(4) What is the effect of the final surface temperature on the carbon dioxide flux data? 1. Introduction

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1 Correcting aarent off-season CO 2 utake ue to surface heating of an oen ath gas analyzer: rogress reort of an ongoing stuy G.G. urba, D. J. nerson, Liukang Xu an D.K. McDermitt LI-COR iosciences, Lincoln, NE, US bstract number of Ey Coariance flux sites with oen ath analyzers hae obsere small aarent CO 2 utake outsie the growing season. In this stuy, we roose an aitional correction ue to instrument surface heating as a art of the Webb-Pearman- Leuning correction. Effects of the roose correction were examine on hourly an seasonal time scales. Signifit reuction in aarent CO 2 utake uring off-season erios was obsere, while fluxes uring the growing season were not noticeably affecte. The roose correction may be useful for future CO 2 flux research, an also be alie to re-existing ata. 1. Introuction Oen-ath infrare gas analyzers are low maintenance high-quality sensors wiely use in fiel research (meriflux, 26). s more yearroun carbon flux stuies become aailable, there is growing concern about aarent off-season CO 2 utake being measure when it is hysiologically unreasonable to exect it (Hirata et al., 25). Soling this roblem is an imortant ste in imroing CO 2 measurements, annual NEE estimates, an subsequent global carbon exchange an climate moeling efforts. One laboratory stuy an two fiel exeriments are currently unerway to examine the influence of the oen-ath infrare gas analyzer surface temerature on the measure CO 2 flux using the Ey Coariance technique. The original concet an some initial results were resente at the meriflux nnual Meeting in October, 25 (urba et al., 25a,b), an has been eeloe further in subsequent months ue to the interest an enthusiasm generate in the flux research community towars resoling this issue. Here we resent the latest uates from these exeriments: a theoretical correction ue to the surface heating is introuce, the relationshi of instrument surface temerature an ambient air temerature is inestigate, an the effect of an aitional term on the CO 2 fluxes is examine. The following secific questions are aresse: (1) Do the electronics insie the sensor hea contribute substantially to instrument surface heating? (2) Is the air in the otical ath of the oen-ath analyzer signifitly warmer then the ambient air ue to the heate instrument surface? (3) How o the electronics an ambient factors combine to affect the final surface temerature? (4) What is the effect of the final surface temerature on the carbon ioxie flux ata? 2. Concet of the correction ue to instrument surface heating The surface of the oen-ath infrare gas analyzers (e.g., LI-75, LI-COR Inc., Lincoln, NE) coul be warmer than the ambient air ue to heat generate by the electronics in an instrument s sensor hea (choer motor, heating of the choer housing, infrare source, thermoelectric coolers, etc.), an by solar raiation. The instrument may also be coole by emission of long-wae raiation an by win-inuce conectie exchange (Fig. 1). The heat flux from an instrument surface is in aition to the ambient heat flux: coler air continuously enters the oen otical ath, an is warme an exane by the warm instrument surface in the same way that the atmoshere is warme by the soil surface (urba et al., 25a). nother way to look at this rocess is on the scale of two measurement ss (Fig. 1) for a situation in which there is no actual flux an no change in actual CO 2 mole fraction. With a ecrease in horizontal win see (s1), air in the ath gets warmer than ambient because there is more time to heat it, an exansion takes lace; so, CO 2 number ensity is lower than ambient. t the same time, w is ositie to assure momentum flux is always towar the surface. With an increase in horizontal win see (s 2), the air is heate less, exansion is less, an CO 2 number ensity is closer to ambient. t the same time w is negatie (to kee momentum transfer towar the surface). Thus, from the coariance calculation ( w 'C'), a false utake woul be obsere when no actual flux is occurring.

2 2 S 1 U weaker S 2 U stronger w w ball bounary layer sars lumes urising warm lume bounary layers U win - u negatie: -w is ositie - more heating - more exansion - reas less CO 2 - u ositie - w is negatie - less heating - less exansion - reas more CO 2 In blue flow atterns of warme air (bounary layers & wakes) In otte re thermal heat issiation (longwae) w u - less CO 2 w own - more CO 2 result - reaing CO 2 utake Figure 1: () Schematics of the stanar LI-75 setu & relate heat issiation; () Visualization of the concet: two measurement ss when there is no actual flux an no change in CO2 mole fraction occurs. S 1: with a ecrease in a horizontal win see, air in the ath gets warmer (more time to heat it), exansion takes lace, CO2 number ensity is lower, an w is ositie. For s 2 the situation is reerse. Therefore, an aarent utake be obsere when there is no actual flux. 3. Deriation of the correction ue to instrument surface heating The effects of such heating coul be inclue in the Webb-Pearman-Leuning correction (WPL: Webb et al., 198) as an aitional heat flux term (urba et al., 25a). The WPL correction, in general, comensates for the fluctuations of temerature an water aor affecting measure fluctuations of CO 2 an H 2 O. In general for an oen-ath CO 2 measurement, utting all constants together, the flux equation coul be written in the following form: Fc = Fc o E qc H C qc Ta (1) where Fc is final correcte CO 2 flux; ry air ensity; Fc o initial uncorrecte CO 2 flux; water aor ensity; E eaotransiration; total air ensity; q c mean ambient CO 2 ensity; C secific heat of air; H sonic sensible heat flux; T a air temerature. In a similar manner for water aor, the flux equation coul be written as follows: E = ( ) E o V H + ( ) C Ta (2) Now, let us imagine that the instrument is warm, an we not istinguish between the multile rocesses shown in Fig 1. Yet, we coul assume some resultant exansion of the air by a sensible heat flux create by instrument surface heating. Such a flux be estimate from the temerature graient using resistance aroach (Montieth, 1963) as follows:

3 3 C ( Ts Ta) HI = (3) ra where H I sensible heat flux from the warm instrument to the ambient air, T s instrument surface temerature, an r a aeroynamic resistance. For an instrument oriente near ertically, only a fraction of H I woul affect the measurement because the win will remoe much of the warme air from the otical ath, carrying away most of H without affecting the measurements. Howeer, a ortion of the warme an exane air will remain in the bounary layer, reucing the measure air ensity an influencing the measurement (Fig. 1). The fraction fr of H I retaine in the otical ath is calle H P an be exresse as follows: H P = frhi (4) So, combining Eqs. 1,3, an 4, the WPL woul be transforme as follows: Fc new E qc = Fco ( H + fr( C ( Ts Ta) / ra )) qc + C Ta (5) The term E in Eq. 5 also nees an aitional heat correction as well (Eq.2), such that: E = ( )Eo + ( H + fr( C ( Ts Ta ) / ra )) V + ( ) C Ta (6) If oen-ath flux ata hae alreay been correcte in the traitional way (frequency resonse an WPL), then the aitional correction for E woul be (from Eqs. 2 an 6): E new = E + fr( ) r a ( T ( T s a Ta) ) (7) an the correction to Fc for ata alreay correcte in the traitional way, woul be (from Eqs. 1,5 an 7): Fc new ( Ts Ta) qc = Fc + fr ( ) (8) r ( Ta ) a where ( Ts Ta) qc fr reresents the instrument ra ( Ta ) heating correction for Fc. The aitional correction to E in Eq. 6 is reresente by ( ), which is tyically ery small, on the orer of.5-2% for a majority of ambient temerature an humiity ranges. For the articular case of the LI-75, each of the two major structural elements of the instrument ( an ball, Fig. 1) may contribute ifferently to the roose aitional term in WPL correction: Fc new ( Ts Ta) qc = Fc + fr ( ) + ra ( Ta ) ball ( Ts Ta) qc + frball ( ) (9) ball r ( Ta ) a Resistances to heat transfer be estimate assuming that force conection reails near the surfaces of the instrument (Cambell an Norman, 1998): r 7. (1) U a 4 r ball 7. (11) U a ball 4 where is the characteristic imension of each element (.42 m for the ball, an.133 m for the ); =.133 m was comute from the raius of the cure making the to surface of the ). oth structural elements are assume here to be cyliners an sheres, so no aitional multilier for is require. ssume that the fraction of the sensible heat flux affecting measurement fr (=H P /H I ) coul be aroximate by the ratio of athlengths containing air influence by H P iie by the total mechanical athlength of the instrument (l ath =.128 m). The

4 4 athlengths affecte by H P will be the sum of the thermal bounary layer thicknesses of the an the ball. Thus: l fr = (12) ath l ball l ball fr = (13) ath l an fr=fr +fr ball. The layers thickness coul be aroximate, assuming laminar bounary layer an ertical instrument osition, as follows: l * Re 2 win + tg( α) (14) ball ball 5.5 l Re (15) x U Re = µ (16) where Re is Reynols number (Gates, 198); =.33 m (raius of the ); α=2 o (angle between the bottom winow an metal shoulers aroun it); win =.12 m (raius of the flat ortion of the winow) ); ball =.22 m (raius of the ball); x is or ball ; µ is ynamic iscosity of the air. The air heate insie the sensor between winows an transucers is CO 2 free, an its temerature fluctuations o not contribute to the flux correction. 4. Results 4.1.Electronics contribution enironmental chamber exeriment To ealuate contribution of the electronics in the sensor hea to the heating of the instrument surface, an exeriment was conucte in an enironmental chamber (Moel F466PC-1, lue M Electronic Comany, lue Islan, Illinois) with a ertical-own recirculating conitioning stream to ensure uniform mixing of the air. Thermocoules were attache to arious arts of the instrument (sie of the cyliner, lower winow area, uer winow area, an sar) an coere with small ieces of foam to minimize irect heating an cooling of the thermocoule by chamber air. Figure 2 shows the instrument surface temeratures lotte ersus ambient temerature for the range from -25 to +5 C. The bottom winow area was consistently warmer than ambient chamber air with an aerage relationshi as follows: T s =.92 T a +4.83, r 2 =1., n>9 (17) Equation 17 hols for the original ersion of the instrument electronic boar, house in the white control box (ersion 1). Eq. 17 emonstrates that ifferences between ambient temerature an the surface temeratures of the lower winow area increase with a ecrease in ambient temerature: the instrument was warmer than ambient by +.8 C at +5 C ambient, an by +6.8 C at -25 C ambient. Other arts of the instrument i not islay signifit ifference from the ambient temerature, though all relationshis were ery tight, with little or no hysteresis. These obserations are corroborate by the location of the majority of the internal instrument electronics, an articularly the heater for the instrument source, near the bottom winow of the instrument. somewhat smaller influence of electronic heating was obsere for the newer ersion of the instrument electronic boar (ersion 2): T s =.93 T a +3.53, r 2 =1., n>9 (18) The instrument was heate by the electronics to +.3 C aboe ambient at +5 C, an by +5.3 C at - 25 C ue to higher efficiency an lower heat issiation of the new boar configuration. Relationshis were linear in all cases, with total number of samlings n>9, groue by ambient temeratures in 9 grous, an yieling r-square of 1.. oth of these equations (Eqs. 17 an 18) emonstrate a signifit influence of the sensor hea electronics on the instrument surface heating, esecially at low ambient temeratures. lso, both are likely to be secific only to the ersion of the electronic boar, an shoul hol for all instrument configurations an for arious exerimental setus. In rincile, these relationshis coul be use, with aroriate assumtions an caution, to ealuate the effect of the surface heating on the measure flux (urba et al., 25a,b).

5 5 5 Enironmental Chamber 1:1 Ts, C sloe offset r 2 Lower winow area er er Uer winow area Ta, C 25 5 Figure 2. Instrument surface temerature (Ts) is lotte ersus air temerature (Ta) in the climate controlle chamber for two ersions of electronic boars. Each oint on the lot is an aerage of 2-3 one-minute ata oints after temerature equilibrium between chamber air an instrument surface was achiee. Data gathere before equilibrium was achiee hae been remoe from the analysis. 4.2 mbient air warming near the instrument surface LI-COR facility exeriment Fiel conitions are ifferent from those in the enironmentally controlle chamber, rimarily ue to factors influencing the instrument surface irectly (not ia electronics heating): sunlight, win an raiatie cooling. fiel exeriment is being conucte at the LI-COR exerimental facility (Lincoln, NE) starting in October, 25 to ealuate the resulting surface-to-air temerature graient in fiel conitions with an without the influence of the electronics. During the exeriment instrument ower was turne off for seeral weeks an on for seeral months. Fine-wire thermocoules were attache to LI-75 analyzer at the center of the lower winow area aroximately.5 mm an 1. mm aboe the lower winow, an in the center of the uer winow area. mbient fine-wire thermocoule was installe 2 cm away from the oen-ath sensor. Figure 3 emonstrates that air coming into the instrument ath is warme u signifitly by the heate instrument surface in the ambient enironment. Temerature graients are ery stee in the first 1 mm aboe the lower winow area: for col air in calm atmosheric conitions, graients may excee 2 C er 1 mm, while uring winy conitions graients seem to ecrease (Fig. 3). During warm ays graients are consierably smaller than in col temerature, while some win effect is still obsere (Fig. 3). Figure 3 C emonstrates that een without electronics heating (with ower off) the instrument surface be seeral egrees warmer than the ambient air in the mile of the ay (likely ue to sunlight), an seeral egrees cooler at night (ue to raiatie cooling). This henomenon has imortant imlications for any oen-ath instrument measuring flux: unless the instrument surface temerature is controlle to be similar to the ambient, instrument surface heating/cooling may signifitly affect the measure flux. 4.3 Combination of all factors affecting instrument surface temerature - exeriment at Mea, NE fiel exeriment to irectly ealuate the influence of instrument heating on the measure CO 2 flux uring the off-season is being conucte (starting Noember, 25) at the Uniersity of Nebraska Carbon Sequestration Program research site near Mea, Nebraska (in collaboration with S.. Verma an. Suyker). This site is equie

6 6 Col ay: 12/9/5 winy -6 T, C calm Lower winow area.5 mm eg C 1. mm eg C Uer winow area Tair Time Warm ay: 1/26/6 12 winy T, C 6 calm T, C 26 27C Time Power off: 12/9/ : 338 6: 12: 18: 24: 339 Central Stanar Time Figure 3. Daily atterns of surface temerature of the lower an uer winow areas; air temerature was measure at aroximately. 5 mm an 1 mm aboe the lower winow, an in the oen air away from the instrument on selecte ays: () col ay, 12/9/5; () warm ay, 1/26/6; an (C) ay with instrument ower turne off, 12/9/5. LI-COR facility exeriment. with a comrehensie Ey Coariance station, which has an oen-ath CO 2 /H 2 O gas analyzer (LI- 75), an a comlete set of instruments for measuring weather an raiation (Suyker et al., 25). Fine-wire thermocoules were attache to an LI-75 oen-ath analyzer near the lower winow, on the sie of the cyliner, on a sar, an near the uer winow. ll thermocoules were ainte with the same titanium oxie aint as the boy of the instrument to assure a similar albeo. Relationshis between the air an instrument surface temeratures near the lower an uer winows are shown in Fig. 4. Similar to the enironmental chamber exeriment, the temerature of the uer winow was not signifitly ifferent from the ambient, at least for the majority of the ata (linear regression yiele sloe of.99 an offset of.23 C). The lower winow area, howeer, was warmer than ambient, but not as much as insie of the enironmental chamber. Linear fits from all exeriments to ate are shown in Table I. mbient exeriments (LI- COR facility an Mea) rouce consistently lower Ts for the same Ta, as comare to the enironmental chamber. We attribute this to better thermal exchange in ambient conitions ue to

7 7 4 Mea exeriment: Noember, 25-March, :1 2 Ts, C equation r 2 Lower winow area linear fit Ts=.9Ta olynomial Ts=.25Ta 2 +.9Ta Uer winow area x Ts=.99Ta Figure 4. Hourly alues of instrument surface temerature (Ts) at the lower an uer winow areas lotte ersus air temerature (Ta) uring the erio from Noember, 25 to March, 26. Ta, C win conection, an ue to raiatie cooling at night (as seen in Fig. 3C). This is somewhat corroborate by frequently obsere conensation an ew formation near the to winow in col an humi fiel enironments. nother imortant feature of the ambient relationshi between Ts an Ta is that the best fit may not actually be linear, because a linear fit woul imly the occurrence of the cooling of the surface at ambient temeratures aboe +2 to +3 C. It is ifficult to justify such a situation, in which the electronics woul cool the instrument below the ambient temerature. Exeriment Ver. Influencing factors lower winow area uer winow area sloe offset r 2 n sloe offset r 2 n Climate control chamber 1 electronics > Climate control chamber 2 electronics > >9 Li-Cor Facility* 2 electronics & ambient > >2 Mea exeriment* 2 electronics & ambient > >15 *non-linear may be better: regression fit will become more comlete after warm-season ata become aailable Table I. Linear fit arameters of the relationshi between ambient air temerature (Ta) an instrument surface temerature (Ts) for the lower an uer winow areas: Ts = sloe Ta+ offset. Influence of the heating by electronics (chamber exeriment) is noticeably offset by atmosheric exchange for the outsie exeriments. Uer winow Ts i not seem to be consistently affecte by Ta. (*) Non-linear may be better: regression fit will become more comlete after warm-season ata become aailable. With only coler temeratures aailable in the Mea exeriment to ate, a olynomial fit not be confiently extraolate beyon +2C. Howeer, a olynomial relationshi for the ambient temerature range from -25 to +2 C oes seem to fit the ata as well or better, than a linear fit (Fig. 4):

8 8 Ts=.25Ta 2 +.9Ta+2.7, r 2 =.99, n>15 (19) Temeratures for the relationshi resente in Eq. 19 were measure irectly, an all factors influencing Ts (e.g., electronics, sunlight, raiatie cooling, an win exchange) hae been incororate into the actual measurement of Ts, howeer, this relationshi may also be less general than those resente in Eqs. 17 an 18, an may ary from site to site an from one setu to another (because of ifferences in shaowing of the instrument from sun an win, temerature ranges, instrument orientation etc.), so it shoul be use with caution..5.5 Maize: October 6, 22 Maize: January 29, 23 Fc, mg m -2 s ol new : 12: : C : 12: : D.5.5 Soybean: October 9, 22 Soybean: February 8, 23. Fc, mg m -2 s : 12: : -.5 : 12: : Central Stanar Time Figure 5. Daily atterns of hourly carbon ioxie flux (Fc) as obtaine using close-ath LI-COR analyzer (6262), traitionally correcte LI-75 (75 ol), an flux correcte with a roose aitional member of WPL correction (75 new): () maize, warm ay; () maize, col ay; (C) soybean, warm ay; (D) soybean, col ay. Utake of CO2 is ositie an release is negatie lying corrections to re-existing ata To take aantage of exactly the same instrument configuration an setu, we hae use Ts relationshi obtaine from Mea exeriment in 25-6 (Eq. 19), an alie it ia Eq. 9 to the traitionally correcte Fc flux ata from 22-3 from the same stuy site uner maize, an from a similar neighboring site uner soybean (ata courtesy of S.. Verma an. Suyker: Suyker et al., 25). Fc from the LI-75 was comare to that from a close-ath instrument, the LI-6262, which was not affecte by instrument surface heating, uring off-season with no green oy, an uring the entire year. Fc from the LI-75 with the traitional form of WPL was comare to that from the LI-75 with an aitional correction ue to instrument surface heating. Figs.5-D comare aily atterns of hourly Fc from the LI-6262 an LI-75 (correcte traitionally an with roose aitional correction) uring warm an col erios without a green oy. Utake of CO 2 is ositie an release is negatie. n aitional term in WPL (Eq. 9) hele to reuce the aarent off-season utake an mae the LI-75 an LI-6262 match reasonably well for such a small oerall flux (fluctuations are in the orer of.5 mg CO 2 m -2 s -1 ). The magnitue

9 9 of the aitional correction itself was een smaller: -.7 mg CO 2 m -2 s -1. The relatie size of the correction increase uring col erios with Fc fluctuations near zero. oth the ost-maize fiel (with substantial resiue) an the soybean fiel (ery little resiue) hae shown similar results. To assess the yearly atterns of the roose correction, hourly alues are shown for the entire year, incluing growing season, for maize an soybean in Figs.6(-). s reicte from Eqs. 9 an 19, the largest corrections were obsere uring colest erios. The majority of col-season corrections range from.25 to.7 mg m -2 s -1. Corrections were signifitly smaller uring warm season, ranging rimarily between.5 an.2 mg m -2 s -1 in both sites..1 Maize: Fc, mg m -2 s /1/2 12/1/2 1/31/3 4/2/3 6/2/3 8/2/3 1/2/3.1 Soybean: Fc, mg m -2 s /1/2 12/1/2 1/31/3 4/2/3 6/2/3 8/2/3 1/2/3 Figure 6. Seasonal atterns of the hourly alues of the roose aitional member of WPL ue to instrument surface heating ( Fc): () irrigate maize; () rainfe soybean. Note the minimal magnitue of this correction - scale on the lot is from to.1 mg CO2 m-2 s-1. Utake of CO2 is ositie an release is negatie. The imact of the roose correction on the resultant hourly Fc is emonstrate in Figs.7(-D). The ratio of Fc from the LI-6262 to that from the traitionally correcte LI-75 is lotte ersus Ta for the erio from October 22 through October 23. In both maize (Fig. 7) an soybean (Fig. 7C), the ratio signifitly eiate from 1when T a was below 2C, an change sign below 1 C, inicating hysiologically unreasonable CO 2 utake. Using the roose correction (Figs. 7,D) eliminate most of the sign fliing, an returne the ratio to nearly 1. It is imortant to note that correction i not just increase the ratio, but greatly reuce ariability on both sies of zero (Figs.

10 1 1 Maize: Fc 75 ol 1 Maize: Fc 75 new Fc 6262/Fc C Soybean: Fc 75 ol Soybean: Fc 75 new D Fc 6262/Fc Ta, C Figure 7. Hourly ratio of carbon ioxie flux measure with LI-6262 (Fc 6262) to that measure with LI-75 (Fc 75) correcte in a traitional way (ol) an correcte with a roose aitional member of WPL correction (new): () maize, traitional WPL; () maize, WPL with aitional term; (C) soybean, traitional WPL; (D) soybean, WPL with aitional term. The erio is from October 22 to October, 23. itional correction imroe both means an istribution of the ata, an hae not just shifte all ata uwar. 7,D). It is also worth mentioning that ery small quantities are iie in these ratios, an therefore some ariability shoul be execte, esecially at low temeratures. The contrast between seemingly small hourly alues of the roose correction an its imact on the integrate fluxes be emonstrate by comaring Figs. 8 (, ) an Table II (-). Using the roose correction reuce the sloe from 3% to 2% an reuce the offset from.3 to.1 mg m -2 s -1 in maze (Fig. 8), an from.3 to zero mg m -2 s -1 in soybean (sloe was not affecte; Fig. 8). This shows that off-season fluxes are somewhat affecte, while growing season fluxes i not signifitly change. In contrast, the imact of the correction on off-season integrations of Fc was extremely imortant (Table II). The roose correction ramatically reuce the number of utake hours (Table II), from 716 to 252 hours in maize, an from 571 to 189 hours in soybean. s a result, the unerestimation of off-season losses of CO 2 (an resulting oerestimation of the yearly utake of CO 2 ) was reuce seeral fol (from -8 g CO 2 to -34 g in maize an from -32 g CO 2 to g in soybean (Table II: utake is ositie an release is negatie). fter alying the roose correction the LI-6262 an LI-75 ata matche to within few ercent, which is well within the accuracy of the Ey Coariance metho. 5. Discussion 5.1 Note of caution The metho escribe aboe for correcting Fc ue to instrument surface heating was relying, on the assumtion that the instrument was oriente nearly ertically. There are two ariables in Eq. 9 that woul be signifitly affecte by ifferent instrument configurations: fr an T s. The

11 11 Fc 75, mg m -2 s Maize: black - Fc 75 ol re - Fc 75 new y =.97x +.3 R 2 =.99 y =.98x +.1 R 2 = Fc 6262, mg m -2 s -1 Fc 75, mg m -2 s Soybean black - Fc 75 ol re - Fc 75 new y = 1.2x +.3 R 2 =.99 y = 1.2x +. R 2 = Fc 6262, mg m -2 s -1 Figure 8. Hourly carbon ioxie flux measure with LI-75 (Fc 75) lotte ersus that measure with LI-6262 (Fc 6262) in: () maize, an () soybean uring the erio is from October 22 to October, 23. 'Fc 75 ol' was correcte in a traitional way, an 'Fc 75 new' was correcte with a roose aitional member of WPL correction. itional correction was so small as comare to the hourly fluxes uring the growing season, that the latter were not noticeably affecte. Utake of CO2 is ositie an release is negatie. Off-season utake hours LI-6262 LI-75 ol LI-75 new Irrigate maize fallow Rainfe soybean fallow Off-season CO2 release, when both 6262 an 75 aailable, g C LI-6262 Irrigate maize fallow Rainfe soybean fallow LI-75 ol LI-75 new Table II. Comarison of the () off-season utake hours an () integrate carbon ioxie release between a close ath analyzer (LI-6262), traitionally correcte oen-ath ata (LI-75 ol) an ata correcte using the roose aitional member of WPL correction (LI-75 new). ll tables an lots inclue only hours with comlete ata: when actual reaings from the LI-62, LI-75, an other instruments use in the roose corrections were aailable. Non-stationary conitions, rain, snow, instrument malfunctions an other fille-in erios were exclue to assure roer comarison between Fc75 an Fc6262. Off-season erios are erios without green foliage area from October 1 to ril 3.

12 12 orientation of the instrument may imact the fr term, since the instrument woul not be ertical, an the ortion of the ath affecte by warm air may be higher than assume for a nearly-ertical instrument. lso, in such a situation, Ts shoul inclue the influence of the surface temerature that woul now also affect the ath, in aition to the winow area temerature. Win irection in a non-ertical configuration woul be an imortant factor as well, since a configuration with a signifitly incline sensor woul not be symmetrical for all win irections. In aition to instrument orientation, the lacement of other instruments an the tower might affect both fr an r a in Eq. 9, thus affecting the correction, esecially when win blows through the structures into the LI- 75 ath. Therefore, the finings resente here shoul be treate with caution, as only an uate on the ongoing inestigation of the imact of instrument surface heating on measurements mae by a nearly-ertical oen-ath CO 2 /H 2 O analyzer. 5.2 lternatie way to ealuate effect of arious sensor configurations In some cases there may be a way to ealuate the effects of a articular comlex sensor configuration on fr an r a in Eq.9, an thus aoi some assumtions (e.g., near-ertical sensor osition, realence of laminar bounary layer aroun sensor arts, absence of flow obstruction from other nearby sensors). If the correct alue of flux is known from close ath instrument or coul be confiently assume for a range of win sees an win irections, Eq.9 may be sole for fr/r a ratio. This woul establish a site-secific fr/r a ratio for ifferent win sees an irections, assuming these relationshis woul not change signifitly with temerature, humiity or ressure. Such solution may inclue the short-term use of the close-ath sensor (e.g., LI-6262 or LI-7) as stanar, or establishing erios when flux alue shoul be hysiologically close to zero (e.g., extremely low temeratures below -25 or -3 C). The rest of the ata coul be correcte for arious sensor configurations using these ratios in Eq.9 in lace of the Eqs This aroach shoul be treate with secial care ue to ossible statistical errors inole in estimating the fr/r a ratio for arious win sees an irections, an ue to the number of ata oints require to construct confient emirical relationshis; howeer, using such methos on re-existing flux ata from sites with comlicate sensor configurations is erhas still more beneficial than rejecting large erios in the measurement year. 5.3 Future research The ata in the ongoing exeriments were collecte rimarily uring col conitions with few regression oints aboe 15C, an with reominantly low fluxes. This force us to aly the emirical relationshi between irectly measure Ts an Ta eeloe in to the Fc ata from With more ata to be collecte uring the warm season of 26, further tests will inclue comarison of fluxes between the close ath LI-6262 an oen ath LI-75 for a wie range of irectly measure surface temeratures. There are seeral other stes that may hel to further stuy an manage the effects of instrument heating. Haing thermocoules on the surface near the lower an uer winows woul hel to secify the roose correction without the ifficulty of haing to estimate Ts (e.g., Eqs ). Further exeriments on the best sensor orientation might also be one to minimize the effects of instrument surface heating. Particularly, usie-own sensor ositioning might reuce the effect of the heating on the flux in all irections. Further research is also neee to ealuate, test an fine-tune the roose concet for arious enironments, an for ifferent manufacturers of oen-ath CO2 analyzers. 6. Summary an conclusion The influence of surface heating on the CO 2 flux measure with the LI-75 oen ath gas analyzer is being examine in one laboratory stuy an two fiel exeriments. Exeriment in a conectie enironmental chamber showe that the sensor hea electronics substantially contribute to surface heating, an that the surface of the oenath analyzer coul be 5-7 C warmer than the ambient air ue to electrical energy issiation in the instrument hea. The ifference was largest at low ambient temeratures an ecrease linearly with an increase in the ambient temerature. fiel exeriment at the LI-COR facility, which is still in rogress, has shown that large temerature graients (u to 2 C er 1mm) exist just aboe the lower winow of the instrument, imlying strong sensible heat fluxes in the first few mm aboe the instrument surface. The thermal

13 13 bounary layer was thicker uring calm conitions, as execte. During the ay, surface heating occurre ue to solar raiation een when the electronics were not owere. Our measurements also suggest that emission of long-wae raiation cause surface cooling at night to temeratures below ambient. n exeriment is in rogress near Mea, NE also in which we are comaring Fc measure with an oen-ath LI-75 to Fc measure by a close ath LI-6262, which is not affecte by surface heating an is iewe as stanar. To ate, this exeriment has emonstrate, that instrument surface temerature is irectly an strongly relate to air temerature (Eq. 19), but lack of ata uring warm season i not allow for confient extraolation of the relationshi beyon +2 C. n aitional term ue to instrument surface heating was introuce as a art of the Webb- Pearman-Leuning correction. The temerature relationshi eeloe in the Mea exeriment in (Eq. 18) was alie to LI-75 ata from 22-3 in maize an soybean, an was comare against LI-6262 ata from the same erio. Een with artial results from the exeriments aailable, a signifit reuction in aarent CO 2 utake uring off-season erios was obsere as a result of alying the aitional correction, while fluxes uring the growing season hae not been noticeably affecte. The correction also resulte in elimination of most of the wrong signs from off-season oen-ath CO 2 measurements, a signifit reuction in ariability of the ata, elimination of the offset between measurements mae with the LI-6262 an the LI- 75, an in a signifit imroement in offseason integrations of CO 2 exchange. framework was create to eelo a site-secific ractical correction ue to instrument heating. The roose aitional term in the Webb-Pearman-Leuning correction may roe useful for future year-roun CO 2 flux research. The technique to estimate the roose correction for re-existing ata may hel in better interretation of reiously collecte information. cknowlegements ata an facilities for the Mea exeriment, an for numerous aluable hel an aice. We also thank Drs. rian miro, Robert Clement, Dai Cook, Marc-nré Giasson, Ryuichi Hirata, Ray Leuning, ill Massman, Walt Oechel, Howar Skinner, an Jon Welles for their interest in eeloing the toic. We acknowlege hel roie by Jerry Ficke of LI-COR on LI-75 s electronics an other roerties, an for assistance with the enironmental chamber exeriment. We also thank Fluxnet, meriflux, Fluxnet-Canaa, an Carbo- Euroe networks for roiing oortunity for the iscussions an for access to flux ata obtaine with LI-75 an LI-6262 instruments. References: meriflux, 26. meriflux Sites Information." <htt://ublic.ornl.go/ameriflux/site-select.cfm> urba G. G., nerson D. J., Xu L., an D. K. McDermitt. 25a. Soling the off-season utake roblem: correcting fluxes measure with the LI-75 for the effects of instrument surface heating. Progress reort of the ongoing stuy. PRT I: THEORY. Poster resentation. meriflux 25 nnual Meeting, ouler, Colorao. urba G. G., nerson D. J., Xu L., an D. K. McDermitt. 25b. Soling the off-season utake roblem: correcting fluxes measure with the LI-75 for the effects of instrument surface heating. Progress reort of the ongoing stuy. PRT II: RESULTS. Poster Presentation. meriflux 25 nnual Meeting, ouler, Colorao. Cambell, G. S., an J. M. Norman, Enironmental iohysics. Secon Eition. Sringer Science, New York: 286. Gates, D. M iohysical Ecology. Sringer-Verlag, NY. Hirata R., Hirano T., Mogami J., Fujinuma Y., Inukai K., Saigusa N., an S. Yamamoto, 25. CO 2 flux measure by an oen-ath system oer a larch forest uring snowcoere season. Phyton, 45: Monteith, J. L., Gas exchange in lant communities. Enironmental Control of Plant Growth. L. T. Eans, e. caemic Press, New York: Suyker,.E., S.. Verma, G.G. urba, an T.J. rkebauer. 25. Gross rimary rouction an ecosystem resiration of irrigate maize an irrigate soybean uring a growing season. gricultural an Forest Meteorology, 131: Verma, S.., an. E. Suyker. Personal Communications. Setember-December, 25. Webb, E. K., Pearman, G. I., an R. Leuning, 198. Correction of flux measurements for ensity effects ue to heat an water aour transfer. Quart. J. R. Met. Soc., 16: 85-1 We woul like to acknowlege a number of scientists who contribute to this stuy ia aluable iscussions, aice, ata sets, an other ersonal communications. Particularly we want to thank Drs. Shashi Verma an nrew Suyker for roiing