Article: How our balloons work
We currently use biodegradable latex balloons filled with hydrogen gas and sulfur dioxide (SO₂) to create reflective clouds in the stratosphere. Here's how it works:
Balloon setup:
The amount of hydrogen gas is calculated based on the balloon size, payload weight, and the desired burst altitude. Our goal is to release the reflective cloud above 20 km (66,000 ft) in the stratosphere. A calculator helps ensure precise measurements.
Ascent and release:
After launch, the balloon expands as it rises, due to decreasing air pressure, and eventually bursts. If the payload bursts above 66,000 ft, we issue Cooling Credits. If telemetry does not confirm stratospheric deployment, we re-deploy as needed.
Article: Make Sunsets is a tiny start-up headquartered in South Dakota that is using balloons to release small quantities of sulfur dioxide into the upper atmosphere, in the hope of reflecting some of the Sun's energy away from the earth. Each gram of SO2, says Andrew Song, one of Make Sunsets' founders, offsets the warming from one metric ton of carbon dioxide released by burning fossil fuels. Not everyone is convinced by Make Sunsets' methods, however—and many researchers and environmentalists worry the startup's unregulated operations are disrupting more responsible research into geoengineering, including a prominent effort at Harvard.
Make Sunsets' name is a reference to the dramatic sunsets that high-altitude SO2 particles can produce, as seen following the eruption of Mount Pinatubo in 1991. That eruption briefly depressed global temperatures by about 0.2 °C for a year, until the particles slowly returned to Earth.
[...]
"There's a lot of disagreement among folks thinking about solar geoengineering, but most agree that Make Sunsets is a bad idea," says Sikina Jinnah, a professor of environmental studies at the University of California Santa Cruz. Jinnah was also the co-chair of Harvard University's Stratospheric Controlled Perturbation Experiment (SCoPEX) advisory committee, one of the first efforts to design a governance framework for an outdoor solar geoengineering experiment.
"A couple of rogue tech bros taking action completely outside the scope of government authority or any public engagement are really embodying the nightmare of what folks think this could be," Jinnah says.
The consensus among most atmospheric scientists is that we have only a very limited understanding of how to inject particles effectively into the upper atmosphere without triggering side effects such as damage to the ozone layer, disrupting weather patterns like monsoons, or causing pollution at ground level. Solar geoengineering could also lock us into having to continue to inject particles essentially forever, in order to avoid the "termination shock" of a sudden temperature rise.
Article: Make Sunsets does not employ any scientists. The employees are just Iseman and Song, who met while Iseman worked at a tech incubator and Song worked as an outreach manager at a crowdfunding website. Iseman says if they scale up the company, they'll hire scientists.
[...]
The United Nations Convention on Biological Diversity – a treaty that protects wildlife – has a moratorium on geoengineering that affects living species. But the moratorium is guidance, and non-binding. And "the U.S. is actually not a party" to the treaty, says Daniel Bodansky, a climate legal expert and law professor at Arizona State University.
And to put that in a broader perspective:
I'm surprised the Asian output doesn't follow China.And to put that in a broader perspective:
dynamically-generated image from: https://ourworldindata.org/grapher/so-emissions-by-world-region-in-million-tonnes
https://miro-analytical.com/so2-molecule-of-the-month-may/External Quote:
If it "reflects solar radiation", I would expect something. Unless the SO2 has gone liquid and coats the ballon scraps now? @Ann K?
I'd presume their droplets are nucleation sites for water vapour, which would be a far more abundant, and thus we're just viewing them how we would any other cloudy thing, including contrails. In which case, it's not the fact that it's specifically SO2 that's the problem, a nucleation site is a nucleation site, no matter what it's made of.
(Disclaimer: not my field.) If there is liquid SO2 when it bursts, that would be a temporary condition. I would expect a small delay until it sublimes, added to whatever delay needed for it to seed and form the clouds. This, of course, causes some cooling ...but the SO2 is a greenhouse gas anyway once it gets to the upper atmosphere, so it's six of one, half a dozen of the other.
I don't think that's true for that altitude.
Article: The stratosphere extends from 4 -12 miles (6-20 km) above the Earth's surface to around 31 miles (50 km). This layer holds 19 percent of the atmosphere's gases but very little water vapor.
In this region, the temperature increases with height. Heat is produced in the process of the formation of ozone, and this heat is responsible for temperature increases, from an average -60°F (-51°C) at tropopause to a maximum of about 5°F (-15°C) at the top of the stratosphere.
This increase in temperature with height means warmer air is located above cooler air. This prevents convection as there is no upward vertical movement of the gases. As such, the location of the bottom of this layer is readily seen by the anvil-shaped tops of cumulonimbus clouds
Article: As is indicated in the above table different types of cloud are formed in different atmospheric conditions – temperature – humidity – dynamics – stability. From a microphysical point of view, clouds may be liquid, mixed phase, or ice and different microphysical processes are involved in each case.
https://www.mide.com/air-pressure-at-altitude-calculator gives altitude 19812m ~ 5.6kPa. pressure. The temperature is ~222K.
Article: 
Sulfur dioxide 197.69 K (−75.46 °C) 1.67 kPa (0.0165 atm)
Article: Effects of Different Stratospheric SO2 Injection Altitudes on Stratospheric Chemistry and Dynamics (2018)
Two different scenarios are explored that produce approximately the same global cooling of 2°C over the period 2042–2049, a high-altitude injection case using 24 Tg SO2/year at 30 hPa (≈25-km altitude) and a low-altitude injection case using 32 Tg SO2/year injections at 70 hPa (between 19- and 20-km altitude), with annual injections divided equally between 15°N and 15°S. Both cases result in a warming of the lower tropical stratosphere up to 10 and 15°C for the high- and low-altitude injection case and in substantial increases of stratospheric water vapor of up to 2 and 4 ppm, respectively, compared to no geoengineering conditions.
On average both simulations reduce surface temperature by about 2°C.
