Atmospheric NO2: In Situ Laser-Induced Fluorescence Detection at

The measured value of NO2 is equal to the true NO2 when C(t) is equal to 1 and Zero(t) is equal to zero. The temporal dependence of C(t) is divided in...
0 downloads 0 Views 173KB Size
Download PDF
Anal. Chem. 2000, 72, 528-539

Atmospheric NO2: In Situ Laser-Induced Fluorescence Detection at Parts per Trillion Mixing Ratios Joel A. Thornton,† Paul J. Wooldridge,† and Ronald C. Cohen*,†,‡,§

Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, Department of Geology and Geophysics, University of California, Berkeley, and Energy and Environment Technologies Division, Lawrence Berkeley National Laboratory

The oxides of nitrogen, NO and NO2 (NOx ≡ NO + NO2), play a central role in atmospheric chemistry, regulating production rates of ozone and OH in the troposphere1 and destruction rates of ozone in the stratosphere.2 Processes that exchange NOx and its oxidation products between the surface (soils, rivers, etc.) and the atmosphere are important to global and regional ecology3 and may be a significant factor in the regulation of terrestrial carbon uptake by nitrogen-limited forests.4 Gas-phase NOx has also been shown to initiate seed germination in fire-sensitive species,5 thus

raising the interesting possibility that NOx emissions may be substantially altering some ecosystems through species-specific biochemical mechanisms as well as by the more commonly recognized effects of nitrate on the nutrient balance and acid rain on the pH balance. Our need to understand these processes is increasing as a result of the enormous growth in anthropogenic NOx emissions. Anthropogenic NOx emissions are now estimated to be 3-4 times natural emissions, and they are expected to double again by 2020.6 Changes to NOx have affected, and will continue to affect, atmospheric chemistry and composition on local, regional, and global scales. In urban areas, NOx mixing ratios of 10-100 parts per billion are typical,7 while in the remote atmosphere, concentrations of a few parts per trillion have been measured.8,9 Techniques used to measure NO at atmospherically relevant mixing ratios (1 ppt 10 ppb), including O3-chemiluminescence10 and two-photon laserinduced fluorescence are accurate (5%), sensitive, and have small uncertainties in their zero.11 In contrast, existing techniques for measuring NO2 are generally less accurate and reliable, especially at concentrations below 1000 ppt and in environments where higher oxides of nitrogen are more abundant by factors of 10100.12 In this paper, we describe the design, development, and initial field trials of a laser-induced fluorescence (LIF) instrument that provides a new, reliable, and accurate method for detection of NO2 throughout the atmosphere. The instrument offers the advantages of high sensitivity, high accuracy, and immunity from interferences associated with photochemical conversion of NO2 to another species prior to detection. It is designed to be capable

* Corresponding author: (tel.) 510-642-2735; (fax) 510-642-6911; C (e-mail) [email protected]. † Department of Chemistry, University of California, Berkeley. ‡ Department of Geology and Geophysics, University of California, Berkeley. § Lawrence Berkeley National Laboratory. (1) Logan, J. A.; Prather, M. J.; Wofsy, S. C.; McElroy, M. B. J. Geophys. Res. 1981, 86, 7210-7254. (2) Wennberg, P. O.; Cohen, R. C.; Stimpfle, R. M.; Koplow, J. P.; Anderson, J. G.; Salawitch, R. J.; Fahey, D. W.; Woodbridge, E. L.; Keim, E. R.; Gao, R. S.; Webster, C. R.; May, R. D.; Toohey, D. W.; Avallone, L. M.; Proffitt, M. H.; Loewenstein, M.; Podolske, J. R.; Chan, K. R.; Wofsy, S. C. Science (Washington, D.C.) 1994, 266, 398-404. (3) Pelley, J. Environ. Sci. Technol. 1998, 32, 462-466. (4) Holland, E. A.; Braswell, B. H.; Lamarque, J. F.; Townsend, A.; Sulzman, J.; Muller, J. F.; Dentener, F.; Brasseur, G.; Levy, H., II.; Penner, J. E.; Roelofs, G. J. J. Geophys. Res. 1997, 102, 15849-15866. (5) Keeley, J. E.; Fotheringham, C. J. Science (Washington, D.C.) 1997, 276, 1248-1250.

(6) Yienger, J. J.; Klonecki, A. A.; Levy, H.; Moxim, W. J.; Carmichael, G. R. J. Geophys. Res., Atmos. 1999, 104, 3655-3667, and references therein. (7) NAS. Rethinking the ozone problem in urban and regional air pollution; National Academy Press: Washington, D. C., 1991. (8) Carroll, M. A.; Ridley, B. A.; Montzka, D. D.; Hubler, G.; Walega, J. G.; Norton, R. B.; Huebert, B. J.; Grahek, F. E. J. Geophys. Res. 1992, 97, 1036110374. (9) Crawford, J. H.; Davis, D. D.; Chen, G.; Bradshaw, J.; Sandholm, S.; Kondo, Y.; Merrill, J.; Liu, S.; Browell, E.; Gregory, G.; Anderson, B.; Sachse, G.; Barrick, J.; Blake, D.; Talbot, R.; Pueschel, R. J. Geophys. Res. 1997, 102, 28447-28468. (10) e.g. Ridley, B. A.; Walega, J. G.; Dye, J. E.; Grahek, F. E. J. Geophys. Res. 1994, 99, 25519-25534. (11) Sandholm, S.; Smyth, S.; Bai, R.; Bradshaw, J. J. Geophys. Res. 1997, 102, 28651-28661. (12) Crawford, J.; Davis, D.; Chen, G.; Bradshaw, J.; Sandholm, S.; Gregory, G.; Sachse, G.; Anderson, B.; Collins, J.; Blake, D.; Singh, H.; Heikes, B.; Talbot, R.; Rodriguez, J. J. Geophys. Res. 1996, 101, 2053-2072.

We describe a time-gated laser-induced fluorescence instrument designed for accurate ((5%, 1σ), continuous, autonomous, in situ observations of NO2 with the sensitivity (15 ppt/10 s at S/N ) 2) and portability necessary to study NO2 anywhere in the troposphere. The technique is advantageous because it is spectroscopically specific and direct in that it does not require conversion of NO2 into another species (e.g., NO) prior to detection, eliminating a class of potential interferences. Performance of the instrument is illustrated with 15 weeks (July-Oct 1998) of observations at the University of California, Blodgett Forest field station located in the foothills of the Sierra Nevada and 4 weeks (June 15 - July 15, 1999) in Nashville, TN during the Southern Oxidants Study. Ambient concentrations of NO2 at Blodgett Forest varied from below 50 ppt to 4000 ppt and NO2 ranged from 5 to 50% of the total reactive nitrogen; while in Nashville, TN, concentrations ranged from 1 to 75 ppb.

528 Analytical Chemistry, Vol. 72, No. 3, February 1, 2000

10.1021/ac9908905 CCC: $19.00

© 2000 American Chemical Society Published on Web 12/22/1999

of making observations from the surface to 40 km, and it contains a series of internal standards to aid in maintaining a precise calibration over months to years. As a result of these features and rapid progress in laser technology, over the next few years we expect this instrument will evolve into a small, lightweight, fully autonomous, inexpensive sensor capable of routine measurements of NO2 from aircraft, balloons, and ground stations. Before describing our new instrument in detail, we briefly review some other approaches to NO2 detection. This discussion emphasizes techniques that have been subject to an atmospheric field trial. There are numerous other techniques that have shown promise in the laboratory (e.g., surface acoustic wave detectors13) under highly controlled conditions. Without a field trial, it is difficult to evaluate the specificity of these methods and their potential to suffer from interferences at concentrations of interest in the atmosphere. The most common technique used to measure atmospheric NO2 is Photo-Fragmentation Chemi-Luminescence, hereafter PF-CL, in which photolytic conversion of NO2 to NO is followed by chemiluminescence detection of NO.14-21 Photolysis of NO2 is accomplished using UV light from a filtered xenon arc lamp (λ ∼ 375 nm) in a flowing sample. O3 is added to the sample flow downstream of the photolysis cell where it reacts with NO to produce electronically excited NO2. The excited NO2* emits a broad continuum of radiation peaking at 1.3 µm.22 The visible portion of this emission 500-900 nm is detected using a redsensitive photomultiplier tube (PMT). Such instruments are commercially available (e.g., Eco Physics, Inc., Ann Arbor, MI) with detection limits of 155 ppt/10 s,23 and custom-built versions have reached still lower detection limits. For example, Del Negro et al. have demonstrated a sensitivity near 25 ppt/10 s with an accuracy ranging from (10-30%.21,24 For the ease of comparisons, count rates and sensitivities will be reported as those achieved in 10-s integration periods at a signal-to-noise ratio of 2:1. We assume (13) Penza, M.; Vasanelli, L. Sens. Actuators, B 1997, B41, 31-36. (14) Fahey, D. W.; Hubler, G.; Parrish, D. D.; Williams, E. J.; Norton, R. B.; Ridley, B. A.; Singh, H. B.; Liu, S. C.; Fehsenfeld, F. C. J. Geophys. Res. 1986, 91, 9781-9793. (15) Ridley, B. A.; Carroll, M. A.; Gregory, G. L.; Sachse, G. W. J. Geophys. Res. 1988, 93, 15813-15830. (16) Fehsenfeld, F. C.; Drummond, J. W.; Roychowdhury, U. K.; Galvin, P. J.; Williams, E. J.; Buhr, M. P.; Parrish, D. D.; Hubler, 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.; Schiff, H. I. J. Geophys. Res. 1990, 95, 3579-3597. (17) Munger, J. W.; Wofsy, S. C.; Bakwin, P. S.; Song-Miao, F.; Goulden, M. L.; Daube, B. C.; Goldstein, A. H.; Moore, K. E.; Fitzjarrald, D. R. J. Geophys. Res. 1996, 101, 12639-12657. (18) Harder, J. W.; Williams, E. J.; Baumann, K.; Fehsenfeld, F. C. J. Geophys. Res. 1997, 102, 6227-6243. (19) Williams, E. J.; Roberts, J. M.; Baumann, K.; Bertman, S. B.; Buhr, S.; Norton, R. B.; Fehsenfeld, F. C. J. Geophys. Res. 1997, 102, 6297-6314. (20) DiasLalcaca, P.; Brunner, D.; Imfeld, W.; Moser, W.; Staehelin, J. Environ. Sci. Technol. 1998, 32, 3228-3236. (21) Gao, R. S.; Keim, E. R.; Woodbridge, E. L.; Ciciora, S. J.; Proffitt, M. H.; Thompson, T. L.; McLaughlin, R. J.; Fahey, D. W. J. Geophys. Res. 1994, 99, 20673-20681. (22) Clough, P. N.; Thrush, B. A. Trans. Faraday Soc. 1967, 63, 915-925. (23) Dias-Lalcaca, P.; Brunner, D.; Imfeld, W.; Moser, W.; Staehelin, J. Environ. Sci. Technol. 1998, 32, 3228-3236. (24) Del Negro, L. A.; Fahey, D. W.; Gao, R. S.; Donnelly, S. G.; Keim, E. R.; Neuman, J. A.; Cohen, R. C.; Perkins, K. K.; Koch, L. C.; Salawitch, R. J.; Lloyd, S. A.; Proffitt, M. H.; Margitan, J.; Stimpfle, R. M.; Bonne, G. P.; Voss, P. B.; Wennberg, P. O.; McElroy, C. T.; Swartz, W. H.; Kusterer, T. L.; Anderson, D. E.; Lait, L. R.; Bui, T. P. J. Geophys. Res. 1999, 104, 2668726703.

that the sensitivity of these other instruments scales as the square root of the integration time and adjust values as reported in the literature accordingly. Bradshaw et al. use two-photon laser-induced fluorescence in place of the O3-chemiluminescence reaction to measure NO derived from the photolysis of NO2. In a recent study, they report a detection limit for NO2 of 8 ppt/10 s for the Photo-Fragmentation Two-Photon Laser-Induced Fluorescence (PF-TP-LIF) instrument which uses the third harmonic of an Nd3+:YAG laser for the photolysis of NO2 and two-photon excitation to the C state of NO followed by fluorescence as a means of NO detection.25 In a number of studies, the accuracy of this instrument has been questioned because of possible interferences.12,26 Bradshaw et al. have attributed the interferences to surface decomposition of other NO2 containing species and have tried to minimize the effect by increasing the sample flow rate. However, several of these species, such as peroxyacetylnitrate (PAN), considered to be the largest interference, are commonly detected using gas chromatography, a method that depends directly on the molecule surviving numerous collisions with surfaces, suggesting that surface decomposition is not likely to be the primary source of the observed interference. Both the PF-CL and PF-TP-LIF detection methods are sensitive enough for observation of NO2 anywhere on the globe. Their shortcomings arise from: (a) the indirect nature of the photolysis step and consequent additional uncertainty about how the instrument will respond to other NO2 containing compounds, (b) the requirement for a simultaneous NO measurement and the associated need to determine NO2 as the difference between NOx and NO, (c) the large size, weight, and electrical power consumption of these instruments, and (d) the difficulty of translating the performance of scientific instruments to commercially available ones that can be operated routinely at the high data quality characteristic of research-grade instrumentation. Another commonly used technique for NO2 detection is the luminol-chemiluminescence method, first developed by Wendel et al. in 1983.27 Instruments based on this method are commercially available, and they have been used by several groups to measure atmospheric NO2,16 NOx fluxes by eddy correlation,28 and NO2 derived from thermal decomposition of PAN and other organic nitrates.29,30 The technique employs reaction between NO2 and a solution of luminol resulting in fluorescence peaking near 425 nm. The technique is direct and sensitive (part-per-trillion detection limits), but it is nonspecific. O3, PAN, and other peroxynitrates are detected with high and sometimes variable efficiency.16 Gaffney et al. have recently reported a gas chromatographic separation of NO2 and C2-C4 peroxy nitrates, followed by detection using luminol with part-per-trillion sensitivity.31 (25) 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. Geophys. Res. Lett. 1999, 26, 471-474, and references therein. (26) Sandholm, S. T.; Bradshaw, J. D.; Dorris, K. S.; Rodgers, M. O.; Davis, D. D. J. Geophys. Res. 1990, 95, 10155-10161. (27) Wendel, G. J.; Stedman, D. H.; Cantrell, C. A. Anal. Chem. 1983, 55, 937940. (28) Hesterberg, R.; Blatter, A.; Fahrni, M.; Rosset, M.; Neftel, A.; Eugster, W.; Wanner, H. Environ. Pollut. 1996, 91, 21-34. (29) Blanchard, P.; Shepson, P. B.; Schiff, H. I.; Drummond, J. W. Anal. Chem. 1993, 65, 2472-2477. (30) Chen, X. H.; Hulbert, D.; Shepson, P. B. J. Geophys. Res. 1998, 103, 2556325568.

Analytical Chemistry, Vol. 72, No. 3, February 1, 2000

529

Although their paper did not discuss possible losses of NO2 on the chromatography column, they did show losses of NO2 on a variety of other common materials (e.g., aluminum and copper), suggesting an advantage for techniques with rapid flow rates and a minimum of surface contact between the instrument and the sample. Numerous other techniques have been used in the atmosphere or suggested as new approachesstunable infrared diode laser absorption spectroscopy (TDLAS) has demonstrated a capability for in situ detection at 400-700 ppt/10 s with approximately 100-m path lengths.32-34 Detection limits using a visible diode laser and 1-m path lengths have reached 5000 ppt/10 s,35 while others remain orders of magnitude higher.36,37 Long-path differential absorption has been used to detect atmospheric NO2 with a sensitivity of 250 ppt/10 s using a 20.6-km folded path length.18 Laser imaging detecting and ranging (LIDAR)38,39 and its relative, differential absorption LIDAR, have detection limits near 2 × 105 ppt for NO2, limiting their current use to studies near the combustion sources of NOx.40 Magnetic rotation spectroscopy has been used in the laboratory to achieve a detection limit of 6000 ppt/10 s.41,42 The above techniques are all spectroscopically specific to NO2. However, none of them has the sensitivity of LIF. LASER-INDUCED FLUORESCENCE DETECTION OF NO2 The fluorescence spectrum of NO2* is known to be broad and unstructured except at the earliest times (single-collision conditions) following excitation. The fluorescence following laser excitation is expected to exhibit a similar wavelength distribution as the NO2* produced by reaction of NO + O3 because of the dominant role of collisional redistribution in determining energy disposal in NO2*. An early attempt at LIF detection of NO2 by Tucker et al. used a He-Cd laser with nonresonant detection and achieved a detection limit of 2040 ppt/10 s but the specificity of the approach was not commented on.43 George and O’Brien used a frequency-doubled Nd3+:YAG laser with detection from 580 to 900 nm to achieve a sensitivity to NO2 of 2500 ppt with 10-s averaging in a more portable configuration.44 They took advantage (31) Gaffney, J. S.; Bornick, R. M.; Chen, Y. H.; Marley, N. A. Atmos. Environ. 1998, 32, 1445-1454. (32) Schiff, H. I.; Karecki, D. R.; Harris, G. W.; Hastie, D. R.; Mackay, G. I. J. Geophys. Res. 1990, 95, 10147-10153. (33) Webster, C. R.; May, R. D.; Allen, M.; Jaegle, L.; McCormick, M. P. Geophys. Res. Lett. 1994, 21, 53-56. (34) Gregory, G. L.; Hoell, J. M.; Torres, A. L.; Carroll, M. A.; Ridley, B. A.; Rodgers, M. O.; Bradshaw, J.; Sandholm, S.; Davis, D. D. J. Geophys. Res. 1990, 10129-10138. (35) Sonnenfroh, D. M.; Allen, M. G. Appl. Opt. 1996, 35, 4053-4058. (36) Mihalcea, R. M.; Baer, D. S.; Hanson, R. K. Appl. Opt. 1996, 35, 40594064. (37) Gianfrani, L.; Gagliardi, G.; Pesce, G.; Sasso, A. Appl. Phys. B 1997, B64, 487-491. (38) Weitkamp, C.; Goers, U. B.; Glauer, J.; Kohler, S.; Rairoux, P.; Immler, F.; Woste, L.; Ulbricht, M.; Weidauer, D. 18th International Laser Radar Conference, Berlin, Germany, 1996; Springer-Verlag; pp 411-414. (39) Strong, K.; Jones, R. L. Appl. Opt. 1995, 34, 6223-6235. (40) e.g., Toriumi, R.; Tai, H.; Takeuchi, N. Opt. Eng. 1996, 35, 2371-2375. (41) Blake, T. A.; Chackerian, C., Jr.; Podolske, J. R. Appl. Opt. 1996, 35, 973985. (42) Smith, J. M.; Bloch, J. C.; Field, R. W.; Steinfeld, J. I. J. Opt. Soc. Am. B 1995, 12, 964-969. (43) Tucker, A. W.; Birnbaum, M.; Fincher, C. L. Appl. Opt. 1975, 14, 14181421. (44) George, L. A.; O’Brien, R. J. J. Atmos. Chem. 1991, 12, 195-209.

530

Analytical Chemistry, Vol. 72, No. 3, February 1, 2000

of the long fluorescence lifetime of NO2 to discriminate against some of the resonant background scatter and used an NO2 scrubbing step (reducing NO2 to NO using FeSO4) to demonstrate specificity of the technique to NO2. Recently, Fong and Brune developed an LIF instrument employing a tunable dye laser as the excitation source.45 They excited NO2 at 564 nm with detection using a red-sensitive PMT (8% quantum efficiency) filtered to a bandwidth of approximately 200 nm. By tuning the laser on and off of a rotational resonance in the excitation spectrum, this instrument demonstrated its specificity for NO2. The sensitivity of the Fong and Brune prototype is limited by the combination of a modest signal rate (0.12 counts/10 s/ppt) and a high background count rate (approximately 10 000 counts/10 s). The reported instrument sensitivity, 1085 ppt/10 s, was about a factor of 2.5 better than that of George and O’Brien. Perkins et al., recently designed an NO2-LIF instrument for detection of NO2 aboard the ER-2 aircraft in the lower stratosphere.46 The instrument, a direct predecessor to the one we describe in this article, uses a dye laser that is tuned on and off an NO2 resonance near 585 nm. Detection of the total fluorescence integrated from 750 nm to the long-wavelength cutoff of a GaAs PMT occurs simultaneously with the pulsed laser. The ER-2 instrument was first demonstrated during NASA’s 1997 Photochemistry of Ozone Loss in the Arctic In Summer (POLARIS) experiment. During this deployment, Perkins et al. demonstrated a detection limit of 10-50 ppt, limited by systematic noise in the background, and a sensitivity of 40-80 ppt/10 s at the ambient pressure (60) Etalon (free spectral range of 0.54 cm-1) during scans of the laser wavelength. b. Detection Cell. The detection cell, see schematic in Figure 2, is built around a 40-pass, 30.5-cm White cell with the design goal of maximizing the NO2 fluorescence collected while minimizing collection of scattered light from other sources. The polarized laser beam propagates through this cell with the E vector parallel to the detector. Fluorescence is collected from a volume that is 1 cm on the long axis of the White cell and approximately 2 × 20 mm in cross section. The fluorescence is collected using a 32mm-diameter, 38-mm focal length lens (L1) and a retroreflecting mirror (M4) with a radius of curvature of 44 mm located opposite the collection lens. These optics intercept roughly 5% of the solid angle for a point at the center of the cell. The extended source (54) Hsu, D. K.; Monts, D. L.; Zare, R. N. Spectral Atlas of Nitrogen Dioxide 5530 to 6480 Angstroms; Academic Press: New York, 1978.

Figure 2. Schematic of the detection axis. White Cell mirrors (M1M3) are separated by their radii of curvature, 30.5 cm. A spherical mirror (M4, radius of curvature 44 mm) is used to double the fraction of the solid angle collected. The limiting aperture to the optical train located in front of the first lens, L1, is 32 mm. The collection lens (L1), with a 50-mm focal length, collimates light through the dielectric filters (F1-F3). Light is focused by a 75-mm focal length lens L2 onto the PMT. F1 is a 610-nm-long pass (LP) filter. F2 is a 700-nm LP filter used during NO2 detection, and F3 is a narrow bandwidth (80%) of light with wavelengths longer than 650 nm with at least 10-5 attenuation at 585 nm and good rejection of any residual 532-nm light. A 700-nm dielectric long pass filter directly follows the 610 nm long pass filter to block Raman scattering of the laser light by N2 and O2. This second filter strongly rejects wavelengths less than 700 nm (10-5 attenuation for wavelengths between 585 and 680 nm) and transmits greater than 80% for wavelengths greater than 750 nm. The attenuation provided by these filters is sufficient to reduce any pressure-dependent scatter (Rayleigh scatter and Raman scatter) to below our detection limit. For calibration purposes, the NO2 filter is occasionally replaced with a filter that (55) Herriott, D. R.; Kogelnik, H.; Kompfner, R. Appl. Opt. 1964, 3, 523-526.

Analytical Chemistry, Vol. 72, No. 3, February 1, 2000

533

transmits at 670 nm (676 ( 8 nm) for detection of N2 Raman. The 700-nm LP (NO2 detection mode) and the 676 nm band-pass (N2-Raman mode) filters are mounted on a motorized swinging platter which, under computer control, rotates the desired filter in and out of the optical path within one second. The swinging platter is temperature controlled to prevent changes in filter transmission, and it is enclosed in a light-sealed aluminum housing. The band-pass filter is used to transmit light from the N2-Raman transition (greater than 80% transmission between 674 and 680 nm), while attenuating light outside this region by 10-5. The light transmitted by the filters is focused (L2, 75 mm) onto the cooled (-20 °C) 10 mm × 4 mm GaAs photocathode (C31034A, Burle Technologies). Pulses from the PMT are sent through an AC-coupled preamplifier and are then counted by a two-channel gated photon counter (SR400, Stanford Research Systems). We calculate the transmission through the optical train to the PMT to be 50% from 700 to 1000 nm. The lenses and two windows used to thermally isolate the photocathode are not A/R coated in this current system. A/R coating could increase the transmission to 64%. With the optical filters in place, the majority of the noise in the instrument originates at the White cell mirrors, where light that is not reflected (