Daily Mean O3 Research Articles

Estimation of ground-level O3 concentration in the Yangtze River Delta region based on a high-performance spatiotemporal model MixNet

In recent years, the escalating ozone (O3) concentration has significantly damaged human health. The machine learning models are widely used to estimate ground-level O3 concentrations, but the spatial and temporal features in the data are less considered. To address the issue, this study proposed a novel framework named MixNet to estimate daily O3 concentration from 2020 to 2021 over the Yangtze River Delta. The MixNet utilized image convolution to extract the potential spatial information related to O3 fully. The temporal features were extracted by a Long Short-Term Memory (LSTM). A U-Net, a new jump connection method with an attention mechanism and residual blocks, facilitated a more comprehensive extraction of spatial features in the data. The extracted temporal and spatial features were fused to estimate ground-level O3. Meanwhile, a novel training method was proposed to enhance the accuracy of MixNet. The daily mean O3 maps have high validation results in comparison with ground-level O3 measurement, with R2 (RMSE) of 0.903 (14.511 μg/m3) for sample-based validation, 0.831 (19.036 μg/m3) for site-based validation, and 0.712 (25.108 μg/m3) for time-based validation. The season-average maps indicate that O3 concentration is summer > autumn > spring > winter. The highest value was 137.41 μg/m3 in the summer of 2021 over the Yangtze River Delta urban agglomeration, and the lowest value was 52.73 μg/m3 in winter 2020. The MixNet showed better performance compared with other models, and thus the “point-plane image thinking” will contribute to future studies in developing better methods to estimate atmospheric pollutants.

Ambient ozone at a rural Central European site and its vertical concentration gradient close to the ground.

The representativeness of ambient air quality of an in situ measurement is key in the use and correct interpretation of the measured concentration values. Though the horizontal representativeness aspect is generally not neglected in air pollution studies, a detailed, high-resolution vertical distribution of ambient air pollutant concentrations is rarely addressed. The aim of this study is twofold: (i) to explore the vertical distribution of ground-level ozone (O3) concentrations measured at four heights above the ground-namely at 2, 8, 50, and 230m-and (ii) to examine in detail the vertical O3 concentration gradient in air columns between 2 and 8, 8 and 50, and 50 and 230m above the ground. We use the daily mean O3 concentrations measured continuously at the Košetice station, representing the rural Central European background ambient air quality observed during 2015-2021. We use the semiparametric GAM (generalised additive model) approach (with complexity or roughness-penalised splines implementation) to analyse the data with sufficient flexibility. Our models for both O3 concentrations and O3 gradients use (additive) decomposition into annual trend and seasonality (plus an overall intercept). The seasonal and year-to-year patterns of the modelled O3 concentrations look very similar at first glance. Nevertheless, a more detailed look through O3 gradients shows that they differ substantially with respect to their seasonal and long-term dynamics. The vertical O3 concentration gradient in 2-230m is not uniform but changes substantially with increasing height and shows by far the highest dynamics near the ground between 2 and 8m, differing in both the seasonal and annual aspects for all the air columns inspected. We speculate that non-linear changes of both seasonal and annual components of vertical O3 gradients are due to atmospheric-terrestrial interactions and to meteorological factors, which we will explore in a future study.

The differential impact of air pollutants on acute urticaria and chronic urticaria: a time series analysis.

Several studies have revealed a relationship between short-term exposure to air pollution and the exacerbation of certain skin conditions. This study was developed to expand on these findings by exploring the potential association between exposure to air pollutants including particulate matter, sulfur dioxide, and ozone and the incidence of acute and chronic urticaria in Shenyang, China, from 2016 to 2018. Exposure-response relationships between daily mean concentrations of these airborne pollutants and visits to outpatient dermatological clinics for acute urticaria and chronic urticaria were evaluated via a time series analysis approach using a generalized additive model. This analysis revealed that a 10 μg/m3 increase in daily mean O3_8h concentrations was associated with a 0.36% (95% CI, 0.31-0.41%), 0.35% (95% CI, 0.30-0.40%), and 0.34% (95% CI, 0.29-0.39%) increase in the number of outpatient visits for acute urticaria on that day (lag0), lagging day 1 (lag1), and lagging day 2 (lag2), respectively. O3 levels also had a similar but weaker effect on the frequency of patients seeking outpatient care for chronic urticaria. These analyses also revealed that estimated 0.47% (95% CI, 0.41-0.52%) and 0.46% (95% CI, 0.40-0.51%) increases in dermatological outpatient acute urticaria visits were observed for every 10μg/m3 rise in O3_8h concentrations on cumulative lagging days (lag01 and lag02). Increases in particulate matter (PM2.5, PM10) levels had a similar cumulative effect on patients with chronic urticaria. In summary, these results suggest that short-term O3, PM2.5, and PM10 exposure can increase the risk of acute urticaria and chronic urticaria.

Effect of COVID-19 Response Policy on Air Quality: A Study in South China Context

Mass suspension of anthropogenic activities is extremely rare, the quarantine due to the coronavirus disease 2019 (COVID-19) represents a natural experiment to investigate the impact of anthropogenic activities on air quality. The mitigation of air pollution during the COVID-19 lockdown has been reported from a global perspective; however, the air pollution levels vary in different regions. This study initiated a novel synthesis of multiple-year satellite observations, national ground measurements towards SO2, NO2 and O3 and meteorological conditions to evaluate the impact of the COVID-19 lockdown in Beihai, a specific city in a less developed area in southwest China, to reveal the potential implications of control strategies for air pollution. The levels of the major air pollutants during the COVID-19 lockdown (LP) and during the same period of previous years (SP) were compared and a series of statistical tools were applied to analyze the sources of air pollution in Beihai. The results show that air pollutant levels decreased with substantial diversity during the LP. Satellite-retrieved NO2 and SO2 levels during the LP decreased by 5.26% and 22.06%, while NO2, SO2, PM2.5 and PM10 from ground measurements during the LP were 25.6%, 2.7%, 22.2% and 22.2% lower than during SP, respectively. Ground measured SO2 concentrations during the LP were only 2.7% lower than during the SP, which may be attributed to uninterrupted essential industrial activities, such as power plants. Polar plots analysis shows that NO2 concentrations were strongly associated with local emission sources, such as automobiles and local industry. Additionally, the much lower levels of NO2 concentrations during the LP and the absence of an evening peak may highlight the significant impact of the traffic sector on NO2. The decrease in daily mean O3 concentrations during the LP may be associated with the reduction in NO2 concentrations. Indications in this study could be beneficial for the formulation of atmospheric protection policies.

Trends in ambient O3 concentrations at twelve sites in the Czech Republic over the past three decades: Close inspection of development

Ambient or ground-level ozone (O3), a key player in atmospheric chemistry, has been studied extensively throughout the world in recent decades due to its harmful effects on human health and the environment. The aim of this study is to examine in detail the time patterns of O3 in the Czech Republic, a Central European country with a long–term history of ambient air pollution. We have examined the interplay between annual trends and within-a-year seasonality in daily O3 concentrations at twelve stations representing different environments, geographical regions and altitudes in the Czech Republic in 1993–2018. We base our analysis on a generalised additive model (GAM) framework as a modern regression approach suitable for addressing non-linear trend shapes in a formalised and unified way. In particular, we use penalised splines. The inconsistent behaviour of the stations prior to 1998, two local maxima around 2003 and 2006 and a steady increase in O3 at all sites from 2014 to the present are the main features of our study. A seasonality O3 pattern with a clear maximum in May–June is similar for all stations. In parallel with the O3 concentration changes over the years, the deformation of seasonality profiles over a long period of time occurs, resulting in a shift of the peak O3 concentrations towards later days in the year. Our statistical modelling offers a detailed view of both long-term trend and seasonality components separately. Additionally, in a model with trend*seasonality interaction, we are able to study the gradual deformation of the seasonality profile over the years.Capsule: Our analysis of daily mean O3 concentrations at twelve Czech sites in 1993–2018 revealed a steady increase in O3 from 2014 and deformation of the seasonality O3 profiles over the year.

Causal association between ambient ozone concentration and mortality in Seoul, Korea

BackgroundThe linearity of concentration-response (C-R) curve between ambient ozone (O3) concentration and mortality has been controversial. The aim of the present analysis was to examine the C-R curve between O3 concentration and mortality with a causal framework approach. MethodsWe extracted data of hourly meteorology, hourly O3 concentration and daily non-accidental mortality in Seoul from 2001 to 2009. We divided the dataset into two, odd-number (training set) and even-number years (testing set). Using the training set, we constructed a prediction model from hourly O3 concentration with support vector regression estimating the daily variations of mean O3 concentration caused by sun irradiation, wind speed and direction, controlling temperature, barometric pressure and temporal trend. With this model we predicted variance of daily O3 from the testing set, thus creating an instrumental variable. We analyzed the association between the instrumental variable and daily mortality. We also analyzed the association according to the quartiles of daily mean O3 concentration to examine the linearity of the association. ResultsThe instrumental variable was significantly and negatively associated with daily mortality in the linear model. In the stratified analysis, the negative slope was observed in the lowest quartile and the negative slope of the association diminished as the quartile increased, and the slope became positive over the 3rd quartile (O3 > 23.3 ppb). The interaction between quartiles and instrumental variable was significant (P = 0.0108). ConclusionWe observed unequal effect of exposure to ambient O3 concentration on mortality according to the different ranges of daily mean O3 concentration with a causal framework approach.

Is the recovery of stratospheric O<sub>3</sub> speeding up in the Southern Hemisphere? An evaluation from the first IASI decadal record (2008–2017)

Abstract. In this paper, we present the global fingerprint of recent changes in middle–upper stratosphere (MUSt; <25 hPa) ozone (O3) in comparison with lower stratosphere (LSt; 150–25 hPa) O3 derived from the first 10 years of the IASI/Metop-A satellite measurements (January 2008–December 2017). The IASI instrument provides vertically resolved O3 profiles with very high spatial and temporal (twice daily) samplings, allowing O3 changes to be monitored in these two regions of the stratosphere. By applying multivariate regression models with adapted geophysical proxies on daily mean O3 time series, we discriminate anthropogenic trends from various modes of natural variability, such as the El Niño–Southern Oscillation (ENSO). The representativeness of the O3 response to its natural drivers is first examined. One important finding relies on a pronounced contrast between a positive LSt O3 response to ENSO in the extratropics and a negative one in the tropics, with a delay of 3 months, which supports a stratospheric pathway for the ENSO influence on lower stratospheric and tropospheric O3. In terms of trends, we find an unequivocal O3 recovery from the available period of measurements in winter–spring at middle to high latitudes for the two stratospheric layers sounded by IASI (>∼35∘ N–S in the MUSt and >∼45∘ S in the LSt) as well as in the total columns at southern latitudes (>∼45∘ S) where the increase reaches its maximum. These results confirm the effectiveness of the Montreal Protocol and its amendments and represent the first detection of a significant recovery of O3 concurrently in the lower, in the middle–upper stratosphere and in the total column from one single satellite dataset. A significant decline in O3 at northern mid-latitudes in the LSt is also detected, especially in winter–spring of the Northern Hemisphere. Given counteracting trends in the LSt and MUSt at these latitudes, the decline is not categorical in total O3. When freezing the regression coefficients determined for each natural driver over the whole IASI period but adjusting a trend, we calculate a significant speeding up in the O3 response to the decline of O3-depleting substances (ODSs) in the total column, in the LSt and, to a lesser extent, in the MUSt, at high southern latitudes over the year. Results also show a small significant acceleration of the O3 decline at northern mid-latitudes in the LSt and in the total column over the last few years. That, specifically, needs urgent investigation to identify its exact origin and apprehend its impact on climate change. Additional years of IASI measurements would, however, be required to confirm the O3 change rates observed in the stratospheric layers over the last few years.

Impact of bay breeze and thunderstorm circulations on surface ozone at a site along the Chesapeake Bay 2011–2016

Coastal regions are frequently subject to higher concentrations of ozone (O3) than areas farther inland due to thermally-direct recirculation (e.g., sea/bay breezes) of pollutants during summer months. Along with high O3 concentrations on polluted days, a bay breeze (in this case, the Chesapeake Bay breeze) causes low-level convergence over the land, which can lead to deep convection even on days without any large-scale convective forcing, altering both the local flow pattern and the concentrations and distributions of pollutants. Presented here is a bay breeze and thunderstorm climatology for Edgewood, MD, a coastal site along the Chesapeake Bay, from June, July, and August 2011–2016 between 11 and 19 EST (UTC-5). Using meteorological data from the Maryland Department of the Environment (MDE), bay breezes are identified by an automated detection algorithm, the Bay-breeze Identification Algorithm (BIA), customized for the complex coastline of river inlets within the Chesapeake Bay area. Additionally, thunderstorm vs. non-thunderstorm days are analyzed using gridded Earth Networks lightning data (ENTLN) within an influential radius of the site. The effects of bay breezes and deep convection on local O3 is quantified. The highest daily conditional mean O3 for all years was observed on bay breeze days, and the lowest on days with thunderstorms only (no bay breeze). Additionally, 39% of O3 exceedance days within the analysis time period were also bay breeze days. However, as ozone precursor emissions have fallen, the relationship between bay breezes and high O3 has diminished. While days with thunderstorms only (no bay breeze) were associated with the lowest daily mean O3, there were O3 events on some thunderstorm days, especially if there was also a bay breeze. Twenty-four percent of the total ozone exceedance days had a thunderstorm, demonstrating that thunderstorm days should not be assumed to be clean days, as boundary-layer venting and downdraft mixing depends on the thunderstorm duration and type (e.g., non-frontal/pop-up convection allowed slightly higher O3 concentrations than did frontal convection). These results illustrate the significant role that localized meteorological events that are difficult to predict play in modulating air pollution where many of the world's cities lie – near bodies of water.

Is Non-Linear Concentration-Response Function between Daily Mean O3 Concentration and Mortality Biased?

Background: The linearity of concentration-response (C-R) function between ambient O3 concentration and mortality has been controversial. Previous reports of non-linear C-R function has been suspec...

Ozone time scale decomposition and trend assessment from surface observations in Switzerland

Regulation for ozone (O3) precursor emissions began in Europe around 1990 with control measures primarily targeting industries and traffic. To understand how these measures have affected air quality, it is important to investigate the temporal evolution of tropospheric O3 concentrations in different types of environments. In this study, we analyze long-term trends of the concentrations of O3 and the sum of oxidants Ox (O3 + NO2) in Switzerland for the last 25 years. Statistical decomposition of the observed time series is used to extract the underlying time scales, i.e. the long-term, seasonal and short-term variability. This allows subtraction of the seasonal variation of O3 and Ox from the observations and estimation of long-term changes of de-seasonalized O3 and Ox with reduced uncertainties. A two-regime trend calculation based on the long-term variability accounts for non-monotonic temporal evolution of O3. In addition, adjustment of the higher frequency meteorological influence was applied, based on the time series containing the short-term variability. This led to an uncertainty reduction in the trend estimation, but only by a small factor. We observe that, despite the implementation of regulations and reduction of nitrogen oxides concentrations, for all studied sites daily mean O3 values increased until mid-2000s. Afterwards, a decline or a leveling-off in the concentrations is observed. The start of change in the trend depends on the site type; the more polluted the site, the later is the onset of the change in trend behavior. At locations close to sources, the observed trend can mainly be explained by the reduced titration of O3 by NO due to the strong reductions in nitrogen oxides emissions. At remote locations (such as the high alpine station in Jungfraujoch) that are influenced by hemispheric transport of O3 an increase during 1990s and a decline after early 2000s is observed. The calculated temporal trends exhibit distinct differences depending on the characteristics and pollution burdens of the measurement sites; such differences have become smaller following emission reductions.

Comparison of Different Regularized and Shrinkage Regression Methods to Predict Daily Tropospheric Ozone Concentration in the Grand Casablanca Area

Tropospheric ozone (O3) is one of the pollutants that have a significant impact on human health. It can increase the rate of asthma crises, cause permanent lung infections and death. Predicting its concentration levels is therefore important for planning atmospheric protection strategies. The aim of this study is to predict the daily mean O3 concentration one day ahead in the Grand Casablanca area of Morocco using primary pollutants and meteorological variables. Since the available explanatory variables are multicollinear, multiple linear regressions are likely to lead to unstable models. To counteract the multicollinearity problem, we compared several alternative regression methods: 1) Continuum Regression; 2) Ridge & Lasso Regressions; 3) Principal component regression (PCR); 4) Partial least Square regression & sparse PLS and; 5) Biased Power Regression. The aim is to set up a good prediction model of the daily ozone in the Grand Casablanca area. These models are fitted on a training data set (from the years 2013 and 2014), tested on a data set (from 2015) and validated on yet another data set data (from 2015). The Lasso model showed a better performance for the prediction of ozone concentrations compared to multiple linear regression and its other alternative methods.

Spatial variability of concentrations of gaseous pollutants across the National Capital Region of Delhi, India

This study presents a systematic evaluation of a year-long, continuous, real-time measurements of gaseous pollutants including oxides of nitrogen (NOx = NO + NO2), carbon monoxide (CO), and ozone (O3) in the National Capital Region (NCR) of Delhi. Data are available from seven air quality monitoring stations (AMS) and meteorological parameters were measured at three meteorological observatories (MO). The daily mean concentrations of NOx and CO across NCR of Delhi (average of all seven AMSs) were 37.1 ± 11.6 ppb (NO: 18.3 and NO2: 18.8 ppb), and 2.3 ± 0.6 ppm, respectively. The highest diurnal CO concentrations were observed during midnight (21:00 to 01:00 h LT) due to nocturnal boundary conditions. The daily mean O3 concentration was 37.5 ± 11.0 ppb. The diurnal variations of O3 concentrations were characterized by the high concentrations during the daytime (07:00 to 17:00 h LT: 43.6 ppb) and low concentrations during the late evening and early morning hours (18:00 to 06:00 h LT: 31.1 ppb). From 07:00 h onwards, the O3 concentrations start increasing gradually after sunrise coinciding with the increasing solar radiation. The highest concentrations (51.7 ppb) were observed around 15:00 h. Thereafter, it is decreased. Local sources of gaseous pollutants were identified by analyzing the surface wind patterns. The annual resultant wind vector were 338° (57%) and 288° (27%) at the Hindon and Palam MOs, respectively. Local NO emissions substantially reduced the urban ozone concentrations as seen by the observed untitrated NO concentrations.

Influence of advections of particulate matter from biomass combustion on specific-cause mortality in Madrid in the period 2004-2009.

Approximately, 20 % of particulate and aerosol emissions into the urban atmosphere are of natural origin (including wildfires and Saharan dust). During these natural episodes, PM10 and PM2.5 levels usually exceed World Health Organisation (WHO) health protection thresholds. This study sought to evaluate the possible effect of advections of particulate matter from biomass fuel combustion on daily specific-cause mortality among the general population and the segment aged ≥ 75 years in Madrid. Ecological time-series study in the city of Madrid from January 01, 2004 to December 31, 2009. The dependent variable analysed was daily mortality due to natural (ICD-10:A00-R99), circulatory (ICD-10:I00-I99), and respiratory (ICD-10:J00-J99) causes in the population, both general and aged ≥ 75 years. The following independent and control variables were considered: a) daily mean PM2.5 and PM10 concentrations; b) maximum daily temperature; c) daily mean O3 and NO2 concentrations; d) advection of particulate matter from biomass combustion ( http://www.calima.ws/ ), using a dichotomous variable and e) linear trend and seasonalities. We conducted a descriptive analysis, performed a test of means and, to ascertain relative risk, fitted a model using autoregressive Poisson regression and stratifying by days with and without biomass advection, in both populations. Of the 2192 days analysed, biomass advection occurred on 56, with mean PM2.5 and PM10 values registering a significant increase during these days. PM10 had a greater impact on organic mortality with advection (RRall ages = 1.035 [1.011-1.060]; RR ≥ 75 years = 1.066 [1.031-1.103]) than did PM2.5 without advection (RRall ages = 1.017 [1.009-1.025]; RR ≥ 75 years = 1.012 [1.003-1.022]). Among specific causes, respiratory-though not circulatory-causes were associated with PM10 on days with advection in ≥ 75 year age group. PM10, rather than PM2.5, were associated with an increase in natural cause mortality on days with advection of particulate matter from biomass combustion, particularly in the ≥ 75 year age group.

Short-term effects of daily air pollution on mortality

The daily variations of air pollutants in the Klang Valley, Malaysia, which includes Kuala Lumpur were investigated for its association with mortality counts using time series analysis. This study located in the tropic with much less seasonal variation than typically seen in more temperate climates. Data on daily mortality for the Klang Valley (2000–2006), daily mean concentrations of air pollutants of PM10, SO2, CO, NO2, O3, daily maximum O3 and meteorological conditions were obtained from Malaysian Department of Environment. We examined the association between pollutants and daily mortality using Poisson regression while controlling for time trends and meteorological factors. Effects of the pollutants (Relative Risk, RR) on current-day (lag 0) mortality to seven previous days (lag 7) and the effects of the pollutants from the first two days (lag 01) to the first eight days (lag 07) were determined. We found significant associations in the single-pollutant model for PM10 and the daily mean O3 with natural mortality. For the daily mean O3, the highest association was at lag 05 (RR = 1.0215, 95% CI = 1.0013–1.0202). CO was found not significantly associated with natural mortality, however the RR's of CO were found to be consistently higher than PM10. In spite of significant results of PM10, the magnitude of RR's of PM10 was not important for natural mortality in comparison with either daily mean O3 or CO. There is an association between daily mean O3 and natural mortality in a two-pollutants model after adjusting for PM10. Most pollutants except SO2, were significantly associated with respiratory mortality in a single pollutant model. Daily mean O3 is also important for respiratory mortality, with over 10% of mortality associated with every IQR increased. These findings are noteworthy because seasonal confounding is unlikely in this relatively stable climate, by contrast with more temperate regions.

Surface ozone concentrations in Agra: links with the prevailing meteorological parameters

Measurements of surface ozone (O3), nitric oxide (NO), nitrogen dioxide (NO2), oxides of nitrogen (NOx=NO+NO2) and meteorological parameters have been made at Agra (North Central India, 27°10’N, 78°05’E) in post monsoon and winter season. The diurnal variation in O3 concentration shows daytime in situ photochemical production with diurnal maximum in noon hours ranging from 51 to 54 ppb in post monsoon and from 76 to 82 ppb in winter, while minimum (16–24 ppb) during nighttime and early morning hours. Average 8-h O3 concentration varied from 12.4 to 83.9 ppb. The relationship between meteorological parameters (solar radiation intensity, temperature, relative humidity, wind speed and wind direction) and surface O3 variability was studied using principal component analysis (PCA), multiple linear regression (MLR) and correlation analysis (CA). PCA and MLR of daily mean O3 concentrations on meteorological parameters explain up to 80 % of day to day ozone variability. Correlation with meteorology is strongly emphasized on days having strong solar radiation intensity and longer sunshine time.

Diurnal and phenological variations of O3 and CO2 fluxes of rice canopy exposed to different O3 concentrations

A dynamic chamber system was designed to measure simultaneously the diurnal and phenological canopy ozone (O3) and carbon dioxide (CO2) fluxes in the paddy field under different O3 concentrations (0, 40, 80 and 120 nmol mol−1). On the diurnal timescale, a decreasing trend of canopy O3 flux was observed from morning to evening and the O3 flux increased with increasing O3 concentration, while canopy CO2 flux generally followed the track of photosynthetic active radiation, with higher values at noon except at the end of the growing season when rice was senescent. The constant CO2 flux among different O3 treatments in this experiment suggested that the photosynthesis of the rice canopy was not affected by short-duration (ca. 10 min) O3 exposure of elevated concentration. The daily mean O3 and CO2 fluxes increased with rice growth until the dough stage and the late jointing stage, respectively, then decreased with rice aging. The peak values of O3 flux appeared later than those of CO2 flux because the latter was closely synchronized with the leaf area index of the rice canopy. Diurnal mean canopy O3 flux varied from 18.7 to 43.3 nmol m−2 s−1, and nocturnal mean canopy O3 flux varied from 2.7 to 17.8 nmol m−2 s−1 and from 7.0 to 25.4 nmol m−2 s−1 for the 40 and 80 nmol mol−1 O3 treatments, respectively. The considerable amount of nocturnal O3 flux indicated a significant contribution of non-stomatal factors to canopy O3 uptake. The adjusted Jarvis multiplicative models were used and well parameterized to fit the measured O3 and CO2 fluxes of our rice cultivar from environmental variables. Although more validation work is needed, the present results suggest that the models can be considered as a tool for canopy flux predictions in the paddy field.

Postharvest Performance Evaluation of Plum (Prunus salicina Lindel., `Casselman') Fruit Grown under Three Ozone Concentrations

We investigated the effects of three seasonal atmospheric ozone (0,) concentrations on fruit quality, internal breakdown, weight loss, cuticle structure, and ripening characteristics of plum fruit from 3-year-old `Casselman' trees in the 1991 season. Trees were exposed to 12-hour daily mean O3 concentrations of 0.034 [charcoal-filtered air (CFA)], 0.050 [ambient air (AA)], or 0.094 [ambient plus O3 (AA+O)] μl·liter-1 from bloom to leaf-fall (1 Apr. to31 Oct. 1991). Fruit quality and internal breakdown incidence measured at harvest and after 2, 4, and 6 weeks of storage at 0C were not affected by any of the O3 treatments. Following an ethylene (C2H4) preconditioning treatment, the rate of fruit softening, C2H4 production, and CO, evolution was higher for plums harvested from the AA + O than from those grown in CFA. Weight loss of fruit from the AA + O exceeded that of fruit from CFA and AA. Anatomical studies of mature plums indicated differences in wax deposition and cuticle thickness between fruit grown in AA + O, AA, and CFA. Differences in gas permeability, therefore, may explain the difference in the ripening pattern of `Casselman' plum fruit grown in high atmospheric O3 partial pressures.