For more than a decade, scientists have observed “ship tracks” in natural-color satellite imagery of the ocean. These bright, linear trails amidst the cloud layers are created by particles and gases from ships. They are a visible manifestation of pollution from ship exhaust, and scientists can now see that ships have a more subtle, almost invisible, signature as well.
In 2009 I went to a summer school in Corsica, and one of the coolest things I did was measuring the concentration of NO2 in the atmosphere. We plugged a USB radiometer to a laptop, measured light at different angles, and from that we could know the concentration of NO2 -- we knew the sun radiation spectrum at the top of the atmosphere and the absorption signature of NO2, and from that it was just math.
We measured the concentration of a gas kilometers away using light. Light! Science is awesome.