“The mass balance IHF method equates the vertical flux of
NH3 from a treated area of limited upwind extent with the
net integrated horizontal flux at a known downwind distance
(Wilson et al., 1983). “
wind
+ urea
x
x
-unfertilized area
masts with ammonia traps
& anemometers
“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”.
downwind
upwind
wind direction
Zp, height of
gas profile
development
diffusion &
turbulence
convection
anemometer
NH3(g)
NH3(g)
NH3(g)
urea prills
NH(3) traps
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
NH3 traps and anemometers to measure wind speed are placed at upwind and
downwind boundaries”
upwind
downwind
Height, m
Height, m
Height, m
Zp
c1
NH3 conc., µg N m-3
c1
Wind speed, m sec-1
c1
NH3 conc., µg N m-3
“These figures illustrate typically profiles of ammonia concentration in the air at
downwind and upwind positions, and wind speed. Source: Ryden et al.,1984”.
i=5
i=4
i=3
i=2
u = wind speed, m/sec
i=1
c = concentration, μg/m3
”Horizontal flux, or the product of wind speed x concentration, is
calculated for each of the five planes (i)”
x
Height
x
x
x
fetch distance, m
x
Horizontal flux
“To calculate Fv 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”.
downwind
wind direction
NH3(g)
NH3(g)
upwind
NH3(g)
urea prills
“Fetch is the distance between downwind and upwind boundaries”.
x
x
downwind mast
upwind mast
x
x
x
Height
Height
x
x
x
x
x
Horizontal flux
x
x
Horizontal flux
“To compute the net vertical flux, ammonia flux at the upwind mast
position is subtracted from the downwind mast position”.
Height
Horizontal flux
developed in Australia by Leuning et. al (1985)
eliminates the need for wind speed measurements with
anemometers
horizontal flux (NH3 concentration x wind speed) is calculated
from the equation!
M = mass of ammonia (µg) collected
T = time
A = effective cross-section sampling area the shuttle
Flow rate (10-5 m3 s-1)
25
20
Slope = 2.42 x 10-5 m2
15
10
5
0
0
2
4
6
8
Wind speed (m s-1)
10
Source: Leuning et al. (1985)
Atmospheric Environ.
..mimic the design of
Leuning et al. (1985).
2.75 m
1.50 m
1.00 m
0.50 m
0.25 m
Shuttles rotate on a pivot
and point into wind
“Five shuttles are placed on all masts in a gradient spacing with the top shuttle
being situated at the approximate Zp height”.
IHF method requires
• known fetch distance (radius = 20 m)
• large unfertilized buffer areas
necessary around plots
urea + Agrotain
background
+ urea (90 lbs N/a)
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
Height
5
(μg N m-2 sec-1) x time period
losses expressed as a fraction of urea-N applied
4
3
2
1
M/AT
“The mass balance IHF method equates the vertical
flux of NH3 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 timeintegrated measurement; widely recognized