Lower Tropospheric Ozone Research Articles

The effect of using limited scene-dependent averaging kernels approximations for the implementation of fast observing system simulation experiments targeted on lower tropospheric ozone

Abstract. Practical implementations of chemical OSSEs (Observing System Simulation Experiments) usually rely on approximations of the pseudo-observations by means of a predefined parametrization of the averaging kernels, which describe the sensitivity of the observing system to the target atmospheric species. This is intended to avoid the use of a computationally expensive pseudo-observations simulator, that relies on full radiative transfer calculations. Here we present an investigation on how no, or limited, scene dependent averaging kernels parametrizations may misrepresent the sensitivity of an observing system. We carried out the full radiative transfer calculation for a three-days period over Europe, to produce reference pseudo-observations of lower tropospheric ozone, as they would be observed by a concept geostationary observing system called MAGEAQ (Monitoring the Atmosphere from Geostationary orbit for European Air Quality). The selected spatio-temporal interval is characterised by an ozone pollution event. We then compared our reference with approximated pseudo-observations, following existing simulation exercises made for both the MAGEAQ and GEOstationary Coastal and Air Pollution Events (GEO-CAPE) missions. We found that approximated averaging kernels may fail to replicate the variability of the full radiative transfer calculations. In addition, we found that the approximations substantially overestimate the capability of MAGEAQ to follow the spatio-temporal variations of the lower tropospheric ozone in selected areas, during the mentioned pollution event. We conclude that such approximations may lead to false conclusions if used in an OSSE. Thus, we recommend to use comprehensive scene-dependent approximations of the averaging kernels, in cases where the full radiative transfer is computationally too costly for the OSSE being investigated.

Lower tropospheric ozone at northern midlatitudes: Changing seasonal cycle

At northern midlatitudes the abundance of tropospheric O3 has increased by a factor of approximately 2 since the 1950s. The cause of this increase is generally attributed to increasing anthropogenic precursor emissions, but present chemical and transport models cannot quantitatively reproduce its magnitude. Here we show another manifestation of changes in O3 abundance—a shift of the seasonal cycle at northern midlatitudes so that the observed peak concentrations now appear earlier in the year than in previous decades. The rate of this shift has been 3 to 6 days per decade since the 1970s. We examine possible reasons to explain this shift and suggest it is due to changes in atmospheric transport patterns combined with spatial and temporal changes in emissions. Detailed modeling is necessary to test these hypotheses; this investigation will provide useful guidance for improving global chemistry‐climate models and stringent tests of the model results.

Analysis of the potential of one possible instrumental configuration of the next generation of IASI instruments to monitor lower tropospheric ozone

Abstract. To evaluate the added value brought by the next generation of IASI (Infrared Atmospheric Sounder Interferometer) instruments to monitor lower tropospheric (LT) ozone, we developed a pseudo-observation simulator, including a direct simulator of thermal infrared spectra and a full inversion scheme to retrieve ozone concentration profiles. We based our simulations on the instrumental configuration of IASI and of an IASI-like instrument, with a factor 2 improvement in terms of spectral resolution and radiometric noise. This scenario, that will be referred to as IASI/2, is one possible configuration of the IASI-NG (New Generation) instrument (the configuration called IASI-NG/IRS2) currently designed by CNES (Centre National d'Études Spatiales). IASI-NG is expected to be launched in the 2020 timeframe as part of the EPS-SG (EUMETSAT Polar System-Second Generation, formerly post-EPS) mission. We produced one month (August 2009) of tropospheric ozone pseudo-observations based on these two instrumental configurations. We compared the pseudo-observations and we found a clear improvement of LT ozone (up to 6 km altitude) pseudo-observations quality for IASI/2. The estimated total error is expected to be more than 35% smaller at 5 km, and 20% smaller for the LT ozone column. The total error on the LT ozone column is, on average, lower than 10% for IASI/2. IASI/2 is expected to have a significantly better vertical sensitivity (monthly average degrees of freedom surface–6 km of 0.70) and to be sensitive at lower altitudes (more than 0.5 km lower than IASI, reaching nearly 3 km). Vertical ozone layers of 4 to 5 km thickness are expected to be resolved by IASI/2, while IASI has a vertical resolution of 6–8 km. According to our analyses, IASI/2 is expected to have the possibility of effectively separate lower from upper tropospheric ozone information even for low sensitivity scenarios. In addition, IASI/2 is expected to be able to better monitor LT ozone patterns at local spatial scale and to monitor abrupt temporal evolutions occurring at timescales of a few days, thus bringing an expected added value with respect to IASI for the monitoring of air quality.

Long-term changes in lower tropospheric baseline ozone concentrations at northern mid-latitudes

Abstract. Changes in baseline (here understood as representative of continental to hemispheric scales) tropospheric O3 concentrations that have occurred at northern mid-latitudes over the past six decades are quantified from available measurement records with the goal of providing benchmarks to which retrospective model calculations of the global O3 distribution can be compared. Eleven data sets (ten ground-based and one airborne) including six European (beginning in the 1950's and before), three North American (beginning in 1984) and two Asian (beginning in 1991) are analyzed. When the full time periods of the data records are considered a consistent picture emerges; O3 has increased at all sites in all seasons at approximately 1% yr−1 relative to the site's 2000 yr mixing ratio in each season. For perspective, this rate of increase sustained from 1950 to 2000 corresponds to an approximate doubling. There is little if any evidence for statistically significant differences in average rates of increase among the sites, regardless of varying length of data records. At most sites (most definitively at the European sites) the rate of increase has slowed over the last decade (possibly longer), to the extent that at present O3 is decreasing at some sites in some seasons, particularly in summer. The average rate of increase before 2000 shows significant seasonal differences (1.08 ± 0.09, 0.89 ± 0.10, 0.85 ± 0.11 and 1.21 ± 0.12% yr−1 in spring, summer, autumn and winter, respectively, over North America and Europe).

Relation between tropospheric and stratospheric ozone at Thumba (8.5°N, 77°E) and Bangalore (13°N, 77.5°E), India and its effect on environment

The paper presents the nature of long-term seasonal and yearly variations of tropospheric and stratospheric ozone over Thumba (8.5°N, 77°E) and Bangalore (13°N, 77.5°E), India. Seasonal variations clearly reveal that stratospheric ozone concentration attained comparatively higher value in the months from July to September while lower tropospheric ozone concentration at the same time over the above stations during the period 1979–2005. Yearly variation shows increasing trend in tropospheric ozone but decreasing trend in stratospheric ozone from 1979 to 2005. Undesirable environmental effects due to such tropospheric rise and stratospheric decline in ozone are also mentioned.

Megacity ozone air quality under four alternative future scenarios

Abstract. The impact of the megacities of the world on global tropospheric ozone, and conversely, the extent to which megacities are influenced by emissions of ozone precursors from outside of the megacities is examined under the four alternative RCP ("Representative Concentration Pathway") emissions scenarios. Despite accounting for about 6% of present-day anthropogenic emissions of ozone precursor species, the contribution of emissions from megacities to global tropospheric ozone is calculated to be 0.84%. By 2100 this contribution falls to between 0.18% and 0.62% depending on the scenario, with the lower value being for the most-polluting of the four future emissions scenarios due to stringent controls on ozone precursor emissions from highly populated areas combined with a stronger tropospheric background ozone field. The higher end of this range is from the least-polluting of the four emissions scenarios, due to lower background tropospheric ozone combined with the use of a simpler downscaling methodology in the construction of the scenario, which results in higher emissions from megacities. Although the absolute impact of megacities on global ozone is small, an important result of this study is that under all future scenarios, future air quality in megacities is expected to be less influenced by local emissions within the cities, but instead more influenced by emission sources outside of the cities, with mixing ratios of background ozone projected to play an increasing role in megacity air quality throughout the 21st century. Assumptions made when downscaling the emissions scenarios onto the grids used in such modelling studies can have a large influence on these results; future generations of emissions scenarios should include spatially explicit representations or urban development suitable for air quality studies using global chemical transport models.

Changing polar environments: Interdisciplinary challenges

In the past few decades, there has been enormous growth in scientific studies of physical, chemical, and biological interactions among reservoirs in polar regions. This has come, in part, as a result of a few significant discoveries: There is dramatic halogen chemistry that occurs on and above the sea ice in the springtime that destroys lower tropospheric ozone and mercury [Simpson et al., 2007; Steffen et al., 2008], the sunlit snowpack is very photochemically active [Grannas et al., 2007], biology as a source of organic compounds plays a pivotal role in these processes, and these processes are occurring in the context of rapidly changing polar regions under climate feedbacks that are as of yet not fully understood [Serreze and Barry, 2011]. Stimulated by the opportunities of the International Polar Year (IPY, 2007–2009), a number of large‐scale field studies in both polar environments have been undertaken, aimed at the study of the complex biotic and abiotic processes occurring in all phases (see Figure 1). Sea ice plays a critical role in polar environments: It is a highly reflective surface that interacts with radiation; it provides a habitat for mammals and micro‐organisms alike, thus playing a key role in polar trophic processes and elemental cycles; and it creates a saline environment for chemical processes that facilitate release of halogenated gases that contribute to the atmosphere's ability to photochemically cleanse itself in an otherwise low‐radiation environment. Ocean‐air and sea ice–air interfaces also produce aerosol particles that provide cloud condensation nuclei.

Assimilation of IASI partial tropospheric columns with an Ensemble Kalman Filter over Europe

Abstract. Partial lower tropospheric ozone columns provided by the IASI (Infrared Atmospheric Sounding Interferometer) instrument have been assimilated into a chemistry-transport model at continental scale (CHIMERE) using an Ensemble Square Root Kalman Filter (EnSRF). Analyses are made for the month of July 2007 over the European domain. Launched in 2006, aboard the MetOp-A satellite, IASI shows high sensitivity for ozone in the free troposphere and low sensitivity at the ground; therefore it is important to evaluate if assimilation of these observations can improve free tropospheric ozone, and possibly surface ozone. The analyses are validated against independent ozone observations from sondes, MOZAIC1 aircraft and ground based stations (AIRBASE – the European Air quality dataBase) and compared with respect to the free run of CHIMERE. These comparisons show a decrease in error of 6 parts-per-billion (ppb) in the free troposphere over the Frankfurt area, and also a reduction of the root mean square error (respectively bias) at the surface of 19% (33%) for more than 90% of existing ground stations. This provides evidence of the potential of data assimilation of tropospheric IASI columns to better describe the tropospheric ozone distribution, including surface ozone, despite the lower sensitivity. The changes in concentration resulting from the observational constraints were quantified and several geophysical explanations for the findings of this study were drawn. The corrections were most pronounced over Italy and the Mediterranean region, we noted an average reduction of 8–9 ppb in the free troposphere with respect to the free run, and still a reduction of 5.5 ppb at ground, likely due to a longer residence time of air masses in this part associated to the general circulation pattern (i.e. dominant western circulation) and to persistent anticyclonic conditions over the Mediterranean basin. This is an important geophysical result, since the ozone burden is large over this area, with impact on the radiative balance and air quality. 1 Measurements of OZone, water vapour, carbon monoxide and nitrogen oxides by in-service AIrbus airCraft (http://mozaic.aero.obs-mip.fr/web/).

Update of long-term trends analysis of ambient 8-hour ozone and precursor monitoring data in the South Central U.S.; encouraging news

In the South Central U.S., lower tropospheric ozone pollution continues to be a challenging problem. This paper provides an update on long-term trends analyses of the ambient ozone and precursor monitoring data collected over the past 25 years (1986-2010) in four South Central U.S. cities, following up on a previous published review of 20 year trends (M.E. Sather and K. Cavender, J. Environ. Monit., 2007, 9, 143-150). The results of these analyses should be useful to air quality scientists, managers, planners, and modelers in assessing the effectiveness of nitrogen oxides (NO(x)) and volatile organic compounds (VOC) pollution controls for ambient ozone concentration reduction programs. Large amounts of quantitative information for each South Central U.S. city are concisely synthesized into one graphic per city. Results reported in this paper show significant long-term decreases in ambient ozone and precursor concentrations in all four South Central U.S. cities, especially over the recent five-year period 2006-2010.

Measurement of western U.S. baseline ozone from the surface to the tropopause and assessment of downwind impact regions

[1] Since 1997, baseline ozone monitoring from the surface to the tropopause along the U.S. west coast has been limited to the weekly ozonesondes from Trinidad Head, California. To explore baseline ozone at other latitudes, an ozonesonde network was implemented during spring 2010, including four launch sites along the California coast. Modeling indicated that North American pollution plumes impacted the California coast primarily below 3 km, but had no measurable impact on the average coastal ozone profiles. Vertical and latitudinal variation in free tropospheric baseline ozone appears to be partly explained by polluted and stratospheric air masses that descend isentropically along the west coast. Above 3 km, the dominant sources of ozone precursors were China and international shipping, while international shipping was the greatest source below 2 km. Approximately 8–10% of the baseline ozone that enters California in the 0–6 km range impacts the surface of the USA, but very little reaches the eastern USA. Within California, the major impact of baseline ozone above 2 km is on the high elevation terrain of eastern California. Baseline ozone below 2 km has its strongest impact on the low elevation sites throughout the state. To quantify ozone production within California we compared inland ozone measurements to baseline measurements. For average daytime conditions, we found no enhancements of lower tropospheric ozone in the northern Central Valley, but enhancements of 12–23% were found in the southern Central Valley. Enhancements above Joshua Tree were greater, 33–41%, while the greatest enhancements occurred over the LA Basin, 32–63%.

Pic 2005, a field campaign to investigate low-tropospheric ozone variability in the Pyrenees

The Pic 2005 field campaign took place from 13 June to 7 July 2005 close to the high-altitude permanent atmospheric observatory Pic-du-Midi (PDM), situated at 2875 m asl in the French Pyrenees. The experimental set-up combined in situ ground-based observations at PDM with ozone lidar measurements at two lower sites in close vicinity (600 m asl/28 km away, and 2380 m asl/500 m away). Such an experimental configuration is appropriate to address the question of the vertical layering of the chemical atmosphere in a mountain area and above the plain nearby, and how this influences measurements conducted on a mountain summit under the influence of horizontal transport at regional scale, and vertical transport at local scale. Forecast tools made it possible to plan and carry out 6 one-day Intensive Observation Periods (IOPs), mostly in anticyclonic conditions favoring local thermally induced circulations, with and without local pollution in the lower troposphere. It was thus possible to document i) ozone diurnal variations at PDM; ii) correlation between ozone measurements at PDM and their counterparts at the same altitude in the free troposphere; iii) ozone variability in the vicinity of PDM. The field campaign provided direct experimental evidence that at daytime in the encountered conditions (mostly anticyclonic), PDM failed in a large extent to be representative of the troposphere above the surrounding flat areas at similar altitude. First, ozone daily averages at PDM were found lower than their free-tropospheric counterpart. Thermally induced circulations and convection pumping clean air from the rural boundary layer can account qualitatively for ozone depletion observed at PDM during daytime. However the surface measurements do not support the hypothesis of direct lifting of near-surface air masses up to PDM. Thus, mixing with free-tropospheric air, photochemistry and surface deposition in the valleys appear to be needed ingredients to account quantitatively for the observed variations (in proportions that further studies should determine). Second, ozone variability was found to be much lower at PDM than in the free troposphere—again an indication of atmospheric mixing. In particular at daytime, the PDM observatory did not allow for detection of ozone-rich layers simultaneously visible above the plain. Beyond these first results, the data set presented here paves way to detailed studies of the IOPs.

Anomalous low tropospheric column ozone over Eastern India during the severe drought event of monsoon 2002: a case study

The present study is an attempt to examine some of the probable causes of the unusually low tropospheric column ozone observed over eastern India during the exceptional drought event in July 2002. We examined horizontal wind and omega (vertical velocity) anomalies over the Indian region to understand the large-scale dynamical processes which prevailed in July 2002. We also examined anomalies in tropospheric carbon monoxide (CO), an important ozone precursor, and observed low CO mixing ratio in the free troposphere in 2002 over eastern India. It was found that instead of a normal large-scale ascent, the air was descending in the middle and lower troposphere over a vast part of India. This configuration was apparently responsible for the less convective upwelling of precursors and likely caused less photochemical ozone formation in the free troposphere over eastern India in July 2002. The insight gained from this case study will hopefully provide a better understanding of the process controlling the distribution of the tropospheric ozone over the Indian region.

Snow-sourced bromine and its implications for polar tropospheric ozone

Abstract. In the last two decades, significant depletion of boundary layer ozone (ozone depletion events, ODEs) has been observed in both Arctic and Antarctic spring. ODEs are attributed to catalytic destruction by bromine radicals (Br plus BrO), especially during bromine explosion events (BEs), when high concentrations of BrO periodically occur. However, neither the exact source of bromine nor the mechanism for sustaining the observed high BrO concentrations is completely understood. Here, by considering the production of sea salt aerosol from snow lying on sea ice during blowing snow events and the subsequent release of bromine, we successfully simulate the BEs using a global chemistry transport model. We find that heterogeneous reactions play an important role in sustaining a high fraction of the total inorganic bromine as BrO. We also find that emissions of bromine associated with blowing snow contribute significantly to BrO at mid-latitudes. Modeled tropospheric BrO columns generally compare well with the tropospheric BrO columns retrieved from the GOME satellite instrument (Global Ozone Monitoring Experiment). The additional blowing snow bromine source, identified here, reduces modeled high latitude lower tropospheric ozone amounts by up to an average 8% in polar spring.

IASI observations of seasonal and day-to-day variations of tropospheric ozone over three highly populated areas of China: Beijing, Shanghai, and Hong Kong

Abstract. IASI observations of tropospheric ozone over the Beijing, Shanghai and Hong Kong areas during one year (2008) have been analysed, demonstrating the capability of space-borne infrared nadir measurements to probe seasonal and even day-to-day variations of lower tropospheric ozone (0–6 km partial columns) on the regional scale of highly populated areas. The monthly variations of lower tropospheric ozone retrieved from IASI clearly show the influence of the Asian summer monsoon that brings clean air masses from the Pacific during summer. They exhibit indeed a sharp ozone maximum in late spring and early summer (May–June) followed by a summer minimum. The time periods and the intensities of the maxima and of the decreases are latitude-dependent: they are more pronounced in Hong Kong and Shanghai than in Beijing. Moreover, IASI provides the opportunity to follow the spatial variations of ozone over the surroundings of each megacity as well as its daily variability. We show here that the large lower tropospheric ozone amounts (0–6 km partial columns) observed with IASI are mainly downwind the highest populated areas in each region, thus possibly suggesting the anthropogenic origin of the large ozone amounts observed. Finally, an analysis of the mean ozone profiles over each region – for selected days with high ozone events – in association with the analysis of the meteorological situation shows that the high ozone amounts observed during winter are likely related to descents of ozone-rich air from the stratosphere, whereas in spring and summer the tropospheric ozone is likely enhanced by photochemical production in polluted areas and/or in air masses from fire plumes.

The impact of traffic emissions on atmospheric ozone and OH: results from QUANTIFY

Abstract. To estimate the impact of emissions by road, aircraft and ship traffic on ozone and OH in the present-day atmosphere six different atmospheric chemistry models have been used. Based on newly developed global emission inventories for road, ship and aircraft emission data sets each model performed sensitivity simulations reducing the emissions of each transport sector by 5%. The model results indicate that on global annual average lower tropospheric ozone responds most sensitive to ship emissions (50.6%±10.9% of the total traffic induced perturbation), followed by road (36.7%±9.3%) and aircraft exhausts (12.7%±2.9%), respectively. In the northern upper troposphere between 200–300 hPa at 30–60° N the maximum impact from road and ship are 93% and 73% of the maximum effect of aircraft, respectively. The latter is 0.185 ppbv for ozone (for the 5% case) or 3.69 ppbv when scaling to 100%. On the global average the impact of road even dominates in the UTLS-region. The sensitivity of ozone formation per NOx molecule emitted is highest for aircraft exhausts. The local maximum effect of the summed traffic emissions on the ozone column predicted by the models is 0.2 DU and occurs over the northern subtropical Atlantic extending to central Europe. Below 800 hPa both ozone and OH respond most sensitively to ship emissions in the marine lower troposphere over the Atlantic. Based on the 5% perturbation the effect on ozone can exceed 0.6% close to the marine surface (global zonal mean) which is 80% of the total traffic induced ozone perturbation. In the southern hemisphere ship emissions contribute relatively strongly to the total ozone perturbation by 60%–80% throughout the year. Methane lifetime changes against OH are affected strongest by ship emissions up to 0.21 (± 0.05)%, followed by road (0.08 (±0.01)%) and air traffic (0.05 (± 0.02)%). Based on the full scale ozone and methane perturbations positive radiative forcings were calculated for road emissions (7.3±6.2 mWm−2) and for aviation (2.9±2.3 mWm−2). Ship induced methane lifetime changes dominate over the ozone forcing and therefore lead to a net negative forcing (−25.5±13.2 mWm−2).

Increase in springtime tropospheric ozone at a mountainous site in Japan for the period 1998–2006

The trend of tropospheric ozone from 1998 to 2006 was examined based on continuous measurements made at a site on Mt. Happo, Japan. We focused our study on springtime ozone, to coincide with the East Asian continental outflow that dominates the lower tropospheric ozone over Japan during this season. The observed increase of ∼1 ppbv yr −1 in the mean ozone level was statistically significant. We also found that the probability distribution of the springtime ozone mixing ratios was substantially modified, with the ozone mixing ratios greatly increasing at the upper end of the probability distribution. This increase has been particularly large since 2003, with larger increases occurring at the higher percentiles. The number of high-ozone days doubled during 2003–2006 compared to 1999–2002. One of the very likely explanations is the enhancement of regional ozone pollution due to rapidly increasing anthropogenic emissions from East Asia.

Tropospheric ozone distributions over Europe during the heat wave in July 2007 observed from infrared nadir spectra recorded by IASI

First partial tropospheric ozone columns (0–6 km) derived from radiances observed by the IASI instrument aboard the MetOp‐A platform over Europe during summer 2007 are presented. They were retrieved using an altitude‐dependent regularization method. Comparison with measurements from balloon sondes shows excellent agreement. Space‐borne observations show large lower tropospheric ozone amounts over South‐Eastern Europe during the heat wave period, which are also displayed by simulations with a regional chemistry‐transport model CHIMERE.

Recent ozone investigations over Bulgaria by remote sensing: Ground-based and satellite data

This presentation deals with the recent investigations of the dynamics of the total ozone content (TOC) and of the lower troposphere ozone over Bulgaria, South-Eastern Europe. For analysis of the TOC we have used data obtained through measurements carried out by using a ground-based spectrophotometer Photon and data from satellite experiments TOMS-Nimbus 7 and TOMS-EP. The spectrophotometer Photon measures the direct solar ultraviolet (UV) radiation in the range 290–360 nm with 1 nm resolution, using which TOC is calculated. The ozone values received from the ground-based and satellite observations in the period 1999–2005 have been compared. In this period the ozone column did not show a statistically significant trend. The TOC data demonstrated typical for the middle latitudes seasonal variations. The monthly mean ozone values observed during the last 9 years have been examined also. It was found that the largest TOC fluctuations were detected in February and the smallest – in July. Ozone behaviour in the planetary boundary layer (PBL) has been analyzed by using data obtained with remote-sensing technique – ground-based aerosol LIDAR (LIght Detection And Ranging), which give information about the evolution of the PBL. The surface ozone concentrations have been measured with a fast and sensitive ozone monitor, based on chemiluminescent method for ozone detection. The investigation shows that the simultaneous observations of the surface ozone and of the boundary layer dynamics provide the opportunity accurately to establish relation between the BL structure and the features of ozone variations. In particular, the influence of the residual layer on the ozone values near the ground is demonstrated.

Strong influence of lowermost stratospheric ozone on lower tropospheric background ozone changes over Europe

Using ozone measurements from two sounding sites and two high‐altitude stations in Central Europe, we show evidence for a dominant influence of changes in lowermost stratospheric ozone on the variability and overall upward trend of background ozone in the lower troposphere (3000–3500 m asl) during the 1992–2004 period. Numerical simulations with a stratospheric chemistry transport model suggest that changes in lower stratospheric ozone were driven by dynamics rather than by changes in stratospheric chlorine loading. In addition, Lagrangian model simulations indicate that changes in downward transport of ozone from the stratosphere into the troposphere were dominated by changes in lowermost stratospheric ozone concentrations rather than by variations of cross‐tropopause air mass transport. This suggests that the positive ozone trends and concentration anomalies in the lower free troposphere over Europe during the 1990s were likely to a large extent due to enhanced stratospheric ozone contributions, particularly in winter–spring.

Trends analysis of ambient 8 hour ozone and precursor monitoring data in the south central U.S.

For the south central U.S., lower tropospheric ozone pollution has been a persistent and challenging problem. This paper provides long-term trends analyses of the ozone and precursor monitoring data collected over the past 20 years in four south central U.S. cities. The results of these analyses should be useful to air quality scientists, managers, planners, and modelers in assessing the effectiveness of ozone pollution control strategies being implemented or being planned for the future. Results of the data analyses show that all areas have monitored significant decreases in ozone and precursor concentrations over the past 20 years, especially in El Paso, Texas. Continuing challenges include the reduction of the percentage of time that monitors record 8 hour ozone concentrations over the U.S. 8 hour ozone standard, and the future control of highly reactive volatile organic compounds.