Thunderstorms: an important mechanism in the transport of air pollutants

Acid deposition and photochemical smog are urban air pollution problems, and they remain localized as long as the sulfur, nitrogen, and hydrocarbon pollutants are confined to the lower troposphere (below about 1-kilometer altitude) where they are short-lived. If, however, the contaminants are rapidly transported to the upper troposphere, then their atmospheric residence times grow and their range of influence expands dramatically. Although this vertical transport ameliorates some of the effects of acid rain by diluting atmospheric acids, it exacerbates global tropospheric ozone production by redistributing the necessary nitrogen catalysts. Results of recent computer simulations suggest that thunderstorms are one means of rapid vertical transport. To test this hypothesis, several research aircraft near a midwestern thunderstrom measured carbon monoxide, hydrocarbons, ozone, and reactive nitrogen compounds. Their concentrations were much greater in the outflow region of the storm, up to 11 kilometers in altitude, than in surrounding air. Trace gas measurements can thus be used to track the motion of air in and around a cloud. Thunderstorms may transform local air pollution problems into regional or global atmospheric chemistry problems.     

Evaluation of the acetylene inhibition method for measuring denitrification in soilless plant culture systems

Denitrification measurements by means of the acetylene inhibition method require a continuous presence of acetylene to block the microbial reduction of N₂0 to N₂. To examine the effect of such steady exposures on the growth of plants, roots of cucumber and tomato seedlings were treated with different acetylene concentrations. Acetylene concentrations of ≥1 vol% in the gas phase, which were necessary for complete inhibition of N₂ formation, led to a significant retardation of root growth. This was partly due to trace amounts of ethylene contained in the acetylene gas which could not be removed with the usual prescrubbing through a sulfuric acid train. As a result of the growth impairment, oxygen consumption in the root zone decreased after 4 days of exposure. In order to avoid these side effects, the denitrification measurements in soilless cultures were performed on individual plants over a limited period of 2–3 days. The flow-through chamber method proved to be suitable for determining the gaseous N losses in a closed-loop system. It avoided greater air variations from the environmental conditions (substrate temperature, airflow and plant composition) and excluded errors in measurement caused by injury to roots and spatial variability of denitrification activity in the root medium. For exact estimation of the gaseous N losses, preceding 24-h acetylene fumigation was necessary. Subsequently at least three gas samples had to be taken throughout the day, because the N₂O+N₂ emissions were subject to a pronounced diurnal variability.     

The snowfall chemistry collector intercomparison test (SCCIT)

The Snowfall Chemistry Collector Intercomparison Test (SCCIT) took place as part of a field experiment of the PRocessing of Emissions by Clouds and Precipitation (PRECP) program during January and February 1986 in western New York. SCCIT compared the chemical composition and equivalent water depth of snow collected with a large, bag-lined can (used by the PRECP community for a concurrent surface snowfall network); the most widely used automatic wet/dry collector; an aerodynamically designed, heated, funnel and bottle system; and a large plastic sheet. The aerodynamically designed system had significantly higher concentrations of analytes than the other systems, particularly H⁺ and SO₄ ^2?, apparently because of evaporative losses. Samples from the bag-lined can were more acidic than those from the automatic wet/dry collector. The aerodynamically designed system and the bag-lined can indicated greater snow amounts than the other methods.     

Iodine uptake and translocation in apple trees grown under protected cultivation

Background and Aims : Agronomic biofortification of food crops with iodine may improve the dietary intake of this trace element, which is essential for human development and health. So far, little is known about the suitability of this technique in pome fruits. The objectives of this study were (1) to investigate uptake and translocation of exogenously applied iodine in apple trees, (2) to identify possible strategies of iodine biofortification for this type of fruit, and (3) to evaluate interactions between foliar applied iodine and selenium. Methods : Apple trees were cultivated in a plastic tunnel for two growing seasons. Iodine was applied via leaves or substrate. During the 2^nd year, simultaneous foliar application of iodine and selenium were tested as well. At harvest time, iodine and selenium content in leaves and fruits were determined. The phytoavailable iodine concentration in the growing medium was analyzed following an extraction with calcium chloride. In addition, the dynamics of iodine applied as potassium iodide and iodate in a peat‐based substrate was investigated in an incubation experiment without plants. Results : The iodine concentration in washed apples increased more than 100‐fold, valuing around 50 µg (100 g FM)^?1 by foliar application of iodine as compared to the control treatment. However, this level was only achieved in fruits which were directly wetted by the spray solution. The translocation of leaf‐absorbed iodine to fruits was negligible. Following a substrate fertilization, the fruit iodine content remained rather low due to a strong retention of iodine in the growing medium. When using foliar sprays, the addition of selenium did not affect the iodine enrichment of the apple fruits. Conclusions : Foliar fertilization of iodine seems to be a promising method to biofortify apples with iodine. The level of I achieved in apple fruits by means of foliar fertilization can significantly contribute to the daily I intake requirement of humans.     

Influence of nitrogen concentration and form in the nutrient solution N20 and N2 emissions from a soilless culture system

The influence of nitrogen concentration and form in the nutrient solution on the N₂O and N₂ emissions from a closed rockwool system with cucumber crops was investigated. At optimum, nitrate-accented nitrogen supply of plants (120 mg N l^?1) on average 0.59 kg N per hectare greenhouse area and day was released. Comparable emission rates occurred with moderate sub- and supraoptimum nitrogen fertilization. Thus, in this range the N supply was non-limiting for the gaseous nitrogen losses. Only in the case of severe suboptimum N supply (40 mg N l^?1) the N₂O N₂ emissions were clearly lower, probably as a result of diminished plant growth and therefore reduced root respiration and exudation of organic carbon sources for microorganisms. The proportion of nitrous oxide in the gaseous nitrogen losses increased on average from 5 to 13% with increasing N supply, possibly because of an impaired N₂O reduction in denitrification due to high nitrate concentrations. Short-term shifting with optimum nitrogen supply from nitrate- accented composition to pure nitrate did not affect the gaseous nitrogen emissions, whereas an exclusive ammonium supply resulted in a considerable decrease. The results indicated that the N₂O emission and the total nitrogen losses from the soilless culture system were predominantly caused by denitrification.     

Evaluation of the acetylene inhibition method for measuring denitrification in soilless plant culture systems

Denitrification measurements by means of the acetylene inhibition method require a continuous presence of acetylene to block the microbial reduction of N₂0 to N₂. To examine the effect of such steady exposures on the growth of plants, roots of cucumber and tomato seedlings were treated with different acetylene concentrations. Acetylene concentrations of 1 vol% in the gas phase, which were necessary for complete inhibition of N₂ formation, led to a significant retardation of root growth. This was partly due to trace amounts of ethylene contained in the acetylene gas which could not be removed with the usual prescrubbing through a sulfuric acid train. As a result of the growth impairment, oxygen consumption in the root zone decreased after 4 days of exposure. In order to avoid these side effects, the denitrification measurements in soilless cultures were performed on individual plants over a limited period of 2–3 days. The flow-through chamber method proved to be suitable for determining the gaseous N losses in a closed-loop system. It avoided greater air variations from the environmental conditions (substrate temperature, airflow and plant composition) and excluded errors in measurement caused by injury to roots and spatial variability of denitrification activity in the root medium. For exact estimation of the gaseous N losses, preceding 24-h acetylene fumigation was necessary. Subsequently at least three gas samples had to be taken throughout the day, because the N₂O+N₂ emissions were subject to a pronounced diurnal variability.     

Factors influencing the efficacy of iodine foliar sprays used for biofortifying butterhead lettuce (Lactuca sativa)

Iodine biofortification of butterhead lettuce (Lactuca sativa) via foliar sprays was investigated in field trials, focusing on assessing the influence of the time and application method. The iodine (I) concentrations in the edible plant parts increased when potassium iodide (KI) and potassium iodate (KIO₃) solutions were sprayed at doses up to 0.25 kg I ha^?1 on different dates close to harvest. Crop yield and marketable quality were not significantly affected by I treatments. A greater efficacy of KI was frequently observed and probably related to its lower point of deliquescence and smaller anion size in comparison with KIO₃. KI sprays on butterhead lettuce at different times of the day resulted in a higher I enrichment when applied at 11:00 and 15:00 h. The diurnal variation in I uptake may reflect the impact of fluctuating climatic conditions at the time of application. Iodine treatments at different application dates near harvest led to an increasing I concentration in the vegetable produce that could be related to the rising shoot fresh mass and leaf area. When KI and KIO₃ were sprayed simultaneously with commercial calcium fertilizers, fungicides or insecticides, I accumulation in butterhead lettuce was not negatively affected or in some cases even significantly enhanced. The results show that foliar sprays of KI and KIO₃ are an effective method to biofortify butterhead lettuce with I and this approach may easily be implemented as a routine method in commercial cultivation.