Index Geophysics

The role of dew as a night-time reservoir and morning source for atmospheric ammonia

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^&minus;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^&minus;2 s^&minus;1 during dew evaporation was calculated using total dew volume (<i>V</i>_dew) and evaporation time (<i>t</i>_evap) 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 NH_<i>x</i> ( ≡  NH₃ + NH₄⁺) wet deposition may be overestimated by models that assume that all NH_<i>x</i> deposited in rain and dew remains at the surface.

Wentworth, Gregory R.

Murphy, Jennifer G.

Benedict, Katherine B.

Bangs, Evelyn J.

Collett Jr., Jeffrey L.

2018-09-20

Text

application/pdf

10.5194/acp-16-7435-2016

https://acp.copernicus.org/articles/16/7435/2016/

eISSN: 1680-7324

eng