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Aerosols (tiny solid or liquid particles suspended in the atmosphere) have been in the forefront of environmental and climate change sciences as the primary atmospheric pollutant and external force affecting Earth’s weather and climate. There are two dominant mechanisms by which aerosols affect weather and climate: aerosol-radiation interactions (ARIs) and aerosol-cloud interactions (ACIs). ARIs arise from aerosol scattering and absorption, which alter the radiation budgets of the atmosphere and surface, while ACIs are connected to the fact that aerosols serve as cloud condensation nuclei and ice nuclei. Both ARIs and ACIs are coupled with atmospheric dynamics to produce a chain of complex interactions with a large range of meteorological variables that influence both weather and climate. Elaborated here are the impacts of aerosols on the radiation budget, clouds (microphysics, structure, and lifetime), precipitation, and severe weather events (lightning, thunderstorms, hail, and tornadoes). Depending on environmental variables and aerosol properties, the effects can be both positive and negative, posing the largest uncertainties in the external forcing of the climate system. This has considerably hindered the ability to project future climate changes and make accurate numerical weather predictions.


Sumit Sharma, Liliana Nunez, and Veerabhadran Ramanathan

Atmospheric brown clouds (ABCs) are widespread pollution clouds that can at times span an entire continent or an ocean basin. ABCs extend vertically from the ground upward to as high as 3 km, and they consist of both aerosols and gases. ABCs consist of anthropogenic aerosols such as sulfates, nitrates, organics, and black carbon and natural dust aerosols. Gaseous pollutants that contribute to the formation of ABCs are NOx (nitrogen oxides), SOx (sulfur oxides), VOCs (volatile organic compounds), CO (carbon monoxide), CH4 (methane), and O3 (ozone). The brownish color of the cloud (which is visible when looking at the horizon) is due to absorption of solar radiation at short wavelengths (green, blue, and UV) by organic and black carbon aerosols as well as by NOx. While the local nature of ABCs around polluted cities has been known since the early 1900s, the widespread transoceanic and transcontinental nature of ABCs as well as their large-scale effects on climate, hydrological cycle, and agriculture were discovered inadvertently by The Indian Ocean Experiment (INDOEX), an international experiment conducted in the 1990s over the Indian Ocean. A major discovery of INDOEX was that ABCs caused drastic dimming at the surface. The magnitude of the dimming was as large as 10–20% (based on a monthly average) over vast areas of land and ocean regions. The dimming was shown to be accompanied by significant atmospheric absorption of solar radiation by black and brown carbon (a form of organic carbon). Black and brown carbon, ozone and methane contribute as much as 40% to anthropogenic radiative forcing. The dimming by sulfates, nitrates, and carbonaceous (black and organic carbon) species has been shown to disrupt and weaken the monsoon circulation over southern Asia. In addition, the ozone in ABCs leads to a significant decrease in agriculture yields (by as much as 20–40%) in the polluted regions. Most significantly, the aerosols (in ABCs) near the ground lead to about 4 million premature mortalities every year. Technological and regulatory measures are available to mitigate most of the pollution resulting from ABCs. The importance of ABCs to global environmental problems led the United Nations Environment Programme (UNEP) to form the international ABC program. This ABC program subsequently led to the identification of short-lived climate pollutants as potent mitigation agents of climate change, and in recognition, UNEP formed the Climate and Clean Air Coalition to deal with these pollutants.