Angelo Finco

Comparison of seasonal variations of ozone exposure and fluxes in a Mediterranean Holm oak forest between the exceptionally dry 2003 and the following year

Ozone and energy fluxes have been measured using the eddy covariance technique, from June to December 2004 in Castelporziano near Rome (Italy), and compared to similar measurements made in the previous year. The studied ecosystem consisted in a typical Mediterranean Holm oak forest. Stomatal fluxes have been calculated using the resistance analogy and by inverting the Penmann-Monteith equation. Results showed that the average stomatal contribution accounts for 42.6% of the total fluxes. Non-stomatal deposition proved to be enhanced by increasing leaf wetness and air humidity during the autumnal months. From a comparison of the two years, it can be inferred that water supply is the most important limiting factor for ozone uptake and that prolonged droughts alter significantly the stomatal conductance, even 2 months after the soil water content is replenished. Ozone exposure, expressed as AOT40, behaves similarly to the cumulated stomatal flux in dry conditions whereas a different behaviour for the two indices appears in wet autumnal conditions. A difference also occurs between the two years.

Stomatal Conductance Modeling to Estimate the Evapotranspiration of Natural and Agricultural Ecosystems

This chapter presents some of the available modelling techniques to predict stomatal conductance at leaf and canopy level, the key driver of the transpiration component in the evapotranspiration process of vegetated surfaces. The process-based models reported, are able to predict fast variations of stomatal conductance and the related transpiration and evapotranspiration rates, e.g. at hourly scale. This high–time resolution is essential for applications which couple the transpiration process with carbon assimilation or air pollutants uptake by plants.

Errors in ozone risk assessment using standard conditions for converting ozone concentrations obtained by passive samplers in mountain regions

Passive samplers are often employed to measure ozone concentrations in remote areas such as mountain forests. The potential ozone risk for vegetation is then assessed by calculating the AOT40 exposure index (accumulated hourly ozone concentration exceedances above 40 ppb, i.e. AOT40 = Σ([O(3)] - 40)Δt for any hourly ozone concentration [O(3)] > 40 ppb). AOT40 is customary calculated on the basis of ozone concentrations expressed as a volumetric mixing ratio, while lab sheets normally report ozone concentrations from passive samplers in mass units per cubic metre. Concentrations are usually converted from mass units to ppb using a standard conversion factor taking SATP (Standard Ambient Temperature and Pressure) conditions into account. These conditions, however, can vary considerably with elevation. As a consequence, the blanket application of a standard conversion factor may lead to substantial errors in reporting and mapping ozone concentrations and therefore in assessing potential ozone risk in mountain regions. In this paper we carry out a sensitivity analysis of the effects of uncertainties in estimations of air temperature (T) and atmospheric pressure (P) on the concentration conversion factor, and present two examples from two monitoring and mapping exercises carried out in the Italian Alps. We derived P and T at each site from adiabatic lapse rates for temperature and pressure and analysed the magnitude of error in concentration estimations. Results show that the concentration conversion is much more sensitive to uncertainties in P gradient estimation than to air temperature errors. The concentration conversion factor (cf) deviates 5% from the standard transformation at an elevation of 500 m asl. As a consequence, the standard estimated AOT40 at this elevation is about 13% less than the actual value. AOT40 was found to be underestimated by an average between 25% and 34% at typical elevations of mountain forest stands in the Italian Alps when a correct conversion factor for transforming ozone concentrations from μg m(-3) to ppb is not applied.

Evapo)Transpiration Measurements Over Vegetated Surfaces as a Key Tool to Assess the Potential Damages of Air Gaseous Pollutant for Plants

The term evapotranspiration (ET) is used to describe the contemporary evaporation of water from surfaces and transpiration of water trough stomata. Evaporation (E) consists in the change of state of water from liquid to vapour that occurs when water molecules momentarily acquire high speed near the surface of water as the result of collisions with other molecules. This kind of process can be enhanced by environmental factors, such as direct solar radiation and temperature, which provide the required energy. Leaf transpiration can be thought of as a necessary cost associated with the opening of stomata to allow the diffusion of carbon dioxide inside the leaf, for photosynthesis and plant growth. However, these low-resistance apertures also provide a favourable diffusional pathway for atmospheric gas pollutants, such as ozone, that can reach the substomatal cavity and the mesophyll inside the leaf, (causing negative effects at different biological and physiological levels). Stomata opening is regulated by environmental factors such as light, air temperature and humidity, wind speed and water availability in the soil (Jarvis, 1976; Stewart, 1988). The interpretation of the ET and the understanding of the potential influencing factors are important topics for ecophysiology, agriculture and agro-meteorology since the past century (Penman, 1948). ET plays a key role in the water cycle, affecting the water balance from local up to regional scale and causing feedback between vegetation and climate (Wilske et al. 2010). Penman (1948), combining the energy balance with the mass transfer method, derived an equation to calculate the evaporation from an open water surface using meteorological data such as radiation, temperature, humidity and wind speed. Today the Penman-Monteith equation is considered the most reliable method to calculate evapotranspiration.