Aerosol particles sulfate layer

As Martell has pointed out (30), in the region of the stratospheric large particle layer near 18-20 km. altitude, radioactive aerosol particles become attached to natural sulfate particles in the size range of about 0.1-0.4 jumeter radius. Subsequent upward transport of the radioactive aerosols is opposed by gravitational sedimentation. This combination of processes affords an explanation for the observed accumulation of 210Pb near 20 km. in the tropical stratosphere (2). At higher latitudes where slow mean motions are directed poleward and downward, no such accumulation is possible. 

All sulfur compounds showed relatively constant profiles throughout the lower fright levels which is consistent with the neutral conditions in the mixed layer. As compared to the data obtained during the ship cruise (Tables I and II) the concentrations in the lowest flight level (30 m) were about 2-3 times lower for DMS, nearly the same for SO2, and about 3 times higher for MSA and nss-SC>42 The fraction of nss-SC>42 to total sulfate at this level was 18%, similar to the results from the ship cruise. The relatively higher concentrations observed for MSA and nss-SC>42 indicate a significant accumulation of aerosol particles in the mixed layer due to the postfrontal inversion between the mixed layer and the free troposphere. 

During periods of intense volcanic activity large quantities of SO2 can be injected into the stratosphere, increasing the concentration of sulfate aerosols. In 1991 Mt. Pinatubo, in the Philippines, injected some 20 Tg of SO2 into the stratosphere. Under normal conditions aerosol sulfate concentrations are 1-10 particles cm , although after eruptions this can rise by as much as two orders of magnimde. Peak sulfate levels in the lunge layer can increase from around 0.1p,gm to 40 p,g m. The sulfate layer appears to take about six months to form through the slow oxidation of SO2 into a sulfate aerosol. 

Chloride levels. A high level of chloride in a marine-industrial environment, such as 30-40 mg/m /day, leads to a high density of pitting of aluminum and its alloys. Incorporation of chlorides into the aqueous layer on an aluminum substrate from deposition of sea salts, aerosol particles or from an organic gas containing chlorine may lead to the formation of AICI3 or A1(0H)2C1, which are soluble in weak acidic solution and do not offer resistance to aluminum dissolution. Sulfides and sulfates. In aqueous layers, SO2 dissolves and ionizes to produce HSO ion. 

FIGURE 4 Aerosol mass spectrometer airborne measurements of fine particle compositional mass loading and size distributions showing two sulfate aerosol layers in air masses from the Ohio River valley. 

Fogs in polluted environments have the potential to increase aerosol concentrations by droplet-phase reactions but, at the same time, to cause reductions because of the rapid deposition of larger fog droplets compared to smaller particles (Pandis et al. 1990). Pandis et al. (1992) estimated that more than half of the sulfate in a typical aerosol air pollution episode was produced inside a fog layer the previous night. 

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