Laserfiche WebLink
<br />- Eil - <br /> <br />diameter of 7.5 and a height of 0.8 cm. As a sample of air comes into equilibrium, small <br />water droplets form on those particles that act as CCN at the peak supersaturation in the <br />chamber. It is assumed that there is sufficient mixing in the chamber so that all the par- <br />ticles will pass through the region of maximum supersaturation. The concentration of droplets <br />is counted photographically, by television techniques or by light-scattering counters <br />(Laktionov, 1968; Radke and Hobbs, 1969; Sinnarwalla and Alofs, 1973i Hudson and Squires, <br />1976). By repeating the above procedure with different temperature distributions in the <br />chamber, the concentrations N of CCN in the air at several different supersaturations can <br />be determined. If this supersaturation spectrum is close to linear on a logarithmic plot, <br />one can write <br /> <br />N = c(S - 1) k, <br /> <br />(7) <br /> <br />where c and k are constants, which characterize the spectra. <br /> <br />lENS U = tell <br />PlATE Wlllf SLIT <br />Pt<<JrOOETECTOR <br />(lIELl 8 I<<lWEL <br />509-501 <br /> <br /> <br />IllUMINJITIl* <br />WINOOI '- <br /> <br />SCALE <br />I 1t<<:H <br /> <br />~ <br /> <br /> <br />1CMi <br /> <br /> <br />CHAMBER <br />WALL <br />SCATTERING AND <br />PHOTOGRAPHIC <br />WlUME <br />PHOTOGRAPHIC <br />WINDOW <br /> <br />. .. ~ ... . <br /> <br />'~ <br />..... ' if. AEROSOL OOllET <br /> <br />~: Schematic diagram of the etatic diffusion chamber showing the location of the <br />scattering volume with respect to the illumination window. photographic window and <br />the scattered light measurement system. (From Lala and Jiusto, 1977). <br /> <br />In Fig. 6 experimentally determined spEctra from aircraft measurements in different geo- <br />graphic regions are plotted together with the distribution of the slope parameter k. The <br />mean value of k obtained from a series of long distance flights was found to be about 1/2. <br />Since the critical supersaturation of cl soluble particle of mass m is proportional to m-1/2 <br />(see above) and thus (Sc - 1) CI. r-3/~while the cumulative number of particles above a <br />minimum radius r ina Junge distribution is proportional to r-3 (see equation (5)), such <br />a distribution would produce N = c(S - 1)2 representing a much steeper slope than has been <br />found in most measured spectra. Values of k around 2 have only been reported by Radke et al., <br />(1976) for a remote site in Alaska. Gererally, though. the CCN do not follow a Junge <br />distribution law or else the proportior of insoluble material in the particles or the <br />proportion of insoluble particles must increase with decreasing size. CCN measurements <br />made at hourly intervals during 5 years (Twomey et al., 1978) show a very clear diurnal <br />pattern with a maximum at approximately 1800 local time and a minimum at 0600. This sug- <br />gests that photochemical reactions playa major role in the generation of CCN. Indeed <br />many CCN consist of sulphates which may have been formed by a so-called gas-to-particle <br />conversion from sulphur dioxide, enhanced by high relative humidity or the presence of <br />liquid water (Easter and Hobbs, 1974). <br />