Environ. Sci. Technol. 1990, 2 4 , 1841-1847
1,2,3,7,8,9-H~CDF, 72918-21-9; 2,3,4,6,7,8-H~CDF,60851-34-5; 1,2,34,6,7,8-HpCDF, 67562-39-4; 2,3,7,8-TCDD, 1746-01-6; 1,2,3,7,8-PeCDD,40321-76-4; 1,2,3,4,7&HxCDD, 39227-28-6; 1,2,3,6,7,8-H~CDD, 57653-85-7; 1,2,3,7,8,9-H~CDD, 19408-74-3; 1,2,3,4,6,7,&HpCDD,35822-46-9OCDD, 326887-9Mg, 7439-95-4. Literature Cited Oehme, M.; Mano, S.; Bjerke, B. Chemosphere 1989,18, 1379-1389. Oehme, M.; Manq S.; Brevik, E. M.; Knutzen, J. Fresenius 2.Anal. Chem. 1989, 335, 987-997. Ahlborg, V. G.; Hikonsson, H.; Waern, F.; Hanberg, A. Nordisk dioxinriskbedomning with English summary., Miljorapport 49:5-111! Nordic Of Ministers: Copenhagen, 1988. Ballschmiter, K.; Buchert, H.; Niemczyk, R.; Munder, A,; Swerev, M. Chemosphere 1986, 15, 901-915. Rappe, C.; Andersson, R.; Bergqvist, P.-A.; Brohede, C.; Hansson, M.; Kjeller, L.-0.; Lindstrom, G.; Marklund, S.; Nygren, M.; Swanson, S. E.; Tysklind, M.; Wiberg, K. Chemosphere 1987, 16, 1603-1618. Swanson, S. E.; Rappe, C.; Malstrom, J.; Kringstad, K. P. Chemosphere 1988, 17, 681-691. Swanson, S. E. Dissertation, Dioxins in the Bleach Plant. Institute of Environmental Chemistry, University of Umei, Umei. 1988. Knutzen, J.; Oehme, M. Chemosphere 1989,18,1897-1909. Norstrom, R. J.; Muir, D. C. G.; Simon, M. Arch. Enuiron. Contam. Toxicol., in press. 1988:79
(10) Knutzen, J.; Oehme, M. Klorerte dibenzofuraner og dioksiner i krabber, fisk og reker fra Frierfjorden, tilstertende omrider og referansestasjoner 1988-1989 Norwegian Institute for Water Research: Oslo, Norway, 1989. (11) Smith, L. M.; Stalling, D. L.; Johnson, J. L. Anal. Chem. 1984,56, 1830-1842. (12) Oehme, M.; Fiirst, P.; Kruger, D.; Meemken, H. A.; Groebel, W. Chemosphere 1988, 17, 1291-1300. (13) Tarkowski, S.; Yrjanheikki, E. Chemosphere 1989, 18, 995-1000. (14) Kuehl, D. W.; Cook, P. M.; Batterman, A. R.; Butterworth, B. C. Chemosphere 1987,16,657-666. (15) Rappe, C.; Bergqvist, P.-A,; Kjeller, L.-0. Chemosphere 1989, 18,651-658. (16) Bergqvist,p, A.; Bergek,S,; Rappe, C.; de With, C.; Jansson, B.; Olsson, M. Chemosphere, in press. (17) Hagenmaier, H.; Brunner, H.; Haag, R.; Berchthold, A. Chemosphere 1986,15, 1421-1428. (18) Zebuhr, Y.; Naf, C.; Broman, D.; LexBn, K.; Colmsjo, A.; Ostman, C. Chemosphere, in press. (19) Czuczwa, J. M.; Hites, R. A. Enuiron. Sci. Technol. 1986, 20, 195-220. (20) Bartonova, A,; Oehme, M. Statistical Analysis of Concentrations of PCDD in Crab from the Southeast Coast of Norway; Norwegian Institute for Air Research: Lillestrom, Norway, 1989. Received for reuiew March 9,1990. Revised manuscript received July 2, 1990. Accepted July 24, 1990.
The Radiochemical Hydroxyl Radical Measurement Method Colin C. Felton,t John C. Sheppard,*ptand Malcolm J. Campbell$
Department of Chemical Engineering and Laboratory for Atmospheric Research, Washington State University, Pullman, Washington 99164 The radiocarbon tracer technique for measuring ground-level OH concentrations is described in detail with special emphasis on the validity of the method and the steps taken to ensure reproducible measurements. The method uses 14C0as a tracer of the tropospheric CO-OH reaction in a photolytic UV-transparent flow reactor. The OH concentration is inferred from the amount of tracer used, the CO-OH reaction rate constant, the reaction time, and the p activity of the resulting 14C02. Forty-two experiments performed on 5 days in October 1987 revealed midday mean OH concentrations of (10.1 f 3.3) X lo6 and (2.9 f 0.5) X lo6 radicals cm-3 in polluted and pure air, respectively. Early morning and evening OH concentra. tions were less than (2.5 f 0.5) X lo5 radicals ~ m - ~The significant differences between the OH concentrations observed in the clean air near Colfax, WA, and the polluted air in Pullman, WA, are attributed to the Washington State University coal-fired power plant plume. ~~
~
~
~
Introduction The hydroxyl radical is widely recognized as the primary oxidant for the removal of pollutants from the earth's atmosphere even though it is 5-6 orders of magnitude less abundant than 0%The importance of the hydroxyl radical is reflected in the almost two decade effort to measure its concentration, which has been estimated to be lo6
-
'Department of Chemical Engineering. Laboratory for Atmospheric Research. 0013-936X/90/0924-1841$02.50/0
for typical midday conditions. Measurements of OH are needed to test models of atmospheric chemistry. Recently we (1) reported the measurement of five diurnal OH cycles for pure and moderately polluted air of eastern Washington using the radiochemical method, which we have been developing since 1979 (2-4). Below we report the details of the I4C tracer method and the factors that influence its precision and accuracy. Method The radiochemical method measures the rate of CO oxidation in the troposphere, which is believed to be almost exclusively by OH (5): CO + OH COZ + H (1) -+
The oxidation rate is determined by injecting 14C0 into ambient air that is drawn, undisturbed, through a UVtransparent cylindrical quartz reactor (Figure 1). Air from the reactor's effluent is analyzed and the OH concentration of the air is inferred from the concentrations of 14C0and l4COZ,the tracer reaction (residence) time, and the CO-OH reaction rate constant (6). The concentration of l4CO2in the reactor formed via reaction 1 is determined by integrating the I4CO2production rate from injection to collection: [14C02]= JTrk[14CO][OH]dt
(2)
Assuming the OH concentration is unperturbed by the presence of the reactor or tracer and an insignificant
0 1990 American Chemical Society
Environ. Sci. Technol., Vol. 24, No. 12, 1990
1841
\
U
V
L
I
G
H
T
/
/
i\ I i J J
SCREEN TO QUENCH OH
100 cm QUARTZ TUBE
AIR TO
\ INJECTION MANIFOLD
TRAP
L
AIR TO VENTURI,
8
MUFFLER AND BLOWER
TRACER
Figure 1. Photochemical reactor.
portion of the 14C0tracer is oxidized in the reactor, then the ambient OH concentration can be calculated from (3)
where k is the CO-OH rate constant; T, is the 14C0 reaction time; [l4C0]is the CO concentration; [14C02]is the 14C02concentration; and [OH] is the OH concentration. The assumptions made when eq 3 is used are as follows: (I) 14C02is formed only by the 14CO-OH reaction; (2) the ambient 14C02concentration is negligible relative to that formed by the I4CO-OH reaction; (3) 14C0 tracer is not significantly consumed during its time in the reactor; (4) I4CO2and 14C0are dispersed a t equal rates in the reactor; and (5) depletion of OH by reactor surfaces, the introduction of tracer, or UV radiation attenuation is negligible. This relatively straightforward concept suggests that the radiochemical method should have been correspondingly easy to develop; however, experience has proven otherwise. Major difficulties in development of the radiochemical method were caused by interferences with the accurate measurement of the 14C02concentration because 14C02 activities can be less than 1 p- count (disintegration) per minute (cpm). Stringent purity requirements for the tracer and effective purification techniques for 14C02 were therfore required. Interfering 14C02comes from radiolytic and catalytic oxidation of the 14C0tracer during its course from production to sample collection. And because gas proportional counters have difficulty distinguishing between different radioactive compounds, the 14C02produced from the CO-OH reaction must be purified from radioactive gases that are unavoidably collected with the 14C02. These include the I4CO tracer, tracer impurities, and ambient compounds present such as 222Rnand 85Kr. Preparation of 14C0 Tracer. Because 14C02is the desired product of the 14CO-OH reaction, 14C0, that is a product of other reactions must not be injected into the reactor with the I4CO tracer. The natural ambient I4CO2 concentration only contributes the equivalent of -2 X lo3 OH radicals cm-3 to the background of the OH instrument. With a typical reaction time of 11 s and a 14C0 tracer concentration of 30 ppb, no more than -0.3 dpm (lo8 molecules or an equivalent OH concentration of lo5 radicals ~ m - 14C02 ~ ) can be introduced without affecting the sensitivity of the instrument. This corresponds to a ''C02/'4C0 ratio of less than 1842
Environ. Sci. Technol., Vol. 24, No. 12, 1990
Our initial attempt to produce 14C0utilized the dehydration of formate by sulfuric acid, but 14C02/14C0ratios varied between and lo4. Hardy (7) and Henry (8) subsequently achieved the required purity with a method in which 14C02is passed over granular zinc a t 400 "C with the 14C0and unreduced 14C02flushed through a series of liquid nitrogen cooled traps to remove 14C02. Because of radiolysis, the purity of 14C0in the storage vessel rapidly deteriorates. radiation emitted by 14C causes oxygen disproportionation via the reaction (7,9,10) l4CO
+ p-
-
+
14C02 14C
+ l4c302
(4)
Hardy (7)found the first-order rate constant to be h-l; thus the tracer purity soon becomes unacceptable for use. An on-line prepurifer was subsequently made (8)that removes >99.8% of the 14C02in the tracer immediately before it flows into the injection manifold. The tracer is currently injected via an array of 37 stainless steel needles that cover the central 50% of the flow area. Reactor. The basic requirement of the technique is that the chemistry in the axial region of the reactor be identical with that in the outside air. This in turn requires that the reactor does not deplete the OH concentration by surface reaction, attenuate photolytic radiation, or oxidize the I4CO tracer. To prevent surface reactions, the reactor was designed to isolate the air flow near the axis of the reactor from the walls under a wide range of meteorological conditions. Tracer is injected only into the air in the center of the reactor and only air near the axis of the reactor is analyzed when the 14C02concentration is determined. This avoids the analysis of air that may be depleted in OH due to wall reactions and also prevents the collection of I4CO2that may have originated from a wall oxidation of the tracer. Equation 3 requires that the 14C0concentration change due to reaction must be negligible during its residence time. Ambient OH concentrations are low enough, and the reaction time sufficiently short, that less than 3 X lo4 of the 14C0is oxidized. The 14C0concentration therefore can be assumed constant. Similarly, the OH concentration must be in a photostationary state. Factors that affect the OH concentration in the reactor include reactions at the tube wall, injection needle surfaces, UV attenuation, and tracer addition. Significant radiation attenuation by quartz occurs only for infrared radiation of wavelength greater than 7 Km (11).
Therefore radiation attenuation should not affect the chemical composition in the reactor because there are no known atmospheric photochemical reactions that can be attributed to radiation of wavelength greater than 1.2 pm (12). The effect of Fresnel reflection on the surfaces of the cylindrical quartz photochemical reactors has been shown to be minimal by Zafonte et al. (13). These authors calculated that the reduction in radiation flux caused by external reflection and the increased internal radiation intensity from internal reflection exactly cancel each other for an infinite tube or spherical reactor. Their measurements in several reactors corroborated their calculations. Our reactor has a sufficiently large length-to-diameter ratio (5 to 1) to ensure negligible radiation losses. Radical loss by contact with the injection needles was estimated by using a heat-transfer analogy. For the needles in a cross flow of air, a Nusselt number of 1.14 was obtained (14). Because the Prandtl (0.72) to Schmidt (0.60) ratio is near 1, the Sherwood number for mass transfer can be substituted for the Nusselt number (15). Use of a Sherwood number of 0.85 yields a radical masstransfer coefficient of 14 cm s-l for a mass diffusivity of 0.3 cm2 s-l and a needle diameter of 0.2 mm. Taking the extreme assumption of complete radical loss upon needle contact, less than 10% is lost during injector passage. Meyn (16) modeled and experimentally observed the flow characteristics of this reactor to estimate OH wall losses. The inherent flow regime of the reactor is laminar with a Reynolds number of 860; therefore, the parabolic flow field would fully develop 2000 cm down the tube (17) if it were that long. Because the tube is 100 cm long, the mean air velocity is nearly uniform with a slowly developing boundary layer, which is the major diffusive resistance. Other diffusive characteristics of the reactor are determined by the turbulence level of the incoming air and the wind velocity in relation to the reactor’s axis. Meyn found that wall losses of OH are less than 5% and that the sensitivity to inlet turbulence is low; angles of up to 15’ between the wind velocity and the reactor’s axis do not significantly increase wall losses. The effect of the 14C0tracer on the OH concentration was estimated by first determining the mixing time of the tracer in air. T o have a minimal effect on the photochemical equilibrium of the air, the 14C0mixing time must be short compared to the l l - s reaction time. Mixing of the 14C0tracer was examined with the Gaussian dispersion model for a continuous point source in a uniform infinite flow field (18). Superposition of 37 injection nozzles, an axial velocity of 6.4 cm s-l, and a mass diffusivity of CO of 0.22 cm2 s-l were used in these calculations. Diffusion was assumed to be molecular, but higher diffusivities are likely for experiments performed during the day when there is thermally induced air turbulence. The modeled dispersion is, therefore, a worst case. These calculations show that the 14C0tracer concentration is almost uniform across the center of the reactor less than 5 cm downstream of the injection plane. Because of the initial high 14C0 concentration, the 14C02production rate is reduced until mixing is complete. To examine this effect, a step change in the I4CO tracer concentration from 0 to 30 ppb was assumed to occur a t 5 cm downstream of the injection needles. Initial photostationary CO and OH concentrations of 150 ppb and lo6 radicals ~ m - respectively, ~, were assumed together with UV radiation as the only source of OH and CO is the only sink. Including the 10% initial OH depletion due to injection needle contact calculated above, and a flow rate of 2 L s-l, the 14C02concentration a t the end of the reactor was found to be underestimated by 20%
if eq 2 was used. The actual reduction will be much less because of the mixing and chemistry in the first 5 cm of the reactor. If other sources of OH are included in the model, the error should approach the fraction of the reactor length required for tracer mixing. This is the length that the 14C0 is not mixed with air, therefore limiting the amount of 14C02formed. The most significant other source of OH is from HOz NO OH + NO2 (5) as well as the photolysis of H202,H N 0 2 , and CH20. For a 5-cm mixing length the minimum OH reduction is 6 % . Concentration gradients produced by noninstantaneous dispersion of tracer could cause a misestimation of the OH concentration if molecular diffusion in the tube is dominating. Since the molecular diffusivity of 14C0 is -25% greater than that of 14C02(11,15),production of 14C02in regions of high 14C0will cause 14C0to diffuse out of the center of the tube faster than the 14C02,resulting in an overestimation of the 14C02production rate and a higher OH concentration. A worst case calculation, assuming both 14C0 and 14C02molecularly diffuse out of a completely mixed 5-cm-diameter reactor core toward the reactor wall where the concentrations are zero, shows a linear relation between [14C0,]/[14CO]and the axial distance, and a t the reactor’s end only a +1% deviation from the case where radial diffusion is not considered. Turbulence in the incoming air will further decrease this effect. Each area of the initial flow reactor was also evaluated for its potential to catalytically produce 14C02(19). Catalytically active areas of the system that were initially constructed of stainless steel were subsequently reconstructed with weakly catalytic aluminum. Because the injection manifold is in contact with high concentrations of 14C0,this surface is expected to have the largest effect. Recalculation of the catalytic oxidation of 14C0by surfaces in the instrument yielded an equivalent OH contribution to the instrument background of less than 6 X lo4 radicals ~m-~. At the end of the reaction zone is a 5-cm cone that directs the central 10% of the air flowing through the reactor through a screen to quench OH and define the reaction time and then through the primary cryocondensing unit (PCU) (Figures 1 and 2). This central air portion is then passed through a venturi for flow measurement, a flow regulation valve, and the blower that drives the flow. The unsampled, outer air portion flows from the end of the reactor through a venturi to measure the flow rate and then through the blower. Sample Purification and Collection. A time-averaged sample of air in the center of the reactor is collected in an evacuated container for I4C counting to determine the 14C0tracer concentration (Figure 2). An orifice controls the air collection rate, and the quantity of air collected is determined by measuring the pressure change in the vessel. Measurement of the 14C02concentration by gas proportional counting requires efficient separation from ambient radioactive compounds such as CH,T, HTO, 85Kr, and 222Rnin addition to the unreacted 14C0 tracer and some ( < 3 % ) 14CH4in the tracer (20). Although the vapor pressures of CO and CHI are very small a t the temperature of liquid oxygen, solid C 0 2 is an excellent adsorbent (21,22). Solid C 0 2 condensed from air containing different concentrations of CHI and CO has the ability to adsorb 1% each of CH4 and CO. Isotopic dilution (23), therefore, was chosen as a purification technique for the 14C02samples. Immediately downstream from the collection cone (Figure 2), 2 cm3 s-l each of the
+
+
-
-
Environ. Sci. Technol., Vol. 24, No. 12, 1990
1843
ISOTOPIC DILUENTS (I2CH4t I2CO)
I4co2
SAMPLE CYLINDER
MIXING COIL
RADON SE PARATl ON
co2 CONDENSER
COLUMN
Flgure 2. Sample collection and purification system.
isotopic diluents 12CH4and l2C0 are continuously injected into the air that passes through the C 0 2 condenser. With isotopic dilution the activity of 14C0 in a C 0 2 sample is reduced by the ratio [i4c0i + [12COlambient+ ['2COladded
(6) [14COI + [12COIambient The addition of the diluents to 2 L s-l of ambient air with 30 ppb 14C0and 0.9 ppb 14CH4results in reduction of 14C0 retention by a factor of 5.6 X lo3 and 14CH4adsorption by a factor of 500. Dilution with 12CH4also purifies the 14C02 of ambient CH3T. Early background measurements displayed unpredictable and often large fluctuations. One C 0 2 sample was subjected to sequential counting for several days and yielded a decay curve with a 3.8-day half-life. This is the half-life of 222Rn(11). Estimates of the amount of ambient 222Rnthat could be collected along with the C02 in air were confirmed by experiment. Ambient 2nRn could contribute an apparent OH concentration of up to 6 X lo7 radicals cm-3 (24, 25). A chromatographic separation stage was added to separate 22zRnfrom 14C02. The 14C02and trace impurities condensed from the air by the PCU are then transferred to a smaller, liquid oxygen cooled trap by heating and flushing with a He-CO carrier gas. Water vapor (H20and HTO) collected along with the C 0 2 is retained in the PCU because it is not warmed above -40 "C during transfer. The mixture of COz, 14C02,2nRn, &Kr, and the less important radioactive trace atmospheric gases is then transferred into a 7 m by 2 mm column packed with Porapak Q (Waters Associates, Inc.) A Nz carrier gas is then used to elute the 14C02,222Rn,and &Kr a t 290 K. The CO, peak comes out a t 5.5 min and the separation time between the radon and COz is 2 min. The purified 14C02is collected in another liquid oxygen cooled trap, transferred to a storage vessel, and later taken to the Washington State University (WSU) radiocarbon dating laboratory where the 14Ccounting rate is determined (26). The 14C02collection efficiency, the fraction of the 14C02 entering the PCU that is ultimately collected by the sample purification and collection system, was found to be 0.45 f 0.05 (lo). Collection efficiencies were determined for water vapor mixing ratios between 0.004 and 0.006 and air temperatures from 4 to 20 OC and found to be independent of these parameters. 14C Analysis. The 14C02 and 14C0 samples were counted in the WSU radiocarbon dating laboratory. Samples of l4CO2and 14C0 are transferred to evacuated 1844
Environ. Sci. Technol., Vol. 24, No. 12, 1990
450-cm3 counting tubes, pressurized to 1 atm with nonradioactive CHI, and counted in a gas proportional counter with a background of 3.5 f 0.1 cpm. The counting system's 95% detection limit of 0.2 cpm for 14C02is equivalent to an OH concentration of -5 X lo4 cm-3 under standard operating conditions. The relative error of a single 14C determination, uR, is (Rt)-1/2(26),where R and t are the 14Ccounting rate and duration, respectively; thus, a 3% precision requires IO3 counts. Calculations. Since the 14C0 and 14C02samples are counted in the same detector, the effect of detector efficiency can be neglected. Therefore, the OH concentration is obtained from eq 3, where ['4C02] =
AC0* - B
(7)
QPCU~coll~CO*
and
TcOu is the sample collection time; AcOzis the gross activity of the 14C02sample; Aco is the gross activity of the 14COair sample; B is the background of the counting system; EcO% is the l4CO2collection efficiency; QpCU is the flow rate of air through the PCU; Vco is the volume of the 14C0 assay cylinder; Po is the initial pressure of the 14C0vessel; Pf is the final pressure of the 14C0 vessel; and Pa is the ambient pressure. Below is an example of the calculations used to obtain an OH concentration from the data obtained on October 15, 1987 a t 11.22 h P D T (see Table I). The 14C0 concentration is calculated from the activity collected in an evacuated cylinder divided by the volume of the air collected. The volume of the tracer air sample is determined from the relative pressure change in the 100 cm3 14C0assay vessel. The rate of pressure change is linear with time so the vessel contains a time-averaged sample of the 14C0 concentration in the flow reactor. The atmospheric, initial, and final pressures were, respectively, 0.945, 0.027, and 0.176 atm; therefore, the volume of air collected in the 14C0 assay vessel is 15.8 cm3. Dividing the net 14C0 activity of 1915 cpm by the volume of the 14C0 assay gives a 14C0 tracer concentration of 121 cpm ~ m - ~The . 14C02concentration is determined by dividing the 14C02counting rate by the volume of air processed and the 45% 14C02 collection efficiency. With a gross 14C02activity of 15.1 cpm, a counter background of 3.5 cpm, a volumetric flow
Table I. Radiochemical Method OH Data time, h (PDT)
x106 cm-3
T,O C
H20 (ratio)
J O('D)
07.75" 10.17 11.58b 12.50 13.50b 15.00 17.13' 19.50
0.16 f 0.03 13.6 f 1.8 10.0 f 1.2 21.7 f 2.5 7.6 f 0.9 7.4 f 0.9 5.8 f 0.7 0.32 f 0.05
24 22 28 30 31 31 29 20
0.0045 0.0059 0.0062 0.0064 0.0058 0.0058 0.0044 0.0056
0.0 1.2 2.0 2.3 1.9 1.4 0.4 0.0
06.66 08.58 10.66b 11.83 12.92' 14.66 16.00 17.25 19.25
0.50 f 0.13 5.12 f 0.71 14 f 1.15 12.1 f 1.44 9.38 f 1.26 13.1 f 1.63 8.81 f 1.12 5.8 f 0.85 1.41 f 0.28
11 16 25 29 30 32 31 27 23
0.0046 0.0053 0.0050 0.0053 0.0055 0.0052 0.0057 0.0057 0.0056
0.0 0.4 1.5 2.0 2.2 1.6 0.8 0.2 0.0
00.13 06.83 09.00 11.00 12.08 14.63 16.50 19.50
0.20 f 0.14 0.29 f 0.17 0.26 f 0.17 6.47 f 0.74 12.0 f 1.5 5.86 f 0.85 0.19 f 0.16 0.19 f 0.16
9 6 12 23 24 25 18 18
0.0043 0.0043 0.0053 0.0054 0.0054 0.0057 0.0060 0.0060
0.0 0.0 0.3 1.4 1.8 1.4 0.0 0.0
11.28 12.13 14.18 15.13 16.08 18.07
3.81 f 0.52 2.35 f 0.30 1.20 f 0.25 2.00 f 0.35 1.27 f 0.27 0.30 f 0.16
18 19 18 17 15 13
0.0042 0.0047 0.0036 0.0041 0.0045 0.0047
1.0 1.2 0.5 0.4 0.2 0.0
07.80 10.25 11.22 12.25 13.18 14.10 15.05 16.07 17.02 18.23
0.32 f 0.15 2.27 f 0.33 2.89 f 0.40 3.08 f 0.45 3.15 f 0.42 2.71 f 0.39 2.33 f 0.33 1.57 f 0.26 3.71 f 0.47 0.20 f 0.14
-2 9 10 13 17 14 14 14 14 6
0.0039 0.0053 9.9950 0.0044 0.0042 0.0040 0.0040 0.0038 0.0044 0.0033
0.1 0.6 0.7 1.2 1.2 0.7 0.5 0.3 0.1 0.0
[OH],
s-l
Aco,, cpm
Aco, cpm
VCO,cm3
October 1, 1987, Pullman, WA 6.0 f 0.2 81.7 f 5.0 114 f 4.0 181 f 5.0 86.0 f 4.0 77.0 f 3.0 67.0 f 3.0 7.3 f 0.3
6327 2405 4663 3307 4374 4060 4755 5267
1.99 1.85 1.84 1.83 1.81 1.79 1.93 1.97
0.260 0.264 0.264 0.264 0.264 0.264 0.264 0.265
100 100 100 100 100 100 100 100
13.6 15.8 16.0 15.5 15.7 16.0 15.8 16.0
2.08 2.08 2.08 2.08 2.07 2.08 2.08 2.09 2.08
0.264 0.266 0.264 0.264 0.267 0.264 0.264 0.264 0.264
100 100
15.6 16.2 16.0 17.4 15.3 15.7 15.0 15.5 16.3
2.08 2.12 2.08 2.09 2.09 2.11 2.12 2.12
0.264 0.264 0.264 0.264 0.264 0.264 0.264 0.264
100 100 100 100 100 100
100 100
15.8 15.2 15.5 15.4 15.4 15.2 15.5 15.5
2.08 2.10 2.09 2.11 2.10 2.14
0.264 0.264 0.264 0.264 0.264 0.264
100 100 100 100 100 100
16.1 15.8 15.4 15.7 15.8 16.2
2.10 2.10 2.12 2.11 2.11 2.14 2.18 2.19 2.18 2.14
0.264 0.264 0.264 0.264 0.264 0.264 0.264 0.264 0.264 0.264
100 100 100 100 100 100
16.1 15.3 15.8 15.8 15.8 15.9 15.6 15.6 15.8 15.9
October 2, 1987, Pullman, WA 8.0 f 24.4 f 25.5 f 58.0 f 38.8 f 45.5 f 38.6 f 22.1 f 10.1 f
1.0 1.6 1.8 2.1 2.5 2.2 2.0 1.6 1.0
4170 1945 1285 2135 1683 1485 1772 1466 1271
100 108
100 100 100 100 100
October 7, 1987, Pullman, WA 4.5 4.8 4.5 24.2 33.6 14.3 4.2 4.2
0.6 0.7 0.5 0.4 f 1.6 f 0.9 f 0.4 f 0.4
f f f f
2235 2081 1790 1463 1153 84 1 1651 1651
October 14, 1987, Colfax, WA 14.9 14.7 8.6 9.5 7.8 4.4
f f f f f f
0.8 0.6 0.7 0.7 0.7 0.4
1427 2249 1816 1421 1605 1839
October 15, 1987, Colfax, WA
Measurement performed inside laboratory.
5.1 f 0.6 12.3 f 0.7 15.1 f 0.9 14.9 f 1.0 15.8 f 0.8 12.9 f 0.8 14.1 f 0.8 9.9 f 0.7 22.1 f 1.1 4.4 f 0.5
2343 1770 1915 1771 1874 1706 2204 1992 2452 2147
100 100 100 100
Reactor was shielded from UV radiation.
rate of 264 cm3 s-l, and a collection time of 100 s, the 14C02 Oclober 15. 1987 concentration is 9.76 X cpm ~ m - The ~ . reaction time, T, is the volume of the reaction zone divided by the volumetric flow rate of air in the reactor. The reaction zone is 10 cm in radius and 80 cm in length with a flow rate of 2120 cm3 s-l; therefore, the reaction time is 11.9 s. Using a OH-CO rate constant of 2.3 X cm3 molecules-'~-~ (6) and eq 3 yields an OH concentration of 2.9 X lo6 ~ m - ~ . Method Summary. Apart from the effect of uncertainty in the OH-CO reaction rate constant, the precision of this method is affected mainly by radical loss with injector needle contact coupled with nonimmediate dispersion of tracer and unequal diffusion of 14C02and 14C0in the reactor. The maximum effects on the calculated OH 1os0 4 0 12 16 20 2 concentration are estimated to be -20% and +1%, reTime (PDT) spectively, and partially cancel. The relative error of an Figure 3. Clean air OH measurements near Colfax, WA. OH concentration is dominated by uncertainties in the 14C02collection efficiency (currently -lo%), and the 14C02count rate. The 14C02count rate error can, in technique is primarily a measure of the rate of ambient CO oxidation, implying the OH concentration introduces practice, be reduced with longer counting times. This
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an additional error due to uncertainty in the CO-OH reaction rate constant (-20%). Our calculated OH concentrations can be corrected as better CO-OH rate constant data become available.
Results Table I contains the results of 42 measurements (I, 22) made in Pullman, WA, (46.7’ N, 117.2’ W) on October 1, 2, and 7, 1987, and near Colfax, WA (46.9’ N, 117.4’ W) on October 14 and 15,1987. Figure 3 shows the expected diurnal variation observed on October 15 in Colfax. The Pullman measurements were conducted on the roof of the WSU engineering building and coincidentally in the fumigating plume of a coal-fired power plant that was -200 m upwind. The Colfax measurements were performed a t a rural “clean air” site maintained by the WSU Laboratory for Atmospheric Research. Table I also includes the results of experiments in which the reactor was shielded from UV radiation. The l u errors in Figure 3 and Table I were calculated by considering all significant experimental errors in data used to calculate the OH concentration. The primary contribution to the error in these data is from 14C02counting, followed by a 10% error from variable 14C02 collection efficiencies. Individual 14C02 counting errors are reported in Table I. Counting errors for 14C0 samples were all less than 2%. Table I also includes measurements of auxiliary data. The O3 photolysis rates were obtained by UV-flux scaling of the theoretical peak value of Demerjian et al. (27) for Colfax. The UV fluxes were measured with a wide-angle Lambert-response International Light radiometer with a spectral response adjusted to approximate the J OPD) production spectrum. Peak photolysis rates during the Pullman measurements were -85% larger than those near Colfax because of the large reflectivity of the roof where the measurements were made. Although not included in Table I, CO, CH,, 03,and NO, concentrations were also measured (22). Ozone mixing ratios peaked a t -35 ppb, CHI was -2 ppm, and CO varied from -150 ppb for the Colfax measurements to -400 ppb for the Pullman measurements. Nitrogen oxides mixing ratios were generally less than 5 ppb, the detection limit of the instrument; however, in Pullman, intermittent spikes in NO, of up to 100 ppb occurred. Water concentrations and air temperatures were also significantly larger during the Pullman measurements. Also there were probably slightly increased non-methane hydrocarbon (NMHC) concentrations and elevated particle and SOz concentrations associated with the power plant plume in Pullman. The causes of the large OH concentrations observed a t 12.50 h on October 1, 15.13 h on October 14, and 17.02 h on October 15 are unknown. If these data are not reflections of the actual OH concentrations, sources for large counts in the 14C02sample could be 14C0or 222Rn,the two major interfering compounds from which the 14C02 is purified. Contamination by 14C0 is unlikely because the simple collection and transfer of the I4CO2sample into the storage vessel would remove the 14C0,unless the dilution gases were mistakenly not turned on. Radon separation, on the other hand, is very operator-dependent (22). Errors may have been caused by a misread stopwatch, leading to collection of nzRn with the 14C02.Other operational errors would lead to a loss of sample and a low OH concentration. An error not considered was the possibility of residual 222Rnin the 14C02sample. The extreme variability of ambient 222Rnconcentrations, and incomplete separation, caused a background (early morning and evening) count rate of collected 14C02between 0.7 and 6.6 cpm above
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counter background. This variability was also evident prior to the experiments in which 14C0was not injected and COz samples were counted. The 222Rneffect for the Pullman and Colfax experiments was relatively small, less than (2.5 f 0.5) X lo5 radicals ~ m - ~ . Precision of the results can be determined by comparing the October 1, 2, and 7 data, and comparing the October 14 and 15 measurements. Very similar results were obtained on days with comparable meteorlogical and chemical conditions. Significance of the results can be claimed because the 12 early morning, evening, and nighttime experiments showed low I4CO2levels, corresponding to OH concentrations of less than (2.5 f 0.5) X lo5 radicals ~ m - ~ , indicating almost no photochemically induced oxidation of 14C0. Table I also shows that the midday maximum OH concentrations observed in Pullman were from 2 to 5 times higher than those measured in the cleaner air of Colfax. No other modelers or experimenters report OH concentrations approaching the mean peak levels measured a t WSU of 10.1 X lo6 radicals cm-3 for comparable photolysis rates in either polluted or clean air. Modelers of power plant plumes (28) suggest that there is increased production of 0, in a plume; however, modeled OH concentrations are insensitive to O3 (29) and large O3 concentrations were not observed. Other significant characteristics of power plant plumes are increased NO,, SO2, NMHC, and aerosol concentrations in addition to the nonequilibrium state of the plume. Aerosols and SO2both provide sinks for radicals so their elevated concentrations should only lower the OH concentration. Models are fairly insensitive to NMHC concentrations as suggested by Weinstock et al. (29) and Perner et al. (30);therefore, the significant aspects of the plume are the elevated NO concentrations and the nonequilibrium state of the air. In contrast to the Pullman results, the Colfax midday mean OH concentration of (2.9 f 0.5) X lo6 radicals cm-3 at an O(lD) photoproduction rate of 1.2 X compares more favorably with data of other experimenters. Perner et al. (30)reported OH midday average OH concentrations between 1 X lo6 and 3 X lo6 radicals cm-3 for O(lD) photoproduction rates between 1 X and 2 X s-l. The diurnal variation measured by Platt et al. (31)shows a peak OH concentration of -6 X lo6 for a photolysis rate of 3 X s-l. After proportional scaling of these two long-path absorption (LPA) technique measurements to the O(lD) photoproduction rates, they are close to the peak OH concentrations shown in Figure 3, but certainly not larger. A good evaluation of OH measurement techniques, however, can only be made by simultaneous tests of the different methods or by testing the models with complete sets of trace gas concentration data. Because power plant plumes are not in photolytic equilibrium with ambient air, OH radical concentrations in plumes would be expected to be less than ambient if UV radiation is the primary source of OH. However, if air with a large NO concentration is rapidly mixed with ambient air containing typically -5 X los HOz radicals cme3 (31), the NO H 0 2 reaction (eq 5) may be a significant, but transient, source of OH. The NO + HOz reaction is also probably the reason that the OH concentrations were not significantly lowered by shielding the reactor from UV radiation because eq 5 is not directly dependent upon UV radiation. The relaxation time for OH decay is on the order of 15-30 s (29), which is larger than the residence time of air in the reactor. Therefore the UV-shielded OH concentrations would not differ significantly from ambient concentrations.
+
Conclusions The repeated observation with the radiochemical method of high CO oxidation rates during the daytime and much lower rates during early mornings and evenings suggests a photochemically induced oxidation of CO. Hydroxyl concentrations are inferred from the CO oxidation rate by assuming only the OH-CO reaction rate constant, which can be determined independently. The future of the radiochemical method for OH detection includes automation to reduce operation errors, more complete auxiliary data collection for modeling, and improved field portability. Acknowledgments
We thank J. Farmer, Y.Welter and H. Elshafei for their valuable contributions to this research. Literature Cited (1) Felton, C. C.; Sheppard,J. C.; Campbell, M. J. Nature 1988, 335, 53. (2) Campbell, M. J.; Sheppard, J. C.; Au, B. F. Geophys. Res. Lett. 1979, 6, 175. (3) Sheppard, J. C.; Hardy, R. J.; Hopper, F. Antarct. J . U S . 1982, 17, 206. (4) Campbell, M. J.; Farmer, J. C.; Fitzner, C. A.; Henry, M. N.; Sheppard, J. C.; Hardy, R. J.; Hopper, J. F.; Muralidhar, V. J . Atmos. Chem. 1986, 4 , 413. (5) Madronich, S.; Calvert, J. G. Technical Note NCAR/TN333+STR, National Center for Atmospheric Research: Boulder, CO, 1989. (6) Hynes, A. J.; Wine, P. H.; Ravishankara, A. R. J. Geophys. Res. D 1986,91, 11815. (7) Hardy, R. J.; Sheppard, J. C.; Campbell, M. J. Znt. J . Appl. Radiat. h o t . 1984, 35, 1071. (8) Henry, M. N. M.S. Thesis, Washington State University, 1984. (9) Dondes, S.; Harteck, P.; von Weyssenhoff, H. 2. Naturforsch. 1964, 19A, 13. (10) Clay, P. G.; Johnson, G. R. A,; Warman, J. M. Discuss. 1974, 36,46. Faraday SOC. (11) CRC Handbook of Chemistry and Physics, 55th ed.; Chemical Rubber Co.: Press: Cleveland, OH, 1974.
(12) Seinfeld,J. H. Atmospheric Chemistry, 1st ed.; John Wiley and Sons: New York, 1981. (13) Zafonte, L.; Rieger, P. L.; Holmes, J. R. Enuiron. Sei. Technol. 1977,11, 483. (14) Kreith, F. Principles of Heat Transfer, 3rd ed.;Harper and Row: New York, 1973. (15) Shenvood,T. K.; Pigford, R. L.; Wilke, C. R. Mass Transfer, 3rd ed.; McGraw-Hill: New York, 1975. (16) Meyn, L. A. M.S. Thesis Washington State University, 1982. (17) Schlichting, H. Boundary Layer Theory, 7th ed.; McGraw-Hill: New York, 1979. (18) Wilson, H. A. Proc. Cambridge Philos. SOC.1904,12,406. (19) Fitzner, C. A. M.S. Thesis, Washington State University, 1984. (20) Elshafei,H. A. I. M.S. Thesis, Washington State University, 1987. (21) Muller, E. Cryogenics 1966, 8, 242. (22) Felton, C. C. Ph.D. Thesis, Washington State University, 1988. (23) Wahl, A. C.; Bonner, N. A., Eds. Radioactivity Applied to Chemistry, 1st ed.; John Wiley and Sons: New York, 1951. (24) Pearson, J. F.; Moses, H. J . Appl. Meteorol. 1966,5, 175. (25) Nero, A. V.; Schwehr, M. B.; Nazaroff, W. W.; Revsan, K. L. Science 1986,234, 992. (26) Sheppard, J. C. A Radiocarbon Dating Primer;Washington State University College of Engineering Bulletin No. 338, 1975. (27) Demerjian, K. L.; Schere, K. L.; Peterson, J. T. Adv. Enuiron. Sci. Technol. 1980, 10, 369. (28) Hov, 0.; Isaksen, I. Atmos. Enuiron. 1981, 15, 2367. (29) Weinstock, B.; Niki, H.; Chang, T. Y. Adu. Enuiron. Sci. Technol. 1980, 10, 221. (30) Perner, D.; Platt, U.; Trainer, M.; Hubler, G.; Drummond, J.; Junkermann, W.; Rudolph, J.; Schubert, B.; Volz, A.; Ehhalt, D. H. J . Atmos. Chem. 1987,5, 185. (31) Platt, U.; Rateike, M.; Junkermann, W.; Hofzumahaus, A.; Ehalt, D. H. Free Radicals Res. Commun. 1987, 3, 165. (32) Logan, J. A.; Prather, M. J.; Wofsy, S. C.; McElroy, M. B. J . Geophys. Res. C 1981, 86, 7210. Received for review February 28, 1990. Accepted July 18, 1990. Early aspects of this research were supported by NSF, EPA, and N A S A grants and contracts. T h e results reported here were obtained with partial support from the WSU department of chemical engineering.
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