When doing a google search I found that this story and claim are over exaggerated. You can find in a couple articles that Bill Gates was doing experiments that sprayed something into the air. Time magazine reports, "Bill Gates, for instance, backed a project by Harvard University scientists to test an idea to spray calcium carbonate into the atmosphere in the skies over northern Scandinavia in 2021 (the project was ultimately canned after outcry from local Indigenous groups and environmentalists)"
https://time.com/6258126/solar-geoengineering-billionaries-george-soros/
https://www.forbes.com/sites/arielcohen/2021/01/11/bill-gates-backed-climate-solution-gains-traction-but-concerns-linger/?sh=141c8be793b6
Harvard's Stratospheric Controlled Perturbation Experiment, or SCoPEx, which is financially backed by Gates. If you look on USA today they talk about the misleading thought behind these claims and how they started from a social media post.
https://www.usatoday.com/story/news/factcheck/2021/04/23/fact-check-bill-gates-not-trying-block-suns-rays/7310134002/
When I look on the actual website Keutsch Group at Harvard. They explain that right now they aren't sending chemicals in our atmosphere to interact but just to observe. According to the main website, "SCoPEx is a scientific experiment to advance understanding of stratospheric aerosols that could be relevant to solar geoengineering. It aims to improve the fidelity of simulations (computer models) of solar geoengineering by providing modelers with experimental results vital to addressing specific science questions....At the heart of SCoPEx is a scientific balloon, fitted with repurposed off-the-shelf airboat propellers. The repurposed propellers serve two functions. First, the propeller wake forms a well mixed volume (roughly 1 km long and 100 meters in diameter) that serves as an experimental ‘beaker’ in which we can add gasses or particles. Second, the propellers allow us to reposition the gondola to different locations within the volume to measure the properties of the perturbed air. The payload can achieve speeds of a few meters per second (walking speed) relative to the surrounding air, generally for about ten minutes at a time.
The advantage of the SCoPEx propelled balloon is that it allows us to create a small controlled volume of stratospheric air and observe its evolution for (we hope) over 24 hours. Hence the acronym, Stratospheric Controlled Perturbation Experiment. If we used an aircraft instead of a balloon, we would not be able to use such a small perturbed volume nor would we be able to observe it for such long durations."
https://www.keutschgroup.com/scopex#h.p_Xru29Emo-OMw