Research Program Area: Ecosystem & Multimedia Effects
The primary objective of this work was to design and build a system which would provide definite, measurable levels of relative humidity in air over test plants so that the effects of this environmental condition could be measured on the response of plants to air pollutants, primarily ambient ozone.
A humidity generation system was designed, constructed, and tested for use in increasing relative humidity within open-top chambers under field conditions. The system consisted of a gas fired steam boiler capable of generating 450 kg of steam at 1.055 kg/cm2 (15 PSI) pressure. This steam source was connected to two manifolds which supplied humidified air to two chambers. A modulating valve, controlled by a humidity sensor, metered steam to the chambers. Two ambient humidity chambers were used f or comparison. One, each of the humidified chambers received carbon filtered air, the other ambient air. The ambient humidity chambers also received filtered or ambient air.
Performance tests of the humidity generation system showed that with air temperatures of 300C and ambient relative humidity of 10% the air humidity could be increased by 60%. At higher ambient humidities a greater maximum chamber humidity was achieved. Injection of steam into the ambient air chamber reduced ozone levels by about one fourth.
Alfalfa plants were exposed to 0.20 ppm ozone for seven hrs/day for two days to determine "acute" effects of ozone with different humidities. Neither growth nor number of nodes were affected by the short exposure but visible injury occurred on more than 50% of the leaves which had the high humidity and leaf drop was increased very significantly.
"Chronic" exposures of three cultivars of alfalfa to ambient ozone at ambient humidity or increased humidity occurred for three successive periods with harvests on April 25, May 16 and June 13, 1986. At the first harvest, visible injury occurred on 40% of the leaves exposed to ambient ozone with increased humidity. No statistically significant effects on plant growth occurred because ambient ozone was low during this period. Humidifying the air in general increased plant growth. The second harvest showed similar results. Stomatal conductance was measured and showed that humidification increases conductance, thus allowing greater gas uptake and consequently greater injury to the plant by pollutants. The third harvest showed that the dry weight of alfalfa was reduced significantly by ambient ozone with high humidity. Defoliation due to ozone was increased significantly with added humidity.
This work showed that the basic engineering and design of the humidification system was sound, reliable, and workable. The facility will serve to obtain reliable information concerning the effects of relative humidity on crop losses from air pollutants.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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