Nitrogen Dioxide Photolysis in the Los Angeles Atmosphere Leo Zafonte”, Paul L. Rieger, and John R. Holmes State of California, Air Resources Board Laboratory, 9528 Telstar Avenue, El Monte, Calif. 91731
Atmospheric measurements of k l , the rate of NO:! photolysis, were made in El Monte, Calif., by use of a tubular quartz reactor to measure directly the conversion of NO2 to NO in an NP atmosphere. A least-squares treatment of the data from September 1975 resulted in the following empirical relationships for predicting k 1 from standard UV radiometric data. 0’ < Solar zenith angle < 40’: k1 (min-l) = 0.079 (l/cos z ) 0.002 X radiometric UV (mW/cm2);40’ < solar zenith angle < 90’: k l (min-l) = 0.16 (1 - cos z ) 0.088 X radiometric UV (mW/cm2). Ultraviolet intensities corresponding to k 1 values greater than 0.4,0.3, and 0.2 min-l can persist for periods of 8, 10, and 12 h, respectively, during the weeks around the summer solstice. Both the high photochemical light intensity observed and its extended time duration indicate that the potential for ozone formation in the atmosphere is greater than utilized in most previous “smog chamber” studies. This result suggests that the application of existing smog chamber data in such areas as solvent reactivity may have to be reinterpreted.
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A number of early measurements of the total ultraviolet radiation present in polluted atmospheres has been reported in previous studies (1, 2 ) . These studies, however, did not measure the primary quantity of interest in our understanding of photochemical air pollution, the intensity of ultraviolet radiation leading directly to the photodissociation of nitrogen dioxide. The quantity is important because the production of oxygen atoms in this process and their subsequent recombination with oxygen via the reactions defined below,
NO2
-
+ hv (A < 4400 A) 0 + 02 + M + O 3 +
NO + 0
(1)
M
(2)
are the only steps known to lead directly to the production of ozone in polluted atmospheres. In most of the earlier investigations, either chemical actinometers or ultraviolet radiometers were used to make the light intensity measurements. These devices have disadvantages in their ability to measure directly the primary quantity of interest because of their limited spectral response and/or their lack of total “volumetric” sensitivity to ultraviolet radiation. With the introduction of chemiluminescent NO, analyzers, however, improved measurements of k l , the rate of NO:! photolysis, have been made. Jackson et al. ( 3 )have reported a continuous technique that determines the rate of NO2 photodissociation by measuring the nitric oxide formed as nitrogen dioxide traverses a tubular quartz, plug flow reactor containing low concentrations (e.g., .1ppm) of nitrogen dioxide in air. Sickles and Jeffries ( 4 ) modified this technique for measuring k l by using a spherical, quartz batch flow reactor and by photolysis of nitrogen dioxide in nitrogen. The use of a cylindrical quartz tube or quartz sphere has the advantage that the reactor itself, to a good approximation, is equally responsive to light from all directions and is therefore less sensitive to the orientation effects that tend to reduce the actual measurement. Jackson et al. ( 3 )estimated the light loss due to orientation effects of the tubular reactor to be less than
5%, while Sickles and Jeffries ( 4 ) suggest that the UV light intensity determined in the sphere is still less dependent on orientation. One study ( 3 ) uses the value 93% as the overall transmission factor for quartz and simply applies a correction factor of 7% to their calculations to account for this radiation which is absorbed or reflected by the reactor walls. In this paper we examine some of the theoretical factors involved in calculating the quantity k 1 in a plug flow reactor. In addition, we report some recent measurements of k l in the South Coast (Los Angeles) Air Basin of California and examine their relationship to the type of ultraviolet radiometer frequently used to measure ultraviolet radiation. The implications of these measurements for the production of ozone are briefly considered. Experimental
Method and Apparatus. The apparatus and method used in this study were similar to those utilized by Jackson et al. ( 3 )where nitrogen dioxide/air mixtures were photolyzed. To increase the sensitivity of the method, however, a mixture of nitrogen dioxide in nitrogen was used in this work. The apparatus consisted of a 2.2 cm X 1 m quartz tube mounted 21 cm above a utility box containing two flow meters (Fisher and Porter Co.) and two low-pressure line regulators (Matheson Gas Products) which were used to maintain a stable and constant flow rate throughout the day. The flow meters were calibrated both in the laboratory and in the ambient atmosphere with a wet test meter. There was little or no apparent change in calibration under these conditions. The utility box (56 X 33 X 26 cm) was brown in color and was not expected to reflect UV radiation. The albedo of the roof of the building upon which the measurements were taken was 7-10% at noon. The instruments were exposed to ambient temperatures during the measurement period. Nitrogen dioxide gas, 100 ppm in nitrogen (Matheson Gas Products), was diluted to 2 ppm for these measurements. All tubing connections were made of Teflon. Measurements of nitric oxide and total nitrogen oxides were made with a chemiluminescent NO-NO, analyzer (Model 14D, Thermo-Electro i Corp.). Solar ultraviolet radiometric measureinents were made with a UV radiometer (the Eppley Laboratories, Inc.) with a full bandwidth sensitivity from 2950 to 3850 A and a half-power bandwidth sensitivity extending from 3100 to 3700 A. This radiometer was calibrated against a second UV radiometer which, at the beginning of the study, had been newly calibrated by the Eppley Laboratories, Inc. The equation used to calculate the rate of nitrogen dioxide photolysis was a modification of the equation used by Jackson et al. ( 3 )for a plug flow reactor. A(N0) F 1 1 x-x-x(N02) V @ T
k l = -
In this equation, A(N0) is the measured increase in the nitric oxide concentration (pprn) as a result of photolysis, (NO*)is the nitrogen dioxide concentration (ppm),F is the total flow rate in the tubular reactor (Urnin), V is the volume of the quartz reactor, @ is the effective quantum yield for the production of nitric oxide upon photolysis, and T i s the overall transmission factor of the quartz tube for UV light. The last two parameters are examined further in the text below. The nitrogen dioxide concentration used in the calVolume l l , Number 5, May 1977
483
culation of the parameter, k l , is given by the following relationship (Nod =
(Nodfinal
+ 1/2 A O " )
where (N02)finalis the resultant NO2 concentration determined after the NOz/N2 gas mixture has traversed the quartz tube. This quantity is a good approximation to the average NO2 concentration during the period that the NO2 gas traverses the photolysis tube. Typical NO2 concentrations were on the order of 1-2 ppm, typical gas flow rates were on the order of 3 l./min, and typical values for A(N0) ranged from 0 to 0.2 ppm. Calculation of Quantum Yield Factor. The calculation of the quantum yield factor, a, given in Equation A is straightforward, based on the chemistry of the reaction system. The primary chemical reactions of interest for the photolysis of nitrogen dioxide in a nitrogen atmosphere are given in Table I. Other reactions were found to be unimportant for our conditions. The effective quantum yield is given by the following expression: Rate of NO appearance 9= Rate of NO2 photolysis 9=
ki(N02) + ks(NOz)(O) - kdO)(NO)(Nz)- kdN03)(NO) k 1(NO21
(B) This equation can be simplified by assuming an instantaneous steady state for the concentrations of 0 and NO3 to yield
The intensity of the refracted ray will equal the quantity (1 - R ) times the intensity of the incident ray. For light transmitted through an air-quartz-air interface, the Fresnel equation predicts the behavior shown in Figure 1,which gives the light transmission properties of quartz. For a normal or zero degree angle of incidence, 93% of the radiation at 3700 A will be transmitted. The percent transmission decreases slowly to 90% as the angle of incidence increases to 40' and drops off rapidly to a value of zero for an incident angle of 90'. The figure indicates that for light incident on a photochemical reactor of any geometry, considerations of both refracted and reflected radiation must be made, particularly for angles of incidence greater than 40'. The model used to calculate the geometry transmission factor for ultraviolet radiation penetrating to the interior of the tubular or the spherical reactor is given in Figure 2. The figure shows an incident ray of light, 10,at an arbitrary angle of incidence, i , striking a circular cross section of the reactor.
Table I. Principal Reactions in Photolysis of NO2 in Nitrogen Reaction
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Rate constant
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(To be determined) (1) NO2 hv (A < 4400 A) NO 0 9.1 x (3) 0 NO2 NO 0 2 8.2 x 10-32b (4) 0 NO2 N2 No3 N2 9.6 x 10-32 = (5) 0 NO N2 NO2 N2 8.7 x 10-'2d (6) NO3 NO 2 NOn a Units: cc/moiecule/s (5). Units: (cc/molecule)*/s (6). Units: (cc/moieculeF'/s (7). Units: cc/molecule/s ( 6 ) .
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+
+
-
k d N 0 z ) - h4(N02)(Nz) - k5(NO)(Nz) (C) k d N 0 z ) + k4(NOz)(N2) + kdNO)(Nz) For a wide range of initial conditions, i.e., for initial nitrogen dioxide concentratibns of from 1 to 4 ppm in nitrogen, and initial nitric oxide concentrations of from 0 to 0.3 ppm in nitrogen, the quantum yield factor reduces to- a constant term
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9=1+
90 80.-
-
The error limits represent the maximum deviation from the value of 1.61 for the concentration ranges defined above. Calculations were made at a temperature of 300 K and an atmospheric pressure of 750 torr. This value of 9 was determined for the set of rate constants given in Table I which was chosen based on the best available data. The critical rate constant is that for Reaction 4 which considerably reduces the quantum yield factor from the maximum value of 2.0. Consideration of other values for this reaction (see, for example, ref. 8) could alter this factor by a small amount. Inclusion of N205 chemistry would alter the value of 9 slightly. However, this effect would be within the experimental errors in the measurements. Calculation of Geometry Transmission Factor. Concern has been expressed ( 4 ) over the most appropriate geometry to be used for the quartz photochemical reactor for the direct determination of k l . The problem arises from a consideration of the Fresnel equation, which can be used to predict the fractions of light refracted and reflected at the interface of two transparent media. For natural sunlight (unpolarized radiation) at a single interface, this equation reduces to the following form:
R=
g
50--
+
40.-
Y
Y
30.-
a
TO-. IO-0
uuuamvc
INUX-I 4 7 1 TEYPERITURE - 2 5 ' C WAVELENGTH - 37WI
!
:
;
;
:
I
;
:
sin2(i - t ) 2 sin*(i t )
tan*(i - t ) + + 2 tan2(i + t )
where R is the reflection coefficient for natural sunlight at the interface, i is the angle of the incident radiation on the surface, and t is the angle of the refracted or transmitted radiation. 484
--
60
m
9 = 1.61 f 0.07
70--
$?
Environmental Science & Technology
1 Figure 2. Light
ray diagram for spherical or infinite tubular reactor
Also shown are the reflected light ray, E, the transmitted light ray, 71, and the internally reflected light rays, f z and 7,. The problem is one of calculating th_e re_lative_contributionof the internally reflected light rays, 12,13,. . . I,, which also contribute to the overall light intensity within the reactor. These rays compensate for the loss of the light ray, E, which is initially reflected from the surface and therefore does not contribute to the photodissociation of nitrogen dioxide within the tube. The intensity of reflected and refracted light from an airquartz-air interface is given by (9)
and 1-R f (refracted) = 70 (1
+R ) = “
Results and Discussion
and the contributions to the total light intensity within the reactor are given by 1-R 7, = 7, l+R 7, = 71 2 R 1+R i, = I , 2 R l+R
(-) (-) (-)
in+l
=
in
for n
>o
The total light intensity can be found from ?tota,
= 7,
+ 7, + 7,.
(-)
. . + 7,
l+R
+...
(-)2 R + . . .
l+R
l+R
2
l+R
(F) The summation in the final expression has a solution commonly encountered in physical problems, Le., m
(X)n=n=O
For X = ( 2 R/1
Experimental Comparison of Tubular and Spherical Reactor Geometries. The effect of reactor geometry on the value of K 1 was studied in a series of measurements whereby a spherical reactor of the type used by Sickles and Jeffries ( 4 ) and a tubular reactor described above were used. The experiments were carried out on relatively cloudless days in March and April 1976 with intensities followed radiometrically. During the course of the experiment the use of the tube and the sphere was alternated with all other parameters kept constant. Another comparison was made indoors within a conventional smog chamber with blacklight fluorescent lighting. The equation used for the calculation of k l with the data from the spherical reactor is as follows:
+ 70 (-)1 - R (-)2 R
= 7, 1 - R l+R
+io(-) 1 - R
an inverted, round bottom quartz flask. Equation G will be valid only when an infinite number of reflections are permitted within the flask. The well at the bottom of the spherical reactor, however, acts as a sink for radiation, thus prohibiting further internal reflections after the light ray has entered the well. The result is a slightly lower light intensity within the flask, the correction for which will be proportional to the ratio of the area of the well and the internal area of the sphere. A priori it is difficult to determine which correction would be smaller since the occurrence of any internal reflection reduces the magnitude of the correction. Furthermore, although it is expected that the sphere will be less directionally sensitive than the cylindrical tube, the curvature and large size of the sphere present much the same problem for the calculation of the light transmission properties of the sphere as for the cylindrical tube.
1-x
for-I
