Environment in Open‐top Field Chambers Utilized for Air Pollution Studies

Cylindrical open‐top chambers were established in an alfalfa field ( Medicago sativa L.) 50 km north of Madison, Wis. Chambers were set up with and without activated charcoal filters. Environmental measurements were made at different times during the summer growing season. The environment of open‐top chambers was found to differ somewhat from open areas. Light intensity was as much as 20% lower, temperature increases as great as 2°C were measured, and evaporative water loss was 10% less in the chambers than open areas. The filtered and nonfiltered chambers had slightly different environments as a result of variations in air flow rates and dust accumulation in the plenums of the two types of chambers. Across the chambers, there was a gradient in light intensity and evaporative water loss, but no significant gradient in temperature or air velocity. There was no evidence of pollution injury to alfalfa in this study. The environmental changes in chambers were shown to cause differences in plant growth which likely caused changes in pollution sensitivity of plants within the chambers. However, it is postulated that this altered plant growth would likely not result in either a consistent increase or decrease in sensitivity at different times during the growing season or in different years. Thus, when open‐top chambers are utilized for the determination of crop yield losses, assessments should be based on studies conducted over several different growing periods.