The manipulation of certain aspects of the environment to bring about changes in weather has been carried out for over half a century. Cloud seeding is a widely used technique that serves various climate objectives, such as altering the amount or type of precipitation, suppressing hailstorms, or weakening hurricanes (List, 2011). This practice involves the dispersal of seeding agent particles into the atmosphere to induce cloud condensation or ice nuclei formation.
Goals of Cloud Seeding
Cloud seeding is employed for several important purposes:
- Enhancing Precipitation: This is particularly beneficial for agricultural areas that suffer from drought. By increasing rainfall, cloud seeding helps boost crop yields and replenish water reservoirs.
- Hail Suppression: In regions prone to hailstorms, cloud seeding is used to minimize damage to crops, infrastructure, and property. By altering the microphysical properties of hail-forming clouds, the size and frequency of hailstones can be reduced.
- Storm Mitigation: Efforts to weaken hurricanes or large storms involve modifying their intensity. This could be done to protect coastal areas from severe damage.
Substances Used in Cloud Seeding
The most commonly used substance in cloud seeding operations is silver iodide (AgI), favored for its crystalline structure that closely resembles that of ice, thus facilitating the freezing of supercooled water droplets (Ćurić & Janc, 2013). Silver iodide is typically dispersed using acetone generators, where AgI is suspended in acetone. The acetone is burned to produce a smoke containing ice nuclei, which is then transported into clouds using natural atmospheric turbulence. Other substances that are sometimes used include sodium chloride (common salt) and dry ice (solid carbon dioxide). These materials also act as condensation or freezing nuclei to enhance precipitation.
Environmental and Health Concerns
Despite its widespread application, cloud seeding has raised concerns about its potential environmental and health impacts. According to the Clean Water Act guidelines of the Environmental Protection Agency (EPA), silver iodide is classified as a hazardous, priority pollutant and a toxic contaminant (dangerous goods). This classification stems from the fact that cloud seeding releases AgI into the atmosphere over specific areas, and cumulative annual releases in some instances may reach up to 3 metric tons (Eisler, 1996).
Organizations like the Office of Environment, Health and Safety and the University of California, Berkeley, have identified AgI as a Class C, insoluble, inorganic hazardous chemical that contaminates both water and soil (WHO, 1977). The Australian Drinking Water Guidelines have established a concentration threshold of 0.43 µM AgI for drinking water to mitigate potential health risks (NHMRC/NRMMC, 2004).
Toxicological Impact and Ecological Risks
The ecotoxicological effects of various silver compounds are mainly associated with the toxicity of free silver ions (dangerous goods). Elemental silver exists in different oxidation states, but in environmental conditions, only the solid silver state (Ag⁰) and the silver ion state (Ag⁺) are common. In water, silver can exist as free silver ions or associate with negative ions (Purcell & Peters, 1998; Cardno ENTRIX, 2011). Free silver ions from soluble silver salts are known to be toxic to fungi, algae, and bacteria, even at relatively low doses (Williams & Denholm, 2009).
However, silver iodide is much less soluble and therefore considered significantly less toxic. Studies have shown that the insolubility of AgI makes it a poor source of free silver ions, rendering it non-toxic or minimally toxic to a variety of land and aquatic organisms (Cooper & Jolly, 1970; Ratte, 1999). This insolubility is critical for cloud seeding success since dissolution would prevent the formation of effective nucleation sites.
Cumulative Environmental Impact
Despite the lower immediate toxicity, the cumulative environmental effects of silver iodide cloud seeding are a concern. The large-scale, repeated introduction of AgI into the atmosphere over the same regions could lead to a gradual buildup of this compound in soil and water systems. After cloud seeding events, significant quantities of seeding materials are deposited onto the ground through wet deposition, posing potential risks to both terrestrial and aquatic ecosystems (Ćurić & Janc, 2013; Li et al., 2011).
These concerns highlight the necessity of evaluating the long-term impact of cloud seeding. Continuous monitoring and rigorous ecotoxicological assessments are required to understand the potential accumulative effects and ensure that environmental and human health risks are minimized. The current research aims to simulate these scenarios and determine the impact of various AgI concentrations on a range of organisms to inform safe and sustainable weather modification practices.
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