Ozone Air Quality Models

Abstract

The field of ozone air quality modeling, or as it is commonly referred to, photochemical air quality modeling, has undergone rapid change in recent years. Improvements in model components, as well as in methods of interpreting model performance, have contributed to this change. Attendant with this rapid change has been a growing need for those developing and using air quality models and policy makers to have a common understanding of the use and role of models in the decision making process. This Critical Review highlights recent advances and continuing problem areas in photochemical air quality modeling. Emphasis is placed on the components and input data for such models, model performance evaluation, and the implications for their use in regulatory decisions.     

Ozone Air Quality Models Prepared Discussion

The APCA Critical Review entitled “Ozone Air Quality Models” was presented by John H. Seinfeld, Louis E. Nohl Professor and Executive Officer for Chemical Engineering at the California Institute of Technology, Pasadena, California. Dr. Seinfeld presented his review at the 81st APCA Annual Meeting & Exhibition, held in Dallas, Texas in June 1988. The Critical Review paper, published in the May 1988 issue of JAPCA, highlighted recent advances and continuing problem areas in photochemical air quality modeling. Prepared discussions and floor comments presented during the Critical Review session in Dallas are published here, along with additional comments and closing remarks by Dr. Seinfeld. Howard M. Ellis, Chairman of the Critical Review Subcommittee of the Publications Committee, served as moderator of the 1988 APCA Critical Review session.     

The Need for a More Robust Ozone Air Quality Standard

The present National Ambient Air Quality Standard for ozone has many statistical problems, including use of extreme values which have inherent large fluctuations, a compliance test that can gradually lower the target of the design value below the standard level, and inconsistencies between the number-of-exceedances criterion and the design value. The above problems can be avoided or minimized by using a more robust statistic, such as the 95th percentile, and applying a statistical compliance test, without sacrificing the stringency of the standard. Analysis of EPA’s ozone data shows that the annual 95th percentiles and their three-year means have less variability than the annual second highest values and the fourth highest values in three years, respectively. A t test for the mean of the annual 95th percentiles is proposed for compliance testing not only to preserve the averaging concept of the present standard, but also to take account of ozone concentration fluctuations in order to increase the stability of the compliance status of a site or a Metropolitan Statistical Area. A procedure is provided to adjust the level of the 95th-percentile standard so that the stringency of the present standard is preserved.     

Stratospheric Ozone Protection: An EPA Engineering Perspective

Chlorine released into the atmosphere is a major factor in the depletion of the protective stratospheric ozone layer. The Montreal Protocol, as amended in 1990, and the Clean Air Act Amendments of 1990, address the limits and reduction schedules to be placed on chlorine- and bromine-containing chemicals. The status of technical solutions to the problem of chlorofluorocarbons, halons, methyl chloroform, and carbon tetrachloride in the major use areas of refrigeration, foam, aerosols, fire protection, and solvents is discussed here. The discussion includes the cooperative efforts involving academia, industry, U.S. governmental organizations, and other nations who are contributing solutions to these problems.     

Statistical Analysis of Trends in Urban Ozone Air Quality

The purpose of this paper is to demonstrate the use of some statistical methods for examining trends in ambient ozone air quality downwind of major urban areas. To this end, daily maximum 1 -hr ozone concentrations measured over New Jersey, metropolitan New York City and Connecticut for the period 1980 to 1989 were assembled and analyzed. This paper discusses the application of the bootstrap method, extreme value statistics and a nonparametric test for evaluating trends in urban ozone air quality. The results indicate that although there is an improvement in ozone air quality downwind of New York City, there has been little change in ozone levels upwind of New York City during this ten-year period.     

Air Quality Modeling and Decisions for Ozone Reduction Strategies

Abstract

Despite the widespread application of photochemical air quality models (AQMs) in U.S. state implementation planning (SIP) for attainment of the ambient ozone standard, documentation for the reliability of projections has remained highly subjective. An “idealized” evaluation framework is proposed that provides a means for assessing reliability. Applied to 18 cases of regulatory modeling in the early 1990s in North America, a comparative review of these applications is reported. The intercomparisons suggest that more than two thirds of these AQM applications suffered from having inadequate air quality and meteorological databases. Emissions representations often were unreliable; uncertainties were too high. More than two thirds of the performance evaluation efforts were judged to be substandard compared with idealized goals. Meteorological conditions chosen according regulatory guidelines were limited to one or two cases and tended to be similar, thus limiting the extent to which public policy makers could be confident that the emission controls adopted would yield attainment for a broad range of adverse atmospheric conditions. More than half of the studies reviewed did not give sufficient attention to addressing the potential for compensating errors. Corroborative analyses were conducted in only one of the 18 studies reviewed. Insufficient attention was given to the estimation of model and/or input database errors, uncertainties, or variability in all of the cases examined. However, recent SIP and policy‐related regional modeling provides evidence of substantial improvements in the underlying science and available modeling systems used for regulatory decision making. Nevertheless, the availability of suitable databases to support increasingly sophisticated modeling continues to be a concern for many locations. Thus, AQM results may still be subject to significant uncertainties. The evaluative process used here provides a framework for modelers and public policy makers to assess the adequacy of contemporary and future modeling work.     

Attainment Flip-Flops and the Achievability of the Ozone Air Quality Standard

Abstract

This paper presents the results of the first reported study on fine particulate matter (PM) chemical composition at Salamanca, a highly industrialized urban area of Central Mexico. Samples were collected at six sites within the urban area during February and March 2003. Several trace elements, organic carbon (OC), elemental carbon (EC), and six ions were analyzed to characterize aerosols. Average concentrations of PM with aerodynamic diameter of less than 10 μm (PM₁₀) and fine PM with aerodynamic diameter of less than 2.5 μm (PM_2.5) ranged from 32.2 to 76.6 μg m^-3 and 11.1 to 23.7 μg m^-3, respectively. OC (34%), SO₄ ⁼ (25.1%), EC (12.9%), and geological material (12.5%) were the major components of PM_2.5. For PM₁₀, geological material (57.9%), OC (17.3%), and SO₄ ⁼ (9.7%) were the major components. Coarse fraction (PM₁₀ –PM_2.5), geological material (81.7%), and OC (8.6%) were the dominant species, which amounted to 90.4%. Correlation analysis showed that sulfate in PM_2.5 was present as ammonium sulfate. Sulfate showed a significant spatial variation with higher concentrations to the north resulting from predominantly southwesterly winds above the surface layer and by major SO₂ sources that include a power plant and refinery. At the urban site of Cruz Roja it was observed that PM_2.5 mass concentrations were similar to the submicron fraction concentrations. Furthermore, the correlation between EC in PM_2.5 and EC measured from an aethalometer was r² = 0.710. Temporal variations of SO₂ and nitrogen oxide were observed during a day when the maximum concentration of PM_2.5 was measured, which was associated with emissions from the nearby refinery and power plant. From cascade impactor measurements, the three measured modes of airborne particles corresponded with diameters of 0.32, 1.8, and 5.6 μm.     

Variability of Ozone Air Quality Indicators in Selected Metropolitan Statistical Areas

This article analyzes numerical variability In ozone air quality data to understand how this variability affects the number of violations seen each year in metropolitan statistical areas (MSAs). Three commonly cited violation indices are used: 1) the annual number of expected exceedances averaged over 3 years is greater than 1; 2) the n+ 1th hourly value in n years of data is greater than 0.12 ppm; and 3) the annual number of expected exceedances is greater than 1. Only the first index is consistent with applicable regulations. The analyses indicate that about 23 percent of all MSAs with valid data had one or more change In their ozone violation status between 1979 and 1987. This change in status occurred for approximately 7 percent of all MSA-years of available data. This statistic was about one-third of the value usually obtained when the two incorrect, but commonly used, criteria of ozone violations are used.     

Consideration of Air Quality Standards for Vegetation With Respect to Ozone

Present evidence suggests that ozone is the most damaging of all air pollutants affecting vegetation. It is the principal oxidant in the photochemical smog complex. Concentrations of ozone have exceeded 0.5 part per million (ppm) in the Los Angeles area. One-tenth of this level for 8 hours is known to injure very sensitive tobacco varieties. Many plant species are visibly affected after a few hours exposure at concentrations much lower than 0.5 ppm. There is also some evidence that ozone reduces plant growth. Many factors must be taken into account when considering standards to protect vegetation from ozone damage. These include ozone concentration and methods of measurement, time of exposure, possible additive effects of other pollutants, sensitivity of plant species, their economic value, and the extent of injury which can be tolerated. The response of a species to the pollutant is conditioned by genetic factors and environmental conditions. Lack of specific routine methods for measuring ozone in ambient air is a handicap. California and Colorado established standards for oxidants at 0.15 and 0.10 ppm, respectively, for 1 hour. How these standards relate to the ozone dosage causing acute and chronic injury to various plant species is discussed.     

Issues Regarding the Ozone Air Quality Standard and the Design Value

Abstract

Two problems exist in the form and the compliance test of the present National Ambient Air Quality Standard (NAAQS) for ozone. One is the use of the number of exceedances in the form of the standard, which generates confusion and unnecessary complexity when the form is translated to the design value. The other is the requirement of a zero percent chance of violation in the compliance test, which makes the NAAQS considerably more stringent than generally assumed. There are also two sample-size problems in the estimation procedure for the design value. One is the upward creeping of the (n+l)th highest value in n years as n increases from one in the table look-up approach. The other is the infinite-sample-size assumption instead of the number of high-ozone season days per year for the daily maximum ozone concentrations in the distribution fitting approach. Both problems lead to an exaggeration of the design value.

The above problems can be removed in a revised NAAQS by (1) using a statistic that is identical to the design value itself in the form of the standard, (2) defining the design value as an n-year mean of, say, the annual mth highest values rather than the xth highest value in n years, and (3) using a simple compliance test like the t test that compares the design value with the level of the standard, taking into account the year-to-year fluctuation of the annual mth highest values. When the design value of an area is close to the level of the standard, the test provides a natural “too close to call” interval, which adjusts itself with the fluctuation of the annual mth highest values, so that as the fluctuation increases, the ability to assign the compliance status of the area decreases. The inclusion of a “too close to call” interval or category in the standard is critical to reduce the tendency toward ozone attainment flip-flops in areas approaching attainment and to assure that the ozone NAAQS is not more or less stringent than it appears.     

The Characterization of Ozone and Sulfur Dioxide Air Quality Data for Assessing Possible Vegetation Effects

Since the 1960s, much effort has been devoted to collecting and formatting air quality data. This paper discusses 1) the availability of air quality data for assessing potential biological impacts associated with ozone and sulfur dioxide ambient exposures, 2) examples of how air quality data can be characterized for assessing vegetation effects, and 3) the limitations associated with some exposure parameters used for developing relevant vegetation doseresponse yield reduction models. Data are presented showing that some ozone monitoring sites not continuously affected by local urban sources experience consecutive hourly ozone exposures ≥0.10 ppm in the late evening and early morning hours. These sites experience their maximum ozone concentrations either in the spring or summer months. Sites influenced by local rural sources experience their maximum ozone concentrations during the summer months. It is suggested that further research be performed to identify whether the sensitivity of a target organism at the time of exposure, as well as the pollutant concentration and chemical form that enters into the target organism, is as important in defining effects as air pollutant exposure alone.     

Simulation of the Environmental Impact of an Airport on the Surrounding Air Quality

It is the purpose of this study to demonstrate the procedure involved in simulating those average and maximum pollutant concentrations at or around an airport which fall under the control of the Clean Air Act. The information is useful, when planning new or expanding existing airports, when estimating the impact of airports on the surrounding air quality, and when assessing the effectiveness of control procedures. Simulation of airport air quality requires the accurate assessment of the temporal and spatial emission patterns. This involves the tabulation of air traffic density by type and engine, make and model of aircraft, and engine mode number; the use of fuel by different aircraft; the pollutant emission rates by engine model and operational mode; the allocation of emission rates to the respective runways, turn-off points, taxi-ways, and parking areas, and the time each aircraft spent in the different operational modes. The resulting emission pattern for the Honolulu International Airport reflects scheduled and unscheduled commercial and military jet and piston aircraft and nonaircraft operations. Using this and the appropriate meteorological information average and maximum surface concentrations were calculated and compared with local ambient air quality standards. The calculation of concentrations is based on a newly developed diffusion model incorporating harmonic mean wind speeds for every degree of wind direction as determined by a Parzen maximum likelihood interpolation technique, and the assumption of log-normal concentration distributions. It is shown that for some pollutants the air quality standards are substantially exceeded, and it is concluded that airports may have a considerable adverse impact on their surrounding air quality.     

The Impact of Migration on Air Quality Dose-Response Functions: A Case Study of Jacksonville,Florida

The purpose of this study was to analyze quantitative relationships between air pollution and mortality, and to examine the impact of migration on pollution-related mortality functions. Dose-response functions were estimated for intra-urban variations in ambient air quality for the city of Jacksonville, Florida. Indices of air pollution used in this study were sulfur dioxide (SO₂) and total suspended particulates (TSP). Ambient air quality was measured by the dispersion of TSP and SO₂ across census tracts using the SYMAP dispersion model in conjunction with air quality monitoring stations.

Holding other things constant, TSP apeared to have no statistically significant association with mortality rates. By contrast, the significance of the estimated coefficient for the pollution variable, SO₂, supported the contention that there is a positive and statistically significant relationship between air pollution and mortality rates. However, after making a limited test of the impact of migration on dose-response functions, the SO₂ pollution variable was no longer statistically significant. That is, recent migrants may have limited exposure to the existing level of SO₂ in Jacksonville, Florida, but carry with them long term exposure to more heavily polluted areas in the Northern United States. The results of this study suggest that further epidemiological studies and economic analysis of the health effects on air pollution should make some attempt to control the migration effect.     

Monte Carlo Simulation Techniques Applied to Air Quality Impact Assessment

Hammersly and Handscomb¹ describe Monte Carlo Methods as a branch of experimental mathematics concerned with experiments on random numbers. The purpose of this paper is to suggest methods through which this powerful technique can be applied to the problem of evaluating the return period of a concentration estimate.     

A Re-Examination of Ambient Air Ozone Monitor Interferences

Abstract

Attaining the National Ambient Air Quality Standard (NAAQS) for ozone (O₃) could cost billions of dollars nationwide. Attainment of the NAAQS is judged on O₃ measurements made by the Federal Reference Method (FRM), ethylene chemiluminescence, or a Federal Equivalent Method (FEM), predominantly ultraviolet (UV) absorption. Starting in the 1980s, FRM monitors were replaced by FEMs so that today virtually all monitoring in the United States uses the UV methodology. This report summarizes a laboratory and collocated ambient air monitoring study of interferences in O₃ monitors. Potential interferences examined in the laboratory included water vapor, mercury, o-nitrophenol, naphthalene, p-tolualdehyde, and mixed reaction products from smog chamber simulations of urban atmospheric photochemistry. UV absorption O₃ monitors modi?ed for humidity equilibration were also collocated with UV FEM O₃ monitors at six sites in Houston, TX, during the 2007 summer O₃ season. The results suggest that humidity and interfering species can positively bias (overestimate) O₃ measured by FEM monitors used to determine compliance with the O₃ standards. The results also suggest that humidity equilibration can mitigate this bias.     

The Impact of Computers Upon Air Pollution Research

The electronic computer has had a profound effect upon air pollution research activities. This paper discusses some of the ways in which Public Health Service scientists routinely make use of the computer. It is used as a data processor to handle the large volumes of data resulting from continuous air monitoring devices. More importantly, it is used to perform the very complex computations involved in various statistical analyses and mathematical model building. In future years computers may be used on a real-time basis to control the sources which emit pollutants to the atmosphere.     

A Quantitative Approach to the Traffic Air Quality Problem: The Traffic Air Quality Index

Abstract

The Traffic Air Quality (TAQ) model is a simple tool to estimate traffic fine particulate emissions on roadways (g/km) and can be used for both real-time analysis and for localized conformity analysis (“hot-spot” analysis for nonattainment areas) as defined by 40 CFR 93.123. This paper is a follow-up to a study published earlier regarding the development of the TAQ model. This paper shows how local air quality levels can be a factor in traffic management in nonattainment areas. Similar to the industrial source quotas measured in tons per year, it is proposed that road segments are to be assigned emission quotas (or TAQ indices) measured in pollutant mass emitted per road length (g/km) above which traffic-measures have to be taken to reduce the fine-particulates emissions on such road links. The TAQ model as well as traffic-rerouting measures along with the Intelligent Transportation System (ITS) protocols can be used to have a real-time control of the traffic conditions along expressways to maintain the fine-particulates emissions below the quota assigned per road link and consequently improving the over all local air quality in nonattainment areas.