Urban Ozone Research Articles

Reliability of optimal control strategies for photochemical air pollution.

This study illustrates how consideration of modeling uncertainties can affect optimal control strategies for urban ozone. Control strategies are investigated for illustrative cases of air parcel trajectories ending at Azusa, CA, and Riverside, CA, on August 28, 1987. The control strategies are designed to achieve a specified air quality target with a given reliability, considering uncertainties in the California Institute of Technology's trajectory model and its inputs, including uncertainties in emissions and in the SAPRC-97 chemical mechanism. A decoupled stochastic optimization scheme is used to solve the chance-constrained programming problem. Least-cost control strategies derived using nominal model inputs and parameter values have low reliability for some target O3 concentrations when uncertainties are taken into account. For the case considered, reducing volatile organic compound (VOC) emissions from motor vehicles is identified as the least-cost approach to meeting O3 targets at Azusa. However, the optimal control strategies for Riverside depend on the target O3 concentrations and the level of reliability required. Consideration of model uncertainty is found to shift the focus from VOC controls to nitrogen oxide controls for the Riverside trajectory.

The evolution of the boundary layer and its effect on air chemistry in the Phoenix area

During a 4‐week period in May and June of 1998, meteorological and chemical measurements were made as part of a field campaign carried out in the Phoenix area. Data from the field campaign provide the first detailed measurements of the properties of the convective boundary layer in this area and of the effects of these properties on ozone levels. The meteorological and chemical measurements have been combined with results from a set of meteorological, particle, and chemistry models to study ozone production, transport, and mixing in the vicinity of Phoenix. Good agreement between the simulations and observations was obtained, and the results have been used to illustrate several important factors affecting ozone patterns in the region. Heating of the higher terrain north and east of Phoenix regularly produced thermally driven circulations from the south and southwest through most of the boundary layer during the afternoon, carrying the urban ozone plume to the northeast. The combination of deep mixed layers and moderate winds aloft provided good ventilation of the Phoenix area on most days so that multiday buildups of locally produced ozone did not appear to contribute significantly to ozone levels during the study period. Sensitivity simulations determined that 20 to 40% of the afternoon surface ozone mixing ratios (corresponding to 15 to 35 ppb) were due to vertical mixing processes that entrained reservoirs of ozone into the growing convective boundary layer. The model results also indicated that ozone production in the region is volatile organic compound limited.

Characteristics of Urban Ground-Level Ozone in Korea

ABSTRACT This study considers the characteristics of ground-level ozone (O3) in five Korean cities over a time period of 6-8 years. The focus of this study is daily maximum 1-hr and 8-hr concentrations. For all the study cities in the period examined, the mean and most of the percentiles (5, 10, 25, 50, 75, 90, and 95) for the daily maximum 1-hr and 8hr concentrations showed increasing trends, although not all trends were statistically significant. The daily maximum 1-hr and 8-hr concentrations slowly increased during late winter, and peaks were attained during the summer season (from May to September). All the selected cities exhibited a high degree of correlation between their daily maximum 8-hr and 1-hr concentrations. The daily maximum 8-hr concentrations, which were climatologi-cally equivalent to the Korean 1 hr/100 parts per billion (ppb) standard, were higher than the current 8 hr/60 ppb by a difference of 8-16 ppb. Compared with other cities in Korea, Seoul recorded a substantially higher frequency of days and hours with concentrations above 1 hr/100 ppb, and a higher frequency of days with concentrations above 8 hr/60 ppb and 8 hr/80 ppb. Seoul also recorded a substantially higher frequency of hours with concentrations above 1 hr/100 ppb than days with concentrations above 1 hr/100 ppb, implying that on some days severe exceedances persisted for more than one hour per day. During multiple-day episodes a North Pacific High dominated Korea, which is quite typical in Korea during the summer season.

On the NO2 + soot reaction in the atmosphere

Recent modeling studies have suggested that soot is a key component of tropospheric chemistry in remote regions, acting to reduce HNO3 to NO2 and possibly NO2 to NO. It may be expected then that soot also affects the chemistry of rural and urban areas, where soot concentrations are typically several orders of magnitude higher than in the remote troposphere. In order to test this assumption, a modeling study was conducted for typical urban and rural areas, with the same HNO3/NO2/soot chemistry proposed in the previous modeling studies of the remote troposphere. Unreasonable results were found (e.g., nearly total suppression of urban ozone, in contradiction to common observations), suggesting that the NO2/soot reaction was considerably overestimated in previous modeling studies. Therefore the NO2/soot chemistry was reconsidered. A new preliminary mechanism is suggested, based on recent laboratory studies of this reaction. Results show that the NO2/soot reaction does not notably affect the Ox‐NOx‐HOx chemistry of the lower continental troposphere, except maybe during nighttime in urban environments. A potential contribution of the NO2/soot interaction to HONO production is noted.

Ozone production during an urban air stagnation episode over Nashville, Tennessee

The highest O3 levels observed during the 1995 Southern Oxidants Study in middle Tennessee occurred during a period of air stagnation from July 11 through July 15. Extensive airborne (two fixed wing and one helicopter) and ground‐based measurements of the chemistry and meteorology of this episode near Nashville, Tennessee, are presented. In situ airborne measurements include O3, NOy, NO, NO2, SO2, CO, nitrate, hydrocarbons, and aldehydes. Airborne LIDAR O3 measurements are also utilized to map the vertical and horizontal extent of the urban plume. The use of multiple instrumented research aircraft permitted highly detailed mapping of the plume chemistry in the vertical and horizontal dimensions. Interactions between the urban Nashville plume (primarily a NOx and hydrocarbon source) and the Gallatin coal‐fired power plant plume (primarily a NOx and SO2 source) are also documented, and comparisons of ozone formation in the isolated and mixed urban and power plant plume are presented. The data suggest that during this episode the background air and the edges of the urban plume are NOx sensitive and the core of the urban plume is hydrocarbon sensitive. Under these worst case meteorological conditions, ambient O3 levels well over the level of the new National Ambient Air Quality Standard (NAAQS) for ozone (80 ppb) were observed over and just downwind of Nashville. For example, on July 12, the boundary layer air upwind of Nashville showed 60 to 70 ppb O3, while just downwind of the city the urban plume maximum was over 140 ppb O3. With a revised ozone standard set at 80 ppb (8 hour average) and upwind levels already within 10 or 20 ppb of the standard, only a slight increase in ozone from the urban area will cause difficulty in attaining the standard at monitors near the core of the urban plume during this type of episode. The helicopter mapping and LIDAR aircraft data clearly illustrate that high O3 levels can occur during stagnation episodes within a few kilometers of and even within the urban area. The extremely light boundary layer winds (1–3 m s−1) contributed to the creation of an ozone dome or blob which stayed very near to the city rather than an elongated plume. The small spatial scale of the zone of high O3 concentrations is mapped in detail demonstrating that the regulatory monitoring network failed to document the maximum O3 concentrations. Modelers using such regulatory data to test photochemical algorithms need to bear in mind that magnitude and frequency of urban ozone may be underestimated by monitoring networks, especially in medium‐sized urban areas under slow transport conditions. Finally, this effort shows the value of collaborative field measurements from multiple platforms in developing a more complete picture of the chemistry and transport of photochemical O3.

Relative production of ozone and nitrates in urban and rural power plant plumes: 1. Composite results based on data from 10 field measurement days

A rather limited number of large power plants are responsible for about 2/3 and 1/3 of the U.S. anthropogenic emissions of SO2 and NOx, respectively. Considerable uncertainty continues to prevail about the local and regional impact of their potentially harmful secondary products (e.g., ozone, sulfates, nitrates), We have analyzed state‐of‐the‐art data of the Southern Oxidant Study (SOS)‐Nashville Field Study (1994, 1995) for 10 days of summer daytime field measurements by instrumented aircraft in the plumes of three large, tall‐stack, base‐load, Tennessee Valley Authority (TVA) coal‐fired power plants in northwestern Tennessee: Gallatin (G), located within the Nashville urban ozone nonattainment area, and Cumberland (C) and Johnsonville (JV) in rural isoprene‐rich forested areas about 100 km to the west of Nashville. The average 1995 emissions of NOx from these three sources ranged over more than an order of magnitude. In this paper, we have explored plume chemical evolution and the magnitude, efficiency, and yield of ozone and NOz, (NOx oxidation products, mostly inorganic and organic nitrates) production in a broad variety of plume transport and chemistry scenarios within the convective boundary layer (CBL) in rural and urban settings. The results show that (1) plume chemical maturity and peak production capacities of ozone and NOz were realized quite close to the sources, within 30–40 km and 4 hours of daytime transport for Gallatin (smallest NOx emission rate, QNOx, and suburban environment) and typically within 100 km and 6 hours of CBL transport for Cumberland (highest QNOx and rural environment rich in isoprene); (2) the ozone impact of Gallatin on Nashville can exceed that of Cumberland, and under favorable transport and chemical conditions, both power plants can contribute as much as 50 ppb of excess ozone to the urban area, raising local peak levels well in excess of 100 ppb; (3) an estimated 3.1±0.7 molecules of ozone and more than 0.6 molecules of NOz, may be produced in large isolated rural power plant plumes (PPPs) per molecule of NOx release, and the corresponding peak yields of ozone and NOz may be significantly greater in urban PPPs; (4) the rate of NOz production ≈ 10–15% h−1 in isolated rural PPPs, and higher in urban PPPs; (5) NOz production is favored in all PPPs at first when the chemistry is VOC‐limited; later, with increasing VOC ingestion from the background, the chemistry increasingly favors NOx‐limited ozone production, starting at plume edges, and ultimately throughout the diluted plume. These results have major implications on outstanding issues related to the environmental impact and regulatory control of electric utility industry NOx emissions.

Daytime buildup and nighttime transport of urban ozone in the boundary layer during a stagnation episode

A 3‐day period of strong, synoptic‐scale stagnation, in which daytime boundary‐layer winds were light and variable over the region, occurred in mid July of the 1995 Southern Oxidants Study centered on Nashville, Tennessee. Profiler winds showed light and variable flow throughout the mixed layer during the daytime, but at night in the layer between 100 and 2000 m AGL (which had been occupied by the daytime mixed layer) the winds accelerated to 5–10 m s−1 as a result of nocturnal decoupling from surface friction, which produced inertial oscillations. In the present study, we investigate the effects of these wind changes on the buildup and transport of ozone (O3). The primary measurement system used in this study was an airborne differential absorption lidar (DIAL) system that profiled O3 in the boundary layer as the airplane flew along. Vertical cross sections showed that O3 concentrations exceeding 120 ppb extended up to nearly 2 km AGL, but that the O3 hardly moved at all horizontally, instead forming a dome of pollution over or near the city. The analysis concentrates on four meteorological processes that determine the 3‐D spatial distribution of O3 and the interaction between urban and rural pollution: (1) daytime buildup of O3 over the urban area, (2) the extent of the drift of pollution cloud during the day as it formed, which controls peak O3 concentrations, (3) nighttime transport by the accelerated winds above the surface, and (4) vertical mixing of pollution layers the next day. Other consequences of very light‐wind conditions were intra‐regional differences in daytime mixed‐layer depth over distances of 50 km or less, and indications of an urban heat‐island circulation.

The urban plume of Vienna: comparisons between aircraft measurements and photochemical model results

A Lagrangian photochemical box model (IMPO model) developed for ozone forecasts and long-term scenario calculations was evaluated by comparing simulated ozone and nitrogen dioxide concentrations with aircraft measurements done around Vienna/Austria during episodes of high ozone concentrations in 1995. The major aim of these measurements was to record position, extent and intensity of the urban ozone plume. The Lagrangian model simulations were done on a grid covering Vienna and surroundings. The simulated concentrations were interpolated to the aircraft positions. In most cases, the horizontal position of the plume was reasonably well reproduced by the model, demonstrating that despite of all the necessary simplifications, a box model based on air-mass trajectories can be a valuable tool in describing the transport of pollutants. The ozone production in the plume was underpredicted by a factor of 2. High nitrogen dioxide concentrations measured near the city centre were reproduced by the model, but there are strong indications that nitrogen dioxide concentrations were underpredicted by the model in rural areas. A clear quantification proved to be impossible due to uncertainties in the measurement data.

The potential of statistical state space models in urban ozone forecasting

State space models for tropospheric urban ozone prediction are introduced and compared with linear regression models. The linear and non-linear state space models make accurate short-term predictions of the ozone dynamics. The average prediction error one hour in advance is 7 microg/m(3) and increases logarithmically with time until it reaches 26 microg/m(3) after 30 days. For a given sequence of solar radiation inputs, predictions converge exponentially with a time scale of 8 hours, so that the model is insensitive to perturbations of more than 150 microg/m(3) O(3). The slow increase of the prediction error in addition to the uniqueness of the prediction are encouraging for applications of state space models in forecasting ozone levels when coupled with a model that predicts total radiation. Since a radiation prediction model will be more accurate during cloud-free conditions, in addition to the fact that the state space models perform better during the summer months, state space models are suitable for applications in sunny environments.

Effects of Chemical Mechanism Uncertainties on the Reactivity Quantification of Volatile Organic Compounds Using a Three-Dimensional Air Quality Model

Accurate quantification of the ozone-forming potential, termed reactivity, of volatile organic compounds (VOCs) is critical for correctly assessing the impacts of emissions on air quality. As reactivity-based regulations are being more carefully considered for urban ozone control strategies, the uncertainties in the ability to quantify reactivity are gaining importance. This study utilized a three-dimensional air quality model to examine the uncertainty in reactivity quantification resulting from a set of reaction rate constant uncertainties. A previous study identified the set of rate constants that were most critical for single-cell model ozone predictions. With the detailed airshed model, uncertainties in rate constants for aldehyde photolysis, nitric acid formation, and decomposition of peroxy acetyl nitrate (PAN) and peroxy propionyl nitrate plus higher PAN analogues (PPN) exhibited the greatest impact on relative compound reactivity values. For the compounds and reactions examined, the combined responses to 2σ changes in reaction rate constants were approximately 15% of the predicted relative reactivity values, with the reactivities of ethylbenzene and toluene exhibiting the greatest response. The choice of reactivity quantification measures and the air quality models used had a greater impact on relative reactivity predictions than did the rate constant uncertainties.

Contribution of biogenic nitric oxide in urban ozone: Raleigh, NC, as a case study

Anthropogenic emissions from industrial and automotive sources within the confines of the city of Raleigh, NC have been documented by the North Carolina Department of Environment, Health and Natural Resources, Division of Environmental Management, but no direct biogenic emissions of nitric oxide (NO) from soils has yet been measured. In this study, emissions of NO were measured in Raleigh, NC, and its surrounding suburbs, in an attempt to determine the portion of the total NO x ( = NO + NO 2) budget which can be attributed to biogenic sources. Residential and commercial lawns, and golf courses receiving normal fertilizer applications were chosen as the primary biogenic source of NO. Soil NO fluxes were measured using a dynamic chamber technique from 11 sites and ranged in value (hourly averages calculated from 15 min readings) from 1.24 to 23.7 ng N m −2 s −1. These hour averages were then combined with estimates of lawn acreage within the city proper, and in the surrounding suburbs, in order to develop a budget for giogenic NO emissions in Raleigh. This budget was then compared to the budget used in the Environmental Protection Agency's (EPA) Regional Oxidant Model (ROM) for photochemical modeling. Results from this comparison suggest that less than 1 % of the total NO x budget for Raleigh, NC is emitted by natural processes, and that approximately 1.2% of the nitrogen applied as fertilizer is lost via soil NO emissions. Thus, the effects of biogenic NO may be neglected in the development of a reliable plan for reducing ozone in the urban atmosphere.

Environmental Policy Analysis, Peer Reviewed: Cost–Benefit and Uncertainty Issues in Using Organic Reactivity to Regulate Urban Ozone

Environmental Policy Analysis, Peer Reviewed: Cost–Benefit and Uncertainty Issues in Using Organic Reactivity to Regulate Urban Ozone

Research Watch: Urban trees and ozone

Research Watch: Urban trees and ozone

Predicting Urban Ozone Levels and Trends with Semiparametric Modeling

Ozone in the planetary boundary layer of the troposphere is considered harmful to plants and human health. Surface ozone levels are determined by the strengths of sources and precursor emissions and by meteorological conditions. Therefore, assessing ozone trends is complicated by meteorological variability. Ozone data in the Chicago area over the period 1981 to 1991 were studied. In this article, ozone concentration was modeled as a year effect plus a nonparametric model in meteorological variables. In this way, long-term trend over the 11-year period was separated from the effects of meteorology. Estimation of long-term trend is important for assessing the effectiveness of environmental policy. The indications were that there was a decrease in ozone levels once the meteorological effects on ozone had been adjusted for. The models based on this technique were also shown to provide predictions that closely match the actual ozone levels.

Inverse modeling of biogenic isoprene emissions

Using current biogenic emission estimates, Urban Airshed Model simulations for Atlanta, Georgia substantially underpredict isoprene concentrations relative to observations. In this work, an inverse method is used to determine the biogenic isoprene emissions for Atlanta that minimize the difference between the model‐simulated and the observed isoprene concentrations. The resulting isoprene emissions are 2 to 10 times higher than any of the accepted emission estimates. Overall, the concurrent ozone simulation improved when the biogenic isoprene emissions were increased. These higher isoprene emissions significantly increase the concentration of model‐simulated ozone within the plumes of nitrogen oxides emitted from large point sources. These results should be considered when developing control strategies for urban ozone.

Accounting for meteorological effects in measuring urban ozone levels and trends

Observed ozone concentrations are valuable indicators of possible health and environmental impacts. However, they are also used to monitor changes and trends in the sources of ozone and of its precursors, and for this purpose the influence of meteorological variables is a confounding factor. This paper examines ozone concentrations and meteorology in the Chicago area. The data are described using least absolute deviations and local regression. The key relationships observed in these analyses are then used to construct a nonlinear regression model relating ozone to meteorology. The model can be used to estimate that part of the trend in ozone levels that cannot be accounted for by trends in meteorology, and to ‘adjust’ observed ozone concentrations for anomalous weather conditions.

Intercomparison of Meteorological Models Applied to the Athens Area and the Effect on photochemical Pollutant Predictions

Abstract In this study, four different meteorological models, one diagnostic and three prognostic, are used to develop meteorological inputs for a photochemical model, as applied to the peninsula of Athens, Greece. The comparison of meteorological models results pointed out significant differences in the calculated wind fields, mainly during the night period. These differences are linked to specific aspects of the models, such as model vertical resolution, hydrostatic versus nonhydrostatic formulation, and numerical diffusion. During the day hours, models produce quite similar wind fields, which agree correctly with the available observations related to the Athens center area. Using the different wind fields as input to a photochemical air quality model led to similar urban ozone levels in the Athens area. Outside of the city, the different wind fields transport the urban plume in different directions in a range of 50°. The more primary pollutants, for example CO and NO2 concentrations, varied significant...

Urban Ozone Plumes and Population Distribution by Income and Race: A Case Study of New York and Philadelphia

Who lives in the downwind areas of urban ozone plumes? Location theory suggests that the poor should disaproportionately populate the downwind area, while the rich are most likely to live outside urban ozone plumes. Dynamically, we would expect that the downwind area would become poorer over time because the rich would "vote with their feet" under the serious impacts of urban ozone plumes. However, the importance of urban ozone pollution could be discounted due to the public's risk perception. According to risk and air quality perception literature, one would expect that the public would likely perceive ozone air pollution as a relatively low risk, compared with other environmental risks and social concerns. Therefore, actual population distribution might not follow what location theory suggests. Who actually does live in the downwind areas of urban ozone plumes? The results from a case study of New York and Philadelphia show that the ozone downwind areas are currently populated by a considerably larger proportion of upper income households and whites than the source areas. Furthermore, the population dynamics data do not provide any evidence for the "vote with their feet" hypothesis. While these findings deviate from the hypotheses prescribed by location theory, they are consistent with what we could expect from theories of risk perception.

A free lunch at higher CAFE? A review of economic, environmental and social benefits

Pressure has been increasing to raise the corporate average fuel economy (CAFE) standard. Some researchers assert many benefits from increasing CAFE, from less dependence on imported petroleum to lower urban ozone. We find that fuel savings from increasing CAFE are subject to diminishing returns, with little or no effect on urban air pollution, and a less than proportional reduction in greenhouse gas emissions. Higher CAFE reduces safety, unless offsetting steps are taken. Should the public demand moderately higher fuel efficiency, the government should raise the CAFE standard and gasoline prices. Given current technology, substantial increases in CAFE have potential costs, and do not appear to offer significant benefits as contended.

Photochemical Modeling of the Impact of Fuels and Vehicles on Urban Ozone Using Auto/Oil Program Data

An extensive set of emission tests has been conducted in the Auto/Oil Air Quality Improvement Research Program on different fuel/vehicle systems. These emission tests have been used to model the impact of fuel/vehicle changes on ozone formation in Los Angeles, Dallas−Fort Worth, and New York in 1995 and 2005/2010. Light-duty vehicles are estimated to contribute 28−37% of the peak ozone in 1980/1985, decreasing to 7−18% in 1995, and further decreasing to 5−9% in 2005/2010. Gasoline changes that show promise in reducing the contribution of light-duty vehicles to ozone formation are reductions in olefin content, 90% distillation temperature, sulfur content, and vapor pressure. Results for a methanol/gasoline blend (M85) used in prototype flexible/variable fuel vehicles depend on the assumptions used to project future M85 emissions. A research test gasoline produced less ozone than the M85 cases in Los Angeles and New York and either more or less ozone than M85 in Dallas−Fort Worth, depending on the assumptio...