Interactive effects of CO2 and O3 on a ponderosa pine plant/litter/soil mesocosm.

To study individual and combined impacts of two important atmospheric trace gases, CO2 and O3, on C and N cycling in forest ecosystems; a multi-year experiment using a small-scale ponderosa pine (Pinus ponderosa Laws.) seedling/soil/litter system was initiated in April 1998. The experiment was conducted in outdoor, sun-lit chambers where aboveground and belowground ecological processes could be studied in detail. This paper describes the approach and methodology used, and presents preliminary data for the first two growing seasons. CO2 treatments were ambient and elevated (ambient + 280 ppm). O3 treatments were elevated (hourly averages to 159 ppb, cumulative exposure > 60 ppb O3, SUM 06 approximately 10.37 ppm h), and a low control level (nearly all hourly averages <40 ppb. SUM 06 approximately 0.07 ppm h). Significant (P < 0.05) individual and interactive effects occurred with elevated CO2 and elevated O3. Elevated CO2 increased needle-level net photosynthetic rates over both seasons. Following the first season, the highest photosynthetic rates were for trees which had previously received elevated O3 in addition to elevated CO2. Elevated CO2 increased seedling stem diameters, with the greatest increase at low O3. Elevated CO2 decreased current year needle % N in the summer. For 1-year-old needles measured in the fall there was a decrease in % N with elevated CO2 at low O3, but an increase in % N with elevated CO2 at elevated O3. Nitrogen fixation (measured by acetylene reduction) was low in ponderosa pine litter and there were no significant CO2 or O3 effects. Neither elevated CO2 nor elevated O3 affected standing root biomass or root length density. Elevated O3 decreased the % N in coarse-fine (1-2 mm diameter) but not in fine (< 1 mm diameter) roots. Both elevated CO2 and elevated O3 tended to increase the number of fungal colony forming units (CFUs) in the AC soil horizon, and elevated O3 tended to decrease bacterial CFUs in the C soil horizon. Thus, after two growing seasons we showed interactive effects of O3 and CO2 in combination, in addition to responses to CO2 or O3 alone for a ponderosa pine plant/litter/soil system.     

Ozone, nitric acid, and ammonia air pollution is unhealthy for people and ecosystems in southern Sierra Nevada, California

Two-week average concentrations of ozone (O₃), nitric acid vapor (HNO₃) and ammonia (NH₃) were measured with passive samplers during the 2002 summer season across the central Sierra Nevada Mountains, California, along the San Joaquin River drainage. Elevated concentrations of the pollutants were determined with seasonal means for individual sites ranging between 62 and 88 ppb for O₃, 1.0-3.8 μg m⁻³ for HNO₃, and 2.6-5.2 μg m⁻³ for NH₃. Calculated O₃ exposure indices were very high, reaching SUM00-191 ppm h, SUM60-151 ppm h, and W126-124 ppm h. Calculated nitrogen (N) dry deposition ranged from 1.4 to 15 kg N ha⁻¹ for maximum values, and 0.4-8 kg N ha⁻¹ for minimum values; potentially exceeding Critical Loads (CL) for nutritional N. The U.S., California, and European 8 h O₃ human health standards were exceeded during 104, 108, and 114 days respectively, indicating high risk to humans from ambient O₃.     

Detecting ozone and demonstrating its phytotoxicity in forested areas of Poland: a pilot study

Ambient concentrations of ozone (O(3)) were measured and O(3) phytotoxicity to tobacco (Nicotiana tabacum L.) was demonstrated in several forest locations in Poland during a pilot study from July-October, 1991. At southern and central locations in Poland, the 24-hour average O(3) concentrations measured with a UV absorption photometer were in the range of 32-55 ppb, and the corresponding 1-hour maxima in the range of 39-83 ppb. At these locations longer period (four to fifteen days) average concentrations were determined using O(3) passive samplers (DGA, Inc.) and were reaching 60 ppb, while at Bialowieza in eastern Poland O(3) concentrations averaged less than 40 ppb. In Szarow, near the Niepolomice Forest in southern Poland, 1-hour O(3) maxima estimated from the data obtained using passive samplers were about 105 ppb in early September. At several locations in southern and central Poland, extensive O(3) injury was determined on O(3)-sensitive Bel W-3 tobacco plants; such injury did not occur in the Bialowieza Forest of eastern Poland. The results of this pilot study indicate that O(3) is present at phytotoxic levels in southern and central Poland.     

Characteristics of urban ground-level ozone in Korea

This study considers the characteristics of ground-level ozone (O3) in five Korean cities over a time period of 6-8 years. The focus of this study is daily maximum 1-hr and 8-hr concentrations. For all the study cities in the period examined, the mean and most of the percentiles (5, 10, 25, 50, 75, 90, and 95) for the daily maximum 1-hr and 8-hr concentrations showed increasing trends, although not all trends were statistically significant. The daily maximum 1-hr and 8-hr concentrations slowly increased during late winter, and peaks were attained during the summer season (from May to September). All the selected cities exhibited a high degree of correlation between their daily maximum 8-hr and 1-hr concentrations. The daily maximum 8-hr concentrations, which were climatologically equivalent to the Korean 1 hr/100 parts per billion (ppb) standard, were higher than the current 8 hr/60 ppb by a difference of 8-16 ppb. Compared with other cities in Korea, Seoul recorded a substantially higher frequency of days and hours with concentrations above 1 hr/100 ppb, and a higher frequency of days with concentrations above 8 hr/60 ppb and 8 hr/80 ppb. Seoul also recorded a substantially higher frequency of hours with concentrations above 1 hr/100 ppb than days with concentrations above 1 hr/100 ppb, implying that on some days severe exceedances persisted for more than one hour per day. During multiple-day episodes a North Pacific High dominated Korea, which is quite typical in Korea during the summer season.     

Ambient ozone in forests of the Central and Eastern European mountains

Ambient ozone (O(3)) concentrations in the forested areas of the Central and Eastern European (CEE) mountains measured on passive sampler networks and in several locations equipped with active monitors are reviewed. Some areas of the Carpathian Mountains, especially in Romania and parts of Poland, as well as the Sumava and Brdy Mountains in the Czech Republic are characterized by low European background concentrations of the pollutant (summer season means approximately 30 ppb). Other parts of the Carpathians, especially the western part of the range (Slovakia, the Czech Republic and Poland), some of the Eastern (Ukraine) and Southern (Romania) Carpathians and the Jizerske Mountains have high O(3) levels with peak values >100 ppb and seasonal means approximately 50 ppb. Large portions of the CEE mountain forests experience O(3) exposures that are above levels recommended for protection of forest and natural vegetation. Continuation of monitoring efforts with a combination of active monitors and passive samplers is needed for developing risk assessment scenarios for forests and other natural areas of the CEE Region.     

Surface ozone at four remote island sites and the preliminary assessment of the exceedances of its critical level in Japan

Analysis of the recent surface ozone data at four remote islands (Rishiri, Oki, Okinawa, and Ogasawara) in Japan indicates that East Asian anthropogenic emissions significantly influence the boundary layer ozone in Japan. Due to these regional-scale emissions, an increase of ozone concentration is observed during fall, winter, and spring when anthropogenically enhanced continental air masses from Siberia/Eurasia arrive at the sites. The O₃ concentrations in the “regionally polluted” continental outflow among sites are as high as 41–46 ppb in winter and 54–61 ppb in spring. Meanwhile, marine air masses from the Pacific Ocean show as low as 13–14 ppb of O₃ at Okinawa and Ogasawara in summer but higher O₃ concentrations, 24–27 ppb, are observed at Oki and Rishiri due to the additional pollution mainly from Japan mainland. The preliminary analysis of the exceedances of ozone critical level using AOT40 and SUM06 exposure indices indicates that the O₃ threshold were exceeded variously among sites and years. The highest AOT40 and SUM06 were observed at Oki in central Japan where the critical levels are distinctly exceeded. In the other years, the O₃ exposures at Oki, Okinawa, and Rishiri are about or slightly higher than the critical levels. The potential risk of crop yields reduction from high level of O₃ exposure in Japan might not be a serious issue during 1990s and at present because the traditional growing season in Japan are during the low O₃ period in summer. However, increases of anthropogenic emission in East Asia could aggravate the situation in the very near future.     

Preliminary background ozone concentrations in the mountain and coastal areas of Bulgaria

Urban and non-urban rural ozone (O3) concentrations are high in Bulgaria and often exceed the European AOT40 ecosystem as well as the AOT60 human health standards. This paper presents preliminary estimates to establish background, non-urban O3 concentrations for the southern region of Bulgaria. Ozone concentrations from three distinctly different sites are presented: a mountain site influenced by mountain-valley wind flow; a coastal site influenced by sea-breeze wind flow; and a 1700-m mountain peak site without 'local' wind flow characteristics. The latter offers the best estimate of 46-50 ppb for a background O3 level. The highest non-urban hourly value, 118 ppb, was measured at the mountain-valley site.     

Analysis of tree rings using inductively coupled plasma mass spectrometry to record fluctuations in a metal pollution episode

Analysis of Acer pseudoplatanus L. (sycamore) tree rings using ICP-MS was used to assess the impact of metal deposition on trees growing in the vicinity of a metal refinery at Prescot, north-west England compared to a reference site at Croxteth 6 km distant receiving minimal deposition. No difference in tree growth between sites was recorded. Large reductions in Cu and Cd deposition since the late 1970s was accompanied by a steep decline in Cd concentrations in the outer rings in trees close to the refinery. A similar reduction in Cu concentrations was less apparent due to a tendency for Cu to increase in the outer rings. Zinc and Ni were higher in xylem at Prescot compared to Croxteth. There was no trend in Zn concentrations at either site although Ni concentrations increased in trees close to the refinery after 1982. Manganese concentrations in xylem were much higher at Prescot; Mn levels declined until the late 1970s and then increased slightly in later years interspersed by large concentration peaks within individual years during the 1980s. There was little change in Mn concentrations in trees at Croxteth during this period. Calcium, Mg and Sr concentrations remained steady or declined slightly in rings formed since 1965 in trees at Croxteth. Concentrations of Ca, Mg and Sr were higher in rings formed prior to the mid 1970s in trees at Prescot, but declined steadily after this period, although peaks in concentrations of each element were recorded in individual years during the 1980s. Phosphorous concentrations in rings increased towards the cambium at Croxteth, although P levels decreased in rings formed after 1982 at Prescot. No difference in K concentrations between sites was recorded. Lead concentrations in xylem at both sites declined steadily in rings formed after 1970, although concentrations were higher at the reference site. Analysis of individual sycamore tree rings appears to record short-term changes in pollution episodes, with little lateral movement of elements occurring. It is suggested that changes in element concentrations in trees close to the refinery are a result of reduced metal deposition combined with increased soil acidity due to reduced buffering capacity of metal ions in rainfall.     

Distribution of ozone and other air pollutants in forests of the Carpathian Mountains in central Europe.

Ozone (O3) concentrations were monitored during the 1997-1999 growing seasons in 32 forest sites of the Carpathian Mountains. At all sites (elevation between 450 and 1320 m) concentrations of O3, nitrogen dioxide (NO2), and sulfur dioxide (SO2) were measured with passive samplers. In addition, in two western Carpathian locations, Vychodna and Gubalówka, ozone was continuously monitored with ultraviolet (UV) absorption monitors. Highest average hourly O3 concentrations in the Vychodna and Guba?ówka sites reached 160 and 200 microg/m3 (82 and 102 ppb), respectively (except for the AOT40 values, ozone concentrations are presented as microg/m3; and at 25 degrees C and 760 mm Hg, 1 microg O3/m3 = 0.51 ppb O3). These sites showed drastically different patterns of diurnal 03 distribution, one with clearly defined peaks in the afternoon and lowest values in the morning, the other with flat patterns during the entire 24-h period. On two elevational transects, no effect of elevation on O3 levels was seen on the first one, while on the other a significant increase of O3 levels with elevation occurred. Concentrations of O3 determined with passive samplers were significantly different between individual monitoring years, monitoring periods, and geographic location of the monitoring sites. Results of passive sampler monitoring showed that high O3 concentrations could be expected in many parts of the Carpathian range, especially in its western part, but also in the eastern and southern ranges. More than four-fold denser network of monitoring sites is required for reliable estimates of O3 distribution in forests over the entire Carpathian range (140 points). Potential phytotoxic effects of O3 on forest trees and understory vegetation are expected on almost the entire territory of the Carpathian Mountains. This assumption is based on estimates of the AOT40 indices for forest trees and natural vegetation. Concentrations of NO2 and SO2 in the entire Carpathian range were typical for this part of Europe and below the expected levels of phytotoxicity.     

Summer-time distribution of air pollutants in Sequoia National Park, California

Concentrations of air pollutants were monitored during the May November 1999 period on a network of forested sites in Sequoia National Park, California. Measurements were conducted with: (1) active monitors for nitric oxide (NO), nitrogen dioxide (NO2) and ozone (O3); (2) honeycomb denuder/filter pack systems for nitric acid vapor (HNO3), nitrous acid vapor (HNO2), ammonia (NH3), sulfur dioxide (SO2), particulate nitrate (NO3-), ammonium (NH4+), and sulfate (SO4(2-)); and (3) passive samplers for O3, HNO3 and NO2. Elevated concentrations of O3 (seasonal means 41-71 ppb), HNO3 (seasonal means 0.4-2.9 microg/m3), NH3 (seasonal means 1.6-4.5 microg/m3), NO3 (1.1-2.0 microg/m3) and NH4+ (1.0-1.9 microg/m3) were determined. Concentrations of other pollutants were low. With increasing elevation and distance from the pollution source area of O3, NH3 and HNO3 concentrations decreased. Ammonia and NH4+ were dominant N pollutants indicating strong influence of agricultural emissions on forests and other ecosystems of the Sequoia National Park.     

Long-term changes of acidifying deposition in Finland (1973-2000)

The long-term changes of acidifying deposition in Finland during the period 1973-2000 were studied using bulk deposition data from 19 stations belonging to the national monitoring network. The regional-scale approach (southern, central and northern Finland) was used for trend assessment with respect to implementation of European sulphur (S) emission reduction amendments involving deposition changes prior to (1973-1985) and after (1986-2000) the agreements (S protocols in 1985 and 1994). There were no marked changes in sulphate deposition between the 1970s and 1980s and consistent trends in 1973-1985 were not observed. Deposition of nitrogen (N) compounds, particularly NO3-N, were increasing between the 1970s and 1980s. Deposition of base cations exhibited a slight decline throughout the 1970s and 1980s. Decrease of calcium and magnesium deposition without corresponding decrease in sulphate resulted in increased acidifying potential (AP) of deposition. Due to successful implementation of S (and N) emission reduction measures, sulphate deposition has decreased substantially (30% in northern and up to 60% in southern Finland) since the late 1980s. N deposition also decreased, but less than S deposition. Base cation deposition has also declined substantially, but this decline appeared to be leveling off during the 1990s, accounting for the decrease of AP in deposition. The observed deposition pattern is in agreement with the on-going biochemical recovery of acidified small Finnish lakes taking place since the early 1990s.     

Consultant Guide/1987

ABSTRACT

This study considers the characteristics of ground-level ozone (O₃) in five Korean cities over a time period of 6-8 years. The focus of this study is daily maximum 1-hr and 8-hr concentrations. For all the study cities in the period examined, the mean and most of the percentiles (5, 10, 25, 50, 75, 90, and 95) for the daily maximum 1-hr and 8hr concentrations showed increasing trends, although not all trends were statistically significant. The daily maximum 1-hr and 8-hr concentrations slowly increased during late winter, and peaks were attained during the summer season (from May to September). All the selected cities exhibited a high degree of correlation between their daily maximum 8-hr and 1-hr concentrations. The daily maximum 8-hr concentrations, which were climatologi-cally equivalent to the Korean 1 hr/100 parts per billion (ppb) standard, were higher than the current 8 hr/60 ppb by a difference of 8-16 ppb. Compared with other cities in Korea, Seoul recorded a substantially higher frequency of days and hours with concentrations above 1 hr/100 ppb, and a higher frequency of days with concentrations above 8 hr/60 ppb and 8 hr/80 ppb. Seoul also recorded a substantially higher frequency of hours with concentrations above 1 hr/100 ppb than days with concentrations above 1 hr/100 ppb, implying that on some days severe exceedances persisted for more than one hour per day. During multiple-day episodes a North Pacific High dominated Korea, which is quite typical in Korea during the summer season.     

Ambient ozone (O3) and adverse crop response: a unified view of cause and effect

This paper presents a cohesive view of the dynamics of ambient O(3) exposure and adverse crop response relationships, coupling the properties of photochemical O(3) production, flux of O(3) from the atmosphere into crop canopies and the crop response per se. The results from two independent approaches ((a) statistical and (b) micrometeorological) were analyzed for understanding cause-effect relationships of the foliar injury responses of tobacco cv Bel-W3 to the exposure dynamics of ambient O(3) concentrations. Similarly, other results from two independent approaches were analyzed in: (1) establishing a micrometeorological relationship between hourly ambient O(3) concentrations and their vertical flux from the air into a natural grassland canopy; and (2) establishing a statistical relationship between hourly ambient O(3) concentrations in long-term, chronic exposures and crop yield reductions. Independent of the approach used, atmospheric conditions appeared to be most conducive and the crop response appeared to be best explained statistically by the cumulative frequency of hourly ambient O(3) concentrations between 50 ppb and 90 ppb (100 and 180 microg m(-3)). In general, this concentration range represents intermediate or moderately enhanced hourly O(3) values in a polluted environment. Further, the diurnal occurrence of this concentration range (often approximately between 0900 and 1600 h in a polluted, agricultural environment) coincided with the optimal CO(2) flux from the atmosphere into the crop canopy, thus high uptake. The frequency of occurrence of hourly O(3) concentrations > 90 ppb (180 microg m(-3)) appeared to be of little importance and such concentrations in general appeared to occur during atmospheric conditions which did not facilitate optimal vertical flux into the crop canopy, thus low uptake. Alternatively, when > 90 ppb (180 microg m(-3)) O(3) concentrations occurred during the 0900-1600 h window, their frequency of occurrence was low in comparison to the 50-90 ppb (100-180 microg m(-3)) range. Based on the overall results, we conclude that if the cumulative frequency of hourly ambient O(3) concentrations between 50-62 ppb (100-124 microg m(-3)) occurred during 53% of the growing season and the corresponding cumulative frequency of hourly O(3) concentrations between 50-74 ppb (100-148 microg m(-3)) occurred during 71% of the growing season, then yield reductions in sensitive crops could be expected, if other factors supporting growth, such as adequate soil moisture are not limiting.     

Assessment of a regulatory model's performance relative to large spatial heterogeneity in observed ozone in Houston, Texas

In Houston, some of the highest measured 8-hr ozone (O3) peaks are characterized by sudden increases in observed concentrations of at least 40 ppb in 1 hr or 60 ppb in 2 hr. Measurements show that these large hourly changes appear at only a few monitors and span a narrow geographic area, suggesting a spatially heterogeneous field of O3 concentrations. This study assessed whether a regulatory air quality model (AQM) can simulate this observed behavior. The AQM did not reproduce the magnitude or location of some of the highest observed hourly O3 changes, and it also failed to capture the limited spatial extent. On days with measured large hourly changes in O3 concentrations, the AQM predicted high O3 over large regions of Houston, resulting in overpredictions at several monitors. This analysis shows that the model can make high O3, but on these days the predicted spatial field suggests that the model had a different cause. Some observed large hourly changes in O3 concentrations have been linked to random releases of industrial volatile organic compounds (VOCs). In the AQM emission inventory, there are several emission events when an industrial point source increases VOC emissions in excess of 10,000 mol/hr. One instance increased predicted downwind O3 concentrations up to 25 ppb. These results show that the modeling system is responsive to a large VOC release, but the timing and location of the release, and meteorological conditions, are critical requirements. Attainment of the O3 standard requires the use of observational data and AQM predictions. If the large observed hourly changes are indicative of a separate cause of high O3, then the model may not include that cause, which might result in regulators enacting control strategies that could be ineffective.     

Vegetation exposure to ozone over the continental United States: Assessment of exposure indices by the Eta-CMAQ air quality forecast model

The main use of air quality forecast (AQF) models is to predict ozone (O₃) exceedances of the primary O₃ standard for informing the public of potential health concerns. This study presents the first evaluation of the performance of the Eta-CMAQ air quality forecast model to predict a variety of widely used seasonal mean and cumulative O₃ exposure indices associated with vegetation using the U.S. AIRNow O₃ observations. These exposure indices include two concentration-based O₃ indices, M7 and M12 (the seasonal means of daytime 7-h and 12-h O₃ concentrations, respectively), and three cumulative exposure-based indices, SUM06 (the sum of all hourly O₃ concentrations ≥ 0.06 ppm), W126 (hourly concentrations weighed by a sigmoidal weighting function), and AOT40 (O₃ concentrations accumulated over a threshold of 40 ppb during daylight hours). During a three-month simulation (July–September 2005), the model over predicted the M7 and M12 values by 8–9 ppb, or a NMB value of 19% and a NME value of 21%. The model predicts a central belt of high O₃ extending from Southern California to Middle Atlantic where the seasonal means, M7 and M12 (the seasonal means of daytime 7-h and 12-h O₃ concentrations), are higher than 50 ppbv. In contrast, the model is less capable of reproducing the observed cumulative indices. For AOT40, SUM06 and W126, the NMB and NME values are two- to three-fold of that for M7, M12 or peak 8-h O₃ concentrations. The AOT40 values range from 2 to 33 ppm h by the model and from 1 to 40 ppm h by the monitors. There is a significantly higher AOT40 value experienced in the United States in comparison to Europe. The domain-wide mean SUM06 value is 14.4 ppm h, which is about 30% higher than W126, and 40% higher than AOT40 calculated from the same 3-month hourly O₃ data. This suggests that SUM06 and W126 represent a more stringent standard than AOT40 if either the SUM06 or the W126 was used as a secondary O₃ standard. Although CMAQ considerably over predicts SUM06 and W126 values at the low end, the model under predicts the extreme high exposure values (>50 ppm h). Most of these extreme high values are found at inland California sites. Based on our analysis, further improvement of the model is needed to better capture cumulative exposure indices.     

Long-term development of inorganic nutrients and chlorophyll alpha in the open northern Baltic Sea

Eutrophication is an ongoing process in most parts of the Baltic Sea. This article reports on the changes during recent decades of several eutrophication-related variables in the open sea areas surrounding Finland (wintertime nutrient concentrations, wintertime nutrient ratios, and summer time chlorophyll alpha concentrations at the surface). The sum of nitrate- and nitrite-nitrogen ([NO3+NO2]-N) was observed to increase nearly fourfold in the Northern Baltic Proper and the Gulf of Finland and almost double in the Bothnian Sea from the 1960s until the 1980s or 1990s. The increase was followed by a decrease, which was modest in the two former subregions. Phosphate-phosphorus (PO4-P) concentrations followed a similar pattern in the Northern Baltic Proper (threefold increase and subsequent slight decrease) and Bothnian Sea (30% increase and subsequent decrease), but increased throughout the study in the Gulf of Finland, with the present concentration being threefold to the measurements made in the early 1970s. The PO4-P concentration decreased throughout the study in the Bothnian Bay. Silicate-silicon (SiO4-Si) concentrations decreased 30-50% from the early 1970s to the late 1990s and increased 20-40% thereafter in the Northern Baltic Proper, the Gulf of Finland, and the Bothnian Sea. Chlorophyll alpha showed an increase of over 150% in the Northern Baltic Proper and the Gulf of Finland from the 1970s until the early 2000s. In the Bothnian Sea the chlorophyll alpha concentration increased more than 180% from the late 1970s until the late 1990s, and decreased thereafter. According to these long-term observations, the Gulf of Finland and Northern Baltic Proper show clear signs of eutrophication, which may be emphasized by hydrographical changes affecting the phytoplankton communities and thus the algal biomass.     

Trends in meteorologically adjusted ozone concentrations in six Kentucky metro areas, 1998-2002

Meteorologically adjusted ozone (O3) concentrations during five recent O3 seasons (1998-2002) were computed for six Kentucky metro areas using a nonlinear regression model originally developed for forecasting ground-level O3 concentrations. The meteorological adjustment procedure was based on modifying actual measured O3 concentrations according to model-predicted responses to climate departures with respect to a reference year. For all six Kentucky metro areas, meteorologically adjusted O3 concentrations declined over the five-year period. The linear best-fit rate of decline in mean adjusted O3 concentrations ranged from 0.9 to 2.6 ppb/yr for these metro areas; the average rate of decline was 1.6 ppb/yr. The rates of decline in meteorologically adjusted extreme value (e.g., 95th percentile) concentrations were approximately the same, but there is greater statistical uncertainty concerning the extreme value trends.     

A numerical analysis of the combined open-top chamber data from the USA and Europe on ambient ozone and negative crop responses

Statistical analysis was performed using selected sets of combined data from the US National Crop Loss Assessment Network and the European Open-Top Chambers Programme to examine the relationships between the occurrences of hourly ambient ozone (O3) concentrations and adverse crop yield responses. The results suggest that the frequency of occurrences of relatively low hourly O3 concentrations ( approximately <35 ppb) are not as important as moderate to higher concentrations in eliciting negative crop biomass responses. They also suggest that daily peak (highest) hourly O3 values ( approximately >90 ppb) may not be as critical, most likely because they frequently do not occur during time periods when conditions that promote atmospheric conductivity (O3 deposition) and plant uptake (O3 absorption) are in coherence.     

Measurements of peroxyacyl nitrates (PANS) in Mexico City: implications for megacity air quality impacts on regional scales

Peroxyacyl nitrates (PANs) were measured using gas chromatography with electron capture detection (GC/ECD) in north central Mexico City during February–March of 1997. Peroxyacetyl nitrate (PAN) was observed to exceed 30 ppb during five days of the study, with peroxypropionyl nitrate (PPN) and peroxybutryl nitrate (PBN) reaching 6 and 1 ppb maximum, respectively. Levels of total PANs typically exceeded 10 ppb during the period of measurement and showed a very strong diurnal variation with PANs maximum during the early afternoon and falling to less than 0.1 ppb during the evening hours. These levels of PANs are the highest reported values in North America (and the world) for an urban center, since levels of approximately 30 ppb were reported during the late 1970s in the Los Angeles area (South Coast Air Basin, Tuazon et al., 1978). Hydrocarbon measurements indicate that the levels of olefins, specifically butenes are significant in Mexico City. A time series taken of source indicator hydrocarbons taken before and during a Mexican National Holiday with reduced automobile traffic clearly show that mobile sources of butenes are as important as liquefied petroleum gas. Observations of 10–40 ppb C methyl-t-butyl ether (MTBE) are consistent with MTBE/gasoline fuel usage as a source of isobutene and formaldehyde. Both these reactive species can lead to increased oxidant and PAN formation. The strong diurnal profiles of PANs are consistent with regional clearing of the Mexico City air basin on a daily basis. Estimates are given using a simple box model calculation for a number of key primary and secondary pollutant emissions from this megacity on an annual basis. These calculations indicate that megacities can be important sources of both primary and secondary pollutants, and that PANs produced in megacity environments are likely to contribute strongly to regional scale ozone and aerosol productions during long range transport.