187, São Paulo, SP , Brazil.

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1 EFFECTS OF BIOMASS BURNING AEROSOLS AND CLOUDS ON THE CO 2 FLUX IN AMAZONIA Paulo H. F. Oliveira 1, Paulo Artaxo 1, Silvia de Lucca 1, Carlos Pires Jr 1, 2 RESUMO: As alterações no balanço radiativo terrestre causado pelas emissões de partículas de queimadas afetam os fluxos de CO 2 (net ecosystem exchange-nee) entre a atmosfera e a biosfera sobre uma vasta área da Amazônia. A associação do NEE com o aumento dos aerossóis na atmosfera foi investigado como parte do Large Scale Biosphere Atmosphere Experiment in Amazonia (LBA). A profundidade óptica dos aerossóis (AOT) foram medidas com os fotômetros solares CIMEL operados pela AERONET, e as medidas de NEE foram obtidas a partir da técnica de eddy-correlation. Os locais estudados foram: Reserva Biológica Jarú (RBJ), em Rondônia; e Floresta Nacional do Tapajós (FLONA-Tapajós), no Pará. O valor do AOT de maior efeito sobre as medidas de NEE na FLONA-Tapajós foi de.83, causando um aumento de 8.4% nas medidas de NEE. Na RBJ, o NEE aumentou em 16.4% para um valor de AOT de O valor de AOT que maximiza as medidas de NEE foi de 1.7, na FLONA-Tapajós, e de 1.64 para o RBJ. O aumento do NEE é atribuído a um melhor aproveitamento da fração difusa da radiação solar pelo dossel da floresta. Outras variáveis como temperatura e umidade relativa podem também influenciar o NEE nas florestas tropicais. ABSTRACT: The changes in the radiation budget by biomass burning aerosol particles emissions affects the CO 2 flux (net ecosystem exchange - NEE) between biosphere and atmosphere over large areas in Amazonia during the dry season. For two sites in Amazonia, the association of NEE with atmospheric aerosol loading was investigated as part of the Large Scale Biosphere Atmosphere Experiment in Amazonia (LBA). Aerosol optical thickness (AOT) measurements were made with CIMEL sun photometers operated by AERONET, and NEE measurements were determined by eddy-correlation techniques. The sites investigated were: Reserva Biológica Jarú (RBJ), in Rondônia; and Floresta Nacional do Tapajós (FLONA-Tapajós), in Pará. The value of AOT with the strongest effect on NEE in the FLONA-Tapajós site was.83, with an average enhancement of NEE of 8.4%. In the RBJ site, the strongest effect was for AOT of 1.26, with an average enhancement of 16.4%. The biomass burning AOT that maximize the NEE value is of 1.7 for FLONA-Tapajós site, and of 1.64 for RBJ site. The enhancement in NEE is attributed to an advantage of diffuse radiation by forest canopy. Other environmental factor such as temperature and relative humidity also influences the NEE in tropical forests. 1 LFA Laboratório de Física Atmosférica, Instituto de Física, Universidade de São Paulo, Rua do Matão, Travessa R 187, São Paulo, SP 558-9, Brazil. Artaxo@if.usp.br 2 DSA - Divisão de Satélites e Sistemas Ambientais, Instituto Nacional de Pesquisas Espaciais

2 WORDS KEYS: radiative budget, CO 2 flux, biomass burning INTRODUCTION During biomass burning events in Amazonia, a great amount of CO 2 and aerosol particles is emitted to the atmosphere. The interaction of the downward solar radiation with these particles and with the clouds in the region, affect directly the atmospheric radiative budget, reducing the direct fraction of downward solar radiation and increasing its diffuse fraction, especially in the wave lengths of the photosyntethically active radiation (PAR) (Schafer et al., 22). The effects of this strong radiation deficit on the CO 2 net flux (net ecosystem exchange-nee), and on the environmental variables that influence these CO 2 flux in the Amazonia, is far from being fully understood. Field measurements have showed that high CO 2 exchange occurs more frequently on cloudy days instead of clear-sky days (Niyogi et al., 24; Gu et al., 22). Using long-term halfhourly eddy covariance flux, meteorological and aerosol optical thickness measurements, this work present additional evidence that the presence of aerosol particles cause enhance on CO 2 uptake in two Amazonian forests sites. The three objectives in this study are as follows: (1) to quantify the influences of clouds and aerosols on the NEE measurements. (2) To establish for which fraction of solar irradiance the NEE measurements is maximized. (3) And finally, to determine for what value of aerosol optical thickness (AOT) the NEE measurements is maximized in the two Amazonian forests. SITES ANALYZED The measurements were made at the Reserva Biológica Jarú (RBJ) Tower Site (1 5' S and 61 55' W), located to approximately 1 km to the north of the urban area of Ji-Paraná, Rondônia, Brazil (figure 1), and at the Floresta Nacional do Tapajós (FLONA-Tapajós) km83 Tower Site (3 1' S and 54 58' W), approximately 7 km south of Santarém, Pará, Brazil. METHODS In the RBJ site, the downward solar radiation was measured with pyranometer CM21 from Kipp & Zonen and downward PAR radiation were measured with quantum sensor LI-19 from LI-COR. In the FLONA-Tapajós site the downward solar radiation was measured with thermopile pyranometer CM6 from Kipp & Zonen, and downward PAR radiation were measured with LI-19

3 from LI-COR. Both sites had been equipped with eddy correlation systems, similar to described by Moncrieff et al. (1997). The cloud free aerosol optical thickness (AOT) measurements were obtained from the NASA AERONET (Aerosol Robotic Network) network (Holben et al., 1998). The two AERONET CIMEL sun-photometer utilized has been operated at the Abracos Hill site, in the Fazenda Nossa Senhora Aparecida (1 45 S and W), a site about 1 Km from the RBJ CO 2 flux tower, and at the Belterra (2 39 S and W) site, close to FLONA-Tapajós CO 2 flux tower (3 1' S and 54 58' W). In both sites it was possible to relate its AOT measurements with the carbon flux data. The data base period utilized in this work in the RBJ site was of January of 2 to December of 21. In the FLONA-Tapajós the data base utilized was of June of 2 to December of 23. Quantification of the aerosol loading and cloud cover In order for quantify the aerosol and clouds effects on NEE, it was calculated the so-called relative irradiance (f) (equation 1): f = S MEAS [AOT, clouds and aerosols] S [AOT. 5, cloudless], (1) = In the equation 1 S MEAS [AOT, clouds and aerosols] is the total downward solar irradiance measured by the pyranometer in actual sky conditions, and S [AOT=.5, cloudless] is the total downward solar irradiance that the same pyranometer would be measuring for cloudless and background aerosol (AOT=.5) condition, calculated from SBDART (Ricchiazzi et al., 1998). Quantifying the aerosol and clouds influence on the NEE To quantify the magnitude of the influence of aerosols and clouds on the NEE measurements in relation to clear-sky NEE measurements (background aerosols with AOT =.5 and cloudless), the following expression was used: % NEE 1 ( NEE( α ) NEECLEAR SKY ( α )) = (3) NEE ( α ) CLEAR SKY NEE (α) is the measurement of NEE made at actual sky conditions for a solar zenithal angle α, and the NEE CLEAR-SKY (α) is the measurement of the NEE in clear-sky days (f equal to 1.). NEE CLEAR-SKY (α) is calculated from the relationship between clear-sky NEE and the solar zenithal angle (α) (it not showed here). To minimize the influence of solar elevation angles on the analyses

4 of NEE or %NEE with relative irradiance (f), the calculations it was grouped into 5 degree intervals of solar zenithal angle. Method for to isolate the aerosol effect on NEE measurements For to observe only radiative aerosol effect on solar irradiance flux (computed from f ), and consequently on NEE measurements, the aerosol effect has to be isolated from that caused by clouds. It for to identify clouds influence on solar irradiance and to stay only the aerosol effect on relative irradiance (f), the aerosol optical model developed by Procópio et al (23) was used. This approach produces the modeled f (named f S ) as a function of the AOT in cloudless conditions in the Amazonia (equation 2): f S = S SIM [AOT, cloudless] S [AOT =. 5, cloudless] (2) O S SIM [AOT, cloudless] is the value of the total downward solar irradiance modeled that would be measured by the pyranometer in cloudless condition for any AOT value, and S [AOT=.5, cloudless] is the solar irradiance modeled for a cloudless and background aerosol (AOT=.5) condition. S SIM [AOT, cloudless] and S [AOT=.5, cloudless] were calculated with the radiative transfer code SBDART. The cloud screening algorithms consist in to remove measurements of experimental irradiance relative (f (AOT)) that has it value in great discrepancy with irradiance relative modeled (f S (AOT)). The criteria considered here it was to remove measurements of the f ± σ (σ is the variance of f) with it value out of the interval f S ± σ S (σ S is the variance of f S ). RESULTS Clouds and aerosol effects on the NEE measurements From equation 3, the relationship between the %NEE and the relative irradiance was calculated for the same 1 solar zenithal angle intervals (of 1 to 7 degrees). For to obtain the value of the relative irradiance that shows largest change in %NEE measurements, an average of the %NEE in bins of.1 of the relative irradiance for all points between 1 and 7 degrees it was realized (figure 1-a). It is concluded that in the FLONA-Tapajós site, the average value of the relative irradiance that maximizes the %NEE in 12.2% is of.65. For RBJ site, the %NEE measurements increase in 18 % for an average value of the relative irradiance of.7.

5 It can be observed that the relative increase in NEE (%NEE) by aerosol effect is of 8.4 % for f =.82 or AOT=.83, in the FLONA-Tapajós site, and of 16.4 % for f =.77 or AOT= 1.26 in the RBJ site (figure 1-b). From (graphic 1-a) and f versus AOT relationship (equation 1 and 2), was determined what AOT maximize the %NEE value. This value is of 1.7 for FLONA-Tapajós site, and of 1.64 for RBJ site. Relative Change of NEE (%) 2-2 Clouds and aerosol effect FLONA-Tapajós -4 RBJ Relative Irradiance Relative Change of NEE (%) RBJ FLONA-Tapajós Relative Irradiance Aerosol effect Figure 1 - Relative change of NEE (%NEE) as a function of the relative irradiance. The data are averaged in bins of.1 of the relative irradiance for all solar zenithal angle intervals (of 1 to 7 ) (a). Relative change of NEE (%NEE) increase by biomass aerosol loading, averaged on bins of.1 of the relative irradiance, for solar zenithal angle between 2 and 3 (b). The behavior of the CO 2 flux measurements as a function of the PAR radiation for clear-sky days (f = 1. and AOT ~.5) and for days with the influence of aerosols and clouds (f < 1.), for FLONA-Tapajós (figure 2-a) and RBJ (figure 2-b) sites it is showed. Of this graph it may be conclude that, for the same PAR value, 14 µmol m -2 s -1, for example, the NEE measurements in the FLONA-Tapajós (or RBJ), is larger (more negative) for days loaded by aerosol and clouds (f < 1.) if compared to NEE clear-sky measurements (f = 1.). 6 4 Floresta Nacional do Tapajós f < 1. (aerosol and clouds) f = 1. (clear-sky) 6 4 Reserva Biológica Jarú f < 1. (aerosols and clouds) f = 1. (clear-sky) NEE (µmolm -2 s -1 ) Total downward PAR (µmolm -2 s -1 ) a) NEE (µmolm -2 s -1 ) Total downward PAR(µmolm -2 s -1 ) Figure 9 - NEE as a function of total downward PAR radiation for FLONA-Tapajós (a) and RBJ (b) sites.

6 CONCLUSIONS The effect of reduction of total downward solar radiation by aerosols and clouds on the CO 2 fluxes was in quantified for two Amazonian forests. In the FLONA-Tapajós site, the average value of the %NEE is of 12.2% for 65 % of clear-sky radiance. For RBJ site, the %NEE increase is of 18 % for 7 % of clear-sky radiance. The biomass burning aerosol effect on CO 2 fluxes was too quantified using aerosol spectral optic properties. Was observed that the relative increase in NEE (%NEE) by aerosol effect is of 8.4 % for AOT=.83 in the FLONA-Tapajós site, and of 16.4 % for AOT=1.26 in the RBJ site. Was find that the biomass burning AOT that maximize the %NEE value is of 1.7 for FLONA-Tapajós site, and of 1.64 for RBJ site. These numbers show that for these two forest ecosystems, clear skies do not provide the environmental condition for the carbon uptake, and clouds and aerosols can enhance forest carbon absorption from an advantage of diffuse radiation by forest canopy or influencing environmental variables that have direct or indirect influence on the processes that control the CO 2 exchanges between the forest and the atmosphere. Acknowledgement: We thank the researchers, students and technicians contributing to LBA science, from keeping up the continuous tower operation. We thank to Brent Holben and Joel Schafer for to keep the AERONET sites studied. We thank the CAPES and CNPq for the Millennium Institute financial support. REFERENCES Artaxo, P., E. T. Fernandes, J. V. Martins, M. A. Yamasoe, P. V. Hobbs, W. Maenhaut, K. M. Longo and A. Castanho Large Scale Aerosol Source Apportionment in Amazonia. Journal of Geophysical Research, 13 (D24), Gu, L., Baldocchi, D. D., Verma, S. B., Black, T. A., Vesala, T., Falge, E. M. and Dowty, P. R. 22. Advantage of diffuse radiation for terrestrial ecosystem productivity. Journal of Geophysical Research, 17 (D6), Holben, B.N., T. F. Eck, I. Slutsker, D. Tanré, J. P. Buis, A. Setzer, E. Vermote, J. A. Reagan, Y. J. Kaufman, T. Nakajima, F. Lavenu, I. Jankowiak and A. Sirmonov AERONET: A federated instrument network and data archive for aerosol characterization. Remote Sensing Environment, 66, Moncrieff, J. B., Massheder, J. M., De Bruin, H., Elbers, J., Friborg, T., Heusinkveld, B., Kabat, P., Scott, S., Soegaard, H. and Verhoef, A A system to measure surface fluxes of momentum, sensible heat, water vapor and carbon dioxide. Journal of Hydrology, 188, Niyogi, D., Chang, H., Saxena, V. K., Holt, T., Alapaty, K., Booker, F., Chen, F., Davis, K. J., Holben, B., Matsui, T., Meyers, T., Oechel, W. C., Pielke, R. A., Wells, R., Wilson, K. and Xue, Y. 24. Direct observations of the effects of aerosol loading on net ecosystem CO 2 exchanges over different landscapes. Geophysical Research Letters, 31, Procópio, A. S., L. A. Remer, P. Artaxo, Y. J. Kaufman and B. N. Holben. 23. Modeled spectral optical properties for smoke aerosols in Amazonia. Geophysical Research Letters, 3 (24),

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