Ambient O3 Concentrations Research Articles

Observations and model simulations link stomatal inhibition to impaired hydraulic conductance following ozone exposure in cotton

ABSTRACTOzone (O3) inhibits plant gas exchange and productivity. Vapour phase (gs) and liquid or hydraulic phase (K) conductances to water flux are often correlated as both change with environmental parameters. Exposure of cotton plants to tropospheric O3reducesgsthrough reversible short‐term mechanisms and by irreversible long‐term disruption of biomass allocation to roots which reducesK. We hypothesize that chronic effects of O3on gas exchange can be mediated by effects onKwithout a direct effect of O3ongsor carbon assimilation (A). Experimental observations from diverse field and exposure chamber studies, and simulations with a model of mass and energy transport, support this hypothesis. O3inhibition ofKleads to realistic simulated diurnal courses ofgsthat reproduce observations at low ambient O3concentration and maintain the positive correlation between middaygsandKobserved experimentally at higher O3concentrations. Effects mediated by reducedKmay interact with more rapid responses ofgsandAto yield the observed suite of oxidant impacts on vegetation. The model extends these physiological impacts to the extensive canopy scale. Simulated magnitudes and diurnal time courses of canopy‐scale fluxes of H2O and O3match observations under low ambient concentrations of O3. With greater simulated concentrations of O3during plant development, the model suggests potential reductions of canopy‐scale water fluxes and O3deposition. This could represent a potentially unfavourable positive feedback on tropospheric O3concentrations associated with biosphere–atmosphere exchange.

Photochemical and dynamical processes affecting gaseous H2O2 concentrations in the lower troposphere

Measurements of gas phase H2O2, O3, and SO2 in ambient air, along with other meteorological parameters, were made at the Summit (1.5 km, above mean sea level, amsl) and at the Lodge (0.6 km, amsl) on Whiteface Mountain situated in the Adirondack region of northern New York state. These measurements were made during the summer of 1997 (July‐August) to study the dynamical and chemical processes that control the distribution of H2O2 at a pristine mountainous site. H2O2 and O3 exhibited considerable variability from day to day. Gas phase H2O2 ranged from 0.3 to 4.3 ppb with a mean of 1.2±0.6 at the Summit, whereas at the Lodge the range was 0.10 to 4.6 ppb with a mean of 1.1±0.8. H2O2 exhibited a weak reverse diurnal variation at the Summit, whereas a diurnal variation was observed in the H2O2 levels at the Lodge. The daytime H2O2 concentrations at the Lodge were significantly higher than those at the Summit, while the nighttime values were higher at the Summit. The observed diurnal variations at the Lodge have been explained in terms of photochemical production of H2O2 during the day and scavenging by aqueous aerosols during the nighttime under the influence of a stable nocturnal boundary layer (NBL). Ozone was observed to have a reverse diurnal variation at both sites though the concentrations at the Summit were higher than at the Lodge. Nocturnal maxima in H2O2 and O3 concentrations under certain meteorological conditions were observed at both sites and are believed to be due to transport of these pollutants above the NBL after they are mixed down in the presence of a weak NBL. Two case studies are presented to elucidate the influence of atmospheric chemical and dynamical processes on the ambient concentration of H2O2 and O3. Results of multivariate statistical analysis show that photochemical production is most important in regulating the formation of H2O2 at the Lodge, whereas at the Summit meteorological processes are most important.

Evidence of primary air pollutant reduction and photochemical pollution enhancement after opening a road tunnel to the public

Measurements of local air pollution and road traffic were taken during a period of one month before and after the opening of a road tunnel to the public in nearby Sachseln, a small city located in a rural region (Obwald) at the center of Switzerland. Two 5-day periods of time (before and after the tunnel opening) of equivalent weather and road traffic features were selected for assessing the tunnel impact on air quality. Despite the road traffic across Sachseln dropped by about 50% after the tunnel opening, its daily profile remained unchanged. After a further time split, statistical analyses allowed determining the modal concentrations of air pollutant at daytime and nighttime (background). Truck traffic was found to be the controlling variable of PM10 concentration. After the tunnel was opened, daytime concentration of PM10, PAH and NO(x) dropped by about 50%. On the contrary, the ambient O3 concentration increased by about 35%, probably as a result of reducing NO(x) in a VOC-limited regime for ozone formation.

Comparison of two numerical models on photosynthetic response ofQuercus mongolica leaves to air pollutants

A multiple-regression model is presented for estimating the effect of major air pollutants on net photosynthetic rate (Pn) ofQuercus mongolica leaves, of which visible injury is not shown. Photosynthetic capacity was found to be primarily a function of PPFD, air temperature (T) and ambient ozone (O3) concentration. The negative direction of photosynthetic capacity response to O3 concentration indicates a potential growth reduction ofQ. mongolica due to ambient O3 concentration in the urban areas of Korea. The model was compared with a non-linear regression model including the same variables. We assessed the contribution of variables to two two models of ambient O3 affecting Pn ofQ. mongolica leaves. The mean Pn difference between the models with and without ambient O3 in the multiple-regression was smaller than that in the non-linear regression. The relative contributions of ambient O3 to multiple-regression and non-linear regression were 12.6% and 5.6%, respectively. The results indicate that multiple-regression models can be applicable for qualitative or quantitative assessment of the effect of air pollutants on Pn response of plant leaves, of which visible injury may not be shown in situ. Also, the assessment of ecophysiological effects using numerical models will have a degree of uncertainty associated with the measuring time/period of the field data used in the modelling, as well as the numerical structure of the models.

Ozone and forest trees

Current estimates of forest yield losses attributable to ozone pollution amount to c. 10% over Europe as a whole. This figure is derived from a synthesis of all European studies using trees for which AOT40 exposure values are available. However, the choice of 40 nl l−1 as the threshold concentration for demonstrable effects has led to debate, and this value might not be low enough to predict ozone effects in Scandinavia, where concentrations are lower than in southern Europe, and chronic injury resulting from cumulative exposure is observed. This ‘level I’ approach provides a useful means for mapping physiologically effective concentrations but has significant shortcomings in that it is unable to take environmental conditions into account. In order to produce a mechanism capable of predicting yield losses resulting from ozone pollution at specific sites and for individual species, the effective ozone dose, a product of conductance and concentration, must be calculated. Process‐based models relating the environment (temperature, humidity/saturation deficit, incident light and soil moisture content) to conductance are available for a number of species (oak, Scots pine, Norway spruce, Sitka spruce, beech and poplar). Effective ozone doses could therefore be calculated, and the relationships between effective dose and yield loss could be determined by revisiting existing data for which only concentrations or AOT40 values are available at present. Yield loss must be the growth parameter with which ozone damage is expressed, since visible injury does not necessarily represent severe injury to the plant, whilst visibly unaffected plants may be significantly compromised in terms of biomass accumulation. For conifers, premature needle drop, although indicative of ozone pollution, might not represent a significant reduction in growth since older needles are, functionally, relatively unimportant.When revisiting old experiments the question should also be asked ‘what is an appropriate control treatment’. It is unrealistic to expect ambient O3 concentrations on a global scale to return to pre‐industrialization levels, and therefore the use of a charcoal‐filtered treatment is probably unrealistic. In addition, charcoal filters will remove some of the NOx, and therefore on nutrient deficient soils (typical of many forest soils) the ‘control’ treatment might represent a reduced supply of nitrogen, making it difficult to disentangle the ozone effect per se.The stage has therefore been reached where ‘level II’ ozone exposure mapping (i.e. based on effective dose at the physiological level) is possible for a number of species. As financial considerations become increasingly important, the scientific community must aim to provide better estimates of O3‐induced yield losses so that long‐term environmental audits can be performed.

Photoassimilate allocation and photosynthetic and biochemical characteristics of two alfalfa (Medicago sativa) cultivars of different ozone sensitivities

Ozone (O3) is a major air pollutant that has been reported to affect growth of alfalfa (Medicago sativa L.). In the present study, an O3-sensitive cultivar, 'Apica', and an O3-tolerant cultivar, 'Team', of alfalfa were exposed to multiple levels of ambient O3 concentrations (i.e., 0x, 1x, 1.5x, and 3x) using open-top chambers. The objectives were to characterize O3 sensitivities at the leaf level and to determine whether stomatal or mesophyll conductance could account for the differences in O3 sensitivity. Differences in photoassimilate allocation were also determined using 14CO2. Effects of O3 exposures on leaf characteristics such as leaf area and dry matter; leaf concentrations of chlorophyll, starch, proteins, amino acids, and nutrients; and stomatal density were evaluated. Gas exchange measurements were also performed. Results showed that O3 exposures produced a greater decline in leaf biomass, chlorophyll concentration, and net assimilation rate for 'Apica' compared with 'Team'. For O3 concentrations of 20-40 nL ·L-1, a 7-15% reduction in net assimilation rate was observed for 'Apica' only. With an increase in O3 concentrations, stomatal limitation did not increase correspondingly, and a strong correlation between net assimilation rate, stomatal conductance, and carboxylation efficiency was observed, suggesting a physiological coordination of the O3 stress response at the leaf level. Possibly the decrease in assimilation rate induced by O3 could be associated with the O3 effects at the mesophyll level. For both cultivars, O3 increased the CO2 compensation point of the leaves, and some effects were also noted in terms of leaf mineral concentrations. O3 did not alter photoassimilate allocation between shoots and roots but stimulated their retention in leaves compared with stems. These results suggest that O3 has a major impact on the carbon metabolism of leaves, which could explain differences in O3 sensitivities.Key words: gas exchange, ozone tolerance, alfalfa cultivars, stomata, photoassimilate allocation.

Kinetics of the Gas-Phase Reactions of OH and NO3 Radicals and O3 with the Monoterpene Reaction Products Pinonaldehyde, Caronaldehyde, and Sabinaketone

Using a relative rate method, rate constants have been measured for the gas-phase reactions of OH and NO3 radicals with pinonaldehyde, caronaldehyde and sabinaketone at 296 ± 2 K. The OH radical reaction rate constants obtained are (in units of 10−12 cm3 molecule−1 s−1): pinonaldehyde, 48 ± 8; caronaldehyde, 48 ± 8; and sabinaketone, 5.1 ± 1.4, and the NO3 radical reaction rate constants are (in units of 10−14 cm3 molecule−1 s−1): pinonaldehyde, 2.0 ± 0.9; caronaldehyde, 2.5 ± 1.1; and sabinaketone, 0.036 ± 0.023, where the error limits include the estimated overall uncertainties in the rate constants for the reference compounds. Upper limits to the O3 reaction rate constants were also obtained, of <2 × 10−20 cm3 molecule−1 s−1 for pinonaldehyde and caronaldehyde, and <5 × 10−20 cm3 molecule−1 s−1 for sabinaketone. These reaction rate constants are combined with estimated ambient tropospheric concentrations of OH radicals, NO3 radicals and O3 to calculate tropospheric lifetimes and dominant transformation process(es) of these and other monoterpene reaction products.

Modeling the Interactions of Ozone with Pulmonary Epithelial Lining Fluid Antioxidants

Water soluble antioxidant—ascorbate (AA), urate (UA), and reduced glutathione (GSH)—consumption by ozone (O3) was investigated in a range of pulmonary epithelial lining fluid (ELF) models. Antioxidants were exposed individually and as a composite mixture, with and without human albumin to a range of ambient O3concentrations: 0–1500 ppb using a continually mixed, interfacial exposure setup. We observed the following: (1) UA constituted the most O3-reactive substrate in each of the models examined. Reactivity hierarchies in each were as follows: UA > AA ⪢ GSH (individual antioxidant), UA > AA > GSH (composite antioxidant), and UA ⪢ AA ≈ GSH (composite antioxidant + albumin). (2) Consumption of GSH as a pure antioxidant solution was associated with a 2:1 stoichiometric conversion of GSH to GSSG. This simplistic relationship was lost in the more complex models. (3) Consumption of antioxidants by O3occurred without alteration of sample pH. (4) Protein carbonyl formation was observed when albumin alone was exposed to O3. However, in the presence of the composite antioxidant solution no evidence of this oxidative modification was apparent. These data indicate that GSH does not represent an important substrate for O3. In contrast, UA displays high reactivity consistent with its acting as a sacrificial substrate in the ELF. As UA concentrations are highest in the ELF of the proximal airways, its localization, allied to its reactivity, suggests that it plays important roles, both in conferring protection locally and also by “scrubbing” O3from inhaled air, limiting its penetration to the more sensitive distal lung.

Updated Photochemical Modeling for California's South Coast Air Basin: Comparison of Chemical Mechanisms and Motor Vehicle Emission Inventories

Large uncertainties remain in photochemical models used to relate emissions of VOC and NOx to ambient O3 concentrations. Bias in motor vehicle emission estimates for VOC has been a long-standing concern. An improved Eulerian photochemical model is described and applied to the August 27-28, 1987, period in southern California. The chemical mechanism used in the model is SAPRC93, which predicts peak ozone concentrations 22% higher on average than the LCC mechanisms used previously. A revised motor vehicle emission inventory was developed using gasoline sales and infrared remote sensing data for CO and measured ambient NMOC/CO and NOx/CO ratios. On-road vehicle emissions for the South Coast Air Basin in summer 1987 were estimated to be (1800 plus or minus 400) x 1000 and (710 plus or minus 160) x 1000 kg day-1 for NMOC nad NOx, respectively. These values are 2.4 and 1.0 times, respectively, the corresponding current official inventory estimates (MVEI 7G). Ozone concentrations predicted using the CIT airshed model matched observations more closely when the revised inventory was used in place of official emission estimates. If the vehicle fleet in 1987 were operating with no emission controls, NMOC and NOx emissions would have been 590 x 1000 and 1200 x 1000 kg day-1 respectively. On average, peak predicted ozone concentrations for the controlled vehicle fleet operating in 1987 were 43% lower than values predicted for the uncontrolled vehicle fleet. The peak predicted ozone concentration with the uncontrolled vehicle fleet was 500 ppb. (A)

Observations of gas‐phase hydrogen peroxide at an elevated rural site in New York

Concurrent measurements of gas‐phase hydrogen peroxide and ozone, along with meteorological parameters, were made from July 1 to July 28, 1995, at Whiteface Mountain, a rural, high‐elevation site in the northeastern United States. H2O2 and O3 exhibited variability from day to day, with concentrations ranging from 0.15 to 5.15 parts per billion by volume (ppbv) (mean, 1.61±0.90 ppbv) and from 29 to 108 ppbv (mean, 50.2±14.6 ppbv), respectively. We present three case studies to elucidate the influence of atmospheric chemical and dynamical processes on the ambient concentrations of H2O2 and O3. Elevated levels of these two photochemical oxidants were observed during July 12–14, owing to an intense photochemical activity in the presence of a synoptic high‐pressure system centered over the northeast. Nocturnal maxima in H2O2 and O3 concentrations were observed and are thought to be due to mixing processes injecting air from aloft into the nocturnal boundary layer. While H2O2 showed a weak diurnal variation with a maximum concentration during daytime, O3 showed a weak reverse diurnal variation with nighttime concentrations higher than daytime. A strong positive correlation was observed between H2O2 and O3, suggesting that O3 serves as a source of hydroperoxy radicals (HO2), the precursor of H2O2. The concentration of HO2 in the atmosphere was estimated to be 23 parts per trillion by volume from the observed rate of production of hydrogen peroxide, which is in good agreement with the values reported based on direct measurements and modeling studies.

Effects of elevated O3 and CO2 concentrations on photosynthesis and stomatal conductance in Scots pine

ABSTRACTNaturally regenerated Scots pines (Pinus sylvestris L.), aged 28–30 years old, were grown in open‐top chambers and subjected in situ to three ozone (O3) regimes, two concentrations of CO2, and a combination of O3 and CO2 treatments From 15 April to 15 September for two growing seasons (1994 and 1995). The gas exchanges of current‐year and 1‐year‐old shoots were measured, along with the nitrogen content of needles. In order to investigate the factors underlying modifications in photosynthesis, five parameters linked to photosynthetic performance and three to stomatal conductance were determined. Elevated O3 concentrations led to a significant decline in the CO2 compensation point (Г*), maximum RuP2‐saturated rate of carboxylation (Vcmax), maximum rate of electron transport (Jmax), maximum stomatal conductance (gsmax), and sensitivity of stomatal conductance to changes in leaf‐to‐air vapour pressure difference (∂gs/∂Dv) in both shoot‐age classes. However, the effect of elevated O3 concentrations on the respiration rate in light (Rd) was dependent on shoot age. Elevated CO2(700 μmol mol−1) significantly decreased Jmax and gsmax but increased Rd in 1‐year‐old shoots and the ∂gs/∂Dv in both shoot‐age classes. The interactive effects of O3 and CO2 on some key parameters (e.g. Vcmax and Jmax) were significant. This may be closely related to regulation of the maximum stomatal conductance and stomatal sensitivity induced by elevated CO2. As a consequence, the injury induced by O3 was reduced through decreased ozone uptake in 1‐year‐old shoots, but not in the current‐year shoots. Compared to ambient O3 concentration, reduced O3 concentrations (charcoal‐filtered air) did not lead to significant changes in any of the measured parameters. Compared to the control treatment, calculations showed that elevated O3 concentrations decreased the apparent quantum yield by 15% and by 18%, and the maximum rate of photosynthesis by 21% and by 29% in the current‐year and 1‐year‐old shoots, respectively. Changes in the nitrogen content of needles resulting from the various treatments were associated with modifications in photosynthetic components.

Ozone Sensitivity of Selected Southeastern Landscape Plants

Abstract Twenty-six species and/or cultivars commonly used in landscapes of the southeastern United States were exposed to three ozone (O3) levels for 3-week periods during spring and summer 1994. Thirteen species or cultivars exhibited visible foliar injury at the highest rate, 2.5× ambient, and two cultivars exhibited foliar injury with ambient O3 concentrations. The most sensitive were two cultivars of buddleia or butterfly bush (Buddleia davidii Franch. ‘Black Knight’ and ‘Royal Red’), with visible injury under ambient and 2.5× ambient O3 levels, and ‘White Star’ zinnia (Zinnia angustifolia HBK ‘White Star’) with visible injury observed under 2.5× ambient O3 levels. Seven other cultivars of buddleia and three cultivars of red maple (Acer rubrum L. ‘Autumn Flame’, ‘October Glory’, and ‘Franksred’ (Red SunsetTM)) exhibited minor foliar injury under 2.5× ambient O3 levels. Visible injury was not present on flowering dogwood (Cornus florida L. ‘Stokes Pink’), hollies (Ilex crenata Thunb. ‘Green Luster’, I. cornuta Lindl. &amp; Paxt. ‘Carissa’, I. x attenuate Ashe ‘Fosteri #2’), cultivars of crapemyrtle (Lagerstroemia indica L. ‘Byers Wonderful White’ and ‘Carolina Beauty’), glossy abelia (Abelia x grandiflora (Andre) Rehd.), southern waxmyrtle (Myrica cerifera L.), sawtooth oak (Quercus acutissima Carruth), begonia (Begonia semperflorens-cultorum Hort. ‘Pizzazz Red’), petunia (Petunia x hybrida Hort. Vilm.-Andr. ‘Celebrity Red’), salvia (Salvia splendens Sello. ‘Hotline’), or gomphrena (Gomphrena globosa L. ‘Strawberry Fields’).

Association between Lifetime Ambient Ozone Exposure and Pulmonary Function in College Freshmen—Results of a Pilot Study

Human health effects due to chronic exposure to ozone (O3) have not been established due to problems with exposure assignment and the use of measures of lung function which may not reflect the site of O3toxicity in the lung. We investigated the feasibility of retrospective assessment of O3exposure-relevant covariates and derived lifetime “effective exposure” to ozone. Mid- and end-expiratory flows (FEF25–75%, FEF75%) were regressed against effective exposure and ecological lifetime exposure. A convenience sample of 130 UC Berkeley freshmen, ages 17–21, participated twice in the same tests (residential history, questionnaire, pulmonary function), 5–7 days apart. Students had to be lifelong residents of Northern (SF) or Southern (LA) California. Monthly ambient O3concentrations (OZ) were assigned based on the lifetime residential history. An “effective time” (T) spent in OZ environments was derived for each residence and age stratum (0–2, 3–5, 6–11, 12+) with the use of questions about “total time spent outdoors” and time spent in “moderate” and/or “heavy” activity. Effective exposure was calculated over the lifetime (OZ ×T) of each subject. Ozone metrics used were 8-hr averages (10am–6pm) and “hours above 60 ppb.” FEF25–75% and FEF75% decreased with both effective exposure and ecologic assignment of O3exposure. For a 20 ppb increase (interquartile range) in 8-hr O3, FEF75% decreased 334 ml/sec (95%Cl:11–657 ml/sec), which corresponds to 14% (1.0–28.3%) of the population mean FEF75%. The corresponding effect on FEF25–75% was −420 ml/sec (95%Cl: +46 to −886,P= 0.08) or 7.2% of the mean. Use of time–activity data to define exposure had no impact on estimates. Negative confounding factors were region (SF vs LA), gender, and ethnicity. Lifetime 8-hr average O3concentrations ranged from 16 to 74 ppb with little overlap between regions. There was no evidence for different O3effects across regions. Effects were independent of lifetime mean PM10, NO2, temperature, or humidity. Effects on FEV1 tended to be negative whereas those for FVC, although negative in some models, where inconsistent and small. The strong relationship of lifetime ambient O3on mid- and end-expiratory flows of college freshmen and the lack of association with FEV1 and FVC are consistent with biologic models of chronic effects of O3in the small airways. Since the present study was designed as a pilot study, these findings have to be confirmed in a larger sample that is representative of the target population.

Ozone destruction on solid particles

Ozone (O3) is an important constituent of the Earth atmosphere, either stratosphere, where it has a beneficial role to protect Earth's surface from harmful UV-B radiation, or troposphere where it is considered an air pollutant. We investigated the ozone destruction on solid particles of natural or anthropogenic origin as: silica-gel, pollen, coal fly ash, titanium dioxide with different specific surface (s) and sodium halides (NaCl, NaBr and NaI). The experiments were conducted in a fluidized bed reactor with elevated ambient concentrations of O3 (100 ppb) employed. The results indicate that the destruction of O3 depends upon: sample quantity (silica-gel with equal s), sample surface (TiO2 with different s) and chemical composition (coal fly ash comparative to wood ash). Interesting results were obtained with sodium halides: no effect on O3 concentrations was detected with NaCl, NaBr shows a certain destruction, while NaI removes completely O3 from the air stream. In the experiments with NaI doped NaCl, the destruction of O3 was dependent on NaI quantity.

Effect of O3 on Hydraulic Architecture in Pima Cotton (Biomass Allocation and Water Transport Capacity of Roots and Shoots)

Abstract Pima cotton (Gossypium barbadense L. cv S-6) exhibits foliar injury and yield reduction at ambient concentrations of O3. We tested the hypotheses that O3 reduces the allocation of biomass to the root system, and that this disrupted carbohydrate allocation impairs root hydraulic capacity relative to transpiring leaf area. Both hypotheses are supported, even though leaf area development is itself reduced by O3. Seedlings were grown in pots in greenhouse fumigation chambers and exposed from planting to sinusoidal O3 profiles with peak concentrations of 0, 0.1, 0.2, and 0.3 [mu]L-1 (12-h averages of 0, 0.037, 0.074, and 0.111 [mu]L L-1). At 8 weeks after planting, stem basal diameter, leaf area, and total plant dry weight decreased by 61, 83, and 88%, whereas root/shoot dry weight ratio declined from 0.16 to 0.09 g/g. Hydraulic conductance decreased per plant by 85%, and per unit leaf area by 35%. Conductance of all organs declined per plant, but only root conductance declined per leaf area by 41%. Root resistance increased from 69 to 82% of whole plant resistance, a functional consequence of reduced carbon allocation to roots. Stomatal conductance declined with root hydraulic conductance, protecting short-term leaf water status. Reduced root hydraulic efficiency may mediate O3 injury to whole plants by reducing shoot gas exchange and biomass productivity through the inhibition of water and nutrient acquisition.

Trace gas measurements and air mass classification from a ground station in Taiwan during the PEM‐West A experiment (1991)

Measurements of the ambient concentrations of O3, NOx (NO + NO2), and PAN (peroxyacetyl nitrate) were collected at Kenting, a rural site near the southern end of Taiwan, during the autumn of 1991 as part of the Pacific Exploratory Mission ‐ West A (PEM‐West A) experiment. Two principal component analyses (PCAs) were performed on the data. The results of the PCA indicated that at least three different air mass types were observed at the site, including marine boundary layer, urban plume, and continental outflow (from the Asian mainland). The average concentrations determined for PAN and O3 in the marine boundary layer are 0.03 ± 0.02 and 13 ± 5 parts per billion by volume (ppbv), respectively. In the continental outflow type air masses the average concentrations of PAN and O3 were 0.33 ± 0.41 and 46 ± 7 ppbv, respectively. The data collected indicate that a minimum of 100–150 parts per trillion by volume (pptv) PAN is required in order to affect the production of ozone in the marine boundary layer.

Measurements of nitric oxide flux from an upper coastal plain, North Carolina agricultural soil

Agricultural soil NO flux measurements (using a dynamic chamber technique) were made from 18 August to 1 September 1993 in the Upper Coastal Plain region of North Carolina in an effort to determine the role of natural emissions of NO on rural atmospheric photochemistry. Overall average NO flux rates increased proportionally to the level of applied fertilizer nitrogen in the agricultural soil. The soybean, cotton, and corn field measurements revealed an average NO flux of 1.79 (range −1.0–6.9) ng N m−2 s−1; 3.77 (range −0.1–38.0) ng Nm−2s−1; and 8.05 (range −0.5−52.8) ng N m−2s−1 respectively. There was a positive correlation between NO concentration near the soil surface (∼ 50 cm) and NO flux. A significant negative correlation between NO flux and ambient O3 concentration, however, supports the hypothesis that soil emissions of NO contribute to local production of O3 in rural areas.

Rate constants for the gas‐phase reactions of cis‐3‐Hexen‐1‐ol, cis‐3‐Hexenylacetate, trans‐2‐Hexenal, and Linalool with OH and NO3 radicals and O3 at 296 ± 2 K, and OH radical formation yields from the O3 reactions

AbstractRate constants for the gas‐phase reactions of the four oxygenated biogenic organic compounds cis‐3‐hexen‐1‐ol, cis‐3‐hexenylacetate, trans‐2‐hexenal, and linalool with OH radicals, NO3 radicals, and O3 have been determined at 296 ± 2 K and atmospheric pressure of air using relative rate methods. The rate constants obtained were (in cm3 molecule−1 s−1 units): cis‐3‐hexen‐1‐ol: (1.08 ± 0.22) × 10−10 for reaction with the OH radical; (2.72 ± 0.83) × 10−13 for reaction with the NO3 radical; and (6.4 ± 1.7) × 10−17 for reaction with O3; cis‐3‐hexenylacetate: (7.84 ± 1.64) × 10−11 for reaction with the OH radical; (2.46 ± 0.75) × 10−13 for reaction with the NO3 radical; and (5.4 ± 1.4) × 10−17 for reaction with O3; trans‐2‐hexenal: (4.41 ± 0.94) × 10−11 for reaction with the OH radical; (1.21 ± 0.44) × 10−14 for reaction with the NO3 radical; and (2.0 ± 1.0) × 10−18 for reaction with O3; and linalool: (1.59 ± 0.40) × 10−10 for reaction with the OH radical; (1.12 ± 0.40) × 10−11 for reaction with the NO3 radical; and (4.3 ± 1.6) × 10−16 for reaction with O3. Combining these rate constants with estimated ambient tropospheric concentrations of OH radicals, NO3 radicals, and O3 results in calculated tropospheric lifetimes of these oxygenated organic compounds of a few hours. © 1995 John Wiley &amp; Sons, Inc.

Relationship of antioxidant enzymes to ozone tolerance in branches of mature ponderosa pine (Pinus ponderosa) trees exposed to long‐term, low concentration, ozone fumigation and acid precipitation

Seasonal activity of superoxide dismutase (SOD, EC 1.15.1.1). ascorbate peroxidase (APOD, EC 1.11.1.11) and guaiacol‐oxidizing enzymes (GPODs, EC 1.11.1.7) was examined in needles of 12‐ to 15‐year‐old ponderosa pine (Pinus ponderosa Laws.) trees which received ozone (O3) and acid precipitation treatment. Individual branches were enclosed in branch exposure chambers delivering either charcoal‐filtered (O3‐reduced) air, ambient air, or air with twice ambient (2 x ambient) concentrations of O3. Acid precipitation treatments were rain of pH 3.0 or 5.1 or no rain. Changes in antioxidant enzyme activity were not a consistent response to O3 fumigation or acid precipitation, but when observed, they occurred most often in the O3‐sensitive clone and in symptomatic, fumigated branches. In the second year of fumigation. O3 fleck symptoms appeared on needles of the sensitive clone as early as July and APOD activities were significantly increased by O3 at all sampling dates. In the tolerant clone, antioxidant enzyme activities were not significantly changed by O3 in the first season of fumigation (March to December 1990), not even during an episode when ambient O3 concentrations reached 125 nl 1−1 (240 nl 1−1 in 2x ambient chambers). No foliar symptoms were observed on needles of the tolerant clone during this year. However, in the second year of fumigation (1992), O3 fleck symptoms were observed on the tolerant clone and APOD activities were significantly increased in previous‐year needles. The tolerant clone had SOD, APOD, and GPOD activities at least 40% higher than those of the sensitive clone before fumigation and 65, 178, and 119% higher, respectively, during both years of fumigation. The higher constitutive levels of these enzymes may have protected against foliar injury in 1990, however in 1992 we concluded that the stimulations in antioxidant enzyme activities observed in symptomatic branches of both clones were a consequence of O3 injury. Total (intra‐ and extracellular) activities of the antioxidant enzymes did not appear to be good indicators of O3 tolerance. Phenotypically, the O3‐tolerant clone was much more vigorous and in both years of fumigation, gas exchange rates were 30 to 71% higher than in the sensitive clone (P. D. Anderson, unpublished data). The greater vigor of the tolerant clone may allow more carbon allocation to protective and repair processes which include, but are not restricted to, the turnover of antioxidant enzymes and metabolites.

Atmospheric lifetimes and fates of a series of sesquiterpenes

Using relative rate techniques, rate constants for the gas phase reactions of OH radicals and NO3 radicals with the sesquiterpenes α‐cedrene, α‐copaene, β‐caryophyllene, α‐humulene, and longifolene have been measured at 296±2 K and atmospheric pressure of air. The rate constants obtained (in units of 10−11 cm3 molecule−1 s−1) for the OH radical and NO3 radical reactions, respectively, are α‐cedrene, 6.7±1.4 and 0.82±0.30; α‐copaene, 9.0±1.9 and 1.6±0.6; β‐caryophyllene, 20+5−9 and 1.9±0.8; α‐humulene, 29+7−10 and 3.5±1.3; and longifolene, 4.7±1.0 and 0.068±0.021, where the indicated errors include the estimated overall uncertainties in the rate constants for the reference compounds. These rate constants and those for the corresponding O3 reactions are combined with estimated ambient lower tropospheric concentrations of OH radicals, NO3 radicals, and O3 to allow the lifetimes of these sesquiterpenes with respect to these chemical loss processes to be calculated. The sesquiterpenes studied are all reactive, with calculated overall lifetimes of a few hours or less. In particular, β‐caryophyllene and α‐humulene are highly reactive toward O3 and NO3 radicals, with calculated lifetimes due to these reactions of 1–2 min, and β‐caryophyllene and α‐humulene are therefore not expected to be present in ambient air at measurable concentrations.