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Surface ozone mixing ratio increase with altitude in a transect in the Catalan Pyrenees
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Atmospheric Environment 40 (2006) 7308–7315 www.elsevier.com/locate/atmosenv
Surface ozone mixing ratio increase with altitude in a transect in the Catalan Pyrenees A`ngela Ribas, Josep Pen˜uelas Unitat d’Ecofisiologia CSIC-CEAB-CREAF, CREAF (Centre de Recerca Ecologica i Aplicacions Forestals), Edifici C, Universitat Auto`noma de Barcelona, 08193 Bellaterra, Barcelona, Espanya, Spain Received 31 January 2006; received in revised form 1 June 2006; accepted 27 June 2006
Abstract Tropospheric ozone mixing ratios and their phytotoxicity and NO2 mixing ratios were measured along an altitudinal gradient at the Meranges valley in the Catalan Pyrenees. Biweekly measurements using Radiello passive samplers were taken along a transect of seven stations ranging from 1040 to 2400 m ASL from May to December 2004. As well, at each station biweekly evaluations were made of the visual symptoms of ozone damage in Bel-W3 and Bel-B tobacco cultivars. Whereas ozone mixing ratios increased with altitude, NO2 mixing ratios decreased from the valley floor upwards. Ozone damage rates were found to vary with time and space depending on local environmental and meteorological conditions, although the highest ozone damage to foliage was found in the stations at greatest altitude, especially wherever altitudinal micrometeorological conditions enhanced plant sensitivity. r 2006 Elsevier Ltd. All rights reserved. Keywords: Altitudinal gradient; Mediterranean region; Nitrogen oxides; Ozone; Phytotoxicity; Tobacco plants
1. Introduction The annual average background ozone concentrations at mid-latitudes in the Northern Hemisphere range approximately from 20 to 45 ppb, with variability depending on geographical location, altitude and the extent of anthropogenic influence (Finlayson-Pitts and Pitts, 2000). In pre-industrial times ozone concentrations stood at approximately 10–15 ppbv and this increase has been attributed to an increase in NOx emissions associated with the
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E-mail addresses: [email protected] (A`. Ribas), [email protected] (J. Pen˜uelas). 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.06.039
switch to fossil fuels during the industrial period (Finlayson-Pitts and Pitts, 1997). There are many rural air-pollution monitoring stations in central and northern Europe providing a satisfactory picture of the spatial distribution of surface ozone. However, fewer systematic measurements are carried out in the Mediterranean basin (Glavas, 1999; de Leeuw, 2000). Previous measurements have revealed high ozone concentrations in this Mediterranean region (Gimeno et al., 1995; Ziomas et al., 1998; Ribas and Pen˜uelas, 2000, 2004; Sanz et al., 2000), although such studies are few and far between. Detailed temporal and spatial analysis is still needed, above all in Spain, where experimental evidence of oxidants and precursors is limited (Duen˜as et al., 2002), including the situation
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in the mountain ranges of the Mediterranean region. In mountains higher solar radiation has a serious impact on the photochemistry of O3 production (Volz and Kley, 1988). Moreover, differences in local recirculation linked to orographic systems (for instance valley–mountain systems) may also change the mixing ratios of photochemical compounds (Lefhon, 1992; Sanz and Milla´n, 2000). The distribution of ozone in mountainous regions has been studied, for example, in the Alps (Puxbaum et al., 1991), White Mountains (Aneja et al., 1994a, b), Appalachian Mountains (Skelly et al., 1997), San Bernardino Mountains (Bytnerowicz et al., 1999) and Sierra Nevada (Van Ooy and Carroll, 1995) and results show on the whole positive relationships between site altitude and O3 mixing ratios. O3 mixing ratios and their effects on plants and animals (their toxicity) can vary in accordance with local conditions at different altitudes and some authors have reported that O3 uptake in trees of similar age increases with altitude (Wieser et al., 1999). The objective of this study was to monitor surface O3 mixing ratios and their phytotoxicity along an altitudinal gradient in the Pyrenees. As well, the possible relationship between O3 mixing ratios and variations in their NOx precursors was studied. A valley-to-mountain transect was studied from Bellver, a valley-bottom village at the base of the altitudinal gradient where previous studies have reported high mixing ratios of O3 (often above current human and plant protection thresholds). 2. Material and methods 2.1. Physico-chemical measurements Seven sites were studied along an altitudinal gradient from 1040 m ASL to 2400 m ASL on the south-facing slopes of Puigpedro´s in La Cerdanya, a region in the Central Pyrenees (Bellver 421 220 N 11 460 E). The seven sampling sites were located approximately every 200 m in altitude (Fig. 1) in a forested area dominated by Pinus uncinata. Ozone mixing ratios and their phytotoxicity as well as NO2 mixing ratios were measured every two weeks from May to December 2004 at all stations. Phytotoxicity could not be measured at 2400 m site because of the irregularity of terrain which did not allow the installation of plants.
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Radiello radial symmetry passive samplers (Cocheo et al., 1996) were used to analyse ozone and nitrogen dioxide at all sampling sites. The use of these samplers is very common in studies conducted in remote areas (Cooper and Peterson, 2000; Krupa and Legge, 2000) where logistic limitations such as power supplies make it difficult to accurately quantify spatial patterns. At 1040 m ASL, at the lowest valley-bottom site of transect, ozone measurements were also conducted with MCV 48 AUV analysers that use light absorption at 253.7 nm. The detection threshold of the instrument is 1 ppbv and it has a precision of 72% and a signal-to-noise level of 1 ppbv. NOx levels were determined by using chemiluminescence. This site is part of a regional network of rural monitoring stations run by the Ministry of the Environment of the Catalan government (Generalitat de Catalunya) that consists of a network of ozone monitors and meteorological stations. Intercalibrations were made among passive samplers and monitor at this point. As in other previous works (Krupa and Legge, 2000; Skelly et al., 2001), close relationships have been found between passive and active measurements (Mixing ratiopassive sampler ¼ 0.6043Mixing ratiomonitor+12.24, R2 ¼ 0:69, po0:05). Nitrogen dioxide from passive samplers was also significantly correlated with NO2 active measurements (Mixing ratiopassive sampler ¼ 2.33Mixing ratiomonitor+1.11, R2 ¼ 0:57, po0:05). Temperature and humidity were measured at three of the sampling sites by means of weatherproof temperature/RH datalogger (HOBO Pro, Sistemas y Instalaciones, Madrid, Spain), with sensors located at 1500, 2200 and 2400 m ASL. These variables were also available for the valleybottom station (1040 m ASL) operated by the Catalan Ministry of the Environment. The temperature (from 30 to +50 1C) and the relative humidity (from 0% to 100%) sensors were accurate to 70.2 1C and 3%, respectively. Extrapolations from linear models for each biweekly period were used to estimate the temperature for the rest of the sites. 2.2. Description of ozone phytotoxical levels: active bioindication Phytotoxical effects were evaluated in terms of the ozone damage present in the tobacco (Nicotiana tabacum) cultivars Bel-W3 (sensitive) and Bel-B (resistant) that are used in most ozone bioindicator
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Fig. 1. Map of the studied altitudinal transect sites in the region of La Cerdanya in the central Catalan Pyrenees.
programmes (Heggestad, 1991; Klumpp et al., 2002). Tobacco seeds were germinated in ozonefree conditions to obtain homogeneous plants, thereby avoiding any possible early ozone contamination. When seedlings were at the fourth leaf stage, they were transplanted to 8 L pots filled with 25% peat, 25% vermiculite, 25% perlite and 25% sand. The soil pH of these pots was adjusted to 6.0 by adding CaCO3. A NPK 15:11:13 slow-release fertilizer (Osmocote plus) was also added. A self-watering system was used in which each pot was placed above individual self-watering reservoirs connected by two wicks. Every fortnight, from May to September, we added six new plants of both cultivars to each of the stations (Fig. 1). Phytotoxic levels were defined according to the percentage of damaged leaf area in the Bel-W3 and Bel-B tobacco cultivars (Bytnerowicz et al., 1993; Ribas and Pen˜uelas, 2003). After each exposure, the percentage of ozone-induced lesions on the oldest four leaves was visually recorded. These percentages were estimated in 5% intervals. 3. Results 3.1. Temperature and relative humidity The expected decrease in temperature with increasing altitude was observed along the studied transect. During the studied period (May–December) biweekly average temperature ranged between 14 1C (at 1040 m ASL) and 7.8 1C (at 2400 m ASL), that is, temperatures decreased approximately by 0.4 1C per 100 m of altitude. Average values of relative humidity for the whole period were as
follows: 71.9% at 1040 m, 63.4% at 1500 m, 68.48% at 2200 m and 65.84% at 2400 m; nevertheless, these differences were not significant. 3.2. O3 Ozone mixing ratios increased with altitude from c. 46 ppbv, measured at the lowest studied site (1040 m ASL) to c.69 ppbv at the highest site (Fig. 2). Seasonal changes in the O3 mixing ratios at all the studied sites occurred. In the valleybottom site the maximum values occurred in spring–summer and the minimal values in winter (7% higher mixing ratios in the warm period), while at the rest of studied sites mixing ratios were on average 18.5% greater during the cold period (corresponding to autumn and winter up to 31 December 2004) than during the warm period (end of spring and summer) (Fig. 2). For both the cold and warm periods in the valleybottom site (Fig. 3) the diurnal profiles of ozone mixing ratios (provided by the ozone monitor) showed a characteristic pattern of minimum values in the early morning, a significant rise during the morning with increasing solar radiation, peak mixing ratios in the afternoon between 13:00 and 15:00 h (local time), and then a decline due to ozone destruction by nitrogen oxide during the night (Fig. 3). 3.3. Threshold excess The plant protection threshold (65 mg m3 or 32.5 ppbv as a mean over 24 h, as defined by the current European Ozone Directive (92/72/EEC)) was exceeded on 62% of the studied days at the
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Fig. 2. May–December mean ozone mixing ratios (O3) 7SEM (standard error of the mean, n ¼ 9212) calculated for values obtained from passive samplers throughout the study period. The lower panel differentiates between the cold (September–December measurements, n ¼ 325) and warm periods (May–August measurements, n ¼ 627) in each site along the altitudinal gradient.
valley-bottom site during the summer period (July–September).
Fig. 3. Diurnal pattern of mean NO2, NO and O3 mixing ratios for the valley-bottom site (Bellver station at 1040 m ASL) 7SEM (standard error of the mean) in the cold (September–December measurements, n ¼ 1342137) and warm periods (May–August measurements, n ¼ 1792184).
3.4. NO2 and NO Mean NO2 mixing ratios remained at about 1 ppbv in both cold and warm periods. Only at the valley-bottom site (at 1040 m ASL) in the town of Bellver did the mean NO2 exceed 2 ppbv (Fig. 4). During the warm period mixing ratios were lower than during the cold period only in the valleybottom site (c.3 versus c.10 ppbv). In the other sites NO2 mixing ratios were generally greater in the
warm period. Maximum mixing ratios were recorded in the morning and at the beginning of the night (Fig. 3), a pattern that became more accentuated during the cold period, when NO2 mixing ratios reached 13 ppbv (Fig. 3). In this period daily mixing ratios only dropped when ozone mixing ratios rose. This pattern also occurred in the warm period but was attenuated by lower and less variable NO2 and NO mixing ratios (Fig. 4).
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Fig. 5. May–September mean damaged leaf area (%) in Nicotiana tabacum Bel-W3 and Bel-B cultivars for each altitude 7SEM (standard error of the mean, n ¼ 228).
Fig. 4. May–December mean nitrogen dioxide mixing ratios (NO2) 7SEM (standard error of the mean, n ¼ 9212) calculated for values obtained from passive samplers throughout the study period. The lower panel differentiates between the cold (September–December measurements, n ¼ 325) and warm periods (May–August measurements, n ¼ 627).
3.5. Phytotoxical levels: spatial variation of leaf damage Phytotoxicity increased with altitude. Substantial ozone injury symptoms were found at all altitudes on the Bel-W3 tobacco plants exposed to the ambient air, although damage was worse in the plants at greatest altitudes (Fig. 5). More than 40% of the surface area was affected at all sites, although at 2200 m ASL the mean damaged leaf area was 80% in the sensitive Bel-W3 tobacco plants and c.
Fig. 6. Relationship between damaged leaf area (%) in Nicotiana tabacum Bel-W3 and mean ozone mixing ratios values (ppbv). Each dot represents a site along the altitudinal gradient.
40% in the Bel-B resistant cultivar. In general, however, the resistant cultivar (Bel-B) was less affected overall and damaged leaf areas ranged from 5% to 15% in the remaining sites (Fig. 5). The percentage of ozone-induced damaged leaf area in the sensitive Bel-W3 cultivar had a significant linear relationship with ozone mixing ratios (Fig. 6). 4. Discussion The ozone mixing ratios measured in this area are similar to those reported by previous studies from
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valley sites (Ribas and Pen˜uelas, 2004) and are comparable to the highest average ozone levels recorded from rural stations in south-central Europe (Emberson et al., 1996; Kalabokas et al., 2000) and to values reported from other Mediterranean regions (Kalabokas et al., 2000; Lelieveld et al., 2002; Nali et al., 2002). These data document the existence of persistent medium-to-high O3 levels that contrast with the peak-type episodes found in central and northern Europe (Sanz and Milla´n, 2000). Surface ozone mixing ratios increased with altitude along the studied transect (Fig. 2); in other alpine regions mean O3 mixing ratios have also been reported to increase with altitude (Stockwell et al., 1997; Cooper and Peterson, 2000). The rate of ozone mixing ratio increase was 2 ppbv per 100 m of altitude, a figure that is not far from the 1.3 ppbv reported in the western Washington Mountains (USA) (Cooper and Peterson, 2000). Our results also agree with the reported O3 mixing ratios from high altitude ecosystems in the European Alps and in the Carpathian mountains, where O3 episodes of 100–120 ppb (Bytnerowicz et al., 2002) and mean growing season values of 40–50 ppb (Bytnerowicz et al., 2004; Wieser et al., 2001, 2006) have been reported. Regionally, topography has been shown to play a significant role in determining O3 exposure levels and the higher levels found at altitude (Cooper and Peterson, 2000; Skelly et al., 2001) may be caused by the increase in irradiance associated with higher altitudes. Some of the highest ozone mixing ratios ever detected have been from high-altitude locations (Brace and Peterson, 1998; Bytnerowicz et al., 1999), with relatively high night-time mixing ratios recorded from rural locations where local production of nitric oxide (NO) is minimal and the titration of ozone is low (Logan, 1985; Lefhon, 1992). Nevertheless, the issue is far more complex: altitudinal differences could also be caused by less vertical mixing, stratospheric intrusions of O3 or by reduced loss processes at greater altitudes. At the studied altitudinal gradient in the Pyrenees, contrasting seasonal changes were found between the valley and mountain cycles of both ozone and NO production. At the valleys, ozone mixing ratios exhibited seasonal cycles with maxima between April and July (warm season) and minima in winter (cold season). On the other hand, unexpected results have been found at greater altitude, where ozone reached maximum levels in
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the cold season (Fig. 2). Maximal values in winter period have been previously reported (Gur Sumiran Satsangi et al., 2004; Naja et al., 2003) including in mountain sites (Khemani et al., 1995). In these high altitude sites, maximum concentrations of O3 could be related to transport of O3 rich air from higher altitudes. The photochemical lifetime of ozone in mid-latitudes in winter is of the order of few months (Liu et al., 1987). Also, the lifetimes of O3 precursors would be considerably greater in winter than summer. Seasonal variation in O3 with a peak in late winter and early spring has been cited as evidence for non-photochemical sources of O3 such as exchange of stratosphere and tropospheric air. However, further studies are clearly needed to confirm and explain these results. Ozone variation throughout the day may be helpful in defining the processes that are responsible for ozone formation or loss at any particular location (Fig. 3). At the valley site the differences between day and night were accentuated by changes in radiation as a result of local photochemical production. During the whole study period the average area of tobacco plant leaf damage was closely related to the ozone mixing ratios expressed as average biweekly ozone mixing ratios (Fig. 5). These mountain sites showed intensive damage response—as was reported for the valley site by a previous study (Pen˜uelas et al., 1999)—that corresponds to ozone mixing ratios above the sensitivity threshold for this cultivar Bel-W3 (40 ppbv) (Heggestad, 1991) and to the microclimatic conditions such as high radiation or humidity that enhance stomatal conductance and ozone uptake (Kaufmann, 1976; Pen˜uelas et al., 1999). The damaged leaf area increased with altitude in response to increased O3 mixing ratios and likely O3 uptake increase with greater altitude (Kaufmann, 1976; Wieser et al., 1999). Moreover, an increase in altitude represents a substantial change in the microclimatic conditions experienced by the plant bioindicator, which might synergically interact with the negative effects increasing O3 mixing ratios. Further research on all these issues is necessary, although our results do show that, in accordance with recent literature on critical levels (Pleijel, 2000), the observed values of ozone could have negative effects on plant life, especially at high altitude sites in the Pyrenees.
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Acknowledgements The authors are grateful to the Ministry of the Environmental of the Catalan government (Generalitat de Catalonia) for providing the ozone and meteorological data from Bellver de la Cerdanya. This research was funded by a GALOPA grant (Gradientes Altitudinales de Ozono en Comunidades Pascı´ colas) from ‘‘Fundacio´n BBVA 2004’’. Partial funding was also provided by ISONET (Marie Curie network contract MC-RTN-CT-2003504720 with the European Union).
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Atmospheric Environment 40 (2006) 7308–7315 www.elsevier.com/locate/atmosenv
Surface ozone mixing ratio increase with altitude in a transect in the Catalan Pyrenees A`ngela Ribas, Josep Pen˜uelas Unitat d’Ecofisiologia CSIC-CEAB-CREAF, CREAF (Centre de Recerca Ecologica i Aplicacions Forestals), Edifici C, Universitat Auto`noma de Barcelona, 08193 Bellaterra, Barcelona, Espanya, Spain Received 31 January 2006; received in revised form 1 June 2006; accepted 27 June 2006
Abstract Tropospheric ozone mixing ratios and their phytotoxicity and NO2 mixing ratios were measured along an altitudinal gradient at the Meranges valley in the Catalan Pyrenees. Biweekly measurements using Radiello passive samplers were taken along a transect of seven stations ranging from 1040 to 2400 m ASL from May to December 2004. As well, at each station biweekly evaluations were made of the visual symptoms of ozone damage in Bel-W3 and Bel-B tobacco cultivars. Whereas ozone mixing ratios increased with altitude, NO2 mixing ratios decreased from the valley floor upwards. Ozone damage rates were found to vary with time and space depending on local environmental and meteorological conditions, although the highest ozone damage to foliage was found in the stations at greatest altitude, especially wherever altitudinal micrometeorological conditions enhanced plant sensitivity. r 2006 Elsevier Ltd. All rights reserved. Keywords: Altitudinal gradient; Mediterranean region; Nitrogen oxides; Ozone; Phytotoxicity; Tobacco plants
1. Introduction The annual average background ozone concentrations at mid-latitudes in the Northern Hemisphere range approximately from 20 to 45 ppb, with variability depending on geographical location, altitude and the extent of anthropogenic influence (Finlayson-Pitts and Pitts, 2000). In pre-industrial times ozone concentrations stood at approximately 10–15 ppbv and this increase has been attributed to an increase in NOx emissions associated with the
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E-mail addresses: [email protected] (A`. Ribas), [email protected] (J. Pen˜uelas). 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.06.039
switch to fossil fuels during the industrial period (Finlayson-Pitts and Pitts, 1997). There are many rural air-pollution monitoring stations in central and northern Europe providing a satisfactory picture of the spatial distribution of surface ozone. However, fewer systematic measurements are carried out in the Mediterranean basin (Glavas, 1999; de Leeuw, 2000). Previous measurements have revealed high ozone concentrations in this Mediterranean region (Gimeno et al., 1995; Ziomas et al., 1998; Ribas and Pen˜uelas, 2000, 2004; Sanz et al., 2000), although such studies are few and far between. Detailed temporal and spatial analysis is still needed, above all in Spain, where experimental evidence of oxidants and precursors is limited (Duen˜as et al., 2002), including the situation
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in the mountain ranges of the Mediterranean region. In mountains higher solar radiation has a serious impact on the photochemistry of O3 production (Volz and Kley, 1988). Moreover, differences in local recirculation linked to orographic systems (for instance valley–mountain systems) may also change the mixing ratios of photochemical compounds (Lefhon, 1992; Sanz and Milla´n, 2000). The distribution of ozone in mountainous regions has been studied, for example, in the Alps (Puxbaum et al., 1991), White Mountains (Aneja et al., 1994a, b), Appalachian Mountains (Skelly et al., 1997), San Bernardino Mountains (Bytnerowicz et al., 1999) and Sierra Nevada (Van Ooy and Carroll, 1995) and results show on the whole positive relationships between site altitude and O3 mixing ratios. O3 mixing ratios and their effects on plants and animals (their toxicity) can vary in accordance with local conditions at different altitudes and some authors have reported that O3 uptake in trees of similar age increases with altitude (Wieser et al., 1999). The objective of this study was to monitor surface O3 mixing ratios and their phytotoxicity along an altitudinal gradient in the Pyrenees. As well, the possible relationship between O3 mixing ratios and variations in their NOx precursors was studied. A valley-to-mountain transect was studied from Bellver, a valley-bottom village at the base of the altitudinal gradient where previous studies have reported high mixing ratios of O3 (often above current human and plant protection thresholds). 2. Material and methods 2.1. Physico-chemical measurements Seven sites were studied along an altitudinal gradient from 1040 m ASL to 2400 m ASL on the south-facing slopes of Puigpedro´s in La Cerdanya, a region in the Central Pyrenees (Bellver 421 220 N 11 460 E). The seven sampling sites were located approximately every 200 m in altitude (Fig. 1) in a forested area dominated by Pinus uncinata. Ozone mixing ratios and their phytotoxicity as well as NO2 mixing ratios were measured every two weeks from May to December 2004 at all stations. Phytotoxicity could not be measured at 2400 m site because of the irregularity of terrain which did not allow the installation of plants.
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Radiello radial symmetry passive samplers (Cocheo et al., 1996) were used to analyse ozone and nitrogen dioxide at all sampling sites. The use of these samplers is very common in studies conducted in remote areas (Cooper and Peterson, 2000; Krupa and Legge, 2000) where logistic limitations such as power supplies make it difficult to accurately quantify spatial patterns. At 1040 m ASL, at the lowest valley-bottom site of transect, ozone measurements were also conducted with MCV 48 AUV analysers that use light absorption at 253.7 nm. The detection threshold of the instrument is 1 ppbv and it has a precision of 72% and a signal-to-noise level of 1 ppbv. NOx levels were determined by using chemiluminescence. This site is part of a regional network of rural monitoring stations run by the Ministry of the Environment of the Catalan government (Generalitat de Catalunya) that consists of a network of ozone monitors and meteorological stations. Intercalibrations were made among passive samplers and monitor at this point. As in other previous works (Krupa and Legge, 2000; Skelly et al., 2001), close relationships have been found between passive and active measurements (Mixing ratiopassive sampler ¼ 0.6043Mixing ratiomonitor+12.24, R2 ¼ 0:69, po0:05). Nitrogen dioxide from passive samplers was also significantly correlated with NO2 active measurements (Mixing ratiopassive sampler ¼ 2.33Mixing ratiomonitor+1.11, R2 ¼ 0:57, po0:05). Temperature and humidity were measured at three of the sampling sites by means of weatherproof temperature/RH datalogger (HOBO Pro, Sistemas y Instalaciones, Madrid, Spain), with sensors located at 1500, 2200 and 2400 m ASL. These variables were also available for the valleybottom station (1040 m ASL) operated by the Catalan Ministry of the Environment. The temperature (from 30 to +50 1C) and the relative humidity (from 0% to 100%) sensors were accurate to 70.2 1C and 3%, respectively. Extrapolations from linear models for each biweekly period were used to estimate the temperature for the rest of the sites. 2.2. Description of ozone phytotoxical levels: active bioindication Phytotoxical effects were evaluated in terms of the ozone damage present in the tobacco (Nicotiana tabacum) cultivars Bel-W3 (sensitive) and Bel-B (resistant) that are used in most ozone bioindicator
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Fig. 1. Map of the studied altitudinal transect sites in the region of La Cerdanya in the central Catalan Pyrenees.
programmes (Heggestad, 1991; Klumpp et al., 2002). Tobacco seeds were germinated in ozonefree conditions to obtain homogeneous plants, thereby avoiding any possible early ozone contamination. When seedlings were at the fourth leaf stage, they were transplanted to 8 L pots filled with 25% peat, 25% vermiculite, 25% perlite and 25% sand. The soil pH of these pots was adjusted to 6.0 by adding CaCO3. A NPK 15:11:13 slow-release fertilizer (Osmocote plus) was also added. A self-watering system was used in which each pot was placed above individual self-watering reservoirs connected by two wicks. Every fortnight, from May to September, we added six new plants of both cultivars to each of the stations (Fig. 1). Phytotoxic levels were defined according to the percentage of damaged leaf area in the Bel-W3 and Bel-B tobacco cultivars (Bytnerowicz et al., 1993; Ribas and Pen˜uelas, 2003). After each exposure, the percentage of ozone-induced lesions on the oldest four leaves was visually recorded. These percentages were estimated in 5% intervals. 3. Results 3.1. Temperature and relative humidity The expected decrease in temperature with increasing altitude was observed along the studied transect. During the studied period (May–December) biweekly average temperature ranged between 14 1C (at 1040 m ASL) and 7.8 1C (at 2400 m ASL), that is, temperatures decreased approximately by 0.4 1C per 100 m of altitude. Average values of relative humidity for the whole period were as
follows: 71.9% at 1040 m, 63.4% at 1500 m, 68.48% at 2200 m and 65.84% at 2400 m; nevertheless, these differences were not significant. 3.2. O3 Ozone mixing ratios increased with altitude from c. 46 ppbv, measured at the lowest studied site (1040 m ASL) to c.69 ppbv at the highest site (Fig. 2). Seasonal changes in the O3 mixing ratios at all the studied sites occurred. In the valleybottom site the maximum values occurred in spring–summer and the minimal values in winter (7% higher mixing ratios in the warm period), while at the rest of studied sites mixing ratios were on average 18.5% greater during the cold period (corresponding to autumn and winter up to 31 December 2004) than during the warm period (end of spring and summer) (Fig. 2). For both the cold and warm periods in the valleybottom site (Fig. 3) the diurnal profiles of ozone mixing ratios (provided by the ozone monitor) showed a characteristic pattern of minimum values in the early morning, a significant rise during the morning with increasing solar radiation, peak mixing ratios in the afternoon between 13:00 and 15:00 h (local time), and then a decline due to ozone destruction by nitrogen oxide during the night (Fig. 3). 3.3. Threshold excess The plant protection threshold (65 mg m3 or 32.5 ppbv as a mean over 24 h, as defined by the current European Ozone Directive (92/72/EEC)) was exceeded on 62% of the studied days at the
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Fig. 2. May–December mean ozone mixing ratios (O3) 7SEM (standard error of the mean, n ¼ 9212) calculated for values obtained from passive samplers throughout the study period. The lower panel differentiates between the cold (September–December measurements, n ¼ 325) and warm periods (May–August measurements, n ¼ 627) in each site along the altitudinal gradient.
valley-bottom site during the summer period (July–September).
Fig. 3. Diurnal pattern of mean NO2, NO and O3 mixing ratios for the valley-bottom site (Bellver station at 1040 m ASL) 7SEM (standard error of the mean) in the cold (September–December measurements, n ¼ 1342137) and warm periods (May–August measurements, n ¼ 1792184).
3.4. NO2 and NO Mean NO2 mixing ratios remained at about 1 ppbv in both cold and warm periods. Only at the valley-bottom site (at 1040 m ASL) in the town of Bellver did the mean NO2 exceed 2 ppbv (Fig. 4). During the warm period mixing ratios were lower than during the cold period only in the valleybottom site (c.3 versus c.10 ppbv). In the other sites NO2 mixing ratios were generally greater in the
warm period. Maximum mixing ratios were recorded in the morning and at the beginning of the night (Fig. 3), a pattern that became more accentuated during the cold period, when NO2 mixing ratios reached 13 ppbv (Fig. 3). In this period daily mixing ratios only dropped when ozone mixing ratios rose. This pattern also occurred in the warm period but was attenuated by lower and less variable NO2 and NO mixing ratios (Fig. 4).
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Fig. 5. May–September mean damaged leaf area (%) in Nicotiana tabacum Bel-W3 and Bel-B cultivars for each altitude 7SEM (standard error of the mean, n ¼ 228).
Fig. 4. May–December mean nitrogen dioxide mixing ratios (NO2) 7SEM (standard error of the mean, n ¼ 9212) calculated for values obtained from passive samplers throughout the study period. The lower panel differentiates between the cold (September–December measurements, n ¼ 325) and warm periods (May–August measurements, n ¼ 627).
3.5. Phytotoxical levels: spatial variation of leaf damage Phytotoxicity increased with altitude. Substantial ozone injury symptoms were found at all altitudes on the Bel-W3 tobacco plants exposed to the ambient air, although damage was worse in the plants at greatest altitudes (Fig. 5). More than 40% of the surface area was affected at all sites, although at 2200 m ASL the mean damaged leaf area was 80% in the sensitive Bel-W3 tobacco plants and c.
Fig. 6. Relationship between damaged leaf area (%) in Nicotiana tabacum Bel-W3 and mean ozone mixing ratios values (ppbv). Each dot represents a site along the altitudinal gradient.
40% in the Bel-B resistant cultivar. In general, however, the resistant cultivar (Bel-B) was less affected overall and damaged leaf areas ranged from 5% to 15% in the remaining sites (Fig. 5). The percentage of ozone-induced damaged leaf area in the sensitive Bel-W3 cultivar had a significant linear relationship with ozone mixing ratios (Fig. 6). 4. Discussion The ozone mixing ratios measured in this area are similar to those reported by previous studies from
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valley sites (Ribas and Pen˜uelas, 2004) and are comparable to the highest average ozone levels recorded from rural stations in south-central Europe (Emberson et al., 1996; Kalabokas et al., 2000) and to values reported from other Mediterranean regions (Kalabokas et al., 2000; Lelieveld et al., 2002; Nali et al., 2002). These data document the existence of persistent medium-to-high O3 levels that contrast with the peak-type episodes found in central and northern Europe (Sanz and Milla´n, 2000). Surface ozone mixing ratios increased with altitude along the studied transect (Fig. 2); in other alpine regions mean O3 mixing ratios have also been reported to increase with altitude (Stockwell et al., 1997; Cooper and Peterson, 2000). The rate of ozone mixing ratio increase was 2 ppbv per 100 m of altitude, a figure that is not far from the 1.3 ppbv reported in the western Washington Mountains (USA) (Cooper and Peterson, 2000). Our results also agree with the reported O3 mixing ratios from high altitude ecosystems in the European Alps and in the Carpathian mountains, where O3 episodes of 100–120 ppb (Bytnerowicz et al., 2002) and mean growing season values of 40–50 ppb (Bytnerowicz et al., 2004; Wieser et al., 2001, 2006) have been reported. Regionally, topography has been shown to play a significant role in determining O3 exposure levels and the higher levels found at altitude (Cooper and Peterson, 2000; Skelly et al., 2001) may be caused by the increase in irradiance associated with higher altitudes. Some of the highest ozone mixing ratios ever detected have been from high-altitude locations (Brace and Peterson, 1998; Bytnerowicz et al., 1999), with relatively high night-time mixing ratios recorded from rural locations where local production of nitric oxide (NO) is minimal and the titration of ozone is low (Logan, 1985; Lefhon, 1992). Nevertheless, the issue is far more complex: altitudinal differences could also be caused by less vertical mixing, stratospheric intrusions of O3 or by reduced loss processes at greater altitudes. At the studied altitudinal gradient in the Pyrenees, contrasting seasonal changes were found between the valley and mountain cycles of both ozone and NO production. At the valleys, ozone mixing ratios exhibited seasonal cycles with maxima between April and July (warm season) and minima in winter (cold season). On the other hand, unexpected results have been found at greater altitude, where ozone reached maximum levels in
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the cold season (Fig. 2). Maximal values in winter period have been previously reported (Gur Sumiran Satsangi et al., 2004; Naja et al., 2003) including in mountain sites (Khemani et al., 1995). In these high altitude sites, maximum concentrations of O3 could be related to transport of O3 rich air from higher altitudes. The photochemical lifetime of ozone in mid-latitudes in winter is of the order of few months (Liu et al., 1987). Also, the lifetimes of O3 precursors would be considerably greater in winter than summer. Seasonal variation in O3 with a peak in late winter and early spring has been cited as evidence for non-photochemical sources of O3 such as exchange of stratosphere and tropospheric air. However, further studies are clearly needed to confirm and explain these results. Ozone variation throughout the day may be helpful in defining the processes that are responsible for ozone formation or loss at any particular location (Fig. 3). At the valley site the differences between day and night were accentuated by changes in radiation as a result of local photochemical production. During the whole study period the average area of tobacco plant leaf damage was closely related to the ozone mixing ratios expressed as average biweekly ozone mixing ratios (Fig. 5). These mountain sites showed intensive damage response—as was reported for the valley site by a previous study (Pen˜uelas et al., 1999)—that corresponds to ozone mixing ratios above the sensitivity threshold for this cultivar Bel-W3 (40 ppbv) (Heggestad, 1991) and to the microclimatic conditions such as high radiation or humidity that enhance stomatal conductance and ozone uptake (Kaufmann, 1976; Pen˜uelas et al., 1999). The damaged leaf area increased with altitude in response to increased O3 mixing ratios and likely O3 uptake increase with greater altitude (Kaufmann, 1976; Wieser et al., 1999). Moreover, an increase in altitude represents a substantial change in the microclimatic conditions experienced by the plant bioindicator, which might synergically interact with the negative effects increasing O3 mixing ratios. Further research on all these issues is necessary, although our results do show that, in accordance with recent literature on critical levels (Pleijel, 2000), the observed values of ozone could have negative effects on plant life, especially at high altitude sites in the Pyrenees.
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Acknowledgements The authors are grateful to the Ministry of the Environmental of the Catalan government (Generalitat de Catalonia) for providing the ozone and meteorological data from Bellver de la Cerdanya. This research was funded by a GALOPA grant (Gradientes Altitudinales de Ozono en Comunidades Pascı´ colas) from ‘‘Fundacio´n BBVA 2004’’. Partial funding was also provided by ISONET (Marie Curie network contract MC-RTN-CT-2003504720 with the European Union).
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