Ozone and urban forests in Italy

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Abstract

Ozone levels along urban-to-rural gradients in three Italian cities (Milan, Florence, Bari) showed that average AOT40 values at rural and suburban sites were 2.6 times higher than those determined at urban sites. However, O3 also exceeded the European criteria to protect forest health at urban sites, even when the standards for human health protection were met. For protecting street trees in Mediterranean cities, the objectives of measurement at urban sites should extend from the protection of human health to the protection of vegetation as well. A review of forest effects on O3 pollution and of O3 pollution on forest conditions in Italian cities showed that it was not possible to distinguish the effect of O3 in the complex mixture of urban pollutants and stressors. A preliminary list of tree species for urban planning in the Mediterranean area shows the average tree capacity of O3 removal and VOC emission.

Introduction

Ground-level ozone (O3) is the most widespread and harmful pollutant to trees (Paoletti, 2007). Trees in cities are confronted by a variety of adversities, including O3 pollution. Yet it frequently occurs that O3 levels at traffic hot spots are lower compared to urban background sites, where O3 concentrations are in turn lower than at rural locations (EEA, 2007). The main reasons for this are (Paoletti, 2007): (a) O3 is a secondary pollutant, in that it is not directly emitted from sources. It is therefore probable that a considerable fraction is formed at a certain distance from the precursor sources; (b) the increased pollution level in urban areas favours return reactions with nitrogen oxides which lead to O3 depletion; (c) in green areas, biogenic hydrocarbons (BVOCs) may contribute to the formation of O3, since they are more reactive than anthropogenic hydrocarbons.

In spite of lower O3 levels at urban than rural sites, from 1996 onwards 13–60% of the urban population in Europe has been exposed to ambient O3 concentrations exceeding the European target value set for the protection of human health (EEA, 2007). The potential of these exceedances to damage urban forest health is unclear, as the recent Directive 2008/50/EC on ambient air quality and cleaner air for Europe uses different O3 metrics to protect human and vegetation health (Table 1). In addition, the Directive 2008/50/EC states that urban monitoring stations have to be used to evaluate the protection of human health, while suburban and rural stations have to be used to evaluate the protection of vegetation. On this basis, it is hard to know the potential of O3 impact on urban trees.

Although emissions of O3 precursors fell substantially during the period 1990–2004 in Europe resulting in improved air quality over the region (EEA, 2007), ambient O3 concentrations have not shown any improvement since 1997 (EEA, 2007). This might be due to meteorological variability and growing long-distance transport of pollution. As a result, O3 is still the air pollutant of major concern for forests (Paoletti et al., 2007a). Ozone's toxic potential to urban forests should be better evaluated, in particular in the Southern European countries around the Mediterranean, where high rural O3 concentrations are most pronounced (EEA, 2007). Due to its central position in the Mediterranean, Italy may be considered as a hot-spot for O3 and representative of O3 impacts on Mediterranean vegetation (Paoletti, 2006). An analysis of 34 O3-exposure metrics showed that O3 pollution in Italy regularly exceeds the European standards at rural sites (Paoletti et al., 2007b). This paper reports O3 levels along urban-to-rural gradients in three Italian cities as a case study, and reviews the state-of-knowledge of O3 pollution effects on urban forest condition in Italy.

As O3-exposure affects trees, trees affect O3 in the air. Pollution mitigation, in fact, is among the many benefits urban forests can provide (Brack, 2002). Urban trees affect O3 pollution through three major processes: 1. cooling of ambient temperature and hence slowing the smog formation process (Akbari, 2002). Trees reduce air temperature through shading and evapotranspiration (Oke, 1989), thus they: reduce emissions of O3 precursors from air-conditioning and other cooling equipment; decrease temperature-dependent emissions of hydrocarbons from biogenic and anthropogenic sources; change the reaction rates of O3-forming chemical reactions; change the depth of the mixing layer (Nowak et al., 2000). The role of trees to improve urban climatic conditions differs across Europe and is particularly important in southern countries (Tyrväinen et al., 2005). 2. Dry deposition (including stomatal uptake and non-stomatal deposition upon plant surfaces) by which O3 is removed from the air (Akbari, 2002). Once inside the leaf, O3 diffuses into the intercellular spaces and dissolves in water films to form toxic reactive oxygen species (Paoletti, 2007). Deposition of gaseous pollutants is greater in woodlands than in shorter vegetation (Fowler et al., 1989) as, in addition to having greater leaf areas than other types of vegetation, trees create more turbulent mixing of the air passing over land. 3. Emission of biogenic volatile organic compounds (BVOCs), which can contribute to O3 formation (Benjamin and Winer, 1998). The amount of BVOCs in major urban areas is minimal or negligible when compared to anthropogenic sources (Carter, 1994). BVOCs, however, are estimated to be 2–3 times more reactive than a weighted average of hydrocarbons from gasoline combustion (Carter, 1994), thus increasing their relative contribution to O3 formation. BVOCs include the isoprenoids (isoprene and monoterpenes as well as sesquiterpenes and homoterpenes) and minor compounds such as alkanes, alkenes, carbonyls, alcohols, esters, ethers, and acids. Isoprenoids protect plant membranes against oxidative stressors, including O3 (Calfapietra et al., this issue). A taxonomic methodology to classify tree and shrub species on the base of hourly emission rates of isoprene and monoterpene, thus identifying low O3-forming potential species, has been proposed (Benjamin et al., 1996, Benjamin and Winer, 1998). Ozone mitigation by urban trees, however, is usually greater than O3 formation (Nowak et al., 2000).

In the US, urban forests were estimated to remove about 711,000 metric ton ($3.8 billion value) of air pollution per year (Nowak et al., 2006). The amount of pollution removed was typically greatest for O3, followed by particulate matter, nitrogen dioxide, sulfur dioxide, and carbon monoxide. Pollution removal varies among cities depending on the amount of tree cover (increased tree cover leading to greater total removal), pollution concentration (increased concentration leading to greater downward flux and total removal), length of the in-leaf season (increased growing season length leading to greater total removal), amount of precipitation (increased precipitation leading to reduced total removal via dry deposition), and other meteorological variables that affect tree transpiration and deposition velocities (factors leading to increased deposition velocities would lead to greater downward flux and total removal) (Nowak et al., 2006). Only two other studies about O3 mitigation by urban forests have been carried out in Mediterranean-type climates (Fuenlabrada, Spain, Vilela Lozano, 2004; Santiago, Chile, De la Maza et al., 2005), where O3 levels are of most concern. This paper reports two new case studies from Italy.

In summary, the aims of this paper are: 1. to summarise O3 levels along urban-to-rural gradients in three cities representative of Northern (Milan), Central (Florence) and Southern (Bari) Italy; 2. to review the state-of-knowledge of forest effects on O3 pollution and of O3 pollution on forest conditions in Italian cities.

Section snippets

Ozone pollution along urban-to-rural gradients in Italian cities

The relationship between O3 levels at rural and urban locations in Italy has never been studied in detail. Quality assured data (annual sampling efficiency ≥ 80%) were collected from monitoring stations run by the local authorities in charge of air pollution monitoring for the cities of Milan, Florence, and Bari, during 2005. These cities were selected to represent three typical climates of Italy, according to the Köppen climate classification (Peel et al., 2007). Milan has a humid subtropical

Urban forest and ozone interactions

In Italy, urban boundaries are referred to by administrative municipalities. Inventories of urban green area refer to public spaces only. Public green open space represents on average 5% of municipal surface, and the availability per person is 17.7 m2 (APAT, 2006).

Only a few studies carried out in Rome, the capital of Italy, investigated functional changes of urban trees in response to O3 in the air. In Pinus pinea, a rise in peroxidase activity and structural alterations of stomata were

Concluding remarks

Ozone levels in and around three Italian cities were shown to be high enough to jeopardise human and forest health. Although the indicators of human and forest health were usually higher at suburban and rural sites than in the cities, the criteria to protect forest health were also exceeded at urban sites, even when the criteria to protect human health were met. Note, however, that the values in the EU Directive are defined as 3–5 year means in order to compensate for annual variations, while

Acknowledgment

Dave Nowak and Bob Hoehn are acknowledged for support with the UFORE model. Caterina Morosi and Eleonora Casini are acknowledged for support with tree inventory in Florence.

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