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Ozone damage to vegetation

Issue 13-02-2014This report describes a study into ozone damage to vegetation commissioned by MIRA and carried out by VITO in collaboration with CODA-CERVA. The study consists of two parts: a literature study and a study into the feasibility of an ozone flux-based area-wide indicator. The study reveals that the building blocks needed to develop a Flemish ozone dose model are available. These building blocks are the DOɜSE model (used to calculate deposition and exchange of ozone), the Chimere model (for air quality calculations) and the RIO model (interpolation model used to calculate area-wide ozone concentrations). The input parameters for these models are available or can be derived.

Critical review of the current indicator

MIRA annually reports on the evolution of the AOT40ppb-veg indicator. This indicator provides insight into the ozone excess affecting plants during the growing season. Exposure to ozone can in fact cause damage to plants, such as leaf discolouration, defoliation, stunted growth and even death. The European Air Quality Directive (2008/50/EG) sets objectives for the protection of vegetation, based on the AOT40ppb-veg. However, this indicator is restricted in that it only takes account of the ozone concentration in the atmosphere, not the ozone flux. In fact, only a portion of the atmospheric ozone is absorbed by the plant through the pores (stomata) on the leaves. The ozone flux is defined as the amount of ozone which is deposited per unit area and time in the plant, and depends on plant species and (environmental) conditions. Accumulation of the ozone flux in time yields the ozone dose.

The figure below illustrates the substantial added value of calculations of ozone damage to vegetation based on ozone fluxes as compared to calculations based on ozone concentrations in the air (AOT40ppb). The figure on the left shows the regions in Europe where the highest AOT40ppb values were calculated in 1999 (blue coloured), and the figure on the right the regions with the highest ozone fluxes (blue coloured). It is clear that the regions with the highest risk of ozone damage calculated on the basis of ozone fluxes are no longer situated in Southern Europe, but are shifting to Central and Northwestern Europe (and therefore also Flanders).

Map AOT40 and ozone flux 2014

Source: http://www.emep.int.

From ozone concentration in the air to effect on plants

Plants absorb ozone through the pores (stomata) in the leaves. The CO2 and H2O exchange also occurs through these stomata. Plants can adapt the transpiration and photosynthesis processes to the ambient conditions by adjusting the degree of opening of these stomata. Ozone uptake is therefore dependent on atmospheric ozone concentration, ambient factors (such as temperature, atmospheric and soil moisture, light intensity, etc.) and specific plant characteristics (number of stomata, stage of development, etc.) (see figure).

Figure ozone uptake 2014

In the past 10 years, international research has been conducted to model the ozone uptake by plants and derive reliable dose-response relationships, to arrive at more realistic estimates of ozone damage. This research is summarised in this report. The research resulted in a new indicator for estimating the risk of crop yield reduction or decrease in biomass of agricultural crops, forest trees and grassland: the Phytotoxic Ozone Dose, PODY. This PODY value represents the accumulated stomatal ozone flux per leaf area during daylight hours and above a specified flux threshold value. The PODY value can be compared to plant-specific limit values to check whether the limit value for a specific harmful effect is or is not exceeded. Based on experiments, reliable dose-response relationships for crop yield reduction could be calculated for such crops as potatoes, wheat and tomatoes.

Feasibility of a new ozone flux-based indicator for Flanders

This study also examined the feasibility of developing a POD model for Flanders. For this, the version of the DO3SE model (Deposition of Ozone and Stomatal Exchange) as implemented in the European air quality model of EMEP (European Modelling and Evaluation Programme) was used as basis. The most obvious method of constructing a model for Flanders would be to link DO3SE to a regional air quality model, as these models already contain a large portion of the data input required for DO3SE.

The study shows that all input parameters needed for DO3SE are either available for Flanders or can be calculated in a parameterised manner. The study further shows that with the Chimere and RIO models and with the version of DO3SE that is implemented in the EMEP model, all major building blocks are in place for developing a Flemish POD model. The Chimere model is an air quality model that is operational at IRCEL and is used both for status descriptions and forecasts. RIO is an intelligent spatial interpolation technique that calculates area-wide ozone concentrations on the basis of ozone concentration measurements in the Belgian telemetry monitoring networks.

Read the English summary of the Dutch report ‘Ozone damage to vegetation: Literature study and study into the feasibility of an ozone flux-based indicator and into the implications for the area-wide calculation via an air quality model’

Study commissioned by MIRA, the Environment Reporting Unit
Research report MIRA/2013/12 - VITO/2013/RMA/R/322

researchers: Felix Deutsch, VITO en Karine Vandermeiren, CODA-CERVA

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