https://www.gamespot.com/articles/last-chance-pokemon-go-bonuses-available-in-game-f/1100-6458441/

Earth Day took place on April 22, and Pokemon Go developer Niantic held a special Clean Up event on the holiday to encourage players to go out and pick up refuse around their community. Now that the event is over, Niantic has announced a special bonus that players can get in the game for a limited time. [Update: It's the final day to take advantage of Pokemon Go's Earth Day bonuses, meaning players only have a few more hours to earn triple the usual amount of Stardust for capturing certain types of Pokemon.]

One of the incentives for participating in the Pokemon Go Clean Up event was to unlock bonuses in the game. Depending on how many players took part in the event, the developer would offer a different amount of Stardust for capturing certain types of Pokemon. If 1,500 trainers participated, everyone would get double the usual amount of Stardust; if 3,000 participated, everyone would get triple.