Robust real-time gas sensing is important for many fields, including agriculture and health care analysis of breath/biofluid volatiles. Ammonia is ambiently present at parts per billion (ppb) to parts per million (ppm) levels, but current detection technologies suffer long measurement times, instability, cost, and issues with selectivity. Here, surface-enhanced Raman spectroscopy (SERS) is markedly improved through precision precleaning protocols, which allow surface sensitization of the metal facets using a water monolayer. Harnessing monolayer aggregates of densely packed gold nanoparticles with sub-nanometer spacing defined by rigid scaffolding molecules gives sub-ppm detection of ammonia at room temperature. Accessing the poorly studied high-wave number [>2500 per centimeter (cm)] region provides much improved discriminatory capabilities, enabling us to generalize this approach to a range of volatile organic carbon (VOC) molecules including ethanol, methanol, and acetone.