Rad. and Environm. Biophys. 11,281--288 (t975) © by Springer-Verlag 1975

Experiments on the Formation of Particles by Chemical Reactions in the Dark and under Influence of Light* S. R a s e a n d J. P o r s t e n d ö r f e r Strahlenzentrum, Institut für Biophysik, Universität Gießen Federal Republie of Germany Received September 25, 1974

Summary. Some experiments concerning the photochemical production of condensation nuclei are described. Preliminary measurements of filtered atmospherie air, initially free of particles yielded high concentrations of particles by reactions in the dark when the air was previously irradiated by sunlight. In further investigations a definite composition of pure gases was used. The formation of nitric acid particles from NO 2 in pure nitrogen of different relative humidities in the dark and under influence of light was investigated. No particle formation was found which could be correlated to any production of nitric acid nuclei. Even within a spectral region in which photolysis of NO 2 takes place no HNO3-nucleation could be found. The particles detected under certain eonditions of irradiation originate from impurities in the walls of the reaction chamber. Particle growth in an irradiated mixture of 1~2 and lqO 2 with benzene is demonstrated and the mean radius ofparticles is calculated from measurements with a diffusion battery. Introduction The increasing a t m o s p h e r i c p o l l u t i o n b y emission of p o s s i b l y deleterious i m p u r i t i e s - - t h e i r origin a n d p r i m a r y c o m p o s i t i o n are sufficient]y k n o w n - causes m a n y different p h y s i c a l a n d chemical reactions. The resulting t y p e of i m m i s s i o n is of g r e a t i m p o r t a n c e for t h e influence t o t h e biosphere. The v a r i e t y of processes can be seen from t h e fo]lowing scheine:

I. Emission D u s t f r o m dispersion processes, liquid a n d soli4 p a r t i c l e s from c o m b u s t i o n , gaseous i m p u r i t i e s (NOx, SO S, etc.), r e a c t i v e h y d r o c a r b o n s . P u r e air, considered as carrier gas, is l o a d e d w i t h all these i m p u r i t i e s a n d r e p r e s e n t s a m i x e d p h a s e of solid, liquid, a n d gaseous c o m p o n e n t s .

I I . Reactions in the Atmosphere 1. Chemical: a) U n s e n s i t i z e d reactions, i.e. r e a c t i o n s b e t w e e n r e a c t a n t s due t o t h e i r chemical affinities. b) Sensitized reactions. Photoly~ic r e a c t i o n s effected b y s u n l i g h t cause t h e p r o d u c t i o n of radicals which i n i t i a t e a series of successive chemical processes, e.g. t h e p h o t o l y t i c dissociation of 1~0~ p r o d u c e s a t o m i c o x y g e n as well as ozone. * Dedicated to Prof. Dr. A. Schraub on the occasion of his 65th birthday. 19

Rad. a n d E n v i r o n m . Biophys., Vol. 11

282

S. Rase and J. PorstendSrfer

Especially in the presence of hydrocarbons a wide spectrum of reaction products arises [8, 9, l i ] . 2. Physical: Formation of condensation nuelei (Aitgen nuclei), agglomeration, log formation. Solubilization of particles into water droplets and solubilization of gaseous components into particles and droplets. Catalytic proeesses on partieles favouring reactions due to their microporous structure. The colloidal particles of haze also ean serve as eenters of reaction [5, 6]. I I I . Immission All reaetions mentioned above result in a composition of more or less toxic components in the environment affecting our biosphere by deposition oB these components in and on plants, animals and man due to washout, rainout, sedimentation, diffusion, and adsorption in the ground region. Equilibrium exists within the chain of environmental events Brom emission of substances up to the final deposition. I t taust be emphasized that particle formation by chemical reactions in the atmosphere, especially the photochemical processes in the sunlight, represents an important source of very small particles (Aitgen nuclei, mean diameter of about i0 -a ~m), whieh ean grow by agglomeration. This eauses a spectrum of partieles in a colloida] phase in the range from l0 -3 ~m up to about i ~m. Of special interest for our investigations are detailed problems of partiele formation by chemieal reactions in the dark and under influence of light, the variation of partiele size and partiele preeipitation under various conditions. Some preliminary experiments are reported. The performance of them was stimulated by the methods and results oB Bricard et al. [1, 2, 3].

Experimental Performance 1. Dosage o/Gas. In Fig. i the experimental e q u i p m e ß is given schematically. The gas atmosphere to be investigated in respect oB nuclei formation was confined in a PVC chamber of a volume of about i m a. The chamber was fiUed either with air or a pure carrier gas passing through two aerosol filters (Sch]eicher & Schüll, Cellulose Asbest) in order to prevent particles from getting into the chamber. Tracing with additional gases (ppm-range) was possible by means of a bypass. We used pure nitrogen (N 2 reinst, Messer Griesheim) with a composition of 99,99 % •2, 50 vpm 02, 30 vpm H20, 20 vpm noble gases. Relative humidity was maintained by wetting part of the N S in a bypass. NO~ was obtained, after stoichiometrieal calculation by heating of lead nitrate p.a. : Pb(NO3)2 -~ PbO~ ÷ 2 NOt. Benzene used as hydroearbon was vapourized in the N~-stream. After adding the trace gases the eontent oB the reaction chamber was stirred shortly to obtain a homogeneous mixture. For all experiments a lownitrogen flow through the reaction ehamber was maintained resulting in a slight overpressure of nitrogen in the chamber, preventing infiltration of unwanted impurities through leaks. 2. Irradiation source was a high pressure Xenon arc lump (XBO 450, Osram), and the window was quartz glass Suprasil (Heraeus). Suitable edge filters (Sehott) were used.

283

Partic]e Formation by Photochemical Reactions Reaction chamber Ouarz

Air N2

Hydro-

I

carbon

I

" ~

Aero,olfilter

\

',(.\

-'~" .

----J

~.

T L--

/'-l'-opti¢~,~/

[ Filter

Diffusion battery Nuclei counter -

Fig. 4. Experimental equipment for dosage and irradiation of gas mixtures and detection of condensation nuclei

3. The particle number coneentration was measured with a condensation nuclei eounter (General Eleetric), which was ca]ibrated with a Scholz counter. Detection of particles bigger t h a n t0-3Bm in diameter was possible (according to the manufaeturer). The mean particle size was calculated [7] flora measurements with a diffusion battery. Results

1. Measurements o/Outside Air Outside air was sucked into the reaction chamber during several hours b y means of a centrifugal p u m p (12 m3/h). Prefiltration prevented particles and nuclei flora getting into the chamber, which was checked with the nuclei counter. Therefore detected nuclei taust have originated in the chamber. The experimental equipment was kept in the dark. A part oB the air stream ]eaving the chamber was tested eontiuuously with the nuclei counter in order to detect particles. The concentration of the detected partic]es varied strongly with daytime (Fig. 2). 2. Measurements with .Known Gas Composition a) Nitric acid/ormation. Because of the undefined conditions deseribed under t. whieh do not allow to describe kind and composition of the reactious, further measurements with pure nitrogen as earrier gas were performed. Bricard et al. had already investigated the formation of particles in pure nitrogen with the addition of ~TO2 and SO S [2]. Concerning the formation oB nitric acid particles we made some experiments using the following compositions: pure ~T~ of different relative humidities, and pure N Swith i0 p p m NO2 of relative humidities ranging flora _< 2% up to 80%. Dark experiment: I n none of the dark experiments concentrations of nuclei above the baekground of the device were detected. 19"

284

S. Rase and J. Porstendörfer

10 5

I

T

17.7.73

II Il II

31.7.73

10 '~

I IWI ~ Iflll

.$ Z

lO 3

711 IR~I 'IIIIII'~I ',,~IHIA!

~_~U~__ Time of the day

9

I

I

l

i

i

i

i

i

i

i

12

15

18

21

24

3

6

9

12

15

I

18 hours

Fig. 2. Continous registration of condensation nuclei in filtered air irradiated by sun]ight and air in the night, respectively. Peaks of condensation nuclei formed by succeeding reactions in the dark chamber

Irradiation with light: Irradiation of the gas mixture with long-waved U V and visible ]ight using the X e n o n are lamp with edge filter W G 320 n m initiated no formation of nuelei when the relative h u m i d i t y was less t h a n 20 %. Increasing the relative h u m i d i t y up to 80 %, concentrations of particles up to 104/cm 8 could be deteeted. The reason for the observed 10article formation could be the influence of impurities, acting in the presence of higher eoncentrations of water vapour, or the formation of nitric acid at sufficiently high h u m i d i t y of the gas. As long as this kind of partiele formation was still unknown, the quantitative relation between particle concentration and relative h u m i d i t y was not determined. The further question concerning this mode of particle formation was the participatioa of NO ÷ 0 from photolysis of NO 2 during irradiation. Scrambling experiments with the oxygen isotope 180 using NO2-1sO~-mixtures have shown t h a t the q u a n t u m yield for the formation of O 2 after photolyse of NO2 ~ vanishes near the wavelength of $ = 435.8 n m [4]. T h a t means t h a t under the pre-mentioned eonditions oB irradiation NO~-photolysis in the reaction ehamber can oecur. During the following investigations the spectral conditions of irradi~ting the gaseous mixture were varied. Using different edge filters the shorter end of the spectrum 1 Serambling of 18--1802 tO 18--1602can Occur by the following way: NO 2 + hv-~ NO + O; O + 02 ( + 1K)~ 03 ( + M) ; 03 + NO -~ NO~ + 02.

Particle Formation by Photoehemical Reactions

~X

0.9

~

X

285

~ X

0.5

"----~ ~"~. \ - -

~02

p

--

\ - -

\

x

-'

0.1 I

I

I

300

340

-I

I

~

l

I

420

380

100

/

/

50

I I

/ 10

./

/

I

/

I ~°

300

340

/

,

380

X

,

_

420 nm

~'ig. 3. Upper part: qSo." = quantum yield for ls01«0 formation after photolysis of NO, and serambling of isotopes (see footnote 1). ~0= yield of partiele formation in the speeial experiment (arbitrary units). Lower part: Normalized spectra of irradiation, a = short-wave region emitted from Xenon art lamp. b = with edge filter 320 nm. c = wich edge filter 360 mn

was out oft stepwise from 320 n m to 440 nm. While testing the gas mixture at a n y step of irradiation no further particle f o r m a t i o n was f o u n d when wavelengths shorter t h a n 360 n m were eliminated. The absenee of particle formation also at high humidities, when irradiated with wavelengths only longer t h a n 360 nm, fell into a spectral region in whieh NO 2 photolysis takes place with a good q u a n t u m yield (sec Fig. 3). I t was therefore coneluded t h a t the formation of particles below 360 n m is due to impurities present within the chamber. b) Benzene: W i t h regard to a smog simulation wc used a mixture of pure N » t0 p p m NO 2 and 10 p p m benzene with a relative h u m i d i t y of 70%. W h e n kept in the dark, no formation of condensation nuelei could be found in the system. B u t during irradiation (XBO 450, edge filter 320 nm) partiele formation with a m a x i m u m eoncentration of a b o u t 8 . t0õ/cm 8 was deteeted. Fig. 4 a shows the deerease of particle eoneentration _hrchin the ehamber and the eoneentration Nd~r after passing of the mixture t h r o u g h the diffusion b a t t e r y as a funetion of time. Fig. 4 b shows the effieieney

l = (i-

Ndi~z~. 2V« / 100

of the diffusion battery, ealeulated ffom the values of Fig. 4a. F r o m this efficieney the mean radius of the particles as funetion of time was determined (see Fig. 4 e).

S. Rase and J. Porstendörfer

286 106

r•xNCh .

N2 +lOppm NO2

~

.lOppm Benzene

X~X~x

HR = 70%

10 5 NDiff

~x.~.X~x~

"

lO 4

~#t *\, ~°I

\~~x ~

50

xx

l

30,

l

l

l

l

i

l

|

.

.

.

l

l

f

'

i

.

.

.

.

.

2

"~ 1.0

-6

n-

6

10

rt ! 50

100

.

.

150 minutes

Fig. 4. Particle formation and growth during irradiation of a defined gas mixture. N~~ = nuelei/em~in the reaet~ionehamber, Naln = nuclei/em3 after passing of the gas streams through the diffusion battery. ~ = effieieney (see text). Nean partiele radius R in cm, ealeulated from ~

Discussion

Previously Bricard et al. [i, 2, 3] have reported that atmospheric air initially free of particles by filtration gives rise to partiele formation when activated before in the snnlight. The results obtained by our own investigations confirm these observations. The atmospherie air, irradiated with high intensity of sunlight was cleaned by aerosol filtration. Subsequently in the dark reaetion ehamber high coneentrations of particles could be deteeted. Atmospherie reaction products passing the filters in a gaseons phase are turned into further reaction products resulting in high coneentrations oB eondensation nuclei. Noticeable flnctuations in partie]e eoncentration with distinet peaks at some times of the day m a y be explained by sudden uneontrolled emissions into the atmosphere. Itowever, correlation with distinct sonrces of emission eould not be found. During comparative continuous registration of outer air in the night no particle formation was detected. .The results eoncerning the I-INO3-formation in pure N e in presence of NO 2 and H20 vapour m a y be discussed us follows. No HNO3-particle formation by reaction between NO~ und water vapour can oecur in the dark, eren at high values of relative humidity. With concentrations up to 20 ppm NO~ und rel. hum. 80 % no particle formation in the dark could be deteeted.

Particle Formation by Photochemical Reactions

287

The fact t h a t irradiation of the mixture results in particle formation required a detaited elucidation. The deteeted nuelei either could originate from reactions of impurities or could be the condensation nuclei of hydrates of ttN0a. Bricard et al. [3] eame to the same conclusion and assumed the creation of hydrates of HN03 to be the more likely. Il, in eontrast to the dark experiment, irradiation leads to the formation of ItNO3-aerosol it should oceur mainly in a spectral region in which NO 2 is photolytieally dissociated. I n this ease reactions of atomic oxygen, mixed oxides of nitrogen and water vapour would generate the nitric acid (Bricard et al. [2]). Simultaneously effects of impurities in the chamber might be possible. For preliminary experiments have shown t h a t the irradiation of the chamber only filled with pure nitrogen in a spectral region below 300 n m results in particle formation. Within the above-mentioned experiments, the shorter part of the UV-irradiation was cut oft stepwise b y edge filters with the effect of preventing any particle formation at a limit of 360 nm (see Fig. 3). Nevertheless, under the eonditions of irradiation (spectrum of the Xenon are, cut with edge filter 360 nm) with no further particle formation, NO2-photodissociation in the speetral region of 360 to 440 n m occurs. Since still no condensation nuelei can be detected it m a s t be coneluded t h a t the ereation of nitric neid partieles does not exist at all. Otherwise, irradiation of the wall material in the speetral region below 360 nm resulted in partiele formation b y impurities exhausting ffom the walls into the reaetion volume. Evidence for this consideration could be given b y eliminating the irradiation of the chamber walls. The unirradiated mixture was kept in the dark within a first reaetion ehamber. Flowing through a high intensively irradiated quartz euvette into a second chamber, also kept in the dark, the mixture could be analysed subsequently. No formation of condensation nuelei was found. Finally the behaviour of photochemieally produced particles can be shown b y irradiation of a smog mixture. The reaetions between irradiated NO 2 and benzene within a wer nitrogen atmosphere resulted in the formation of about l0 «nuclei/cm 3. Fig. 4 demonstrates the growth of particles by agglomeration within a time of 90 min. The caleulated mean radius of the particles increases from 0.005 ~m to 0.03 fLm. The agglomeration of such partieles m a y proceed to higher sizes, especially in the free atmosphere. Within the reaetion chamber the observation of particle growth was limited b y the decrease of concentration due to precipitation at the walls and b y dilution due to replacing the nitrogen in the flow system. l~eferences 1. Bricard, J., Billard, F., ~adelaine, G. : Formation and evolution of nuclei of condensation that appear in air initially free of aerosols. J. geophys. Res. 73, 4487--4496 (t968) 2. Bricard, J., Cabane, M., Madelaine, G., Viglia, D. : Production d'aerosols en relation avec la photo-oxidation de l'anhydride sulfureux. Aerosol Sci. 2, 275--280 (1971) 3. Bricard, J., Cabane, M., 1Vfadelaine, G., Viglia, D. : ~'ormation and properties of neutral ultrafine particles and small ions conditioned by gaseous impurities of the air. In: Aerosols and atmospheric chemistry (ed. G. M. l:[idy,) lap. 27--43. New York, London: Academic prss 1972 4. Calvert, J. G., Pitts, J. N., Jr.: Photochemistry, 13. 209. New York, London, Sydney: Wiley 1966 5. Goetz, A., Pueschel, R.: Basic mechanisms of photochemical aerosols formation. Atmosph. Envir. 1, 287--306 (1967)

288

S. l~ase and J. Porstendörfer

6. Goetz, A.: Über den Vorgang der Dunstbildung reaktanter Kohlenwasserstoffe in der Atmosphäre und deren Nachweis. Staub - - Reinhalt. Luft 29, 357--360 (1969) 7. Gormley, P. G., Kennedy, M. : Diffusion from a stream flowing through a eylindrical tube. Proc. Roy. Irish Aead. A 52, 163--169 (t949) 8. Haagen-Smit, A. J., Wa:~ne, L. G. : Atmospherie reactions and seavenging processes. In: Air Pollution, vol. I (eds. Stern, C. Arthur), p. 149. New York, London: Academic Press 1968 9. Jaffe, S. : Photooxidation of ethylene oxide and propionaldehyde in the presence of NO2 and light. Seeond Int. Clean Air Congr. CP I ß , 316--324 (1970) 10. Levy, A., Miller, S. E., Scoffield, F.: The photoehemical smog reactivity of solvents. Second Int. Clean Air Congr. CP lA, 305--3t6 (1970) Dr. S. Rase Institut f. Biophysik I)-63 Gießen Strahlenzentrum Leihgesterner Weg 217 Fed. t~ep. Germany