Anal. Chem. 1994,66, 3384-3393
Free Radical Detector for Tropospheric Measurement Using Chemical Amplification Jla Hu and Donald H. Stedman' Department of Chemistty, Universiw of Denver, Denver, Colorado 80208 An improved chemical amplifier for atmospheric RO, (HO + HO2 RO ROz) radical measurement is reported. The reaction chamber in this version was equipped with a Teflon filter-nitrogen wall to maintain the nature of the wall constant and reduce the free radical wall loss. The results of laboratory studies were consistent with theoretical calculations assuming a radical wall loss coefficient of 1 s-l. Radical calibration gave relatively stable chain lengths, Le., 26%variability over a period of 3 months. In addition, the zero modulationand the interferences from the ambient PAN and PNA were reduced by using a smaller reactor which yielded a short residence time. The details of the instrumentation and chain length measurement procedure are described. Results of a local field study in the summer of 1993 using this chemical amplifier are presented.
+
+
Free radicals in the earth's atmosphere play significant roles in tropospheric chemistry. The hydroxyl radical, HO, the most important tropospheric gas-phase oxidant, comes from the ultraviolet photolysis of ozone (03), nitrous acid (HONO), and various precursors. H O can initiate the reactions of hundreds of trace gases, both natural and anthropogenic, in many instances as the first and ratedetermining step.' For example, nitrogen and sulfur oxides are converted respectively to nitric and sulfuric acids, and nonmethane hydrocarbons (NMHC) are oxidized to organic acids after reacting with H0.2 Reactions of H O with hydrocarbons and CO lead to the production of hydroperoxy, HO2, organic oxy, RO, and organic peroxy, R02, radicals. HO2 is also formed following formaldehyde photolysis, nitrate radical (NO3) reactions, and the reactions of alkenes with ~ z o n e . ~These - ~ RO, radicals (HO + HO2 + R O + R02) are known to be involved in the production and removal of ozone in photochemical cycles through the competition between the oxidation of NO by RO2 and 0 3 , which govern the so-called NO-N02-03 photostationary-state system in the atmo(1) Ehhalt, D. H.; Dorn, H.-P.; Poppe, D. Proc. R . SOC.Edinburgh 1991.97, 17.
(2) Thompson, A. M. Science 1992, 256, 1157. (3) Calvert, J. G. Proceedings of the NATO Advanced Study Institute on Atmospheric Ozone: Irs Variation and Human Influences; Report No. FAAEE-80-20; U S . Department of Transportation, Federal Avktion Administration, US.Government Printing Office: Washington, DC, 1979; p 153. (4) Moortgat, G. K.; Cox, R. A.; Schuster, G.; Burrows, J. P.; Tyndall, G. S. J . Chem. Soc.. Faraday Trans. 2 1989,85, 809. ( 5 ) Atkinson, R.; Plum, C. N.; Carter, W. P. L.; Winter, A. M.; Pitts, J. N., Jr. J. Phys. Chem. 1984.88, 1210. (6) Cantrell, C. A.; Stockwell, W. R.; Anderson, L. G.; Busarow, K. L.; Perner, D.; Schmeltekopf, A.; Calvert, J. G.; Johnston, H. S. J . Phys. Chem. 1985, 89, 139. (7) Cantrell, C. A.; Davidson, J. A,; Busarow, K. G.; Calvert, J. G. J. Geophys. Res. 1986, 91, 5347. (8) Su, F.; Calvert, J. G.; Shaw, J. H. J . Phys. Chem. 1980, 84, 239. (9) Kan, C. S.;Su, F.; Calvert, J. G.; Shaw, J. H. J . Phys. Chem. 1981,85,2359.
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Analytical Chemistry, Vol. 66,No. 20, October 75, 1994
sphere.1° In addition, HO2 and RO2 are the only gas-phase precursors for peroxide compounds H202 and ROOH, which also contribute to acid precipitation.2 Finally, HO2 can participate in a number of oxidation reactions in the aerosol phase because it is soluble in aqueous aerosols.ll Many methods have been attempted to measure RO, radicals. As a consequence of their high reactivity, the tropospheric RO, concentrations arevery low. Thecalculated total midday maximum varies from about 7 X 108 molecule/ cm3 (28 parts per trillion by volume (pptv) mixing ratio) for clean conditions to around 5 X lo9 molecule/cm3 (200 pptv) for the more polluted case.12 The daytime value for HO2 was predicted to be 10-100 pptv, and a 1991 measurement suggested that atmospheric RO2 was at the same level as HO2.I3 HO concentrations are perhaps 100-1000 times less, with midday photostationary-state concentrations ranging from -1 X lo7 molecule/cm3 (0.5 pptv) to -1 X lo6 molecule/cm3 or less.14J5 RO concentrations are estimated to be much smaller, about 1 X lo4 molecule/cm3 (0.0005 pptv).16 Consequently, the measurement of RO, radicals represents a challenge. Determining the concentration of HO generally makes use one of the following four techniques: laser-induced fluorescence (LIF),17-20long-path absorption,21-26 a radiochemical tracer method in which H O reacts with I4CO to produce 14C02,27-29and titration/selected-ion chemical (IO) Crutzen, P. J. Tellus 1974, 26, 47. (1 1) Mozurkewich, M.; McMurry, P. A.; Gupta, A.; Calvert, J. G. J. Geophys. Res. 1987, 92, 4163. (12) Mozurkewich, M.; Calvert, J. G. J. Geophys. Res. 1990, 95, 5697. (13) Cantrell, C. A.; Lind, J. A.; Shetter, R. E.; Calvert, J. G.; Goldan, P. D.; Kuster, W.; Fehsenfeld, F. C.; Montzka, S.A,; Parrish, D. D.; Williams, E. J.; Buhr, M. P.; Westberg, H. H.; Allwine, G.; Martin, R. J. Geophys. Res. 1992, 97, 2067 1. (14) Eisele, F. L.; Bradshaw, J. D. Anal. Chem. 1993, 65, 927A. (15) Kanakidou, M.; Singh, H. B.; Valentin, K.M.;Crutzen,P. J. J. Geophys. Res. 1991, 96, 15395. (16) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry: Fundamentals and Experimental Techniques;John Wiley and Sons, Inc.: New York, 1986. (17) Hard, M. T.; OBrien, R. J.; Chan, C. Y.; Mehrabzadeh, A. A. Enuiron. Sci. Technol. 1984, 18, 768. (18) Rodgers, M. 0.;Bradshaw, J. D.; Sandholm, S. D.; KeSheng, S.; Davis, D. D. J . Geophys. Res. 1985, 90, 12819. (19) Davis, L. I., Jr.; Gao, C.; James, J. V.; Morris, P. T.; Postiff, R.; Wang, C. C. J. Geophys. Res. 1985, 90, 12836. (20) Chan, C. Y.; Hard, T. M.; Mehrabzadeh, A. A.; George, L. A.; OBrien, R. J. J . Geophys. Res. 1990, 95, 18569. (21) Perner,D.;Ehhalt,D.H.;Patz,H. W.;Pllat,U.;Roth,E.P.;Volz,A.Geophys. Res. Lett. 1976, 3, 466. (22) Dorn, H.-P.; Callies, J.; Pllat, U.; Ehhalt, D. H. Tellus 1988, 40B, 437. (23) Hofzumahaus, A.; Dorn, H.-P.; Callies, J.; Ehhalt, D. H. Atmos. Enuiron. 1991, 25A, 2017. (24) Mount, G. H. J. Geophys. Res. 1992, 97, 2427. (25) Hiibler, G.; Perner, D.; Platt, U.; Tonnissen, A.; Ehhalt, D. H. J. Geophys. Res. 1984, 89, 1309. (26) Armerding, W.; Herbert, A.; Spiekcrmann, M.; Walter, J.; Comes, F. J. Fresenius'-Z. Anal. Chem. 1991; 340, 654. (27) Campbell, M. J.; Sheppard, J. C.; Au, B. F. Geophys. Res. Len. 1979,6,175. (28) Campbell, M. J.; Farmer, J. C.; Fitzer, C. A,; Henry, M. N. J. Armos. Chem. 1986, 4, 413. 0003-2700/94/0368-3384$04.50/0
0 1994 Amerlcan Chemical Society
ionization MS.30 Measuring R 0 2 and H 0 2 concentration has been accomplished in the atmosphere and in the laboratory with techniques including matrix isolation with electron paramagnetic resonance (MIEPR),31-33which may measure HO2 and speciated RO2, and some spectroscopic methods. LIF may also measure H02 by converting HO2 to HO.” More details of the measurement methods for RO, radicals in the atmosphere can be found in review article^.^^,^^.^^ Cantrell and Stedman36and Cantrell et al.37developed a technique for the measurement of total RO, radicals in the early 1980s. They utilized a chemical amplification method in which each initial radical produced a large number of NO2 molecules through a chain reaction involving HO, HO2, RO, and R02 with N O and CO. The chain reaction may be initiated, for example, by letting HO2 or RO2 react with N O to form NO2 and an H O or RO: HO,
+ NO
RO,
+ NO
-
+
+ HO NO, + RO NO,
(1) (2)
Through the addition of a large amount of CO, the HO radical produced in reaction 1 reforms HO2 by the following reactions: HO + CO
-
H + 0, + M
+H HO, + M
CO,
+
(3)
(4)
R O radical from eq 2 reacts efficiently with 0 2 to form an HO2 or RO2 radical and a carbonyl molecule: RO
+ 0,
-
R’CHO (or R’”’C0)
+ HO, (or R’”0,)
(5)
There are several reactions which remove free radicals from the system. The most significant chain termination processes were considered to be the following reactions: HO + NO
+M HO, + NO, + M HO,
+ wall
+
-
HONO + M
(6)
+M
(7)
nonradical products
(8)
-.+
HO,NO,
Actually, the chain can be initiated at any of reactions 1-5 by any of the radicals, and thus the sum of HO, H 0 2 , RO, and RO2 is determined by this technique. In practice, N2 gas is added periodically to replace CO so that the chain reaction (29) Felton, C. C.; Sheppard, J. C.; Campbell, M. J. Enuiron. Sci. Technol. 1990, 24, 1841. (30) Eisele, F. L.; Tanner, D. J. J . Geophys. Res. 1991, 96(D5), 9295. (31) Mihelcic, D.; Ehhalt, D. H.;Kulessa, G. F.; Komfass, J.;Trainer, M.; Schmidt, A.; Rohrs, H. Pure Appl. Geophys. 1978, 116, 530. (32) Mihelcic, D.; Musgen, P.; Ehhalt. D. H. J. Atmos. Chem. 1985, 3, 341. (33) Mihelcic, D.; Volz-Thomas,A.; Patz, H. W.; Kley, D.; Mihelcic, M. J. Atmos. Chem. 1990, 11, 271. (34) Cantrell, C. A.; Shetter, R. E.; McDaniel, A. H.;Calvert, J. G. In Measurement Challenges in Armospheric Chemistry; Newman, L., Ed.; Advances in Chemistry Series 232; American Chemical Society: Washington, DC, 1993; p 291. (35) OBrien, R. J.; Hard, T. M. In Measurement Challenges in Atmospheric Chemistry; Newman, L., Ed.; Advances in Chemistry Series 232; American Chemical Society: Washington, DC, 1993; p 323. (36) Cantrell, C. A.; Stedman, D. H. Geophys. Res. Lert. 1982, 9(8), 846. (37) Cantrell, C. A.; Stedman, D. H.; Wendel, G. J. Anal. Chem. 1984,56, 1496.
is turned off, and a blank signal resulting from 03,NO2, and other possible interfering molecules is recorded. The difference between the signals with and without CO, or the modulated NO2 signal, is proportional to the radical concentration. The NO2 produced is monitored, in most cases, using a sensitive detector based on the chemiluminescent reaction of NO2 with lumin01.~*,3~ The number of NO2 molecules produced per radical, or the chain length (CL), is a crucial factor in determining measured radical concentrations. This number is dependent on the rates of reactions 1-5, the processes that remove the radicals from the system, and the experimental conditions such as the flow rates of the reagent gases and the reaction time. In the past decade, several groups around the world have adopted this chemical amplification method in their research. In addition to measuring free radicals, this technique was employed to detect PAN and radon decay in the ambient air.4U3 One of the major problems often encountered is the uncertainty of the free radical wall loss in the reaction chamber, resulting in the instability of measured chain lengths. Another is so-called zero modulations, which are fake radical signals occurring when no free radicals are considered to be present. Described in this paper is an improved free radical detector which, to a certain extent, solved the two problems mentioned above. The results of laboratory studies and ambient air measurements are presented and discussed.
EXPERIMENTAL SECTION Chemical Amplifier. The instrumentation is composed of three parts: a reactor where the chemical amplification takes place and NO2 is produced from the chain reactions, a detector in which the NO2 concentration is determined, and a gas system to supply the reagent gases and appropriately ventilate the exhaust. (i) Reactor or Chemical Amplifier. Several reactor designs were used in our study. The original configuration was developed by both Buhr and Ghim.44*45Later, a cone-shaped chamber with a Teflon filter wall was c o n ~ t r u c t e d .After ~~ an informal comparison between these two reactors and a reactor designed by Hastie et al.$0,46a new version of the University of Denver chemical amplifier was configured as shown in Figure 1. In the original reactor used in this laboratory during 1980s, the reaction chamber (250 mL volume) was fabricated from stainless steel coated with halocarbon wax. This configuration yielded chain length estimates ranging from 10s to over 1000.44345After a series of tests on the factors affecting the chain length determination, it was found that the major problem was the radical wall loss. In our final configuration, ,
(38) Maeda, Y.; Aoki, K.; Munemori, M. Anal. Chem. 1980, 52, 307. (39) Wendel, G. J.; Stedman, D. H.; Cantrell, C. A. Anal. Chem. 1983, 55, 937. (40) Hastie, D. R.; Weissenmayer, M.; Burrows, J. P.; Harris, G. W. Anal. Chem. 1991, 63, 2048. (41) Anastasi, C.; Gladstone, R. V.;Sanderson, M. G. Enuiron. Sci. Techno/.1993, 27, 474. (42) Blanchard, P.; Shepson, P. B.; Schiff, H. I.; Drummond, J. W. Anal. Chem. 1993,65, 2472. (43) Ding, H.; Hopke, P. K. J . Atmos. Chem. 1994, 17(4), 375. (44) Buhr, M. P. M.S. Thesis, University of Denver, Denver, CO, 1986. (45) Ghim, B. T. M.S. Thesis, University of Denver, Denver, CO, 1988. (46) Stedman, D. H.; Hu, J. Proc. Annu. Meet.-Air Waste Manage. Assoc., 86th 1993, Paper No. 93-WA-68A.03, 16 pp.
Analytical Chemistry, Vol. 66,No. 20, October 15, 1994
3385
GAS MIXTURE
CO
-
(a)
-*1
*
A
N2
A
e
A
(C)
N2
,
A
*
A
B/ ,
* A ,
*
7
A
* CO
A - -
*
A
A
~I
* A
A
*
A
B/ A
7
* -
I
A
BI
-
A
*
A
*
*
B/ A
A
,
A
B ,
+EXHAUST
t
co
t
N2
Figure 1. Schematic diagram of the chemical amplifier. The Teflon filter or nitrogen wall reaction chamber is drawn with the dashed lines. Letters A and B refer to the two carbon monoxide addition points.
the reaction chamber is a tubular Teflon filter (Fiberfilm T60A20, Pallflex Inc.). A piece of 20.4 cm X 7 cm Teflon filter was vertically rolled up and sprung inside a coarse galvanized wire mesh (mesh size, 1 in.) cylinder which was mounted inside a 20.8 cm long, 5.1 cm i.d. leak-free PVC pipe. The final dimension of the Teflon filter reactor is 20.4 cm long with 1.85 cm i.d., corresponding to a volume of 55 cm3. An N2 gas flow of about 400 mL/min enters the PVC pipe and passes through the Teflon filter wall, or nitrogen wall as it is called, into the chamber so that a nitrogen layer is formed around the wall, in an attempt to prevent the free radical molecules from contacting with the wall, thereby reducing the wall loss of the radicals. It is quite possible that some radicals might still diffuse into the nitrogen layer and be lost on the wall. However, the fraction of the radical molecules colliding directly with the wall is expected to be decreased with this design. At a typical total flow of 3 L/min, the residence time of radicals in the reactor is 1.1 s, which is the same as that of Hastie et ala’sreactor.40 This smaller chamber was adopted partly to reduce the zero modulation and the interferences caused by PAN and PNA in the atmosphere, as discussed later. Another change we made was to take off the air or gas mixture “inlet”. In the original RO, detector, air was drawn into the reactor through an inlet tube of 1/4 in. stainless steel where the reagent gases NO, CO, and/or N2 were added. In the new version, the reaction chamber itself is a Teflon filter “tubing” of less than 3/4 in. i.d., which also functions as an air inlet. The reagent gases are sent to the chamber through theTeflon filter wall at the front of the reactor as demonstrated in Figure 1. This improvement eliminates possible free radical loss in the narrow stainless steel inlet. (ii) NO2 Detector. The luminol detector used in our work is similar to those described el~ewhere.~*J~ The luminol solution is pumped by a peristaltic pump at a rate of 0.3 mL/ min and flows over a 4.7 cm diameter fiberglass cloth which is held on the back wall of the detection cell. The gas mixture from the reactor is passed between the fiberglass cloth and a glass window that seals the cell from the photomultiplier tube (RCA 4507). The blue chemiluminescence occurring at the 3388
Analytical Chemistry, Vol. 66, No. 20, October 15, 1994
gas-liquid surface from the reaction of NO2 with luminol is measured, and the photocurrent generated is amplified by an electrometer. The output is displayed on a chart recorder (Cole-Parmer Instrument 8387-32) and is also collected by a personal computer using the Lab Master program. (iii) Gas System. All gas flows are controlled by mass flow controllers (Tylan Corp.) and calibrated using a bubble flow meter for slow flow rates or a gas sampling test meter (Singer, American Meter Division) for larger ones. The total flow is measured between the reactor and the detector. The solenoid valve timing sequence is controlled by a Chrontrol CT-4 table top timer. Since a high concentration of carbon monoxide is used, care is taken to ensure that the system is leak-free and that the laboratory area is equipped with an alarm (Enmet ISA-3) to warn of dangerous levels. The exhaust from the detector and CO/N2 switch system is properly vented. (iv) Modulation Experiments. As described in the introduction section, in the conventional system, a radical zero was obtained by replacing the flow of CO with an equal flow of N236337344345 (Figure 2a). In Hastie et al.3 system, the chain mechanism was suppressed by moving the point at which the CO was added closer to the NO2 detectorm (Figure 2b). Besides these two modulation patterns, a new one combining the features of conventional and Hastie’s systems (Figure 2c) was also tested. As shown in Figure 1, three 3-way solenoid valves were used in the RO, detector. The modulation frequency was 0.25 min-I since experiments showed that about 90 s was needed for a signal to reach its maximum. The first 2 min in each modulation cycle supported the chain reaction, CO being added into the reactor at the front of the chamber and N2 going to the vent. At the third minute, CO and N2 were switched, Nz being sent to the chamber and a signal zero being achieved. For the last minute, N2 was sent to the vent again, and CO was added between the reactor and the detector. No chain reaction was expected in this situation. Notice that the chain reaction was shut off during the last 2 min by both replacing CO with Nz (third minute) and moving the CO addition point (fourth minute). Without Nz, CO flow is temporarily doubled at point B when CO is switched from point A to B. This causes a spike in the output reading which we essentially eliminated using the N2 addition. Data were collected as 1 s average values every 2 min (1 s before the switch of with/without chain reactions) for all three modula-
-
BLACK LIGHT
H2+Jbb\
c12
AIR
1
LIGHT SHIELD
DILUENT AIR
/,-
3
/-
7
--w
TO DETECTOR
LIGHT _._. . .
SHIELD
L l -
BLACK LIGHT
Flguro 3. Schematic diagram of C12/H2/alr photolysis system used as a free radical (HO?)source.
tion patterns as indicated by the asterisk marks in Figure 2. With this data acquisition system, a point-to-point precision of &3% was observed using a stable free radical source in the laboratory. Free Radical Source. A free radical or HO2 source was established using the Clz/Hz/air photolysis system which was studied by Cox et al. in 1977.47 For [Hz]/[Clz] > 20, HO2 radicals are produced in a flow tube by the following mechanism: C1,
+ hv (320 nm) -.2C1
j
(9)
experiments were performed under three conditions: with NO2 only, with 6 ppm N O added, and in the radical detection mode. Radical Calibration Technique. Free radical calibration or chain length measurement was made in this study using the Clz/H2/air photolysis system. When the HO2 concentration is large enough that homogeneous radical destruction dominates over wall loss, the following equations hold from reactions 9-13: d[Cl]/dt = 2j[C12]- klo[C1l[H,] = 0
(14)
C1+ H2-. HCl+ H H + 0, + M -.,HO, 2H02 2H02 + M
+M
kll
(11)
H 2 0 2 0,
k12
(12)
H 2 0 2 0, + M
k13
(1 3)
- - +
For the initial period after all flows are established and the light shields are rapidly removed, the term 2(k12 + k13) [H02I2 is assumed negligible. Then: d[H021/dt = kl,[Hl[021 = klo[C11[H21= 2j[Cl21 (17)
The radical destruction processes also include wall loss, eq 8, when the [HOz] is low. The photolysis flow tube setup is shown in Figure 3. The large portion of the tube is 7.5 cm i.d. X 23.5 cm, and the narrower region is 1.65 cm i.d. X 37 cm, giving a total volume of 1.09 L and reaction time of 150 s when a flow of 410 mL/min is used. The inner surface of the flow tube is coated with halocarbon wax (Halocarbon Products Corp.) to reduce wall reactions. Four G E black lights rich in 320 nm radiation are arranged around the tube. The air flow providing 0 2 for the reactions and hydrogen flow are maintained in excess to minimize undesirable reactions. The final flow is diluted with a large air flow near the reactor inlet. The free radical concentrations can be varied by changing either Cl2 or dilution air flow. In the latter case, the radical source itself remains unchanged. NO2 Calibration Technique. NO2 was added in concentrations from 0 to 250 ppbv at the front of the reaction chamber with thereagent gases NO, CO, and/or N2. The photocurrents from the electrometer were recorded. As a comparison, the (47) COX,R.A.; Derwent, R.G.J . Chem. SOC.,Faraday Trans. I 1977, 73, 272.
For the steady state: d[H02]/dt = 2j[C12]- 2(k,,
+ k1,)[HO2l2= 0
(18)
Combining eqs 17 and 18 and letting the chain length, CL, equal [NO21/ [H02], the final chain length calculation formula is given: CL = 2tk1.2 + ki3)[NO2lS:/IS
(19)
where [N021ss refers to NO2 concentration at steady state and IS stands forinitialslope, d[NOt]/dt, whichequals2j[Cl+ CL. Thus, in this method, we measured two NO2 signals from which the chain lengths or instrumental sensitivity to free radicals might be derived from eq 19: (1) the initial change of NO2 signal with time and (2) the steady-state signal. Experimentally, the initial slope was obtained by uncovering Analytical Chemisfty, Vol. 66, No. 20, October 15, 1994
3387
Table 1. Modeling Conditions
temperature pressure relative humidity
CH3C002 (PA) CH2O 0 3
HNOi
Environment 298 K 630 mmHg (Denver area) 15% ambient reagent Initial Conditions 30 pptv 0.2 pptv 2 PPbV 40 ppbv 20 ppbv 2 PPmv 2 PPbV 10 pptv 50 ppbv 60 ppbv 20 ppbv 5 PPbV 0.5 ppbv 1.5 ppmv 50 pptv 30 pptv 0.0005 pptv 3 PPtV 0.003 pptv 4690 ppmv 20%
6 PPmv 8%
the black lights to start the photolysis reactions and measured during the first 10 s. During this period, the signal rose steeply and nearly linearly with time. Steady-state NO2 signals were measured about 20 min later after the steady state was achieved. In practice, the chain lengths were measured for a range of HO2 concentrations in which eq 19 held. This range was determined experimentally. Local Field Measurements. The field measurement site was located in a parking lot near our laboratory building. The free radical detector was set in a trailer. Atmospheric ozone concentrations and solar UV fluxes were measured simultaneously with an 0 3 monitor and an Eppley radiometer, respectively. Computer Simulation. The behavior of the chemical amplifier under the conditions of laboratory research and ambient air study was modeled using the PHASEPLANE computer program.48 The program input was in the form of differential equations or rate expressions. The reaction mechanism and rate coefficients were largely based on recommendations of reference^.^,^^,^^,^^ Table 1 lists the model conditions for the ambient air study with the initial gas mixing ratios for the case of the field measurement in the parking lot. Simulations were performed for daytime only, so the reactions concerning species like N2Os and NO3 were not included. The modeling results are presented in the formof chain lengths. Reagents. The gases used in this work were CO (General Air, CP grade); Nz (General Air, C P grade); 1300 ppm N O (48) Ermentrout, B. Phaseplane: The Dynamical Systems Tool; Brooks/Cole
Publishing Co.: Pacific Grove, CA, 1989. (49) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. Chemical Kinetics and Photochemical Dura for Use in Stratospheric Modeling, NASA Panel for Data Evaluation, Evaluation Number 10, JPL (Jet Propulsion Laboratory) Publication 92-20; Pasadena, California, 1992. (50) Cantrell, C. A.; Shetter, R. E.; Lind, J. A,; McDaniel, A. H.; Calvert, J. G.; Parrish, D. D.; Fehsenfeld, F. C.; Buhr, M. P.; Trainer, M. J . Geophys. Res. 1993,98, 2891.
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Analytical Chemistry, Vol. 66, No. 20, October 15, 1994
in N 2 (Scott Specialty Gases, Certified Master Gas); 50 ppm NO2 in air (Scott Specialty Gases, Certified Master Gas); H2 (General Air, CP grade); and 0.1% Cl2 in N2 (General Air). The CO was passed through an activated charcoal/iodine trap to remove metal carbonyl compounds, principally Ni(C0)dand Fe(C0)6.44 TheNO was passed throughdry FeS04 to reduce to N O any NO2 formed in the cylinder. Stainless steel instead of Teflon tubing was used to carry N O to prevent the inward diffusion of 0 2 which led to formation of N02. Building compressed air was used and passed through activated charcoal to remove hydrocarbons and NOx, through ascarite to remove C02 and water, and through Purafil to remove CO before entering the photolysis tube. The composition of the luminol operating solution was 0.05 M NaOH, 0.2 M Na2S03, 1 X 10-4 M luminol (3aminophthalhydrazide, Aldrich, 97%), 1.5 X 10-4 M EDTA, and 0.1% (v/") Triton, as recommended by both Cantrell et al.so and Blanchard et Triton, a surfactant, was added to cause the solution to flow homogeneously over the fiberglass cloth and thus reduce the base line noise, as reported by Blanchard et al. The base line noise of the system with this solution was 0.05 mA, compared to 0.1 mA with the solution we previously46 used. In addition, we observed higher sensitivity and better linearity for the NO2 detector. All reagents were used without further purification with the exception of the luminol, which was recrystallized from alkaline aqueous solution as described by B ~ r k h a r d t . ~ '
RESULTS AND DISCUSSION Theoretical Behavior of the Chemical Amplifier. A general description of chemical amplifier behavior as a function of reagent concentrations, initial radical concentrations, residence time, and rate constants for pertinent reactions, especially the wall loss of radicals, can be found in the Presented in this paper are model results concerning our chemical amplifier under the specific operating conditions used, in order to get a better understanding of the instrument. For example, 1.1 s was used as the reaction time in the chamber for all the simulations with the exception of chain lengths as a function of reaction time. (i) Influence of Instrumental and/or Operational Parameters. The calculated chain lengths versus [NO] added to the system as a reagent are shown in Figure 4 by the solid line. is chosen to be 1 The wall loss coefficient for radicals, s-l, as discussed later. The optimum N O concentration appears at 4 ppmv. Normally, we workat an N O concentration of 6 ppmv. The modeled chain lengths versus [CO] added in the chemical amplifier is displayed by the dotted line in Figure 4. Here, again, the calculation is performed with radical wall loss equal to 1 s-l. This result indicates that the sensitivity of the instrument is continuously increased by raising the carbon monoxide concentration within the range of our calculations. Considering the lower explosion limit of CO in air, 12.5%,52a CO concentration of 8% is used in practice for safety reasons. Shown as the downward sloping line in Figure 4 is the chain length as a function of kwall. The RO, concentrations (51) Burkhardt, M. R. Ph.D. University of Denver, Denver, CO, 1989. ( 5 2 ) CRC, Handbook of Chemistry and Physics; Weast, R. C., Ed., CRC Press: Cleveland, OH, 1974.
I
._.^_C--
" I
0
__.__.. -
reaction time _,,,_.__._._. ,__...-..,-....".-.--
1
2
3
[NO],ppmv;
5 6 7 B [CO],%;Kwall,l/sec; t,sec
4
9
3
Figure 4. Calculated chain lengths as a function of the InstrumentaVoperationalparameters: reagent gases [NO] (solM line) and [CO] (dotted line), radical wall-loss coefficlent, kwell (downward sloping Ilne), and reaction tlme (dashed line).
are 30 pptv for both H02 and RO2, and the normal operating conditions are applied, that is, the reagents are 6 ppmv and 8% for N O and CO, respectively. The kwallof our system is estimated by comparing the chain lengths measured in the laboratory and calculated at various kwallusing the laboratory model. At the point of kwall = 1 s-l, the calculated chain length, 160, from the simulation of laboratory study is consistent with those obtained before and after the field study (average value of 158). For ambient air modeling, the calculated chain length for a kwallof 1 s-l is 151, very close to both the observed data and calculated value based on the laboratory model. Therefore, a radical wall loss of 1 s-l is chosen for all the model calculations. The calculated effect of varying the residence time on the chain lengths is demonstrated by the dashed line in Figure 4. Notice that the chain reaction continues to increase in length even at t = 10 s in this simulation. The calculated value of chain length from this model is lower than those in the literature because a polluted ambient air condition was adopted. For example, high concentrations of O3 and NO2 are involved as found over a typical urban parking lot, and both may cut chain length down as discussed below. If [O3]and [NO21 were set at zero, a calculated chain length of 3 10 would be achieved for kwall= 0 at reaction time of 1.1 s, which would be consistent with the results of Hastie et al., 270,40 and Cantrell et al., 450.50 (ii) Interferences from Atmospheric Species. The ambient air is a complex mixture that contains a number of species to which the luminol NO2 detector responds. For example, NO2 is often present, and ozone will convert the added NO to NO2 in the instrument. Both may contribute directly to the signal and act as interferents. This background signal is determined by shutting off the chain reactions for a part of the measurement time. NO2 and ozone may also alter chain lengths as indicated by the modeled results. Summarized in Figure 5 by solid line and dotted line are the calculated impacts on chain length by ozone and NO2,
respectively, in the ambient air. The chain lengths are reduced as [O,] increases, because of the formation of NO2 by the reaction between ozone and reagent NO. However, this background-dependent sensitivity is small compared to the effect of other factors. Atmospheric NO2 has a more significant effect on calculated chain lengths. This is because of the extra termination of the chain reactions by the reaction between NO2 and H O radicals when the NO2 concentration is high. Two atmospheric molecules have been identified as the principal interferents in both the radical measurement and the NO2 detection system. PAN (peroxyacetyl nitrate, CH3C002N02) and PNA (pernitric acid, H02NO2) could decompose in the reactor, releasing NO2 as well as peroxyacetyl radicals and H02. Because these contributions to the signal are comparable to the signal from RO, radicals, it is necessary that they be determined regularly to correct the measured radical signals. However, the decomposition ratios are dramatically decreased by shortening the reaction time.40The simulated results show that both PAN and PNAconcentrations increase at the very beginning of the chain reactions, and the decompositions occur after about 0.2 and 0.25 s, respectively. At 1.1 s residence time, [PAN] is still higher than its initial concentration, and the fractional decomposition of PNA is 0.8%, corresponding to an HO2 concentration of 4 pptv at the ambient [PNA] of 500 pptv as illustrated by the modeling conditions in Table 1. Interference from PAN and PNA is not expected to be a problem in this system under the normal operating conditions. By contrast, for a radical reactor of 10 s residence time, the fractional decomposition would be 0.13% and 30%for PAN and PNA, respectively,which would produce free radicals of 2.6 pptv from PAN and 150 pptv from PNA (if 2 ppbv PAN and 500 pptv PNA were presented in the ambient air as shown in Table 1). Under these conditions, one would overestimate the radical concentration by more than 200% when the chemical amplifier was measuring 60 pptv RO, radicals. Anal~icalChemlstry, Vol. 66,No. 20, October 15, 1994
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., R02
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[031,ppbv; [N02l,ppbv; [R02l,pptv Figure 5. Calculated chain lengths as a function of concentrationsof ambient species: line).
Finally, the calculated chain lengths versus total initial free radical concentrations, which are the sum of [HOz] and [ROz] with the ratio of [H02] to [ROz] set at 1:1, are plotted on the dashed line in Figure 5. The nonlinear response in the model is due to the PNA chemistry. PNA decomposes when both HO2 and NO2 concentrations are low and is formed at high H02 and NO2 levels. Laboratory Studies. (i) Modulation Experiments. The output from the radical detector when HO2 is measured from a stable radical source with three modulation patterns is shown in Figure 6 . Spikes were observed for both Hastie’s and the combined pattern when CO was switched between the two addition points. Replacing CO with N2 (the conventional style) resulted in a slowly changing, time-dependentfree radical signal. Fundamentally, the radical sensitivitieswere the same for all three. The combined modulation pattern generated by three valves was employed throughout our studies, both laboratory and field, since more stable readings were obtained with small spikes. (ii) NO2 Calibration. Figure 7 shows the results of calibrations under three experimental conditions: with NO2 only, with addition of 6 ppm NO, and in the radical detection mode. It has been reported that for large concentrations of N O (>-5 ppmv) a decrease in sensitivity and a nonlinear behavior for the NO2 detector o c c ~ r r e d .These ~ ~ ~ results ~~ were also observed in our system. Moreover, the sensitivity decreased further when the radical detection mode was applied. The reason for this phenomenon is not clear yet. It is inferred that the following reaction might occur at the gas-liquid surface: CO + NO,
+ H,O
-
H,CO,
+ NO
(20)
The calibration curve used is that for the radical detection mode. The linear response region for NO2 is from 40 to 250 ppb in this case. In practice, a small, constant flow of NO2 corresponding to 40 ppb is added into the system downstream 3390 Anelfiical Chemistry, Vol. 66, No. 20, October 15, 7994
O3(solid line), NOz(dotted line), and free radicals (dashed
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u 0
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Time, minutes Flgure 8. Output from the radlcal detector for the measurement of HOnradicals from a stable source using the three modulation patterns described in the text. Left, this study; middle, conventional system: right, Hastie et al.
of the reactor in order to ensure the linearity of the detector. The sensitivity is about 0.1 mA/ppb NO*, and the system is able to detect ppt levels of NO2. An apparent free radical modulation was still observed in the absence of free radicals but at a much lower level than with other configurations (typically about 2.5% of NO2 signals). This zero signal was subtracted from all data except
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100 150 [N021, ppbv
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Table 2. Chain Lengths Determlned Using Ci2/H2/Alr Photoiysls System as Radlcal Source date no. w 2 1 , PPtV chain length
July, 1993
October, 1993
Flgure 7. NO2 calibration of reactor and detector system for three conditions: with NO2 only (m), 6 ppmv [NO] added (A),and In the radlcal detectlon mode (0).
those from some laboratory studies in which a constant NO2 background signal existed. Hastie et al.40 found that the magnitude of these zero signals was dependent on the amplifier reaction time and attributed it to the production of radicals from the thermal decomposition of PAN in the chemical amplifier. They designed a radical reaction chamber with consideration to the lifetime of PAN, and their final configuration had a much shorter residence time than conventional reactors. As described previously, one of the improvements we made on our chemical amplifier was to build a small reaction chamber which gave a reaction time of 1.1 s. With this design, the zero modulation observed in NO2 calibration decreased from 7.5% (yielded by a Teflon filter reactor with a reaction time of 3.5 s46) to 2.5% of NO2 signals. In typical field measurements, a background reading of 140 ppb NO2 (40 added at the reactor and 100 from ambient NO2 plus 0 3 ) gave a background modulation of about 3.5 ppb. The ambient free radicals provided an added modulation of about 20 ppb. NO2 calibration in this study was performed in the laboratory with the building compressed air in which no PAN was supposed to exist. The zero modulation observed during the experiments indicated some other causes besides the decomposition of peroxy nitrates. The NO2 calibration curves (Figure 7) were plotted using the average of the signals in the radical modulation mode. (iii) Response to a Stable Radical Source. A stable radical source was set up using the C12/Hz/air photolysis system. The chlorine, hydrogen, and reagent air flows were constant, and the flow rates were 2.7, 29.8, and 378.8 mL/min, respectively. The large dilution air flow acted as an independent variable, varying from 2.4 to 9.8 L/min. The results show that the chemical amplifier system is linear to changes in [H02]. The linearity of response is fairly good, with R2 values of 99.8%. (iv) Relationship between HOz Produced and Clz in the Photolysis Tube. In this study, the flow rates of hydrogen, reagent air, and dilution air were 29.8, 378.8, and 3400 mL/ min, respectively, and fixed, but that of chlorine varied from 0.4 to 8.0 mL/min. At low Cl2 concentrations, a linear relationship between [Clz] and final NO2 signals was obtained since the dominant radical removal process was wall loss. When the chlorine flow rate was greater than 2.5 mL/min, cor-
1 2 3 4 5 6 7 8 9 10 11
35.9 39.4 53.1 156.5 137.2 101.6 58.4 25.2 3.4 7.2 49.0 average
133.7 131.5 118.8 112.7 157.7 146.5 131.91 138.9 150.4 162.1 151.5 139.6 A 10.5
1 2 3
69.0 62.9 57.5 average
176.5 181.4 169.7 175.8 f 10.8
responding to over 6.1 ppmv [Clz] in the flow tube, the plot of radical signals versus C12 concentrations was not linear, but the relationship between the signals and [Clz] (square root of [Clz]) was. This is consistent with the radical production mechanism of HO2-HO2 combination dominating over free radical wall loss. From eq 18, the following expression may be derived:
where K = u/(kl2 + k13)]l/~. (v) Determination of Chain Lengths. Chain length calibrations were performed in the laboratory before and after the ambient air measurement with H 0 2 produced in the Cl2/ Hz/air photolysis tube. The flow rates of hydrogen and reagent air were held constant at 29.8 and 378.8 mL/min, respectively, and that of chlorine changed from 2.7 to 8.0 mL/min, which was within the linear region of radical signal versus [C12]1/2. At the outlet of the tube, the gas mixture was diluted by the large air flow (flow rates varied from 3 to 13 L/min) so that the magnitude of the recorded modulated signals were within the range observed in the field measurement. Only a small portion of the flow from the photolysis tube entered the amplifier, where it was further diluted by the reagent gases, principally N2 and CO. Both the initial slope and the steadystate NO2 signal were corrected for zero modulation. The results of the chain length calibration are presented in Table 2. HO2 concentrations listed areobtained by dividing the steady-state [NO21 by the calculated chain lengths and represent the final radical mixing ratios in the chemical amplifier. The chain length determined prior to the field study was 140, with a variability of f10.5 at the 95% confidence interval. The radical calibration repeated after the field study gave a chain length of 176, with a variability of f10.8 at the same confidence interval. These are consistent with a free radical wall loss rate coefficient of about 1 s-l, based on the calculations discussed earlier. In comparison, Hastie et al. reported a chain length of 240, which was consistent with a wall loss coefficient of 2.5 s-l for their system$Oand the radical calibration made by Cantrell et al. during a 1-month field measurement yielded chain lengths from 5 1 f 22 to 131 f Anal’ical
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18 19 20 21 Date in August, 1993
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Figure 8. 30-min averages of RO, concentrations measured in the parking lot outside the S. 0 . Mudd Building at the University of Denver, August 13-25, 1993.
30, which corresponded to a calculated kwa1lof 5 s-1.13,50,53 Combining the results of both NO2 and free radical calibrations, the sensitivity of this free radical detector is estimated to be 0.016 mA/pptv RO,, with a detection limit of about 5 pptv with a stable background signal. As we expected, the wall loss of the radicals at the Teflonnitrogen gas wall was fairly low. Furthermore, stable chain lengths resulting from a relatively invariant wall loss rate coefficient are achieved. These results imply that the character of the "nitrogen wall" formed on the Teflon filter in the reactor seems constant, with the fresh nitrogen continuously carried in. This improvement appears to solve a major problem of the marked and variable effect of the reactor wall on the sensitivity of the instrument. Ambient Air Measurements. A local field measurement was carried out in the summer of 1993 from August 8 to September 12 outside our laboratory building at the University of Denver, about 7 miles southeast of downtown Denver. The free radical detector, along with an ozone monitor and an Eppley radiometer, were set up in a trailer with the inlet pointed northward to avoid sunshine. The temperature inside the trailer was kept at about room temperature using an air conditioner or, occasionally, a heater. The operating conditions were the same as for the laboratory studies. All instruments were calibrated before and after the measurement. Since free radicals are easily destroyed by any surface, no trap or filter was employed for the ambient air before it entered the chemical amplifier. This caused a problem in that the cleanliness of the system deteriorated rapidly during the field study, particularly in thereactor exit tubing. As a precaution, the Teflon filter and its galvanized wire cylinder support were changed and the tubing around the reactor and the cell in the NO2-luminol detector were cleaned about every 2 weeks. The (53) Cantrell, C. A.; Shetter, R. E.; Calvert, J. G.; Parrish, D.D.;Fehsenfeld, F. C.; Goldan, P. D.;Kuster, W.; Williams, E. J.; Westberg, H. H.; Allwine, G; Martin, R. J . Geophys. Res. 1993,98, 18355.
3392 Analytiml Chemistry, Vol. 66, No. 20, October 15, 1994
tubing, especially the ferrules, became dirty with some kind of dark green material apparently from the atmosphere. In the detection cell, some solid material was formed from the alkaline luminol solution. However, the Teflon filter of the chemical amplifier was relatively clean. Only a pale brown spot was found on the position opposite the reagent gas inlet. The air flow drawn into the radical detection system and NO2 sensitivity of the luminol detector decreased with the measurement time. Both changes were determined by measuring the total flow rate of the system and making the NO2 calibration right before and after each successive run. Our site was located in a parking lot, leading to high and often rapidly fluctuating NO, mixing ratios that appeared in the free radical base line. This fluctuation made discerning the modulated signal due to ambient RO, difficult, and the data were therefore collected, averaged, and analyzed using a personal computer. The system was operated from August 13 to September 12. Figure 8 shows the RO, concentrations measured from August 13 to August 25. The free radical detector ran continuously from the morning of August 13 (Friday) to early morning on August 26 (Thursday) with occasional suspension for maintenance. In the data processing, the backgroundrelated zero modulation was subtracted from the radical signals, and the RO, mixing ratios were reduced to 30 min average values. The weather for this period was quite changeable, with frequent thunder storms, but generally clear and sunny and sometimes very hot. The conditions were conducive to radical formation. The daytime free radical mixing ratios ranged from 15 to 140 pptv, with around 60 pptv typical, peaking in the early afternoon at about 2:OO p.m. each day. The nighttime mixing ratios of RO, were more variable than we expected, from -1 7 to 15 pptv, with average >5 pptv, which was within the nighttime radical range theoretically predicted by Cantrell et al.' This might reflect thegenerationof free radicals through
nighttime chemistry by the reactions of NO3 radicals with CH20,CH3CH0, and other biogenic hydrocarbons.6~7~54.55 Negative values also were reported occasionally during daytime because of the rapid fluctuation of the background which resulted from 03 and NO2 in the atmosphere, whose fluctuations were sometimes faster than the 4 min instrument cycle time. (54) Stockwell, W. R.; Calvert, J. G. J. Geophys. Res. 1983, 88, 6673. (55) SabljiC, A,; Giisten, H. Atmos. Enuiron. 1990, 24.4, 73.
Only part of the data from the RO, radical measurement are displayed here to demonstrate the ability of this improved chemical amplifier to detect normal atmospheric free radical levels under difficult conditions. Data analysis, comparison to ozone concentrations, solar UV fluxes, j(NO2) values, etc. will be presented in a later paper. Received for review April 15, 1994. Accepted July 6, 1994.' *Abstract published in Advance ACS Abstracts, August 15, 1994.
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