Aerosols are an important part of the Earth system they interact with incoming solar radiation, outgoing long wave radiation and atmospheric chemistry, and when deposited impact snow albedo and biogeochemistry. Aerosols are also highly heterogenous in space, time, composition and size, and any of these heterogeneities can complete change the impact of aerosols in the Earth System. New tools and networks are expanding our knowledge of aerosols, but it remains one of the largest uncertainties in understanding how global change is impacting human health, ecosystems and climate. Here we explore the constraints that observations can provide on the distribution, composition and size of aerosols, focusing on in situ observations and satellite remote sensing. We focus not only on where we have information, but also identify places where more information would be more valuable.