Submitted to: Geophysical Research Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/29/2004
Publication Date: 10/29/2004
Citation: Niyogi, D., Chang, H.I., Sexana, V.K., Holt, T., Alapaty, K., Booker, F.L., Chen, F., Davies, K.J., Holben, B., Matsui, T., Meyers, T., Oechel, W.C., Pielke, R.A., Wells, R., Wilson, K., Yongkang, X. 2004. Direct observations of the effects of aerosol loading on net ecosystem CO2 exchanges over different landscapes. Geophysical Research Letters 31:L20506, doi: 10.1029/2004GL020915. Interpretive Summary: Atmospheric concentrations of aerosols (very small air pollution particles of soot and smoke-stack emissions) continue to increase globally. A critical issue is how these changes will affect agricultural productivity. Atmospheric aerosols can affect plant growth by changing the amount and quality of light that plants receive, the so-called global dimming effect. Increasing levels of aerosols cause sunlight to become more diffuse, and it is thought that this better illuminates plant canopies, leading to higher rates of plant production. Results from this study showed that photosynthesis of forests and croplands increased with increased levels of diffuse radiation, presumably because of better light interception within the plant canopy. Additional decreases in solar radiation from aerosol pollution would be expected to suppress plant production. The study showed that aerosols exert a significant impact on photosynthesis that needs to be considered in estimates of regional terrestrial carbon exchange responses to future climate conditions.
Technical Abstract: We present the first direct, multisite observations in support of the hypothesis that atmospheric aerosols affect the regional terrestrial carbon cycle. The daytime growing season (summer) CO2 flux observations from six sites (forest, grasslands, and croplands) with collocated aerosol and surface radiation measurements were analyzed for: high and low diffuse radiation, effect of cloud cover, and effect of high and low aerosol optical depths (AOD). Results indicate that aerosols exert a significant impact on net CO2 exchange, and their effect may be even more advantageous than that due to clouds. The response appears to be a general feature irrespective of the landscape and photosynthesis pathway. The CO2 sink increased with aerosol loading for forest and crop lands, and decreased for grassland. The analysis of the direct field measurements indicates that aerosol loading plays a significant role in the variability of the regional terrestrial carbon exchange by altering the amount of diffuse solar radiation.