Humidity as a Modifier of Vegetation Responses to Ozone: Design and Testing of a Humidification System for Open-Top Field Chambers

Abstract

A system was designed, constructed, and tested for increasing the relative humidity in open-top field chambers used for air pollution studies. The system consisted of a propane-fired steam boiler capable of generating 454 kg hr⁻¹ of dry steam at 0.1055 g cm⁻² (15 PSI), ducting to supply humidified air to open-top chambers, and humidistats to control the humidity level In each chamber. Performance tests showed that under warm, dry conditions (air temperatures of 30°C and ambient relative humidity of 10 percent) the humidity level could be increased by 43 percent in each of two chambers or 25 percent in each of six chambers. At higher ambient humidities or cooler ambient temperatures, greater increases in relative humidity could be achieved. Temperature rise in the humidified chambers was 2.0-4.5°C vs. nonhumidified chambers on warm days due to the injected steam and solar heating of the galvanized steel ducting. Insulating the ducting reduced this difference to 1.6- 2.3°C. Bean plants (Phaseolus vulgaris) were grown in twelve chambers for four months in a 2 × 2 factorial design to detect interactions between ambient air pollutants, principally ozone, and relative humidity on leaf injury (senescence) or yield (total dry weight). The treatments were- charcoal-filtered air and added humidity; charcoal-filtered air and ambient (dry) humidity; nonfiltered air and added humidity; and nonfiltered air and ambient (dry) humidity. There was a significant humidity x ozone interaction on leaf injury and vegetative growth with ambient ozone, causing more necrosis in the humidified than in dry chambers. In contrast, there was no interaction between humidity and ozone on bean production.