• 1900–1959: the atmosphere became cleaner in most European cities;
• 1960–1985: anthropogenic aerosols were responsible for the dimming phenomenon in Europe. AOD increased over Southeast Brazil and decreased noticeably over Japan while a small negative trend was found over Oceania;
• 1986–2015: generally, the atmosphere has become much cleaner everywhere after the reversal trend around 1980s mainly observed in Europe.

Airborne particles affect clouds, climate, and air quality, but it is difficult to determine how their chemical components are mixed within individual particles. We tested a method that estimates this mixing from water-uptake measurements using detailed computer simulations. The method works well in many cases, but can overestimate particle mixing when moderately water-attracting material exists in separate particle types. We then applied this uncertainty framework to long-term observations.