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Isoprene and monoterpene fluxes measured above Amazonian rainforest and their dependence on light and temperature
Atmospheric Environment 36 (2002) 2421–2426
Isoprene and monoterpene fluxes measured above Amazonian rainforest and their dependence on light and temperature H.J.I. Rinne1, A.B. Guenther*, J.P. Greenberg, P.C. Harley National Center for Atmospheric Research, Atmospheric Chemistry Division, 1850 Table Mesa Dr., Boulder, CO 80305, USA Received 28 February 2001; received in revised form 10 August 2001; accepted 23 September 2001
Abstract Canopy scale emissions of isoprene and monoterpenes from Amazonian rainforest were measured by eddy covariance and eddy accumulation techniques. The peak mixing ratios at about 10 m above the canopy occurred in the afternoon and were typically about 90 pptv of a-pinene and 4–5 ppbv of isoprene. a-pinene was the most abundant monoterpene in the air above the canopy comprising E50% of the total monoterpene mixing ratio. Measured isoprene fluxes were almost 10 times higher than a-pinene fluxes. Normalized conditions of 301C and 1000 mmol m2 s1 were associated with an isoprene flux of 2.4 mg m2 h1 and a b-pinene flux of 0.26 mg m2 h1. Both fluxes were lower than values that have been specified for Amazon rainforests in global emission models. Isoprene flux correlated with a lightand temperature-dependent emission activity factor, and even better with measured sensible heat flux. The variation in the measured a-pinene fluxes, as well as the diurnal cycle of mixing ratio, suggest emissions that are dependent on both light and temperature. The light and temperature dependence can have a significant effect on the modeled diurnal cycle of monoterpene emission as well as on the total monoterpene emission. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Tropical rainforests are estimated to be a major source of volatile organic compounds (VOC) into the atmosphere (Guenther et al., 1995). However, VOC emission estimates of tropical rainforests are highly uncertain due to enormous species diversity, uncharacterized landscapes, and the limited amount of emission data on tropical vegetation. Based on leaf level emission measurements and boundary layer concentration profiles, isoprene emission from tropical rainforests is expected to dominate over monoterpene emission (e.g., Guenther et al., 1995). Biogenic VOC (BVOC) mixing ratios above tropical rainforests are usually dominated by isoprene, but monoterpenes and other
*Corresponding author. Fax: +1-303-497-1477. E-mail address: [email protected] (A.B. Guenther). 1 Present address: CNRM/GMEI - M!et!eo-France, 42 avenue G. Coriolis, 31057 Toulouse, Cedex, France.
BVOCs are also observed (e.g., Zimmerman et al., 1988; Helmig et al., 1998; Kesselmeier et al., 2000). Isoprene emission from tropical forests has previously been treated as depending on light and temperature, whereas monoterpene emissions have generally been treated as depending solely on temperature (e.g., Guenther et al., 1995). However, monoterpene emission of certain Mediterranean and tropical plant species has been shown to be dependent on both light and temperature, similar to isoprene emission behavior (Staudt and Seufert, 1995; Loreto et al., 1996, Kuhn et al., 2001). The emission parameters previously used in global emission models for tropical rainforest have been derived from tethered balloon mixing ratio measurements and thus rely heavily on chemistry models. This introduces a source of uncertainty to the emission parameters. This paper presents first results of terpenoid flux measurements conducted by eddy flux techniques above neo-tropical rainforest, together with their dependencies on light and temperature.
1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 5 2 3 - 4
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The measurements were conducted on a 45 m tower located about 5 km west of the Santar!em-Cuiab!a highway (BR-163) at km 67, in Floresta Nacional do ! Para! , Brazil (21510 S, 541580 W). The measureTapajos, ment site is situated on the upper plateau, planalto, with old acidic soils and little organic matter. The forest at the site is primary terra firme rainforest, with closed canopy structure and emergent trees. The terra firme forest covers nearly 60% of the land area of the Amazon region (Parrotta et al., 1995). The canopy height around the measurement tower was 35–40 m with some nearby gaps, probably due to large fallen trees. The measurements presented here were conducted between 6 and 8 July 2000. ! is almost The annual precipitation in Tapajos 2000 mm. Measurements were conducted in July 2000, near the end of the rainy season. The distinct dry period, usually between August and October, has monthly rainfall o60 mm (Parrotta et al., 1995 and references therein). The ozone mixing ratios in Amazonian tropics outside biomass burning season are very low, the daytime mixing ratios generally varying between 10 and 15 ppbv (Vanni Gatti et al., 2000; Cordova Leal et al., 2000). During the measurements no local or largescale biomass burning occurred. The vertical fluxes of terpenoids were measured by the disjunct eddy accumulation (DEA) method described by Rinne et al. (2000a). Isoprene fluxes were also measured using a fast isoprene sensor (FIS) described by Guenther and Hills (1998). The DEA technique accumulated hydrocarbons onto solid adsorbent cartridges filled with 200 mg of Tenax TA and 200 mg of Carbosieve SIII. Cartridges were refrigerated before and after sampling and returned to the National Center for Atmospheric Research (Boulder, CO) for analysis by gas chromatography with mass spectroscopy. An ozone scrubber was not used for sampling because of low ozone mixing ratios. According to Calogirou et al. (1996) the recovery of a-pinene at these ozone mixing ratios and our sampling rates is better than 95%. A more detailed description of the chemical analysis is given by Greenberg et al. (1999). The measurements were made at the top of the tower at a height of 47 m. The acoustic anemometer (Applied Technologies Inc., SATI 3 K) was located 1 m above the DEA sampling system.
abundant monoterpene, was measured at mixing ratios up to 120 pptv, comprising 50% of the total monoterpene mixing ratio. The peak isoprene concentrations determined by the FIS system were about 5 ppbv. Limonene and b-pinene each comprised of 15–20% of the total monoterpene mixing ratio. Other monoterpenes observed included camphene, sabinene and myrcene. These observations are consistent with those of Zimmerman et al. (1988) and Helmig et al. (1998). Isoprene and a-pinene showed similar diurnal patterns, with lowest mixing ratios in the morning and highest in the afternoon with a correlation of r2 ¼ 0:41 between them. Previously, Zimmerman et al. (1988) reported apinene mixing ratios to be lower at night and higher during daytime above Amazonian rainforest. Fig. 2 presents measured air temperatures, photosynthetic photon flux densities and isoprene, a- and bpinene fluxes on one of the measurement days. The isoprene fluxes measured by the FIS (Fig. 2B) showed a diurnal pattern with higher emissions during daytime and negligible or negative fluxes during nighttime. The peak midday flux was about 2 mg m2 h1. This is lower than emissions currently assigned to the Amazon forest by global emission models. Midday isoprene fluxes reported for other vegetation types range from o1 mg m2 h1 to >4 mg m2 h1 (e.g., Guenther et al., 1996; Zhu et al., 1999). The fluxes observed early in the night
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The isoprene and a-pinene mixing ratios above the ! km 67, determined from cartridges canopy at Tapajos collected by DEA, are shown in Fig. 1. The above canopy mixing ratios are dominated by isoprene with mixing ratios reaching about 4 ppbv. a-Pinene, the most
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Fig. 1. Mixing ratios of isoprene and a-pinene above the forest canopy during 6 (diamonds), 7 (circles) and 8 July (triangles), 2000.
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Fig. 2. (A) Photosynthetically active photon flux density, PPFD (dashed line) and temperature (solid line) on 7 July at the top of the measurement tower at km 67; (B) measured isoprene (circles), a-pinene (solid squares) and b-pinene (open squares) fluxes.
were usually downward, which may be caused by dry deposition or within-canopy NO3 chemistry. The peak measured a-pinene fluxes were around 0.1 mg m2 h1 (Fig. 2B) and they were on average about 10% of the isoprene flux. This is the same order of magnitude as the daytime monoterpene fluxes observed above European boreal forests (Christensen et al., 2000; Rinne et al., 2000b, c) but much lower than observed above some temperate and Mediterranean vegetation types (Ciccioli et al., 1999; Christensen et al., 2000; Gallagher et al., 2000). The ratio of b-pinene and limonene fluxes to the a-pinene fluxes was similar to the ratios of concentrations. The isoprene fluxes measured by DEA and FIS were of the same magnitude, but there was a large scatter in the DEA isoprene flux data, with some very high flux values and large negative values, which were absent in the DEA a-pinene flux data. Thus, the overall correlation between isoprene fluxes measured by the two techniques was poor. Isoprene fluxes measured by the FIS, however, correlated reasonably well with a-pinene fluxes measured by the DEA with r2 ¼ 0:44: One possible explanation for this discrepancy is a more heterogeneous source distribution for isoprene, which might affect DEA more, since it takes only a few
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samples with a relatively long interval between them. Another explanation is that the large volume (4–6 L) of air sampled onto the DEA cartridges probably caused larger uncertainties in isoprene, than a-pinene, due to breakthrough of isoprene on the cartridges. The analytical uncertainties in the determination of isoprene concentration, other than those associated with the actual volume sampled, are o5% (Greenberg et al., 1999). The major chemical losses for isoprene and monoterpenes in the atmosphere are reactions with O3, OH and NO3 radicals. At the canopy level the OH and O3 concentrations are very low, since they are destroyed quickly by vegetation and surfaces. NO3 radical concentration is negligible during daytime. The effect of O3 on isoprene in the cartridge would be much less than on monoterpene concentrations, since isoprene travels deep into the absorbent, where it is trapped on the second-stage absorbent (Carbotrap), while terpenes are trapped within the first stage (Tenax). Since O3 is likely destroyed on initial contact with the Tenax, little effect on isoprene is expected. In regional and global models the VOC emission flux (F ; mg m2 h1) is usually calculated as F ¼ EN Dg;
ð1Þ
where EN is the emission flux normalized to Ts ¼ 301C 1 and L ¼ 1000 mmol m2 s1 (mg g1 dw h ), D is biomass 2 density (gdw m ) and g is the nondimensional emission activity factor (e.g., Guenther, 1997). The light and temperature dependence of isoprene is usually described by an activity factor 2 3 CT1 ðT Ts Þ " # exp 6 7 aCL1 L RTs T 7 gL ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 6 ð2Þ 4 5; 2 2 C ðT T Þ T2 M 1þa L CT3 þ exp RTs T where L is the photosynthetic photon flux density (PPFD), R is the gas constant, and CL1 ; a; CT1 ; CT2 and CT3 are empirical constants (Guenther, 1997). Although this algorithm was originally developed for isoprene emission, it has also been applied to calculation of light-dependent monoterpene emissions (e.g., Ciccioli et al., 1997; Schuh et al., 1997; Simpson et al., 1999; Lindfors et al., 2000). Fig. 3 shows measured isoprene fluxes plotted against the light- and temperaturedependent activity factor calculated using Eq. (2) and averaged over the entire canopy using the canopy environment model of Guenther et al. (1995). The normalized emission flux according to these measurements was E2.4 mg m2 h1. Isoprene flux correlated better with sensible heat flux than with the activity factor. This is probably due to discrepancies between the actual temperature and light environment in the canopy and that calculated by the canopy model. Also, the sensible heat flux measurement and isoprene flux
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measurement are not independent since both use the same sonic anemometer data. Monoterpene emission is often assumed to be solely dependent on temperature and its dependence is described by activity factor gT ¼ expðb½T Ts Þ;
0 0
ð3Þ
where T is the leaf temperature, Ts is the leaf temperature at standard condition (usually 301C), and b is an empirical coefficient usually taken to be 0.091C1 (Guenther, 1997). Eq. 3, with b ¼ 0:09; does not explain the variation in the measured a-pinene fluxes very well (r2 ¼ 0:21). Fitting an exponential curve to measured fluxes gives b¼ 0:501C1 ; with r2 ¼ 0:53: This b-coefficient is, however, much higher than the usually observed b¼ 0:0720:21C1 (e.g., Guenther et al., 1991; Schween et al., 1997; Rinne et al., 2000b) and implies a physiologically unrealistic Q10 of 150 (Nobel, 1991). The temperature- and light-dependent activity factor (Eq. (2)) better explains the variation in fluxes with r2 ¼ 0:57 (Fig. 4). This leads to a normalized emission flux of 0.26 mg m2 h1. At this site the monoterpene mixing ratios were low in the morning, increasing during the day, which was similar to the diurnal cycle of isoprene mixing ratio. This gives further evidence for the light-dependent nature of the monoterpene emission. The morning of 6 July was cloudy and rainy which explains the low mixing ratios observed at noon. b-pinene and limonene mixing ratios showed similar diurnal cycles.
4. Discussion The ratio of isoprene to monoterpene fluxes observed here is similar to that calculated by Guenther et al.
Fig. 4. Observed fluxes of a-pinene against temperature and light activity factor described by Eq. (2) (solid circles and solid line) and sensible heat flux (open circles and dashed line).
(1995) for tropical forests. According to the model of Guenther et al. (1995), the midday average isoprene emissions from Brazilian tropics is around 4 mg m2 h1, which is higher than the 1–2 mg m2 h1 observed at the Tapajos km 67 research site. The monoterpene emissions calculated by Guenther et al. (1995) range from 0.3 to 0.6 mg m2 h1, which is also higher than the observed monoterpene fluxes of about 0.2 mg m2 h1. The measurements reported here are associated with a small footprint area. This together with the enormous diversity of neo-tropical rain forests makes it impossible to reliably generalize these results to the regional scale. Aircraft-based flux measurements are thus needed to obtain direct measurements of BVOC fluxes at larger scales. The behavior of both fluxes and mixing ratios of apinene indicates that emissions of this compound from this site are likely to be light-dependent. If they were solely temperature-dependent, the b-coefficient would be much higher than anything previously observed. Since temperature and light correlate in nature, it is hard to get conclusive evidence on the possible light dependence using surface layer flux measurement techniques only. However, Kuhn et al. (2001) have observed lightdependent behavior of monoterpene emissions from an Amazonian species using an enclosure technique. Also, diurnal variation of monoterpene mixing ratios reported previously by Zimmerman et al. (1988) showed behavior typical of BVOCs emitted in a light-dependent manner. This is in contrast to the boreal zone, where monoterpene emissions generally depend solely on temperature and thus their concentrations are observed to be higher at night than during the daytime (e.g., Janson, 1993; Rinne et al., 2000b, c; Hakola et al., 2000).
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Part of the scatter in the correlations can be explained by the relatively large uncertainties typical of micrometeorological VOC flux measurements. The natural geophysical variability of the flux can easily be 30% for a single data point (Wesely and Hart, 1985), in addition to analytical uncertainties. A part of the scatter may also be related to possible heterogeneities in the source distribution and other factors, such as physical stress affecting the emission. Monoterpene emissions from tropical rainforests have previously been estimated using exclusively temperaturedependent parameterization (e.g., Guenther et al., 1995). A light and temperature-dependent behavior of terpene emission, if common among the tropical flora, would have a strong influence on the predicted temporal distribution of monoterpene emission. Moreover, if we compare the emission estimated using a temperaturedependent algorithm with that estimated using a lightand temperature-dependent algorithm and assume the same basal emission rate, the daily emission calculated using the light- and temperature-dependent algorithm in tropical conditions is only about one-third of the emission calculated using the temperature-dependent algorithm.
5. Conclusions Canopy scale isoprene emissions from this primary rainforest site were almost 10 times higher than a-pinene emissions, which is consistent with previous measurements. The observed isoprene and monoterpene fluxes were lower than those estimated by a global emission model. The fluxes reported here, however, represent emissions from a rather small footprint area compared to the area of even a single grid in a global model. This together with the high species diversity of neo-tropical rainforests makes this comparison inconclusive. Measured isoprene fluxes correlated reasonably well with a light- and temperature-dependent emission activity factor and even better with measured sensible heat flux. The temperature-dependent monoterpene emission algorithm, generally used in emission modeling, did not explain the variation of a-pinene fluxes well. Both the variation of measured a-pinene fluxes and the diurnal cycle of mixing ratios indicate the likelihood of light-dependent behavior of the emission. b-pinene and limonene, which were the next most abundant monoterpenes, showed similar behavior. The possible light dependence of monoterpene emissions can have a strong impact on calculated instantaneous and daily monoterpene emission totals. Enclosure studies to develop emission algorithms are needed because the light and temperature activity factors for monoterpenes may differ from those for isoprene.
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Acknowledgements This work was conducted as a part of the LBAEcology research project (NASA grant LBA/97-0058). We thank Luciana Vanni Gatti and the people at the LBA office in Santare! m for their assistance during the measurement campaign. Janne Rinne acknowledges the Academy of Finland for financial support. The National Center for Atmospheric Research is sponsored by The National Science Foundation.
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Isoprene and monoterpene fluxes measured above Amazonian rainforest and their dependence on light and temperature H.J.I. Rinne1, A.B. Guenther*, J.P. Greenberg, P.C. Harley National Center for Atmospheric Research, Atmospheric Chemistry Division, 1850 Table Mesa Dr., Boulder, CO 80305, USA Received 28 February 2001; received in revised form 10 August 2001; accepted 23 September 2001
Abstract Canopy scale emissions of isoprene and monoterpenes from Amazonian rainforest were measured by eddy covariance and eddy accumulation techniques. The peak mixing ratios at about 10 m above the canopy occurred in the afternoon and were typically about 90 pptv of a-pinene and 4–5 ppbv of isoprene. a-pinene was the most abundant monoterpene in the air above the canopy comprising E50% of the total monoterpene mixing ratio. Measured isoprene fluxes were almost 10 times higher than a-pinene fluxes. Normalized conditions of 301C and 1000 mmol m2 s1 were associated with an isoprene flux of 2.4 mg m2 h1 and a b-pinene flux of 0.26 mg m2 h1. Both fluxes were lower than values that have been specified for Amazon rainforests in global emission models. Isoprene flux correlated with a lightand temperature-dependent emission activity factor, and even better with measured sensible heat flux. The variation in the measured a-pinene fluxes, as well as the diurnal cycle of mixing ratio, suggest emissions that are dependent on both light and temperature. The light and temperature dependence can have a significant effect on the modeled diurnal cycle of monoterpene emission as well as on the total monoterpene emission. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Tropical rainforests are estimated to be a major source of volatile organic compounds (VOC) into the atmosphere (Guenther et al., 1995). However, VOC emission estimates of tropical rainforests are highly uncertain due to enormous species diversity, uncharacterized landscapes, and the limited amount of emission data on tropical vegetation. Based on leaf level emission measurements and boundary layer concentration profiles, isoprene emission from tropical rainforests is expected to dominate over monoterpene emission (e.g., Guenther et al., 1995). Biogenic VOC (BVOC) mixing ratios above tropical rainforests are usually dominated by isoprene, but monoterpenes and other
*Corresponding author. Fax: +1-303-497-1477. E-mail address: [email protected] (A.B. Guenther). 1 Present address: CNRM/GMEI - M!et!eo-France, 42 avenue G. Coriolis, 31057 Toulouse, Cedex, France.
BVOCs are also observed (e.g., Zimmerman et al., 1988; Helmig et al., 1998; Kesselmeier et al., 2000). Isoprene emission from tropical forests has previously been treated as depending on light and temperature, whereas monoterpene emissions have generally been treated as depending solely on temperature (e.g., Guenther et al., 1995). However, monoterpene emission of certain Mediterranean and tropical plant species has been shown to be dependent on both light and temperature, similar to isoprene emission behavior (Staudt and Seufert, 1995; Loreto et al., 1996, Kuhn et al., 2001). The emission parameters previously used in global emission models for tropical rainforest have been derived from tethered balloon mixing ratio measurements and thus rely heavily on chemistry models. This introduces a source of uncertainty to the emission parameters. This paper presents first results of terpenoid flux measurements conducted by eddy flux techniques above neo-tropical rainforest, together with their dependencies on light and temperature.
1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 5 2 3 - 4
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The measurements were conducted on a 45 m tower located about 5 km west of the Santar!em-Cuiab!a highway (BR-163) at km 67, in Floresta Nacional do ! Para! , Brazil (21510 S, 541580 W). The measureTapajos, ment site is situated on the upper plateau, planalto, with old acidic soils and little organic matter. The forest at the site is primary terra firme rainforest, with closed canopy structure and emergent trees. The terra firme forest covers nearly 60% of the land area of the Amazon region (Parrotta et al., 1995). The canopy height around the measurement tower was 35–40 m with some nearby gaps, probably due to large fallen trees. The measurements presented here were conducted between 6 and 8 July 2000. ! is almost The annual precipitation in Tapajos 2000 mm. Measurements were conducted in July 2000, near the end of the rainy season. The distinct dry period, usually between August and October, has monthly rainfall o60 mm (Parrotta et al., 1995 and references therein). The ozone mixing ratios in Amazonian tropics outside biomass burning season are very low, the daytime mixing ratios generally varying between 10 and 15 ppbv (Vanni Gatti et al., 2000; Cordova Leal et al., 2000). During the measurements no local or largescale biomass burning occurred. The vertical fluxes of terpenoids were measured by the disjunct eddy accumulation (DEA) method described by Rinne et al. (2000a). Isoprene fluxes were also measured using a fast isoprene sensor (FIS) described by Guenther and Hills (1998). The DEA technique accumulated hydrocarbons onto solid adsorbent cartridges filled with 200 mg of Tenax TA and 200 mg of Carbosieve SIII. Cartridges were refrigerated before and after sampling and returned to the National Center for Atmospheric Research (Boulder, CO) for analysis by gas chromatography with mass spectroscopy. An ozone scrubber was not used for sampling because of low ozone mixing ratios. According to Calogirou et al. (1996) the recovery of a-pinene at these ozone mixing ratios and our sampling rates is better than 95%. A more detailed description of the chemical analysis is given by Greenberg et al. (1999). The measurements were made at the top of the tower at a height of 47 m. The acoustic anemometer (Applied Technologies Inc., SATI 3 K) was located 1 m above the DEA sampling system.
abundant monoterpene, was measured at mixing ratios up to 120 pptv, comprising 50% of the total monoterpene mixing ratio. The peak isoprene concentrations determined by the FIS system were about 5 ppbv. Limonene and b-pinene each comprised of 15–20% of the total monoterpene mixing ratio. Other monoterpenes observed included camphene, sabinene and myrcene. These observations are consistent with those of Zimmerman et al. (1988) and Helmig et al. (1998). Isoprene and a-pinene showed similar diurnal patterns, with lowest mixing ratios in the morning and highest in the afternoon with a correlation of r2 ¼ 0:41 between them. Previously, Zimmerman et al. (1988) reported apinene mixing ratios to be lower at night and higher during daytime above Amazonian rainforest. Fig. 2 presents measured air temperatures, photosynthetic photon flux densities and isoprene, a- and bpinene fluxes on one of the measurement days. The isoprene fluxes measured by the FIS (Fig. 2B) showed a diurnal pattern with higher emissions during daytime and negligible or negative fluxes during nighttime. The peak midday flux was about 2 mg m2 h1. This is lower than emissions currently assigned to the Amazon forest by global emission models. Midday isoprene fluxes reported for other vegetation types range from o1 mg m2 h1 to >4 mg m2 h1 (e.g., Guenther et al., 1996; Zhu et al., 1999). The fluxes observed early in the night
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The isoprene and a-pinene mixing ratios above the ! km 67, determined from cartridges canopy at Tapajos collected by DEA, are shown in Fig. 1. The above canopy mixing ratios are dominated by isoprene with mixing ratios reaching about 4 ppbv. a-Pinene, the most
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Fig. 1. Mixing ratios of isoprene and a-pinene above the forest canopy during 6 (diamonds), 7 (circles) and 8 July (triangles), 2000.
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Fig. 2. (A) Photosynthetically active photon flux density, PPFD (dashed line) and temperature (solid line) on 7 July at the top of the measurement tower at km 67; (B) measured isoprene (circles), a-pinene (solid squares) and b-pinene (open squares) fluxes.
were usually downward, which may be caused by dry deposition or within-canopy NO3 chemistry. The peak measured a-pinene fluxes were around 0.1 mg m2 h1 (Fig. 2B) and they were on average about 10% of the isoprene flux. This is the same order of magnitude as the daytime monoterpene fluxes observed above European boreal forests (Christensen et al., 2000; Rinne et al., 2000b, c) but much lower than observed above some temperate and Mediterranean vegetation types (Ciccioli et al., 1999; Christensen et al., 2000; Gallagher et al., 2000). The ratio of b-pinene and limonene fluxes to the a-pinene fluxes was similar to the ratios of concentrations. The isoprene fluxes measured by DEA and FIS were of the same magnitude, but there was a large scatter in the DEA isoprene flux data, with some very high flux values and large negative values, which were absent in the DEA a-pinene flux data. Thus, the overall correlation between isoprene fluxes measured by the two techniques was poor. Isoprene fluxes measured by the FIS, however, correlated reasonably well with a-pinene fluxes measured by the DEA with r2 ¼ 0:44: One possible explanation for this discrepancy is a more heterogeneous source distribution for isoprene, which might affect DEA more, since it takes only a few
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samples with a relatively long interval between them. Another explanation is that the large volume (4–6 L) of air sampled onto the DEA cartridges probably caused larger uncertainties in isoprene, than a-pinene, due to breakthrough of isoprene on the cartridges. The analytical uncertainties in the determination of isoprene concentration, other than those associated with the actual volume sampled, are o5% (Greenberg et al., 1999). The major chemical losses for isoprene and monoterpenes in the atmosphere are reactions with O3, OH and NO3 radicals. At the canopy level the OH and O3 concentrations are very low, since they are destroyed quickly by vegetation and surfaces. NO3 radical concentration is negligible during daytime. The effect of O3 on isoprene in the cartridge would be much less than on monoterpene concentrations, since isoprene travels deep into the absorbent, where it is trapped on the second-stage absorbent (Carbotrap), while terpenes are trapped within the first stage (Tenax). Since O3 is likely destroyed on initial contact with the Tenax, little effect on isoprene is expected. In regional and global models the VOC emission flux (F ; mg m2 h1) is usually calculated as F ¼ EN Dg;
ð1Þ
where EN is the emission flux normalized to Ts ¼ 301C 1 and L ¼ 1000 mmol m2 s1 (mg g1 dw h ), D is biomass 2 density (gdw m ) and g is the nondimensional emission activity factor (e.g., Guenther, 1997). The light and temperature dependence of isoprene is usually described by an activity factor 2 3 CT1 ðT Ts Þ " # exp 6 7 aCL1 L RTs T 7 gL ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 6 ð2Þ 4 5; 2 2 C ðT T Þ T2 M 1þa L CT3 þ exp RTs T where L is the photosynthetic photon flux density (PPFD), R is the gas constant, and CL1 ; a; CT1 ; CT2 and CT3 are empirical constants (Guenther, 1997). Although this algorithm was originally developed for isoprene emission, it has also been applied to calculation of light-dependent monoterpene emissions (e.g., Ciccioli et al., 1997; Schuh et al., 1997; Simpson et al., 1999; Lindfors et al., 2000). Fig. 3 shows measured isoprene fluxes plotted against the light- and temperaturedependent activity factor calculated using Eq. (2) and averaged over the entire canopy using the canopy environment model of Guenther et al. (1995). The normalized emission flux according to these measurements was E2.4 mg m2 h1. Isoprene flux correlated better with sensible heat flux than with the activity factor. This is probably due to discrepancies between the actual temperature and light environment in the canopy and that calculated by the canopy model. Also, the sensible heat flux measurement and isoprene flux
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measurement are not independent since both use the same sonic anemometer data. Monoterpene emission is often assumed to be solely dependent on temperature and its dependence is described by activity factor gT ¼ expðb½T Ts Þ;
0 0
ð3Þ
where T is the leaf temperature, Ts is the leaf temperature at standard condition (usually 301C), and b is an empirical coefficient usually taken to be 0.091C1 (Guenther, 1997). Eq. 3, with b ¼ 0:09; does not explain the variation in the measured a-pinene fluxes very well (r2 ¼ 0:21). Fitting an exponential curve to measured fluxes gives b¼ 0:501C1 ; with r2 ¼ 0:53: This b-coefficient is, however, much higher than the usually observed b¼ 0:0720:21C1 (e.g., Guenther et al., 1991; Schween et al., 1997; Rinne et al., 2000b) and implies a physiologically unrealistic Q10 of 150 (Nobel, 1991). The temperature- and light-dependent activity factor (Eq. (2)) better explains the variation in fluxes with r2 ¼ 0:57 (Fig. 4). This leads to a normalized emission flux of 0.26 mg m2 h1. At this site the monoterpene mixing ratios were low in the morning, increasing during the day, which was similar to the diurnal cycle of isoprene mixing ratio. This gives further evidence for the light-dependent nature of the monoterpene emission. The morning of 6 July was cloudy and rainy which explains the low mixing ratios observed at noon. b-pinene and limonene mixing ratios showed similar diurnal cycles.
4. Discussion The ratio of isoprene to monoterpene fluxes observed here is similar to that calculated by Guenther et al.
Fig. 4. Observed fluxes of a-pinene against temperature and light activity factor described by Eq. (2) (solid circles and solid line) and sensible heat flux (open circles and dashed line).
(1995) for tropical forests. According to the model of Guenther et al. (1995), the midday average isoprene emissions from Brazilian tropics is around 4 mg m2 h1, which is higher than the 1–2 mg m2 h1 observed at the Tapajos km 67 research site. The monoterpene emissions calculated by Guenther et al. (1995) range from 0.3 to 0.6 mg m2 h1, which is also higher than the observed monoterpene fluxes of about 0.2 mg m2 h1. The measurements reported here are associated with a small footprint area. This together with the enormous diversity of neo-tropical rain forests makes it impossible to reliably generalize these results to the regional scale. Aircraft-based flux measurements are thus needed to obtain direct measurements of BVOC fluxes at larger scales. The behavior of both fluxes and mixing ratios of apinene indicates that emissions of this compound from this site are likely to be light-dependent. If they were solely temperature-dependent, the b-coefficient would be much higher than anything previously observed. Since temperature and light correlate in nature, it is hard to get conclusive evidence on the possible light dependence using surface layer flux measurement techniques only. However, Kuhn et al. (2001) have observed lightdependent behavior of monoterpene emissions from an Amazonian species using an enclosure technique. Also, diurnal variation of monoterpene mixing ratios reported previously by Zimmerman et al. (1988) showed behavior typical of BVOCs emitted in a light-dependent manner. This is in contrast to the boreal zone, where monoterpene emissions generally depend solely on temperature and thus their concentrations are observed to be higher at night than during the daytime (e.g., Janson, 1993; Rinne et al., 2000b, c; Hakola et al., 2000).
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Part of the scatter in the correlations can be explained by the relatively large uncertainties typical of micrometeorological VOC flux measurements. The natural geophysical variability of the flux can easily be 30% for a single data point (Wesely and Hart, 1985), in addition to analytical uncertainties. A part of the scatter may also be related to possible heterogeneities in the source distribution and other factors, such as physical stress affecting the emission. Monoterpene emissions from tropical rainforests have previously been estimated using exclusively temperaturedependent parameterization (e.g., Guenther et al., 1995). A light and temperature-dependent behavior of terpene emission, if common among the tropical flora, would have a strong influence on the predicted temporal distribution of monoterpene emission. Moreover, if we compare the emission estimated using a temperaturedependent algorithm with that estimated using a lightand temperature-dependent algorithm and assume the same basal emission rate, the daily emission calculated using the light- and temperature-dependent algorithm in tropical conditions is only about one-third of the emission calculated using the temperature-dependent algorithm.
5. Conclusions Canopy scale isoprene emissions from this primary rainforest site were almost 10 times higher than a-pinene emissions, which is consistent with previous measurements. The observed isoprene and monoterpene fluxes were lower than those estimated by a global emission model. The fluxes reported here, however, represent emissions from a rather small footprint area compared to the area of even a single grid in a global model. This together with the high species diversity of neo-tropical rainforests makes this comparison inconclusive. Measured isoprene fluxes correlated reasonably well with a light- and temperature-dependent emission activity factor and even better with measured sensible heat flux. The temperature-dependent monoterpene emission algorithm, generally used in emission modeling, did not explain the variation of a-pinene fluxes well. Both the variation of measured a-pinene fluxes and the diurnal cycle of mixing ratios indicate the likelihood of light-dependent behavior of the emission. b-pinene and limonene, which were the next most abundant monoterpenes, showed similar behavior. The possible light dependence of monoterpene emissions can have a strong impact on calculated instantaneous and daily monoterpene emission totals. Enclosure studies to develop emission algorithms are needed because the light and temperature activity factors for monoterpenes may differ from those for isoprene.
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Acknowledgements This work was conducted as a part of the LBAEcology research project (NASA grant LBA/97-0058). We thank Luciana Vanni Gatti and the people at the LBA office in Santare! m for their assistance during the measurement campaign. Janne Rinne acknowledges the Academy of Finland for financial support. The National Center for Atmospheric Research is sponsored by The National Science Foundation.
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