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Particles in the atmosphere—natural and man-made
PARTICLES
IN THE ATMOSPHERE--NATURAL FAN-JADE*
AND
C. N. DAVIES Department
of Chemistry,
University
of Essex, Wivenhoe
Park, Colchester,
Essex
Abstract-The existence of particles in the atmosphere was established by John Aitken whose work is related to recent studies. The concentration and composition of Aitken nuclei are discussed and contemporary measurements of the output of natural and human sources are reviewed. Larger particles are not adequately described by the Junge size distribution, and do not produce a self-preserving size distribution; they result from a variety of independent sources. Reduction of visual range is achieved most economically by particles of 03-O-4 birn radius. Estimates are quoted for the proportion of matural particles in the atmospheric aerosol.
AITKEN
NUCLEI:
COUNTERS
W. Thomson (later Lord Kelvin) showed theoretically in 1870 that the vapour pressure of smali droplets tended to infinity as the radius tended to zero. In 1875 P. J. Coulier expanded moist air so that it cooled adiabatically and found that condensation as a mist took place. He suggested that the condensation of all droplets of mist occurred on small solid nuclei. Later experiments showed that air which was inactive and would not produce mist, since it was free of nuclei, could be activated by heating and would then form mist when cooled by expansion. This shook his belief in the hypothesis of condensation upon nuclei which, by their finite radii, avoided the difficulty of Kelvin’s formula. He did not realise that heating the vessel generated nuclei from its walls. In 1880 John Aitken showed that steam condensed and formed a mist when injected into a vessel filled with ordinary room air, but that no mist appeared if the air had been filtered. He pursued this observation and demonstrated the reason for Coulier’s loss of fdith in the nucleation hypothesis (1881). By 1890 Aitken had designed and constructed several types of apparatus for counting the number of condensation nuclei in the air, culminating in a portable nucleus counter (Fig. 1) which enabled him to study the concentration under a variety of geographical and meteorological conditions as well as the artificial production of nuclei. In 1888 J. J. Thomson proved theoretically that the large increase in vapour pressure of a droplet with diminishing size, predicted by Kelvin’s formula, did not take place if the droplet was electrically charged. The possibility therefore existed for mist formation to take place upon gaseous ions as well as Aitken condensation nuclei. This was demonstrated experimentally by C. T. R. Wilson in 1895 and led those who doubted the existence of Aitken particles in the atmosphere to claim that the greater number of the nuclei counted were ions. However, Aitken had already demonstrated the existence of hygroscopic nuclei which caused haze in air well below saturation, of nuclei which were pro* Based on a lecture given at a symposium on Air Pollution, of the Loughborough University of Technology. 7-X January
the Leverhulme 1974.
Project
for Education
in Industry
IO70
(‘. h. t>\\lts
duced in large numbers by towns and travelled many miles on the u ind into the unpolluted air of the open country, of nuclei formed from suiphur dioxide bq the action of sunlight in the presence of polluting particles and of nuclei produced bq heating a number of substances.
portable counlel- (IXW). Sampksof a~,-or suitable \LI~UIIIC 211~‘taken m the bore taps. M. They are transferred and diluted with clean air lion1 the tiltcr. I-, bq means of the pump A. to the chamber R which is lmcd with moist blotting p:tpc!-. A raped expansion. Fig. 1. Aitken’s
of one of the 3
of desired amount,
with the pump causes the air in R to cool and water droplets to condense upon the particles. The number in a known volume of the original air can lx found I?? counting the number of droplets which settles on a known xca of the countmp stage In R. using a magnifying lens which is n(lt shown.
Condensation upon most gaseous ions requires a greater fill1 in temperature than is required for Aitken nuclei and there is thus no difficulty in distinguishing between gaseous ions and Aitken nuclei; ions are only activated by a much greater expansion, as shown in Table 1. Soluble nuclei are more readily activated to produce droplets by condensation than insoluble and liquify below saturation unless extremely small. Only the very largest of the insoluble Aitken nuclei can act as condensation centrcs in the open air because the saturation ratio of the atmosphere rarely rises above 1.01. For 60 years the Aitken type of counter persisted as the sole means of studying condcnsation nuclei. Then an important advance came in 1940 with the construction of the first photoelectric counter by Pollak. The mist was produced by expansion. as in the original
Particles
in the atmosphere
1071
instruments of Aitken, but instead ofcounting droplets visually, the turbidity of the mist was measured with a lamp and photocell. Calibration of this instrument is described by Nolan and Pollak (1946) and by Hayes (1970). Since then, automatic recording instruments, working by light scattering (Rich, 1961) have been developed which make it easy to foilow the changes in concentration of nuclei with time. Table
1. Condensation
nucleus
counters
Expansion Uncharged particles
insoluble
Small ions
AITKEN
ratio
Saturation
0.2 pm radius 0,006
1.001 1.03
lGo5 1.20
Negative Positive
I.25 1.32
4 6
NUCLEI:
COMPOSITION
AND
ratio
CONCENTRATION
Aitken made a very large number of observations with his dust counters. He counted the nuclei in the atmosphere under all kinds of weather conditions, from sea level to mountain summits, from the Atlantic coast to the Mediterranean. At his home in Falkirk, about half way between Glasgow and Edinburgh, he showed that particles from the towns might still give rise to high concentrations, when the wind carried them 20 miles (approx. 32 km) from the source, and that such particles were the cause of red skies at sunset. He associated atmospheric haze with the concentration of particles, showing that the product of visual range and concentration was constant at a given humidity of the atmosphere but that the product increased in dry air. Visual ranges were estimated from 1 to 250 miles (approx. 1.6 to 400 km). He described the areas of the globe where most cloud forms and most rain falls as purifying areas, showing that the four great purifying areas covered by his observations of particle counts were the Mediterranean, the Alps, the Atlantic and the Highlands of Scotland. He proved that the sun was the producer of haze in air masses which had travelled from densely populated areas. Sulphur dioxide, when released in clean air, produced no nuclei in the dark; exposure to light, especially sunshine, resulted in the formation of hygroscopic nuclei which grew in size during prolonged exposure. He commented on the waste by combustion of some 5000 tons of sulphur a day, from the coal consumed in the British Isles, and the need for a process to prevent this. With modern apparatus, Aitken’s observations have been extended; his conclusions remain valid. The concentration of Aitken nuclei diminishes at night and increases after sunrise; it falls during windy weather. Small nuclei are formed during daytime by photochemical action and are lost by coagulation with larger particles. The Pollak counter indicates geometric mean concentrations of Aitken nuclei as follows (Hayes, 1970). Oceans Remote headland of Galway coast Dublin, centre
260 cm- 3 590-2300 24000-178000
These figures are very similar to those obtained by Aitken in similar locations. The use of recording apparatus and data processing enables large numbers of results to be pooled and analvsed so as to reveal diurnal changes in spite of large day to day variations; the results of Pedder (1971) give the concentrations of electrically charged nuclei
I071
(‘. I\r. DA\ll
\
and also the total concentration, charged plus uncharged. There is ;I relationship connecting the proportion of nuclei which are charged with the particle size and Peddcr used this to demonstrate that during calm nights the concentration fell and the particle size increased as would be expected from Brownian coagulation. However. during windy nights there was a large fall in concentration and hardI? an! incrcasc in particlc radius. the maximum being 0.025,~n: the effect of the wind is to incrcasc the rate 01‘ loss of particles bq diffusion to foliage and the ground. Aitken showed that domestic combustion. and industrial processes and combustion. produced condensation nuclei. both dircctlj and through photo-chemical gas to particle conversion. To these sources arc non added the ctlluents from internal combustion engines. His regarding the concentration of nuclei ah an index of atmospheric pollution is often valid but in addition to the obvious volcanoes and forest fires. there are considcrable natural sources which have only recent11 been rcvealcd.
John Tyndall (1875) showed that the presence of line particles in air was indicated bq the light scattered sideways from a sharply focused beam of light of high intensity: turbidity of the “Tyndall cone” indicated the presence of airborne particles. When certain organic vapours were added to clean air the beam became visible due to the appearance of blueish scattered light. proving that very small particles oxhibiting Raylcigh scattering (radius less than 0.1 /trn) had been formed photochemically from the vnpour. It occurred to Went (1960) that volatile emanations from vegetation could undergo a similar gas to particle conversion under the action of sunlight. A repetition of Tqndall’s experiment with terpene vapours showed that no particles were form4 unless light absorbing catalysts such as nitrogen oxide or iodine were also present. This is similar to the well known photochemical mechanism of smog formation from olefines derived from motor car exhausts and petrol vapour (Haagen-Smit. 1952). The particles which produce the blueish “Tyndall cone” are similar in size to Aitkcn nuclei and their concentration was measured with a Rich (1961) counter. Went showed that condensation nuclei were formed in clean country air when vapour of pinene and nitrogen dioxide were released Mhilt the sun was shining. Compounds having conjugated double bonds occur in plant vapours (Fig. 2). They include isoprene and terpenes pinene. mqrcene and carotene. Their biological function is uncertain; they may be by-products of metabolism which habe to be eliminated and possibly arc derived from dead cells. The presence in the air of tcrpcne bapours of vegetable origin was demonstrated by Rasmussen and Went (1965) in concentrations of 2 20 pp 1000 iii ((pi. IO 100 jig in- ’ for a molecular weight of 100). The concentration is lowest in cold \bcathcr and peaks in autumn. The vapours arc responsible for various characteristic country and seaside odours. including that of pinewoods. Algae and plankton also release terpencs. One of the dificultics of determining the Ievcl of Aitkcn nuclei of vegetable origin is the avoidance of man-made emissions. Any fire products overwhelming concentrations. all kinds of engines contribute and cities as far as 100 km upwind of the sampling point are still detectable sources. Taking suitable precautions against such contamination, Went operated a Rich counter over long periods in a number of isolated locations. The lowest recorded concentration of nuclei was taken from a vessel 10 km off the (‘alifornian coast at night in calm weather with an onshore brcetc; ;I I& hundred nuclei per cm3 wcrc
Fig. 2. Blue haze in Great Smoky Mountains, Tennessee; particles are formed by the polymerization of vapour of terpenes and other compounds which are emitted by the dense forest area. The visibility is about 50 km which corresponds to a particle concentration of 100 pg ma3 at a radius below 0. t pm. The number concentration is of the order of 25 OJJ@CZ~-~.
1073
Particles in the atmosphere
found. At ground level, inland in remote wooded valleys 1000-5000 cm- 3 were recorded as being definitely of natural origin. Some years of study of Aitken nuclei in Toulouse and south-west France (Fig. 3) have been carried out by Lopez (1974); he used an automatic counter (Cabrol ef al., 1972). The expansion ratio could be adjusted and the optical sensitivity was such that concentrations exceeding 300 particles cm - 3 could be detected, after condensation. Q 9 t . Limoees Clermbnt
\
Ferrand
i
P
Bo?deaux
I
Landes Toulouse
Fig. 3. Study of Aitken nuclei by Lopez (1974) in south-western France. He measured concentrations in the effluents from Toulouse. Lacq, the Landes pinewoods and in a rural area.
A number of observations were made from a Cessna aeroplane which was flown in clear weather across the plume of effluent which was swept downwind from the urban complex of Toulouse. The complex covers an area of 110 km2 and contains a population of 400 000 people. A typical traverse at 10 km from the centre of the town, in a wind of 1.5-3.5 m s- 1 and normal adiabatic temperature lapse, showed a well-defined plume about 18 km wide at an altitude of 150 m and 12 m wide at 900 m. The peak concentration was about 20 000 nuclei cm- 3 over a span of some 7 km. No plume was detectable at a height of 1200 m. From a review of such observations it was possible to integrate over the cross-section of the plume and calculate the total flux of Aitken nuclei which was generated in the urban complex. Six such values were obtained ranging from 2.5 x 10” particles s- ’ at 11:OOh on Wednesday 26 April 1972 to 7.0 x LO’7 particles s- ’ at 11:OOh on Friday 26 November 1971. When conditions favoured dilution of the plume by atmospheric turbulence, lower values of the total flux were obtained at distances of 10 and 30 km downwind. Similar observations on the total effluent from the industrial complex at Lacq, which has grown up in recent years because of the availability of natural gas, gave a total flux of particles of some lo’* s- ‘, the peak concentration in the plume being above 100000 Aitken nuclei cm- 3. Search was made for a rural site where the count would be minimal and unaffected by man-made effluents. The location decided upon was 30 km south-east of Toulouse, in gently undulating country with no village of more than 2500 inhabitants closer than 10 km; the vegetation was mainly cereal crops, Observations were made between April and
(‘. N. DA\‘II.S
I074
June 1971. North-westerly winds caused increases in the Aitken nuclei but, otherwise, the region was unpolluted, as indicated by the X-day mean concentration of lead in the atmosphere which was 0,02 ,~lg rnp3 compared with I pg m -3 at a distance of 5 km from the centre of Toulouse. In south-easterly winds the 3 months average concentration of nuclei winds being nearly double. was 8500 cm- 3, the figure for north-westerly From an analysis of his observations in this rural area. Lopez was able to conclude that there were two main local sources of Aitken nuclei. During the night there was formation near the ground at a rate of 3.5 x IO” IO4 particles cm ’ s. In daytime the rate of formation depended upon solar radiation and varied between 3 x IO” and 5 x lo4 particles Cm-’ s. The particles formed in the dark were larger than those measured in daylight. the mean radii being 0.036 and 0.028 /o-n, respectively. A part of this difference might be due to the higher relative humidity at night. Measurements of electric charge showed that the particles were in Boltzmann equilibrium with the ions of the air, some 40 50 per cent of the Aitken particles being charged. Such charged particles are described as giant or Langevin ions and have much lower mobilities than the ordinary small ions which are charged aggregates of only a few molecules. A study of the production of Aitken nuclei by the Landes forest was also carried out by Lopez (1974). This area of 7000 km2 borders the Atlantic coast from the Gironde estuary to the Spanish border, a distance of some 200 km. The trees are all pines. The observations showed that the forest was an intense source of Aitken nuclei: during the daytime in spring the mean rate of formation was 1.2 x 10’ particles cm -’ s. or about three times that over a country area of cereal crops. During the night time there was a temperature inversion up to about 400 m altitude below which the air was calm. Concentrations in this inversion layer u’erc measured at IO:00 h and found to be near to 3 x to I6 pg nC 3 for particles of radius 0.05 /nn and IO” particles cm- 3. which corresponds unit density. Above 400 m the concentration rapidly declined. During the morning, the inversion layer disappeared and the resulting atmospheric turbulence mixed the particle-rich air below 400 m with the relatively particle-free air above. At 13:OO h the concentration of nuclei was constant at IO” cm 3 up to an altitude of I .S km. It therefore seemed that the particles were formed by gas to particlc conversion in air below 400 m and that none originated at greater altitudes. The lifetime of one or more of the participants in the conversion must have been less than 12 11. Table 2 summarises the position over the Pyrenees and Aquitaine regions of the Midi, south-west France. The total population of the area of 87000 km’ is 4.10”. or ten times that of Toulouse, Table 2. Total generation
____
Lone
TouloLlsc complrx Other town:. Lacq complex RuXll Landes forc\t Total
of Altken nuclei
m the atmosphere
of south-west
France (Lop.
19741
Area I IO km’ Point sources Point source x0000 7000
x7 000
Of the atmospheric particulates in the entire regions made effluents and 85 per cent from natural ones.
some I5 per cent result from man-
Particles
in the atmosphere
LARGER
1075
PARTICLES
Atmospheric particles cover a range of size of about 105; the number of particles in a given size ratio decreases rapidly as the mean size increases above a radius of 01 pm. The size distribution was expressed by Junge (1963) as (dN/d In r) = (NrmP/333),
r, > r > 0.1,
N is the total number of particles cm- 3 and dN is the number cm- 3 in the range d In Y,about r; p is near to 3 for r < 10,um and 6 for Y > 10pm (Junge and Jaenicke, 1971); the value of the constant depends on the concentration. The Aitken nuclei, being below @l pm radius do not follow this law. However, when considering larger particles Junge’s formula may be misleading because the distribution which it predicts is mainly influenced by particles between 0.1 and 1 pm radius. Those between 1 and 10 pm have only a slight effect and those between 10 and 100 pm have no effect whatsoever. This is easily seen from Table 3 where the values of characteristic radii are shown for distributions with maximal particle radii, r, = 1 and 10 pm. The values for rm = 100 pm are the same as those for 10 pm. Table 3. Junge’s size distribution Mean particle (Fy
(pm) surface area
formula
for atmospheric
particles
radii (;5)1/3 volume
Maximum radius I’,,,(/cm)
P
f, number
3
0.15 0.15
0.17 016
0.24 0.19
10 and 100 1
6
0.12 @12
0.12 0.12
0.13 0.13
10 and 100 1
The reason for this is that, just like the Aitken nuclei, the particles of the atmosphere which are larger than 0.1 pm in radius come from multiple sources which act independently (Davies, 1974). For some years attempts were made to explain the Junge size distribution as a self-preserving one; this means that there were opposing mechanisms of particle formation and particle removal which established an equilibrium size distribution. This concept is erroneous because, like all properties of aerosols, the processes affecting the particles each operate over a limited range of particle size. In fact, as a broad generalisation, if particles cover more than a lOO-fold ratio of size, then multiple mechanisms of generation should be suspected; removal mechanisms, likewise, will differ. Only at high altitudes are the imprints of the various generating and removal mechanisms of the larger atmospheric particles obliterated. Sampling from aircraft indicates that above an altitude of 67 km, as far as the tropopause, the atmospheric aerosol is constant over large global areas of ocean and continent (Blifford, 1970). The particles have aged and have been homogenized by various processes. The concentration is low, being of the order of 1 particle cm- 3 in the range 0.13 < r < 5.5 pm. In 1954 the U.S. established a number of routine sampling stations, using high volume samplers, both at home and abroad. Samples were analysed chemically and for radioactivity. The Environmental Protection Agency now run the National Air Surveillance Network which includes particle size determination with cascade impactors (Lee, 1972); these are effective over a range 0.25-18 pm radius. Lognormal size distributions were found at various situations; in Hertford, Connecticut, m.m.d. were from 0.12 (18.2 pg mp3) to 1.9 pm (138 pg me3). This sampling technique, it should be noted, reveals neither the
C‘.ix DA\It,
1070
Aitkcn particles nor the coarse dust (I’ > 1.X j(m): the latter. of course, adds considerably to the concentration by weight. The large particles include ash. smoke aggregates and droplets of acid and salt solution (Wailer, Brooks and Cartwright. 1963); sea salt in particles or droplets according to the humidity of the atmosphere (Moore and Mason. 1954): dust from the ground. Lvhich ma! trnvcl for long distances (Junge and Jaenicke, 1971: Hagen and Woodrutf 1963): pollen and l’ungal spores (Gregory. 1973): textile fibres and skin scales (Clark. 1974). IO”
IO’
IO 0’
10-5
IO
gme3 M
i\ about
01x ofthc
0.3 pm.
most obvious effects produced by atmospheric particles is a decrease in visual This depends not only upon the concentration of the particles but also on their sile, the most efTective radii being between 0.1 and 1 Llrn (Fig. 4). On a yearly average the distribution ofatmospheric turbidity in the U.S. has a maximum centred along the axis of the Allegheny Mountains (Flowers. McCormick and Ku&, 1969) which is due to the Aitken nuclei generated by vapours from the dense forest cover (Fig. 5). Arid regions have cxtrcmely clear air, and even the industrial areas and centres of population do not produce more than 2;3 of the mean turbidity of the forest regions. A completely different picture (Fig. 6) results when the contours of annual mean mass concentration are plotted (Robinson and Robbins, 1971). Above the Alleghenies the concentration averages only half the figures for the populated and industrial areas. The arid western regions of Utah. Colorado and Wyoming yield still less. The difference between the geographical distribution of atmospheric turbidity and mass concentration of particulates cannot be explained purely in terms of particle size and mass I-an&c.
Particles in the atmosphere
---‘7._.-._~.-~. I
1077
0.08
\
c-...
s(1 \_
:
In
-..
Lc C0.f
:
--.
i J
Fig. 5. Annual mean turbidity of the atmosphere at a wavelength of 5000 A from measurements at 43 non-urban stations in the U.S. using a Volz sun photometer. The turbidity due to the atmospheric aerosol is the log to base IO of solar intensity expressed as a fraction of the intensity outside the earth’s atmosphere, allowance being made for absorption by air and ozone. (Flowers, McCormick and Kurfis, 1969.)
0 0
Fig. 6. Annual mean mass concentrations of airborne particles from non-urban stations of the U.S. National Air Sampling Network, 19641965. (Robinson and Robbins, 1971.)
107X
General Industry Coal combust
‘(
Fig. 7. Relative
Mineral
and
masses of man-made emkons
\
Ion
/
m the U.S.A. IRobinson
;tnd Rohhinb.
1971.1
concentration, as study of Fig. 4 will confirm. It seems likely that the Aitken nuclei of natural origin extend to much greater altitudes than the man-made coarser particles so that the optical effects of the latter are predominantly within a kilometre above ground level. This. in turn. implies that the removal of the coarser, man-made particles by natural processes is more efficient than it is for the fine Aitken nuclei. The relative contributions of various sources to the man-made atmospheric aerosol have been estimated, as shown in Fig. 7. by Robinson and Robbins ( 197 1). They have also made an evaluation of the contribution of natural and man-made sources to global emission; this is reproduced in Table 4.
Particles
in the atmosphere
1079
Although the proportion of man-made particles on this estimate, like that of Lopez for the Aitken nuclei (15 per cent by number) is small, it is nevertheless appreciable and the generation of particles by man usually takes place in densely populated areas. The large contribution from SO,, about half of the entire man-made quantity, is notable but since these particles are hygroscopic they grow in moist air and are speedily eliminated; other particles which originate from photochemical reaction can persist and spread much further. REFERENCES Aitken John (1880) On dust, fogs and clouds. Proc. R. Sot. Edin. 11, 14-18; Collected Papers, Cambridge (1923). Blifford I. H. (1970) Tropospheric aerosols. J. geophys. Res. 75 (15), 3099-3103. Cabrol C., Lopez A., Chapuis A. and Fontan J. (1972) A self-contained automatic and portable apparatus for the counting of particles. J. Aerosol Sci. 3 (4), 281-287. Clark R. P. (1974) Skin scales among airborne particles. J. Nyy. Cambridge, 72,47-51. Coulier P. J. ( 1875) Note on a new property of the air. J. Pharm. Chin+ Ser. 4. 22 (l), 165-173; (2). 254255 (1875). Davies C. N. (1974) Size distribution of atmospheric particles. J. Aerosol Sci. 5 (3), 293-300. Davies C. N. (1975) Pollution, particle size and visibility. J. Aerosol Sci. To be published. Flowers E. C., McCormick R. A. and Kurfis K. R. (1969) Atmospheric turbidity over the United States (1961.1966). J. Appl. Meteorol. 8, 955-962. Gregory P. H. (1973) Microbiology of the Atmosphrrr. Leonard Hill, London. Haagen-Smit A. J. (1952) Chemistry and physiology of Los Angeles smog. Ind. Enyny C’hern. 44, 1342-1346. Hagen L. J. and Woodruff N. P. (1973) Air pollution from duststorms in the Great Plains. Atmospheric Emirontnent 7, 323-332. Hayes E. I. (1970) The frequency distribution of atmospheric condensation nucleus concentration. Proc. R. Ir. Acud. 70 (A8), 59-69. Junge C. (1963) Air Chrmistry and Rudioactiuity. Academic Press, New York. Junge C. and Jaenicke R. (1971) New results in background aerosols studies from the Atlantic expedition of the R.V. Meteor. spring, 1969. J. Aerosol Sci. 2 (3), 30%314. Lee R. E. (1972) The size of suspended particulate matter in air. Sckwcr 178 (4061). 567--575. Lopez A. (1974) Contribution a l’ktude de I’aCrosol atmosphbrique. These. Docteur ts Sciences. Paul Sabatier Universitt?. Toulouse. Moore D. J. and Mason B. J. (1954) The concentration, size distribution and production rate of large salt nuclei over the ocean. Q. J. R. mrt. Sot. 80, 583-590. Nolan P. J. and Pollak L. W. (1946) The calibration of a photoelectric nucleus counter. Proc. R. Ir. Acad. 51A (2), 9-31. Pedder M. A. (1971) On the analysis of continuous records of total concentration and equivalent radius in rural aerosol. J. Aerosol Sci. 2, 175-l 83. Rasmussen R. A. and Went F. W. (1965) Volatile organic material of plant origin in the atmosphere. Proc. Nat. Acad. Sci. 53, 215-220. Robinson E. and Robbins R. C. (1971) Emission, concentration and fate of particulate atmospheric pollutants. Final Report, S.R.I. Project SCC-8507. Stanford Research Institute, Menlo Park, California, U.S.A. Rich T. A. (1961) Continuous recorder for condensation nuclei. 4th Int. Sytnp. 011 Corldensatiort Nuclei. Heidelberg, May. Thomson J. J. (I 888).ilpplicafion of‘Dynumics to Physics and Chemistry. Chap. 2, pp. 158-I 79. Cambridge University Press, Cambridge. Thompson W. (1X70) On the equilibrium of vapour at a curved surface of a liquid. Proc. R. Sot. Edit]. 7, 63. Tyndall J. (1875) Si.\- Lrcrures on Light. Longmans Green, London. Waller, R. F.. Brooks A. G. F. and Cartwright J. (1963) An electron microscope study of particles in town air. Int. J. Air Wut. Poll. 7, 779-786. Went F. W. (1960) Organic matter in the atmosphere and its possible relation to petroleum formation. Proc. Nat. Acad. Sci. 46, 212-221. Went F. W. (1966) On the nature of Aitken condensation nuclei. Tellus 18 (2). 549-556. Wilson C. T. R. (195X) On the formation of cloud in the absence dust. Nature 52, 144.
IN THE ATMOSPHERE--NATURAL FAN-JADE*
AND
C. N. DAVIES Department
of Chemistry,
University
of Essex, Wivenhoe
Park, Colchester,
Essex
Abstract-The existence of particles in the atmosphere was established by John Aitken whose work is related to recent studies. The concentration and composition of Aitken nuclei are discussed and contemporary measurements of the output of natural and human sources are reviewed. Larger particles are not adequately described by the Junge size distribution, and do not produce a self-preserving size distribution; they result from a variety of independent sources. Reduction of visual range is achieved most economically by particles of 03-O-4 birn radius. Estimates are quoted for the proportion of matural particles in the atmospheric aerosol.
AITKEN
NUCLEI:
COUNTERS
W. Thomson (later Lord Kelvin) showed theoretically in 1870 that the vapour pressure of smali droplets tended to infinity as the radius tended to zero. In 1875 P. J. Coulier expanded moist air so that it cooled adiabatically and found that condensation as a mist took place. He suggested that the condensation of all droplets of mist occurred on small solid nuclei. Later experiments showed that air which was inactive and would not produce mist, since it was free of nuclei, could be activated by heating and would then form mist when cooled by expansion. This shook his belief in the hypothesis of condensation upon nuclei which, by their finite radii, avoided the difficulty of Kelvin’s formula. He did not realise that heating the vessel generated nuclei from its walls. In 1880 John Aitken showed that steam condensed and formed a mist when injected into a vessel filled with ordinary room air, but that no mist appeared if the air had been filtered. He pursued this observation and demonstrated the reason for Coulier’s loss of fdith in the nucleation hypothesis (1881). By 1890 Aitken had designed and constructed several types of apparatus for counting the number of condensation nuclei in the air, culminating in a portable nucleus counter (Fig. 1) which enabled him to study the concentration under a variety of geographical and meteorological conditions as well as the artificial production of nuclei. In 1888 J. J. Thomson proved theoretically that the large increase in vapour pressure of a droplet with diminishing size, predicted by Kelvin’s formula, did not take place if the droplet was electrically charged. The possibility therefore existed for mist formation to take place upon gaseous ions as well as Aitken condensation nuclei. This was demonstrated experimentally by C. T. R. Wilson in 1895 and led those who doubted the existence of Aitken particles in the atmosphere to claim that the greater number of the nuclei counted were ions. However, Aitken had already demonstrated the existence of hygroscopic nuclei which caused haze in air well below saturation, of nuclei which were pro* Based on a lecture given at a symposium on Air Pollution, of the Loughborough University of Technology. 7-X January
the Leverhulme 1974.
Project
for Education
in Industry
IO70
(‘. h. t>\\lts
duced in large numbers by towns and travelled many miles on the u ind into the unpolluted air of the open country, of nuclei formed from suiphur dioxide bq the action of sunlight in the presence of polluting particles and of nuclei produced bq heating a number of substances.
portable counlel- (IXW). Sampksof a~,-or suitable \LI~UIIIC 211~‘taken m the bore taps. M. They are transferred and diluted with clean air lion1 the tiltcr. I-, bq means of the pump A. to the chamber R which is lmcd with moist blotting p:tpc!-. A raped expansion. Fig. 1. Aitken’s
of one of the 3
of desired amount,
with the pump causes the air in R to cool and water droplets to condense upon the particles. The number in a known volume of the original air can lx found I?? counting the number of droplets which settles on a known xca of the countmp stage In R. using a magnifying lens which is n(lt shown.
Condensation upon most gaseous ions requires a greater fill1 in temperature than is required for Aitken nuclei and there is thus no difficulty in distinguishing between gaseous ions and Aitken nuclei; ions are only activated by a much greater expansion, as shown in Table 1. Soluble nuclei are more readily activated to produce droplets by condensation than insoluble and liquify below saturation unless extremely small. Only the very largest of the insoluble Aitken nuclei can act as condensation centrcs in the open air because the saturation ratio of the atmosphere rarely rises above 1.01. For 60 years the Aitken type of counter persisted as the sole means of studying condcnsation nuclei. Then an important advance came in 1940 with the construction of the first photoelectric counter by Pollak. The mist was produced by expansion. as in the original
Particles
in the atmosphere
1071
instruments of Aitken, but instead ofcounting droplets visually, the turbidity of the mist was measured with a lamp and photocell. Calibration of this instrument is described by Nolan and Pollak (1946) and by Hayes (1970). Since then, automatic recording instruments, working by light scattering (Rich, 1961) have been developed which make it easy to foilow the changes in concentration of nuclei with time. Table
1. Condensation
nucleus
counters
Expansion Uncharged particles
insoluble
Small ions
AITKEN
ratio
Saturation
0.2 pm radius 0,006
1.001 1.03
lGo5 1.20
Negative Positive
I.25 1.32
4 6
NUCLEI:
COMPOSITION
AND
ratio
CONCENTRATION
Aitken made a very large number of observations with his dust counters. He counted the nuclei in the atmosphere under all kinds of weather conditions, from sea level to mountain summits, from the Atlantic coast to the Mediterranean. At his home in Falkirk, about half way between Glasgow and Edinburgh, he showed that particles from the towns might still give rise to high concentrations, when the wind carried them 20 miles (approx. 32 km) from the source, and that such particles were the cause of red skies at sunset. He associated atmospheric haze with the concentration of particles, showing that the product of visual range and concentration was constant at a given humidity of the atmosphere but that the product increased in dry air. Visual ranges were estimated from 1 to 250 miles (approx. 1.6 to 400 km). He described the areas of the globe where most cloud forms and most rain falls as purifying areas, showing that the four great purifying areas covered by his observations of particle counts were the Mediterranean, the Alps, the Atlantic and the Highlands of Scotland. He proved that the sun was the producer of haze in air masses which had travelled from densely populated areas. Sulphur dioxide, when released in clean air, produced no nuclei in the dark; exposure to light, especially sunshine, resulted in the formation of hygroscopic nuclei which grew in size during prolonged exposure. He commented on the waste by combustion of some 5000 tons of sulphur a day, from the coal consumed in the British Isles, and the need for a process to prevent this. With modern apparatus, Aitken’s observations have been extended; his conclusions remain valid. The concentration of Aitken nuclei diminishes at night and increases after sunrise; it falls during windy weather. Small nuclei are formed during daytime by photochemical action and are lost by coagulation with larger particles. The Pollak counter indicates geometric mean concentrations of Aitken nuclei as follows (Hayes, 1970). Oceans Remote headland of Galway coast Dublin, centre
260 cm- 3 590-2300 24000-178000
These figures are very similar to those obtained by Aitken in similar locations. The use of recording apparatus and data processing enables large numbers of results to be pooled and analvsed so as to reveal diurnal changes in spite of large day to day variations; the results of Pedder (1971) give the concentrations of electrically charged nuclei
I071
(‘. I\r. DA\ll
\
and also the total concentration, charged plus uncharged. There is ;I relationship connecting the proportion of nuclei which are charged with the particle size and Peddcr used this to demonstrate that during calm nights the concentration fell and the particle size increased as would be expected from Brownian coagulation. However. during windy nights there was a large fall in concentration and hardI? an! incrcasc in particlc radius. the maximum being 0.025,~n: the effect of the wind is to incrcasc the rate 01‘ loss of particles bq diffusion to foliage and the ground. Aitken showed that domestic combustion. and industrial processes and combustion. produced condensation nuclei. both dircctlj and through photo-chemical gas to particle conversion. To these sources arc non added the ctlluents from internal combustion engines. His regarding the concentration of nuclei ah an index of atmospheric pollution is often valid but in addition to the obvious volcanoes and forest fires. there are considcrable natural sources which have only recent11 been rcvealcd.
John Tyndall (1875) showed that the presence of line particles in air was indicated bq the light scattered sideways from a sharply focused beam of light of high intensity: turbidity of the “Tyndall cone” indicated the presence of airborne particles. When certain organic vapours were added to clean air the beam became visible due to the appearance of blueish scattered light. proving that very small particles oxhibiting Raylcigh scattering (radius less than 0.1 /trn) had been formed photochemically from the vnpour. It occurred to Went (1960) that volatile emanations from vegetation could undergo a similar gas to particle conversion under the action of sunlight. A repetition of Tqndall’s experiment with terpene vapours showed that no particles were form4 unless light absorbing catalysts such as nitrogen oxide or iodine were also present. This is similar to the well known photochemical mechanism of smog formation from olefines derived from motor car exhausts and petrol vapour (Haagen-Smit. 1952). The particles which produce the blueish “Tyndall cone” are similar in size to Aitkcn nuclei and their concentration was measured with a Rich (1961) counter. Went showed that condensation nuclei were formed in clean country air when vapour of pinene and nitrogen dioxide were released Mhilt the sun was shining. Compounds having conjugated double bonds occur in plant vapours (Fig. 2). They include isoprene and terpenes pinene. mqrcene and carotene. Their biological function is uncertain; they may be by-products of metabolism which habe to be eliminated and possibly arc derived from dead cells. The presence in the air of tcrpcne bapours of vegetable origin was demonstrated by Rasmussen and Went (1965) in concentrations of 2 20 pp 1000 iii ((pi. IO 100 jig in- ’ for a molecular weight of 100). The concentration is lowest in cold \bcathcr and peaks in autumn. The vapours arc responsible for various characteristic country and seaside odours. including that of pinewoods. Algae and plankton also release terpencs. One of the dificultics of determining the Ievcl of Aitkcn nuclei of vegetable origin is the avoidance of man-made emissions. Any fire products overwhelming concentrations. all kinds of engines contribute and cities as far as 100 km upwind of the sampling point are still detectable sources. Taking suitable precautions against such contamination, Went operated a Rich counter over long periods in a number of isolated locations. The lowest recorded concentration of nuclei was taken from a vessel 10 km off the (‘alifornian coast at night in calm weather with an onshore brcetc; ;I I& hundred nuclei per cm3 wcrc
Fig. 2. Blue haze in Great Smoky Mountains, Tennessee; particles are formed by the polymerization of vapour of terpenes and other compounds which are emitted by the dense forest area. The visibility is about 50 km which corresponds to a particle concentration of 100 pg ma3 at a radius below 0. t pm. The number concentration is of the order of 25 OJJ@CZ~-~.
1073
Particles in the atmosphere
found. At ground level, inland in remote wooded valleys 1000-5000 cm- 3 were recorded as being definitely of natural origin. Some years of study of Aitken nuclei in Toulouse and south-west France (Fig. 3) have been carried out by Lopez (1974); he used an automatic counter (Cabrol ef al., 1972). The expansion ratio could be adjusted and the optical sensitivity was such that concentrations exceeding 300 particles cm - 3 could be detected, after condensation. Q 9 t . Limoees Clermbnt
\
Ferrand
i
P
Bo?deaux
I
Landes Toulouse
Fig. 3. Study of Aitken nuclei by Lopez (1974) in south-western France. He measured concentrations in the effluents from Toulouse. Lacq, the Landes pinewoods and in a rural area.
A number of observations were made from a Cessna aeroplane which was flown in clear weather across the plume of effluent which was swept downwind from the urban complex of Toulouse. The complex covers an area of 110 km2 and contains a population of 400 000 people. A typical traverse at 10 km from the centre of the town, in a wind of 1.5-3.5 m s- 1 and normal adiabatic temperature lapse, showed a well-defined plume about 18 km wide at an altitude of 150 m and 12 m wide at 900 m. The peak concentration was about 20 000 nuclei cm- 3 over a span of some 7 km. No plume was detectable at a height of 1200 m. From a review of such observations it was possible to integrate over the cross-section of the plume and calculate the total flux of Aitken nuclei which was generated in the urban complex. Six such values were obtained ranging from 2.5 x 10” particles s- ’ at 11:OOh on Wednesday 26 April 1972 to 7.0 x LO’7 particles s- ’ at 11:OOh on Friday 26 November 1971. When conditions favoured dilution of the plume by atmospheric turbulence, lower values of the total flux were obtained at distances of 10 and 30 km downwind. Similar observations on the total effluent from the industrial complex at Lacq, which has grown up in recent years because of the availability of natural gas, gave a total flux of particles of some lo’* s- ‘, the peak concentration in the plume being above 100000 Aitken nuclei cm- 3. Search was made for a rural site where the count would be minimal and unaffected by man-made effluents. The location decided upon was 30 km south-east of Toulouse, in gently undulating country with no village of more than 2500 inhabitants closer than 10 km; the vegetation was mainly cereal crops, Observations were made between April and
(‘. N. DA\‘II.S
I074
June 1971. North-westerly winds caused increases in the Aitken nuclei but, otherwise, the region was unpolluted, as indicated by the X-day mean concentration of lead in the atmosphere which was 0,02 ,~lg rnp3 compared with I pg m -3 at a distance of 5 km from the centre of Toulouse. In south-easterly winds the 3 months average concentration of nuclei winds being nearly double. was 8500 cm- 3, the figure for north-westerly From an analysis of his observations in this rural area. Lopez was able to conclude that there were two main local sources of Aitken nuclei. During the night there was formation near the ground at a rate of 3.5 x IO” IO4 particles cm ’ s. In daytime the rate of formation depended upon solar radiation and varied between 3 x IO” and 5 x lo4 particles Cm-’ s. The particles formed in the dark were larger than those measured in daylight. the mean radii being 0.036 and 0.028 /o-n, respectively. A part of this difference might be due to the higher relative humidity at night. Measurements of electric charge showed that the particles were in Boltzmann equilibrium with the ions of the air, some 40 50 per cent of the Aitken particles being charged. Such charged particles are described as giant or Langevin ions and have much lower mobilities than the ordinary small ions which are charged aggregates of only a few molecules. A study of the production of Aitken nuclei by the Landes forest was also carried out by Lopez (1974). This area of 7000 km2 borders the Atlantic coast from the Gironde estuary to the Spanish border, a distance of some 200 km. The trees are all pines. The observations showed that the forest was an intense source of Aitken nuclei: during the daytime in spring the mean rate of formation was 1.2 x 10’ particles cm -’ s. or about three times that over a country area of cereal crops. During the night time there was a temperature inversion up to about 400 m altitude below which the air was calm. Concentrations in this inversion layer u’erc measured at IO:00 h and found to be near to 3 x to I6 pg nC 3 for particles of radius 0.05 /nn and IO” particles cm- 3. which corresponds unit density. Above 400 m the concentration rapidly declined. During the morning, the inversion layer disappeared and the resulting atmospheric turbulence mixed the particle-rich air below 400 m with the relatively particle-free air above. At 13:OO h the concentration of nuclei was constant at IO” cm 3 up to an altitude of I .S km. It therefore seemed that the particles were formed by gas to particlc conversion in air below 400 m and that none originated at greater altitudes. The lifetime of one or more of the participants in the conversion must have been less than 12 11. Table 2 summarises the position over the Pyrenees and Aquitaine regions of the Midi, south-west France. The total population of the area of 87000 km’ is 4.10”. or ten times that of Toulouse, Table 2. Total generation
____
Lone
TouloLlsc complrx Other town:. Lacq complex RuXll Landes forc\t Total
of Altken nuclei
m the atmosphere
of south-west
France (Lop.
19741
Area I IO km’ Point sources Point source x0000 7000
x7 000
Of the atmospheric particulates in the entire regions made effluents and 85 per cent from natural ones.
some I5 per cent result from man-
Particles
in the atmosphere
LARGER
1075
PARTICLES
Atmospheric particles cover a range of size of about 105; the number of particles in a given size ratio decreases rapidly as the mean size increases above a radius of 01 pm. The size distribution was expressed by Junge (1963) as (dN/d In r) = (NrmP/333),
r, > r > 0.1,
N is the total number of particles cm- 3 and dN is the number cm- 3 in the range d In Y,about r; p is near to 3 for r < 10,um and 6 for Y > 10pm (Junge and Jaenicke, 1971); the value of the constant depends on the concentration. The Aitken nuclei, being below @l pm radius do not follow this law. However, when considering larger particles Junge’s formula may be misleading because the distribution which it predicts is mainly influenced by particles between 0.1 and 1 pm radius. Those between 1 and 10 pm have only a slight effect and those between 10 and 100 pm have no effect whatsoever. This is easily seen from Table 3 where the values of characteristic radii are shown for distributions with maximal particle radii, r, = 1 and 10 pm. The values for rm = 100 pm are the same as those for 10 pm. Table 3. Junge’s size distribution Mean particle (Fy
(pm) surface area
formula
for atmospheric
particles
radii (;5)1/3 volume
Maximum radius I’,,,(/cm)
P
f, number
3
0.15 0.15
0.17 016
0.24 0.19
10 and 100 1
6
0.12 @12
0.12 0.12
0.13 0.13
10 and 100 1
The reason for this is that, just like the Aitken nuclei, the particles of the atmosphere which are larger than 0.1 pm in radius come from multiple sources which act independently (Davies, 1974). For some years attempts were made to explain the Junge size distribution as a self-preserving one; this means that there were opposing mechanisms of particle formation and particle removal which established an equilibrium size distribution. This concept is erroneous because, like all properties of aerosols, the processes affecting the particles each operate over a limited range of particle size. In fact, as a broad generalisation, if particles cover more than a lOO-fold ratio of size, then multiple mechanisms of generation should be suspected; removal mechanisms, likewise, will differ. Only at high altitudes are the imprints of the various generating and removal mechanisms of the larger atmospheric particles obliterated. Sampling from aircraft indicates that above an altitude of 67 km, as far as the tropopause, the atmospheric aerosol is constant over large global areas of ocean and continent (Blifford, 1970). The particles have aged and have been homogenized by various processes. The concentration is low, being of the order of 1 particle cm- 3 in the range 0.13 < r < 5.5 pm. In 1954 the U.S. established a number of routine sampling stations, using high volume samplers, both at home and abroad. Samples were analysed chemically and for radioactivity. The Environmental Protection Agency now run the National Air Surveillance Network which includes particle size determination with cascade impactors (Lee, 1972); these are effective over a range 0.25-18 pm radius. Lognormal size distributions were found at various situations; in Hertford, Connecticut, m.m.d. were from 0.12 (18.2 pg mp3) to 1.9 pm (138 pg me3). This sampling technique, it should be noted, reveals neither the
C‘.ix DA\It,
1070
Aitkcn particles nor the coarse dust (I’ > 1.X j(m): the latter. of course, adds considerably to the concentration by weight. The large particles include ash. smoke aggregates and droplets of acid and salt solution (Wailer, Brooks and Cartwright. 1963); sea salt in particles or droplets according to the humidity of the atmosphere (Moore and Mason. 1954): dust from the ground. Lvhich ma! trnvcl for long distances (Junge and Jaenicke, 1971: Hagen and Woodrutf 1963): pollen and l’ungal spores (Gregory. 1973): textile fibres and skin scales (Clark. 1974). IO”
IO’
IO 0’
10-5
IO
gme3 M
i\ about
01x ofthc
0.3 pm.
most obvious effects produced by atmospheric particles is a decrease in visual This depends not only upon the concentration of the particles but also on their sile, the most efTective radii being between 0.1 and 1 Llrn (Fig. 4). On a yearly average the distribution ofatmospheric turbidity in the U.S. has a maximum centred along the axis of the Allegheny Mountains (Flowers. McCormick and Ku&, 1969) which is due to the Aitken nuclei generated by vapours from the dense forest cover (Fig. 5). Arid regions have cxtrcmely clear air, and even the industrial areas and centres of population do not produce more than 2;3 of the mean turbidity of the forest regions. A completely different picture (Fig. 6) results when the contours of annual mean mass concentration are plotted (Robinson and Robbins, 1971). Above the Alleghenies the concentration averages only half the figures for the populated and industrial areas. The arid western regions of Utah. Colorado and Wyoming yield still less. The difference between the geographical distribution of atmospheric turbidity and mass concentration of particulates cannot be explained purely in terms of particle size and mass I-an&c.
Particles in the atmosphere
---‘7._.-._~.-~. I
1077
0.08
\
c-...
s(1 \_
:
In
-..
Lc C0.f
:
--.
i J
Fig. 5. Annual mean turbidity of the atmosphere at a wavelength of 5000 A from measurements at 43 non-urban stations in the U.S. using a Volz sun photometer. The turbidity due to the atmospheric aerosol is the log to base IO of solar intensity expressed as a fraction of the intensity outside the earth’s atmosphere, allowance being made for absorption by air and ozone. (Flowers, McCormick and Kurfis, 1969.)
0 0
Fig. 6. Annual mean mass concentrations of airborne particles from non-urban stations of the U.S. National Air Sampling Network, 19641965. (Robinson and Robbins, 1971.)
107X
General Industry Coal combust
‘(
Fig. 7. Relative
Mineral
and
masses of man-made emkons
\
Ion
/
m the U.S.A. IRobinson
;tnd Rohhinb.
1971.1
concentration, as study of Fig. 4 will confirm. It seems likely that the Aitken nuclei of natural origin extend to much greater altitudes than the man-made coarser particles so that the optical effects of the latter are predominantly within a kilometre above ground level. This. in turn. implies that the removal of the coarser, man-made particles by natural processes is more efficient than it is for the fine Aitken nuclei. The relative contributions of various sources to the man-made atmospheric aerosol have been estimated, as shown in Fig. 7. by Robinson and Robbins ( 197 1). They have also made an evaluation of the contribution of natural and man-made sources to global emission; this is reproduced in Table 4.
Particles
in the atmosphere
1079
Although the proportion of man-made particles on this estimate, like that of Lopez for the Aitken nuclei (15 per cent by number) is small, it is nevertheless appreciable and the generation of particles by man usually takes place in densely populated areas. The large contribution from SO,, about half of the entire man-made quantity, is notable but since these particles are hygroscopic they grow in moist air and are speedily eliminated; other particles which originate from photochemical reaction can persist and spread much further. REFERENCES Aitken John (1880) On dust, fogs and clouds. Proc. R. Sot. Edin. 11, 14-18; Collected Papers, Cambridge (1923). Blifford I. H. (1970) Tropospheric aerosols. J. geophys. Res. 75 (15), 3099-3103. Cabrol C., Lopez A., Chapuis A. and Fontan J. (1972) A self-contained automatic and portable apparatus for the counting of particles. J. Aerosol Sci. 3 (4), 281-287. Clark R. P. (1974) Skin scales among airborne particles. J. Nyy. Cambridge, 72,47-51. Coulier P. J. ( 1875) Note on a new property of the air. J. Pharm. Chin+ Ser. 4. 22 (l), 165-173; (2). 254255 (1875). Davies C. N. (1974) Size distribution of atmospheric particles. J. Aerosol Sci. 5 (3), 293-300. Davies C. N. (1975) Pollution, particle size and visibility. J. Aerosol Sci. To be published. Flowers E. C., McCormick R. A. and Kurfis K. R. (1969) Atmospheric turbidity over the United States (1961.1966). J. Appl. Meteorol. 8, 955-962. Gregory P. H. (1973) Microbiology of the Atmosphrrr. Leonard Hill, London. Haagen-Smit A. J. (1952) Chemistry and physiology of Los Angeles smog. Ind. Enyny C’hern. 44, 1342-1346. Hagen L. J. and Woodruff N. P. (1973) Air pollution from duststorms in the Great Plains. Atmospheric Emirontnent 7, 323-332. Hayes E. I. (1970) The frequency distribution of atmospheric condensation nucleus concentration. Proc. R. Ir. Acud. 70 (A8), 59-69. Junge C. (1963) Air Chrmistry and Rudioactiuity. Academic Press, New York. Junge C. and Jaenicke R. (1971) New results in background aerosols studies from the Atlantic expedition of the R.V. Meteor. spring, 1969. J. Aerosol Sci. 2 (3), 30%314. Lee R. E. (1972) The size of suspended particulate matter in air. Sckwcr 178 (4061). 567--575. Lopez A. (1974) Contribution a l’ktude de I’aCrosol atmosphbrique. These. Docteur ts Sciences. Paul Sabatier Universitt?. Toulouse. Moore D. J. and Mason B. J. (1954) The concentration, size distribution and production rate of large salt nuclei over the ocean. Q. J. R. mrt. Sot. 80, 583-590. Nolan P. J. and Pollak L. W. (1946) The calibration of a photoelectric nucleus counter. Proc. R. Ir. Acad. 51A (2), 9-31. Pedder M. A. (1971) On the analysis of continuous records of total concentration and equivalent radius in rural aerosol. J. Aerosol Sci. 2, 175-l 83. Rasmussen R. A. and Went F. W. (1965) Volatile organic material of plant origin in the atmosphere. Proc. Nat. Acad. Sci. 53, 215-220. Robinson E. and Robbins R. C. (1971) Emission, concentration and fate of particulate atmospheric pollutants. Final Report, S.R.I. Project SCC-8507. Stanford Research Institute, Menlo Park, California, U.S.A. Rich T. A. (1961) Continuous recorder for condensation nuclei. 4th Int. Sytnp. 011 Corldensatiort Nuclei. Heidelberg, May. Thomson J. J. (I 888).ilpplicafion of‘Dynumics to Physics and Chemistry. Chap. 2, pp. 158-I 79. Cambridge University Press, Cambridge. Thompson W. (1X70) On the equilibrium of vapour at a curved surface of a liquid. Proc. R. Sot. Edit]. 7, 63. Tyndall J. (1875) Si.\- Lrcrures on Light. Longmans Green, London. Waller, R. F.. Brooks A. G. F. and Cartwright J. (1963) An electron microscope study of particles in town air. Int. J. Air Wut. Poll. 7, 779-786. Went F. W. (1960) Organic matter in the atmosphere and its possible relation to petroleum formation. Proc. Nat. Acad. Sci. 46, 212-221. Went F. W. (1966) On the nature of Aitken condensation nuclei. Tellus 18 (2). 549-556. Wilson C. T. R. (195X) On the formation of cloud in the absence dust. Nature 52, 144.