Short answers, pulled from the story.
An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas, where the term refers to the mixture of particles and suspending gas together, not the particles alone. The liquid or solid particles in an aerosol typically have diameters less than 1 micrometre. The term was coined by Frederick G. Donnan during World War I, built by analogy with the word hydrosol.
Volcanic aerosols form in the stratosphere as droplets of sulfuric acid after an eruption and can persist for up to two years. They reflect incoming sunlight back toward space, which lowers surface temperatures. This cooling effect can extend across broad regions of the planet for as long as the aerosol layer remains aloft.
In 2020, international fuel regulations cut sulfur dioxide emissions from international shipping by approximately 80%. The sudden reduction removed a large source of aerosol-driven cloud formation that had been partially cooling the planet, an effect scientists described as an unexpected global geoengineering termination shock.
Particles with an effective diameter smaller than 2.5 micrometres, classified as PM2.5 or fine particulate matter, can penetrate all the way to the gas exchange region of the lungs, which poses the greatest hazard to human health. Particles smaller than 10 micrometres reach the bronchi. The US Environmental Protection Agency introduced PM2.5 standards in 1997.
Aerosols act as cloud seeds, providing surfaces onto which water molecules collect and grow into droplets. More aerosol particles in a given volume of air tend to produce clouds with many smaller droplets, which are brighter and longer-lived than clouds seeded by fewer, larger particles. This indirect effect alters the way clouds reflect and absorb solar radiation.
Aerosols in the 20-micrometre range exhibit "jet rider" behaviour in air-conditioned rooms, following air jets when air moves fast but falling out when air slows, which gives them an unusually long persistence indoors. This size range is the most effectively deposited in the human nose, which scientists identified as the primary infection site in COVID-19, suggesting these aerosols may have contributed to transmission of the disease.