L.W.A. van Hove

A study of the adsorption of NH3 and SO2 on leaf surfaces.

The adsorption of NH3 and SO2 on the external leaf surface of bean (Phaseolus vulgaris L.) and poplar (Populus euramericana L.) was studied. The adsorbed quantities increased strongly with increasing air humidity, indicating that water on the leaf surface plays a major role in the interaction of these gases with the leaf surface. On the other hand temperature in the range between 15 and 26°C had no significant influence. The adsorbed quantities of NH3 at a specific air humidity appeared to be proportional to NH3 concentration. This proportionality was less clear for SO2. The affinity of SO2 for the leaf surface was found to be approximately twice that of NH3. A mixture of these gases in the air mutually stimulated their adsorption on the leaf. No significant desorption or uptake of these gases through the cuticle could be detected, indicating that the bulk of the adsorbed gases remains associated with the cuticle.

Analysis of the uptake of atmospheric ammonia by leaves of Phaseolus vulgaris L.

Abstract Individual leaves of Phaseolus vulgaris L. were exposed for 9 h in a leaf chamber to different NH 3 concentrations at different light intensities. The rates of NH 3 -uptake, transpiration and photosynthesis were measured simultaneously. The flux density of NH 3 increased linearly with concentration in the range of 4–400μg m −3 . Flux densities also increased with light intensity. Resistance analysis indicated that NH 3 transport into the leaf is via the stomata: transport via the cuticle is negligible under the experimental conditions. There is no internal resistance against NH 3 transport. The NH 3 flux was found not to influence the photosynthesis.

Spatial variability of the Rotterdam urban heat island as influenced by urban land use

Novel bicycle traverse meteorological measurements were made in Rotterdam to assess the spatial variation of temperature during a tropical day. Nocturnal spatial urban temperature differences of 7?K were found to be related to city morphology. The coolest residential areas were green low-density urban areas. During midday measurements the downtown was up to 1.2?K warmer than the surrounding rural area while a city park was 4.0?K cooler than downtown. A regression analysis showed that the nocturnal measured urban heat island (UHI) can be linked to land use, namely plan area fraction of vegetation, built up area water and is most significant for vegetation. The vegetated area was derived from visible and near infrared aerial images. Neighbourhoods with vegetation (within an upwind radius of 700?m) had a significantly reduced UHI during the night. From the traverse observation data a multiple linear regression model was constructed and independently validated with 3-year summertime UHI statistics derived from 4 urban fixed meteorological stations. In addition, two fixed rural stations were used; a WMO station at Rotterdam airport and a rural station further away from the city. Wind rose analysis shows that UHI is strongest from easterly directions and that the temperature signal of the WMO station is influenced by an UHI signal from both the airport runways and urban directions. A regression model reproduced the nighttime spatial variability of the UHI within a fractional bias of 4.3% and was used to derive an UHI map of Rotterdam and surroundings. This map shows that high density urban configurations lacking greenery or close to large water bodies are vulnerable to high nocturnal temperatures during heat waves. This warming effect of water bodies is also evident for an urban weather station located in the harbor area, which had a similar nocturnal UHI frequency distribution as the downtown urban weather station. The UHI map can be used as a valuable planning tool for mitigating nocturnal urban heat stress or identifying neighborhoods at risk during heat waves.

The effective thickness of water films on leaves

Abstract There are indications that the adsorption of water-soluble gases like NH3 and SO2 on a leaf surface may be compared with the dry deposition on a freely accessible water layer. The aim of our study was to quantify the thickness of the apparent water layer on the leaf surface. The thickness was calculated from NH3 adsorption data on leaves. Also we performed weighing experiments with dried leaves of different plant species at relative air humidities of 20 and 95% (at 20°C), respectively. From the increase in weight the water film thickness was calculated. The thickness calculated from NH3 adsorption data strongly depends on air humidity and varied from 10 μm at low relative humidities to 100 μm at high relative humidities. However, the maximum water layer thicknesses obtained for the dried leaves were much lower (8.8–17.9μm). We postulate that the cuticular membrane may behave as a “valve” between the inner and outer region of the leaf and that its permeability is controlled by the relative humidity of the air. This view may be a fundamental contribution to the representation of the mechanism of the dry deposition of gaseous compounds to the vegetation.

A leaf chamber for measuring the uptake of pollutant gases at low concentrations by leaves, transpiration and carbon dioxide assimilation

Abstract A leaf chamber has been developed for analyzing the uptake of pollutant gases (NO, NO 2 , O 3 , SO 2 , NH 3 ), in ambient concentrations, into leaves and the effects thereof on stomatal behaviour and photosynthesis. Performance studies showed a negligible permanent reaction of these pollutant gases with the internal surfaces. With SO 2 and NH 3 , however, a considerable time was necessary until the desired concentration within the leaf chamber was reached. This adsorption of NH 3 and SO 2 might be related to the presence of a thin water film on the internal surfaces of the leaf chamber. A wide range of air temperatures and humidities can be applied in the leaf chamber. The wind velocity across both leaf surfaces is homogeneous and can be varied, up to a maximum of about 3ms −1 . Consequently, the relation between the boundary layer resistance and uptake of a pollutant gas can be studied properly. Measurements with a thermovision camera have shown that the control of leaf temperature distribution at the leaf surface is improved. This enables a higher accuracy in the determination of the stomatal resistance.