text: The great majority of photogeochemical research is performed in the laboratory, as it is easier to demonstrate and observe a particular reaction under controlled conditions. This includes confirming the identity of materials, designing reaction vessels, controlling light sources, and adjusting the reaction atmosphere. However, observation of natural phenomena often provides initial inspiration for further study. For example, during the 1970s it was generally agreed that nitrous oxide (NO) has a short residence time in the troposphere, although the actual explanation for its removal was unknown. Since NO does not absorb light at wavelengths greater than 280 nm, direct photolysis had been discarded as a possible explanation. It was then observed that light would decompose chloromethanes when they were absorbed on silica sand, and this occurred at wavelengths far above the absorption spectra for these compounds. The same phenomenon was observed for NO, leading to the conclusion that particulate matter in the atmosphere is responsible for the destruction of NO via surface-sensitized photolysis. Indeed, the idea of such a sink for atmospheric NO was supported by several reports of low concentrations of NO in the air above deserts, where there is a high amount of suspended particulate matter. As another example, the observation that the amount of nitrous acid in the atmosphere greatly increases during the day lead to insight into the surface photochemistry of humic acids and soils and an explanation for the original observation.