The role of dew as a night-time reservoir and morning source for atmospheric ammonia
Abstract. Several field studies have proposed that the volatilization of NH3 from evaporating dew is responsible for an early morning pulse of ammonia frequently observed in the atmospheric boundary layer. Laboratory studies conducted on synthetic dew showed that the fraction of ammonium (NH4+) released as gas-phase ammonia (NH3) during evaporation is dependent on the relative abundances of anions and cations in the dew. Hence, the fraction of NH3 released during dew evaporation (Frac(NH3)) can be predicted given dew composition and pH. Twelve separate ambient dew samples were collected at a remote high-elevation grassland site in Colorado from 28 May to 11 August 2015. Average [NH4+] and pH were 26 µM and 5.2 respectively and were on the lower end of dew [NH4+] and pH observations reported in the literature. Ambient dew mass (in g m−2) was monitored with a dewmeter, which continuously measured the mass of a tray containing artificial turf representative of the grass canopy to track the accumulation and evaporation of dew. Simultaneous measurements of ambient NH3 indicated that a morning increase in NH3 was coincident in time with dew evaporation and that either a plateau or decrease in NH3 occurred once the dew had completely evaporated. This morning increase in NH3 was never observed on mornings without surface wetness (neither dew nor rain, representing one-quarter of mornings during the study period). Dew composition was used to determine an average Frac(NH3) of 0.94, suggesting that nearly all NH4+ is released back to the boundary layer as NH3 during evaporation at this site. An average NH3 emission of 6.2 ng m−2 s−1 during dew evaporation was calculated using total dew volume (Vdew) and evaporation time (tevap) and represents a significant morning flux in a non-fertilized grassland. Assuming a boundary layer height of 150 m, the average mole ratio of NH4+ in dew to NH3 in the boundary layer at sunrise is roughly 1.6 ± 0.7. Furthermore, the observed loss of NH3 during nights with dew is approximately equal to the observed amount of NH4+ sequestered in dew at the onset of evaporation. Hence, there is strong evidence that dew is both a significant night-time reservoir and strong morning source of NH3. The possibility of rain evaporation as a source of NH3, as well as dew evaporation influencing species of similar water solubility (acetic acid, formic acid, and HONO), is also discussed. If release of NH3 from dew and rain evaporation is pervasive in many environments, then estimates of NH3 dry deposition and NHx ( ≡ NH3 + NH4+) wet deposition may be overestimated by models that assume that all NHx deposited in rain and dew remains at the surface.
