Comparison of ozone temporal scales for large urban, small urban, and rural areas in Georgia

Ground-level ozone (O3) time series are characterized by the sum of several distinct temporal scales: long-term, seasonal, synoptic, diurnal (daily), and intraday variation. In this study, the authors use a Kolmorogov-Zurbenko filter to separate the 1981-2001 O3 time-series from many sites in and around Georgia into these various components. The authors compare the temporal components to examine differences between small and large metropolitan areas and between urban and rural areas. They then focus on the synoptic component to define a predominant transport region or airshed for each site.     

Identification and Interpretation of Representative Ozone Distributions in Association with the Sea Breeze from Different Synoptic Winds over the Coastal Urban Area in Korea

Abstract

To aid the studies of long-term impact assessment of cumulative ozone (O₃) exposures, the representative 8-hr O₃ pollution patterns have been identified over the Greater Seoul Area (GSA) in Korea. Principal component analysis and two-stage clustering techniques were used to identify the representative O₃ patterns, and numerical and observational analyses were also used to interpret the identified horizontal distribution patterns. The results yielded three major O₃ distribution patterns, and each of the three patterns was found to have strong correlations with local and synoptic meteorological conditions over the GSA. For example, pattern 1, accounting for 46% of O₃ concentration distributions, mostly occurred under relatively weak westerly synoptic winds. The predominant features of this pattern were infrequent high O₃ levels but a distinct gradient of O₃ concentration from the western coastal area to the eastern inland area that was mainly induced by the local sea breeze. Pattern 2, accounting for 31% of O₃ concentration distributions, was found with higher O₃ levels in the western coastal area but lower in the eastern inland area. This is due to the modified sea breeze under the relatively stronger easterly opposing synoptic wind, affecting the high O₃ occurrence in the western coastal area only. However, pattern 3, accounting for 21% of O₃ concentration distributions, showed significantly higher O₃ concentrations over the whole GSA mainly due to the retarded and slow-moving sea-breeze front under the weak opposing synoptic flow. Modeling study also indicated that local and synoptic meteorological processes play a major role in determining the high O₃ concentration distribution patterns over the GSA.     

Evaluating the performance of regional-scale photochemical modeling systems. Part III—Precursor predictions

Two regional-scale photochemical modeling systems, RAMS/UAM-V and MM5/MAQSIP, are used to simulate precursor concentrations for 4 June–31 August 1995 period. The time series of simulated and observed precursor concentrations are spectrally decomposed into intra-day, diurnal, synoptic, and longer-term (baseline) forcings and compared on each time scale. The results reveal that the observed and modeled precursor concentrations are uncorrelated on the intra-day time scale for both modeling systems while correlations are higher on longer time scales. In observations, the variability in NO_x concentrations is dominated by the diurnal and synoptic-scale processes, while NO_y is found to vary most on the synoptic time scale. In observations of volatile organic compounds (VOCs), the variability is dominated by the diurnal and synoptic processes for both biogenic and anthropogenic hydrocarbons. The pattern seen in the observations, in terms of the relative contribution of different temporal components to the total variance, is better captured by the two modeling systems for NO_y than for NO_x and VOCs. There are differences between model predictions and observations in terms of the actual magnitudes of variances of each temporal component. These results suggest the need to cover longer time periods in modeling simulations to resolve longer-term processes, because they seem to play a dominant role in dictating the precursor variability.     

Extending the Kolmogorov–Zurbenko Filter: Application to Ozone,Particulate Matter,and Meteorological Trends

Abstract

Tropospheric ozone (O₃) and particulate matter (PM) are pollutants of great concern to air quality managers. Federal standards for these pollutants have been promulgated in recent years because of the known adverse effects of the pollutants on human health, the environment, and visibility. Local meteorological conditions exert a strong influence over day‐to‐day variations in pollutant concentrations; therefore, the meteorological signal must be removed in order for air quality planners and managers to examine underlying emissions-related trends and make better air quality management decisions for the future. Although the Kolmogorov–Zurbenko (KZ) filter has been widely used for this type of trend separation in O₃ studies in the eastern United States, this article aims to extend the method in three key ways. First, whereas the KZ filter is known as a useful tool for O₃ analysis, this study also evaluates its effectiveness when applied to PM. Second, the method was applied to Tucson, AZ, a city in the semi‐arid southwestern United States (Southwest), to evaluate the appropriateness of the method in a region with weaker synoptic weather controls on air quality than the eastern United States. Third, additional forms of output were developed and tailored to be more applicable to decision-makers’ needs through a partnership between academic researchers and air quality planners and managers. Results of the study indicate that the KZ filter is a useful method for examining emissions‐related PM trends, resulting in small, but potentially significant, differences after adjustment. For the Tucson situation with weaker synoptic controls, the KZ method identified mixing height as a more important variable than has been found in other cities.     

Defining the Photochemical Contribution to Particulate Matter in Urban Areas Using Time-Series Analysis

Abstract

The objective of this project is to demonstrate how the ambient air measurement record can be used to define the relationship between O₃ (as a surrogate for photochemistry) and secondary particulate matter (PM) in urban air. The approach used is to develop a time-series transfer-function model describing the daily PM₁₀ (PM with less than 10 μm aerodynamic diameter) concentration as a function of lagged PM and current and lagged O₃, NO or NO₂, CO, and SO₂. Approximately 3 years of daily average PM₁₀, daily maximum 8-hr average O₃ and CO, daily 24-hr average SO₂ and NO₂, and daily 6:00 a.m.-9:00 a.m. average NO from the Aerometric Information Retrieval System (AIRS) air quality subsystem are used for this analysis. Urban areas modeled are Chicago, IL; Los Angeles, CA; Phoenix, AZ; Philadelphia, PA; Sacramento, CA; and Detroit, MI. Time-series analysis identified significant autocorrelation in the O₃, PM₁₀, NO, NO₂,CO, and SO₂ series. Cross correlations between PM₁₀ (dependent variable) and gaseous pollutants (independent variables) show that all of the gases are significantly correlated with PM₁₀ and that O₃ is also significantly correlated lagged up to two previous days. Once a transfer-function model of current PM₁₀ is defined for an urban location, the effect of an O₃-control strategy on PM concentrations is estimated by calculating daily PM₁₀ concentrations with reduced O₃ concentrations. Forecasted summertime PM₁₀ reductions resulting from a 5 percent decrease in ambient O₃ range from 1.2 μg/m³ (3.03%) in Chicago to 3.9 μg/m³ (7.65%) in Phoenix.     

Evaluation of the Community Multiscale Air Quality (CMAQ) model version 4.5: Sensitivities impacting model performance: Part I—Ozone

This study examines ozone (O₃) predictions from the Community Multiscale Air Quality (CMAQ) model version 4.5 and discusses potential factors influencing the model results. Daily maximum 8-h average O₃ levels are largely underpredicted when observed O₃ levels are above 85 ppb and overpredicted when they are below 35 ppb. Using a clustering approach, model performance was examined separately for several different synoptic regimes. Under the most common synoptic conditions of a typical summertime Bermuda High setup, the model showed good overall performance for O₃, while associations have been identified here between other, less frequent, synoptic regimes and the O₃ overprediction and underprediction biases. A sensitivity test between the CB-IV and CB05 chemical mechanisms showed that predictions of daily maximum 8-h average O₃ using CB05 were on average 7.3% higher than those using CB-IV. Boundary condition (BC) sensitivity tests show that the overprediction biases at low O₃ levels are more sensitive to the BC O₃ levels near the surface than BC concentrations aloft. These sensitivity tests also show the model performance for O₃ improved when using the global GEOS-CHEM BCs instead of default profiles. Simulations using the newest version of the CMAQ model (v4.6) showed a small improvement in O₃ predictions, particularly when vertical layers were not collapsed. Collectively, the results suggest that key synoptic weather patterns play a leading role in the prediction biases, and more detailed study of these episodes are needed to identify further modeling improvements.     

Trends and Variability of Ground-Level O3 in Connecticut over the Period 1981–1997

Abstract

The temporal and spatial characteristics of ground-level (tropospheric) O₃ measured at 10 monitoring stations in Connecticut were studied from 1981 to 1997. To detect the O₃ trend caused by changes in precursor emissions, moving average filters and a linear least-squared regression model were used to eliminate the short-term variation and effects of temperature from raw time-series O₃ data. The results showed a significant decrease in the number of days exceeding the National Ambient Air Quality Standards (NAAQS) and a small change in total O₃ concentration. The analysis indicated that the variation of daily maximum O₃, caused by changes in emissions, explained more than 10% of the total O₃ variation in Bridgeport and East Hartford during the past 17 years. Meanwhile, a strong weekly cycle was also found in O₃ time-series data, resulting in lower O₃ concentration in urban areas than in nonur-ban areas, implying that land use and land cover have impacts on the spatial distribution of ground-level O₃ in Connecticut.     

Parameter Evaluation and Model Validation of Ozone Exposure Assessment Using Harvard Southern California Chronic Ozone Exposure Study Data

Abstract

To examine factors influencing long‐term ozone (O₃) exposures by children living in urban communities, the authors analyzed longitudinal data on personal, indoor, and outdoor O₃ concentrations, as well as related housing and other questionnaire information collected in the one‐year‐long Harvard Southern California Chronic Ozone Exposure Study. Of 224 children contained in the original data set, 160 children were found to have longitudinal measurements of O₃ concentrations in at least six months of 12 months of the study period. Data for these children were randomly split into two equal sets: one for model development and the other for model validation. Mixed models with various variance‐covariance structures were developed to evaluate statistically important predictors for chronic personal ozone exposures. Model predictions were then validated against the field measurements using an empirical best‐linear unbiased prediction technique.The results of model fitting showed that the most important predictors for personal ozone exposure include indoor O₃ concentration, central ambient O₃ concentration, outdoor O₃ concentration, season, gender, outdoor time, house fan usage, and the presence of a gas range in the house. Hierarchical models of personal O₃ concentrations indicate the following levels of explanatory power for each of the predictive models: indoor and outdoor O₃ concentrations plus questionnaire variables, central and indoor O₃ concentrations plus questionnaire variables, indoor O₃ concentrations plus questionnaire variables, central O₃ concentrations plus questionnaire variables, and questionnaire data alone on time activity and housing characteristics. These results provide important information on key predictors of chronic human exposures to ambient O₃ for children and offer insights into how to reliably and cost‐effectively predict personal O₃ exposures in the future. Furthermore, the techniques and findings derived from this study also have strong implications for selecting the most reliable and cost‐effective exposure study design and modeling approaches for other ambient pollutants, such as fine particulate matter and selected urban air toxics.     

Air Quality Modeling of Interpollutant Trading for Ozone Precursors in an Urban Area

Abstract

Emission trading is a market‐based approach designed to improve the efficiency and economic viability of emission control programs; emission trading has typically been confined to trades among single pollutants. Interpollutant trading (IPT), as described in this work, allows for trades among emissions of different compounds that affect the same air quality end point, in this work, ambient ozone (O₃) concentrations. Because emissions of different compounds impact air quality end points differently, weighting factors or trading ratios (tons of emissions of nitrogen oxides (NO_x) equivalent to a ton of emissions of volatile organic compounds [VOCs]) must be developed to allow for IPT. In this work, IPT indices based on reductions in O₃ concentrations and based on reductions in population exposures to O₃ were developed and evaluated using a three‐dimensional gridded photochemical model for Austin, TX, a city currently on the cusp of nonattainment with the National Ambient Air Quality Standards for O₃ concentrations averaged over 8 hr. Emissions of VOC and NO_x from area and mobile sources in Austin are larger than emissions from point sources. The analysis indicated that mobile and area sources exhibited similar impacts. Trading ratios based on maximum O₃ concentration or population exposure were similar. In contrast, the trading ratios did exhibit significant (more than a factor of two) day‐to‐day variability. Analysis of the air quality modeling indicated that the daily variability in trading ratios could be attributed to daily variations in both emissions and meteorology.     

Effects of Instrument Precision and Spatial Variability on the Assessment of the Temporal Variation of Ambient Air Pollution in Atlanta,Georgia

Abstract

Data from the U.S. Environmental Protection Agency Air Quality System, the Southeastern Aerosol Research and Characterization database, and the Assessment of Spatial Aerosol Composition in Atlanta database for 1999 through 2002 have been used to characterize error associated with instrument precision and spatial variability on the assessment of the temporal variation of ambient air pollution in Atlanta, GA. These data are being used in time series epidemiologic studies in which associations of acute respiratory and cardiovascular health outcomes and daily ambient air pollutant levels are assessed. Modified semivariograms are used to quantify the effects of instrument precision and spatial variability on the assessment of daily metrics of ambient gaseous pollutants (SO₂, CO, NO_x, and O₃) and fine particulate matter ([PM_2.5] PM_2.5 mass, sulfate, nitrate, ammonium, elemental carbon [EC], and organic carbon [OC]). Variation because of instrument imprecision represented 7–40% of the temporal variation in the daily pollutant measures and was largest for the PM_2.5 EC and OC. Spatial variability was greatest for primary pollutants (SO₂, CO, NO_x, and EC). Population–weighted variation in daily ambient air pollutant levels because of both instrument imprecision and spatial variability ranged from 20% of the temporal variation for O₃ to 70% of the temporal variation for SO₂ and EC. Wind rose plots, corrected for diurnal and seasonal pattern effects, are used to demonstrate the impacts of local sources on monitoring station data. The results presented are being used to quantify the impacts of instrument precision and spatial variability on the assessment of health effects of ambient air pollution in Atlanta and are relevant to the interpretation of results from time series health studies that use data from fixed monitors.     

Ozone and Other Air Quality-Related Variables Affecting Visibility in the Southeast United States

Abstract

An analysis of ozone (O₃) concentrations and several other air quality-related variables was performed to elucidate their relationship with visibility at five urban and semi-urban locations in the southeast United States during the summer seasons of 1980-1996. The role and impact of O₃ on aerosols was investigated to ascertain a relationship with visibility. Regional trend analysis over the 1980s reveals an increase in maximum O₃ concentration coupled with a decrease in visibility. However, a similar analysis for the 1990s shows a leveling-off of both O₃ and visibility; in both cases, the results were not statistically significant at the 5% level. A case study of site-specific trends at Nashville, TN, followed similar trends. To better understand the relationships between O₃ concentration and visibility, the analysis was varied from yearly through daily to hourly averaged values. This increased temporal resolution showed a statistically significant inverse relationship between visibility and O₃. Site-specific hourly r² values ranged from 0.02 to 0.43. Additionally, by performing back-trajectory analysis, it was found that the visibility degraded by air mass migration over polluted areas.     

A synoptic climatology for surface ozone concentrations in Southern Ontario, 1976–1981

The synoptic climatology of ozone (O₃) for S Ontario has shown that, over the 1976–1981 period, average summer O₃ concentrations follow a relationship similar to that reported for event analysis during periods of high O₃ concentration. Highest average concentrations, 36 parts per billion (ppb), occur with ‘back of the high’ situations while lowest average concentrations (20 ppb) occur with ‘front of the high’ situations.With similar weather events in the winter, the pattern is reversed with highest average O₃ concentrations on the ‘front of the high’ (19 ppb) and lowest average concentrations on the ‘back of the high’ (13 ppb). Concentration of O₃ in the ‘front of the high’ sector is due in part to the intrusion of O₃ in the vicinity of storms from the stratosphere. The seasonal variation of average concentrations in these situations is low, ranging from 14 to 26 ppb.The very low average concentration during the winter and fall for the ‘back of the high’ situation may be the result of scavenging by NO_x from the urban/industrial areas around the Great Lakes. During the spring and summer, solar energy and warm temperatures cause the photochemical production of O₃ from NO_x and HCs precursors. In the fall and winter, photochemical production of O₃ is either very low or absent, and the NO_x consume O₃ rather than produce it. Thus, average O₃ concentrations for winter ‘back of the high’ situations are one-third of those in the summer months.The synoptic climatology of events during the months from May to September with maximum O₃ concentrations in excess of 80 ppb indicates that 78 % of these events occur under synoptic weather classes generally indicative of back or centre of the high situations.     

Characterization and Identification of Trends in Average Ambient Ozone and Fine Particulate Matter Levels through Trajectory Cluster Analysis in Eastern Canada

Abstract

In many locations in Eastern Canada, ambient levels of fine particulate matter (PM_2.5) and surface ozone (O₃) depend on airflow direction and synoptic scale meteorological conditions. In this study, a cluster analysis was performed on 10 yr (1994 –2003) of back-trajectory data for 11 locations in Eastern Canada, resulting in the identification of 10 unique back-trajectory clusters (or airflows) for each location. The airflows were then used to characterize and identify spatial and temporal trends in the daily maximum 8-hr average O₃ (dmax 8-hr O₃) and the daily average PM_2.5 levels. Results showed that airflows from the southwest passing over Michigan and Southern Ontario were associated, on average, with the highest O₃ levels at most locations in Eastern Canada.For PM_2.5, the highest levels occurred with airflows from the Eastern Ohio River Valley. At major urban locations in Ontario and Quebec, the warm season mean (May to September) dmax 8-hr O₃ and the annual mean PM_2.5 were, on average, 12 parts per billion and 7.6 μg/m³ higher, respectively, than airflows from the north. Elevated levels of O₃ and PM_2.5 also occurred under light airflows, and, on average, the levels under light airflows were higher than their nonlight counterparts. At several locations in Canada, including Toronto, Montreal, Quebec City, and Kejimkujik, the annual warm season mean dmax 8-hr O₃ experienced a statistically significant (95% confidence) increasing trend over the 10-yr period. When airflow direction was considered, a number of locations experienced statistically significant upward trends in O₃ for airflow from the north and northwest. Several locations also showed significant upward trends associated with airflow from the southwest passing over Michigan and Southwestern Ontario. Although there are no statistically significant downward trends, airflows from the southwest have shown a reduction in O₃ levels in Southwestern Ontario in more recent years.     

An Ozone Event in Indianapolis

There are doubts concerning the sources of the pollutant ozone in an urban location. Current control regulations,¹ for example, are based on local sources. Many studies suggest that plumes of pollutants released by upwind areas are the cause. For example, New York City has been associated with high O₃ in New England,² Chicago with O₃ in Milwaukee,³ and St. Louis with O₃ in rural Illinois.⁴ Others^5,6 have suggested the problem must be treated on a synoptic scale. In an effort to understand the problem at Indianapolis better, a series of experiments involving aircraft flights were conducted in the Indianapolis area and their results are herein reported. Specifically, a cross country flight of over 100 mi both to the northeast and southwest of Indianapolis, a vertical spiral to 28,000 ft, low level data associated with takeoff and landing of the aircraft, and ground data at four sites, are available for the afternoon of June 9,1976.     

Effect of Liquefied Petroleum Gas on Ozone Formation in Guadalajara and Mexico City

Abstract

Leakages of liquefied petroleum gas (LPG) are suspected to contribute greatly to ozone (O₃) formation in Mexico City. We tested such a hypothesis by outdoor captive-air irradiation (CAI) experiments in the two largest Mexican metropolitan areas: Guadalajara (GMA) in 1997 and Mexico City (MCMA) in 2000. O₃ was monitored in each city for 20 days (8:00 a.m.–6:00 p.m.) in smog chambers containing unaltered morning air or morning air enriched with either commercial LPG or LPG synthetic mixture 60/40 (propane and butane). Tested additions of both components were 35% (by volume) in GMA and 60% (by volume) in MCMA. The addition effects on O₃ (max) were compared with effects from diluting LPG components or total nonmethane hydrocarbons (tNMHCs) by 50%. Diluting tNMHCs had the greatest absolute effect at both cities: it lowered O₃ (max) by 24% in GMA and 55% in MCMA. Adding commercial LPG increased O₃ (max) by 6% in GMA and 28% in MCMA; whereas adding LPG synthetic mixture 60/40 caused a similar increase in O₃ (max), 4 and 21% in GMA and MCMA, respectively. Compared with dilution of tNMHCs, dilution of LPG-associated compounds had a smaller decreasing effect on O₃ (max), only 4% in GMA and 15% in MCMA. These results show that commercial LPG and LPG synthetic mixture 60/40 affect O₃ formation to a lesser extent than estimated previously.     

Trends in Meteorologically Adjusted Ozone Concentrations in Six Kentucky Metro Areas, 1998–2002

Abstract

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

Forecasting Peak Daily Ozone Levels: Part 2—A Regression with Time Series Errors Model Having a Principal Component Trigger to Forecast 1999 and 2002 Ozone Levels

Abstract

A modified time series approach, a Box-Jenkins regression with time series errors (RTSE) model plus a principal component (PC) trigger, has been developed to forecast peak daily 1-hr ozone (O₃) in real time at the University of Wisconsin-Milwaukee North (UWM-N) during 1999 and 2002. The PC trigger acts as a predictor variable in the RTSE model. It tries to answer the question: will the next day be a possible high O₃ day? To answer this question, three PC trigger rules were developed: (1) Hi-Low Checklist, (2) Discriminant Function Approach I, and (3) Discriminant Function Approach II. Also, a pure RTSE model without including the PC trigger and a persistence approach were tested for comparison. The RTSE model with DFA I successfully forecast the only two 1-hr federal exceedances (124 ppb), one in 1999 and one in 2002. In terms of the O₃ 100-ppb exceedances, 60–80% of the incorrect forecasts occurred with incorrect PC decisions. A few others may have been caused by unexpected O_3- weather relations. When the three approaches used UWM-N data to forecast a 100-ppb exceedance somewhere in the Milwaukee, WI, metropolitan area, their performance was dramatically improved: the false alarm rate was reduced from 0.89 to 0.44, and the probability of detection was increased from 0.71 to 0.88.     

Development of a Speciated,Hourly, and Gridded Air Pollutants Emission Modeling System—A Case Study on the Precursors of Photochemical Smog in the Seoul Metropolitan Area,Korea

ABSTRACT

A speciated, hourly, and gridded air pollutants emission modeling system (SHEMS) was developed and applied in predicting hourly nitrogen dioxide (NO₂) and ozone (O₃) levels in the Seoul Metropolitan Area (SMA). The primary goal of the SHEMS was to produce a systemized emission inventory for air pollutants including ozone precursors for modeling air quality in urban areas.

The SHEMS is principally composed of three parts: (1) a pre-processor to process emission factors, activity levels, and spatial and temporal information using a geographical information system; (2) an emission model for each source type; and (3) a post-processor to produce report and input data for air quality models through database modeling. The source categories in SHEMS are point, area, mobile, natural, and other sources such as fugitive emissions. The emission database produced by SHEMS contains 22 inventoried compounds: sulfur dioxide, NO₂, carbon monoxide, and 19 speciated volatile organic compounds. To validate SHEMS, the emission data were tested with the Urban Airshed Model to predict NO₂ and O₃ concentrations in the SMA during selected episode days in 1994. The results turned out to be reliable in describing temporal variation and spatial distribution of those pollutants.     

Understanding the Effectiveness of Precursor Reductions in Lowering 8-Hr Ozone Concentrations—Part II. The Eastern United States

Abstract

Analyses of ozone (O₃) measurements in conjunction with photochemical modeling were used to assess the feasibility of attaining the federal 8-hr O₃ standard in the eastern United States. Various combinations of volatile organic compound (VOC) and oxides of nitrogen (NO_x) emission reductions were effective in lowering modeled peak 1-hr O₃ concentrations. VOC emissions reductions alone had only a modest impact on modeled peak 8-hr O₃ concentrations. Anthropogenic NO_x emissions reductions of 46–86% of 1996 base case values were needed to reach the level of the 8-hr standard in some areas. As NO_x emissions are reduced, O₃ production efficiency increases, which accounts for the less than proportional response of calculated 8-hr O₃ levels. Such increases in O₃ production efficiency also were noted in previous modeling work for central California. O₃ production in some urban core areas, such as New York City and Chicago, IL, was found to be VOC-limited. In these areas, moderate NO_x emissions reductions may be accompanied by increases in peak 8-hr O₃ levels. The findings help to explain differences in historical trends in 1- and 8-hr O₃ levels and have serious implications for the feasibility of attaining the 8-hr O₃ standard in several areas of the eastern United States.     

Atrazine removal by ozonation processes in surface waters

Abstract

Atrazine (6‐chloro‐N‐ethyl‐N'‐isopropyl‐1,3,5‐triazinedyl‐2,4‐diamine) was treated with ozone alone and in combination with hydrogen peroxide or UV radiation in three surface waters. Experiments were carried out in two bubble reactors operated continously. Variables investigated were the ozone partial pressure, temperature, pH, mass flow ratio of oxidants fed: hydrogen peroxide and ozone and the type of oxidation including UV radiation alone. Residence time for the aqueous phase was kept at 10 min. Concentrations of some intermediates, including deethylatrazine, deisopropylatrazine and deethyldeisopropylatrazine, were also followed. The nature of water, specifically the alkalinity and pH were found to be important variables that affected atrazine (ATZ) removal. Surface waters with low alkalinity and high pH allowed the highest removal of ATZ to be reached. There was an optimum hydrogen peroxide to ozone mass flow ratio that resulted in the highest ATZ removal in each surface water treated. This optimum was above the theoretical stoichiometry of the process. Therefore, to reach the maximum removal of ATZ in a O₃/H₂O₂ process, more hydrogen peroxide was needed in the surface waters treated than in ultrapure water under similar experimental conditions. In some cases, UV radiation alone resulted in the removal of ATZ higher than ozonation alone. This was likely due to the alkalinity of the surface water. Ozonation and UV radiation processes yield different amounts of hydrogen peroxide. Combined ozonations (O₃/H₂O₂ and O₃/UV) lead to ATZ removals higher than single ozonation or UV radiation but the formation of intermediates was higher.