island. This high level of NO is reasonable concern ing the regional emission from anthropogenic sources in the populated area of the island. On the other hand, lower NO resulted from unpolluted air masses coming from the Paci"c Ocean. NO variation was caused by a combination of daytime photolysis of NO , reaction with O , and local emissions from the sugar cane "eld near the observatory. 5.5. Agreement between LIF and PLC-CL As shown in Fig. 5, the ambient data from the LIFNO instrument agreed with those of the PLC-CL instrument. A cross plot of all these data is shown in Fig. 6(a). The correlation between LIF and PLC-CL is represented by the unweighted linear regression line with a slope of 0.98$0.01 and an intercept of 88$25 pptv.
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J. Matsumoto et al. / Atmospheric Environment 35 (2001) 2803}2814 Table 4 The results of regression analyses for the LIF and PLC-CL data. Acquired values are shown for (A) all data and (B) less than 500 pptv of NO . Errors are given for 95% con"dential interval
Slope Intercept R
Fig. 5. 12 min averaged data observed in Okinawa, 1999: (a) LIF-NO ; (b) PLC-CL-NO ; (c) O for correction. Dotted lines represent midnight (00:00) by Japan Standard Time (JST).
Errors are shown for a 95% con"dence interval. The results of the regression analyses are summarized in Table 4. Because the correlation coe$cient, R, is 0.99, the excellent linearity between two instruments is shown in the range of observed NO concentration. The inter cept is not zero, but it is insigni"cant in this correlation because the full scale is still large (8000 pptv) and the intercept is in#uenced by the data of ppbv NO . The intercept &88 pptv' is su$ciently small considering the full scale (8000 pptv). In order to discuss the di!erence between two instruments at lower concentration level, data below 500 pptv are compared as shown in Fig. 6(b). The regression line with a slope of 1.17$0.20 and an inter-
(A)
(B)
0.98$0.01 88$25 0.99
1.17$0.20 19$63 0.57
cept of 19$63 pptv is acquired. R is small (0.57) and the correlation is not good. Data points are widely scattered in the plot. Such a large scatter may be caused by the LODs of the LIF (94 pptv) and the PLC-CL (54 pptv). Considering the deviation in the correlation, the slope is regarded as 1 and the intercept is 0 within errors. As a result, the agreement between two instruments is found in this measurement. This agreement suggests that the responses of LIF and PLC-CL are quite the same and reliable. In addition, the reasonability of correction for water vapor and ozone is also con"rmed. It is important that ozone can be highly in#uential especially for PLC-CL, due to large residence time. However, NO concentration in the marine boundary layer is generally expected to be below 100 pptv. Thus, improvement of the instrument is important in order to discuss the intercomparison more precisely. Although the LOD still needs improvement, the direct measurement by the LIF instrument has an advantage over the PLC-CL in its shorter residence time to diminish the reaction of NO#O . 5.6. Problems and future capability The LIF instrument here still has to be improved to quantify NO below 100 pptv. Now the LOD is deter mined mainly by a large background signal. Thus, it is
Fig. 6. Intercomparison between LIF-NO and PLC-CL-NO in Okinawa: (a) comparison of 12 min averaged data; (b) same as (a), except for below 500 pptv. Solid lines represent the regression lines.
J. Matsumoto et al. / Atmospheric Environment 35 (2001) 2803}2814
most important to reduce background signal. A multipass cell can potentially increase background due to laser beam scattering and emissions from the laser mirrors. In addition, the present multi-pass alignment is less e!ective because of the dependence of sensitivity on beam position in the cell. Only the central area of the cell is e!ective due to the characteristic of the lens system. Thus, it may be necessary to adopt and modify a single pass alignment to reduce background. Once this is achieved, a more powerful laser system can be utilized in order to increase sensitivity. If the improvement of the instrument could be explored, much more precise discussion of tropospheric photochemistry in the marine atmosphere could be possible. On the other hand, short-term phenomena such as radical chemistry can be conveniently studied after improvement of the LIF instrument to measure NO at the level of pptv with high temporal resolution.
6. Conclusions Laser-induced #uorescence technique was adopted to measure NO directly in the marine atmosphere. In order to increase sensitivity, a multi-pass excitation cell was utilized as the prototype instrument. Considering the mechanism of excitation and quenching, the cell pressure and the gate time of photon counting were determined, following calculation and validation experiments. After the optimization of other conditions, the LIF instrument's LOD was 94 pptv for 60 s integration. Field measurement was also conducted in Okinawa, in summer, in order to con"rm the reliability of the LIF instrument in the marine atmosphere. Continuous measurement for 10 days was conducted and the stability of the instrument was shown. Intercomparison with a commercial PLC-CL instrument resulted in no signi"cant di!erence between the two instruments in spite of a large correction for ozone e!ect. As a result, the LIF instrument can be seen as a simple, reliable, and promising method of measuring NO directly in the marine atmosphere. In future, although further improvement of the instrument is necessary, the LIF technique will be useful to investigate photochemistry in the marine atmosphere more precisely.
Acknowledgements We are grateful to Dr. Y. Kanaya, Dr. S. Kato (the University of Tokyo) and all other members of ORION'99 measurement team for their advice and practical help in the campaign. This work was supported by the Grant-in-Aid for Scienti"c Research (No. 09558066) from the Ministry of Education, Science, and Culture, and by Core Research for Evolutional Science and Technology from Japan Science and Technology Corporation.
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