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Several field studies have proposed that the volatilization of NH3 fromevaporating dew is responsible for an early morning pulse of ammoniafrequently observed in the atmospheric boundary layer. Laboratory studiesconducted on synthetic dew showed that the fraction of ammonium(NH4+) released as gas-phase ammonia (NH3) during evaporation isdependent on the relative abundances of anions and cations in the dew. Hence,the fraction of NH3 released during dew evaporation (Frac(NH3)) canbe predicted given dew composition and pH. Twelve separate ambient dewsamples were collected at a remote high-elevation grassland site in Coloradofrom 28 May to 11 August 2015. Average [NH4+] and pH were26 µM and 5.2 respectively and were on the lower end of dew[NH4+] and pH observations reported in the literature. Ambient dewmass (in g m−2) was monitored with a dewmeter, which continuouslymeasured the mass of a tray containing artificial turf representative of thegrass canopy to track the accumulation and evaporation of dew. Simultaneousmeasurements of ambient NH3 indicated that a morning increase inNH3 was coincident in time with dew evaporation and that either aplateau or decrease in NH3 occurred once the dew had completelyevaporated. This morning increase in NH3 was never observed on morningswithout surface wetness (neither dew nor rain, representing one-quarter ofmornings during the study period). Dew composition was used to determine anaverage Frac(NH3) of 0.94, suggesting that nearly all NH4+ isreleased back to the boundary layer as NH3 during evaporation at thissite. An average NH3 emission of 6.2 ng m−2 s−1 during dewevaporation was calculated using total dew volume (Vdew) andevaporation time (tevap) and represents a significant morning fluxin a non-fertilized grassland. Assuming a boundary layer height of 150 m,the average mole ratio of NH4+ in dew to NH3 in the boundarylayer at sunrise is roughly 1.6 ± 0.7. Furthermore, the observed lossof NH3 during nights with dew is approximately equal to the observedamount of NH4+ sequestered in dew at the onset of evaporation. Hence,there is strong evidence that dew is both a significant night-time reservoirand strong morning source of NH3. The possibility of rain evaporation asa source of NH3, as well as dew evaporation influencing species ofsimilar water solubility (acetic acid, formic acid, and HONO), is alsodiscussed. If release of NH3 from dew and rain evaporation is pervasivein many environments, then estimates of NH3 dry deposition and NHx( ≡  NH3 + NH4+) wet deposition may be overestimatedby models that assume that all NHx deposited in rain and dew remains atthe surface

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