AUGUST, 1935

Light Absorption in the Atmosphere and Its Photochemistry*

OLIVER R. WULF, Bureau of Chemistry and Soils, Washington, D. C.(Received May 25, 1935)

S UNLIGHT, as it penetrates into the earth's'atmosphere, is absorbed first at very short

wavelengths. The chief molecular constituents ofthe high air,1 hydrogen, nitrogen and oxygen, arearranged in the earth's gravitational field in thisorder with descending height, and absorb, re-spectively, to longer wavelengths, namely, hy-drogen beginning at about 1100A, nitrogen at1450A and oxygen at 1850A. None of these gasesabsorb in the infrared or in the visible, except forweak absorption by oxygen. Thus for some dis-tance into the atmosphere the solar radiationprobably remains pretty much unaffected towavelengths longer than 1900A, unless othergases, produced and maintained by photo-chemical action on these three, show furtherabsorption. Water vapor, while lying principallyin the lower altitudes, probably reaches some-what into the higher ones. It absorbs, however,first below 1800A, and is therefore pretty wellblanketed by oxygen, except for what little mayget to very great altitudes. The small amount ofCO2 in the atmosphere lies close to the earth'ssurface, and aside from its infrared absorption,absorbs first at 1700A. It is thus of no directphotochemical importance, being blanketed bythe gases above. Both CO2 and H20 absorbstrongly in the infrared, of course, but infraredabsorption, insofar as it is simple vibration-rotation absorption, produces no photochemicalchange, only heat. Thus one must look to thegases hydrogen, nitrogen, oxygen and possiblywater vapor as the original substances from whichother absorbing gases may be produced byphotochemical reaction.

The radiation available for this purpose con-sists of the quanta coming from the sun in thedeep ultraviolet, a region inaccessible to directstudy. In lieu of direct information we may

* Presented by invitation at the Symposium on Atmos-pheric Optics held under the joint auspices of the AmericanPhysical Society and the Optical Society of AmericaFebruary 22, 1935.

1Humphreys, Physics of the Air, (McGraw-Hill BookCo., New York, 1929).

assume the ultraviolet radiation of the sun to bethat given by a Planck curve for 6000'K, whichis fairly well approximated out to 3000A. How-ever, deviations from this, such as might occurbecause of general ultraviolet absorption in thehigh solar atmosphere or an excess of ultravioletemission, could affect radically photochemicalconsiderations in the earth's atmosphere. Con-sidering the radiation as black body, however,Fig. 1 gives some idea of how this is distributed.It is a Planck curve for 6000'K but drawn insomewhat unusual units, namely, in quanta in-stead of ergs, since in photochemical processes itis of course the number of quanta that is ofinterest. The number of quanta falling on asquare centimeter of earth's surface per secondper unit frequency band is plotted against fre-quency. The ordinates correspond to abscissaevalues in true frequency units (sec.-'), but theabscissae are indicated in terms of the morecommon wave numbers (cm-'). In estimating thenumber of quanta within any frequency intervalby means of the area under the curve in thisinterval, the given ordinates must be used withthe true frequency units. The curve conveys littleas an illustration other than the fact that theintensity falls away very rapidly in the ultra-violet. Beyond 2000A the rate of decrease is sogreat that it is represented logarithmically in theupper curve for the rest of the way. The regionsabsorbed by H2, N2, and 02 are roughly indi-cated, and of these, accurate and extensivemeasurements are available only for oxygen.2 It ischiefly to radiations in these regions that we mustlook for the photochemical production of newsubstances. These new substances will be pro-duced over a range of altitudes where the activeradiation is absorbed. They may of course betransported also to other altitudes, but it is ofprimary importance to understand where theyare formed.

Because of the approximate exponential dis-tribution of the gases in the atmosphere, the

2 Ladenburg and Van Voorhis, Phys. Rev. 43, 315 (1933).

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J. . S. A. VOLUME 25

OLIVER R. WULF

Frequency (cm-,)

i

cf

41

N

Ye

~E

16

Is

a

10,000 20,000 30,000 40,000 50,000

Frequency (cm-')

FIG. 1. Distribution of solar radiation at the earth assuming the sun to be a black body at 6000'K.

absorption of radiation of one wavelength isrestricted to a relatively narrow region. Thatpossessing a large absorption coefficient will beabsorbed at high altitudes. But neighboringwavelengths carrying the same or even greatertotal energy but of different absorption coeffi-cients will be absorbed at other altitudes, and inparticular those having smaller absorption co-efficients penetrate deeper and will be absorbed atlower altitudes. For the case of a photochemicalsteady state, where one set of wavelengths leadsto the formation of a new molecule and anotherset of wavelengths absorbed by this new moleculeleads to its destruction, this distribution ofabsorption is very important.3

Where the absorbing gas is distributed ex-ponentially with height, as in a gravitationalfield, the fraction of the initial intensity absorbedper unit height is the same for all values of theabsorption coefficient, except for a transpositionin height. Thus Eqs. (lb) and (3a) on page 260 ofreference (3) may be written

3Wulf, Phil. Mag. (7) 17, 251 (1934). This article con-tains a treatment of such photochemical steady states.

(1/I.). (dI/dz) = a noe-Ie-anocZ/P,

where ne-Pz (molecules/cc) expresses the dis-tribution in height of the absorbing molecularspecies and a is the molecular absorption co-efficient of the particular frequency of radiationbeing considered. This is the fraction of the initialintensity which is absorbed per cm of height. Ithas its maximum' at Zmx= -(I/p) log (p/lno)and the constant value of p/e at this maximum.The rate of increase in the height of this maxi-mum with a is equal to 1/pa. Thus the curve ofthe fraction absorbed per unit height remains thesame with changing a except for transposition inheight. This fact was overlooked in the argumentof reference (3). In Fig. 2, simply as an illustra-tion, three such curves are given, computed forthe values of a indicated and a distribution of theabsorbing gas = 1.57 X 1018e-1.73X106z z beingthe height in cm measured from z= 0 at a heightabove the earth's surface of L.1X106 cm, thisdistribution being practically that for atmos-pheric oxygen given in Table II, page 70 ofreference 1. The area under each curve is, ofcourse, unity. (In Fig. 2 k is used for a.)

232

LIGHT ABSORPTION IN THE ATMOSPHERE

The distribution of absorbed radiation may befairly layer-like, if the curve of the absorptioncoefficient possesses a rather flat and broad topand has steep sides. But the new molecule formedin a photochemical steady state will not ingeneral be distributed at all like this but will bebroadly spread out owing to its formation atmany altitudes 3 until new means of decomposi-tion come into play, such as occur at the surfaceof the earth. Moreover, curves of absorption co-efficients usually do not possess broad and flattops with steep sides, but, rather, rise steadily toa definite maximum and then decrease again.That this is ordinarily to be expected can be seenqualitatively from simple considerations ofcommon molecular potential energy curves andthe Franck-Condon principle. What is probablythe most important absorption in the atmos-phere, that due to oxygen,2 possesses this latterform of curve.

This distribution of the product is of impor-tance only where absorption takes place in longpaths with intensities of the same order of mag-nitude over a range of wavelengths embracingwidely different absorption coefficients. But justthese conditions obtain in atmospheres,3 bothstellar and planetary. In the latter case, for thesame ratio of absorption coefficients but withdecreasing absolute values, the concentration ofthe new product at its maximum increases, and

140

120

= 1.85 10 1100

80

60

40

20

0 20 40 60 80

Fraction of initial intensity absorbed per cm x 108

FIG. 2.

the height of this maximum falls. The rates of theprocesses maintaining the steady state will ingeneral vary with altitude, depending upon thecurve of the absorption coefficient. For the caseswhere the producing radiation is completelyabsorbed and the new molecule concentration isalways small compared with that of the originalmolecule, the total amount of new moleculeformed by the two radiations increases with de-creasing value of the absorption coefficients.Where, moreover, the absorbing gas is distributedin a gravitational field, such as in the earth'satmosphere, this increase becomes very great.Where the steady state is maintained by veryslow processes, we would expect it to be mosteasily disturbed and to show variations. Further-more, it is clear from the above that the region ofhigh temperature due to this chemically activeabsorption is not to be expected where the newmolecule is, but where the processes of formationand decomposition proceed at the highest rates,after the heat capacity per unit volume has beenduly taken into consideration. This may ingeneral be at high altitudes, quite distinct fromthe average height of the new substance pro-duced.

At very high altitudes absorption can produceonly dissociation, ionization or fluorescence, sincethere are too few collisons to yield reactionsleading to molecule formation. Here also, how-ever, there are some points worthy of note. Thecharacters of the absorptions differ. Oxygen, ascontrasted with nitrogen and hydrogen, possessesabsorption leading easily to dissociation. This canbe seen from the diagrammatic representation ofthe potential curves of the oxygen and nitrogenmolecules given in Fig. 3. The absorptions ofinterest here are in each case transitions from thelowest curve to the uppermost one. Transitionsfrom the lowest curve to the middle one are veryweak in oxygen (giving rise to the so-called"atmospheric bands"), and are as yet unknown inabsorption in nitrogen. The chief absorption dueto these gases at ordinary temperatures comesfrom regions of the lower curve in the immediatevicinity of the minimum passing to the upper-most curves on paths spreading only a little oneach side of the vertical. Thus in oxygen thesetransitions are to points on the uppermost curvewell above the level of dissociation. Oxygen

k = 3.52 x 10

k = 352 x 1023

I I I I

233

OLIVER R. WULF

16

14

I 0

.n

4

2

0

16

14

10

0

1.00 1.40 .1.60 Z20 2.60 3.0 .00 1.40

r (cm x O-')

FIG. 3. Diagrammatic potential energy curves.

atoms should be relatively prevalent, therefore,at high altitudes. With nitrogen, dissociation ismuch less likely, the absorption resulting chieflyin simple excitation. Since hydrogen may not bepresent in large absolute amounts, and only atvery high altitudes, we may consider nitrogen andoxygen alone, though hydrogen is more similar inthis respect to nitrogen.

It is interesting that oxygen atoms and ni-trogen molecule ions are conspicuous thingsobserved in the light of the night sky and oftwilight. The green line and probably the two redlines due to oxygen atoms are prominent radia-tions observed4 in the light of the night sky.These are normally forbidden transitions, being'So to 'D2 in the first case and D2 to P2 , 1 in thesecond case. However, absorption of sunlight,since they lie in the most intense region of thesolar radiation, might account for step-wise exci-tation of these in large amounts. One might raisethe question as to whether it is stretching the life-time of these metastable states much too far to

4 Slipher, Trans. Am. Geophys. Union (1933), page 125.

suggest that they then slowly come back duringthe night. The mechanism of the emission of theoxygen atom lines in the light of the normal nightsky is a matter of much interest. The nitrogenmolecule ion radiation is, on the other hand,a permitted transition, a 2 to 2Z transition, and itis observed in the absence of aurora only whenthe first or last traces of sunlight touch the highatmosphere.4 It seems thus that it is a fluores-cence phenomenon, due to the absorption ofsunlight by the N2+ ion produced in the highatmosphere photochemically.

Deeper in the atmosphere collisons becomefrequent enough so that molecule formation canoccur in appreciable amounts. The outstandingexample is the reaction of ozone formation. Thisreaction brings into existence the possibility ofthe opposite photochemical reaction that de-stroys ozone, because of the absorption of otherlight by it.' Since, in sunlight, there is radiation in

5 For the present status of the results on the amount anddistribution of atmospheric ozone see Ladenburg, J. Opt.Soc. Am. in press.

I I I I I I°

02

.1 I!A DI I I IL A

r (cm x 10-8 )

234

8

LIGHT ABSORPTION IN THE ATMOSPHERE

both the region of oxygen absorption and ozoneabsorption, a photochemical steady state isset up.

In this steady state, owing to the importance ofweakly absorbed radiations which penetrate tolow altitudes, the ozone should be distributedover a wide range of altitudes 3 as has been indi-cated above. Possibly it is brought to low valuesat low altitudes only by new methods of de-composition entering, such as chemical reactionwith the constituents of the earth's surface.Ozone is a very reactive gas, being presumablyrapidly removed by contact with vegetated orpopulated portions of the earth's surface. It mayalso be that there exists no appreciable amountof ozone-forming sunlight which possesses anabsorption coefficient sufficiently small to pene-trate to the surface of the earth.

An important photochemical effect also tend-ing to raise the amount of ozone in low altitudesis the pressure-dependent oxygen absorption. 6

This is something more than weak absorptionoccurring at low altitudes. This absorption de-pends upon the square of the oxygen pressure andupon the presence of other gases and henceincreases much more rapidly than simply theexponential increase of the oxygen. It, further-more, leads to ozone formation. There occurs,therefore, a region where considerable amountsof ozone-forming radiation are absorbed at quitelow altitudes. 7 The troposphere is a stirredatmosphere and hence this ozone may be ob-served even at the ground. Furthermore, this lowpart of the ozone produced by this pressure-dependent absorption, sometimes interpreted asdue to the molecule 04, should be dependentupon atmospheric pressure conditions, that is tosay, upon weather conditions, as well as possiblyupon geographical latitude.

The 04 or pressure-dependent oxygen absorp-tion can account7 for the failure of all but verysmall amounts of radiation' to get through the2100-2200A region between the ozone and the

6 Salow and Steiner, Nature 134, 463 (1934); Wulf,Proc. Nat. Acad. Sci. 14, 609 (1928); J. Am. Chem. Soc.50, 2596 (1928). In these papers references will be foundto most of the other work on this subject.

7 Wulf, Phys. Rev. 41, 375 (1932).8 Meyer, Schein and Stoll, Helv. Phys. Acta 7, 670

(1934).

main oxygen absorption. It is of interest, how-ever, to inquire what other absorbing substancesmight also be present. Since small amounts ofwater vapor presumably reach to high altitudes,the absorption of this substance must be con-sidered. Radiation in the region of 1600-1700Adissociates water into H and OH. There thenexists the possibility of hydrogen peroxide. Thisabsorbs continuously across the 2100-2200Aregion.' Existing photochemical evidence indi-cates, however, in the opinion of the writer, thatthe photochemical formation of hydiogen perox-ide is not apt to occur to any great extent espe-cially at high altitudes.

The results of Gotz and Maier-Leibnitz10 indi-cate that there is banded absorption in the sur-face layers beyond the ozone maximum, due tosomething further than ozone and oxygen. Itbecomes an important problem to determinewhat the gas or gases may be that lead to this.They may be of surface origin, but it is perhapsimprobable that they are due to artificial originin the high altitudes of Switzerland. It may benecessary to look for some other photochemicallyproduced atmospheric constituent.

Of the possibilities mentioned earlier an oxideof nitrogen seems most probable. The absorptionof nitrogen in the ultraviolet could lead tochemical reactivity with oxygen, though there isas yet no direct experimental demonstration ofthis photochemical effect. It might be inappre-ciable at very low pressures. But there is alsophoto-ionization to be considered as a means ofinducing chemical reaction. It seems probablethat the absorption which leads to ionizationbeyond the limit of the molecular electroniclevels is not unusually strong, and it would berather surprising to find it much stronger, or evenas strong, as the maximum oxygen absorption at1450A. In other words, it seems probable thatradiation producing photo-ionization penetratesreasonably deeply. An ionized layer can bemaintained, of course, only where the pressuresare so low as to make recombination very infre-

9 Urey, Dawsey and Rice, J. Am. Chem. Soc. 51, 1371(1929).

10 Gotz and Maier-Leibnitz, Zeits. f. Geophysik 9, 253(1933). See also Chalonge and Vassy, J. de phys. et rad. (7)5, 309 (1934).

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OLIVER R. WULF

quent. But the ions produced in the deeper alti-tudes can lead to chemical reaction after themanner studied by Brewer" and by Lind'2 withtheir co-workers. Briefly, one ion leads to theformation of the order of one new molecule inseveral simple reactions where chain mechanismsdo not enter, for instance, of nitric oxide, in thecase of a nitrogen-oxygen mixture. This wouldseem to be also a possible source of nitric oxide,which gas would be fairly stable photochemicallyat low pressures. At the higher pressures in lowaltitudes where radiation below 2000A might leadto its decomposition, it is considerably shieldedby oxygen. In the presence of oxygen it could beoxidized to nitrogen dioxide and in the presenceof ozone to nitrogen pentoxide. The possibility ofthe existence of such gases, especially ozone andthe oxides of nitrogen in the presence of oneanother, can hardly be judged from laboratoryexperience because of the entirely differentorder of concentrations here concerned, and theuncertainty of the conditions of the steady state.

Both nitric oxide and nitrogen dioxide possessbanded absorption beyond that of ozone. How-ever, NO and NO2 unite to a small extent toform N2 3

13 which possesses a strong continuousabsorption, covering the region between theoxygen and the ozone absorption. The concen-trations of NO and NO2 would be very small

11 Brewer, et al, J. Phys. Chem. Papers I to XIV onChemical Action in the Glow Discharge. From 1929 to date.

12 Lind, Chemical Effects of Alpha Particles and Electrons(Chemical Catalog Co., New York, 1928).

13 Melvin and Wulf, Phys. Rev. 45, 751 (1934).

and of N203 very much smaller, but, as in thecase of ozone, in long paths might be perceptible.

It is interesting that with water vapor thissubstance N203 causes another new absorption inthe near ultraviolet, a group of diffuse bands, dueprobably to the molecule HNO2.'4 It would beinteresting to compare the transparency of thelayers near the earth in the ultraviolet 2150Aregion with the humidity to see if there is anycorrelation between water vapor and trans-parency in the 2150A region.

In conclusion it may not be out of place topoint out that ozone (and nitric oxide also ifpresent) may conceivably exercise an importantbiological effect, despite its low concentration.The atmosphere, being constantly stirred againstthe surface of the earth, probably suffers therapid removal of such chemically active gases byplant and animal life. That ozone exists at all inthe lower atmosphere would seem to indicate thatits rate of production in the steady state is rela-tively high. It may, therefore, be that the rateof supply of such gases makes them biologicalfactors of some consequence despite their lowconcentration, and this possibility in itself wouldmake desirable an investigation as to theiramounts, their variations and their geographicaldistributions.

14 Results obtained by Dr. Melvin and the writer, nowbeing prepared for publication. These indicate that thebanded portion of the above spectrum is due to a complexinvolving N203 and H20, not to N203 alone as originallybelieved.

236