Measures of ozone concentrations using passive sampling in forests of South Western Europe

Environmental Pollution 145 (2007) 620e628 www.elsevier.com/locate/envpol

Measures of ozone concentrations using passive sampling in forests of South Western Europe M.J. Sanz a,*, V. Calatayud a, G. Sa´nchez-Pen˜a b b

a Fundacio´n CEAM, Charles R. Darwin 14, Parc Tecnolo`gic, E-46980 Paterna, Valencia, Spain Servicio de Proteccio´n de los Montes contra Agentes Nocivos, Direccio´n General para la Biodiversidad, Ministerio de Medio Ambiente, Gran Vı´a de San Francisco, 4, E-28005, Madrid, Spain

Received 31 October 2005; accepted 27 February 2006

Ozone concentrations in forested areas of SW Europe during the period 2000e2002 showed highest values in 2001, as well as a tendency to increase towards the South and with altitude. Abstract Ambient ozone concentrations were measured with passive samplers in the framework of the EU and UN/ECE Level II forest monitoring programme. Data from France, Italy, Luxembourg, Spain and Switzerland are reported for 2000e2002, covering the period from April to September. The number of plots increased from 67 in 2000 to 83 in 2002. The year 2001 experienced the highest ozone concentrations, reflecting more stable summer meteorological conditions. Average 6-month ozone concentrations above 45 ppb were measured this year in 40.3% of the plots, in contrast with the less than 21% measured in the other 2 years. Gradients of increasing ozone levels were observed from North to South and with altitude. Comments are made on the regional trends and on the time frame of the higher ozone episodes. Also, some recommendations enabling a better comparison between plots are provided. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Ozone; Passive samplers; Intensive Monitoring Plots; SW Europe; Gradients

1. Introduction Tropospheric ozone (O3) is the most important regional air pollutant in Europe. It is the product of complex photochemical processes involving nitrogen oxides (NOx) and volatile organic compounds (VOCs) as the main ozone precursors. In the last decades, an increase of this pollutant in the Northern Hemisphere has been observed, and attributed primarily to the increase in anthropogenic ozone precursors (Volz and Kley, 1988). Although European controls on nitrogen oxides and volatile organic compounds emissions have led to a decrease in ozone peak concentrations in recent years, in rural stations of Europe the overall average AOT40 (Accumulated dose over a threshold of 40 ppb, Fuhrer et al., 1997) value

* Corresponding author. Tel.: þ34 961318227; fax: þ34 961318190. E-mail address: [email protected] (M.J. Sanz). 0269-7491/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2006.02.031

has shown a slight tendency to increase (Fiala et al., 2003). Ambient ozone concentrations in rural and forested areas can reach concentrations high enough to produce phytotoxic effects in native vegetation (e.g. Velissariou et al., 1992; Bussotti and Ferretti, 1998; Sanz and Milla´n, 1998; Skelly et al., 1999; Sanz et al., 2000; Innes et al., 2001; de Vries et al., 2003; Ferretti et al., 2007). In Southern Europe, ozone formation is particularly favoured due to the intense solar radiation, high temperatures and re-circulation of the polluted air masses (Milla´n et al., 1997, 2000; Sanz and Milla´n, 1998). In National Air Quality Networks, ozone concentrations are measured with continuous monitors that are costly and mostly concentrated in urban or suburban areas, making it very difficult to infer the ozone spatial patterns, especially in complex mountainous areas. Passive samplers are considered an alternative, or a complement to continuous monitors, e.g. when a higher density in the measuring network is required. Their use in assessing the ecological effects of tropospheric ozone

M.J. Sanz et al. / Environmental Pollution 145 (2007) 620e628

has significantly increased in recent years (e.g. Manning et al., 1996; Blum et al., 1997; Brace and Peterson, 1998; Krupa and Legge, 2000; Sanz et al., 2001). In the framework of the activities carried out on the Intensive Monitoring Plots (ICP Forests International Cooperation Programme for the evaluation and monitoring of air pollution effects in forests, http://www.icp-forests.org/), ozone measurements were recommended, with the following objectives: to produce information on ambient air quality in forest ecosystems, to gain knowledge into the spatial and temporal distribution of concentrations of a secondary gaseous pollutant, ozone, and to evaluate the potential risk to forest ecosystems (Sanz and Krause, 2001). In the present paper, the ozone concentrations measured in the Intensive Monitoring Plots from South Western Europe (involving France, Italy, Luxembourg, Spain, and Switzerland) are documented for the period 2000e2002. 2. Methods and materials 2.1. Passive sampler types Ozone concentrations were measured using different passive sampling systems, following the recommendations of the Submanual for Monitoring of Air Quality (Lo¨vblad et al., 2000). Samplers were placed in open field stations within the forest area, at a height of 2e2.2 m (Italy, Spain, Switzerland) or 3 m (France) above ground level, with the distance from the measuring point to the surrounding stands or other obstacles being at least two times the height of a mature tree/obstacle. The samplers were provided with rain shelters that also protected them from the wind. In France, passive samplers developed by the IVL (Institutet fo¨r Vatten-och Luftva˚dsforskning) were used, the analysis was also carried out by IVL. In the year 2000, Italy employed passive samplers developed by the University of Munich (Department of Forest Bioclimatology and Immission Research), but changed to Passam ag passive samplers in 2001 and 2002. Passam ag samplers were also used by Switzerland during the 3 years of this study. In Spain, exposure to ambient concentrations of ozone were sampled by Ogawa and Co., Inc. passive ozone sampler analysis, and calculation of ozone concentrations were carried out as suggested by Koutrakis et al. (1993). Blanks were always employed in Spain.

2.2. Number of Intensive Monitoring Plots The number of Intensive Monitoring Plots (IMP) with passive samplers considered for this study increased over the 3-year period, from 67 in 2000 to 83 in 2002 (Table 1). France and Italy had the highest number of plots (more than 20), followed by Spain (12), Switzerland (6 in 2000 and 2002, but increased to 16 in 2002) and Luxembourg (2). A list of these plots and some of their characteristics is given by Ferretti et al. (2004).

2.3. Exposure periods The period of the year selected for this study is 1 Aprile30 September, which is considered the vegetative period for trees, and the time window Table 1 Number of Intensive Monitoring Plots where passive-sampler ozone measurements are carried out, and were considered in this study 2000

2001

2002

France Italy Luxembourg Spain Switzerlanda

26 21 2 12 6

26 26 2 12 6

27 26 2 12 16

Total

67

72

83

a

In Switzerland, one of the plots included is a nursery facility, not an IMP.

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recommended for calculating the AOT40 for forests (Fuhrer et al., 1997). Although in Mediterranean conditions, the vegetative period may be longer (for this reason in Spain passive samplers are exposed the whole year), only the measurements from this 6-month period have been taken into account for this study. The starteend dates are reported in Table 2. During the periods considered, passive samplers were collected on a regular basis every 1 or 2 weeks (occasionally 3). In 2000 and 2001, in Italy and Switzerland, the passive samplers were changed weekly while in France, Luxembourg, Spain and Switzerland (2002) for most of the monitoring plots the usual exposure period of the passive samplers was 2 weeks. Table 2 gives the average number of days of the exposures and associated standard deviations

2.4. Data completeness, consistency, and cover for the 6-month period Time consistency of the exposure series was checked by considering the following aspects: (1) completeness of the whole data set, (2) when two or three replicates are exposed in a given site, the cases with a relatively high variation within replicates are examined more carefully, and cross-checked with the meteorological information at the sites, if available, and (3) dubious data are represented and their plausibility checked. Only a few cases did not fulfil these criteria. After clarification of some problematic or dubious data, the non-valid data for the period 2000e2002 and the participating countries were scarce, below 1% per year. Technical problems (e.g. loss of the passive sampler), and the few non-valid data removed from the data set explain the gaps (Table 2).

2.5. Data calculations Measured ozone concentrations are weighed using the number of exposure days, so that differences in duration between exposure periods in a given plot (e.g. 19 days instead of the usual 14 days) are taken into account to calculate the 6-month average. Duration of the exposure periods, which in some countries is 1 week (Italy, Switzerland P.P) and in others mainly 2 weeks (France, Spain, Switzerland p.p.), may lead to biased results if the maximum measured values are compared for the different countries, since shorter exposure periods capture high ozone episodes more accurately. For this reason, to facilitate a better comparison of the maxima among plots, data have been harmonised to 2week periods (e.g. by combining 2 consecutive 1-week periods). The months with seasonal maximum ozone concentrations were determined on the basis of these harmonised 2-week periods, and as single exposure periods may cover part of 2 months, the central day of the period was used to assign it to a given month.

2.6. Comparison against continuous monitors To test passive sampler performance, samplers were co-located at six stations equipped with continuous UV-photometric ozone analyzers (reference analysis method, Directive 2002/3/EC). These analyzers were calibrated on a regular basis by regional air quality network technicians or by instrumental maintenance companies. Passive samplers were installed next to the intake of the continuous monitors. Two sites were located in Italy, three in Spain, and one in Switzerland. While in Italy these sites were permanent monitoring plots, in Spain they were two rural stations in the Valencian Community Air Quality Network (http://www.cma.gva.es/cidam/emedio/atmosfera/), with different ozone daily cycles (Milla´n et al., 2000), and one OTC facility (CEAM). In Switzerland, co-located passive samplers were exposed in Canton Ticino, South of Switzerland, in an OTC facility (WSL) placed in Lattecaldo nursery. Some data periods were excluded from the data set because of malfunctioning of the active monitors.

3. Results 3.1. Comparison with continuous monitors When the complete data set (years 2000e2002, period from April to September) was considered, significant correlations

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Table 2 Starting and ending days for the series of exposure periods, average number of days (and standard deviations) of the periods, data completeness from the first to the last exposure period, and cover of the period 1st Aprile30th September (only validated data have been considered) Start daya

End daya

No. days

% Data completeness from the first to the last exposure period

% Days covered in the period 1st Aprile30th September Mean

Year

Country

First starting day of all plots

Last starting day of all plots

First end day of all plots

Last end day of all plots

Mean  standard deviation

Mean  standard deviation

Number of plots with 90% completeness

2000

France Italy Spain Luxembourg Switzerland

11/04/00 02/05/00 01/04/00 12/04/00 16/05/00

13/04/00 16/05/00 30/06/00 12/04/00 11/07/00

27/09/00 26/09/00 30/09/00 30/09/00 20/09/00

30/09/00 30/09/00 30/09/00 30/09/00 30/09/00

14.0  0.9 7.0  0.0 16.4  4.7 14.0  2.0 7.0  0.5

97.9  4.4 99.3  1.7 98.4  4.6 100.0  0.0 96.6  8.4

96 100 92 100 83

93.8  0.5 81.7  2.0 94.9  14.2 94.0  0.0 63.6  13.9

100 100 92 100 0

2001

France Italy Spain Luxembourg Switzerland

11/04/01 02/04/01 01/04/01 11/04/01 14/05/01

25/04/01 10/04/01 01/04/01 25/04/01 30/05/01

27/09/01 12/06/01 30/09/01 30/09/01 24/09/01

30/09/01 30/09/01 30/09/01 30/09/01 30/09/01

14.0  1.0 7.0  0.6 16.0  4.4 14.6  2.9 7.7  2.1

99.7  1.6 96.7  3.1 100.0  0.0 100.0  0.0 97.5  4.2

100 96 100 100 83

94.1  1.5 96.4  12.6 100.0  0.0 90.7  5.4 74.4  3.6

100 96 100 100 67

2002

France Italy Spain Luxembourg Switzerland

09/04/02 01/04/02 01/04/02 10/04/02 29/04/02

15/04/02 23/04/02 01/04/02 25/04/02 19/06/02

30/09/02 24/09/02 30/09/02 30/09/02 16/09/02

30/09/02 30/09/02 30/09/02 30/09/02 16/09/02

13.9  1.1 7.1  0.9 15.9  4.7 14.0  1.5 14.3  2.7

99.0  2.6 97.7  3.7 100.0  0.0 100.0  0.0 98.8  4.8

100 92 100 100 94

94.9  0.6 96.0  3.3 100.0  0.0 90.9  5.8 77.1  7.7

100 100 100 100 75

Number of plots with 75% cover

a

For exposure periods that only partly cover the period 1st Aprile30th September (i.e. starting before 1st April and ending in April, or starting in September but ending after 30th September), their starting or ending days have been considered to be 1st April and 30th September, respectively.

geographical location and meteorology (Logan, 1985). While it is beyond the scope of this article to carry out an in-depth analysis of the dependence of ozone concentrations on meteorological variables, at least some general climatic characteristics have to be taken into account to explain the interannual differences. Maps of fronts and of temperature at 850 hPa for the 3 years considered in this study show contrasting differences between summers (not shown, available at http:// www.wetterzentrale.de/topkarten/). Both synoptic weather maps and ground data from meteorological stations in Southern and Central Europe indicate that the summer of 2002 was particularly unstable and rainy; in August, a surface low moving from the Mediterranean Basin into Central Europe produced exceptional rainfall events that caused floods in central Europe (Ulbrich et al., 2003). In contrast, synoptic data from 2001 indicate that during this year weather conditions were particularly stable from the second half of July to

between passive sampler and active monitor measurements were found for all the plots ( p < 0.05, R2 ¼ 0.201e0.874 in the different stations, Table 3). However, if individual years were taken separately, some of the correlations were not significant ( p > 0.05), e.g. in cases with only a few valid exposure periods. The strength and significance of the correlations improved substantially if longer data sets were considered. For example, in stations from Spain, if we consider the whole year instead of only the period AprileSeptember, the correlations are stronger (In Table 3, compare the whole year [in italics], with the Aprile September period [regular print]), and they are always significant ( p  0.001, except p ¼ 0.029 at one station). 3.2. Ozone concentrations Ozone concentrations depend on a multitude of factors, such as proximity to large source areas of ozone precursors,

Table 3 Comparison between co-located passive samplers and continuous monitors at two plots in Italy, three in Spain and one in Switzerland from April to September, 2000e2002 Country

Plot

2000e2002 R

Italy Italy Spain Spain Spain Spain Spain Spain Switzerland

VAL1 BOL1 La Peira La Peiraa Sant Jordi Sant Jordia Gandia Gandiaa Lattecaldo

2

0.201 0.654 0.415 0.771 0.874 0.856 0.527 0.839 0.506

2000 2

p

n

R

0.000 0.000 0.040 0.000 0.000 0.000 0.001 0.000 0.000

68 47 18 34 15 25 18 30 44

0.970 e 0.255 0.810 e e 0.401 0.750 0.889

2001 2

p

n

R

0.004

19 e 6 11 e e 6 10 15

0.638 0.764 e 0.848 0.910 0.869 0.899 0.970 0.088

0.307 0.000 e e 0.177 0.001 0.000

2002 p

n

R2

p

N

0.000 0.000 e 0.000 0.014 0.000 0.004 0.001 0.000

23 22 2 10 5 11 6 12 18

0.202 0.584 0.265 0.664 0.814 0.520 0.103 0.575 0.088

0.024 0.000 0.128 0.000 0.000 0.000 5.081 0.029 0.406

24 24 10 13 10 14 6 8 9

a On the Spanish plots, passive samplers were exposed not just for 6 months, but also for the rest of the year; these more complete data are given in italics. Linear regression; crossing of line at the origin of the co-ordinates is not required.

M.J. Sanz et al. / Environmental Pollution 145 (2007) 620e628

the end of August in the Mediterranean area covered by this study. This stability was associated with periods of persistent high temperatures (e.g. both the period 24 Julye4 August, and the period 23e30 August showed areas of high-air-pressure over western and central Mediterranean Sea). The year 2000 was similar to 2001, but with less-marked stability conditions. Average and maximum accumulated values for each sampler exposure for 6-month periods in the IMPs from 2000 to 2002 are calculated and expressed in six classes: class 1, ozone concentrations <30 ppb; class 2, 30 and <45 ppb; class 3, 45 and <60 ppb; class 4, 60 and <75 ppb; class 5, 75 and <90 ppb, and class 6, ozone concentrations 90 ppb. Fig. 1 shows the percentage of plots belonging to the six above defined classes on the basis of their 6-month average ozone concentration. In Southern and Western Europe, 2002 can be considered a year with lower ozone concentrations during spring and summer (classes with average concentrations 45 ppb being only 16.9% of the plots), in contrast with the years 2000 (20.9%) and 2001 (40.3%). Considering all the plots of the network over all 3 years, the majority showed a 6-month average with concentrations between 30 and 45 ppb (class 2). Class 3, with values of 45e60 ppb, represented 36.1% of the plots in 2001, while it was below 18% in 2000 and 2002. High 6-month average values, above 60 ppb, appeared in 2000 and 2001, but representing less than 5% of the plots. Average concentrations in the range of 60e75 ppb (class 4) have been measured in Italy (1 plot in 2000, 2 plots in 2001), Spain (1 plot in 2001) and Switzerland (1 plot in 2000). Fig. 2 (left) shows the spatial distribution of 6-month average concentrations for the years 2000e2002. From an ecological perspective, both chronic and peak exposure concentrations can be equally relevant (Lephon and Musselman, personal communication). For this reason, in the context of ozone effects studies within the IMPs, special attention has to be paid not only to those plots exhibiting high average values during the whole vegetative period but also to those in which short episodes of high ozone occur. Relatively short fumigation episodes under adequate ambient conditions (e.g. a good water supply, allowing stomata to be open) may

70 <30 ppb

60

30-45 ppb

% of plots

50

45-60 ppb

40

60-75 ppb

30 20 10 0 2000

2001

2002

year Fig. 1. Percentages of plots in terms of the average ozone concentration for the years 2000e2002. Data refer to the average value measured with ozone passive samplers from 1st April to 30th September.

623

be enough to affect the most sensitive plants. For this reason, the maximum of all the 2-week ozone exposure periods, has been selected for each plot and year. Data have previously been harmonised to 2-weeks, i.e. in the case of 1-week exposure periods, an average of every two consecutive exposures has been calculated, to provide mean 2-week values. The percentages of plots in the six classes are shown in Fig. 3. In 2002, fewer plots reached values over 60 (class 4 or higher, 10.8%) than in the previous years. However, 2001 is a higher ozone year with 31.9% of the plots showing maximum values 60 ppb. Fig. 2 (right) represents the spatial distribution of these maximum values for each year. France and Luxembourg do not show any 2-week period with ozone concentrations 60 ppb. In these countries, half of the plots are in class 30e45 ppb, while the other half are in class 45e60 ppb. Italy showed higher maximum exposure values. In 2001, two plots (7.7%) experienced at least one 2-week period over 90 ppb, two others (7.7%) belonged to class 75e90 ppb, and half of the plots are over 60 ppb in that year, and about 23% in both years. Spain shows slightly lower values in 2001, with one plot in class 5 (75e90 ppb) and with a high percentage of plots (66.7%) in classes 4 or higher (60 ppb). Even more markedly than in Italy, in 2001, Spain experiences higher maximum ozone levels than in 2000 and 2002: 58.3% of the plots are in the range 60e75 ppb in 2001, as against only 8.3% in 2000 and 2002. In Switzerland, the highest 2-week values belong to class 4 (36e75 ppb). An analysis of the average ozone concentrations per month (average of the 3 years) in plots from four different areas distributed from NW Europe to SE (Fig. 4), shows a tendency for Atlantic plots from France to experience higher concentrations at the beginning of spring. Towards the Mediterranean area (i.e. Mediterranean France or Italy in Fig. 4, or Spain-data not shown), however, higher ozone values were recorded both in spring and summer. Central parts of France show an intermediate situation between the most Atlantic plots and Mediterranean France. These results might illustrate some regional trends from NW to Mediterranean Europe. Nevertheless, given the considerable interannual variation in the plots, more years of measurements are needed before any definitive conclusions can be derived. The month for the highest 2-week ozone episode was determined for each plot and year (Fig. 5). The analysis was restricted to plots common for the 3 years. While in 2000 and 2001, about half of the plots reached their maximum 2-week values in spring or early summer (AprileJune) and the other half in summer (JulyeSeptember), in 2002 about 76% of the plots experienced their highest values from April to June. Consistent with the above results, in France, about 90% of the plots showed their highest values from April to June for the 3 years considered, although in 2001 the maximum values occurred at the end of June for many plots. In contrast, in years 2000 and 2001 for the Mediterranean and central European territories the highest 2-week ozone episode mostly occurred in summer (about 70% of the plots of Italy, Spain and Switzerland). The year 2002 was distinct in that the maximum period

624

M.J. Sanz et al. / Environmental Pollution 145 (2007) 620e628

Fig. 2. Average (left) and 2-week maximum values (right) measured per plot from April to September 2000, 2001 and 2002 (top to bottom).

shifted to spring in many Spanish, Italian and Swiss plots (64% of the plots). The 2-week harmonization of the exposure periods applied to Italy and Switzerland resulted in a decrease in the percentage of plots within the highest ozone classes with regard to the original 1-week periods (data not shown): as an example for Italy, in 2001 classes 5 and 6 (75 ppb) represent 34.6% of the plots if we consider the 1-week measurements, and only 15.4% if averaged 2-week periods, made up of two consecutive 1-week periods are taken. This is because 1-week sample period captured better individual episodes. 3.3. Latitudinal and altitudinal gradients Regression analyses of the 6-months average values from years 2000e2002 show the existence of both latitudinal and altitudinal ozone gradients for the plots considered in SW

Europe (Fig. 6). The trend is stronger in the case of altitude (R2 ¼ 0.43, p < 0.0001), than for latitude (R2 ¼ 0.37, p < 0.0001). Plots from Northern France and Luxembourg experience lower ozone concentrations, and also a lower interannual variability than Mediterranean locations, where high radiance and higher temperatures offer better conditions for photochemical reactions leading to ozone production. In Fig. 4 (top), a group of plots with relatively high ozone values is also evident at about 46e47  , consisting of Northern Italy and Southern Switzerland localities. These plots belong to an area well-known for high ozone levels due to a combination of both topographical characteristics and by receiving emissions from the industrial region of the Po Valley. Although there is considerable variability, results from regression analysis also suggest that in SW Europe seasonal mean ozone concentrations measured with passive samplers tend to increase with altitude (Fig. 6, bottom). In order to

M.J. Sanz et al. / Environmental Pollution 145 (2007) 620e628 70 60 <30 ppb

% of plots

50

30-45 ppb

40

45-60 ppb

30

60-75 ppb 75-90 ppb

20 >=90 ppb

10 0 2000

2001

2002

2000-2002

year Fig. 3. Percentages of plots in different classes of 2-week maximum ozone concentrations measured for the years 2000e2002, and for the 3 years (2000e2002). Data refer to the maximum 2-week value measured with ozone passive samplers from 1st April to 30th September of each year, and for the combined data (2000e2002).

check if there was a possible effect of altitude on the observed latitudinal gradient (as some of the highest altitudes occurred in plots from Central and Southern Europe), the latitudinal analysis was restricted to more comparable plots at altitudes below 1000 m. The exponential regression between latitude and mean ozone concentrations was even stronger in this case (R2 ¼ 0.46, p < 0.000, n ¼ 46), suggesting that the tendency to increase ozone towards the South was not mainly determined by differences in altitude. 4. Conclusions and recommendations

in which this pollutant may represent a risk to native vegetation. Overall, France and Luxembourg experienced lower ozone levels than Italy, Southern Switzerland and Spain. Some of the highest values measured occurred in central Italy and in Southern SwitzerlandeNorthern Italy. These results are consistent with reports based on continuous monitors, indicating that these areas experience the highest number of exceedances of the threshold value for information to the public (de Leeuw and Bogman, 2001; Fiala et al., 2002; Fiala et al., 2003). The hot-spot at Northern Italy/Southern Switzerland is under the influence of the industry and traffic emissions in the Po Valley, including the city of Milan (e.g. Novak et al., 2003), with the polluted air masses undergoing re-circulation processes that permit the cumulative build-up of ozone (Bacci et al., 1990). The occurrence and persistence of re-circulatory processes has been described for the Mediterranean Basin; they are important mechanisms that contribute to the relatively high ozone concentrations in spring and summer (Milla´n et al., 1997). The ozone concentrations recorded in this area are high enough to regularly produce visible injury in many native species (e.g. Innes et al., 2001; Cozzi et al., 2000). As expected, interannual changes in ozone concentrations were observed during the 3 years, in relation to the different climatic conditions. It is well-known that ozone concentrations are highly dependent on meteorological variables, increasing with higher air temperature and solar radiation and decreasing in cloudy and rainy periods (e.g. Amoriello et al., 2003). Warm and sunny weather enhances the ozone concentration because of the emission of volatile hydrocarbons (including vegetation emissions) increases with temperature, higher solar radiation increases photochemical processes, and high temperature results in more rapid chemical ozone formation (Fiala et al., 2003). As indicated in the result section, synoptic and temperature maps over Europe show that the year 2002 was characterised as having a particularly unstable and rainy summer in Southern Europe, in contrast with the year 2001, in which very stable and hotter conditions were prevalent in summer. The year 2000 showed an intermediate situation. The climatic characteristics of 2002 were reflected in lower ozone concentrations than in 2001, but also, since the instability was centred mainly on the summer months, in a lower

Ozone concentration (ppb)

Passive sampling represents a reliable tool for measuring ozone concentrations in rural and remote areas across Europe, and contributes to the knowledge of ozone levels in forested regions not covered by continuous monitors. The results from this study are particularly relevant as they provide data from some areas that are not well covered by continuous monitoring stations (e.g. EMEP), as is the case of the Mediterranean region (Hjellbrekke and Solberg, 2004). With this sampling technique, it has been possible to improve the knowledge on the spatial patterns of ozone concentrations in SW Europe in different years, and to detect hot-spots in forested areas

625

70

D (Medit. Italy) C (Medit. France)

65

B (Middle France)

60

A (Atlantic France)

55 50 45 40 35 30 25 20 Apr

May

Jun

Jul

Aug

Sep

Fig. 4. Distribution of four selected areas from NW to SE parts of SW Europe, and their corresponding monthly average ozone values (each data represents the mean of the 3 years  standard deviation), used to illustrate trends. Only plots common for the 3 years, 2000e2002, have been selected.

M.J. Sanz et al. / Environmental Pollution 145 (2007) 620e628

626

100

18

France

Year 2000

Ozone concentration, ppb

16

Italy

14

Spain

12

Luxembourg Switzerland

10 8 6 4 2 0 18

F I

80

E

LU

CH

70 60 50 40 30

y = 249.8e-0.000004x

20

R2 = 0.37 P<0.0001, N=67

10

0 350000

400000

Year 2001

450000

500000

Latitude

14 12

80

10

70

Ozone concentration, ppb

Number of Plots

16

90

8 6 4 2 0 18 16

Year 2002

14 12

60 50 40 30 20

y = 30.81e0.0003x

10

R2 = 0.43 P<0.0001, N=67

0 0

10

250

500

750

1000

1250

1500

1750

2000

Elevation, m asl

8 6 4 2 0 Apr

May

Jun

Jul

Aug

Sep

Fig. 5. Number of plots per country classified on the basis of the month in which they experienced their highest seasonal 2-week ozone episode. Each 2-week exposure period was assigned to a given month on the basis of the central day of the period. Only plots common for the 3 years, 2000e2002, have been selected (n ¼ 67).

percentage of plots showing their maximum 2-week ozone episodes in summer. In 2001, the highest ozone levels of all 3 years were recorded. During this year, 40.3% of plots had an average ozone concentration above 45 ppb for the sampling period, while in 2002 they were 16.9%, and in 2000, 20.9%. Also in 2001, the 2-week maximum exposure values were over 60 ppb in 31.9% of the plots, as against 11% in 2000 and 2002. Hotter years may lead to even higher ozone concentrations than in 2001, as is exemplified by the year 2003, in which Europe experienced a heat wave and extreme drought (Ciais et al., 2005). The particular meteorological conditions of 2003 produced the highest ozone values for Central Europe since the 80s (Solberg et al., 2005), leading some countries to take measures against elevated ozone concentrations (Fiala et al., 2003). Also this year, an increase in ozone concentration in the forest areas of Europe was detected with passive samplers (Lorenz et al., 2005). Climate change towards hotter and drier summers is expected to result in periods of higher ozone levels in the future (IPCC-DDC, 2004). However, higher ozone episodes may not necessarily produce increased

Fig. 6. Average ozone values (standard deviations) for passive samplers for the years 2000, 2001 and 2002 plotted against latitude (top) and elevation (bottom). Regression equations, coefficients of determination, level of significance and number of cases (N ) are reported. Plots with less than 3 years of measurements have not been included. Horizontal bars indicate the range of latitudes covered by the different countries (CH ¼ Switzerland, E ¼ Spain, F ¼ France, I ¼ Italy, and LU ¼ Luxembourg).

adverse effects on the plants, as these episodes may be associated with more severe drought conditions, which may limit the ozone uptake by plants, therefore preventing the development of visible injury (Fisher et al., 2005) and other effects. A preliminary analysis of the data based on the limited number of years available suggests that maximum values may occur either in spring or in summer, depending on the geographical situation of the plot and the climatic characteristics of these seasons in the different years. Several authors found apparent latitude/longitude patterns in the shape of the seasonal cycle over Europe (Esser, 1993; Monks, 2000). On the western edge of the continent there are sites with a spring maximum/summer minimum, while in the interior or south it is more frequent to find a broad summer maximum. Transitions between these forms can also be found. Results from this study might be in agreement with this scheme, as in the 3 years considered, plots from the most Atlantic part of France usually showed higher values in spring, while in the Mediterranean high levels occurred in spring and summer. Passive samplers operating all year long in Spain (the present study uses a subset of this database restricted to the period Aprile September) show that minimum ozone values occur in winter,

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and that two relative maxima occur frequently on the plots in spring and summer (Sanz et al., 2001). Similarly, two maxima have also been reported from the Apennines, using continuous monitor measurements (Bonasoni et al., 2000). These conclusions must be considered preliminary, however, as longer measurement series are still required to confirm and better understand the above-described seasonal and regional patterns measured with passive samplers in SW Europe, and to enable a better classification of the plots according to their ozone seasonal profile. In this study, increasing gradients of ozone concentrations have been observed both from Northern to Southern Europe, and in altitude with respect to forested areas of SW Europe. The increase from North to South was expected, as conditions for ozone formation, i.e. solar radiation and temperature, are more favourable in the Mediterranean area. Moreover, the Northern territories covered in this study are more exposed to depressions that usually enter the continent from the Atlantic ocean, cleaning the atmosphere of pollutants. An altitudinal gradient was also observed in SW Europe. At a regional scale, in rural or remote areas of the Mediterranean, mountain-top locations may experience relatively high summer ozone levels all day, including at night. Conversely, lower altitude stations usually show a clear diurnal cycle, with higher values in daytime and notably lower values at night. This has been explained by the fact that, at night, these mountain areas may remain inside reservoir layers, i.e. air masses rich in ozone (Milla´n et al., 2000). Lower ozone concentrations at high altitudes may occur in areas that are either outside the transport paths of polluted air masses or advected with clean air. Although there are many factors determining local ozone levels (e.g. distance from the sources, levels of emissions, transport, topography, climatology), night-time ozone concentrations probably contribute to the observed elevation gradient in SW Europe. Similar altitudinal gradients have been identified at a regional scale outside Europe as well, e.g. in western USA (Lee, 2003). Finally, some recommendations are suggested for the monitoring programme as future objectives. It is recommended that the period from 1st April to 30th September should be covered completely and that the starting and ending time should ideally be the same for all the participant countries (or at least take place during the same week) for all the countries. Although 2-week periods represent a good cost-effective option, as relatively short fumigation periods may produce effects (e.g. symptoms) in very sensitive species, and these episodes are better described by 1-week exposures, periods of 1 week should be a future target, provided resources permit it. This is especially relevant for countries showing a higher variability in their ozone concentrations, and would permit a better characterization of the ozone episodes, and to derive more precise empirical models to emulate ozone concentrations. Acknowledgements The NFCs of the countries involved, and particularly N. Kra¨uchi, B. Petriccione, E. Ulrich, are thanked for providing us with the data. F. Bussotti, M. Ferretti, M. Schaub and three

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