ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-8205-2010 The effects of clouds and aerosols on net ecosystem CO<sub>2</sub> exchange over semi-arid Loess Plateau of Northwest China Jing X. 1 Huang J. 1 Wang G. 1 Higuchi K. 2 Bi J. 1 Sun Y. 1 Yu H. 1 Wang T. 1 Key Laboratory for Semi-Arid Climate Change of the Ministry of Education, College of Atmospheric Sciences, Lanzhou University, Lanzhou, 730000, China Adaptation and Impacts Division, Environment Canada, Toronto, Canada 02 09 2010 10 17 8205 8218 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/8205/2010/acp-10-8205-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/8205/2010/acp-10-8205-2010.pdf

The impacts of clouds and atmospheric aerosols on the terrestrial carbon cycle at semi-arid Loess Plateau in Northwest China are investigated, by using the observation data obtained at the SACOL (Semi-Arid Climate and Environment Observatory of Lanzhou University) site. Daytime (solar elevation angles of larger than 50°) net ecosystem exchange (NEE) of CO<sub>2</sub> obtained during the midgrowing season (July–August) are analyzed with respect to variations in the diffuse radiation, cloud cover and aerosol optical depth (AOD). Results show a significant impact by clouds on the CO<sub>2</sub> uptake by the grassland (with smaller LAI values) located in a semi-arid region, quite different from areas covered by forests and crops. The light saturation levels in the canopy are low, with a value of about 434.8 W m<sup>−2</sup>. Thus, under overcast conditions of optically thick clouds, the CO<sub>2</sub> uptake increases with increasing clearness index (the ratio of global solar radiation received at the Earth surface to the extraterrestrial irradiance at a plane parallel to the Earth surface), and a maximum CO<sub>2</sub> uptake and light use efficiency of vegetation occur with the clearness index of about 0.37 and lower air temperature. Under other sky conditions, CO<sub>2</sub> uptake decreases with cloudiness but light use efficiency is enhanced, due to increased diffuse fraction of PAR. Additionally, under cloudy conditions, changes in the NEE of CO<sub>2</sub> also result from the interactions of many environmental factors, especially the air temperature. In contrast to its response to changes in solar radiation, the carbon uptake shows a slightly negative response to increased AOD. The reason for the difference in the response of the semi-arid grassland from that of the forest and crop lands may be due to the difference in the canopy's architectural structure.

 References % vor jede Referenz Abakumova, G. M., Feigelson, E. M., Russak, V., and Stadnik, V. V.: Evaluation of long-term changes in radiation, cloudiness, and surface temperature on the territory of the former Soviet Union, J. Climate, 9, 1319–1327, 1996. Baldocchi, D. D., Hicks, B. B., and Meyers, T. P.: Measuring biosphere-atmosphere exchanges of biologically related gases with micrometeorological methods, Ecology, 69, 1331–1340, 1988. Burba, G. G., McDermitt, D. K., Grelle, A., Anderson, D. J., and Xu, L.: Addressing the influence of instrument surface heat exchange on the measurements of CO<sub>2</sub> flux from open-path gas analyzers, Glob. Change Biol., 14, 1854–1876, doi:10.1111/j.1365-2486.2008.01606.x, 2008. Chen, Y. H., Huang, J. P., Chen, C. H., Zhang, Q., Feng, J. D., Jin, H. C., and Wang, T. H.: Temporal and spatial distribution of cloud water resources over Northwestern China, Plateau Meteorology, 24(6), 905–912, 2005. Chinese Soil Taxonomy Cooperative Research Group, Institute of Soil Science, Academic Sinica, Chinese Soil Taxonomy (revised proposal), Chinese Agricultural Sci-Tech Press, 218~pp., 1995 (in Chinese). Collatz, G. J., Ball, J. T., Grivet, C., and Berry, J. A.: Physiological and environmental regulation of stomata conductance, photosynthesis and transpiration: A model that includes a laminar boundary layer, Agr. Forest Meteorol., 54, 107–136, 1991. Fitzjarrald, D. R., Moore, K. E., Sakai, R. K., and Freedman, J. M.: Assessing the impact of cloud cover on carbon uptake in the northern boreal forest, in: Proceedings of the AGU Meeting, Spring 1995, H32E-5, S215, 1995. Freedman, J. M., Fitzjarrald, D. R., Moore, K. E., and Sakai, R. K.: Boundary layer cloud climatology and enhanced forest-atmosphere exchange. 23rd Conference on Agricultural and Forest Meteorology, American Meteorological Society, Alberquerque, NM, 1998. Freedman, J. M., Fitzjarrald, D. R., Moore, K. E., and Sakai, R. K.: Boundary layer clouds and vegetation-atmosphere feedbacks, J. Climate, 14, 180–197, 2001. Gao, Z., Lenschow, D. H., He, Z., and Zhou, M.: Seasonal and diurnal variations in moisture, heat and CO<sub>2</sub> fluxes over a typical steppe prairie in Inner Mongolia, China, Hydrol. Earth Syst. Sci., 13, 987–998, doi:10.5194/hess-13-987-2009, 2009. Goudriaan, J.: Crop Micrometeorology: A Simulation Study. Center for Agricultural Publication and Documentation, Wageningen, 1977. Gu, L., Fuentes, J. D., Shugart, H. H., Staebler, R. M., and Black, T. A.: Responses of net ecosystem exchanges of carbon dioxide to changes in cloudiness: Results from two North American deciduous forests, J. Geophys. Res., 104, 31421–31434, 1999. Gu, L., Baldocchi, D., Verma, S. B., Black, T. A., Timo, V., Falge, E. M., and Dowty, P. D.: Advantages of diffuse radiation for terrestrial ecosystem productivity, J. Geophys. Res., 107(D6), 4050, doi:10.1029/2001JD001242, 2002. Gu, L., Baldocchi, D. D., Wofsy, S. C., Munger, J., Michalsky, J., Urbanski, S. P., and Boden, T. A.: Response of a deciduous forest to the Mount Pinatubo eruption: Enhanced photosynthesis, Science, 299, 2035–2038, 2003. Holben, B. N., Eck, T. F., Slutsker, I., Tanre, D., Buis, J. P., Setzer, A., Vermote, E., Reagan, J. A., Kaufman, Y. J., Nakajima, T., Lavenu, F., Jankowiak, I., and Smirnov, A.: AERONET–-A federated instrument network and data archive for aerosol characterization, Remote Sens. Environ., 66, 1–16, 1998. Holben, B. N., Tanre, D., Smirnov, A., Eck, T. F., Slutsker, I., Abuhassan, N., Newcomb, W. W., Schafer, J. S., Chatenet, B., Lavenu, F., Kaufman, Y. J., Castle, J., Setzer, A., Markham, B., Clark, D., Frouin, R., Halthore, R., Karneli, A., O'Neill, N., Pietras, C. Pinker, R., Voss, K., and Zibordi, G.: An emerging ground-based aerosol climatology: Aerosol optical depth form AERONET, J. Geophys. Res., 106, 12067–12097, 2001. Hollinger, D. Y., Kelliher, F. M., Byers, J. N., Hunt, J. E., McSeveny, T. M., and Weir, P. L.: Carbon dioxide exchange between an undisturbed old-growth temperate forest and the atmosphere, Ecology, 75, 134–150, 1994. Huang, J. P., Zhang, W., Zuo, J., Bi, J., Shi, J., Wang, X., Chang, Z., Huang, Z., Yang, S., Zhang, B., Wang, G., Feng, G., Yuan, J., Zhang, L., Zuo, H., Wang, S., Fu, C., and Chou, J.: An overview of the semi-arid climate and environment research observatory over the Loess Plateau, Adv. Atmos. Sci., 25(6), 906–921, doi:10.1007/s00376-008-0906-7, 2008. Jarvis, P. G. and Leverenz, J. W.: Productivity of temperate, deciduous and evergreen forests, in: Physiological Plant Ecology IV, (Encyclopedia of Plant Physiology, NS, vol 12D), edited by: Lange, O. L., Nobel, P. S, Osmond, C. B., and Ziegler, H., Springer, Berlin Heidelberg New York, 233–280, 1983. Ji, G. L., Ma, X. Y, Zou, J. L., and Lv, L. Z.: Characteristics of the photosynthetically active radiation over Zhangye region, Plateau Meteorology, 12(2), 141–146, 1993 (in Chinese). Krakauer, N. Y. and Randerson, J. T.: Do volcanic eruptions enhance or diminish net primary production? Evidence from tree rings, Global Biogeochem. Cy., 17(4), 1118, doi:10.1029/2003GB002076, 2003. Letts, M. G., Lafleur, P. M., and Roulet, T.: On the relationship between cloudiness and net ecosystem carbon dioxide exchange in a peatland ecosystem, Ecoscience, 12, 53–59, 2005. Liu, H. Z., Tu, G., Dong, W. J., Fu, C. B., and Shi, L. Q.: Seasonal and diurnal variations of the exchange of water vapor and CO<sub>2</sub> between the land surface and atmosphere in the semi-arid area, Chinese, J. Atmos. Sci., 30(1), 108–117, 2006 (in Chinese). Liu, X. D., Bai, H., Ning, H., Guo, J., and Dong, A.: Response of arid meteorological disaster to climatic warming in Gansu Province, J. Glaciol. Geocryol., 28(5), 707–712, 2006 (in Chinese). Long, C. N. and Ackerman, T. P.: Identification of clear skies from broadband pyranometer measurements and calculation of downwelling shortwave cloud effects, J. Geophys. Res., 105(D12), 15609–15626, 2000. Long C. N., Ackerman, T. P., Gaustad, K. L., and Cole, J. N. S.: Estimation of fractional sky cover from broadband shortwave radiometer measurements, J. Geophys. Res., 111, D11204, doi:10.1029/2005JD006475, 2006. Mercado, L., Bellouin, N., Sitch, S., Boucher, O., Huntingford, C., Wild, M., and Cox, P. M.: Impact of changes in diffuse radiation on the global land carbon sink, Nature, 458, 1014–1017, 2009. Michalsky, J.: The Astronomical Almanac's algorithm for approximate solar position (1950–2050), Sol. Energy, 40(3), 227–235, 1988. Min, Q.: Impacts of aerosols and clouds on forest-atmosphere carbon exchange, J. Geophys. Res., 110, D06203, doi:10.1029/2004JD004858, 2005. Niyogi, D., Chang, H., Saxena, V. K., Holt, T., Alapaty, K., Booker, F., Chen, F., Davis, K., Holben, B., Matsui, T., Meyers, T., Oechel, W., Pielke Sr., Wells, R., Wilson, K., and Xue, Y. K.: Direct observations of the effects of aerosol loading on net ecosystem CO<sub>2</sub> exchanges over different landscapes, Geophys. Res. Lett., 31, L20506, doi:10.1029/2004GL020915, 2004. Norman, J. M. and Arkebauer, T. J.: Predicting canopy light-use efficiency from leaf characteristics, in: Modeling Plant and Soil Systems, edited by: Hanks, J. and Ritchie, J., Agronomy Monograph, ASA-CSSA-SSSA, 125–143, 1991. Oliphant, A. J., Schmid, H. P., Grimmond, C. S. B., Su, A.-B., Scott, S., and Vogel, C.: The role of cloud cover in net ecosystem exchange of CO$_2 $ over two mid-western mixed hardwood forests, Abstracts of 25th Conference on Agricultural and Forest Meteorology, American Meteorological Society, Norfolk, Virginia, 2002. Oliveira, P. H. F., Artaxo, P., Pires, C.,~Lucca, S. D., Procopio, A., Holben, B., Schafer, J., Cardoso,~L. F., Wofsy, S. C., and Rocha, H. R.: The effects of biomass burning aerosols and clouds on CO<sub>2</sub> flux in Amazonia, Tellus B, 59, 338–349, 2007. Price, D. T. and Black, T. A.: Effects of short-term variation in weather on diurnal canopy CO$_2 $ flux and evapotranspiration of a juvenile Douglas-Fir stand, Agr. Forest Meteorol., 50, 139–158, 1990. Roderick, M. L., Farquhar, G. D., Berry, S. L., and Noble, I. R.: On the direct effect of clouds and atmospheric particles on the productivity and structure of vegetation, Oecologia, 129, 21–30, 2001. Schwartz, S. E.: The Whitehouse Effect – Shortwave radiative forcing of climate by anthropogenic aerosols: an overview, J. Atmos. Sci., 27, 359–382, 1996. Song, L. C. and Zhang, C. J: Changing features of precipitation over Northwest China during the 20th century, J. Glaciol. Geocryol., 25(2), 143–148, 2003 (in Chinese). Spitters, C. J. T., Tussaint, H. A. J. M., and Goudriaan, J.: Separating the diffuse and direct component of global radiation and its implications for modeling canopy photosynthesis, Part I, Components of incoming radiation, Agr. Forest Meteorol., 38, 217–229, 1986. Suraqui, S., Tabor, H., Klein, W. H., and Goldberg, B.: Solar radiation changes at Mt. St. Katherine after forty years, Sol. Energy, 16, 155–158, 1974. Wang, T. H.: Retrieving optical and microphysical properties of mixed-phase and dusty cloud over Northwestern China from MFRSR, Ph. D. dissertation, Lanzhou Univ., Lanzhou,~China, 90–92, 2009 (in Chinese). Yao, Y. B., Li, Y. B., Zhang, M. C., Wang, W. T., and Zhang, X. Y.: Response on climate in Longdong loess plateau to globe warming and fruit development impacted, Journal of Nanjing Forestry University (Natural Sciences Edition), 29(4), 73–77, 2005a (in Chinese). Yao, Y. B., Wang, Y. R., Li, Y. H., and Zhang, X. Y.: Climate warming and drying and its environmental effects in the Loess Plateau [J], Resour. Sci., 27(5), 146–152, 2005b (in Chinese). Zhou, Y. H., Xiang, Y. Q., and Luan, L. K.: Climatological estimation of quantum flux densities. Acta Meteorol. Sin., 54(4), 447–455, 1996 (in Chinese). Zuo, J., Huang, J., Wang, J. M., et al: Surface turbulent flux measurements over the Loess Plateau for a semi-arid climate change study, Adv. Atmos. Sci., 26(4), 679–691, doi:10.1007/s00376-009-8188-2, 2009.