From Toon and Pollack (1976), one can make a crude estimate of the mass of lower stratospheric dust by noting that they give the density at 20 km as about 1 mg/m3. The "all sizes" curve in their Figure 9 suggests a reasonably constant concentration in the 8 km from 12 to 20 km. So the mass can be taken to be roughly 5 × 1014 m2 × 8 × 103 × 10-6 g/m3 = 40 × 1011 g = 4 × 1012 g = 4 × 109 kg, or roughly half the amount to be injected to form the screen.