Driving forces for ammonia fluxes over mixed forest subjected to high deposition loads

Ammonia exchange was measured over a mixed suburban forest near a rural area. An average net ammonia flux of -90 ng m^-2 s^-1 was measured with corresponding concentration ([NH₃]) and deposition velocity of 4.1±6.5 µg m^-3 and 3.0±4.6 cm s^-1, respectively. Subdivision into categories of day/nighttime, wind sector and canopy wetness helped explain fluxes and concentrations. Net fluxes were approximately double for the wind sector exposed to high ammonia levels and during the day. To a certain extent, fluxes (F)followed the maximum flux permitted by turbulent transfer (F_max), which was the highest for a dry canopy. When expressed as relative deposition flux (F/F_maxx), a wetted canopy seemed to be a more efficient sink than a dry one, especially at nighttime (20–80% increase compared to dry canopy).

Of the net fluxes, 14% represented emission. Emission fluxes occurred mainly during daytime and were important in magnitude for the high [NH₃] wind sector. Emission episodes generally occurred at low ammonia concentrations although high concentrations during dry, warm episodes were also associated with emission events.

The lower deposition efficiency and higher canopy resistance (R_c) at high ammonia levels and at night were indicative of the reduced capacity for leaf surface to retain ammonia, especially when the canopy was dry. It was found that relative humidity (RH) and temperature (T) strongly codetermined the sink strength of the canopy. A warm and humid atmosphere favoured ammonia uptake while conditions with low RH and T impeded rapid canopy uptake, especially at high ammonia levels. Strong interactions between RH and T with the (NH₃/SO₂) molar ratio occurred for certain categories of canopy wetness and day/nighttime. Canopy uptake was further optimized when this ratio was maintained within a certain range.