From AMS Glossary
Any small liquid droplet (less than a μm in diameter) contributing to an atmospheric haze condition.
In certain industrial areas, such droplets may be entirely nonaqueous (largely hydrocarbons). Most haze droplets are salts dissolved in water in an environment below water saturation (<100% relative humidity) with equilibrium size given by the Köhler equation. Near seacoasts, droplets of seawater are responsible for haze; even far inland, some haze conditions have been shown to be due to such salt-solution droplets. Many inland haze particles are composed of ammonium sulphate (bisulphate), probably produced from industrial air pollution or locally in some areas with volcanic activity as sulfuric acid, reacting with ammonia from agricultural activities.