Deposition and re-emission of ammonia by dew
Jeffrey Collett1, Greg Wentworth2, Jennifer Murphy3, Katherine Benedict4 and Evelyn Bangs5
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+.
1Colorado State University, email@example.com 2University of Toronto 3University of Toronto 4Colorado State University 5Colorado State University