Air transport between continents contributes nearly the same amount of aerosols to the atmosphere above North America as do domestic emissions, according to satellite measurements (Science, DOI: 10.1126/science.1217576). Near the ground, the airborne particles affect air quality, whereas higher in the atmosphere they may reflect or absorb sunlight and seed clouds. Prior estimates of intercontinental aerosol transport were largely based on modeling studies. In the new work, a group led by Hongbin Yu of the University of Maryland and NASA used data from two satellites to quantify aerosols transported across the Pacific and Atlantic Oceans to North America. The team also determined the aerosols’ elevation and identified their sources: dust, the ocean, or combustion, either from biomass fires or burning fossil fuels. The researchers found that 64 million tons of aerosols are transported annually to North America, compared with 69 million tons of domestic emissions. Most of the imported aerosols are dust, indicating that increasing desert area elsewhere, which could be caused by climate change or land-use practices, may increase aerosols over North America. At the same time, aerosols may be deposited into the oceans or otherwise lost from the lowest part of the atmosphere during transport, so local air quality is largely dependent on domestic emissions control.