Atmospheric Chemistry Saturated fatty acids are considered to be inert, but they can be surprisingly reactive when present as a coating at an air-water interface. Rossignol et al. show that nonanoic acid is photochemically active when it is present as a monolayer on a water surface (see the Perspective by Vaida). Fatty acids are ubiquitous in the environment, and their photochemical processing could have a substantial impact on local ozone and particle formation. Science , this issue p. [699][1]; see also p. [650][2] [1]: /lookup/doi/10.1126/science.aaf3617 [2]: /lookup/doi/10.1126/science.aah4111