04_01_Xin - Enhanced Ecosystem Productivity in Cloudy or...

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    Enhanced Ecosystem Productivity in Cloudy or Aerosol-laden Conditions Xin Xi April 1, 2008
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    Radiation process Players: atmospheric gases; aerosols; clouds Processes: absorption and scattering (both back and forward) in both shortwave and longwave; emission (only longwave) Role of atmospheric aerosols : Direct effect: aerosol scattering and absorption Indirect effect: aerosol particles as CCN; 1st indirect effect (enhanced cloud albedo); 2nd indirect effect (enhanced cloud lifetime and amount); semi- direct effect (reduced cloud fraction due to aerosol absorption) Implications for plant growth: 1. Available PAR (total amount) 2. Direct and diffuse components of PAR (400nm~700nm) 3. Induced environmental change (temperature, wind, humidity, CO2, etc) 4. Precipitation and soil moisture 5. Aerosol deposition on vegetation (e.g., poisoning effect) 6. Other effects e.g., photolysis (O 3 , NO, etc)
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    Previous findings 1. Crop scientists found higher radiation use efficiency (RUE, ratio of accumulated biomass to total intercepted solar radiation) or light use efficiency (LUE, same as RUE but based on PAR) for diffuse radiation than for direct radiation. 2. Flux measurements shows higher RUE in cloudy days than clear days for coniferous and deciduous forests 3. Highest NEE rate occurs in cloudy days though incident radiation is largely reduced. …… Explanations: 1. Clouds can increase the diffuse radiation at the surface if the sky is not too cloudy. This will enhance carbon assimilation, if the photosynthesis gains of increased diffuse radiation exceed the photosynthetic loss of reduced beam direct radiation. 2. The presence of clouds can be also the causes or consequences of changes in many atmospheric factors, like air/soil temperature, moisture, latent heat and rainfall, etc 3. Reduced leaf temperature, reduced vapor pressure deficit, stomatal dynamics
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04_01_Xin - Enhanced Ecosystem Productivity in Cloudy or...

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