The influence of south foehn on the ozone mixing ratios at the high alpine site Arosa

Within 2 years of trace gas measurements performed at Arosa (Switzerland, 2030 m above sea level), enhanced ozone mixing ratios were observed during south foehn events during summer and spring (5–10 ppb above the median value). The enhancements can be traced back to ozone produced in the strongly industrialized Po basin as confirmed by various analyses. Backward trajectories clearly show advection from this region during foehn. NO_y versus O₃ correlation and comparison of O₃ mixing ratios between Arosa and Mt. Cimone (Italy, 2165 m asl) suggest that ozone is the result of recent photochemical production (+5.6 ppb on average), either directly formed during the transport or via mixing of air processed in the Po basin boundary layer. The absence of a correlation between air parcel residence times over Europe and ozone mixing ratios at Arosa during foehn events is in contrast to a previous analysis, which suggested such correlation without reference to the origin of the air. In the case of south foehn, the continental scale influence of pollutants emission on ozone at Arosa appears to be far less important than the direct influence of the Po basin emissions. In contrast, winter time displays a different situation, with mean ozone reductions of about 4 ppb for air parcels passing the Po basin, probably caused by mixing with ozone-poor air from the Po basin boundary layer.     

Diurnal and seasonal variation of short-lived radon progeny concentration and atmospheric temporal variations of 210Pb and 7Be in Egypt

During a one-year period, from November 1998 upto October 1999, the atmospheric activity concentrations of the short-lived (²²²Rn)-progeny (²¹⁸Po, ²¹⁴Pb, ²¹⁴Po) were measured every 4 h in the open air, using α-spectrometry. The concentration data of short-lived radon progeny together with meteorological variables (relative humidity, air temperature, and wind speed) were used for a comprehensive regression analysis of daily time variation of radioactivity in the air. The seasonal concentration pattern of all short-lived radon progeny shows the same trend for diurnal variation with higher values at night and early morning hours compared with lower values at noon and in afternoon. The activity concentrations were observed to be higher during the winter months (November–January) than in other seasons. The mean activity concentrations of ²¹⁸Po, ²¹⁴Pb and ²¹⁴Po within the whole year were found to be 6.7±0.8, 4.9±0.5 and 4.4±0.3 Bq m⁻³, respectively.Also, within that time period, approximately 120 samples were analysed to determine the concentrations of the long-lived radon decay product ²¹⁰Pb and the cosmogenic radionuclide ⁷Be using a single-filter technique. The course of ²¹⁰Pb air concentration is characterized by higher values in autumn/winter season and lower values in spring/summer season. The seasonal concentration pattern of ⁷Be reaches regular maximum values in the spring to early summer months. The annual average concentration values of ²¹⁰Pb and ⁷Be have been found to be 0.37±0.06 and 2.0±0.09 mBq m⁻³, respectively. A mean aerosol mass concentration of 36.6±6.2 μg m⁻³ was also determined during the measurements of the long-lived radionuclides. The majority of attached ²¹⁰Pb and ⁷Be were observed at lower aerosol mass concentrations while small fractions of attached activities were found to be associated with the higher mass concentrations.     

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.     

Transport of nitrogen oxides, carbon monoxide and ozone to the Alpine Global Atmosphere Watch stations Jungfraujoch (Switzerland), Zugspitze and Hohenpeissenberg (Germany), Sonnblick (Austria) and Mt. Krvavec (Slovenia)

The Alpine stations Zugspitze, Hohenpeissenberg, Sonnblick, Jungfraujoch and Mt. Krvavec contribute to the Global Atmosphere Watch Programme (GAW) of the World Meteorological Organization (WMO). The aim of GAW is the surveillance of the large-scale chemical composition of the atmosphere. Thus, the detection of air pollutant transport from regional sources is of particular interest. In this paper, the origin of NO_x (measured with a photo-converter), CO and O₃ at the four Alpine GAW stations is studied by trajectory residence time statistics. Although these methods originated during the early 1980s, no comprehensive study of different atmospheric trace gases measured simultaneously at several background observatories in the Alps was conducted up to present.The main NO_x source regions detected by the trajectory statistics are the northwest of Europe and the region covering East Germany, Czech Republic and southeast Poland, whereas the main CO source areas are the central, north eastern and eastern parts of Europe with some gradient from low to high latitudes. Subsiding air masses from west and southwest are relatively poor in NO_x and CO.The statistics for ozone show strong seasonal effects. Near ground air masses are poor in ozone in winter but rich in ozone in summer. The main source for high ozone concentration in winter is air masses that subside from higher elevations, often enhanced by foehn effects at Hohenpeissenberg. During summer, the Mediterranean constitutes an important additional source for high ozone concentrations.Especially during winter, large differences between Hohenpeissenberg and the higher elevated stations are found. Hohenpeissenberg is frequently within the inversion, whereas the higher elevated stations are above the inversion.Jungfraujoch is the only station where the statistics detect an influence of air rich in CO and NO_x from the Po Basin.     

Trends over a 20-year period from 1987 to 2007 in surface ozone at the atmospheric research station,Mace Head,Ireland

Hourly measurements of baseline ozone at the Mace Head Atmospheric Research Station on the Atlantic Ocean coast of Ireland are observed when unpolluted air masses are advected to the station from across the North Atlantic Ocean. Monthly mean ozone mixing ratios in baseline air masses have risen steadily during the 1980s and 1990s reaching unprecedented levels during the early months of 1999. During the 2000s, baseline ozone mixing ratios have shown evidence of decline and stabilisation. Over the entire 20-year 1987–2007 period, the trend in annual baseline ozone has been +0.31±0.12(2−σ) ppb year⁻¹ and is highly statistically significant. Trends have been highest in the spring months and lowest in the summer months, producing a significant increase in the amplitude of the seasonal cycle. Over the shorter 1995–2007 period, we demonstrate how the growth to peak in 1999 and the subsequent decline have been driven by boreal biomass burning events during 1998/1999 and 2002/2003. The 2000s have been characterised by relatively constant baseline ozone and CH₄ levels and these may be a reasonable guide to future prospects, at least in the short term.     

VOC concentration profiles in an ozone non-attainment area: A case study in an urban and industrial complex metroplex in southern Taiwan

VOCs are important precursors of the atmospheric ozone formation species. This study investigated the airborne concentrations of 52 VOCs at two air quality monitoring stations, Daliao and Tzouying, during wintertime in southern Taiwan. Airborne VOCs samples were taken in stainless steel canisters four times per day and analyzed via gas chromatography/mass spectrometry. Maximum increment reactivity (MIR) was used to evaluate the ozone formation potential in this ozone non-attainment region. Toluene, propane, isopentane, propene, n-butane, n-pentane and isoprene contributed 78–79% of the 52 VOCs in Daliao. Toluene, 1-butene, isopentane, propene, propane, n-undecane, and n-butane contributed 71–77% of the 52 VOCs in Tzouying. The VOCs concentrations were higher in Daliao due to the high toluene emissions from a paint plant and a solvent plant in the nearby industrial district. The 24-h VOC concentrations averaged 25 ppb higher in Tzouying than in Daliao. The ozone formation potential of airborne VOCs was 1687–2730 and 1717–2261 μg-O₃/g-VOCs in Daliao and Tzouying, respectively. Ozone concentrations in Tzouying were 44 ppb higher than in Daliao during the 1200–1600 sampling period.     

Transport of SO2 and aerosol over the Yellow sea

Aircraft measurements of air pollutants were made to investigate the characteristic features of long-range transport of sulfur compounds over the Yellow Sea for the periods of 26–27 April and 7–10 November in 1998, and 9–11 April and 19 June in 1999, together with aerosol measurements at the Taean background station in Korea. The overall mean concentrations of SO₂, O₃ and aerosol number in the boundary layer for the observation period ranged 0.1–7.4 ppb 32.1–64.1 ppb and 1.0–143.6 cm⁻³, respectively. It was found that the air mass over the Yellow Sea had a character of both the polluted continental air and clean background air, and the sulfur transport was mainly confined in the atmospheric boundary layer. The median of SO₂ concentration within the boundary layer was about 0.1–2.2 ppb. However, on 8 November, 1998, the mean concentrations of SO₂ and aerosol number increased up to 7.4 ppb and 109.5 cm⁻³, respectively, in the boundary layer, whereas O₃ concentration decreased remarkably. This enhanced SO₂ concentration occurred in low level westerly air stream from China to Korea. Aerosol analyses at the downstream site of Taean in Korea showed 2–3 times higher sulfate concentration than that of other sampling days, indicating a significant amount of SO₂ conversion to non sea-salt sulfate during the long-range transport.     

Monitoring of ozone in a marine environment in Tenerife (Canary Islands)

In the Aguere Valley (in the oceanic boundary layer at Tenerife, 28°N, 16°W, 580 m a.s.l.) the ozone levels were monitored for ambient air quality assessment. Although precursors are emitted in this area, the strong correlation between ozone levels and wind velocity indicates that ozone is transported into the valley from the ocean. The inland ozone supply along the valley is induced by an orographic channelling effect of the northern oceanic air masses. The highest ozone concentrations are mostly recorded during the nocturnal stage under the influence of fresh oceanic air masses, and during high wind speed events. The seasonal cycle is characterised by elevated ozone mixing ratios in the spring (nighttime levels >45 ppbv) and low mixing ratios in the summer (nighttime levels in the range 20–35 ppbv). Back-trajectory analysis shows that the ozone monitored in the Aguere Valley is associated with long-range transport processes. High ozone events in the spring are associated with transport from upper tropospheric levels, both over the North Atlantic-high latitudes (>45°N) and Europe. This downward transport was observed in the western edge of upper tropospheric cyclones, which suggests that the upper tropospheric/low stratospheric ozone sources play a significant role. In summer, ozone is mainly transported from the North Atlantic-high latitudes (>45°N) and from mid- to low-tropospheric levels. In autumn and winter, the high ozone concentrations are transported from sources located a few km above the North Atlantic-high latitudes (>45°N) and over Europe. The Central-North Atlantic (<45°N) and North Africa are not significant sources of ozone. The high spring and lower summer ozone events in the Aguere Valley agree with other North Atlantic ozone observation in the oceanic boundary layer. However, this behaviour contrasts with the high ozone events frequently recorded at Izaña BAPMoN station (located in the free troposphere in Tenerife) during the summer, which have been attributed in the literature to downward transport from upper levels. An intensification of the inversion layer that separates the oceanic boundary layer of the free troposphere during the summer in Canary Islands is interpreted as the cause of this different behaviour between ozone in the Aguere Valley and Izaña BAPMoN station.     

Long-range transport of gaseous and particulate oxidized nitrogen compounds

Measurements of NO₂, PAN, HNO₃ and particulate NO₃⁻ as 24-h mean concentration values have been carried out at a clean air station. The station is situated on the Swedish west coast 40 km south of Gothenburg. Measurements from November 1981 to October 1982 are presented. NO₂ was measured with the Saltzman method after removal of ozone and PAN with a gas Chromatographic method. HNO₃ was sampled in a denuder and particulate NO₃⁻ was sampled on an impregnated filter behind the denuder. The denuder and filter were then analysed by ion chromatography. Episodes of high concentrations occurred several times during the year. On these occasions the air was transported over a long range from the continent. 72-h back trajectories are available for this station at 6-h intervals during the whole period of measurements. Monthly mean concentrations of these four species, together with data on SO₂ and particulate sulphate are presented. Analysis of the data as a function of the direction of the back trajectory and correlations between nitrogen and sulphate compounds are also given.     

Analysis of ozone and VOCs measured in Shanghai: A case study

Shanghai Meteorological Administration has established a volatile organic compounds (VOCs) laboratory and an observational network for VOCs and ozone (O₃) measurements in the city of Shanghai. In this study, the measured VOCs and O₃ concentrations from 15 November (15-Nov) to 26 November (26-Nov) of 2005 in Shanghai show that there are strong day-to-day and diurnal variations. The measured O₃ and VOCs concentrations have very different characterizations between the two periods. During 15-Nov to 21-Nov (defined as the first period), VOCs and O₃ concentrations are lower than the values during 22-Nov to 28-Nov (defined as the second period). There is a strong diurnal variation of O₃ during the second period with maximum concentrations of 40–80 ppbv at noontime, and minimum concentrations at nighttime. By contrast, during the first period, the diurnal variation of O₃ is in an irregular pattern with maximum concentrations of only 20–30 ppbv. The VOC concentrations change rapidly from 30–50 ppbv during the first period to 80–100 ppbv during the second period. Two chemical models are applied to interpret the measurements. One model is a regional chemical/dynamical model (WRF-Chem) and another is a detailed chemical mechanism model (NCAR MM). Model analysis shows that the meteorological conditions are very different between the two periods, and are mainly responsible for the different chemical characterizations of O₃ and VOCs between the two periods. During the first period, meteorological conditions are characterized by cloudy sky and high-surface winds in Shanghai, resulting in a higher nighttime planetary boundary layer (PBL) and faster transport of air pollutants. By contrast, during the second period, the meteorological conditions are characterized by clear sky and weak surface winds, resulting in a lower nighttime PBL and slower transport of air pollutants. The chemical mechanism model calculation shows that different VOC species has very different contributions to the high-ozone concentrations during the second period. Alkane (40 ppbv) and aromatic (30 ppbv) are among the highest VOC concentrations observed in Shanghai. The analysis suggests that the aromatic is a main contributor for the O₃ chemical production in Shanghai, with approximately 79% of the O₃ being produced by aromatic. This analysis implies that future increase in VOC (especially aromatic) emissions could lead to significant increase in O₃ concentrations in Shanghai.     

Increasing interannual and altitudinal ozone mixing ratios in the Catalan Pyrenees

Interannual, seasonal, daily and altitudinal patterns of tropospheric ozone mixing ratios, as well as ozone phytotoxicity and the relationship with NO_x precursors and meteorological variables were monitored in the Central Catalan Pyrenees (Meranges valley and Forest of Guils) over a period of 5 years (2004–2008). Biweekly measurements using Radiello passive samplers were taken along two altitudinal transects comprised of thirteen stations ranging from 1040 to 2300 m a.s.l. Visual symptoms of ozone damage in Bel-W3 tobacco cultivars were evaluated biweekly for the first three years (2004–2006). High ozone mixing ratios, always above forest and vegetation protection AOT40 thresholds, were monitored every year. In the last 14 years, the AOT40 (Apr–Sept.) has increased significantly by 1047 μg m^?3 h per year. Annual means of ozone mixing ratios ranged between 38 and 67 ppb_v (38 and 74 ppb_v during the warm period) at the highest site (2300 m) and increased at a rate of 5.1 ppb_v year^?1. The ozone mixing ratios were also on average 35–38% greater during the warm period and had a characteristic daily pattern with minimum values in the early morning, a rise during the morning and a decline overnight, that was less marked the higher the altitude. Whereas ozone mixing ratios increased significantly with altitude from 35 ppb_v at 1040 m–56 ppb_v at 2300 m (on average for 2004–2007 period), NO₂ mixing ratios decreased with altitude from 5.5 ppb_v at 1040 m–1 ppb_v at 2300 m. The analysis of meteorological variables and NO_x values suggests that the ozone mainly originated from urban areas and was transported to high-mountain sites, remaining aloft in absence of NO. Ozone damage rates increased with altitude in response to increasing O₃ mixing ratios and a possible increase in O₃ uptake due to more favorable microclimatic conditions found at higher altitude, which confirms Bel-W3 as a suitable biomonitor for ozone concentrations during summer time. Compared to the valley-bottom site the annual means of ozone mixing ratios are 37% larger in the higher sites. Thus the AOT40 for the forest and vegetation protection threshold is greatly exceeded at higher sites. This could have substantial effects on plant life at high altitudes in the Pyrenees.     

Behavior, distribution and variability of surface ozone at an arid region in the south of Iberian Peninsula (Seville, Spain)

In order to improve our knowledge of the surface ozone in the south of the Iberian Peninsula, annual, monthly, weekly and daily ozone concentrations have been closely monitored in the Seville metropolitan area highlighting those episodes that exceed the European Ozone Directive. A three-year period (2003-2005) and eight ozone stations were used; five of them located in the city's busiest areas and the rest in adjacent zones ( approximately 25km). In addition, the wind regime was also studied in order to understand the main characteristics of the surface atmospheric dynamics. The lowest ozone concentrations 17-33microgm(-3) took place in January while the highest 57-95microgm(-3) occurred in June. The ozone concentration week-weekend differences from May to September indicate that this phenomenon does not affect the ozone stations analysed. Daily cycles show minimum values between 7:00 and 8:00 UTC and maximum at noon, exceeding 90microgm(-3) during summer months. From March to October the ozone concentrations were above the target value for the protection of human health, especially during the summer months, with values up to 30% over the limit. The information threshold has been exceeded at all ozone stations studied but with greater frequency in the stations far from the city centre. In addition, at these latter stations the alert threshold was also exceeded on six occasions. This study in the city of Seville indicates that the high ozone levels are due to local atmospheric effects, mainly since the ozone air masses may undergo recirculation processes. The ozone is transported to the city from the S-SW, having a major impact in the NE areas.     

Enhanced ozone over western North America from biomass burning in Eurasia during April 2008 as seen in surface and profile observations

During April 2008, as part of the International Polar Year (IPY), a number of ground-based and aircraft campaigns were carried out in the North American Arctic region (e.g., ARCTAS, ARCPAC). The widespread presence during this period of biomass burning effluent, both gaseous and particulate, has been reported. Unusually high ozone readings for this time of year were recorded at surface ozone monitoring sites from northern Alaska to northern California. At Barrow, Alaska, the northernmost point in the United States, the highest April ozone readings recorded at the surface (hourly average values >55 ppbv) in 37 years of observation were measured on April 19, 2008. At Denali National Park in central Alaska, an hourly average of 79 ppbv was recorded during an 8-h period in which the average was over 75 ppbv, exceeding the ozone ambient air quality standard threshold value in the U.S. Elevated ozone (>60 ppbv) persisted almost continuously from April 19–23 at the monitoring site during this event. At a coastal site in northern California (Trinidad Head), hourly ozone readings were >50 ppbv almost continuously for a 35-h period from April 18–20. At several sites in northern California, located to the east of Trinidad Head, numerous occurrences of ozone readings exceeding 60 ppbv were recorded during April 2008. Ozone profiles from an extensive series of balloon soundings showed lower tropospheric features at ～1–6 km with enhanced ozone during the times of elevated ozone amounts at surface sites in western Canada and the U.S. Based on extensive trajectory calculations, biomass burning in regions of southern Russia was identified as the likely source of the observed ozone enhancements. Ancillary measurements of atmospheric constituents and optical properties (aerosol optical thickness) supported the presence of a burning plume at several locations. At two coastal sites (Trinidad Head and Vancouver Island), profiles of a large suite of gases were measured from airborne flask samples taken during probable encounters with burning plumes. These profiles aided in characterizing the vertical thickness of the plumes, as well as confirming that the plumes reaching the west coast of North America were associated with biomass burning events.     

Evaluation of the effect of long-range transport of air pollutants on coastal atmospheric monitoring sites in and around Taiwan

Long-range transport of pollution outflow from Asian mainland has been noticed and expected to play a significant role in Pacific background. Since 1993 the Taiwanese Environmental Protection Administration (TEPA) is conducting ground-based observations of various particulate and gaseous pollutants at 74 monitoring stations in Taiwan. One of these stations, Heng-Chun at the south coast of Taiwan can be considered as a background station with only negligible amounts of local pollution, and another one, Wan-Li at the north coast, predominantly receives air that has not passed over Taiwan, so that background air can be analysed by means of sectorisation. In this work, the sectorised 13-year time series of measurements of CO, SO₂, O₃, NO_x and PM₁₀, from the Wan-Li station are presen and compared to data from the Heng-Chun station and another TEPA background station off the coast of mainland China, Ma-Zu. The CO and O₃ measurements are also compared to data from the Yonaguni station, a Pacific island site, part of the Global Atmospheric Watch (GAW) network.The similarity of the sectorised data from the Wan-Li station with the data of the other station indicates that atmospheric measurements from the Wan-Li site can be used to make inferences about trends in western Pacific background air pollution and the effect of long-range transport of pollutants. The measurement time series from 1993 to 2006 do not indicate a significant trend in the monthly mean O₃ concentrations in accordance with other research about ozone in tropical latitudes. An increasing trend in CO concentrations of 2.8% per annum is observed between 1999 and 2006 for long-range transport to northern Taiwan, and a doubling of the SO₂ and NO_x concentrations observed at the Wan-Li and Heng-Chun sites within the period 2001–2006. SO₂ concentrations are found to quadruple at Ma-Zu within the same period. The data suggest that pollution from the Asian mainland enhances significantly the background air pollution over the Pacific.     

Preliminary study of the determination of ambient carbonyls in Xalapa City,Veracruz, Mexico

A preliminary study of ambient carbonyls was performed in Xalapa City to measure carbonyls in the atmosphere of this City, because it has an explosive increase in population and traffic density, but few industries. The city is located at the eastern flanks of the Sierra Madre Oriental, between 1350 and 1550 m above sea level. Acetone was the most abundant carbonyl in June, followed by formaldehyde and acetaldehyde, whereas acetaldehyde was the most abundant one in November. Higher concentrations were observed in autumn than in spring, probably due to stagnation conditions in autumn and heavy rains from late spring to early autumn. The very high concentrations of acetaldehyde found in November could have been caused by an accidental leak or spill from a truck, since no stationary sources were identified and acetaldehyde concentrations steeply rose and constantly decreased after few days. Moreover, a highly transited highway traverses Xalapa. The most important ozone and carbon monoxide concentrations were below the Mexican Air Quality Standards; 216 μg m⁻³ (0.11 ppm) for 1 h average and 12.6 mg m⁻³ (11 ppm) for 8 h moving average, respectively. The low concentrations of the main carbonyls, compared with the values reported for other urban areas, and of carbon monoxide, seem to indicate that air quality is still satisfactory in Xalapa City.     

Analysis of the effects of combustion emissions and Santa Ana winds on ambient ozone during the October 2007 southern California wildfires

Combustion emissions and strong Santa Ana winds had pronounced effects on patterns and levels of ambient ozone (O₃) in southern California during the extensive wildland fires of October 2007. These changes are described in detail for a rural receptor site, the Santa Margarita Ecological Reserve, located among large fires in San Diego and Orange counties. In addition, O₃ changes are also described for several other air quality monitoring sites in the general area of the fires. During the first phase of the fires, strong, dry and hot northeasterly Santa Ana winds brought into the area clean continental air masses, which resulted in minimal diurnal O₃ fluctuations and a 72-h average concentration of 36.8 ppb. During the second phase of the fires, without Santa Ana winds present and air filled with smoke, daytime O₃ concentrations steadily increased and reached 95.2 ppb while the lowest nighttime levels returned to ～0 ppb. During that period the 8-h daytime average O₃ concentration reached 78.3 ppb, which exceeded the federal standard of 75 ppb. After six days of fires, O₃ diurnal concentrations returned to pre-fire patterns and levels.     

Observational study of ozone pollution at a rural site in the Yangtze Delta of China

Ozone and related trace gases (CO, NO_x, and SO₂) were measured from June 1999 to July 2000 at a rural site in the Yangtze Delta of China, a region of intensive anthropogenic activity. Elevated ozone levels were frequently observed during the study period, with the highest frequency in late spring and early summer. Over a 1 yr period, 21 d were found to have ozone concentrations exceeding the new US 8-h 80 ppb health standard. Calculation of the “SUM06” exposure index also shows relatively high (>15 ppm h) values for each season except winter. At these levels ozone may have adverse effects on human health as well as agricultural crops. Analysis of meteorological data shows that the high ozone days were associated with large-scale stagnation, intense solar radiation, and minimum rainfall. Large-scale back trajectories indicate a slow-moving/re-circulating airmass during the episodic days. Examination of chemical data shows that the observed daytime high ozone concentrations were due to downward mixing of ozone-rich air, in situ photochemical formation, and in some cases, advection to the site of aged plumes. The very high CO levels (and high CO to NO_x ratios) were found to coincide with many of the ozone episodes, suggesting a contribution from sources of emission involving incomplete combustion. It is suggested that the burning of biomass (e.g., biofeuls and crop residues) may be an important source for the observed high CO and O₃ values.     

Inhomogeneities and trends in the surface ozone record (1988–1996) at Jungfraujoch in the Swiss Alps

Ozone data at the Jungfraujoch Observatory (3580 m asl) in the Swiss Alps have been recorded continuously since 1986 in the framework of the Swiss National Air Pollution Monitoring Network (NABEL), operated by the Swiss Federal Laboratories for Materials Testing and Research (EMPA). The long-term ozone record (1988–1996) was examined with regard to potential inhomogeneities using Zugspitze (2960 m asl), a neighbouring alpine site in Germany, as a reference site. The Alexandersson test (for shift) and the Easterling–Peterson test (for trend and shift) were applied to a difference times series, calculated from monthly ozone means at Jungfraujoch and Zugspitze. The Alexandersson test revealed a significant shift discontinuity in November 1989 and the Easterling–Peterson test a trend discontinuity in September 1991. It is assumed that the shift discontinuity may be related to an instrument change, but there is not firm evidence due to lack of detailed information from the instrumental history in the earlier years of the ozone record at Jungfraujoch. Monthly ozone means at Davos (1640 m asl) in eastern Switzerland indicate that part of the trend discontinuity in September 1991 may be of natural origin. Adjustment of the monthly ozone means at Jungfraujoch for the observed shift discontinuity removed the inhomogeneity in November 1989, and reduced the trend discontinuity by a factor of 4.8. A trend analysis on both adjusted (homogenised) and unadjusted monthly ozone means (1988–1996) at Jungfraujoch showed no statistically significant linear trend. The boundaries encompassing the true linear trend are indicated by two linear regression calculations on both adjusted and unadjusted data. Trends in the cold season are positive albeit not significant. The only statistically significant trends are found in October, December, and February for the adjusted data.     

World-wide ambient measurements of peroxyacetyl nitrate (PAN) and implications for plant injury

Peroxyacetyl nitrate (PAN) is a major phytotoxic component of the photochemical oxidant complex in southern California, but its distribution and effects in other parts of the world are very poorly understood. Based on ambient air sampling at Riverside, California, in 1980, and published reports of PAN concentrations from other parts of the world, PAN concentrations in southern California appear to be five to ten times higher than those reported from eastern North America, western Europe, or Japan. PAN in Riverside, California, averaged 8.1 ppb in July, 9.4 ppb in September and 8.8 ppb in October 1980. These values represent the average of 48 measurements made at 15-min intervals between 0800 and 2000 LT. Potentially phytotoxic episodes, defined herein as PAN > 15 ppb for 4 h during the a.m. period or > 25 ppb for 4h in the p.m. period occurred 27 times in Riverside in 1980. These data were similar to PAN concentrations recorded in the mid 1960s in the Los Angeles Basin, indicating that PAN continues to be a serious air pollution problem in southern California. In contrast, PAN normally averages 1 ppb or less outside of southern California, and because of this low concentration, PAN by itself does not appear to be a major cause of photochemical oxidant injury to vegetation in those regions.     

Characteristics of springtime profiles and sources of ozone in the low troposphere over northern Taiwan

To quantify the possible sources of the high ambient ozone concentration in the low troposphere over Taiwan, ozone sounding data from a two-year intensive field measurement program conducted in April and early May of 2004 and 2005 in northern Taiwan has been examined. We found that the vertical ozone distributions and occurrence of enhanced ozone in the lower troposphere (below 6 km) mainly resulted from (1)Type NE: the long-range transport of ozone controlled by the prevailing northeasterly winds below 2 km, (2)Type LO: the local photochemical ozone production process, and (3)Type SW: the strong southwest/westerly winds aloft (2–6 km). In the boundary layer (BL), where Asian continental outflow prevails, the average profile for type NE is characterized by a peak ozone concentration of nearly 65 ppb at about 1500 m altitude. For type LO, high ozone concentration with an average ozone concentration greater than 80 ppb was also found in the BL in the case of stagnant atmospheric and sunny weather conditions dominated. For type SW, significant ozone enhancement with average ozone concentration of 70–85 ppb was found at around 4 km altitude. It is about 10 ppb greater than that of the types NE and LO at the same troposphere layer owing to the contribution of the biomass burning over Indochina. Due to Taiwan's unique geographic location, the complex interaction of these ozone features in the BL and aloft, especially features associated with northeasterly and south/southwesterly winds, have resulted in complex characteristics of ozone distributions in the lower troposphere over northern Taiwan.