Ammonia (NH₃) is a highly water soluble gas-phase pollutant yet its interaction with dew is poorly understood. Dew evaporation is one hypothesis used to rationalize a frequently observed but currently unexplained morning increase in NH₃ occurring between ~7:00-10:00 local time.  In this study simultaneous quantification of dew composition, dew amount and NH₃ gas-phase concentrations was completed to address the following: 1) if dew is a significant reservoir for NH₃, 2) how much NH₄⁺ in dew is released as NH₃ during evaporation, 3) whether or not dew evaporation can explain observed increases in morning NH₃.  Measurements were carried out over a 90-day period in a remote high elevation grassland site in Colorado. Dew was collected off a Teflon sheet covering a horizontal styrofoam block. Dew amount was continuously monitored using a “dew meter” consisting of a tray with artificial turf resting atop an analytical balance. Atmospheric NH₃ was measured via cavity ringdown spectroscopy at a time resolution of 1 min.

Roughly two-thirds of boundary layer NH₃ was sequestered in dew overnight suggesting it is a significant reservoir. Dew composition and pH were used to estimate that, on average at this location, 94% of dew NH₄⁺ is released to the atmosphere as NH₃ during evaporation. Coincident timing of dew evaporation and NH₃ increase, as well as calculated fluxes and approximate mass balance closure of NH₄⁺ and NH₃ provide strong evidence that, at least at this site, dew evaporation is responsible for the morning increase in NH₃. Increases in NH₃ following light rain events suggest a need to also examine possible re-emission of NH₃ originally wet deposited as NH₄⁺.