Homogeneous nucleation rate measurements of 1-butanol in helium: A comparative study of a thermal diffusion cloud chamber and a laminar flow diffusion chamber A Laminar Flow Tube Reactor ͑LFTR͒ was used to study the nucleation of water vapor. Computational analysis was carried out to define the operating conditions of the LFTR suitable for water nucleation measurements. An interface between the LFTR and a mass spectrometer was developed to analyze the chemical content of the freshly nucleated water particles. Contaminants were detected in the initial configuration of the LFTR. As a result, improvements were made to the LFTR to achieve ultrahigh purity conditions in the system. The nucleation rate of water vapor as a function of supersaturation was measured over the temperature range 210-250 K. The first measurement of the nucleation rate of water at a temperature of 210 K was obtained. Reasonable agreement with the classical theory predictions is observed for temperatures in the range 230-250 K. However, below 220 K, classical theory begins to overestimate nucleation rates compared to experimental data and the disagreement grows with decreasing temperature. The experimental data obtained provide an excellent benchmark for further nucleation studies of binary, ternary, and more complex systems.

For more than a century it has been known that vegetative growth is affected by the products of fossil fuel combustion. Recently, the acidic deposition phenomenon has gained increasing attention, especially when implicated as a factor responsible for economic loss. Research has been directed toward quantification of effects; however, limitations in the design of many crop studies restricts the usefulness and applicability of the results. Acidic deposition varies in a number of ways that may affect crop yield (i.e., level of acidity, amount and intensity of rain, S and N doses). The responses to these variables may be nonlinear. To allow comparisons of published acid precipitation effects research, calculations for this review were made (based on information in the literature or from personal communication) to describe the experimental conditions, the various dose parameters, and the responses in comparable units for each investigation.The majority of crop species studied in field and controlled environment experiments exhibited no effect on growth nor yield as a result of simulated acidic rain. The growth and yield of some crops, however, was negatively affected by acidic rain; others exhibited a positive response. This analysis of the current literature concludes that the effects of acidic precipitation on crops appear to be minimal and that when responses are observed, they may be positive or negative. More complex experimental designs and analyses may be necessary in order to examine and describe the possible subtle responses of agricultural systems to acidic precipitation.

Sulfur dioxide (SO2)‐fumigated and unfumigated field plots of soybeans (Glycine max cv. Wells) were exposed to acid (pH 3.1) or control (pH ∼ 5.3) precipitation simulants to determine effects on growth and productivity. The precipitation simulants were applied at approximately 5‐day intervals in July and August with a total of 3.4 cm applied in 1977 and 4.5 cm in 1978. Sulfur dioxide fumigations of ∼ 4‐hour durations were performed 24 times in 1977 and 17 times in 1978, resulting in an average fumigation concentration of 0.79 ppm (89.6 ppm · hour dose) SO2 the 1st year and 0.19 ppm (13.$ ppm · hour) the 2nd. The acid precipitation simulant produced no statistically significant effect on seed yield in either year and a 4% increase in seed size in 1978. The simulated acid rain may have contributed to the nutritional requirements of soybeans by providing S and N during the critical pod‐filling stage. Sulfur dioxide exposure decreased seed yields in both 1977 and 1978 by > 35 and 12%, respectively. Aceelerated senescence, as suggested by increased leaf fall, may be responsible for the decreased yield in the SO2‐exposed plants.The SO2 exposure appeared to negate the positive acid rain effect on seed size observed in 1978, when the two treatments were combined. Acid precipitation apparently increased the reduction in seed weight resulting from SO2 exposure in 1977. Although visible injury was induced by acid rain exposure in a chamber study, only a small percentage (< 1%) of tissue was affected and there was no apparent effect on plant growth. The results of these studies suggest that the possibility for harmful effects on soybean yield from acid precipitation of a magnitude used in this study are minimal; however, soybean yields may be decreased by SO2 exposures > 13.5 ppm · hour occurring during the growing season.

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