Aerosols, tiny solid or liquid particles in the atmosphere, influence climate by absorbing and reflecting sunlight and modifying cloud properties. Light-coloured aerosols, such as sulphate and sea salt, cool the atmosphere, whereas dark aerosols, such as black carbon, cause warming. These interactions also affect cloud formation and properties. Unlike greenhouse gases, aerosols have a net cooling effect; however, their impact on Earth’s radiative budget remains uncertain in quantifying human contributions to global warming. Additionally, aerosols pose health risks, emphasizing the need to understand their sources, formation pathways, and chemical composition. This chapter examines the impacts of aerosol particles on Earth’s climate and human health and reviews ambient measurements of aerosol sources across different geographies, focusing on the significance of extensive observations and modelling efforts for addressing climate change and improving air quality.