testing of the abiotic degradation of chemicals in the atmosphere: the smog chamber approach

ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY 15,298-3 19 (1988)

Testing of the Abiotic Degradation of Chemicals in the Atmosphere: The Smog Chamber Approach’

WALTER KL~PFFER, FRANZ HAAG, ERNST-GUNTHER KOHL, ANDRUDOLF FRANK*

Battelle-lnstitut e. V., Am Riimerhof35, D-6000 Frankfurt am Main, West Germany

ReceivedSeptember 14,1987

Methods for measuring the hydroxyl-, ozone-, and direct photochemical reactivity of a substance in one specially designed medium size smog chamber are described. Rate coefficients for the reaction of OH with n-hexane, n-heptane, ethene, ethyne, chloroform, trichloroethene, methanol, 2-propanol, benzene, o-xylene, 1,4-dichlorobenzene, I ,2,4-trichlorobenzene, pchloroaniline, naphthalene, acenaphthene, l&dichloronaphthalene, biphenyl, and fluorenone are given and discussed. An upper limit of 5 X lo-” cm3/sec is given for the sum penta- and hexachlorobiphenyls (PCB). Rate coefficients for the ozone reaction are given for @-pinene, limonene, A3-carene, cineol, vinyl chloride and I ,3-butadiene. In cases where the literature data are available for comparison, the rate coefficients (b” and !QJ reported here compare favorably with the best data reported. The direct photochemical reactivity has been shown to be measurable if the chamber is cleaned carefully. Preliminary results on benzophenone are reported. The methods described here, except that of direct photochemical reactivity, are in agreement with those proposed to OECD. Moreover, part of the Draft OECD Test Guideline (Berlin, 1987) on “Photochemical-0xidative Degradation in the Atmosphere” is based on work described here and on closely related work in other laboratories (Becker et a/.. 1984). 0 1988 Academic Ptw Inc.

INTRODUCTION

Abiotic degradation and transformation processes contribute to the removal of chemicals from water, soil, and air. In the atmosphere, these processes are nature’s only means of efficient and irreversible destruction of chemicals originating from anthropogenic as well as natural sources.

Chemical legislation demands the analysis of exposure and the fate of “new” and selected “existing” chemicals, implying that suitable testing methods are available. In the following we restrict ourselves to degradation/transformation testing in the gas phase of the atmosphere, which is reached by volatile chemicals.

The most important processes to be considered include (Finlayson-Pitts and Pitts, 1986):

(a) Reactions with hydroxyl radicals (b) Reactions with ozone molecules (c) Direct photochemical reactions (d) Reactions with nitrogentrioxide.

’ Presented in part at the 29th Bunsenkolloquium “Photochemische Abbau- und Transformationspro- zesse in der Atmosphiire,” March 29, 1985, in Frankfurt am Main.

2 Present address: Universitiit Hohenheim, Institut f . Chemie 130, D-7000, Stuttgart 70.

0147-6513/88 $3.00 Copyright 8 1988 by Academic Res. Inc. AU rights ofreprcduction in any form reserved.

298

ABIOTIC DEGRADATION IN THE ATMOSPHERE 299

There are significant differences between these reactions with regard to their importance as sinks for chemicals in the atmosphere. Whereas OH radicals (a) react with most organic molecules in the troposphere-although with highly different rates-, ozone (b) reacts efhciently only with molecules containing double bonds. Direct photochemical reactions (c) in the troposphere are restricted to substances absorbing at wavelengths longer than 290 to 300 nm, the cut-off region of tropospheric UV radiation. NO3 (and Nz05 which is in equilibrium with N03) (d) can contribute signifi- cantly to degradation and transformation during night for certain classes of compounds (e.g., olefins, terpenes, phenols, PAH, sulfides). Reactions (a) and (c), on the other hand, are restricted to day time, whereas the less reactive (longer lived) ozone is present and can react also during the night.

Other reactive species, as OOH radicals, singlet oxygen molecules, and oxygen atoms, seem to be of minor importance. It is generally believed that the reaction with OH (a) is the most important sink for chemicals in the troposphere due to the H abstraction power of hydroxyl (1) and its high electrophilicity leading to addition reactions (2), both types of reactions reaching diffusion controlled rates in suitable cases (Atkinson, 1986; Becker et al., 1984; Atkinson et af., 1979).

RH+OH+R’+H,O (1)

RH + OH + ‘RHOH (2)

In order to calculate the rate of disappearance of a chemical, the bimolecular rate coefficient bH has to be known as a minimum requirement. Preferentially, ko,, kNos, and the photochemical quantum efficiency (which can be transformed into the reaction rate if the absorption spectrum of the substance and the spectral solar irradi- ante is known) should also be known if there is an indication that reactions (b) to (d) may contribute to the tropospheric degradation of the substance considered.

The rate coefficients can be converted into half-life times (r,J familiar from radio- active decay according to

In 2 t

"* = ~HIOHI + bJO31 + kmJNO31 + khr @a)

- In 2/ko,[oH] (for most substances) (3b)

The average tropospheric residence time may be shorter than t,,2 calculated according to (3) (even if appropriate average concentrations [OH], etc., have been used) due to additional degradation/transformation pathways (e.g., transformation at aerosols or in water droplets) and due to the transfer to the surface of the earth by wet and dry deposition. The deposition processes may be reversible and, hence, do not represent real sinks (Kliipffer et al., 1982).

The methods for measuring bH can be divided into absolute and relative (A&in- son, 1986; Becker et al., 1984; OECD, 1987), the absolute methods requiring a higher degree of sophistication with regard to the equipment, especially ifnatural conditions in the test atmosphere are to be maintained (Wahner et al., 1985; &hmi& et al., 1984, 1985).

Relative measurements of bn and measurements ofbS (relative and absolute) can be performed in large to medium sized reaction vessels (“smog chambers”) or Teflon bags using chemical analysis as the main tool (Atkinson, 1986; Becker et al., 1984; OECD, 1987; Atkinson and Carter, 1984).

300 KL6PFFER ET AL.

FIG. 1. View of the ‘Smog Chamber” developed and constructed in close cooperation with Original Hanau Quarzlampen GmbH (Heraeus) in Hanau. The glass parts (DURAN) are standard products of Schott, Mainz.

The quantum efficiency of direct photochemical reactions can be determined using gas-phase photochemical techniques (OECD, 1987), provided that the substance is suffciently volatile.

In this article, a method allowing the separate determination of bn, kcj, and khy in one apparatus is described, although in the case of the direct photochemical rate only preliminary results can be presented. The determination of the rate coefficients of the OH and 03 reaction is in accord with the Draft OECD ( 1987) Test Guideline, whose OH part has been written on the basis of this and related work (Becker et al., 1984; Wahner et al., 1985; Schmidt et al., 1984, 1985; Klopffer et al., 1986). The determination of khy, outlined in this work, may serve as an additional approach after improvement and evaluation.

MATERIALS AND METHODS

The “smog chamber” constructed specifically for measuring the photodegradation and transformation of organic chemicals is shown in Fig. 1 (Kliipffer et al., 1986). It consists of a 0.46-m3 cylindrical Pyrex vessel of 60 cm in diameter surrounded by 28 UV-A lamps ( 1.5 m long, 65 W Hg low pressure lamps by Philips, TL 65-80 W/05). The chamber is fit with a glass fan and measuring devices for temperature, pressure, relative humidity, ozone, NO, and NO*. The measurements are performed at slightly increased pressure (about 0.105 to 0.110 MPa) and temperature (about 300°K). The maximum volume-averaged UV intensity, as measured by kl of NO2 photolysis amounts to about k, = 0.9 min-‘, corresponding to twice the solar UV intensity at sea


FIG. 2. Spectral k-radiance within the chamber (position of the measuring device indicated); number of UV-A lamps used indicated.

level and midlatitudes (Holmes et al., 1973; Heicklen, 1976). The spectral intensity distribution measured with a IL Research Radiometer (Intern. Light Inc.) is shown in Fig. 2.

Gaseous and highly volatile substances are introduced into the chamber using standard methods of gas analysis, whereas for compounds with lower vapor pressure (down to about 0.0 1 to 1 Pa) a special vaporizer has been used (Kliipffer et al., 1986) which is particularly helpful for bringing solid test substances into the gas phase of the reaction vessel.

For the first experiments, a hydrocarbon/NO, photochemical smog has been used as OH source. The concentrations of propene (as the reactive hydrocarbon) and NO were in the range of 0.1 to I .O ppm. Average OH concentrations up to [OH] = 5 X lo6 cme3 could be obtained using this method.

The bulk ofthe experiments has been performed photolyzing gaseous HONO, produced by acidification of solutions of NaN02 with sulfuric acid (Cox et al., 1980, 198 1). The HONO gas is transferred into the reaction vessel with a stream of inert gas. The addition of HONO is repeated every 10 min in order to “recharge” the reaction mixture. Using this method, initial OH concentrations up to [OH] = 10’ cmT3 and average OH concentrations over several hours of [OH] = 5 X lo7 cme3 were obtained.

The photolysis of hydrogen peroxide, using an initial concentration of [H20210 = 136 ppm yielded a half-life of H202 of 72 min and average OH concentrations of only 1 to 3 X lo6 cme3. Other OH sources described in the literature have not been used in this work since HONO photolysis turned out to be the most convenient method.

Analytical determinations of the concentrations of test and reference substances have in most cases been performed by gas chromatography using a suitable nonde- grading tracer substance in order to take into account dilution effects.

The substances used were “Oekanal grade” (Riedel de Haen) or purest form available. Pure synthetic air has been used for filling the chamber.

For measuring the direct photochemical degradation, the formation of reactive species (OH, 03, . . .) has to be excluded rigorously by washing all glass parts in

302 KLiiPFFER ET AL.

wbv

2 4 6

FIG. 3. Smog development using 20 UV-A lamps and an excess of the main smog-forming alkene (propene) relative to NO,.

contact with the illuminated gas. Especially all traces of NO, and HN03 have to be avoided in order to minimize smog formation (see Results and Discussion).

The experiments with ozone have been performed using externally generated ozone, the concentration within the chamber has been monitored with a chemilumi- nescence detector.

RESULTS AND DISCUSSION

1. REACTION WITH HYDROXYL

1.1. Preliminary Work using Alkene/NO, Smog as OH Source

1.1.1. Reaction conditions. The first experiments have been performed-in the hope of obtaining very high OH concentrations (Akimoto et a/., 1980)-using an excess of the highly reactive propene as smog-forming hydrocarbon. Figure 3 shows the typical pattern of [NO], [NO& [O& and [propene] as a function oftime: propene decays rapidly and ozone increases already after 1 hr, most of the reducing NO being consumed at that time. The concentration of NO2 has a maximum after 2 hr, fol- lowed by a general decrease of [NO,1 - [NOJ due to side reactions (formation of PAN, HN03, etc.).

The average concentration of OH has been calculated according to Eq. (3b) from the linear part of the log [propene] vs time plot shown in Fig. 4. The OH rate coefficient of propene at 298°K recommended by Atkinson ( 1986)-koH = 2.63 X lo-” cm3/sec-agrees with a value obtained by an absolute method operating in air at about 0.1 MPa (Schmidt et a/., 1985): koH = (2.2 & 0.4) X lo-” cm3/sec. Using the recommended value and t1,2 = 5400 set, an average hydroxyl concentration of [OH] = 4.9 X lo6 crnm3 is obtained.

Ethene has been added as a minority compound in this experiment (Figs. 2 and 3) and shows exponential decay over 7 hr. Using the half-life t1/2 = 20.700 set and the average [OH] calculated from the decay of propene we obtain

koH (ethene) = 6.9 X lo-” cm”/sec.

This value should be compared with the recommended value koH = 8.54 X lo-‘*


FIG. 4. Semilogarithmic plot of the propene and ethene mixing ratios as a function of irradiation time (20 lamps), see Fig. 3.

cm3/sec at 298°K (Atkinson, 1986) and that obtained with the direct method at 0.1 MPa in air: koH = (7.3 1 1) X lo-i2 cm3/sec (Schmidt et al., 1985). Other data obtained by the relative (“Smog Chamber”) method are 8.06 and 8.45 X 1 O-l2 cm3/sec (Becker et al., 1984).

The main disadvantage of high [a.Ikene]/[NO,] ratios can he seen from Fig. 4: propene starts to decay faster after the onset of strong ozone production (Fig. 2), due to this additional degradation channel (see Eq. (3a)). The rate coefficient of the ozone reaction given by Atkinson and Carter (1984) amounts to kos = 1.13 X 10-i’ cm3/ set with an estimated uncertainty of +40% and -20%. This value yields a half-life of only 5000 set due to the reaction of propene with ozone, if the highest ozone concentration in Fig. 2 (500 ppbv = 1.2 X lOI cm-‘) is used.

A further disadvantage of a high [aIkene]/[NO,] ratio is the drop in [OH] after about 5 hr observed in some ozone-rich artificial smogs. Thus, only OH-reactive substances can be measured under this condition.

Since contrary to expectation, no benefit in the form of increased [OH] counterbal- ances the disadvantages of high ozone concentrations, the [alkene]/[NO,] ratio has been decreased in the following experiments. A typical evolution of the most important species under low ozone conditions in our smog chamber has already been published (KIiipffer et al., 1986). Ozone formation can efficiently be suppressed for up to 10 hr.

The next lesson to be learned concerns the reference substances which should nei- ther react with ozone nor undergo direct photochemical reactions at wavelengths >290 nm. Toluene turned out to be very suitable for this purpose.

I. 1.2. Rate coeficients of high/y volatile substances. In a further experiment using ethene as a smog-forming alkene and reference substance, the OH rate coefficient of benzene has been determined to be

ki~.~ (benzene) = 1.2 X lo-l2 cm3/sec.

This value should be compared with 1.28 X lo-‘* cm3/sec (Atkinson, 1986) and

304 KLijPFFER ET AL.

very similar data obtained by Becker et al. (1984) and Perry et al. (1977). It should be mentioned that this reaction has been studied in considerable detail (r p) by Lo- renz and Zellner ( 1983) and Witte et al. (1986).

It is a particular advantage of the smog chamber method that several substances can be measured at the same time, using one or more reference compounds of known OH rate coefficient. Preferably, the OH reactivity of the substances and the analytical procedures required should be similar. In these preliminary experiments, groups as benzene, toluene, trichloroethene, hexane, and heptane; toluene, o-xylene, mesitylene, o- and p-dichlorobenzene, 2,4dichlorotoluene and 1,2,4-trichlorobenzene; and similar combinations have been investigated. The average [OH] determined from the exponential decay of the reference compounds has mostly been found in the range of 3 to 5 X lo6 cmp3. This relatively low hydroxyl concentration made it necessary to use reaction times on the order of 24 hr. Toluene has been used as a reference substance in most cases, using

koH (toluene) = 6.1 X lo-I2 cm3/sec (Atkinson et al., 1979; OECD, 1987).

The recently recommended value of (6.19 _+ 20%) X lo-i2 cm’/sec (Atkinson, 1986) for 298°K is in accordance with the above value and with indirectly measured rate coefficients (5.7 + 0.9) and (6.1 k 0.6) X IO-l2 cm’/sec at 300°K (Becker et al., 1984).

In the following, some rate coefficients obtained from simultaneous measurements of several compounds will be discussed.

n-Heptane has been measured by us in 198 1 as

koH (heptane) = 8.5 X lo-i2 cm3/sec (Kliipffer et al., 1986),

in good agreement with data reported in the literature (7.18 f 0.17) (Atkinson et al., 1982), (7.42 + 0.09) (Behnke et al., 1984), (8.6 & 0.6) X lo-” cm3/sec (Bruckmann in Becker et al., 1984).

The rate coefficient of trichloroethene has been measured to be

koH (trichloroethene) = (2.8-3.0) X 1O-‘2 cm3/sec,

which compares favorably with (2.36 -I- 0.7) X lo-l2 cm3/sec recommended by Atkin- son (1986).

In an experiment which could only partly be evaluated, o-xylene has been measured relative to mesitylene using kdH = 5.5 X lo- cm3/sec (average of data reported by Atkinson et al., 1979). The rate coefficient of OH + o-xylene is calculated to be

kH (o-xylene) = 1.3 X 10-l’ cm3/sec.

Absolute rate coefficients reported to be temperature-independent between 198 and 320°K average to (1.47 f 25%) X lo-*’ cm3/sec (Atkinson, 1986). The lowest rate coefficients which could be measured under low [OH] conditions are around 5 X lo-l3 cm3/sec, e.g.,

&u (1,2,4-trichlorobenzene) = 6 X lo-l3 cm3/sec,

to be compared with the directly measured value (5.32 _+ 0.50) X IO-l3 cm’& (296°K) by Rinke and Zetzsch ( 1984); see also Becker et al. (1984). Furthermore, the OH rate coefficient of pdichlorobenzene has been determined as

&u (1 ,Cdichlorobenzene) = 4.8 X lo-l3 cm3/sec (relative to ethene).


Wahner and Zetzsch (1983) measured for this compound (3.2 + 0.2) X lo-l3 cm3/ set at 295°K using the resonance fluorescence method which is more accurate for dowIy reacting substances.

Other data of volatile compounds measured in this type of experiment comprise methanol (kou = 1.0 X lo-l2 cm3/sec) and n-hexane (Icon = 4.8 X lo-l2 cm3/sec), both rate coefficients being in good agreement with the literature data.

1.1.3. Measurements on less volatile compounds. The alkene/NO, smog method has also been used for some preliminary experiments with solid substances. The evap- oration has been performed with the aid of a special vaporizer described elsewhere (Klijpffer et al., 1986). First, p-chloroaniline (PCA), vapor pressure pz5 = 2.5 Pa (Rip- pen, 1984), has been studied using [PCAlo = 133 ppbv and [OHjo = 7 X lo6 cmw3. From the first, rapid decay of [PCA] vs (t), kOH - 4 X lo-” cm’/sec could be estimated. For the sake of comparison, an absolute value of 8.4 X 10-l ’ cm3/sec at 295°K and 400 hPa has been reported by Zetzsch (in Becker et al., 1984).

Naphthalene, p25 = 9.48 Pa at 296°K (Handbook, 1973-1974) has been measured repeatedly in 198 1 (Klopffer et aL., 198 1, 1986). In one particular experiment, a purely exponential decay of [ naphthalene] vs (t) has been observed over 10 hr yielding

bH (biphenyl) = 7.4 X lo-l2 cm3/sec.

relative to ethene ([OH] = 2.4 X lo6 cme3). This value is in good agreement with the other measurements in our laboratory and with results by Lorenz and Zellner (1983) Atkinson et al. (1984) and Biermann et al. (1985). Lorenz and Zellner observed a negative activation energy in the range 337 (lowest measuring temperature) to 4 10°K according to

~OH = (2.2 -t 1) X 1Or2 exp(440 f 50)/Tcm3/mol sec. (4)

Extrapolation of (4) to 300°K and conversion to cm3/sec yields &n = 1.6 X lo-” cm3/sec. These and other recent data led to the recommendation of /Q,H (naphthalene) = (2.17 + 30%) X lo-” cm3/sec (Atkinson, 1986), in excellent agreement with our data.

Further experiments on aromatic hydrocarbons involved biphenyl (p25 = 0.94 Pa), acenaphthene (pzS - 0.6 Pa) and fluorene (~25 = 0.09 Pa); vapor pressure data after Mackay er al. (1981). Phenanthrene (p25 = 0.014 Pa, Mackay et al., 1983) could not be evaluated quantitatively. The initial concentrations measured before turning on the UV lamps were in the range of 20 ppbv, corresponding to a total amount of gaseous test substance of about 0.1 mg. The mass balance made by comparison with the amount of substance evaporated always indicates losses, most probably to the walls of the chamber. Since desorption has been observed only in few experiments, which could not been evaluated, the losses observed do not prohibit the degradation test of solid substances. Larger scattering of the analytical data, however, makes the rate coefficients less reliable. This is especially true for substances with vapor pressures below about 1 Pa.

The OH rate coefficient of biphenyl has been determined as

koH (biphenyl) = 7.4 X IO-l2 cm3/sec.

On the basis of an absolute measurement by Zetzsch (Becker et al., 1984) using the


resonance fluorescence method-(5.8 f 0.8) X IO-‘* cm3/sec at 296 K-and two similar relative rate coefficients Atkinson (1986) recommends for biphenyl koH = (7 f 2) X lo-‘* cm3/sec.

For the two other aromatics the following rate coefficients have been measured:

k-on (acenaphthene) = 5.4 X lo-*’ cm”/sec

kou (fluorene) = 1.2 X 10-i’ cm3/sec.

No literature data are available for comparison. Both compounds can react in princi- ple by H abstraction (aliphatic C-H bonds only) or OH addition to the aromatic rings. The high values of koH measured by us in both cases point to addition as the predominant mechanism.

1.2. Experiments using Nitrous Acid as OH Source

I .2.1. OHgeneration. The photochemical splitting of HONO according to Eq. (5), (Cox, 1974; Cox and Sheppard, 1980; Cox and Derwent, 1980; Cox et al., 198 1) yields concentrations of OH higher than those obtained with the smog method described in the previous section.

HO-NO+hv+OH+NO (5)

The gas phase absorption spectrum of HONO shows six vibronic bands (maximum absorption cross section on base e: 26.5 X lo-*’ cm* at 355 nm, Baulch et al., 1980) which excellently fit the spectral intensity distribution shown in Fig. 2. The quantum efficiency of reaction (5) is unity (Baulch et al., 1980), the rate constant measured in our chamber is kS - 0.15 min-‘, in accordance with the empirical relation k,(HONO) - 0.2 kl(N02) - 0.18 min-’ (Whitten et al., 1980). This relation, however, depends on the (wavelength-dependent) radiation intensity and the very good coincidence observed may therefore be fortuitous. The high initial concentration of [OHIO - 10’ cme3 after the first injection of the externally generated HONO gas lasts only for about 10 min. Repeated injection of HONO gas ensures continuous OH generation although reactions (6) to (8) remove OH. One reaction (7), however, regener- ates HONO.

OH + HONO + H20 + NO2 (6)

OH + NO(+M) + HONO (7)

OH + NOz(+M) + HONOz (8)

The corresponding rate coefficients at room temperature and pressure are

k6 = 6.6 X lo-‘* cm3/sec (Graedel, 1980)

k, = 1 .O X 10-l’ cm3/sec (Baulch et al., 1980)

ks = 1.6 X lo-” cm3/sec (Baulch et al., 1980)

These data, together with results from analytical measurements, have been used to estimate OH production rates and concentrations. It turned out that the secondary consumption of HONO by reaction (6) is approximately counterbalanced by (7). The formation of NO during the photochemical splitting of HONO (5) in the pres- ence of test and reference substances inevitably leads to the formation of smog. The

ABIOTIC DEGRADATION IN THE ATMOSPHERE

[Test lt, _ k&Test 1 CRef lo ln - - ~ In -

[Test], koHlRet I CRef If

307

FIG. 5. Evaluation of relative b” determinations after Cox and Sheppard (1980).

reference substances, therefore, must not react with ozone, as discussed in the preced- ing section.

The average concentration of hydroxyl over several hours is 2 to 4 X lo7 cmU3. Accordingly, the reaction time is reduced by I : 10 compared to the smog method described in 1.1. Rate coefficients down to about lo-l3 cm3/sec can be measured sufficiently accurate, provided that the analytical procedures allow the relative concentrations of the test substance to be measured precisely.

1.22. Evaluation. If the concentration of OH is approximateIy constant during the whole experiment or during a sufficiently long period, koH (test substance) may be evaluated from the ratios of the slopes of log[RefJ vs time/log[Test] vs time and the known bH (reference substance). This is equivalent to calculating [OH] from log[Refj vs time and using Eq. (3b) and the measured half-life of the test substance. One of these procedures is recommended if the decay of test and reference substance is exponential, indicating constant [OH).

If [OH] is not constant, an evaluation procedure proposed by Cox and Sheppard (1980) can be applied advantageously (Fig. 5). The precision of the results is largely determined by the analytical method available, including sampling and transfer to the GC or GC/MS apparatus.

2.2.3. Results obtained with volatile compounds. Chloroform has been used as a relatively stable, although not persistent, test substance. The OH reaction is simple H abstraction (9), no other reaction channel being available.

OH + CHC13 -. HOH + CC13 (9) The OH rate coefficient has been determined in two experiments as

308 KL6PFFER ET AL.

koH (chloroform) = (2.9-3.0) x lo-l3 cm3/sec

to be compared with an absolute value obtained by the resonance fluorescence method (2.3 -+ 0.5) X lo-i3 cm’/sec (Zellner in Becker et al., 1984) and the recommended value (from temperature dependence, Atkinson, 1986) of (I .03 + 20%) X 1 O-l3 cm’/sec.

AS representatives of the alcohols, methanol and isopropanol have been investigated. Methanol offers two reaction channels (1 Oa, b) where hydrogen abstraction from the hydroxyl group (lob) is less important due to the bond strength of RO-H which is 40 kJ/mol higher compared to C-H.

OH + CHsOH --+ HOH + CH20H

OH + CH30H + HOH -I- CH30

W-W

(lob)

Recent absolute data and product identification (CH30) by Hagele et al. (1983) showed that (lob) is responsible for about 10% of the total reaction. The rate coefficient (8.0 _+ 1.6) X lo-I3 cm3/sec at 297°K is in accordance with our data,

bH (methanol) = (1.0-I .2) X lo-** cm3/sec,

and other indirect (smog chamber) data reported by Becker et al. (1984). The same is true for isopropanol,

koH (2-propanol) = 5.1 X lo-l2 cm3/sec,

to be compared with other data obtained near 300°K (5.48 f 0.55) and (6.9 + 2.1) X lo-l2 cm’/sec reported by Atkinson (1986) and an absolute value by Zellner (4.1 L- 0.8) X 1 O-i2 cm”/sec (Becker et al., 1984).

The pressure dependence of the reaction OH + ethyne (acetylene) caused some confusion in older literature (Atkinson et al., 1978). Our value

bH (ethyne) = 8.7 X lo-i3 cm3/sec

is in excellent agreement with the high pressure limit in absolute determinations (7.8 x 1O-‘3 cm3/sec, Michael et al., 1980) and more recent data reported by Becker et al. (1984). Rate coefficients measured at low pressure would yield a too low kOn and, hence, predict a considerable longer atmospheric half-life.

In two cases of highly volatile compounds, only upper limits of the OH rate coefficients could be determined:

bH (ethylenoxide; acetonitrile) < lo-l3 cm3/sec.

Absolute rate coefficients are (5-8) X lo-i4 and (1.7-2.7) X lo-l4 cm3/sec at 296% according to Becker et al. (1984). In these cases, the absolute methods are clearly superior to the indirect method described here.

1.2.4. Semivolatile organic compounds. Many substances, termed recently “semivolatile organic compounds (SOC)” (Bidleman et al., 1986), occur in the gas phase or adsorbed at the particulate phase of the atmosphere depending on their vapor pressure and the aerosol content of the air mass considered (Junge, 1975; Gill et al., 1983). The vapor pressure (at 20 to 25°C) of the SOC comprises roughly the region between 1 and 1 Oe6 Pa. At the lower limit, all substances occur adsorbed at particles, except in extremely pure atmosphere. At the upper limit (around 1 to low2 Pa) all


compounds are in the free gas phase, except in heavily (dust/aerosol) polluted areas and if the substance has a specific affinity to certain surfaces.

This latter effect can also be observed in smog chamber studies, where the surface- to-volume ratio is much higher than in the atmosphere (8 m2/m3 in our chamber compared to about 1O-4 m2/m3 in rural air, Gill et al., 1983). It was one major aim of the present work to find out the lower vapor pressure limit, down to which the I& of a substance can be measured in a smog chamber.

The following substances (and one mixture) have been investigated ( p2* in Pa):

-Phenanthrene (0.0 14, Mackay et al., 1983) -Benzoic acid (0.06 at 20°C OECD Interlaboratory Test) -Pentachlorobenzene (>2.5 X 10-3) -Hexachlorobenzene/HCB (2.5 X 10e3, Rippen, 1984) -y-Hexachlorocyclohexane/lindane ( 1.9 X 10d3, DFG, 1982) - I -Chloronaphthalene/ 1 CN (c9.5) - l,4-Dichloronaphthalene/ 1,4DCN (49.5) -Polychlorinated biphenyls/PCB “Aroclor 1254” (2.9 X 1 Oe3, Murphy et al., 1987).

Some of these compounds could not be measured adequately due to adsorption/ desorption effects and slow reaction rates. Due to strong scattering of the phenanthrene data, only a lower limit of the OH rate coefficient could be measured:

koH (phenanthrene) = > 1 X 10-l’ cm3/sec.

Based on two studies of the temperature dependence of I& of this substance by Lorenz and Zellner (I 983) and Biermann et al. (1985), koH (300°K) = 3 X 10-l ’ cm3/ set can be considered as the best value at present. The rate coefficient decreases with temperature as in the case of naphthalene (Atkinson, 1986).

Benzoic acid shows strong desorption from the walls so that koH could not be determined. The reaction of OH with pentachlorobenzene, HCB, and Lindane is too slow to be measurable with this method. In addition, desorption effects disturb the experiments, especially for HCB and Lindane. Dichloronaphthalene could be measured, see Fig. 6.

~OH (1,4DCN) = 6 X IO-l2 cm3/sec

as an average of two experiments giving nearly identical results. No comparison with literature data is available. 1 CN could not be evaluated due to artifacts.

PCB (Aroclor 1254) has been evaluated as the sum of pentddorobiphenyls and as the sum of hexachlorobiphenyls. As can be seen from Fig. 7, practically no degradation has been observed. An upper limit of

koH (penta- and hexachlorobiphenyls) < 5 x lo-l3 cm3/sec

can be deduced from this (single) experiment with confidence. From the pentachloro curve in Fig. 7, even bH < lo-l3 cm3/sec can be estimated. No literature data are available for comparison.

Summing up the experience gained with SOC, substances with vapor pressures at room temperature down to about 1 cPa have been measured in our smog chamber.


1oJ

5-

0 30 60 so 120 min

FIG. 6. Relative concentrations of 1,4&chloronaphthalene ( 1 ,4-DCN, upper curve), compared to toluene as reference compound (lower curve) as a function of irradiation time; [OH]., = 3.2 X 10’ cmW3 produced by repeated addition of HONO gas (av = average); internal analytical standard: hexachloroeth- ane.

Since several substances are inclined to show specific adsorption effects, a more mod- erate lower limit of 1 Pa has been proposed to OECD (1987).

Several~experiments performed in smaller vessels, aimed at reducing the adsorb- ability at the walls, used silanation or, others, saturation with water vapor. These experiments did not show any measurable improvement.

FIG. 7. PCB (Aroclor 1254) experiment. Relative concentration of the sum of hexachlorobiphenyls (curve 1) and pentachlorobiphenyls (curve 2) and of toluene as reference substance (curve 3) as a function of irradiation time; [OH]., = 5 X 10’ cm -“; hatched curve 4 indicates the hypothetical PCB decrease with using by = 5 X 10-l’ cm’/sec.


TABLE 1

RATE COEFWIENTS koH (300 K) AND PARTIAL (OH) TROWSPHERIC HALF-LIVES’

Substance kOH (Cm3hdb f1/2 @aYS)’

Aliphatic Hydrocarbons n-Hexane n-Heptane Ethene Ethyne

Halogenated aliphatic substances, alcohols, etc.

Chloroform Trichloroethene Methanol 2-Propanol Ethylene oxide Acetonitrile

Aromatics and halogenated aromatics

Benzene o-Xylene 1,4-Dichlorobenzene 1,2,4-Trichlorobenzene pChloroaniline Naphthalene Acenaphthene 1,4-Dichloronaphthalene Biphenyl PCB (sum of penta- and

hexachlorobiphenyls) Fluorene Phenanthrene

6.8 x lo-l2 2.4 8.5 x lo-l2 1.9 6.9 x lo-‘* 2.3 8.7 x lo-l3 18.4

(2.9-3.0) x lo-l3 54.4 (2.8-3.0) x lo-‘* 5.5 (1.0-1.2) x lo-‘* 14.6

5.1 x lo-l2 3.1 <lo-‘3 >160 <lo-‘3 >160

1.2 x lo-l2 1.3 x 10-l’ 4.8 x lo-I3

6 x lo-l3 -4 x 10-I’ 2.0 x lo-” 5.4 x 10-l’

6 x lo-I2 7.4 x 10-12

4 x 10-13 1.2 x lo-” >l x 10-I’

13.4 1.2

33.4 26.7 0.4 0.8 0.3 2.7 2.2

>32 1.3

<1.6

’ For comparison with literature see the text. b Most reliable value. ‘From f’12 = In 2/(86400 kH [OH],) days, using an average tropospheric OH

concentration of 5 X IO5 cmA3.

The ~OH data obtained in this work have been compiled in Table 1 together with the “partial,” in this case OH-related (Wagner and Zellner, 1979), tropospheric half-life.

2. REACTION WITH OZONE

.?.I. General

A first hint to an efficient reaction with ozone can be gained as a by-product of ,&., measurements after the onset of O3 production (see Fig. 4) or from examination of the molecular structure (electron-rich double bonds). If there is an indication of ozone reactivity, the bimolecular rate coefficient of reaction ( 1 l), ko,, can be measured sepa-

312 KL6PFFER ET AL.

rately in the dark, since ozone can be produced conveniently by nonphotochemical methods.

O3 + RH + Products (11)

The literature on lo, has been reviewed recently by Atkinson and Carter (1984). Both direct and indirect methods determining keg are described in the Draft Test Guideline, OECD (1987). The direct method works preferably with a known constant surplus of test substance ([Test] = const $ [O&J, [O,] vs time being measured. The exponential decay of [O,] with time is used for calculating ko, according to first-order kinetics.

The advantage of the direct method is its independence of substance-specific ana- lytics, provided the volatility of the substance is so high that the concentration of the test substance in the gas phase, [Test], is given by the amount of test substance introduced into the system; otherwise, [Test] has to be determined precisely by analytical means.

The disadvantages of the direct method are:

-Traces of highly reactive impurities may yield too high b, values, especially in the case of less reactive test compounds -Less volatile compounds (e.g., WC) cannot be measured.

The measurable range of koj has been reported to be lo-r5 to lo-” cm3/sec (Mill and Davenport in OECD, 1987), corresponding to partial tropospheric half-lives between 0.3 hr and 3 years. The vapor pressure should exceed 10 Pa.

The indirect method avoids the disadvantages of the direct one, using one or several reference compounds of known koj (Nolting et al., in OECD, 1987). If the concentration of ozone can be kept constant at an exactly known level, the rate coefficient can also be calculated directly according to

ko3 = ln 2/tdJ’W Pd. (12)

In this case, the reference compound only serves to check the correctness of the experimental conditions. In general, koJ is evaluated from the ratio of slopes log[Test] vs time/log[Refl vs time or in a procedure similar to the bH determination after Cox and Sheppard ( 1980), see Fig, 5. The reference compound’s &, should lie within a factor of 10 of the expected koj of the test substance(s).

The disadvantage of [Test] $ [O,] is in some cases the occurrence of chain reactions leading to complicated kinetics (Atkinson and Carter, 1984).

2.2. Measurements

It is a prerequisite of ko) determinations that no significant ozone losses occur due to reactions at the walls, etc. This (slow) dark loss of ozone in our chamber has been found to be exponential with a half-life of 90 hr ([0310 = 500 ppbv in dry, synthetic air).

The measurements have been performed on several terpenes (some of which are known to be highly reactive) and alkenes using the indirect method. As reference substances, cr-pinene (Heicklen, 1976) and propene (Atkinson et al., 1982) have been Used:


TABLE 2

RATE COEFFICIENTS /co) (295°K) AND PARTIAL (03) TROPOSPHERIC HALF-LIVES

Substance

Terpenes p-Pinene Limonene A3-Carene Cineol

Unsaturated aliphatic substances Vinyl chloride 1.3-Butadiene

b (cm3/sec)

3.9 x 10-l’ 5.4 x lo-l6

:;; $8’

1.7 x lo-j9 6.1 x IO-‘*

tl,2a

7 hr 0.5 hr 3.3 hr

~2.3 days

67 days 1.9 days

’ Using the average tropospheric concentration [O,],, = 7 X 10” cme3 (30 ppbv) (Atkinson and Carter, 1984) and tlj2 = In 2/(86400 ko, 10,l.J days, or In 2 (3600 ko, [O&J hr.

bj (a-pinene) = (1.62 r+ 1) X lo-l6 cm3/sec.

This value is an average of 5 room temperature (294-298°K) data reported without recommendation by Atkinson and Carter ( 1984).

For propene,

ko3 (propene) = (1.22 f 0.15) X lo-” cm3/sec

has been used as an average of 9 literature values (Atkinson et al., 1982). The more recently recommended value of (1.13 + 0.45 - 0.22) X lo-l7 cm3/sec for 298°K (Atkinson and Carter, 19 84) agrees well.

The relative determinations of b, have been performed at 295°K and 0.105 MPa. The concentration of ozone was kept constant at 0.1 to 1 ppmv in case of the reactive terpenes by refilling 03 in intervals of 10 min. The evaluation has been performed after Cox and Sheppard ( 1980), the correlation coefficients of the plots being >0.98.

For the less reactive alkenes, [OS] of 1 and 5 ppmv have been used. The starting concentrations [Test]0 were in most cases about 1 ppmv.

The results are compiled in Table 2. The partial tropospheric half-lives are in the range between about 30 min (limonene) and more than 2 months (vinyl chloride). The data compare favorably with literature values (294-300°K) reported by Atkinson and Carter (1984) [cm3/sec]: 2.1 and 3.6 X IO-” (/3-pinene); 1.2 X lo-l6 (A3-carene); 6.4 X lo-I6 (limonene); 2.3 to 2.5 X lo-l9 (vinyl chloride), and 8.1 to 8.4 X lo-‘* (butadiene).

3.1, General

3. DIRECT PHOTOGHEMICAL REACTIONS

The rate coefhcient khv of a direct photochemical reaction at low absorbance is given by

314 KL6PFFXR ET AL.

where

khu = s

&)i(x)@(x)dx, (13)

a(X) = Absorption cross section, base e [cm*] as a function of wavelength mm] I(X) = Spectral photon it-radiance [photons set-’ cme2 nm-‘1 as a function ofwave-

length 40) = Quantum efficiency of disappearance of test substance.

If the quantum efficiency is independent on the exciting wavelength and the inte- gration can be approximated by a summation over appropriate intervals, ~q. (13) is simplified to

khy = q5 ; &@(X)AX, (14) Xl

where

$X1= Absorption cross section at the center of the wavelength interval I= Spectral photon irradiance averaged over the wavelength interval chosen for

summation.

The summation extends over the whole overlap region of spectral n-radiance and absorption (X1 to X2).

One of the simplest direct photochemical reactions is photolysis which consists in the fragmentation of a molecule by breaking a bond after absorption of a photon. Reactions of this kind have been used throughout this work in order to produce OH radicals (photolyzing HN02 or H20$ or to measure the average UV irradiation intensity (NO2 photolysis). The overall kinetics in most cases are complicated by secondary reactions of the reactive radicals produced in the bond-breaking primary reaction. Due to the high dilution of chemicals in the atmosphere these reactions seem not to be so important for the first step of phototransformation of air pollutants; khpl according to Eq. ( 13) or ( 14) only refers to this primary, genuinely photochemical process.

Studying the direct photochemical reactivity of organic gaseous substances is best performed with monochromatic radiation > 300 nm (OECD, 1987; ECETOC, 1983). Knowing the quantum efficiency and its wavelength-dependence (if there is any), khy can be calculated for a whole range of natural or artificial spectral photon ii-radiances using either Eq. (13) or (14). b-radiance data relevant for central Europe have recentIy been presented by Frank and Klijpffer ( 1986). Theoretical solar irradi- antes for the lower troposphere have been published by Demerijan et al. ( 1980).

Alternatively, the disappearance of a compound during illumination with LJV/VIS radiation similar to a natural one can be measured, yielding an approximate (partial) lifetime of the substance tested. Knowing the spectral irradiance in the chamber (Figs. 2, 8) and assuming that the quantum efficiency is independent on the wavelength, the quantum efficiency can be determined also from this kind of experiment using Eq. (14); khu in this case is the reciprocal of the experimentally observed lifetime or hi2/(ln 2).

3.2. Experimental and Results In these measurements it is of paramount importance that any trace of NO, inside

the chamber has to be avoided. Otherwise invariably OH radicals (and ozone) are


f%G. 8. Absorption spectrum of benzophenone-u(X)-measured in hexane and spectral inadiance inside the smog chamber-I’(X)-averaged at lo-nm intervals.

formed as described in Section 1. In order to monitor the absence of OH and Or, a substance which easily reacts with these species but does not absorb > 300 nm has to be added as an OH/O3 detector: no measurable decay of this substance (e.g., propene) indicates suitable reaction conditions. Problems may arise if a significant fraction of the substance resides at the (illuminated) walls.

We are not aware of any systematic study performed in order to explore the limits of the method with regard to vapor pressure, k hv, etc. In the following, therefore, preliminary results obtained on one substance, benzophenone, are presented.

Benzophenone, p = 0.226 Pa at 305°K (DeKruif et al., 1983) absorbs in the near- UV due to a transition which is mainly localized in the carbonyl group. The spectrum in hexane (Fig. 8) is supposed to be very similar to the not yet measured gas phase spectrum. Benzophenone has been selected as a test substance on account of its well- known photochemical reactivity in solution, very little being known about its gas phase reactivity (Turro, 1978; Calvert and Pitts, 1966).

In order to exclude residual NO,, HN03, etc. (NO,,), the chamber has been purified with 0.1 A4 NaOH, washing three times with hot water and once with cold, deionized water. The chamber has been evacuated several times to about 0.0 1 MPa and refilled with synthetic air.

Two irradiation experiments, using toluene and propene, respectively, as OH detector, yielded approximately exponential decay of benzophenone with a half-life t1/2 = 200-400 min. Unfortunately, the determination of the concentration of benzophenone by mass spectrometry was not accurate enough to give a more precise result. Therefore, deviations from simple first-order kinetics cannot be excluded.

316 KLiiPFFER ET AL.

The concentrations of toluene and propene did not decay over the whole experiment, indicating the absence of any measurable amounts of OH (and ozone in the case of propene). The experiments showed that it is possible to measure khu in a clean “smog chamber,” prevented to behave as such! Assuming first-order kinetics in our experiments,

khy (benzophenone) = 10m5 to 1O-4 set-‘.

The primary reaction is most likely a Norrish-I splitting to give phenyl and benzoyl radicals. The quantum efficiency can be estimated with Eq. (14) to be in the order of

$J (benzophenone) - 5 X 10d3.

For the sake of comparison, the quantum efficiency of photochemical disappearance of acetone can be taken into account:

(b (acetone) = 0.077 (Gardner et al., 1984) at 298°K and 0.10 1 MPa.

This quantum efficiency has been shown to be that of the primary photodissociation into CH3C0 and CH3 (Gardner et a/., 1984). The atmospheric chemistry has recently been reviewed by Carlier et al. (1986).

CONCLUSIONS

In this paper, the possibility of measuring the photochemical-oxidative degradation of chemicals with vapor pressures down to 1 Pa (in favourable cases down to about 1 cPa) in a single, medium size smog chamber has been demonstrated. Due to its leading role in photodegradation and transformation, the reaction with OH has been investigated most thoroughly. Recently, the importance of higher nitrogen ox- ides, especially of N03, has been shown for atmospheric reactions during the night (see Finlayson-Pitts and Pitts, 1986). It is recommended that test procedures for these processes should be developed in order to expand the range of methods available in a standardized form (OECD).

It is an assumption inherent to most testing procedures that the disappearance of the test compound finally leads to complete mineralization. Although this assumption may be justified in most cases, some substances may be transformed into more stable products potentially even more dangerous to the environment than those ini- tially released. This is especially to be expected, if electron-withdrawing groups, e.g., NO*, are introduced into the parent molecule, making the transformation product more stable toward OH and 03. If there is an indication of the formation of stable transformation products, a product analysis of the reaction mixture should be performed.

Another limit constitutes the low vapor pressure of most SOC which may partly occur in the free gas phase down to about pz5 = 10e6 Pa (Junge, 1975; Gill et al., 1983). For these cases, not measurable in the gas phase, liquid phase tests are being developed (Kltipffer, 1980; KlSpffer and Frank, 1986; Dilling, 1986) to be included into the OECD Test Guideline.

The fate of chemicals adsorbed at the aerosol has to be tested separately. The method of determining the direct photochemical reactivity outlined in this

paper deviates from suggestions made to OECD (1987). It still has to be improved considerably and tested with many more substances. It may turn out useful for less volatile substances, as in the case of our example. Calculating khy according to (13)


and (14) and assuming a quantum efficiency of unity only help decide whether an appropriate experiment should be done at all. It does not give, however, any useful estimate of the (partial) lifetime of the substance under tropospheric conditions.

ACKNOWLEDGMENTS

The work presented here was performed under contract of BMFT, Bonn, project management “Environ- mental Chemicals” KFA, Jiihch. We thank ah members of the project management, the project referees, and the other working groups within the project for good cooperation and many stimulating discussions. We also thank Dr. Boxhammer and Mr. Wecht from W. C. Heraeus/Original Hanau for excellent cooperation during the construction of the smog chamber.

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testing of the abiotic degradation of chemicals in the atmosphere: the smog chamber approach

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