Trends in near-surface ozone concentrations in Switzerland: the 1990s

A time series analysis of ozone monitoring data from several locations in Switzerland from 1991 to 1999 is presented. Different methods are used to address changes in the ozone level during these years and to account for the influence of changing meteorological conditions. The results show a slight decrease of the peaks but a highly significant increase of the mean value of around 0.5–0.9 ppb yr⁻¹. The frequency distribution has changed in the sense that very low values have become less frequent and that there is a strong increase in frequency of occurrence of half-hourly mean values between about 45 and 55 ppb. A selection procedure reveals slight tendencies towards different trends of afternoon ozone peaks in summer depending on weather and pollution situations. Ozone peaks tend to decrease on fair weather days at rural sites (but increase at urban sites) and show a small increase on cloudy and windy days. A non-linear regression model is used to estimate trends of summertime afternoon ozone peaks in the presence of meteorological variability. In the model, the long-term signal is additively split into a linear part and a part which is modulated by global radiation. The coefficients for both terms are statistically significant at many sites, with an increasing linear trend at the sites north of the Alps of around 1 ppb yr⁻¹ and a decrease of ozone peaks under fair weather conditions relative to cloudy conditions. When additionally considering the effect of precursor concentrations in the regression models, both trends are weakened, which means that they can partly be explained by changes in local to regional emissions. However, at the sites north of the Alps remains a tendency towards a positive linear “base trend” of around 0.4 ppb yr⁻¹. This could possibly be due to increasing background ozone concentrations.     

Spatial variations of ground level ozone concentrations in areas of different scales

The spatial variation of ground level ozone concentrations was investigated for areas of three different scales: (1) an air quality management district (a region about 100×70 km²) in northern Taiwan, (2) the neighborhood (about 2 km in radius) of an air quality monitoring station, and (3) an open field (about 400×600 m²) surrounded by 3- and 4-story buildings in an elementary school. Analysis of data on hourly ozone concentration, obtained at 13 m above the ground at 21 monitoring stations in the air quality management district, showed that the stations downwind of the urban center in the district had significantly higher ozone concentrations. Measurements for 8-h average ozone concentrations at 1.5 m above the ground by passive samplers showed that, in a flat area about 2 km in radius, the ratios of the ozone concentration at open areas to that at the monitoring station (0.86–0.93) were significantly higher than those obtained at areas with higher traffic flow and density of buildings (0.60–0.68). For the open field in an elementary school, the 8-h average ozone concentrations at 1.5 m above the ground at sites less than 10 m from the nearest building were considerably lower than those at sites farther away from buildings. The results indicated that, in areas of small scales, the spatial distributions of ozone concentration were highly non-uniform and there were appreciable day-to-day variability in spatial distribution. Such variability should be taken into account in determining the extent to which an individual is exposed to ozone.     

A model for predicting maximum and 8 h average ozone in Houston

The objective of this research was to develop a statistical model to predict one day in advance both the maximum and 8 h (10 am–5 pm) average ozone for Houston (TX). A loess/generalized additive model (GAM) approach was taken to model development. Ozone data (1983–1991) from ten stations in the immediate Houston area were used in the study. The meteorological data came from the Houston International Airport. The models were developed using data for April through October for 1983–1987 and 1989–1990. Forecasts were developed for 1988 and 1991. The final model, which was multiplicative in nature, contained three interaction terms for the west/east and south/north wind components (average of hourly values from 8 pm to 5 am, 6 am to 9 am, and 10 am to 5 pm). Opaque cloud cover (averaged over the period 10  am to 5 pm), yesterday’s maximum ozone, today’s maximum temperature and morning mixing depth were also important variables in the model.Individual forecasts were generated for all ten stations in the Houston area using observed meteorology. In addition forecasts were produced for three measures of the network as a whole. The root-mean-square prediction error for the 8 h average forecasts ranged from 13.2 to 16.3 ppb (with R² ranging from 0.66 to 0.73) for the individual stations and from 18.5 to 22.0 ppb (with R² ranging from 0.61 to 0.68) for maximum ozone. A detailed examination was undertaken for a day on which the forecast was much too low.     

On springtime high ozone events in the lower troposphere from Southeast Asian biomass burning

Ozone peaks with mixing ratios as high as 138 ppbv were observed in the lower troposphere (2.5–4.5 km) over Hong Kong in spring. Simultaneously observed high humidity suggests that this enhanced ozone was not the result of transport from the upper troposphere. Back trajectory analysis suggests that these enhancements resulted from lateral transport. Air masses arriving at the altitude of the ozone peaks appear to have passed over continental Southeast Asia where the bulk of biomass burning occurs at this time of the year (February–April). We hypothesize that biomass burning in this region provided the necessary precursors for the observed ozone enhancement. As far as we know this is the first observation of highly enhanced ozone layers associated with biomass burning in continental Southeast Asia.     

Validation of a hybrid forecasting system for the ozone concentrations over the Paris area

A simplified hybrid statistical-deterministic chemistry-transport model, is used in real time for the prediction of ozone in the area of Paris during Summer 1999. We present here a statistical validation of this experiment. We distinguish the forecasts in the urban area from forecasts in the pollution plume downwind of the city. The validation of model forecasts, up to 3 days ahead, is performed against ground based observations within and up to 50 km outside of Paris. In the urban area, ozone levels are fairly well forecast, with correlation coefficients between forecast and observations ranging between 0.7 and 0.8 and root mean square errors in the range 15–20 μg m⁻³ at short lead times. While the bias of urban forecast is very low, the largest peaks are somehow underestimated. The ozone plume amplitude is generally well reproduced, even at long lead times (root mean square errors of about 20–30 μg m⁻³), while the direction of the plume is only captured at short lead times (about 70% of the time). The model has difficulties in forecasting the direction of the plume under stagnant weather conditions. We estimate the model ability to forecast concentrations above 180 μg m⁻³, which are of practical relevance to air quality managers. It is found that about 60% of these events are well forecast, even at long lead times, while the exact monitoring station where the exceedance is observed can only be forecast at short lead times. Finally, we found that about half of the forecast error is due to the error in the estimation of the boundary conditions, which are forecast by a simple linear regression model here.     

Surface ozone and NOx concentrations at a high altitude Mediterranean site,Greece

Ozone was measured in six- and NO_x in five sampling periods in 1996–97, mostly during summer, at a 1070 m altitude site in northern Peloponnese. Mean values in each sampling period ranged from 43–48 ppb exceeding the European Union 24 h plant protection standard. The background ozone concentration of 43 ppb derived from the correlation of ozone with NO^′_x also exceeded the EU plant protection standard. Ozone exhibited maxima in the afternoon and minima during the night; in certain 24–48 h periods, however, the ozone concentrations remained practically constant; in these short periods air mass back trajectories indicated air masses which originated in north Africa. NO^′_x concentrations had maximum of 24 h around noon. Their mean concentrations ranged from 0.5–0.7 ppb, smaller than respective concentrations in north-central Europe.     

Reactions between ozone and building products: Impact on primary and secondary emissions

Reactions of ozone on common building products were studied in a dedicated emission test chamber system. Fourteen new and unused products were exposed to 100–160 ppb of ozone at 23 °C and 50% RH during 48 h experiments. Ozone deposition velocities calculated at steady state were between 0.003 cm s⁻¹ (alkyd paint on polyester film) and 0.108 cm s⁻¹ (pine wood board). All tested product showed modified emissions when exposed to ozone and secondary emissions of several aldehydes were identified. Carpets and wall coverings emitted mainly C₅–C₁₀ n-aldehydes, typical by-products of surface reactions. Linoleum, polystyrene tiles and pine wood boards also showed increased emissions of formaldehyde, benzaldehyde and hexanal associated with reduced emissions of unsaturated compounds suggesting the occurrence of gas-phase reactions. The ozone removal on the different tested products was primarily associated with surface reactions. The relative contribution of gas-phase reactions to the total ozone removal was estimated to be between 5% and 30% for pine wood boards depending on relative humidity (RH) and on the incoming ozone concentration and 2% for polystyrene tiles. On pine wood board, decreasing ozone deposition velocities were measured with increasing ozone concentrations and with RH increasing in the range 30–50%.     

Responses of two birch (Betula pendula Roth) clones to different ozone profiles with similar AOT40 exposure

Saplings of two clones of European white birch (Betula pendula Roth) were exposed to three different ozone profiles resulting in same AOT40 value of 13–14 ppm h in a chamber experiment. The sensitive clone 5 and the more tolerant clone 2 were growing (1) under filtered air (=control), or (2) were exposed to 70 ppb ozone for 24 h d⁻¹ (=profile 1), (3) to 100 ppb ozone for 12 h d⁻¹ at 8:00–20:00 (=profile 2), or (4) to 200 ppb ozone for 4.5 h d⁻¹ at 9:30–14:00 (=profile 3) for 20 d. The saplings were determined for growth, visible leaf injuries, stomatal conductance, and concentrations of Rubisco, chlorophyll and carotenoids. Growth responses and induction of visible foliar injuries under different ozone profiles were variable, resulting in 4–17% lower dry mass of shoot, 16–46% reduction in stem height increment and 11–43% increase in visible injuries in clone 5, which was accompanied by higher leaf turnover rate under profile 3 indicating compensation growth. In clone 2, ozone-induced responses ranged from slight stimulation in stem height growth to 13% decrease in dry mass of shoot and 2–16% increase in visible injuries. Daytime stomatal conductance rates were lowered by 14–54% in clone 5 and 9–74% in clone 2, depending on profile. The additional power-weighted analyses revealed that high peak concentrations and exposure shape were important for induction of visible injuries in both clones and reduction in stomatal conductance in clone 5, whereas growth reductions were rather related to total cumulative exposure. The results indicate that profile of ozone exposure, night-time stomatal conductance (24 h flux), and recovery time for defence and compensations reactions should not be ignored in plant response and ozone flux modelling.     

A review of surface ozone in the polar regions

Surface ozone records from ten polar research stations were investigated for the dependencies of ozone on radiative processes, snow-photochemisty, and synoptic and stratospheric transport. A total of 146 annual data records for the Arctic sites Barrow, Alaska; Summit, Greenland; Alert, Canada; Zeppelinfjellet, Norway; and the Antarctic stations Halley, McMurdo, Neumayer, Sanae, Syowa, and South Pole were analyzed. Mean ozone at the Northern Hemisphere (NH) stations (excluding Summit) is ∼5 ppbv higher than in Antarctica. Statistical analysis yielded best estimates for the projected year 2005 median annual ozone mixing ratios, which for the Arctic stations were 33.5 ppbv at Alert, 28.6 ppbv at Barrow, 46.3 ppbv ppb at Summit and 33.7 ppbv at Zeppelinfjellet. For the Antarctic stations the corresponding ozone mixing ratios were 21.6 ppbv at Halley, 27.0 ppbv at McMurdo, 24.9 ppbv at Neumayer, 27.2 ppbv at Sanae, 29.4 ppbv at South Pole, and 25.8 ppbv at Syowa. At both Summit (3212 m asl) and South Pole (2830 m asl), annual mean ozone is higher than at the lower elevation and coastal stations. A trend analysis revealed that all sites in recent years have experienced low to moderate increases in surface ozone ranging from 0.02 to 0.26 ppbv yr⁻¹, albeit none of these changes were found to be statistically significant trends. A seasonal trend analysis showed above-average increases in ozone during the spring and early summer periods for both Arctic (Alert, Zeppelinfjellet) and Antarctic (McMurdo, Neumayer, South Pole) sites. In contrast, at Barrow, springtime ozone has been declining. All coastal stations experience springtime episodes with rapid depletion of ozone in the boundary layer, attributable to photochemically catalyzed ozone depletion from halogen chemistry. This effect is most obvious at Barrow, followed by Alert. Springtime depletion episodes are less pronounced at Antarctic stations. At South Pole, during the Antarctic spring and summer, photochemical ozone production yields frequent episodes with enhanced surface ozone. Other Antarctic stations show similar, though less frequent spring and summertime periods with enhanced ozone. The Antarctic data provide evidence that austral spring and summertime ozone production in Antarctica is widespread, respectively, affects all stations at least through transport events. This ozone production contributes to a several ppbv enhancement in the annual mean ozone over the Antarctic plateau; however, it is not the determining process in the Antarctic seasonal ozone cycle. Although Summit and South Pole have many similarities in their environmental conditions, this ozone production does not appear to be of equal importance at Summit. Amplitudes of diurnal, summertime ozone cycles at these polar sites are weaker than at lower latitude locations. Amplitudes of seasonal ozone changes are larger in the Southern Hemisphere (by ∼5 ppbv), most likely due to less summertime photochemical ozone loss and more transport of ozone-rich air to the Arctic during the NH spring and summer months.     

Evidence of impact of aerosols on surface ozone concentration in Tianjin,China

In this study, we will present evidence that aerosol particles have strong effects on the surface ozone concentration in a highly polluted city in China. The measured aerosol (PM10), UV flux, and O₃ concentrations were analyzed from 1 November (1 Nov) to 7 November (7 Nov) 2005 in Tianjin, China. During this period, the aerosol concentration had a strong day-by-day variation, ranging from 0.2 to 0.6 mg m⁻³. The ozone concentration also shows a strong variability in correlation with the aerosol concentration. During 1 Nov, 2 Nov, 6 Nov, and 7 Nov, the ozone concentration was relatively high (about 30–35 ppbv; defined as a high-ozone period), and during 3 Nov to 5 Nov, the ozone concentration was relatively low (about 5–20 ppbv; defined as a low-ozone period). The analysis of the measurement shows that the ozone concentration is strongly correlated to the measured UV flux. Because there were near cloud-free conditions between 1 Nov and 7 Nov, the variation of the UV flux mainly resulted from the variation of aerosol concentration. The result shows that higher aerosol concentrations produce a lower UV flux and lower ozone concentrations. By contrast, the lower aerosol concentration leads to a higher UV flux and higher ozone concentrations. A chemical mechanism model (NCAR MM) is applied to interpret the measurement. The model result shows that the extremely high aerosol concentration in this polluted city has a very strong impact on photochemical activities and ozone formation. The correlation between aerosol and ozone concentrations appears in a non-linear feature. The O₃ concentration is very sensitive to aerosol loading when aerosol loading is high, and this sensitivity is reduced when aerosol loading is low. For example, the ratio of Δ[O₃]/Δ[AOD] is about −16 ppbv AOD⁻¹ when AOD is less than 2, and is only −4 ppbv AOD⁻¹ when AOD is between 2 and 5. This result implies that a future decrease in aerosol loading could lead to a rapid increase in the O₃ concentration in this region.     

Impact of biogenic VOC emissions on a tropical cyclone-related ozone episode in the Pearl River Delta region,China

For quantitative estimate of biogenic volatile organic compound emissions (BVOCs) in South China and their impact on the regional atmospheric chemistry, a 3-day tropical cyclone-related ozone episode was modeled using chemical transport model CMAQ, which was driven by the mesoscale meteorological model MM5. Hourly biogenic emission inventories were constructed using the Sparse Matrix Operator Kernel Emissions (SMOKE) model. The simulation results show good agreement with observation data in air temperature, ozone and NO_x levels. The estimated biogenic emissions of isoprene, terpene, and other reactive VOCs (ORVOCs) during this tropical cyclone-related episode are 8500, 3400, and 11 300 ton day⁻¹, respectively. The ratio of isoprene to the total BVOCs was 36.4%. Two test runs were carried out with one incorporated biogenic emissions and the other without. The simulations show that Guangdong province, particularly the Pearl River Delta (PRD) region, was the area most reactive to biogenic emissions in South China. More ozone was produced in all layers under 1500 m when biogenic emissions were included in comparison to that without BVOCs. The net formation of ozone from 9:00 to 15:00 h was the highest near the surface and could reach 38 ppb, which include 4 ppb attributed to biogenic impact. The enhanced ozone due to biogenic emissions first appeared in the PRD region and slowly spread to a greater area in South China. Process analysis indicated that the surface ozone budget was dominated by the vertical transport and dry deposition. The horizontal transport and gas-phase chemical production were relatively small in the surface layer. Presumably, ozone was produced in upper layers within the atmospheric boundary layer and convected down to surface where it is destroyed. When BVOCs was included, apart from the enhancement of gas-phase chemical production of ozone, both the surface deposition and vertical transport were also augmented.     

Sensitivity of ozone to summertime climate in the eastern USA: A modeling case study

The goal of this modeling study is to determine how concentrations of ozone respond to changes in climate over the eastern USA. The sensitivities of average ozone concentrations to temperature, wind speed, absolute humidity, mixing height, cloud liquid water content and optical depth, cloudy area, precipitation rate, and precipitating area extent are investigated individually. The simulation period consists of July 12–21, 2001, during which an ozone episode occurred over the Southeast. The ozone metrics used include daily maximum 8 h average O₃ concentration and number of grid cells exceeding the US EPA ambient air-quality standard. The meteorological factor that had the largest impact on both ozone metrics was temperature, which increased daily maximum 8 h average O₃ by 0.34 ppb K⁻¹ on average over the simulation domain. Absolute humidity had a smaller but appreciable effect on daily maximum 8 h average O₃ (−0.025 ppb for each percent increase in absolute humidity). While domain-average responses to changes in wind speed, mixing height, cloud liquid water content, and optical depth were rather small, these factors did have appreciable local effects in many areas. Temperature also had the largest effect on air-quality standard exceedances; a 2.5 K temperature increase led to a 30% increase in the area exceeding the EPA standard. Wind speed and mixing height also had appreciable effects on ozone air-quality standard exceedances.     

A synthesis of AOT40-based response functions and critical levels of ozone for agricultural and horticultural crops

Crop-response data from over 700 published papers and conference proceedings have been analysed with the aim of establishing ozone dose-response functions for a wide range of European agricultural and horticultural crops. Data that met rigorous selection criteria (e.g. field-based, ozone concentrations within European range, full season exposure period) were used to derive AOT40-yield response functions for 19 crops by first converting the published ozone concentration data into AOT40 (AOT40 is the hourly mean ozone concentration accumulated over a threshold ozone concentration of 40 ppb during daylight hours, units ppm h). For any individual crop, there were no significant differences in the linear response functions derived for experiments conducted in the USA or Europe, or for individual cultivars. Three statistically independent groups were identified: ozone sensitive crops (wheat, water melon, pulses, cotton, turnip, tomato, onion, soybean and lettuce); moderately sensitive crops (sugar beet, potato, oilseed rape, tobacco, rice, maize, grape and broccoli) and ozone resistant (barley and fruit represented by plum and strawberry). Critical levels of a 3 month AOT40 of 3 ppm h and a 3.5 month AOT40 of 6 ppm h were derived from the functions for wheat and tomato, respectively.     

Impact of large scale circulation on European summer surface ozone and consequences for modelling forecast

In this study, we investigate the benefit for European ozone simulation of using day-to-day varying chemical boundary conditions produced by a global chemical weather forecast platform instead of climatological monthly means at the frontiers of a regional model. We performed two simulations over Europe using the regional (0.5 × 0.5°) CHIMERE CTM forced by global scale simulations based on the LMDz-INCA CTM. For summer 2005, ozone differences exceeding 20 ppb can be punctually found between these two simulations in the borders of the domain. The mean of the differences ranges between 0 and 3 ppb beyond 15° of the frontiers of the regional model.Correlations with ground-based ozone measurements at more than 400 stations are slightly increased by the use of daily boundary conditions. The simulation of the temporal variability is significantly enhanced in particular for the daily means and daily maxima. As expected, the gain is higher at the borders of the regional domain.The change of percentile distribution shows that the net impact of high temporal resolution boundary conditions is not of major concern for surface ozone peaks which are mainly due to local photochemistry. The use of daily boundary conditions is however necessary to correctly simulate concentrations in the 20–35 ppb range which are of crucial interest for human and vegetation exposure effects.     

Dry deposition of ozone on building materials. Chamber measurements and modelling of the time-dependent deposition

Tests of the dry deposition of ozone to the surfaces of a concrete floor tile and an activated carbon cloth (ACC) sample were performed in a deposition chamber. The time-dependent deposition of ozone to the material surfaces was modelled with an adsorption, desorption, reaction model. This made it possible to find deposition velocities at equilibrium, at t=∞, from shorter time runs of 48 h. The total equilibrium deposition velocity on the concrete floor tile was found to decrease from 0.08(10) to 0.057(10) cm s⁻¹ in three consecutive runs on the same sample, and was found to be 0.137(8) cm s⁻¹ on an ACC. All at a linear airflow velocity of 0.092 cm s⁻¹, RH=50% and T=22°C. Varying the airflow in the deposition chamber, the surface deposition velocity was found to equal to the total deposition velocity for the concrete floor tile. A surface deposition velocity of 0.186(8) cm s⁻¹ was found for the ACC sample. The total real area and the reaction rate constant for the decomposition of ozone was found to be larger, and the adsorption rate constant, the desorption rate constant and the mass of ozone on the surface smaller, on the ACC sample than on the concrete floor tile.     

Benzene,toluene, ozone,NO2 and SO2 measurements in an urban street canyon in Thessaloniki,Greece

Benzene, toluene, sulphur dioxide, ozone and nitrogen dioxide were measured at a mean level of 13.5 m above ground in a narrow, four-lane street canyon (height 30 m, width 20 m) in Thessaloniki, Greece during the period January–July 1997 by means of a commercial differential optical absorption spectrometer (OPSIS DOAS). Primary pollutant levels were found to be 2.5–4.4 times higher during the cold part of the year than during the warm part of the year, the winter/summer ratio increasing with the reaction rate constant with OH for each of the measured species. Ozone, on the other hand, exhibited a winter/summer ratio of 0.36. NO₂ originates from both primary and secondary sources; its winter/summer concentration ratio of 1.4 lies, therefore, between those of primary pollutants and ozone. Pollution levels were influenced considerably by wind speed, while for the street canyon under study wind direction did not influence pollutant levels considerably. While primary pollution was found to decrease with increasing wind speed, ozone increased. Benzene mean levels during the study period were around 6 ppb and hence much higher than the EU annual limit value of 5 μg m⁻³ (1.44 ppb at STP). Toluene mean levels were around 14 ppb and hence also several times above the WHO recommendation of 2 ppb for 24 h. The apportionment of traffic emissions in four time zones used in most inventories in urban airshed models was tested using benzene and toluene measurements at low (<1 m s⁻¹) wind speeds. The agreement between model emissions and calculated emissions apportionment into the four time zones was good, except for Zone D (23:00–1:59), where model inventory emissions were somewhat too low.     

Ozone uptake by citrus trees exposed to a range of ozone concentrations

The Citrus genus includes a large number of species and varieties widely cultivated in the Central Valley of California and in many other countries having similar Mediterranean climates. In the summer, orchards in California experience high levels of tropospheric ozone, formed by reactions of volatile organic compounds (VOC) with oxides of nitrogen (NO_x). Citrus trees may improve air quality in the orchard environment by taking up ozone through stomatal and non-stomatal mechanisms, but they may ultimately be detrimental to regional air quality by emitting biogenic VOC (BVOC) that oxidize to form ozone and secondary organic aerosol downwind of the site of emission. BVOC also play a key role in removing ozone through gas-phase chemical reactions in the intercellular spaces of the leaves and in ambient air outside the plants. Ozone is known to oxidize leaf tissues after entering stomata, resulting in decreased carbon assimilation and crop yield. To characterize ozone deposition and BVOC emissions for lemon (Citrus limon), mandarin (Citrus reticulata), and orange (Citrus sinensis), we designed branch enclosures that allowed direct measurement of fluxes under different physiological conditions in a controlled greenhouse environment. Average ozone uptake was up to 11 nmol s^?1 m^?2 of leaf. At low concentrations of ozone (40 ppb), measured ozone deposition was higher than expected ozone deposition modeled on the basis of stomatal aperture and ozone concentration. Our results were in better agreement with modeled values when we included non-stomatal ozone loss by reaction with gas-phase BVOC emitted from the citrus plants. At high ozone concentrations (160 ppb), the measured ozone deposition was lower than modeled, and we speculate that this indicates ozone accumulation in the leaf mesophyll.     

Synoptic weather patterns and their relationship to high ozone concentrations in the Taichung Basin

Frequent high ozone days (defined as daily maximum ozone concentration ⩾80 ppb) during recent years in the Taichung Basin have caused much concern. High ozone days occur mainly during autumn and spring. Statistically, there is no clear linear relationship between a single meteorological variable and ozone concentration. In this study, data from 1996–2000 has shown that high ozone concentrations occur during two types of synoptic weather patterns. The first type is a continental cyclone emanating from mainland China, the southern part of it swept towards Taiwan by easterly winds. The second pattern is a tropical depression moving northwards toward the region, the northern part of it affecting Taiwan via easterly winds. Both types cover Taiwan with easterly winds, which are blocked by the Central Mountain Ranges (altitude of 2000–3000 m). The ranges create lee cyclogenesis to the west, which is unfavorable for pollutant dispersion and leads to serious air pollution episodes.The statistical results of the synoptic weather patterns in relation to ozone concentrations are based on the 5 yr data (1996–2000). This was obtained from a network of air-pollution monitoring sites in the study area, while the vertical data come from two 3-day tethersonde experimental campaigns conducted during March and October 2000, measuring air pressure, air temperature, relative humidity, wind speed and direction, non-methane hydrocarbons, NO_x and O₃.     

Surface ozone at the Swiss Alpine site Arosa: the hemispheric background and the influence of large-scale anthropogenic emissions

An innovative and effective method using isentropic trajectory analysis based on the residence time of air masses over the polluted region of Europe was successfully applied to categorize surface ozone amounts at Arosa, Switzerland during 1996–1997. The “European representative” background ozone seasonal cycle at Arosa is associated with long-range transport of North Atlantic air masses, and displays the spring maximum–summer minimum with an annual average of 35 ppb. The photochemical ozone production due to the intense large-scale anthropogenic emission over Europe is estimated as high as 20 ppb in summer, whereas it is insignificant in winter. European sources contribute an annual net ozone production of 9–12 ppb at Arosa. Comparison with the selected regional representative site in Western Europe shows similar results indicating that the categorized ozone data at Arosa by this technique could be regarded as a representative for northern hemispheric mid-latitudes.     

Ozone and meteorological boundary-layer conditions at Summit,Greenland, during 3–21 June 2000

The temporal and spatial distributions of boundary-layer ozone were studied during June 2000 at Summit, Greenland, using surface-level measurements and vertical profiling from a tethered balloon platform. Three weeks of continuous ozone surface data, 133 meteorological vertical profile data and 82 ozone vertical profile data sets were collected from the surface to a maximum altitude of 1400 m above ground.The lower atmosphere at Summit was characterized by the prevalence of strong stable conditions with strong surface temperature inversions. These inversions reversed to neutral to slightly unstable conditions between ∼9.00 and 18.00 h local time with the formation of shallow mixing heights of ∼70–250 m above the surface.The surface ozone mixing ratio ranged from 39 to 68 ppbv and occasionally had rapid changes of up to 20 ppb in 12 h. The diurnal mean ozone mixing ratio showed diurnal trends indicating meteorological and photochemical controls of surface ozone. Vertical profiles were within the range of 37–76 ppb and showed strong stratification in the lower troposphere. A high correlation of high ozone/low water vapor air masses indicated the transport of high tropospheric/low stratospheric air into the lower boundary layer. A ∼0.1–3 ppb decline of the ozone mixing ratio towards the surface was frequently observed within the neutrally stable mixed layer during midday hours. These data suggest that the boundary-layer ozone mixing ratio and ozone depletion and deposition to the snowpack are influenced by photochemical processes and/or transport phenomena that follow diurnal dependencies. With 37 ppb of ozone being the lowest mixing ratio measured in all data no evidence was seen for the occurrence of ozone depletion episodes similar to those that have been reported within the boundary layer at coastal Arctic sites during springtime.