The effects of clouds and aerosols on net ecosystem CO<sub>2</sub> exchange over semi-arid Loess Plateau of Northwest China
Author:
Jing X.,Huang J.,Wang G.,Higuchi K.,Bi J.,Sun Y.,Yu H.,Wang T.
Abstract
Abstract. 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°) NEE of CO2 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 and aerosols on the CO2 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 canopy are lower, with a value of about 434.8 W m−2. Thus, under overcast conditions of optically thick clouds, the CO2 uptake increases with increasing clearness index, and a maximum CO2 uptake and light use efficiency of vegetation occur with the clearness index of about 0.37 and lower air temperature. Under other sky conditions the CO2 uptake decreases with the cloudiness but the light use efficiency is enhanced, due to increase in the fraction of diffuse PAR. Additionally, under cloudy conditions, changes in the NEE of CO2 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 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.
Publisher
Copernicus GmbH
Reference43 articles.
1. 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. 2. 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. 3. 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 CO2 flux from open-path gas analyzers, Glob. Change Biol., 14, 1854–1876, https://doi.org/10.1111/j.1365-2486.2008.01606.x, 2008. 4. 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. 5. 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).
|
|