Atmospheric particulate pollution, which consists of airborne solid or liquid particles generated from a variety of sources, is most often associated with a visible haze and adverse impacts on human health. Particles, also referred to as aerosols, in the lower atmosphere have natural sources such as volcanic emissions, wild fires, and sea spray, as well as anthropogenic sources such as fossil fuel combustion, prescribed forest fires, and road dust. Once generated, tropospheric particles usually reside in the atmosphere for days to weeks before being removed by precipitation or dry deposition to surfaces. It is well-known that particles impact climate  , inducing both warming and cooling forcings depending on the aerosol composition, meteorological conditions, and location over the Earth surface. Particulate matter can directly affect climate by scattering (cooling effect) or absorbing (warming effect) solar radiation. Particles may also indirectly impact climate by serving as cloud condensation nuclei (CCN) and altering cloud formation. Increased CCN leads to smaller cloud droplets, creating brighter and longer-lasting clouds that would increase planetary albedo .
The study of atmospheric particles is an active field of research, with a diversity of scientific disciplines studying the chemical and physical mechanisms of particle formation, impacts on human health and the ecosystem, and climate linkages. Particulate pollution in the lower atmosphere is currently regulated in the United States under the EPA National Ambient Air Quality Standards (NAAQS) for specific size fractions known as "fine" particulate matter (PM25 or particles smaller than 2.5 mm) and "coarse" particulate matter (PM10 or particles smaller than 10 mm). Fine and coarse particle standards are also in place in other countries, although the ambient concentration limits vary.
Ambient air pollution and its associated climate effects are unintentional by-products of fossil fuel powered industrialization. In addition to this inadvertent climate forcing, the long-standing practice of weather modification purposefully injects CCN into already existing clouds in an attempt to control precipitation. First introduced in the 1940s by researchers in the USSR and in the United States ( and references therein), weather modification is still in use today worldwide. Weather modification, also commonly referred to as "cloud seeding", is usually practiced by dispersing a chemical compound to serve as CCN using aircraft or ground-based rockets. Two different particle types have been used - glaciogenic particles (e.g., silver iodide) which are used as ice nuclei in cold clouds and hygroscopic particles (e.g., salt) which serve as water droplet nuclei in warm clouds. Over its history, cloud seeding was used and then banned as a military strategy  and has also been peacefully applied worldwide to meet specific regional goals. Cloud seeding has been applied as a strategy to enhance snow fall for ski resorts, boost precipitation for agriculture or drought-stricken regions, prevent hail formation, and even reportedly to reduce pollution and control rainfall during the 2008 Olympics in Beijing, China .
While cloud seeding has been widely practiced to induce precipitation, results have been mixed. Particle-cloud interactions are complex and the study of these interactions has been limited until recently. Geoengineering climate using a cloud seeding process has a nearly opposite goal of traditional cloud seeding operations - to reduce cloud droplet size and extend the lifetime of a cloud that overlies a region with low solar reflectivity, leading to higher planetary albedo. Rather than enhance precipitation which is the general goal of weather modification, this strategy would lower the chance of precipitation by preventing the growth of large cloud droplets.
Mitigation Strategy: Tropospheric cloud-seeding would be implemented over the ocean, increasing the reflectivity of marine stratocumulus clouds [27-306, 22]. As ocean-covered portions of Earth have low albedo, increased reflective cloud coverage would have a significant cooling effect.
Feasibility: Several methods have been proposed to support long-term seeding of marine stratocumulus clouds. One suggested method would be to continuously emit sulfur dioxide over the ocean, which would convert to sulfate particles and seed clouds overlying the ocean . A more recent proposal is to aerosolize seawater using unmanned wind-powered vessels, resulting in salt particles to operate as CCN [30, 31]. Both studies projected implementation costs, which would equate to approximately $10-100 billion if operated at a level sufficient to offset a doubling of CO2 and maintained for a time horizon of 50 years (providing time to lower greenhouse gas emissions). However, it should be noted that these cost estimates depend on underlying assumptions about the fraction of Earth's surface covered with marine stratocumulus clouds and the required amount of CCN to change cloud albedo, which are of debate . Cloud seeding for climate application is only a concept at this point, with little data existing to show whether extensive cloud-seeding over the ocean would substantially alter the planetary albedo and sustain a climate forcing on timescales of years to decades.
Co-benefits and undesirable consequences: No co-benefits have been identified for this strategy. Potential consequences include the reduction of PAR and subsequent suppression of ocean phytoplankton growth, deposition of acidic particles to the ocean, modification of precipitation patterns, altered storm frequency, and intensity, and human exposure to respiratory pollutants. Human exposure risk is of substantial concern for particulate matter injected into the troposphere, as particles can be transported up to thousands of kilometers from the point of injection before removal from the atmosphere by wet or dry deposition. While these consequences have not been well studied for the application of large-scale cloud-seeding operations, research on anthropogenic particulate pollution may provide some understanding of the potential impacts. One important aspect of a geoengineering cloud seeding program is the capability to rapidly "turn off' the climate forcing, as tropospheric particles have a short lifetime of days to weeks. This can be seen as a benefit in allowing greater control over the process and capability to respond to unintended consequences. However, the need for continuous operation also implies that a major interruption in large-scale geoengineering via cloud seeding may lead to a sudden change in global climate.
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