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"From Tang and Munkelwitz (1993, 1994a, 1994b); note there is a hysteresis so that the RH at which the salts recrystallize as the RH drops (the effluorescence RH) is lower than these values. 6 From Goldberg (1981).

"From Tang and Munkelwitz (1993, 1994a, 1994b); note there is a hysteresis so that the RH at which the salts recrystallize as the RH drops (the effluorescence RH) is lower than these values. 6 From Goldberg (1981).

Paschen Breakdown Mars Gas
FIGURE 9.40 Calculated changes in particle size as a function of relative humidity at 25°C from particles with four different chemical compositions (adapted from Tang, 1980).

size as expected. They suggest that treating light scattering by such aerosols in terms of an external mixture rather than an internal mixture is satisfactory.

It should be noted that hysteresis occurs as the particles are dried out; i.e., the liquid solution does not form a solid particle as the water evaporates at the same RH as it went through the solid —> liquid transition. Typically one must reach RHs 20-30% or more below the deliquescence point in order to "dry" the particle. Figure 9.41, for example, shows the uptake of water by solid (NH4)2S04 and its subsequent dehydration (Tang et al., 1995). At 80% RH the solid deliquesces but does not solidify (effluoresce) on drying until an RH of 37% is reached. (The presence of other species has been shown to increase this effluorescence

Relative humidity (%)

FIGURE 9.41 Uptake of water by (NH4)S04 and its drying as a lunction of relative humidity at 25b3C (adapted from Tang et at., 1995).

Relative humidity (%)

RH (e.g., Oatis et al., 1998). in addition, there is evidence that new metastable solid states can form in these droplets (Tang et al., 1995). Similar studies of ammonium bisulfate, NH4HS04, have been carried out over a range of temperatures and relative humidities relevant to the atmosphere, and its phase diagram, including metastable states and a crystalline hydrate phase at low temperatures has been reported (Imre et al., 1997).

The uptake of water by the particles as the RH is increased can be followed by measuring the light scattering of the aerosol with an integrating nephelometer. Figure 9.42 shows the light scattering coefficient 6sp (as a ratio to that at 30%) as a function of RH for a pure H2S04 aerosol and for one to which NH3 has been added. 6sp increases monotonically for the hygroscopic H2S04 particles. However, when NH3 is added, converting the aerosol to (NH4)2S04, a sharp inflection point is seen at ~80% RH, as expected for ammonium sulfate. One can thus distinguish between H2S04 and (NH4)2S04 by the dependence of the light scattering properties on RH, with and without added NH3 (Charl-son et al., 1974a). While this has the advantage of providing an in situ measurement of the chemical nature of the sulfates, the method responds mainly to those particle sizes that scatter light efficiently (i.e., ~ 0.1-1 /¿m); fortunately, this also appears to be in the range of greatest importance for health and visibility effects. When the particles are not pure sulfates, but mixtures with other species, the (NH4)2S04 must be at least 30-50 mol% to show a clear inflection point in the humidogram.

Using this humidograph technique, the presence of H2S04 (NH)4HS04 or (NH4)2S04 as a major component of ambient light scattering aerosols has been established in a number of locations. (NH4)2S04 was frequently observed, suggesting that there is often sufficient NH3 present in ambient air to completely neu

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