Transactions on Ecology and the Environment vol 21, 1998 WIT Press, ISSN

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1 Assessment of ozone levels at alpine sites in Northern Italy Roberta Vecchi, Gianluigi Valli Istituto di Fisica Generate Applicata - University of Milan Wa Cc/ona, Mz/m?, 7^/y valli@imiucca.csimnimi. it Abstract Increased tropospheric ozone concentrations are currently a matter of large concern, not only as a local problem in urban areas but especially as aregionalone. Moreover comparison of old and recent data indicates that surface ozone concentrations in rural areas in Europe have doubled during the past 100 years. The great interest given to ozone levels assessment is due to its role in controlling the chemical composition of the troposphere and climate; furthermore recent studies liave been shown tliat elevated ozone concentrations can affect human health and vegetation. The largest mountainous region in Central Europe are the Alps, surrounded by heavily industrialised and densely populated regions, the Alps cover a wide area in Northern Italy, representing the greatest natural patrimony of this territory. Information about ozone levels is generally not easily available in mountainous terrain, even if ozone lias been mentioned as a possible cause of the decline of forests also in Europe. In literature data from the northern slope of Alps are available but only a few ones can be found about ozone levels in the southern part of this mountainous region. The aim of our research is to evaluate ozone levels and study the behaviour of ozone at higher altitudes in the Italian region of Central Alps. In tliis paper we will provide recent results obtained during different ozone monitoring campaigns at mountain locations analysing seasonal ozone levels, daily patterns, exceedances of the ozone guide lines and some episodes with high ozone concentrations. The stations are placed at altitudes between 800 metres and 1900 metres, far away from local sources of pollution.

2 198 Air Pollution 1 Introduction The ozone monitoring stations are located in the Italian region of Central Alps, on the southern slope of the alpine chain, where measurements are missing. The investigated region is characterised by a complex topography, with large lakes, valleys with irregular structure and high mountains. The monitoring sites were chosen according to some parameters: altitudes ranging from 800 metres (typical pre-alpine sites) up to 1900 metres (alpine sites, next to mountain-tops); distance from local sources of pollution, such as roads with heavy traffic, industrial settlements and cities (the only exception is Brunate, which is facing the city of Como, at 800 metres ); locations situated in areas where no precedent air quality measurements were performed. A map of the investigated region is given in the table below: Station B innate (province of Como) Mezzoldo (province of Bergamo) Piazzatorre (province of Bergamo) Castello (province of Bergamo) Alpe Motta (province of Sondrio) Passo S Marco (province of Bergamo) Elevation 800 m 880 m 1000 m 1350m 1800m 1900m Latitude N 46 00' N 45 59' N 46 02' N 46 25' N 46 02' N Longitude 9 06' E 9 40' E 9 41'E 9 39' E 9 22' E 9 38' E All the measurements presented in this paper were realised using commercial U V. - photometers. They were checked weekly and intercomparisons were performed twice a year. In May 1994 and 1995 these analysers were also calibrated during an inter-laboratory study organised by the European Reference Laboratory of Air Pollution (European Commission - Joint Research Centre, Ispra, Italy). This type of monitors has measurements errors of about 10 % and a detection limit at 2 ppb.

3 Air Pollution Results and discussion 2.1 Daily patterns In fig. 2 the 5, 25, 50, 75 and 95 percentiles of ozone concentration are represented both for the period April - September and for October - March at all locations. The 50 percentiles are in the range of ppb in summertime and ppb in winter, with a positive trend with increasing altitudes. The inter-quartile range (defined as the difference between the 75 and the 25 percent!le) shows a tendency to decrease with the elevation of the station, indicating a minor influence of extreme episodes on the average values recorded at higher altitude sites. It also interesting to observe the presence of a "background" concentration (represented by the 5 percentile) which is about 30 ppb in summer and 20 ppb in winter. Winter o 60 '8 S o. 0 A A kl : : i t i : : ;» ' + i T i bru mez pzt cst 3 n A 95 % 75 # 50 _+_ A psm Summer Izone concentration (ppb) Ul A * L A / \j/ ik [['.. i v ;. '?;. : j-> i bru mez pzt cst a n psm A 95 ;t( Fig. 1; 5, 25, 50, 75 and 95 percentiles of ozone concentration.

4 200 /*/ The vertical profiles represented in fig. 3 were obtained averaging the ozone hourly concentrations measured at the three stations placed in the same alpine valley (Mezzoldo, Castello and Passo S. Marco). In this context the summer season is conventionally defined as the period June - August and winter as December - February. It is clear a difference between wintertime and summertime ozone patterns. During wintertime the ozone profile shows ozone concentrations varying between 25 and 35 ppb and remains almost the same in the course of the day with a positive gradient towards higher altitudes. It is worthy noting that the two stations located at middle and high elevations have almost the same behaviour and concentrations and differ from the low-elevation one. It is likely that the low convective activity fosters a decoupling of the atmospheric layers. Anyway, at LT (Local Time) a certain degree of uniformity in ozone levels (which differ of less than 5 ppb) at all altitudes can be observed. In summer the profile shows a gradual transition from the night-time positive gradient to a negative one during the LT time interval, and again it comes to a positive gradient in the evening. During night-time the high-altitude station experiences ozone levels greater than those found at the lower-altitudes ones, which show ozone values comparable of about ppb. During daylight hours the ozone levels at the lower-elevation monitoring sites increase and, in the late afternoon, also the station located at mountain-tops records the same ozone concentration providing the evidence of a substantial uniformity of the atmosphere on a large scale. Polluted air masses originating from southern areas (most likely the Po Valley) are spread over a large territory, and during transport processes to northern sites the precursors are converted in photo-chemical products, thus air masses with high ozone levels can be found also at alpine sites. At sunset a progressive decrease in ozone levels is evident at lower-elevation sites, this is due mainly to dry deposition; by contrast, at higher-elevations locations the presence of a residual layer, which maintains ozone levels nearly identical to those found at lower altitudes during former hours, can be observed. It is noteworthy that after LT, the site at 2000 metres a.s.l appears to be decoupled from the layers beyond as only a slow decrease in ozone concentrations is registered. At these mountainous sites, the night-time ozone levels are likely dominated by the concentrations existing in the residual layer, and by the role of vertical transports of air from aloft. A combination of dynamical atmospheric processes and reduced photochemical activity, as well as the

5 Air Pollution 201 TJ 3+* '+3 < Hour: 0000 LT Ron ""^**^" Winter Summer 1 TJ 3 < ROD Hour: 0400 LT i i ^^^ Winter Summer Hour: 0800 LT Altitude (m) Ron 2 i ""^^^" Winter Summer Fig. 2: ozone concentrations at Mezzoldo (880 m), Castello (1350 m) and Passo S. Marco (1900 m).

6 202 Air Pollution ^ 1500 T Hour: 1200 LT < ' Winter «Summer ~ 2000 % 1500 o Hour: 1600 LT »Winter Summer Hour: 2000 LT _ 2000 ~ 1500 o 2 1 ooo ' Winter Summer Fig. 3: ozone concentrations at Mezzoldo (880 m), Castello (1350 m) and Passo S. Marco (1900 m).

7 Air Pollution 203 BRUNATE CM c\j CO CM ALPE MOTTA 1995 ID CM CD CM L ) M CM PIAZZATORRE VAL BREMBANA Mezzoldo Castello Passo S. Marco Fig. 4: ozone concentrations monitored at alpine and prealpine sites during high-pressure systems,

8 204 Air Pollution characteristics of the mountainous complex orography, is the cause of the ozone pattern typical of high-altitude sites. 2.2 Episodes In Europe the highest ozone concentrations occur in summer during stable high pressure systems (local meteorological conditions are generally characterised by clear skies, warm temperatures and light winds); ozone levels during these episodes reach concentrations of about ppb, that may last for several days. During our monitoring campaigns the highest ozone concentrations have been measured during episodes which developed on a regional scale and characterised by stable high-pressure situations. It is well known that anticyclonic subsidence may transport downwards the ozone-rich air, which is delivered in the lower troposphere over periods of days, because of the minimised dispersion associated with high-pressure cells. In fig. 4 some examples are shown; it is important that during these episodes both maxima and minima ozone values grow progressively until the complete dissolution of the stable high-pressure system, when a sudden drop of ozone concentrations can be observed. The ozone level of a certain day is heavily affected by the concentrations reached the day before, fostering an ozone accumulation within the planetary boundary layer from day to day, as also during the night part of the ozone is stored in the residual layer. 2.3 Excecdances The assessment of ozone effects on vegetation is based on the "critical level" concept, which is defined using the AOT40 index. The AOT40 (Average Over Threshold 40 ppb for ozone) is calculated subtracting the threshold value of 40 ppb from each hourly ozone concentration and adding the residuals together. Different critical levels were agreed^^ to prevent damage to the most sensitive crops, forests and natural vegetation. The critical level for forest trees (of major interest for the alpine sites) is an AOT40 value of ppb h, calculated over a 6 months growing season (April - September inclusive), based on a 5 years mean. There was a debate on which hours had to be considered for the evaluation of the critical value; in a first time it was agreed that, unlike the critical level for crops, for forest trees the AOT40 should be calculated over all 24 hours. Afterwards, it was recommended that the AOT40 for forest trees should be calculated for daylight hours only.

9 205 In tab. 2 the AOT40 has been computed only for those years with largest data capture, so it has to be considered an indicative value; the comparison between the AOT40 calculated over 24 hours and for daylight hours is also given. As expected, there is a strong decrease in the AOT40 computed on daylight hours, as at these mountainous sites nighttime ozone concentrations do not undergo an appreciable decrease, reflecting the free troposphere background ozone concentration of about ppb. It is noteworthy that the degree of exceedance of the critical level cannot be used directly to assess the extent of damage to vegetation as it only indicates the potential risk of such damage. In this paragraph the number of hours exceeding the critical value for health protection is briefly discussed. The WHO^ proposed an ozone concentration of ppb for an hourly average to assess an evidence of health effects; in table 3 the number of hours exceeding 75 ppb and 100 ppb, the number of hours validated and the relative percentage of exceedances are summarised. At the prealpine and alpine sites, monitored in the years , percentages of exceedances lower than 4 % have been recorded and only in a very few cases ( %) the value of 100 ppb has been exceeded. 3 Conclusion The ozone levels monitored at alpine and prealpine sites in the Southern part of Alps show a substantial uniformity in the average values. Daily patterns appear to be influenced both by advection processes such as breezes (mountain/valley winds but also slope winds) and by convective ones (as the mixing layer growth); especially in the alpine valley the influence of these thermally-induced processes is evident. The O^ exposure of vegetation at these alpine sites is high, up to times the provisional critical level of ppb h. The analyses performed on ozone concentrations at alpine and prealpine sites, show that processes at a regional scale are responsible for the observed levels, and weekly variations of emissions strongly influence the ozone patterns and levels. This point may be of great importance in the assessment of an appropriate emissions control strategy. References [1] Fuhrer J. & Ackermann B, Critical levels for ozone; a UNECE workshop report. FAC Report no. 16, Swiss Federal Research Station for

10 206 Air Pollution Agricultural Chemistry and Environmental Hygiene, Liebefeld - Bern, [2] Karenlampi L. & Skarby L., Critical levels for ozone in Europe: testing and finalising the concepts, [3] WHO, Air Quality Guidelines for Europe, European series No. 23, Copenhagen, Station B innate Mezzolclo Piazzatorre Castello Alpe Mottn Passo S. Marco Year AOT40 (all day) No. Hours AOT40 (daylight hours) No. hours Table 2: AOT40 calculated both for all day and for daylight hours only. Station B innate Mezzoldo Piazzatorre Castello Alpe Motta Passo S. Marco Station Brnnate Mezzoldo Piazzatorre Castello Alpe Motta Passo S. Marco No. Hours > 75 ppb No. Hotirs > 576 ( ( ( (1.9 No. tot hours No. tot hours ppb %) %) %) %) 75 (1.3 «/o) 144 (1.9 %) No. hours > 100 ppb No. hours > 100 ppb 118(0.6%) 9(0.1 %) 96 (0.7 %) 15(0.2%) 0 (0 %) 12 (0.2 %) Table 3: number of hours exceeding 75 ppb, the number of hours validated and the relative percentage of exceedances.