Since NASA researchers began assembling the Aerosol Robotic Network in the 1990s, the worldwide network of ground-based aerosol sensors has grown to 400 sites across seven continents. The trouble is that two-thirds of the planet is covered by ocean. And aerosols — the tiny atmospheric particles that can have an outsized impact on the climate — are just as likely to be found in the air above the oceans as they are over land.