Inferring gas fluxes from point or line- averaged concentrations Tom Denmead Fellow, CSIRO Land and...

Inferring gas fluxes from point or line-averaged concentrations

Tom Denmead

Fellow, CSIRO Land and Water & University of Melbourne

Ozflux Conference, 4 February 2008

CSIRO. Inferring gas fluxes from point or line-averaged concentrations

A backward Lagrangian stochastic (bLs) dispersion model

• The model traces particles backwards from sensor to origin using a Lagrangian dispersion model

• Surface fluxes calculated from number of touchdowns inside and outside source area in many simulations:(C/Q)sim = (1/N) Σ |2/w0|

C is downwind concentration Q is the surface flux N is the number of

trajectories commonly, 50,000

w0 is the vertical velocity of particles at touchdown

Q = (C-Cbackground) / (C/Q)sim

Micromet.

Source area

wind

Point concentration sensor

Touchdowns


A backward Lagrangian stochastic (bLs) dispersion model

• Suitable for point, line or area sources (any shape)

• Inputs: geometry of source area height and location of sensor, wind speed and direction, atmospheric stability,gas concentrations upwind and downwind

• Uses a software package called WindTrax to calculate surface fluxes from concentration and micrometeorological data

Micromet.

Source area

wind

Point concentration sensor

Touchdowns


Point concentration measurements: an example from grazing (315 dairy cows)

Ammonia concentrations measured with passive samplers


WindTrax map

2 adjoining pasture bays grazed in 6 sessions, one-third of a bay at a time

Sensors located at heights of 1.4 and 2m on 12 masts on the corners of each grazed section

Chemicalsensors Meteorological

Sensors:2 anemometersWind vaneAtmos. stabilityBackgroundconcentrationunknown

Grazedsections


Sensor numbers: measuring NH3 emissions after N fertiliser applied to the whole bay

2.66

Average fluxes (μgNH3-N m-2 s-1), 0900-1800, usingdifferent sensor combinations; wind direction 170o

24 sensors,2 to each mast,at 1.4 and 2m

2 sensors, oneupwind & onedownwind, eachat 1.4m

2.05

2.33

1.55

If backgroundunknown, need2 sensors

If >2 sensors,problem isover-determined& model returnsleast-squares,best-fit background and flux


Multiple source areas (using 16 sensors)

0.14

0.30

Average fluxes, 0800-1730, μgNH3-N m-2 s-1

Grazed yesterday →

Grazed today →

Ungrazed → -0.02


An example result: emissions from one grazed section

NH3 fluxes Kyabram 2004 - top Bay 8

-1

0

1

2

3

4

5

26-Mar 28-Mar 30-Mar 1-Apr 3-Apr 5-Apr 7-Apr 9-Apr

time

ug

NH

3-N

/m2/

s

50 kgN/haUrea

• Before grazing: small NH3 uptake

• Continuous NH3 emission during & after grazing

• Large NH3 emissions after fertilizing

• Emissions cease after irrigation


Line-averaged concentrations: laser and Fourier Transform Infrared (FTIR) systems

• Lasers measure line-averaged gas concentrations up to 1km, FTIR less

• Lasers: tripod-mounted, stand alone, battery-operated units; FTIR requires mains power

• Suitable for point, line and small area sources

LaserFTIR

ReflectorLine-average concentration

Open-path FTIR (CO2,CH4, N2O, NH3) Open-path laser (CO2, CH4, NH3)


Tests: releases and recoveries

CH4, N2O, NH3 released from cylinders through mass-flow controllers

Tests conducted of recoveries from point source and plane source emissions

40m x 15m grid of permeable pipes

Daisy – our virtual cow

40m x 15m grid of

permeablepipe


Tests: releases and recoveries_ point sources

Average NH3 concentrations measured by a laser instrument at 1.5m height along a line of 123m, 10m downwind of a point source of ammonia 0.5m above ground.

Release rates 0f NH3 and downwind NH3 concentrations, 29/07/05

0

2

4

6

8

10

1052-1130

1130-1205

1205-1242

1242-1351

1351-1426

1426-1500

1500-1530

1530-1600

1600-1630

1630-1700

Re

lea

se

ra

te (

L m

in-1

)

0

40

80

120

160

200

NH

3 c

on

ce

ntr

ati

on

(p

pb

)

Release rate Concentration


Tests: releases and recoveries_ areal sources

• Top:• Recovery by laser

of NH3 released from ground level grid, 25m x 25m

• Laser 2m downwind of grid

• Path 128m• NH3 released at 5L

min

• Bottom:• Recovery by 2

lasers and FTIR of CH4 released from ground level grid, 40m x 15m

• Path 140m

Ammonia laser 2m downwind of grid, Aug 2, 2005

0

20

40

60

80

12:00 13:00 14:00 15:00 16:00

mg

NH

3 s

-1

Released Measured

Recovery tests for CH4 over 1 hour, Aug 3, 2005

0

20

40

60

80

Cylinder Laser #1012 Laser #1013 FTIR-CH4

mg

CH

4 s

-1


Example application of open-path systems: CH4 emission from a feedlot with 14,000 cattle

WindTrax map of feedlot layout Laser paths

Micromet. tower


Strengths and weaknesses

• bLs technique + WindTrax represent a powerful new tool for measuring gas emissions from well-defined source areas

• Main advantage: fluxes determined from just one concentration measurement and knowledge of the background concentration + turbulence statistics

• Both closed and open-path measuring systems possible• Path lengths of up to 1 km possible, but 100 to 300m seem more reliable• Open –path systems:

• Lasers tuned to individual gases: CO2, CH4, NH3 and H2O • FTIR units measure many of the gases of interest in the context of

landscape-atmosphere exchanges simultaneously: CO2, CH4, NH3, H2O, N2O and CO

• The main disadvantage of the bLs technique may be in its parameterisation of turbulent transport, but many tests have shown that with appropriate precautions, gas emissions can be measured with acceptable accuracy (Flesch et al., 2004; McBain and Desjardins, 2005; Laubach et al., 2008).


Acknowledgements

• Collaborators

University of Melbourne:

Deli Chen, Debra Turner, Yong Li, Zoe Loh, Julian Hill

University of Wollongong:

David Griffith, Mei Bai, Glenn Bryant, Travis Naylor

DPI Victoria:

Kevin Kelly, Frances Phillips

Charlton Feedlot

Sandalwood Feedlot

• Funding

Australian Greenhouse Office

Meat and Livestock Australia

Contact UsPhone: 1300 363 400 or +61 3 9545 2176

Email: [email protected] Web: www.csiro.au

Thank you

CSIRO Land and Water and University of MelbourneTom DenmeadFellow

Phone: +61 2 6246 5568Email: [email protected]: www.csiro.au

Inferring gas fluxes from point or line- averaged concentrations Tom Denmead Fellow, CSIRO Land and...

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