I think you misunderstood what the authors are suggesting. The idea is to attach an extremely long hose to the smokestacks of factories, and support the hose with balloons every few hundred feet, so that it reaches the stratosphere. Essentially it would be extending the factories' smokestacks a few miles up, so that the SO2 is in the stratosphere instead of the troposphere.
Hmmmm......well, smokestacks work because smoke is, well, hot, and the ambient pressure is essentially equal at the top and bottom of the chimney. Thus the warm air . So the chimeny is initially warmed and the heat warms the air which expands and thus becomes bouyant relative to the air surrounding the chimney and rises through the chimney.
A seven mile tall chimney isn't going to work that way, if it works at all. Any of y'all ask why there's no clouds in the stratosphere? Of course you have. It's because the warm air from the ground cools as it expands, and eventually there's a point reached where it's at ambient temperature and pressure, stops expanding, and there's it's ceiling.
Now, anyone taking the junior heat transfer course in their engineering curriculum learns that the pin-fin is one of the most effective radiators and convective surfaces (certainly the easiest to analyze), and one can clearly see that a seven mile long tube is nothing more than a flexible pin fin for analytical purposes, and that puppy will cool the exhaust gases most effectively.
I don't think a chimney opened in the stratosphere will work, unless it's heated to keep it drawing.
That's not even considering the fact that well, the Earth has what the natives call "weather" and makes securing this monstrosity somewhat problematical. Remember, making something stronger means it's going to be heavier, so the balloons supporting it will be bigger, which means they'll be more subject to aerodynamic drag forces, which means the assembly will need additional strength for that...
...and what happens when a "hose" long enough to stretch the length of Manhattan comes down? How many people does it kill, houses does it break, and roads does it block?
How does "Freakonomics" account for these engineering difficulties and hazards to public safety?