Intercomparison of NO2 photodissociation and U.V. Radiometer Measurements

Armosphrrrr

.Enrrronmmr

Vol

21.

No

3. pp

569-J78.

ow4-6981/87

1987. c

Pnnted I” Greal Brium

INTERCOMPARISON OF NO;! PHOTODISSOCIATION U.V. RADIOMETER MEASUREMENTS

1987 Pergrmon

13.00+0.~ Journals Ltd.

AND

SASHAMADRONKH National Center for Atmospheric Research, P.O. BOX3000, Boulder, CO 80307, U.S.A. (First received 3 February

1986 and infinalform

19 June 1986)

Abstract-A theoretical estimate of the ratio of direct to diffuse solar radiation is used to derive a simple correlation between U.V.irradiance measurements and atmospheric values of the NO1 photodissociation rate coefficient, J. This correlation permits a critical evaluation of the performance of actinometers which measure J. Most reported J measurements in the troposphere agree to better than + 5 % for clear skies and to i: 20 % for overcast skies, but photodissociation rates measured at the University of North Carolina outdoor smog chamber are not consistent with a majority of other studies. It is shown that calibrated standard U.V. radiometers may be used routinely, in place of the more complex actinometers, to obtain reasonably accurate estimates of J.

I. INTRODUCTION

The dissociation of NO2 molecules by atmospheric near-ultraviolet radiation, NO,+hv+NO+O

(280420 nm),

(RI)

plays a key role both in the troposphere, where it is the major source of odd oxygen (0-t 0,). and in the stratosphere, where it controls the partitionjng of odd nitrogen (NO $ NOzf. In some respects, reaction R 1 is the archetypal atmospheric photodissociation process, and it is certainly the most studied, so that photodissociation processes for other important species (e.g. HJO, OS)are often expressed relative to this reaction. As such, NOz photodiss~iation is important in all models of the chemistry of the atmosphere and must be accurately quantified; this parameterization is obtained via the first order photodissociation rate coefficient, J (s- ‘). Computation of J from first principles is possible, i.e. by integrating the product of the ambient actinic flux, the NO;, absorption cross section and the NO2 photodissociation quantum yield, over the 280420 nm wavelength region. Unfortunately, this calculation may not be accurate enough for many modeling applications. The available NO2 quantum yield and absorption cross section data alone give an estimated combined uncertainty of k 30% (NASA, 1985). Calculations of the ambient actinic flux must include large contributions from multiple molecular scattering, as well as scattering by aerosols, clouds and the Earth’s surface. Because these calculations are intricate and relatively untested, total uncertainties in theoretical J values may be significantly higher than the + 30 Y0error due to the NO2 spectral data alone. Perhaps even more importantly, calculated J values may be entirely inapprop~ate if relevant factors (e.g. clouds, local reflections, etc.) are not accurately known for the time and location of a specific application. For 569

example, modeling of the extensive photochemical smog simulation experiments at the University of N Carolina (UNC) outdoor smog chamber cannot benefit from theoretical J values unless the light environment inside the chamber is accurately known. Direct measurements of J are thus highly desirable to test the theoretical calculations and also to complement other simultaneous measurements, e.g. of chemical species concentrations. Several methods to measure J have been developed,and a sizable data base is now available. However, the actinometers employed in these measurements are complex instruments, and each may suffer from a number of possible errors. These experimental problems could result in erroneous J measurements, but because the ambient light can be so highly variable, direct comparison of the J data cannot distinguish between experimental artifacts and natural variations in J. Side-by-side simultaneous measurements with the different actinometers have not been made, so an objective assessment of the reIiabi~ity of these J measurements is lacking. The purpose of this paper is to evaluate the quality of the available experimental data on J. The evaluation is carried out by intercomparing J measurements and simultaneous measurements of the downward U.V. irradiance, which are available from many of the studies, but which had not previously been used for intercomparisons due to the difficulties in accounting for the cosine response of the U.V.radiometers. Here, a simplified treatment of diffuse and direct sunlight is used lo correlate the U.V.irradiamzand J. When the availabte experimental data are tested against this correlation, the variability of the ambient light is largely eliminated, and instrument-specific differences are emphasized. Generally, fairly good agreement between measurements is found, but some exceptions are noteworthy. The correlations of a majority of studies further suggest that routine field determinations of J may be obtained using standard U.V.

SASHAMADRONICH

570

radiometers actinometers.

rather

than

2. ACIINOMETRIC

the

more

complex

AND RADJOMEX’RK

MEASUREMENTS The studies selected for the intercomparisons are given in Table 1. A necessary selection criterion was the availability, from each study, of simultaneous solar zenith angle, J, and U.V.radiometer data: these are required for the correlations described below. Other selection criteria included the extent of citation in the open literature and relevance to current work in atmospheric modeling. Thus, the data of Sickles et al. were discussed extensively in other J studies (Dickerson et al.; Parrish er al.) and are therefore included here. Likewise, the measurements at the UNC chamber are currently used in photochemical mode1 simulations of UNC chamber experiments (e.g. Killus

and Whitten, 1982; Gery and Whitten, 1986) to estimate inchamber photodissociation rates for NO1 and, by

ratio to N02, for all other photoactive species. Errors in these J values may have a considerable effect on the development and testing of models which predict such key atmospheric species as O3 and H202, so it is of considerable interest to examine the consistency of the UNC J data with other J measurements. J measurements by some other workers are not included here either because they were superseded by later measurements (e.g. Jackson et al., 1975) or because broad-band (280nm-2.8 pm) rather than U.V. irradiance was measured (e.g. Bahe et al., 1980). Some processing of the data for intercomparisons was required. For three studies (Zafonte et al., Sickels et al., and UNC chamber), solar zenith angles were calculated from reported times and location. For the

1.8 km study of Dickerson et al., U.V.radiometer data were taken from Dickerson (1980). Additional unpublished U.V.radiometer data for the Parrish er al. study were obtained from Parrish. The UNC data were originally obtained by Sacger (1976) both outside and inside the smog chamber, but obviously only outdoor measurements are used in the present intercomparisons. Furthermore, hecause the data of Saeger are unpublished, it was decided to consider only those

measurements which are also reported elsewhere: the three clear-sky days used by Jeffries et al. (1982) and the single day (13 October 1976) used by Gery and Whitten (1986) in their recent estimate of how the chamber structure perturbs the ambient light. This second citation requirement reduces the possibility of unfair consideration of old data and specifically addresses those measurements which are currently used in estimating photodissociation rates for the chamber. Nonetheless, the other clear-sky measurements obtained by Saeger are qualitatively similar. All J measurements listed in Table 1 were based on exposing a quartz vessel containing NOz gas to ambient light, but it is clear from the table that the actinometers differ in several important respects, including the geometry of the quartz vessel, gas composition and flow rate, and principal diagnostic measurements. Detailed discussions of some possible experimental errors may be found in the original papers describing each instrument (see also Dickerson and Stedman, 1980). These errors generally arise from optical effects (absorption, reflection, and refraction at cell walls, shadowing and reflections by accessory instrumentation and nearby structures), chemical effects (NOJsurface interactions, secondary reactions) and gas transport effects (diffusion, mixing). All of the measurements in Table 1 were obtained

Table 1. Actinometric and radiometric studies of NO2 photodissociation Reference Year of study UNC chamber 19741976 Zafonte cf al. (1977) 1975 Sickles er al. (1978) 1975 Harvey er al. (1977) 1976 Dickerson er al. (1982) 1978: 1979 Parrish er al. (1983) 1980 Madronich ef al. (1984) 1980-1981

Location Elevation (km)* Pittsboro, NC 0 Los Angeles, CA 0 Pittsboro, NC 0 Ann Arbor, MI 0.3 Boulder, C’O I.8 Niwot Ridge, CO 3.0 Niwot Ridge, CO 3.0 Toronto, ON1 0.3

Method Geometry NO,, flowt sphere NO,, flow cylinder NO,, flow sphere

NO,, flow cylinder NO,, flow cylinder

NO,, flow cylinder A P, static 8 double cylinder

Gas mix N02: l-2 ppm NZ: 1 atm NO2 : l-2 ppm N,:l atm NO1 : l-2 ppm N,: I atm NOI:25ppm Air: 1 atm NO2 : 20 ppm 02: I atm NOs : 30 ppm Nz ~45.7Torr NO2 : 1-3 Torr

Background grass brown paint black paint white paint (zinc oxide) black paint

black paint grass, black cloth

*Above sea level, approximate. t Irradiation of flowing gas for various residence times, followed by chemiluminescence detection of NO and/or Nos. 2 Radiometer data for 1.8 km taken from Dickerson (1980). 5 Pressure change from reaction 2N0, + hv -V 2N0 + 02.

571

Intercomparison of NO2 photodissoeiation and U.V.radiometer measurements from ground-based stations, but under significantly different environmental conditions. For example, the seasonal dependence of the Earth-Sun distance, urban versus rural environments, elevation above sea level of the monitoring station, cloud and haze conditions, overhead Oa column, local background light, and regional surface albedo may all contribute to systematic differences among the studies. This point is illustrated in Fig. 1, which shows representative clearsky measurements from each study of Table 1. All of these measurements may seem “reasonable” insofar as they show near zero values at sunrise or sunset (solar zenith angle x0 m 90”) and high-sun values not unlike those calculated by, say, Leighton (1961). From this plot, it is unclear how much of the variability of the measurements (e.g. 5-9 x 10e3 s- ’ at x0 = 45”) can be attributed to differences in the natural ambient light rather than to experimental problems with the actinometric measurements. To provide a measure of the U.V. ambient light during the J measurements, simultaneous irradiance measurements were made in all of the studies listed in Table 1. These measurements were made with a standard U.V. radiometer manufactured by the Eppley Laboratory, Inc. (Newport, RI.). The radiometer consists of a U.V.filter and photocell placed behind a flat, opaque quartz diffuser plate. The dill-user plate produces a cosine (Lambertian) response to the directions of the incoming light due to the reduced projected area of the plate for angles of incidence other than 0”. The manufacturer specifies sensitivity over the 295-385 nm wavelength region, with peak response at 5 340 nm and fwhm of ~60 nm, and adherence to the cosine law to within + 2.5% for light incoming at angles less than 70” from the plate normal. In the studies listed in Table 1, the radiometer was pointed directly up. (The Eppley U.V.radiometer is known to exhibit some long-term drift and should be recalibrated frequently.) Typical correlations of the U.V.irradiance (E) and J measurements are shown for clear-sky conditions in Fig. 2. Notable features are the large scatter of the data

-.

CB LJr,

0 0

30

60

90

Solar zenith angle, deg. Fig. 1. Clear-sky J value measurements: l , UNCchamber (10/13/X); 0, Zafonte cr al. (g/17/75); m, Sickles et ol. (10/12/75);0, Harvey cr al.; A, Dickerson et 01.(3 km); A,

Parrish PI al. (8/21/80); x1 Madronich er al.

OF 0

2

1

UV

3

.

5

6

1

lrradiance E, mW cm-*

Fig. 2. Clear-sky correlation between J value measurements and U.V.irradiance. Symbols as in Fig. 1. Solid line is calculation by Harvey er ul.

and the strong curvature with increasingly higher values of the U.V.irradiance at large J. The earlier study of Jackson et al. (1975) did not show this curvature, probably due to the larger scatter of their measurements. Overcast sky data also result in generally linear correlations. It is now well-recognized that the curved correlations result from the cosine response of the radiometer (Nader and White, 1969; Harvey et al., 1977), but the parameterization of the curvature has been elusive. Expressing J as a polynomial function of the irradiance (e.g. Walega et al., 1980; Whitten and Gery, 1986) may work on any single day, but different values of the fitting coefficients will be observed on different days. For example, even for perfectly clear sky conditions the seasonal variation of the Sun-Earth distance may produce equal J values at quite different values of the solar zenith angle, x0 (e.g. 40” on 1 January, 30” on 1 July, Demerjian et al., 1980); the cosine factor of the radiometer will then be significantly different (e.g. 13 Y0for the above zenith angles), so that two different values of the irradiance are measured for a single value of J. Clouds, variable local background, and station elevation above sea level will have even larger effects on the fitting coefficients. Zafonte et al. (1977) attempted to include the effect of the cosine response by correlating J/E ratios with l/cos x0 or with 1 - cos x0, but the quality of the fit is relatively poor, and there is no theoretical basis for using these zenith angle functions. Harvey er al. (1977) calculated the dependence of J on the irradiance (see solid curve of Fig. 2) based on the diffuse and direct actinic fluxes of Leighton (1961). This calculation agrees well with their measurements, but the discrepancies with the other measurements in Fig. 2are difficult to interpret. They may be due to such factors as the need of using different values for direct and diffuse actinic fluxes for measurements obtained at different altitudes or regional albedoes. In addition, since the calculation requires the computation of absolute values of J, sizable errors may be introduced from, e.g. the rather uncertain NO2 spectral data. More recently, the J vs U.V. correlation measured by

572

SASHA MADRONICH

Madronich et al. (1984) has been rationalized using a method which, while similar in principle to that of Harvey et al., depends only on ratios of direct and diffuse sunlight and is essentially independent of the accuracy of the NO2 spectral data. In the following section, this method is described in detail and extended to allow for variations in local and regional albedo as well as elevation of the measurement station. In Section 4, correlations based on this method are used to evaluate the consistency of the reported actinometric measurements.

3. MODEL

lationship exists between the components J, = A,jJ,, + 250 cos xo).

= dEl/(cos 6 dw dt di. ds),

E=

(3)

where u(i) and ~(1.) are the NO2 absorption cross section and quantum yield, respectively, and he/l is the energy of a single photon. Here, the angular integration is performed over 4n steradians. The cosine factor does not appear in the expression for J because NO, molecules in the atmosphere (and in actinometers) have random spatial orientation. Because of the factor case, the expressions for J and for E cannot be related unless the angular dependence of the radiance L is known. It is therefore convenient to separate the radiance into the three components, L=

L,+L,+Ld

(4)

associated with direct sunlight (L,) and with the diffuse contributions bf the upward and downward propagating light (L. and L,,, respectively). A similar separation can be made for J, J = Jo+J.+J&i

While this assumption is not strictly correct, Luther (1980) has shown that multiple scattering calculations in which the diffuse light is treated as isotropic agree with more detailed calculations (e.g. Luther and Gelinas, 1976) to better than f 10% for most altitudes and zenith angles. Second, equation (7) can be further simplified to E 5 a[ Jo cos x0 + J,/2]

(2)

where A(R) is the spectral response function of the radiometer, and the angular integration is performed over 2x steradians (overhead hemisphere). The cosine response of the radiometer is shown explicitly in this expression. The NO2 photodissociation coefficient, J, is related to the spectral radiance by J = 56(%)cp(j.)(~/hc)SL(A,B,~)dwdl.

Jocosxo~W)Lo(~)d~ Sa(~)~(~)(~/hc)l;,(~)dl

(1)

where dEi is the radiant energy in the wavelength interval 1+ 1+ dl transported across a surface element of area ds, in time dt, from a solid angle element do which is at an angle 8 to the surface normal. The U.V.irradiance, E, measured with the standard Eppley radiometer, can be expressed in terms of the spectral radiance as E= 5A(;1)IL(i.,e,~)cosedodl,

(6)

The 2 cos x0 factor ensures conservation of energy and shows that the photolysis rate can be strongly affected by the local background albedo, e.g. for overhead sun and zero atmospheric optical thickness, changing A, from zero to unity can increase J by a factor of 3 (McElroy and Hunten, 1966). To evaluate the angular and spectral integrals in the above equations, two approximations are required. First, by assuming that the diffuse light is isotropic, the angular integrations can be performed, and it is readily shown by combining equations (2)-(S) that

The spectral radiance, L (I.,@), units = J me2 x s-’ nm-’ St-‘, is defined (see “specific intensity,” Chandrasekhar, 1960) by the expression L(i.&$)

of J:

(5)

where the meaning of the subscripts is as in equation (4). If a reflecting background surface of Lambertian albedo A, is present, an additional re-

(8)

where u is a constant, if the two ratios of wavelength integrals in equation (7) are sufficiently similar. This will certainly be the case (1) if the wavelength dependence of the radiometer response, A(1), is similar to that of the product cr(I,)cp(l) (Yhc), or (2) if the spectral distribution ofthe diffuse light, Ld, is similar to that of direct sunlight, Lo. To first approximation, both of these conditions (only one is necessary) are satisfied. Equation (8) can be rewritten in terms of the total photolysis rate J by using equations (5) and (6): J/E = C/-f- ’ + 2A,]

(9)

where f= CJo/(Jo+ Jdlcosxo+[~ CEO-1.

- JoltJo + J,)lP

With experimental values of J and E on the left hand side, equation (9) provides the desired correlation of J and E measurements, as long as the contribution to J from direct sunlight, Jo/(Jo + J,,) is known. Since only the fractional contribution to J is required, errors due to uncertainties in the NO2 spectral data are largely eliminated. For clear-sky conditions, the ratio Jo/(Jo + Jd) was evaluated using a simplified multiple scattering program (Madronich et al., 1984) based on a two-stream successive orders method (Isaksen er (II., 1977) but adapted to isotropic scattering and Lambertian ground reflections (Luther, 1980). Values of the direct sun fractional contribution are shown in Figs 3 and 4

inte~om~~son

of NO2 photodiss~ation

and U.V. radiometer measurements

573

0.6

0.4 Jd(Jo+ Jd) 0.3

0.0 1 0

10

20

30 40 50 60 Solarzenilh angle,deg

70

80

90

Fig. 3. Clear-sky fractional contributions to J from direct sunlight (zero local albedo), for different values of the regional albedo: 0.0; \%* 0.15; - 0.3; ***0.5; 1.0.

0.6

Jo/(Jo+ Jet)0.4

0.0 0

10

20

30

40

50

60

70

80

90

Solar zenith angle,deg Fig. 4. Clear-sky fractional contributions to J from direct sunlight (zero local atbedo, 0.15 regional albedo), for different surface elevation above sea level: -Okm;*--3km;-6km.

for several values of the regional surface albedo and elevation above sea level. This surface albedo should not be confused with the local background albedo A, of equation (6) which reflects the immediate surroundings of the experiment and is negligible (dark paint, black cloth) in most experiments considered here, Direct sun cont~butions are seen to be n~rly constant for x0 < 70” and, for moderate regional albedo and surface elevation, vary by onfy a few percent from an average value of ca 0.6. The variation of the function /- ’ with surface elevation and albedo is shown in Fig. 5. The zenith angle dependence of f-r is in qualitative agreement with the data of Nader and White (1969) for the ratio of the total signal from six orthogonal radiometers to that of a single upward looking radiometer obtained in an urban atmosphere. Figure 5 shows that the function f- ’ is not very

sensitive to albedo or station elevation. For example, at x0 = O”, changing the surface elevation from 0 to 3.0 km decreases J-’ by 5%, while changing the regional albedo from 0.15 to 1.0 increasesf- ’ by 14%. For each study in the present evaluations ofJ measurements, values off- 1 were calculated at the nearest km surface elevation and an assumed regional surface albedo of 0.15. For an overcast sky, the direct sun contribution vanishes and equation (9) reduces to

J/E = 2C(l + A,_)

(10)

where C should, at least in principle, have the same numerical value as for the clear sky data. This correlation is linear, in agreement with the observations by Parrish er a/., Dickerson PI al., Harvey et al. and Sickles el ai.

SASHA MADRONICH

574

1.0 0

10

20

30

40

SO

60

70

80

90

Solar zenith angle, deg

Fig. 5. Clear-sky conversion factor for~/i~adian~co~lations different values of surface elevation and regional albedo: -0 -0 km, 1.0 alb; - 3 km, 0.15 alb.

(seetext), for km, 0.15 alb;

l

4. INTERCOMPARISONS

A. Clear-sky measurements Figure 6 shows the J and E measurements plotted according to equation (9). The correlations are much more linear than Fig. 2, with a majority of the studies (Zafonte er al., Harvey et al., Dickerson et al., Parrish et al. and Madronich et al.) lying on a single narrowly defined band. Two other sets of measurements (UNC chamber and Sickles et al.) are clearly separated from these data. Equation (9) also predicts that the vahte of C, i.e. the ratio of J to the hemispheri~lly converted irradiance, is independent of the solar zenith angle. Values of this ratio are shown in Figs 7a-7g, calculated for the clearsky data reported by each group. For x0 < 70”. all but one of the studies (UNC) show values of C nearly inde~ndent of solar zenith angle. At larger zenith angles, values of C show increased scatter and, in two of the studies, a decrease from the high sun value. At these zenith angles, the measured values of both J and the irradiance are smaller and therefore less accurate;

C,,

2.. a Y

0. 0

XKU

1

2

E(f-l+

z

4

2A,_),

5

c

7

0

mWcmm2

Fig. 6. Clear-sky correlation of J value measurementswith converted irradiance measurements. Symbols as in Fig. I.

the cosine response of the Eppley radiometer is obeyed less accurately; and the calculation of Jo/fJo + Jd) is subject to greater uncertainty from the complexity of modeling multiple scattering in spherical geometry and with greater optical depths. Therefore these deviations at large zenith angles are not unexpected. Table2 gives the average values of the ratio C, measured by each group for x0 -z 70”. Five of the studies agree to better than f 5%, and yield an unweighted average value, C(clear sky) w 1.35% 0.05 cm2 J-r.

(1 la)

This value of C wilf be referred to as the “consensus” value for clear skies, and will be used below as a standard for evaluating the performance of the actinometers. The choice appears well justified by the experimental confirmation, in six out of seven studies, of the independence of C with x0. and by the remarkably close agreement in the average value of C in five of the seven studies. Indeed, the agreement among different studies indicates that direct actinometric measurements of J can now be made to about _+5 P/ accuracy, in contrast to the much larger uncertainty associated with theoretical determinations ( > M%, NASA, 1985). Some observations specific to each study follow. UNCchamber, Fig. 7a: values of C range from 0.8 to 2.8 cm’J - ‘, with a standard deviation of 2 23 % and a mean value 30% higher than the consensus average (see Table 2). Jeffries (private communication) has suggested that the higher mean value ofccould bedue to Eppley U.V.radiometer calibration errors, and that the large variations observed on any single day may come from spurious light ~rtur~tions (e.g. from trees or buiidin~). The alternative explanation is that the actinometer itself performed incorrectly. It should be mmhmbered that while the measurements shown in Fig. 7a were obtained outside the UNC smogchamber, simultaneous measurements with a similar actino-

Intercomparison of NO2 photodissociation and U.V. radiometer measurements

575

(b)

(df

Solar zenith

angle,

deg.

Fig. 7. Clear-sky ratios of J measurementsand converted irradiance measurements. Several additional m~surements omitted from Figs 1,2 and 6 for clarity arealso shown. Solid horizontal lineisconsensus value, 1.35 cm* J- I. (a) UNC chamber, l 12/3/7&O 1l/19/76, w 816176,o 9/19/74, A 10/l 3176;(b) Zafonteer al., l 9/18/75, 0 9/19/75, n 9/11/75; (c) Sickles et al., + 10/12/75,0 4/21/15, W 4/22/75; (d) Harvey er ol., (e) Dickerson er ol.. l 1.8 km, 0 3.0 km; ff) Parrish er uf., l 7/28/80, 0 s/4/80, l 8/21/80: (g) Madronich et al., 4 6/26/80, 0 8121J8I,

meter were made inside the chamber to “calibrate” the effect of the structure (Teflon film walls, support beams, aluminum foil covered floor)on the actinic flux. If the actinometric technique is at fault, the large erratic trends seen in Fig. 7a indicate that even relative indoor/outdoor measurements may be inaccurate. If, on the other hand, spurious light perturbations affected the outside actinometer. the relative indoor/outdoor measurements cannot be interpreted unless

identical perturbations were also present in the indoor data. In either case, it is difficult to use these actinometric measurements to estimate the in-chamber photodiss~iation rate coefficients for NO2 and implicitly for all other photoactive species. Since the extensive smog chamber simulation experiments are used to test photochemical models, it may be useful to re-measure the J inside and outside this smog chamber, for a variety of light conditions, using

SASHA MADRONKH

576

Table 2. Average value of C over

Reference UNC chamber Sickles et al. Dickerson el al. SZafonte er al. $ Harvey et al. t Dickerson ef al. i Parrish er al. $ Madronich el al. Averaget

C (cm*J-l) Overcast Clear sky 1.7 kO.4 1.10+0.06 1.51 f 0.07 1.35f 0.09 1.38 + 0.05 1.32 t 0.06 1.37 & 0.06 l.35& 0.10

1.2 kO.1 0.93 f 0.06 1.15+0.19 1.04+0.10 1.08+0.10 1.30+0.10

1.35 f 0.04

1.14kO.08

f&70”

Deviation * (clear sky) +30% -20% + 12% (1.8 km) 0% +2% -2% (3 km) +2% 0%

l Deviation from consensus average. t Average of consensus studies, marked:.

an actinometer with a proven record of reliability. Zafonte et al., Fig. 7b: the values of C are somewhat scattered, but their average agrees well with the consensus value. Dickerson and Stedman (1980) have argued that these measurements may beaffected by O2 impurities in the Nz carrier gas, but no evidence of such a perturbation is found in the present analysis. Sickles PI al., Fig. 7c: the values of C are independent of solar zenith angle as predicted, but fall some 20% lower than the consensus value. Although the actinometer used in this study is similar to that used by Jeffries et al. (Fig. 7a), it does not show the same irregular zenith angle dependence. The lower values of C may be due to errors in Eppley radiometer calibration, but they may also be due to general problems associated with spherical actinometers (see, for example, the discussion by Zafonte et al.). Demerjian et al. (1980) have calculated theoretical J values significantly higher than those measured by Sickles et al. (about 15 % for clear sky, and considerably more for overcast and partly cloudy sky), and they have attributed the discrepancies to possible flow problems with the actinometer. Their conclusion agrees quantitatively with the present work. Harvey et ol., Fig. 7d: the average value of C is in excellent agreement with the consensus value, but a slight zenith angle dependence is apparent even for x0 < 70”. The measurements are complicated by uncertainty in the (local) background albedo, which Harvey et al. determined to be 9.6 %, but Dickerson et al. recalculated as 7 %. The latter value was used in the present calculations. Use of the higher albedo alters the average value of C from 1.38 to 1.35cm2J-‘, with negligible effect on the curvature of Fig. 7d. While the source of this curvature is unknown, the resulting deviations from the average value of C are less than rf: lo%, and the actinometer may be considered reliable within these limits. Dickerson er al., Fig. 7e: the measurements at 3.0 km give a C value in excellent agreement with the consensus, while the 1.8 km values, obtained one year earlier, are some 10-15 y0 higher. A comparison of J

values obtained at 1.8 and 3.0 km led Dickerson et al.

to conclude that .I is not dependent on station elevation. However, the analysis given here suggests instead that a systematic error was present in the 1.8 km measurements, resulting in higher values of C. Possible sources of this discrepancy may include a drift in the U.V.radiometer calibration,an unreported bright background at the 1.8 km station, or simply an improvement in the accuracy of the actinometer during the one year period separating the two sets of measurements. Parrish et al., Fig. 7f: for x0 c 70”, the measurements show close adherence to the predicted constant value of C and are in good agreement with the consensus value. Parrish er al. have also carried out theoretical calculations of J and have shown that if the NO, quantum yields measured by Harker et al. (1977) are used, calculated J values are some 20 Y0lower than the measurements, and therefore these quantum yield measurements are probably in error (see also Demerjian et al., 1980). The present analysis supports this conclusion, to the extent that this 20 % discrepancy is much greater than the differences between the five “consensus”studies (see Table 2),and cannot therefore be attributed to experimental uncertainties. For large zenith angles, a decrease in C similar to that observed in the data of Harvey et al. is seen. The high precision of the data and measurements over several days indicate that this fall-off is reproducible. However, it is difficult without further study to determine whether this is due to systematic experimental errors (e.g. edge effects at the radiometer for high angles of incidence) or difficulty in accurately calculating Jo/(Jo + Jd) at large solar zenith angles. Madronich et al., Fig. 7g: the & 10 % scatter of these data is somewhat larger than that of other recent studies, but the average value of C agrees well with the consensus value. This is of some interest because the actinometer employed in this study is the only one based on pressure change rather than NO, detection and was used in recently reported measurements of J in the stratosphere (Madronich et al., 1985).

Intercomparison of NOI photodissociation

B. Overcast sky measurements Cloudy sky data are summarized in Fig. 8. Some of these measurements (Parrish er al., Madronich et al.) were obtained on partly cloudy days with the sun blocked by clouds, while others (Sickles et a!.) were obtained under completely overcast conditions. The values of C were obtained from the measured values of J and E, via equation (lo), which assumes that clouds redistribute the light isotropically over the entire sky. Table 2 gives the average values of C for studies performed under cloudy conditions (including several studies not shown in Fig. 8 be-cause zenith angle data were not available). In all cases, the values of C are lower than those observed for clear skies (see Table 2). The average of five studies (the consensus studies of clear skies) is C(overcast) _ 1.14+0.08cm2J-’

(lib)

or about 16 Y0lower than clear-sky value. The reason for this difference is not clear, but it is probably not due to a failure of the isotropy assumption, since this would systematically underestimate C for x0 < 60 and overestimate it for x0 > 60”. No such trend is evident in Fig. 8. It is possible that the wavelength distribution of light that reaches the ground is strongly changed relative to the clear sky, resulting in different spectral responses for the J and E instruments. Alternatively, geometric perturbations could be more important for overcast skies because the relative contribution from light near the horizon can be greater than for clear skies (if isotropic, one-third of the light is incident at angles greater than 70”). At these large angles of incidence, edge effects of the radiometer plate would increase the signal relative to the cosine law, while extended shadows at the ends of cylindrical actinometers would decrease measured .I values; both effects tend to decrease C. This last explanation is consistent with the lower clear sky values of C observed by Harvey et al. and Parrish et al. for large zenith angles. Nonetheless, considering the large variability in the extent and distribution of cloud cover to which

-

2 4

t

t

Oi

0

30

60

I 90

Solar zenith angle, deg.

Fig. 8. Overcast sky ratios of J measurements IO converted irradiance measurements. Parrish er ol.: 4 , 8/23/80; 0, 8/l-4/80; n ,7/21/80. q.7/30/80; A, 7/18/80: x, Sickles er al., 4/28/75: A, Madronich er al.. 6/26/80.

and U.V.radiometer measurements

571

these data pertain, the agreement between overcast and clear sky values of C is satisfactory. 5. ESTIMATING J FROM U.V. RADIOMETER MEASUREMENTS

Measurements of photodissociation rates are often desired simultaneously with other studies such as trace species measurements from ground stations, airplane or balloon platforms,or smog chambers. For such field measurements, routine use of simple standard U.V. radiometers offers obvious practical advantages over the use of the more complex actinometers. If a single upward-looking radiometer is employed, the method presented in this paper can be used directly to estimate J from the U.V. irradiance. As a simple example, for clear sky, solar zenith angle < 60”, negligible local background (black paint, grass, etc.), and moderate (e.g. 0.15) regional albedo, J can be estimated from the expression .I-

1.35 E (0.56 + 0.03 z) cos x0 + 0.21’

(12)

Here z is the station elevation above sea level in km, J is expressed in s- ‘, and E is measured in W cm-*. For other regional albedoes or zenith angles, the fractional contributions of direct sunlight J&Jo + J,+) can be read from Figs. 3 and 4, or re-calculated for the specific conditions of the experiment. As Fig. 5 shows, the functionj- ’ is not very sensitive to the details of this calculation, so that the values given here should have a fairly wide range of applicability. However, the local background can have a rather large effect on photolysis rates [see equation (6)] and must therefore be accurately quantified in each experimental environment. Thus, for example, use of equation (12) to estimate J in the UNC smog chamber would be inappropriate due to the high reflectivity of the chamber floor. The accuracy can be improved by using several radiometers, in one of several possible arrangements (e.g. downward-looking to quantify local albedo, masking to observe direct sun or diITuse light alone, etc.). Extension of the present analysis to these configurations is straightforward. The accuracy to which J can be determined from the radiometer measurements should be comparable to the agreement among the consensus studies. Thus, for ideal clear-sky conditions, use of equation (10) with C h 1.35 cm* J- ’ should yield .I to about f 5 %. Even allowing for some non-ideal conditions, and minor calibration drifts of the radiometer, an accuracy of f 10% should be easily achieved. For overcast skies, the lower valueofCshould be used (1.14cm2 J-‘)and should yield J to f 20 %. In order to maintain this level of accuracy, the radiometers should be recalibrated frequently, either against a standard light source, or, preferably, against a reliable NO2 actinometer. These accuracy estimates apply only to solar zenith angles

SASHAMADRONICH

578

less than about 70”. For larger zenith angles, some of the discrepancies between the measurements still need to be resokd. Acknowledgements-The author wishes to thank Dr D. D. Parrish of NOAA for providing unpublished radiometer data, and Drs J. C. Calve&, B. A. Ridley and W. R. Stockwell of NCAR. Profs D. R. Hastie and H. I. SchiK of York University; and Prof. H. E. Jetliies of the University of North Carolina for many useful discussions. Although the research described in this article has been funded wholly or in part as part of the National Acid Precipitation Assessment Program by the U.S. Environmental Protection Agency under Interagency Agr~nt DWQ3Ol~l-2 to ihe National Center for Atmospheric Research, it has not been subjected to the Agency’s peer and policy review and therefore does not necessarily reflect the views of the Agency and no otficial endorsement should be inferred.

Warner P. 0. (1975) Direct NO2 photolyis rate monitor. Rev. Sci. Instrum. 46, 3X-378. Jell&s H. E., Kamens R. M, Sexton K. G. and Gerhardt A. A. (1982) Outdoor smog chamber experiments to test photochemical models. EPA renort No. EPA-600/3-82-016a. Leighton P. A. (1961) Phor&emistry of Air Pojfirtion, p. 1IS. Academic Press, New York. Luther F. M. and Gelinas R. J. (1976) EtTect of molecular multiple scattering and surface albedo on atmospheric photodissociation rates. .J. geophys. Res. 81, 1125-l 132. Luther F. M. (1980) Annual rcporf of Lawrena Livermore National Laboratorv to the FAA on the Hiah Altitude Pollution Programil980. Lawrence Livermoie National Laboratory, Livermore. Madronich S., Hastie D. R, Ridky B.A. and Schiff H. 1. (1984) Measurement of the phot~i~~iation coefficient of NO2 in the atmosphere: 1. Method and surface measurements. J. Atmos. Chem. 1, 3-25.

Madronich S.. Hastie D. R., Ridley B. A. and Schiff H. 1. (1985) Measurement of the photodissociation coefficient of NO2 in the atmosphere: II. Stratospheric measurements. J. Atmos. Chem. 3,233-245.

REFERENCES Bahe F. C., Schurath U. and Becker K. H. (1980) The frequency of NO2 photolysis at ground level, as recorded by a continuous actinometer. Atmospheric ~~uironrne~r 14, 71 l-718. Chandrasekhar S. (1960) Radiative 7’runsfer. pp. I-4. Dover, New York. Demerjian K. L., Scherc K. L. and Peterson J. 1. (1980) Theoretical estimates ofactinic (spherically integrated) flux and photolytic rate constants of atmospheric species in the lower troposphere. Ado. Envir. Sci. Tekmol. i0, 396-459. Dickerson R. R. (1980) Direct measurement of ozone and nitrogen dioxide photolvsis rates in the atmosphere. Ph.D. The&, University of Michigan, Ann Arbor. Dickerson R. R. and Stedman D. H. (19801Precision of NO, photolysis rate m~u~ments. .&vir.‘Sci Techno~. 14 1261-1262. Dickerson R. R., Stedman D. H. and Delany A. C. (1982) Direct measurement of ozone and nitrogen dioxide photolysis rates in the troposphere. .I. geophys. Res. 87, 4935-4946.

Harker A. B., Ho W. and Ratto J. J. (1977) Photodissociation quantum yield of NO1 in the region 375 to 420 nm. Chem. lihys. lb. 50, 394-397. Harvey R. B., Stedman D. H. and Chameides W. (1977) Determination of the absolute rate of solar photolysis of _ NO*. J. Air Ponder. Control Ass. 27, 66-66. Isaksen 1. S. A.. Midtbo K. H.. &ride J. and Crutzen P. J. (1977)A simplified method to include molecular scattering and reflection in calculations of photon fluxes and photodissociation rates. Geophysics Norwgico 31, 1 l-26. Jackson J. 0.. Stedman D. H., Smith R. G., Heckcr L. H. and

McElroy M. B. and Hunten D. M. (1966) A method of estimating the Earth albedo for daygiow measurements. J. geophys. Res. ff,

3635-3638.

NASA (1985) Chemical kinetics and phot~hem~l

data for use in stmtosphe~c modeling. Evaluation No. 7, JPL Publ. 8262, National Aeronautics and Spaa Administration,Jet Propulsion Laboratory, Pasadena. Nader J. S. and White N. (1969) Volumetric measurement of ultraviolet energy in an urban atmosphere. Wvir. Sci. Technol. 3, 848-854.

Parrish D. D, Murphy P. C., Albritton D. L. and Fehsenfeld F. C. (1983)The measurement of the photodissociation rate of NO2 in the atmosphere. Atmospheric Environment 17, 1365-1379.

Saeger, M. (1977) An experimental determination of the specific photol~is rate of nitrogen dioxide. M. S. Thesis. University of North Carolina, Chapel Hill. Sickles J. E. 11,Ripperton LA., Eaton W. C. and Wright R. S. (1978) Nitrogen dioxide photolytic, radiometric and meteorological field data. Report to EPA No. EPA-600/ 7-78-053. Walega J. G., Stedman D. H., Shetter R. E., Fukuda R. S. and Bahls, T. A. (1980) Final report: Chemiluminescent/laser diode measurement technique intercomparison and captive air measurement program. Coordinating Research Council Report No. CAPA-17-80. Whitten G. 2. and Gery M. (1986) Further development and evaluation of the carbon bond mechanism: Monthly Technical Narrative to the EPA IJanuarv~. Svstem Applications, Inc., San Rafael CA, ’ -” * Zafonte L., Rieger P. L. and Holmes J. R. (1977) Nitrogen dioxide photolysis in the Los Angeles atmosphere. Enuir. Sci. Technol. l&483-487.