Great Smoky Mountains National Park (GRSM) is the most visited National Park in the United States and has the highest levels of biodiversity of any park unit. It is a relatively small park (similar to 210,433 ha), but topographically complex, with an elevational range of 1757 m. The Park has historically been subject to elevated levels of pollutants, including sulfur dioxide (SO2), ozone (O-3), and nitrogen oxides (NOx). Ozone trends are analyzed from 1989 to 2016 for six monitoring sites in and adjacent to GRSM and ranging in elevation from 564 m to 2030 m. Low-elevation sites have minimum O-3 concentrations in early morning and maximum concentrations in mid-to late afternoon. High-elevation sites have flatter profiles, smaller diurnal ranges, and maxima that occur in early evening or at night. The W126(24-h) exposure index increases with elevation up to 823 m, after which it plateaus. W126(24-h) exposures increased between the years 1989-similar to 2002, and have substantially decreased afterwards. The highest 1-h concentration ever recorded in the Park was 135 ppb, which occurred on 25 August 1998. At most sites the maximum 3-month W126(24-h) exposures have shifted from mid-summer to spring (Apr-Jun). Decreases in exposure result primarily from reduction of hourly averaged O3 concentrations >= 60 ppb. Ozone episodes (3 or more consecutive hours when O-3 >= 60 ppb) have decreased in frequency, magnitude, and duration from 1999 to 2016. Decreases in W126 exposures are correlated with lowered NOx emissions from regional TVA power plants and appear to be a direct result of the State Implementation Plan (SIP) call associated with the Clean Air Act Amendments, resulting in the cleanest air in GRSM over the period of record. Lower cumulative W126 O-3 exposures and reduction in high O-3 concentrations appear to be having beneficial effects on the vegetation within the Park.

Apoplastic ascorbate (ASCapo) is an important contributor to the detoxification of ozone (O3). The objective of the study is to explore whether ASCapo is stimulated by elevated O3 concentrations. The detoxification of O3 by ASCapo was quantified in tobacco (Nicotiana L), soybean (Glycine max (L.) Merr.) and poplar (Populus L), which were exposed to charcoal-filtered air (CF) and elevated O3 treatments (E-O3). ASCapo in the three species were significantly increased by E-O3 compared with the values in the filtered treatment. For all three species, E-O3 significantly increased the malondialdehyde (MDA) content and decreased light-saturated rate of photosynthesis (Asat), suggesting that high O3 has induced injury/damage to plants. E-O3 significantly increased redox state in the apoplast (redox stateapo) for all species, whereas no effect on the apoplastic dehydroascorbate (DHAapo) was observed. In leaf tissues, E-O3 significantly enhanced reduced-ascorbate (ASC) and total ascorbate (ASC+DHA) in soybean and poplar, but significantly reduced these in tobacco, indicating different antioxidative capacity to the high O3 levels among the three species. Total antioxidant capacity in the apoplast (TACapo) was significantly increased by E-O3 in tobacco and poplar, but leaf tissue TAC was significantly enhanced only in tobacco. Leaf tissue superoxide anion (O2•-) in poplar and hydrogen peroxide (H2O2) in tobacco and soybean were significantly increased by E-O3. The diurnal variation of ASCapo, with maximum values occurring in the late morning and lower values experienced in the afternoon, appeared to play an important role in the harmful effects of O3 on tobacco, soybean and poplar.

Assessment of spatial and temporal variation in the impacts of ozone on human health, vegetation, and climate requires appropriate metrics. A key component of the Tropospheric Ozone Assessment Report (TOAR) is the consistent calculation of these metrics at thousands of monitoring sites globally. Investigating temporal trends in these metrics required that the same statistical methods be applied across these ozone monitoring sites. The nonparametric Mann-Kendall test (for significant trends) and the Theil-Sen estimator (for estimating the magnitude of trend) were selected to provide robust methods across all sites. This paper provides the scientific underpinnings necessary to better understand the implications of and rationale for selecting a specific TOAR metric for assessing spatial and temporal variation in ozone for a particular impact. The rationale and underlying research evidence that influence the derivation of specific metrics are given. The form of 25 metrics (4 for model-measurement comparison, 5 for characterization of ozone in the free troposphere, 11 for human health impacts, and 5 for vegetation impacts) are described. Finally, this study categorizes health and vegetation exposure metrics based on the extent to which they are determined only by the highest hourly ozone levels, or by a wider range of values. The magnitude of the metrics is influenced by both the distribution of hourly average ozone concentrations at a site location, and the extent to which a particular metric is determined by relatively low, moderate, and high hourly ozone levels. Hence, for the same ozone time series, changes in the distribution of ozone concentrations can result in different changes in the magnitude and direction of trends for different metrics. Thus, dissimilar conclusions about the effect of changes in the drivers of ozone variability (e.g., precursor emissions) on health and vegetation exposure can result from the selection of different metrics.

This study quantifies the present-day global and regional distributions (2010–2014) and trends (2000–2014) for five ozone metrics relevant for short-term and long-term human exposure. These metrics, calculated by the Tropospheric Ozone Assessment Report, are: 4th highest daily maximum 8-hour ozone (4MDA8); number of days with MDA8 > 70 ppb (NDGT70), SOMO35 (annual Sum of Ozone Means Over 35 ppb) and two seasonally averaged metrics (3MMDA1; AVGMDA8). These metrics were explored at ozone monitoring sites worldwide, which were classified as urban or non-urban based on population and nighttime lights data. Present-day distributions of 4MDA8 and NDGT70, determined predominantly by peak values, are similar with highest levels in western North America, southern Europe and East Asia. For the other three metrics, distributions are similar with North–South gradients more prominent across Europe and Japan. Between 2000 and 2014, significant negative trends in 4MDA8 and NDGT70 occur at most US and some European sites. In contrast, significant positive trends are found at many sites in South Korea and Hong Kong, with mixed trends across Japan. The other three metrics have similar, negative trends for many non-urban North American and some European and Japanese sites, and positive trends across much of East Asia. Globally, metrics at many sites exhibit non-significant trends. At 59% of all sites there is a common direction and significance in the trend across all five metrics, whilst 4MDA8 and NDGT70 have a common trend at ~80% of all sites. Sensitivity analysis shows AVGMDA8 trends differ with averaging period (warm season or annual). Trends are unchanged at many sites when a 1995–2014 period is used; although fewer sites exhibit non-significant trends. Over the longer period 1970–2014, most Japanese sites exhibit positive 4MDA8/SOMO35 trends. Insufficient data exist to characterize ozone trends for the rest of Asia and other world regions.

The impacts of surface ozone (O3) on human health and vegetation have prompted O3 precursor emission reductions in the European Union (EU) and United States (US). In contrast, until recently, emissions have increased in East Asia and most strongly in China. As emissions change, the distribution of hourly O3 concentrations also changes, as do the values of exposure metrics. The distribution changes can result in the exposure metric trend patterns changing in a similar direction as trends in emissions (e.g., metrics increase as emissions increase) or, in some cases, in opposite directions. This study, using data from 481 sites (276 in the EU, 196 in the US, and 9 in China), investigates the response of 14 human health and vegetation O3 exposure metrics to changes in hourly O3 concentration distributions over time. At a majority of EU and US sites, there was a reduction in the frequency of both relatively high and low hourly average O3 concentrations. In contrast, for some sites in mainland China and Hong Kong, the middle of the distribution shifted upwards but the low end did not change and for other sites, the entire distribution shifted upwards. The responses of the 14 metrics to these changes at the EU, US, and Chinese sites were varied, and dependent on (1) the extent to which the metric was determined by relatively high, moderate, and low concentrations and (2) the relative magnitude of the shifts occurring within the O3 concentration distribution. For example, the majority of the EU and US sites experienced decreasing trends in the magnitude of those metrics associated with higher concentrations. For the sites in China, all of the metrics either increased or had no trends. In contrast, there were a greater number of sites that had no trend for those metrics determined by a combination of moderate and high O3 concentrations. A result of our analyses is that trends in mean or median concentrations did not appear to be well associated with some exposure metrics applicable for assessing human health or vegetation effects. The identification of shifting patterns in the O3 distribution and the resulting changes in O3 exposure metrics across regions with large emission increases and decreases is an important step in examining the linkage between emissions and exposure metric trends. The results provide insight into the utility of using specific exposure metrics for assessing emission control strategies.

Tropospheric ozone (O3) pollution is a major air quality issue for human health, vegetation, and climate worldwide. Through an extensive literature review, this paper reports robust short-term O3 trends over the last three decades and provides insights on the effect of regional emission control policies on O3 levels. Since the 1990s, anthropogenic O3 precursor emissions have decreased in North America and Europe, while Eastern Asian emissions slightly decreased recently. A reduction in O3 mean concentrations was observed in rural areas (on average 0.23 ppb year−1) in North America and Europe since the 1990s, while slight decreases were recently reported in East Asia. Most studies have reported significant increases in urban areas worldwide (on average + 0.31 ppb year−1) and at regional background stations (on average + 0.15 ppb year−1) since the 1990s. The increase in urban O3 concentrations can be attributed to decreased local NOx emissions, due to, e.g., vehicle emission controls, resulting in lower O3 titration by NO. The global background O3 increase can be driven by the net impacts of climate change, such as an increase in stratospheric O3 inputs, higher CH4 emissions, changing lightning NOx emissions, and weakened NO titration.

We analyze background surface ozone (O-3) concentrations as estimated by coupled GEOS-Chem/CAMx models for 23 monitoring sites across the US at high- and low-elevation, rural and urban locations during 2006. Specifically, we consider hourly contributions from global tropospheric O-3 entering North America, stratospheric O-3 over North America, and natural O-3 formed from continental biogenic, fire, and lightning sources according to CAMx source apportionment calculations. Unlike historical modeled background definitions that reflect the absence of anthropogenic emissions, we define "EmissionsInfluenced Background" (EIB), which includes chemical interactions with anthropogenic emissions and thus reflects "current" background levels at the sites analyzed. We further define global background O-3 (GBO(3)) as the sum of the global tropospheric and stratospheric components and find that higher modeled GBO(3) occurs during the spring at sites across the US. At many of the sites during the spring, fall, and winter months higher GBO(3) is associated with more frequent stratosphere-to-troposphere transport to the surface (SIT-S) events according to independent three-dimensional trajectories based on global meteorological analyses. Patterns of higher spring EIB O-3 are followed by lower values during the summer, due to heightened chemical interaction with anthropogenic sources, which are then followed by rising EIB O-3 during the fall and winter months. For some high-elevation western US sites, this seasonal pattern is less discernible due to relatively small anthropogenic contributions and the high EIB O-3 estimated throughout the year. EIB O-3 at all high-elevation sites contributes a significant proportion to total O-3 throughout the year and throughout the observed total O-3 frequency distribution, while EIB O-3 at most urban sites contributes a major portion to total O-3 during non-summer months and to the midrange concentrations (30-50 ppb) of the frequency distribution. (C) 2013 Elsevier Ltd. All rights reserved.

In this study, we quantify the frequency of stratosphere-troposphere exchange (STE) events that result in ozone (O-3) concentration enhancements (i.e., hourly average concentrations >= 50 ppb) observed at 39 high- and low-elevation monitoring sites in the US during the years 2007-2009. We employ a refined forward trajectory-based approach to address the relationship between stratospheric intrusions and enhancements in hourly average O-3 concentrations. The model is applied to high-resolution European Center for Medium-Range Weather Forecasting (ECMWF) analyses to identify specific days when the potential for stratosphere-to-troposphere transport (Sir) exists to affect surface O-3 levels. Our results indicate that SIT down to the surface (STT-S) frequently contributes to enhanced surface O-3 hourly averaged concentrations at sites across the US, with substantial year-to-year variability. The O-3 concentrations associated with the SIT-S events appear to be large enough to enhance the measured O-3 concentrations during specific months of the year. Months with a statistically significant coincidence between enhanced O-3 concentrations and STT-S occur most frequently at the high-elevation sites in the Intermountain West, as well as at the high-elevation sites in the West and East. These sites exhibit a preference for coincidences during the springtime and in some cases, the summer, fall, and late winter. Besides the high-elevation monitoring sites, low-elevation monitoring sites across the entire US experience enhanced O-3 concentrations coincident with SIT-S events. (C) 2012 Elsevier Ltd. All rights reserved.

Longer-term (i.e., 20–40 years) tropospheric ozone (O3) time series obtained from surface and ozonesonde observations have been analyzed to assess possible changes with time through 2010. The time series have been selected to reflect relatively broad geographic regions and where possible minimize local scale influences, generally avoiding sites close to larger urban areas. Several approaches have been used to describe the changes with time, including application of a time series model, running 15-year trends, and changes in the distribution by month in the O3 mixing ratio. Changes have been investigated utilizing monthly averages, as well as exposure metrics that focus on specific parts of the distribution of hourly average concentrations (e.g., low-, mid-, and high-level concentration ranges). Many of the longer time series (∼30 years) in mid-latitudes of the Northern Hemisphere, including those in Japan, show a pattern of significant increase in the earlier portion of the record, with a flattening over the last 10–15 years. It is uncertain if the flattening of the O3 change over Japan reflects the impact of O3 transported from continental East Asia in light of reported O3 increases in China. In the Canadian Arctic, declines from the beginning of the ozonesonde record in 1980 have mostly rebounded with little overall change over the period of record. The limited data in the tropical Pacific suggest very little change over the entire record. In the southern hemisphere subtropics and mid-latitudes, the significant increase observed in the early part of the record has leveled off in the most recent decade. At the South Pole, a decline observed during the first half of the 35-year record has reversed, and O3 has recovered to levels similar to the beginning of the record. Our understanding of the causes of the longer-term changes is limited, although it appears that in the mid-latitudes of the northern hemisphere, controls on O3 precursors have likely been a factor in the leveling off or decline from earlier O3 increases.

Stratospheric–tropospheric exchange (STE) processes contribute at both high and low-elevation monitoring sites to background ozone (O3) concentrations. This study addresses the importance of stratospheric intrusions contributing to enhanced hourly average surface O3 concentrations (i.e., ≥50ppb) at 12 O3 monitoring stations in the western and northern tier of the US for 2006, 2007, and 2008. The Lagrangian Analysis Tool (LAGRANTO) trajectory model identified specific days when stratosphere-to-troposphere transport was optimal to elevate surface O3 levels. The coincidences between the number of days with a daily maximum hourly average O3 concentration≥50ppb and stratosphere-to-troposphere transport to surface (STT-S>0) were quantified. The high-elevation site at Yellowstone National Park (NP) in Wyoming exhibited the most coincidences (i.e., more than 19 days a month) during the spring and summer for hourly average O3 concentrations≥50ppb with STT-S>0 of the 12 monitoring sites. At this site, the daily maximum hourly springtime average O3 concentrations were usually in the 60–70ppb range. The maximum daily 8-h average concentrations mostly ranged from 50 to 65ppb. At many of the lower-elevation sites, there was a preference for O3 enhancements to be coincident with STT-S>0 during the springtime, although summertime occurrences were sometimes observed. When statistically significant coincidences occurred, the daily maximum hourly average concentrations were mostly in the 50–65ppb range and the daily maximum 8-h average concentrations were usually in the 50–62ppb range. For many cases, the coincidences between the enhancements and the STT-S events occurred over a continuous multiday period. Supplementary observations, such as (1) the greater frequency of O3 concentration enhancements occurring during the springtime versus other times of the year, (2) the elevation dependency of the frequency of enhancements, (3) the year-to-year variability, (4) the timing of the hour-by-hour occurrences of the O3 concentration enhancements within and across monitoring sites, and (5) the detailed analyses of O3 enhancement events at specific sites, provide additional support for our modeling and statistical results. Our analysis provides an important step in better understanding the variability of natural background O3 concentrations. The study has provided insight into stratospheric intrusions, with emphasis on the combined role of quasi-isentropic large-scale advection and mesoscale boundary layer turbulence for stratospheric air influencing enhanced surface O3.

Policy Relevant Background (PRB) ozone concentrations are defined by the United States (U.S.) Environmental Protection Agency (EPA) as those concentrations that would occur in the U.S. in the absence of anthropogenic emissions in continental North America (i.e., the U.S, Canada, and Mexico). Estimates of PRB ozone have had an important role historically in the EPA's human health and welfare risk analyses used in establishing National Ambient Air Quality Standards (NAAQS). The margin of safety for the protection of public health in the ozone rulemaking process has been established from human health risks calculated based on PRB ozone estimates. Sensitivity analyses conducted by the EPA have illustrated that changing estimates of PRB ozone concentrations have a progressively greater impact on estimates of mortality risk as more stringent standards are considered. As defined by the EPA, PRB ozone is a model construct, but it is informed by measurements at relatively remote monitoring sites (RRMS). This review examines the current understanding of PRB ozone, based on both model predictions and measurements at RRMS, and provides recommendations for improving the definition and determination of PRB ozone.

Controlled human laboratory studies have shown that there is a disproportionately greater pulmonary function response from higher hourly average ozone (O3) concentrations than from lower hourly average values and thus, a nonlinear relationship exists between O3 dose and pulmonary function (FEV1) response. The nonlinear dose-response relationship affects the efficacy of the current 8-h O3 standard to describe adequately the observed spirometric response to typical diurnal O3 exposure patterns. We have reanalyzed data from five controlled human response to O3 health laboratory experiments as reported by Hazucha et al. (1992), Adams (2003, 2006a, 2006b), and Schelegle et al. (2009). These investigators exposed subjects to multi-hour variable/stepwise O3 concentration profiles that mimicked typical diurnal patterns of ambient O3 concentrations. Our findings indicate a common response pattern across most of the studies that provides valuable information for the development of a lung function (FEV1)-based alternate form for the O3 standard. Based on our reanalysis of the realistic exposure profiles used in these experiments, we suggest that an alternative form of the human health standard, similar to the proposed secondary (i.e., vegetation) standard form, be considered. The suggested form is an adjusted 5-h cumulative concentration weighted O3 exposure index, which addresses both the delay associated with the onset of response (FEV1 decrement) and the nonlinearity of response (i.e., the greater effect of higher concentrations over the mid- and low-range values) on an hourly basis.

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

In this analysis, we characterize urban and rural ozone (O3) trends across the US for the periods 1980–2008 (29 years) and 1994–2008 (15 years) using three exposure metrics, which summarize daily O3 concentrations to reflect different ways O3 may affect human health and vegetation. We observe that a statistically significant trend at a specific monitoring site, using one exposure metric, does not necessarily result in a similar trend using the other two metrics. The two most common trends among the monitoring sites are either a continuation of negative trending over the 29-year period or a shift from negative to no trend status, indicating a leveling off of the trending. Very few sites exhibit statistically significant increases in the exposure indices. In characterizing the statistically significant changes in the distribution of hourly average O3, we observe subtle statistically significant changes in the lower part of the distribution (i.e., below 50ppb) that are not necessarily captured by the trending patterns associated with the three exposure metrics. Using multisite data from 12 metropolitan cities, we find that as the frequency of higher hourly average concentrations is reduced, the lower hourly average concentrations also move upward toward the mid-level values. The change in the number of the hourly average concentrations in the lower range is consistent with decreased NO scavenging. We recommend assessing possible subtle shifts in O3 concentrations by characterizing changes in the distribution of hourly average concentrations by month. Identifying statistically significant monthly changes in the mid- and low-level hourly average concentrations may provide important information for assessing changes in physical processes associated with global climate change, long-range transport, and the efficacy of models used for emission and risk reductions. Our results indicate that it is important to investigate the change in the trending pattern with time (e.g., moving 15-year trending) in order to assess how year-to-year variability may influence the trend calculation.

Ozone interacts with plant tissue through distinct temporal processes. Sequentially, plants are exposed to ambient O3 that (1) moves through the leaf boundary layer, (2) is taken up into plant tissue primarily through stomata, and (3) undergoes chemical interaction within plant tissue, first by initiating alterations and then as part of plant detoxification and repair. In this paper, we discuss the linkage of the temporal variability of apoplastic ascorbate with the diurnal variability of defense mechanisms in plants and compare this variability with daily maximum O3 concentration and diurnal uptake and entry of O3 into the plant through stomata. We describe the quantitative evidence on temporal variability in concentration and uptake and find that the time incidence for maximum defense does not necessarily match diurnal patterns for maximum O3 concentration or maximum uptake. We suggest that the observed out-of-phase association of the diurnal patterns for the above three processes produces a nonlinear relationship that results in a greater response from the higher hourly average O3 concentrations than from the lower or mid-level values. The fact that these out-of-phase processes affect the relationship between O3 exposure/dose and vegetation effects ultimately impact the ability of flux-based indices to predict vegetation effects accurately for purposes of standard setting and critical levels. Based on the quantitative aspect of temporal variability identified in this paper, we suggest that the inclusion of a diurnal pattern for detoxification in effective flux-based models would improve the predictive characteristics of the models. While much of the current information has been obtained using high O3 exposures, future research results derived from laboratory biochemical experiments that use short but elevated O3 exposures should be combined with experimental results that use ambient-type exposures over longer periods of time. It is anticipated that improved understanding will come from future research focused on diurnal variability in plant defense mechanisms and their relationship to the diurnal variability in ambient O3 concentration and stomatal conductance. This should result in more reliable O3 exposure standards and critical levels.

Using statistical trending on a site-by-site basis of the (1) health-based annual 2nd highest 1-h average concentration and annual 4th highest daily maximum 8-h average concentration and (2) vegetation-based annual seasonally corrected 24-h W126 cumulative exposure index, we have investigated temporal and spatial statistically significant changes that occurred in surface O3 in the United States for the periods 1980–2005 and 1990–2005 and explored whether differences in trending occur depending upon the selection of the exposure metric. Using the trending results, the analyses quantitatively explore the evidence for the higher hourly average O3 concentrations decreasing faster than the mid- and lower-values. Most of the monitoring sites analyzed in our study experienced decreasing or no trends. Few monitoring sites experienced increasing trends. For those monitoring sites with declining O3 levels, an initial pattern of rapid decrease in the higher hourly average concentrations, followed by a much slower decrease in mid-level concentrations was observed. In some cases, we observed shifts from the lower hourly average O3 concentrations to the mid-level values. On a site-by-site basis, the majority of monitoring sites (1) changed from negative trend to no trend, (2) continued a negative trend, or (3) remained in the no trend status, when comparing trends for the 1980–2005 to the 1990–2005 time periods. For all three exposure metrics, approximately 60% of the monitoring sites shifted from negative trending to no trending status. It appeared that all regions of the United States were equally affected by the shift in status. The greatest statistically significant decreases in the 2nd highest 1-h average concentrations and the annual 4th highest daily maximum 8-h average concentration for the two temporal periods occurred in southern California. Monitoring sites in other portions of the United States experienced lesser decreases than this geographic area. In contrast to the two exposure indices, the vegetation-based 24-h W126 O3 cumulative index for 1980–2005 experienced significant declines in the midwestern states and the northeastern United States as well as in southern California. For the 1990–2005 period, monitoring sites in southern California and the northeastern United States experienced the greatest decreases in the W126 exposure metric. Testing for statistically significant changes in the number of hourly average concentrations within specified concentration intervals identified specific months that experienced shifts in the distribution of the hourly average concentrations. We observed that a statistically significant trend at a specific monitoring site, using one exposure index, did not necessarily result in a similar trend using the other two indices. Because different trending patterns were observed when applying the various exposure indices, a careful selection of O3 exposure metrics is required when assessing trends for specific purposes, such as human health, vegetation, and climate change effects.