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is proposed that CCN emissions should be released over the oceans, that the release should produce an increase in the stratocumulus cloud albedo only, and that the clouds should remain at the same latitudes over the ocean where the surface albedo is relatively constant and small.

Albrecht (1989) estimates that a roughly 30 percent increase in CCN would be necessary to increase the fractional cloudiness or albedo of marine stratocumulus clouds by 4 percent. Albrecht's idealized stratocumulus cloud, which he argues is typical, has a thickness of 375 m, a drizzle rate of 1 mm per day, and a mean droplet radius of 100 mm, and he assumes that each droplet is formed by the coalescence of 1000 smaller droplets. The rate at which the CCN are depleted by his model is 1000/cm3 per day. Consequently, about 300/cm3 per day (30 percent of 1000) of additional CCN would have to be discharged per day at the base of the cloud to maintain a 4 percent increase in cloudiness. This assumes that the perturbed atmosphere would also remain sufficiently close to saturation in the vicinity of the CCN that additional cloud cover would be formed every time the number of CCN increased.

Mass Estimates of Cloud Condensation Nuclei

With Albrecht's assumption in mind that cloudiness in a typical ocean region is limited by the small number of CCN, we now extrapolate to the entire globe. On the average, 31.2 percent of the globe is covered by marine stratiform clouds (Charlson et al., 1987). If no high-level clouds are present, the number n of CCN that need to be added per day is 1.8 × 1025 CCN/day. The mass of a CCN is equal to 4/3pr3 × density, and it is assumed that the mean radius r is equal to 0.07 × 10-4 cm (Charlson et al., 1987). Because the density of sulfuric acid (H2SO4) is 1.841 g/cm3, the CCN mass is 2.7 × 10-15 g. The total weight of H2SO4 to be added per day is 31 × 103 t per day SO2 if all SO2 is converted to H2SO4 CCN.

To put this number in perspective, a medium-sized coal-fired U.S. power plant emits about this much SO2 in a year. Consequently, the equivalent emissions of 365 U.S. coal-burning power plants, distributed homogeneously, would be needed to produce sufficient CCN.

To estimate the value of the sulfur directly, the total weight of SO2 to be added per day would equal 32 × 103 t, or about 16 × 103 t of sulfur (S) per day, which is equivalent to about 6 × 106 t S/yr. If the average market price of sulfur delivered at the mine or plant is taken as $96.60/t for the years 1983 to 1987, the cost would be about $580 million per year. Equating this yearly cost to the 300 parts per million by volume (ppmv) of CO2 necessary for full compensation gives $580 × 106/yr/(3890 × 106 t C/ppmv CO2 × 300 ppmv CO2), or about a fraction of 1 cent/t CO2. To obtain an equivalence to conserved carbon, known emissions of carbon in 1978, 1979, and 1980