“The mass balance IHF method equates the vertical flux of NH 3 from a treated area of limited upwind extent with the net integrated horizontal flux at a known downwind distance (Wilson et al., 1983). “

wind -unfertilized area masts with ammonia traps & anemometers x x + urea “ Try to imagine we have applied urea to a small area (green shaded) inside a larger unfertilized field. Assume the wind is blowing from right to left”.

NH 3(g) wind direction urea prills diffusion & turbulence convection NH 3(g) downwind upwind Z p, height of gas profile development NH (3) traps anemometer x x “The following diagram is modified from Ryden et al. (1984) and illustrates ammonia production from the soil surface (treated with urea). Ammonia moves upward as a result of diffusion and turbulence, and moves laterally due to convection. Masts with NH 3 traps and anemometers to measure wind speed are placed at upwind and downwind boundaries”

NH 3 conc., µg N m -3 Height, m ZpZp c1c1 downwind NH 3 conc., µg N m -3 Height, m c1c1 upwind Wind speed, m sec -1 Height, m c1c1 “These figures illustrate typically profiles of ammonia concentration in the air at downwind and upwind positions, and wind speed. Source: Ryden et al.,1984”.

c = concentration, μg/m 3 u = wind speed, m/sec ”Horizontal flux, or the product of wind speed x concentration, is calculated for each of the five planes (i)” i=5 i=3 i=2 i=1 i=4

fetch distance, m “To calculate F v a plot of height vs. horizontal flux is made at each of the five planes. The horizontal flux is then integrated over the height intervals and divided by the fetch distance”. Horizontal flux Height x x x x x

“Fetch is the distance between downwind and upwind boundaries”. NH 3(g ) wind direction urea prills NH 3(g ) downwind upwind x x

Horizontal flux Height x x x x x x x x x x Horizontal flux Height downwind mast upwind mast “To compute the net vertical flux, ammonia flux at the upwind mast position is subtracted from the downwind mast position”.

Horizontal flux Height

developed in Australia by Leuning et. al (1985) eliminates the need for wind speed measurements with anemometers horizontal flux (NH 3 concentration x wind speed) is calculated from the equation! M = mass of ammonia (µg) collected T = time effective cross-section sampling area the shuttle A = effective cross-section sampling area the shuttle

Wind speed (m s -1 ) Flow rate (10 -5 m 3 s -1 ) Slope = 2.42 x m 2 Source: Leuning et al. (1985) Atmospheric Environ.

..mimic the design of Leuning et al. (1985).

Shuttles rotate on a pivot and point into wind 0.25 m 0.50 m 1.00 m 1.50 m 2.75 m “Five shuttles are placed on all masts in a gradient spacing with the top shuttle being situated at the approximate Z p height”.

+ urea (90 lbs N/a) background 200 m urea + Agrotain known fetch distance (radius = 20 m) large unfertilized buffer areas necessary around plots IHF method requires

shuttles on mast exchanged weekly with recharged shuttles

ammonia trapped in shuttles is eluted in the lab and elutants run on a Timberline ammonia analyzer calculations of ammonia are done in Excel spreadsheets (μg N m -2 sec -1 ) x time period losses expressed as a fraction of urea-N applied M/AT Height

“ The mass balance IHF method equates the vertical flux of NH 3 from a treated area of limited upwind extent with the net integrated horizontal flux at a known downwind distance (Wilson et al., 1983). “ Key points  minimal disturbance to site and provides time- integrated measurement; widely recognized