What 8 chemicals are sprayed in Geo-engineering
Executive summary
Public discussion and peer-reviewed work show that proposed solar geoengineering schemes have centered on a small set of aerosol materials and engineered particles—most prominently sulfur compounds and a handful of particulate candidates such as black carbon and aluminum derivatives—but there is no authoritative list of eight “sprayed chemicals” in operational use because large-scale deployment has not occurred and research papers discuss hypothetical or modeled materials rather than an agreed deployment recipe [1] [2] [3]. Based on government, academic and review reporting, eight chemicals or material classes commonly named in the literature are: sulfur dioxide, sulfate/sulfuric acid aerosols, sulfate aerosol precursors, black carbon/soot, aluminum/alumina (aluminum oxide), barium titanate, engineered nanoparticles (various designed aerosols), and generic “sulfate aerosol” materials used in models; each carries different modeled risks and much uncertainty [4] [5] [3] [6].
1. Sulfur dioxide (SO2) — the most discussed precursor for stratospheric aerosols
Sulfur dioxide is repeatedly cited as the primary candidate for stratospheric aerosol injection because volcanic analogues show SO2 oxidizes to form reflective sulfate aerosols, and multiple science reviews and government summaries highlight SO2 injection as a leading theoretical approach to reflect sunlight and cool the planet [4] [5] [2].
2. Sulfate aerosols / sulfuric acid — the actual reflective particles models use
The material that would scatter sunlight in most modeled SAI scenarios is sulfate aerosol (including sulfuric acid droplets formed from SO2), and studies emphasize that the chemistry of sulfate in the stratosphere is complex, with consequences for ozone, stratospheric heating and tropospheric chemistry [5] [6] [7].
3. Sulfate aerosol precursors (modeling language distinction)
Technical literature distinguishes between the precursor gas (SO2) and the resulting sulfate burden; regulatory and modeling documents therefore list “sulfate aerosol precursors” separately because different injection strategies, altitudes and chemical pathways change outcomes—this is why some sources describe sulfate precursors as a distinct subject of study [3] [8].
4. Black carbon / soot — modeled but widely cautioned against
Black carbon (soot) has been modeled as a possible stratospheric additive and is explicitly discussed in health-impact and modeling papers, but authors warn that black carbon would absorb sunlight, warm the stratosphere, and worsen health impacts—models show even modest black carbon injection has outsized atmospheric and health costs [3] [2].
5. Aluminum and aluminum oxide (alumina) — particulate alternatives under debate
Several commentaries and risk-focused pieces list aluminum-based particles—including aluminum metal and alumina (aluminum oxide)—as proposed engineered aerosols because they scatter sunlight, and some researchers and public-health commentators raise concerns about potential ecological and neurological effects if large quantities were deposited [9] [3].
6. Barium titanate — cited as a speculative engineered aerosol
Barium titanate appears in the literature as an example of a high-refractive-index engineered particle proposed in concept studies; authors note, however, that models have not robustly estimated global burdens or long-term impacts for barium-titanate deployment, leaving its risks largely unquantified [3].
7. Designed nanoparticles / “engineered” particles — a catch‑all for proposed novel aerosols
Beyond plain sulfates and soot, the literature repeatedly refers to “designed nanoparticles” or bespoke aerosol chemistries that might optimize reflectivity and lifetime; these remain conceptual, are discussed in modeling and lab work, and are flagged as requiring much more chemistry and safety study before any field deployment [3] [10].
8. Sulfate-related terminology and modeled sulfate burdens (sulfate vs. sulfate burden)
Because many sources use slightly different terminology—sulfate aerosols, sulfate burden, sulfuric acid droplets—counting them separately highlights that geoengineering discourse often treats the gas, the particle, and the modeled mass burden as distinct entities for policy and risk assessment, which is why “eight” named items in reporting can reflect conceptual categories rather than eight discrete chemicals actually being sprayed [6] [1].
Conclusion and limits of reporting
The available sources agree on the leading candidates (SO2 → sulfate aerosols, black carbon, aluminum-based particles) and also name speculative engineered materials such as barium titanate and generic nanoparticles, but there is no evidence of an operational global spraying program with a definitive eight-chemical manifest; the documents referenced are modeling studies, government summaries and risk reviews that discuss candidates and potential impacts rather than reporting an implemented recipe [4] [3] [8]. Where sources name materials they also emphasize large uncertainties: different injection strategies, chemical feedbacks, ozone impacts and health risks remain active research topics that prevent simple cataloguing of “what is sprayed” as if deployment were settled science [6] [5].