Research Aircraft Measurements of Nitrogen Dioxide and Peroxyacyl Nitrates Using Luminol Chemiluminescence with Fast Capillary Gas Chromatography JEFFREY S. GAFFNEY,* NANCY A. MARLEY, AND H. DONNAN STEELE Environmental Research Division, Argonne National Laboratory, Argonne, Illinois 60439 PAUL J. DRAYTON Department of Geophysical Sciences, University of Chicago, Chicago, Illinois 60637 JOHN M. HUBBE Atmospheric Sciences Technical Group, Pacific Northwest National Laboratory, Richland, Washington 99352
Fast capillary gas chromatography with luminol detection has been used to make airborne measurements of nitrogen dioxide (NO2) and peroxyacetyl nitrate (PAN). The analysis system allows for the simultaneous measurement of NO2 and peroxyacyl nitrates (PANs) with time resolution of less than 1 min, an improvement of a factor of 4-5 over previously reported methods using electron capture detection. Data presented were taken near Pasco, Washington, in August 1997, during a test flight onboard the U.S. Department of Energy G-1 aircraft. We report measurements of NO2 in the boundary layer in a paper mill plume and a plume from a grass fire, in addition to analyses for free tropospheric NO2 and PAN. Ratios of PAN/ NO2 were observed to increase with altitude (decreasing temperature) and to reach values of 2-4 above the boundary layer, consistent with the thermal equilibrium of the peroxyacetyl radical and NO2 with PAN. Estimates for the peroxyacetyl radical in the continental free troposphere, calculated from this equilibrium, were found to be in the range of 104-105 molecules per cubic centimeter. These results demonstrate the application of this approach for airborne measurements of NO2 and PAN in a wide range of field study scenarios.
Introduction Peroxyacyl nitrates (PANs) and nitrogen dioxide (NO2) are important trace gas species associated with photochemical air pollution. The PANs are a class of organic oxidants having the general chemical structure RCdOO-O-NO2. The most common members of the PANs family are peroxyacetyl nitrate (PAN; RdCH3-), peroxypropionyl nitrate (PPN; RdCH3CH2-), and peroxybutyryl nitrate (PBN; RdCH3CH2CH2-) (1, 2). * Corresponding author: phone (630) 252-5178; fax: (630) 2527415; e-mail:
[email protected]. 10.1021/es981102g CCC: $18.00 Published on Web 08/14/1999
1999 American Chemical Society
The PANs are in thermal equilibrium with the peroxyacetyl radical (RCdO-OO• or RCO3) and NO2 (1, 2). Because PANs are trapped peroxy radicals, they are an important indicator species of the photochemical age of an air parcel as well as being a means of long-range transport of NO2, leading to the formation of regional ozone (O3) and other oxidants (1-3). Typically, PANs are measured by using a gas chromatograph with electron capture detection (ECD). Once automated, this method has been shown to be reliable and quite sensitive, allowing tropospheric levels of PANs to be measured at low parts per trillion (ppt) (2). Unfortunately, a number of other atmospheric gases that also have strong ECD signals can act as inferences (e.g., O2, Freons, H2O), limiting the speed of the analysis. Currently, the shortest analysis time for PAN with ECD is approximately 5 min (4, 5). The use of luminol chemiluminescence to detect NO2 has been known for some time (6), but the method has been found to suffer from interferences, particularly interference due to PAN (7). The use of packed columns to separate the NO2 and PAN signals has been demonstrated, but analyses cannot be done in less than 4-5 min (8-10). We recently reported the use of luminol detection with gas capillary chromatography for low-level measurement of NO2 and PANs in rapid monitoring of these important trace gases (11). We have demonstrated the analysis of the PANs (PAN, PPN, and PBN) and NO2 can be performed in 1 min in laboratory studies with this approach. Recent studies have reexamined the kinetics of the reactions of the peroxyacetyl radical with nitrogen dioxide and its thermal decomposition back to the starting reactants, clearly indicating that the peroxyacetyl radical can play an important role in the chemistry of the troposphere (12). Measuring the temperature and pressure when the samples are taken, and then assuming that PAN is in thermal equilibrium with NO2, enables direct estimation of the peroxyacetyl concentration in the troposphere. Reported herein are modifications of the instrument described previously (11) for aircraft operation, together with results of simultaneous measurements of NO2 and PAN during test flights near Pasco, Washington, in August 1997. Estimates of peroxyacetyl radical concentrations are also presented and compared with previous measurements and calculations from other sources for total peroxy radicals.
Experimental Section The details of the instrumental design have been given elsewhere (11) and are described only briefly here. A luminol NO2 detector (Scintrex LMA-3) was modified for the measurement of NO2 and PANs. An air sample is pulled through an inlet tube into a six-port sampling valve that is electronically actuated (Valco timer and Cheminert valve). For these studies, a 5 cm3 sampling loop was used, and the sample was injected into a 3 m DB-1 capillary column for separation of the PANs and NO2. Retention times for NO2 and PAN at a carrier gas (5% O2 in He) flow of 125 cm3/min were 0.23 and 0.32 min, respectively. The recording integrator (HewlettPackard 3395) was interfaced to a laptop computer (Toshiba) by using Hewlett-Packard Peak 96 software, which allows automated collection of data onboard the aircraft. The instrument consisted of a mounting rack, the modified LMA-3 with capillary column operated at ambient temperature, a sampling valve and associated electronics (automated timing system), the carrier gas bottle and regulator, a peristaltic pump for the LMA-3 (external), a recording integrator, and the laptop computer. The instrument’s weight, including the rack, was approximately 190 lb. VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3285
FIGURE 1. A 3-D graphic representation of the G-1 aircraft flight track (bold line(-)), for flight taken on August 13, 1997, showing the location of the Snake River and Pasco, Washington (PSC). GPS altitude is regressed from pressure. A 2D representation of the flight track is also shown (thin line (-)), which indicates the horizontal extent of the flight. Retention times for PAN and NO2 were checked by using a synthesized PAN standard (13). Calibrations were performed with certified NO2 standards and standard dilution techniques. On the basis of laboratory comparisons, PAN was assumed to have approximately the same sensitivity in these preliminary analyses with the 5% O2 in He carrier gas (11). Samples were taken into the sampling loop during flight by using a small air pump operated at a flow rate of 825 cm3/ min to pull outside air through a 1 in. Teflon sampling pipe inside an external sampling inlet (ram-air) on the aircraft. The 5 cm3 samples were injected for 10 s; 50 s was allowed for completion of the analysis before the next analysis began. Thus, analysis of NO2 and PAN was completed in 1 min during the flights in this testing program. The aircraft used for these studies was the Gulfstream G-1, operated for the U.S. Department of Energy by the Battelle Memorial Institute at the Pacific Northwest National Laboratory. The G-1 is a large twin turboprop aircraft that nominally samples at 200 knots or 100 m s-1. The flight described in this work originated at Pasco airport on August 13, 1997 and conducted aircraft sampling from approximately 1700 to 1945 GMT. The instrument was initially started on the ground and was operated from takeoff until shortly before landing, giving results at 1 min intervals. Figure 1 shows the flight track that flew primarily over the Snake River in Washington and the Columbia River near the WashingtonOregon Border. Global Positioning System (GPS) altitude was regressed based upon pressure measurements taken on board the G-1 aircraft.
Results and Discussion Figure 2 shows the potential temperature measured as a function of aircraft altitude during the final descent for the August 13, 1997 flight from Pasco airport. The potential temperature is the temperature an air parcel would obtain if it were taken adiabatically to 1 bar. We present the potential temperature (rather than the temperature) as a function of height because the adiabatic lapse rate appears as a vertical line, and, therefore, well-mixed layers are easier to identify. 3286
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 19, 1999
FIGURE 2. Potential temperature (K) given as a function of altitude (ft) for the G-1 final descent and landing for the flight taken from Pasco airport on August 13, 1997. From Figure 2, it is clear that the boundary layer mixing height was below 3000 ft for most of the flight as these data was taken on the final descent at the end of the 3 h flight. The data also indicate that there are three or four bands of air that are not well-mixed between 3000 and 8500 ft altitude. These bands may be air parcels that were from previous days over the area. The average air speed of the flight was approximately 100 m s-1, corresponding to a speed of 360 km hr-1. Our nitrogen dioxide and PAN measurements, taken at a resolution of 1 min, correspond to a spatial resolution of 6 km. This is a considerable improvement over the 30 km or so spatial resolution that would have been achieved with a 5 min electron capture detection system (8-10). Figure 3a and 3b present the results of 1 min analyses of NO2 during the flight on August 13, 1997, from Pasco airport. Figure 4 shows the same results for PAN. As expected, the initial NO2 levels observed at the airport and during takeoff were quite high (Figure 3a); the concentrations fell rapidly to background levels (Figure 3b). As the flight path approached a paper mill in the area, we were able to detect NO2 in the plume from the mill at an altitude of 1,500 ft (Figure
TABLE 1. Peroxyacetyl Radical (CH3CO3) Concentrations at Three Altitudes, Estimated by using the PAN Equilibrium with NO2 for the G-1 Aircraft Sampling Flight of August 13, 1997, near Pasco, Washingtona number of altitude measure(ft) ments 2,000 4,000 8,000
43 35 51
CH3CO3• (molec/cm3) × 104
T (°C)
P (mbar)
2.8 ( 2.9 (0.02-11.1) 28.7 ( 0.75 936.4 ( 3.3 8.4 ( 3.1 (2.8-15.6) 26.1 ( 0.60 869.9 ( 0.63 4.5 ( 1.7 (0.39-9.5) 16.1 ( 0.28 746.6 ( 0.50
a Values are means and standard deviations. Ranges for the peroxyacetyl radical estimates are in parentheses.
FIGURE 3. (A) Nitrogen dioxide levels (-) and the sampling altitude (9) observed during the flight from Pasco, Washington, on August 13, 1997. The detection of NO2 from a paper mill plume is noted (M) as well as from a grass fire (G). A portion of the graph is expanded in (B) showing the NO2 levels at 4,000 ft and 8,000 ft and indicates the detection of an NO2 source from a grass fire below (G).
FIGURE 4. Peroxyacetyl nitrate (PAN) levels (-) and the altitude (9) at the time of measurement during the flight from Pasco, Washington, on August 13, 1997. 3a, denoted by M). Later in the flight, we made measurements at altitudes of 4,000 ft and 8,000 ft. At 4,000 ft, the instrument detected a small source of NO2 (approximately 100 ppt) in a plume from a grass fire below (Figure 3b denoted by G), consistent with a low-temperature, low-pressure combustion source. The PAN levels observed were quite low (Figure 4), probably because the sampled air was very clean, and the temperatures were relatively high during the flight (approximately 40 °C at the ground, cooling to 16 °C at 8,000 ft; see Table 1). Measured PAN concentrations ranged from the detection limit of 10-80 ppt during most of the sampling period, with 200 ppt levels observed during the final flight descent and landing when the aircraft moved into a polluted
FIGURE 5. Ratios of PAN/NO2 (-) calculated from simultaneous measurements of these two species by fast capillary gas chromatography with luminol detection on August 13, 1997 at various altitudes (9). layer, likely, the one indicated at around 5200 ft in altitude on the potential temperature profile taken during the final descent and landing (Figure 2). Figure 5 shows the PAN/NO2 ratios calculated from the PAN and NO2 concentrations measured at one minute time resolution during the flight. The increase in the PAN/NO2 ratio with time follows the altitude of the flight path, consistent with thermal stabilization of PAN at the lower temperatures of the higher altitudes. The PAN/NO2 ratios range from approximately 0.1 to over 4.0. The lower ratios were observed below the boundary layer at higher temperatures (and where local NO2 sources occurred). The ratios increased at the higher altitudes to approximately 0.8 at 4,000 ft and 2.0 at 8,000 ft, respectively. The air quality during these studies over this area was very clean. The NO2 levels outside the plumes, on the order of 100 ppt at 1,500 ft, dropped to about 25-50 ppt at 8,000 ft. These results are in reasonable agreement with PAN data obtained over the Pacific Ocean by using electron capture detection (14). The PAN and NO2 levels observed indicate that at 8,000 ft the air was predominantly sourced from the marine environment, consistent with the lack of major sources of pollution in the area and the relative proximity of the Pacific Ocean. The distribution of PAN levels (higher at higher altitudes; at or below detection limits at lower altitudes) probably reflects the weather conditions during the flight (a ground temperature of approximately 40 °C) and the increase stability of PAN at lower temperatures aloft. Because the gas chromatograph-luminol system allows simultaneous measurement of PAN and NO2, application of the temperature and pressure data recorded during the aircraft sampling yields an estimate of the peroxyacetyl radical concentration. PAN is formed from the reaction of peroxyacetyl radical with NO2 via reaction 1, where M is a third body (N2 or O2): VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3287
CH3CO3 + NO2 + M f CH3CO3NO2 (PAN) + M (1) PAN can undergo unimolecular thermal decomposition back to the peroxyacetyl radical and NO2 via reaction 2:
PAN f CH3CO3 + NO2
(2)
With the recommended gas phase rate data for these reactions (15, 16) and our measurements of PAN and NO2 as a function of temperature and pressure, we calculated the peroxyacetyl radical concentrations at equilibrium using the following equation:
[CH3CO3] ) k2/k1 [PAN]/[NO2]
(3)
Since our instrument samples both NO2 and PAN simultaneously, and the analysis is done in a very short time, the equilibrium concentration of the peroxyacetyl radicals can be calculated for each sample collected. The results of these calculations are shown in Figure 6, and the mean values, standard deviations, and ranges of values from the determinations are in Table 1. Our estimated values for peroxyacetyl radical concentrations during this flight are on the order of 4 × 104-8 × 104 cm-3. These values can be compared with recently reported total peroxy radical (HO2, RO2, and RCO) concentrations, measured over Germany, of 2 × 108-7 × 108 cm-3 at 3.5-8 km altitude in the summer over the mid-latitudes (17). As well, model calculations have yielded estimates of approximately (2-6) × 108 cm-3 for total peroxy radical concentrations for mid-latitude summer air at these altitudes (18). Our estimate of the peroxyacetyl radical concentration indicates that this one species contributes less than 1% of the total peroxy radical levels, primarily because of the ability of the peroxyacetyl radical to form the stable PAN adduct with NO2 at lower temperatures. These results, we believe, represent the fastest PAN analyses accomplished to date. The improvement from 5 min to 1 min resolution dramatically improves the potential for aircraft measurement of PANs and for low-level, unambiguous NO2 measurements. Further improvements in analysis time might be accomplished by redesigning the luminol detection chamber to remove the dead volume associated with the LMA-3 detection cell. In addition, a cooled column could be used to improve separation and minimize retention time. We note that the luminol system is inherently more sensitive than the chemiluminescent reaction of NO with O3. This is because the luminol emission is in the visible range (emission maximum at 425 nm), while the NO2* emission is in the far-visible-near-infrared (emission maximum at 1.25 µm) (11). Most photomultiplier tubes are at their maximum sensitivity where the luminol emission occurs, giving an inherent advantage to chemiluminescence detection using luminol. Coupling chromatography to the luminol detector allows a number of past interferences in NO2 measurement (e.g., PANs) to be used to advantage because they can be measured along with the species of interest. The luminol system does not require high vacuum or toxic reagents and thus lends itself to aircraft operation by allowing safer operation with lighter equipment. Detection sensitivities in the low tens of parts per trillion are readily accomplished. The use of chromate converters or the conversion of NO to peroxynitric acid should allow this type of instrumentation to be used for NO as well as NO2 and PANs. Thermal conversion of organonitrates, nitroaromatics, etc. to NO or NO2, which could also be accomplished with this approach, would be useful in unraveling the various contributions of these species to tropospheric nitrogen oxides. 3288
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 19, 1999
FIGURE 6. Peroxyacetyl radical concentrations calculated on the basis of the PAN equilibrium reaction with NO2. Indeed, a gas chromatograph-luminol detection system with thermal cracking to NO2 has recently been applied to the investigation of organic nitrates produced in the troposphere (19).
Acknowledgments The authors wish to thank the aircraft operations group of the Pacific Northwest National Laboratory for their assistance in making these test measurements. We wish to acknowledge the continuing support of the U.S. Department of Energy, Office of Energy Research, Office of Biological and Environmental Research, Atmospheric Chemistry Program. Thanks also to Drs. Paul Doskey and Yoshiko Fukui of Argonne National Laboratory for their assistance and helpful discussions.
Literature Cited (1) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry, Wiley & Sons: New York, 1986. (2) Gaffney, J. S.; Marley, N. A.; Prestbo, E. W. In Handbook of Environmental Chemistry, Volume 4/Part B (Air Pollution); Hutzinger, O., Ed.; Springer-Verlag: Berlin, 1989; pp 1-38. (3) Singh, H. B. Environ. Sci. Technol. 1987, 21, 320-327. (4) Williams, J.; Roberts, J. M.; Fehsenfeld, F. C.; Bertman, S. B.; Buhr, M. P.; Goldan, P. D.; Hubler, G.; Kuster, W. C.; Ryerson, T. B.; Trainer, M.; Young V. Geophys. Res. Lett. 1997, 24, 10991102. (5) Schrimpf, W.; Muller, K. P.; Johnen, F. J.; Lienaerts, K.; Rudolph, J. J. Atmos. Chem. 1995, 22, 303-317. (6) Wendel, G. J.; Stedman, D. H.; Cantrell, C. A. Anal. Chem. 1983, 55, 937-940. (7) Fehsenfeld, F. C., Drummond, J. W.; Roychowdhury, U.K.;. Galvin, P. J.; Williams, E. J.; Buhr, M. P.; Parish, D. D.; Hu ¨ 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.; Schiff, H. I. J. Geophys. Res. 1990, 95, 3579-3597. (8) Burkhardt, M. R.; Maniga, N. I.; Stedman, D. H.; Paur, R. J. Anal. Chem. 1988, 60, 816-819. (9) Blanchard, P.; Shepson, P. B.; Schiff, H. I.; Bottenheim, J. W.; Gallant, A. J.; Drummond, J. W.; Wong, P. Atmos. Environ. 1990, 24A, 2839-2846. (10) Blanchard, P.; Shepson, P. B.; Schiff, H. I.; Drummond, J. W. Anal. Chem. 1993, 65, 2472-2477. (11) Gaffney, J. S.; Bornick, R. M.; Chen, Y.-H.; Marley, N. A. Atmos. Environ. 1998, 32, 1145-1154. (12) Sehested, J.; Christensen, L. K.; Møgelberg, O. J.; Nielsen, O. J.; Wallingon, T. J.; Guschin, A.; Orlando, J. J.; Tyndall, G. S. J. Phys. Chem. 1998, 102, 1779-1789. (13) Gaffney, J. S.; Fajer, R.; Senum, G. I. Atmos. Environ. 1984, 18, 215-218. (14) Koike, Y.; Kondo, S.; Kawakami, S.; Singh, H. B.; Ziereis, H.; Merrill, J. T. J. Geophys. Res. 1996, 101, 1829-1851.
(15) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr, J. A.; Troe, J. J. Phys. Chem. Ref. Data 1992, 21, 1125. (16) Atkinson, R. J. Phys. Chem. Ref. Data, Monograph No. 2, 1994, 74-75. (17) Reiner, T.; Hanke, M.; Arnold, F. Geophys. Res. Lett. 1998, 25, 47-50. (18) Logan, J. A.; Prather, M. J.; Wofsy, S. C.; McElroy, M. B. J. Geophys. Res. 1981, 86, 7210-7254. (19) O’Brien, J. M.; Shepson, P. B.; Wu, Q.; Biesenthal, T.; Bottenheim,
J. W.; Wiebe, H. A.; Anlauf, K. G.; Brickell, P. Atmos. Environ. 1997, 31, 2059-2069.
Received for review October 26, 1998. Revised manuscript received May 27, 1999. Accepted June 24, 1999. ES981102G
VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3289
