3. Aerosol and Nucleation

A new method of characterizing aerosol particles using an acoustic perturbation (6.25khz and 25khz) and Doppler velocimetry, gives size distribution in the range 0.3--30 m (Cole and Tennal, 1993). Velocity and phase measurements give independent size and density information for spherical particles. It has been known for some time that aerosol losses at sampling inlets can, on occasion, be important. This results, not only from settling and inertial capture, but on a `lift' force, shown to be important when particles cross stream lines to a highly sheared boundary layer, and decelerate rapidly. Wind tunnel studies show a 10-20% loss at 10-20 m s velocity (Fan et al, 1992). Particles may or may not bounce on wall impact, and a technique for exploring this has been developed. (Xu and Willeke, 1993).

Important properties of aerosol depend on their composition and their degree of saturation. Tang and Munkelwitz (1991) have extended earlier work on supersaturated solution droplets to determine refractive index and density of ammonium sulfate, sodium sulfate and sodium nitrate. Droplets were suspended electrostatically and could be evaporated in vacuum and grown under controlled water vapor density subsaturation (eg ammonium sulfate grows at 84%, crystallizes at 39%). A Mie calculation of 90 scattered light showed that the mole refractivity gives prediction of observed refractivity index to 1% accuracy. Water activity of ammonium nitrate/ammonium sulfate solutions and mixtures were measured in an electro-dynamic balance (Chan et al, 1992), with concentration up to 108 molal at relative humidity down to 35%. Activity of these solutions is halved for a solute mass fraction of 0.7, and even lower at higher concentrations. Further measurements with ammonium sulfate--sulfuric acid showed similar results (Kim et al, 1994). This has implications for the freezing behavior and chemical reaction rates in such haze particles in the atmosphere at low relative humidities and temperatures.

Thin films of condensate are studied by infrared spectroscopy using fast fourier transform analysis (Middlebrook et al 1993). This shows that sulfuric acid films, after being deposited at stratospheric conditions on a substrate, crystallized as tetrahydrate and that melting occurred at a much lower temperature than suggested by earlier laboratory studies.

A further aspect of reactivity on aerosol is the observation that thin water films readily affected oxidation of SO to sulfate in the presence of NH (Benner et al, 1992). This points in general to the importance of surface films on aerosol particles and their role in such reactions under a typical atmospheric conditions. Surface films are also important in controlling the shape of particles. As found earlier for acetylene soot, diesel soot aggregates collapse on forming cloud droplets (Huang et al, 1994) thus significantly changing their optical properties.

Mixing chamber studies (Song and Lamb, 1994b) of latex sphere scavenging showed that particles in the range 0.11 to 0.55 m diameter were removed to ice crystals growing from the vapor in the presence of cloud liquid water in a way consistent with an inhibition of the scavenging process by thermophoretic processes (preferred particle diffusion away from warmer regions). This resulted from the growing drop being at a slightly higher temperature because of release of latent heat of condensation, an effect enhanced at high liquid water contents. The ice growth habit and rate had little influence. Chýlek and Hallett (1992) found that soot particles scavenged by cloud droplets remain partly immersed on the surface, thus giving strong absorption for the surface optical wave and a significantly enhanced absorption compared with soot particles uniformly distributed throughout the volume. This could give a significant increase of cloud absorption over that calculated for a model with droplets containing uniformly distributed particles.

Characterization of the ratio of cloud condensation nuclei (CCN) active at less than 1% as a fraction of all nuclei (CN), is important in assessing the role of combustion and photolytically produced aerosol on the particle size distribution and hence optical properties of both water and ice clouds, particularly cirrus. CCN measurements in a dynamic thermal vapor diffusion chamber relate nucleus size and supersaturation for activation of laboratory produced clouds, to be related to the natural aerosol (Hudson et al, 1991).

There is a subtle discussion on the role of a varying deposition coefficient in calibration of such instruments (Dingesen, 1991). Hudson (1993) argues that, providing the chamber samples the same air as the cloud sees, and reproduces the cloud process, there is justification for carrying the chamber measurement to the cloud. Complications can arise only in the case of multiple evaporation and condensation---but as discussed above (Hoppel et al, 1994), these processes may change the nucleus characteristics by chemical reaction in the liquid and also in the much higher concentration of the haze droplet.

Soot with low nitrate (low combustion temperature) and without other water soluble components (low sulfur fuel; lack of salts in fuel) give low (1-2%) CCN/CN ratios. Soot from jet aircraft and combustion of low sulfur crude oil fall in this category (Pitchford et al, 1991; Rogers et al, 1991). Smoke from combustion of wood and vegetation which contain soluble salts on the other hand give ratios approaching 100%, are highly active even at supersaturation as low as 0.3%.

Laboratory studies have demonstrated the importance of the presence of a large number of small particles in controlling supersaturation in a cloud forming process (Hagen et al, 1991). Scavenging of these particles by larger hydrophobic (soot type) particles, result in an effective lower critical supersaturation.

Questions arise concerning mixed phase haze particles and their ability to supersaturate in solution---do soot or mineral particles inhibit such solution supersaturation by providing nuclei. The importance of these effects is demonstrated by a comparison of aerosol size distribution measured by two commercially available aerosol spectrometers---FSSP 300 and the deiced PCASP-100x probe (Strapp et al 1992). The FSSP (forward scattering spectrometer probe) gives the size by intensity of forward light scattering at a selected angle of a cloud sampled in the free air stream flowing between two probes; the PCASP (Passive Cavity Aerosol Spectrometer Probe) takes aerosol into a cavity, which changes its environment. Thus, the latter cooks the particles to dehydration and fails to show changes in size spectra with changes of relative humidity; the former responds as would be expected, showing growth to haze size with increase of relative humidity. It is noted that particles when crystallized to cubes give a slightly different response because of different shape and refractive index, which leads to a calibration shift by 40 to 15% to larger sizes (Liu et al, 1992). Supersaturation is inferred to be a common state for atmospheric haze particles. The importance of such haze particles in ice nucleation is suggested in the field studies of Heymsfield and Miloshevich (1993) who find evidence for such cold lenticular clouds to near --40C, where homogeneous ice nucleation occurs. Thus the ability of a nucleus to dilute with a threshold of supersaturation may be a measure of its ability to freeze. The CCN spectrum may therefore serve as surrogate for ice forming ability when (as is usually the case at levels where temperature approaches --35C) mineral ice forming particles may be absent. A further suggestion, based on reprocessing of aerosol collected on filter papers, is that evaporation of selected cloud droplets precipitates insoluble particles which subsequently act as ice nuclei when subjected to an ice supersaturation (Rosinski and Morgan, 1991). The role of high ice supersaturation in ice nucleation is also of potential importance (Baker 1991). These situations need to be reproduced in any laboratory study.

Ability of particles to both supercool and supersaturate with respect to different hydrate forms is an important attribute which although known for sometime, is just being realized as being of major importance in clouds at low temperature. Sulfuric acid solutions exist in liquid form to temperatures well below --40C and, indeed below at least --80C in sufficient concentration (Ohtake 1993; Zhang et al, 1993). Molina et al (1993) point out the importance of surface reactions in Polar Stratospheric Clouds composed of water ice and nitric acid trihydrate; sufficient surface area leads to significant catalysis of chlorine containing molecules, which aided by photolysis leads to ozone loss. The ability of such particles to nucleate or remain in liquid form is largely unexplored, as is their potential for glass formation.. At these low temperatures other phenomena become important; laboratory studies demonstrate the existence of large clusters of water molecules (40-50) with positive charge (Yang and Castleman, 1991). These could be important as nuclei, as could the small sulfuric acid droplets from volcanic activity. Should these particles be subjected to low relative humidity and evaporate, high supersaturation with respect to hydrates may be present and chemical reactions may well take place which are unknown in the standard chemistry laboratory. Indeed there may be a whole body of knowledge of atmospheric reactions which are currently unsuspected and could be important in haze layers in the earth's atmosphere as well as other planets.

A debate over application of laboratory studies of nucleation by AgI--AgCl aerosol mixed crystals to aircraft studies of seeded clouds raises interesting questions (Demott, 1991; Deshler and Reynolds 1991; Finnegan and Pitter, 1991). Confusion arises because of differing time scales in laboratory and cloud, where distinct physical process occur---Brownian diffusion of aerosol to supercooled droplets (contact nucleation) and a condensation freezing process. Further uncertainties arise because of different conditions in burner combustion in laboratory and aircraft aerosol generation. This debate characterizes the care with which it is necessary to carry over laboratory results to the atmosphere, and the necessity of a thorough understanding of the physical processes in each case. It also characterizes the necessity of such laboratory studies, as the aircraft results by themselves do not yield sufficient data for a full appraisal of the processes.

A cautionary note for aircraft studies of the ice phase in clouds has been the nucleation of ice by the aircraft itself. Sassen (1991) describes a case where a King Air aircraft produced ice in supercooled alto cumulus, temperature between --28 to --31C. A systematic aircraft study in supercooled fog formed over Mono Lake, California (Woodley et al 1991) showed that ice was produced by the King Air only when flying in a high drag configuration, with landing gear and flaps down. A possible process is adiabatic cooling behind propeller tips enhanced at high torque. This hypothesis was demonstrated in the laboratory in simple adiabatic expansion experiments of saturated air from cylinder, volume a few cm (Foster and Hallett, 1993). This showed a cooling to some --45C being necessary in the short time available during the expansion to produce homogeneous nucleation of liquid cloud droplets followed by homogeneous ice nucleation. This situation is more likely to occur in high propeller torque situations, under high drag or maneuver conditions.