Air Pollution as a Climate Forcing: A Workshop

Air Pollution as a Climate Forcing: A Workshop

The Influence of Aerosols on Plant Growth

Mike Bergin
Georgia Institute of Technology

Abstract

Many of the world's agricultural regions are co-located with highly populated urban areas. As a consequence, crop plants are often exposed to a wide range of pollutants that are potentially harmful including ozone, nitric acid, and aerosol particles. These pollutants potentially impact crop productivity by depositing on plants and damaging plant surfaces as well as by changing the amount of radiation available for plant photosynthesis. Aerosol particles are also believed to modify climate by perturbing the radiation balance of the earth both directly by scattering and absorbing solar radiation as well as indirectly by modifying cloud reflectivity and lifetime. It is also suspected that aerosols influence precipitation. For these reasons, aerosols may not only be influencing crop productivity but also carbon sequestration by other terrestrial plants. Therefore, aerosols may be effecting atmospheric carbon dioxide concentrations. As the population of the World continues to grow along with the emissions of air pollutants and their precursors it is likely that the influence of air pollutants on plants will be exascerbated, particularly in developing countries that will continue to industrialize.

This presentation focuses on the impact of atmospheric aerosol particles on plant growth. Aerosol particles influence not only the quantity but quality (i.e. fraction of diffuse to total) of photosynthetically active radiation (PAR) reaching the surface. Many models that estimate crop production and net primary production (NPP) assume that plant growth is linearly proportionate to the amount of PAR reaching the plant canopy. Recent field observations suggest that under moderately cloudy and/or hazy conditions, a decrease in the amount of PAR reaching the surface is associated with an increase in the flux of CO₂ to plants (Gu et al., 1999). A viable explanation for this is an increase in the amount of PAR reaching leaves within the plant canopy due to an increase in the relative amount of diffuse PAR from the scattering of light by aerosols. Model results suggest that atmospheric aerosols can have either a positive or negative influence on plant growth through the modification of PAR reaching the surface, with the sign and magnitude depending on several factors including cloudiness and aerosol physical and chemical properties. In addition, water insoluble aerosol particles (such as organic compounds and elemental carbon) can deposit on plants and build up over time. These particles can scatter and absorb radiation resulting in less PAR available for plant photosynthesis. Model results as well as experimental evidence suggest that in the Yangtze delta region of China deposited aerosol particles may be decreasing crop production through the attenuation of PAR by as much 20% over a growing season. Overall, results indicate that carefully designed field experiments that include both atmospheric scientists and plant physiologists are needed to understand the link between aerosols and plant growth.