Discussion: Flux correction for cross-contamination. 1. Before measuring: practical considerations for open paths

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1 Disussion: Flux orretion for ross-ontamination Open Path / Closed Path IRGA omparison 1. Before measuring: pratial onsiderations for open paths 2. Appliation of WPL orretion and omparison with losed path 3. Reommendations Historial review of WPL density orretion: Fuehrer and Friehe (2002) Flux orretions revisited, BLM 102, Sott Miller and Mike Goulden (UC Irvine)

2 Pratial Considerations for Open Path Advantages: Low Power Co-loated with soni No high frequeny orretions/delays Disadvantages Density orretions an swamp true fluxes Performane in rain? Dirt on windows

3 Considerations from First Priniples 1. In terms of molar density, (measured by open path) ρ F = ρ ' w' + ρ w Corretion term proportional to bakground CO2 onentration - need to know the mean aurately. Use well-alibrated sensor that doesn t drift Calibrate a drifty sensor regularly Use simultaneous with well-alibrated nearby referene sensor

4 Considerations from First Priniples 1. In terms of molar density, (measured by open path) ρ F = ρ ' w' + ρ w F 6 [ 0.54λ E ] 10 = ρ' w' + ρ H Corretion is ~5 times more sensitive to sensible heat flux than it is to latent heat flux. In theory, sites with small Bowen ratio will have smaller orretions. Also, sine orretion is independent of CO2 flux, the orretion is relatively smaller for sites with large CO2 fluxes.

5 Considerations from First Priniples 1. In terms of molar density, ρ (measured by open path) F = ρ ' w' + ρ w 2. In terms of mixing ratio, = ρ ρ a F = ρ a 'w' Requires simultaneous measurement of dry air molar density, ρ a Fast response, o-loated temperature measurement

6 Tapajos National Forest, Para, Brazil Tower Top Instruments

7 Sample by Sample dry air molar density, ρa ρ a = P a RT P P a - total air pressure - partial pressure dry air P = P a P v P v - partial pressure water vapor P v v = ρ M v R u T R u T T s - gas onstant - dry temperature - soni temperature T Ts P v P Water vapor density measured by open path

8 Considerations from First Priniples ρ = ρ a measured by open path For onstant mixing ratio, δρ = δρ a Spurious flutuation in measured density due to air density flutuation Corretion will be smaller for speies with large flutuations relative the the bakground onentration value.

9 Tapajos National Forest, Para, Brazil Tower Top Instruments EFFECT OF DIFFERENT HARDWARE Using losed path to alibrate open path

10 Performane in Rain Tapajos National Forest, Para, Brazil 2000 mm rain yr -1 Open path: solid Closed path: dashed 1 year Dry Season Wet Season peaks are system downtime

11 Results Applying WPL orretion sample by sample or using 30 minute ovarianes are equal.

12 Tapajos National Forest, Para, Brasil Open Path - Closed Path regression of CO2 flux

13 Averaged over 22 months... Tapajos National Forest, Para, Brasil Fo2 Open path: solid Closed path: dash

14 How Big are Flux orretions? Tapajos National Forest, Para, Brasil Open path: solid Closed path: dash Expressed as gain to the raw flux. Expressed as additative orretion to raw flux

15 Cumulative Sum for 1 Year (NOT u * FILTERED!) Closed path Open path raw

16 Cumulative Sum for 1 Year (NOT u * FILTERED!) Closed path Add w/wpl orr Open path raw

17 Open Path AGC versus PAR: missing data biased toward rainy intervals Appy par model to NEE (not turbulent flux): ag NEE = f(par) Note: listen to the AGC (status) output of the instument par

18 Cumulative Sum for 1 Year (NOT u * FILTERED!) Closed path add PAR model Add w/wpl orr Open path raw

19 u * filter 0.2 ms T C ha -1 yr -1 Although 0.8 T C ha -1 yr -1 may seem large, it is small relative to the effet of a u * filter. We onlude that using an open-path instrument in a tropial rainforest is feasible.

20 Reommendations: 1. Keep well alibrated 2. Corretions derease for high CO2 fluxes, small Bowen ratios, and when flutuations are larger ompared to bakground. 3. Ensure soni temperature has suffiient response 4. Can apply orretion either sample by sample or using 30 minute temperature and water vapor fluxes. 5. Make sure gap-filling is not biased toward sunny periods if missing flux intervals are orrelated with loudy periods.

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23 30 MINUTES u,v,w Soni T CO2 open path (blue) losed path (red) H2O w,co2 ospetrum w,o2 ogive w,h2o ospetrum w,h2o ogive