Urban air quality

Since 1950 the world population has more than doubled, and the global number of cars has increased by a factor of 10. In the same period the fraction of people living in urban areas has increased by a factor of 4. In year 2000 this will amount to nearly half of the world population. About 20 urban regions will each have populations above 10 million people.Seen over longer periods, pollution in major cities tends to increase during the built up phase, they pass through a maximum and are then again reduced, as abatement strategies are developed. In the industrialised western world urban air pollution is in some respects in the last stage with effectively reduced levels of sulphur dioxide and soot. In recent decades however, the increasing traffic has switched the attention to nitrogen oxides, organic compounds and small particles. In some cities photochemical air pollution is an important urban problem, but in the northern part of Europe it is a large-scale phenomenon, with ozone levels in urban streets being normally lower than in rural areas. Cities in Eastern Europe have been (and in many cases still are) heavily polluted. After the recent political upheaval, followed by a temporary recession and a subsequent introduction of new technologies, the situation appears to improve. However, the rising number of private cars is an emerging problem. In most developing countries the rapid urbanisation has so far resulted in uncontrolled growth and deteriorating environment. Air pollution levels are here still rising on many fronts.Apart from being sources of local air pollution, urban activities are significant contributors to transboundary pollution and to the rising global concentrations of greenhouse gasses. Attempts to solve urban problems by introducing cleaner, more energy-efficient technologies will generally have a beneficial impact on these large-scale problems. Attempts based on city planning with a spreading of the activities, on the other hand, may generate more traffic and may thus have the opposite effect.     

Indoor air quality and health

During the last two decades there has been increasing concern within the scientific community over the effects of indoor air quality on health. Changes in building design devised to improve energy efficiency have meant that modern homes and offices are frequently more airtight than older structures. Furthermore, advances in construction technology have caused a much greater use of synthetic building materials. Whilst these improvements have led to more comfortable buildings with lower running costs, they also provide indoor environments in which contaminants are readily produced and may build up to much higher concentrations than are found outside. This article reviews our current understanding of the relationship between indoor air pollution and health. Indoor pollutants can emanate from a range of sources. The health impacts from indoor exposure to combustion products from heating, cooking, and the smoking of tobacco are examined. Also discussed are the symptoms associated with pollutants emitted from building materials. Of particular importance might be substances known as volatile organic compounds (VOCs), which arise from sources including paints, varnishes, solvents, and preservatives. Furthermore, if the structure of a building begins to deteriorate, exposure to asbestos may be an important risk factor for the chronic respiratory disease mesothelioma. The health effects of inhaled biological particles can be significant, as a large variety of biological materials are present in indoor environments. Their role in inducing illness through immune mechanisms, infectious processes, and direct toxicity is considered. Outdoor sources can be the main contributors to indoor concentrations of some contaminants. Of particular significance is Radon, the radioactive gas that arises from outside, yet only presents a serious health risk when found inside buildings. Radon and its decay products are now recognised as important indoor pollutants, and their effects are explored. This review also considers the phenomenon that has become known as Sick Building Syndrome (SBS), where the occupants of certain affected buildings repeatedly describe a complex range of vague and often subjective health complaints. These are often attributed to poor air quality. However, many cases of SBS provide a valuable insight into the problems faced by investigators attempting to establish causality. We know much less about the health risks from indoor air pollution than we do about those attributable to the contamination of outdoor air. This imbalance must be redressed by the provision of adequate funding, and the development of a strong commitment to action within both the public and private sectors. It is clear that meeting the challenges and resolving the uncertainties associated with air quality problems in the indoor environment will be a considerable undertaking.     

Association between air quality and quality of life

Air quality—or its converse, air pollution—is a significant risk factor for human health. Recent studies have reported association between air pollution and human health. There are numerous diseases that may be caused by air pollution such as respiratory infection, lung cancer, cardiovascular disease, chronic obstructive pulmonary disease, and asthma. In this study, the relationship between air quality and quality of life was examined by using canonical correlation analysis. Data of this study was collected from 27 countries. WHO statistics were used as the main source of quality of life data set (Y variables set). European Environment Agency statistics and (for outdoor air-PM10) WHO statistics were used as the main source of air quality data set (X variables set). It is found that there are significant positive correlation between air quality and quality of life.     

Assessment of air quality benefits from national air pollution control policies in China. Part II: Evaluation of air quality predictions and air quality benefits assessment

Following the meteorological evaluation in Part I, this Part II paper presents the statistical evaluation of air quality predictions by the U.S. Environmental Protection Agency (U.S. EPA)’s Community Multi-Scale Air Quality (Models-3/CMAQ) model for the four simulated months in the base year 2005. The surface predictions were evaluated using the Air Pollution Index (API) data published by the China Ministry of Environmental Protection (MEP) for 31 capital cities and daily fine particulate matter (PM_2.5, particles with aerodiameter less than or equal to 2.5 μm) observations of an individual site in Tsinghua University (THU). To overcome the shortage in surface observations, satellite data are used to assess the column predictions including tropospheric nitrogen dioxide (NO₂) column abundance and aerosol optical depth (AOD). The result shows that CMAQ gives reasonably good predictions for the air quality.The air quality improvement that would result from the targeted sulfur dioxide (SO₂) and nitrogen oxides (NO_x) emission controls in China were assessed for the objective year 2010. The results show that the emission controls can lead to significant air quality benefits. SO₂ concentrations in highly polluted areas of East China in 2010 are estimated to be decreased by 30–60% compared to the levels in the 2010 Business-As-Usual (BAU) case. The annual PM_2.5 can also decline by 3–15 μg m^?3 (4–25%) due to the lower SO₂ and sulfate concentrations. If similar controls are implemented for NO_x emissions, NO_x concentrations are estimated to decrease by 30–60% as compared with the 2010 BAU scenario. The annual mean PM_2.5 concentrations will also decline by 2–14 μg m^?3 (3–12%). In addition, the number of ozone (O₃) non-attainment areas in the northern China is projected to be much lower, with the maximum 1-h average O₃ concentrations in the summer reduced by 8–30 ppb.     

Tunnel air quality and vehicle emissions

A general relationship between roadway tunnel air quality and vehicle emissions has been derived. The model includes the effect of pollutant deposition on the tunnel surfaces and dilution from ventilation. The model is applied to air quality measurements of SO₂ and particulates obtained at the Tuscarora Mountain Tunnel. It is found that, if deposition is neglected, SO₂ and sulfate emission factors for both gasoline and Diesel vehicles may be underestimated by ~ 10%. The derived deposition velocities are 0.07cms⁻¹ for SO₂, 0.03cms⁻¹ for sulfate, and ~ 0.001 cms⁻¹ for total suspended particulates and paniculate components (except sulfate). The last value is lower than smooth-surface values quoted for aerosol deposition, and the difference between the last two values presumably reflects the approximations in the model and/or the uncertainty in its input data.     

interpretation of air quality data with respect to the national ambient air quality standards

San Diego Gas &; Electric has developed a quality assurance program for continuous emission monitors (CEM). Extractive, rather than in situ, monitors were selected as a result of an in-house evaluation program. Two extractive systems have been certified and a good operating and maintenance record has been established on these systems. A successful program requires the involvement and support of all affected personnel. It is desirable to have one or two key personnel coordinate the development of the program. It is also highly desirable to have good in-house source testing capabilities.     

Sequential box models for indoor air quality: Application to airliner cabin air quality

In this paper we present the development and application of a model for indoor air quality. The model represents a departure from the standard box models typically used for indoor environments which has applicability in residences and office buildings. The model has been developed for a physical system consisting of sequential compartments which communicate only with adjacent compartments. Each compartment may contain various source and sink terms for a pollutant as well as leakage, and air transfer from adjacent compartments. The mathematical derivation affords rapid calculation of equilibrium concentrations in an essentially unlimited number of compartments. The model has been applied to air quality in the passenger cabin of three commercial aircraft. Simulations have been performed for environmental tobacco smoke (ETS) under two scenarios, CO₂ and water vapor. Additionally, concentrations in one aircraft have been simulated under conditions different from the standard configuration. Results of the simulations suggest the potential for elevated concentrations of ETS in smoking sections of non-air-recirculating aircraft and throughout the aircraft when air is recirculated. Concentrations of CO₂ and water vapor are consistent with expected results. We conclude that this model may be a useful tool in understanding indoor air quality in general and on aircraft in particular.     

Meteorological and climatological factors affecting Denver air quality

In a study of Denver air pollution, regional climatological factors are related primarily to diurnal and seasonal characteristics of air quality. Meteorological factors are assessed mainly for interpreting deviations from the mean, or the frequency of pollution episodes or clean air days. Local topography also influences some of the factors affecting Denver air quality. Emphasis is given to the importance of mixing depth, ventilation, and in particular to the stability above the well-mixed (adiabatic) layer. Emission characteristics and variations in the distribution of pollutants are discussed briefly. Approximately ten years of surface and radiosonde data obtained from Stapleton Airport in Denver are used for a climatological base. Special soundings taken by radiosondes released at noon form the nucleus of the meteorological data base.     

Modeling relationships between indoor and outdoor air quality

Information about the ratio between indoor and outdoor concentrations (IO ratios) of air pollutants is a crucial component in human exposure assessment. The present study examines the relationship between indoor and outdoor concentrations as influenced by the combined effect of time patterns in outdoor concentrations, ventilation rate, and indoor emissions. Two different mathematical approaches are used to evaluate IO ratios. The first approach involves a dynamic mass balance model that calculates distributions of transient IO ratios. The second approach assumes a linear relationship between indoor and outdoor concentrations. We use ozone and benzene as examples in various modeling exercises. The modeled IO ratio distributions are compared with the results obtained from linear fits through plots of indoor versus outdoor concentrations.     

Inherent uncertainty in air quality modeling

This paper summarizes the discussions of a working group that was charged with the task of examining inherent uncertainty in air quality modeling. The major topics of the paper are:

• 1.1. Definition of inherent uncertainty in air quality models;
• 2.2. Determination of inherent uncertainty;
• 3.3. Role of inherent uncertainty in model evaluation.

The concepts introduced here are illustrated through a numerical simulation with Gifford's fluctuating plume model.     

Sulfate air quality control strategy design

An approach to the design of emission control strategies for sulfate air quality improvement is described. Methods developed are tested within a case study of the nature and causes of the high sulfate levels observed in the Los Angeles area.An air quality model for sulfate formation and transport is developed which computes long-term average sulfate concentrations using a Lagrangian marked particle technique. The air quality model is verified by application to the Los Angeles air basin during each month of the years 1972 to 1974. The time sequence of observed sulfate air quality is reproduced closely in spite of the fact that emission peaks are six months out of phase with observed peak sulfate concentrations. An important finding is that there is a seasonal trend in the rate of SO₂ oxidation to form sulfates in the Los Angeles atmosphere with conversion rate averaging about 6% per hour in the late spring, summer and early autumn, and declining to between 3% and 0.5% per hour in winter months.The problem of identifying the least costly combination of emission controls needed to achieve a major sulfate concentration reduction is addressed using the air quality model results. Example calculations show that close to a 50% improvement in sulfate air quality could be achieved in downtown Los Angeles at a cost of circa $100 million annually. Since the effect on visibility of such a sulfate concentration reduction has been estimated previously, a partial remedy for the Los Angeles visibility problem is described.     

Background levels of air and precipitation quality for Europe

This paper is intended to be used by specialists engaged in air and precipitation quality management on regional and continental scales. Major goals are to establish definition, methodology and specific values of background air and precipitation quality for sulfur (S) and nitrogen (N) species to be used in practical applications of air resources management. Major findings are the following:

• 1.(a) 69% of SO₂ and 63 % of NO₂ concentration over Europe originate from continental scale anthropogenic sources,
• 2.(b) 15% of precipitation sulfate and 11% of precipitation nitrate over Europe are contributed by hemispheric background,
• 3.(c) hemispheric background pollution values for Europe were found as 1.25 μg (SO₂-S)m⁻³, 0.80 μg (SO₄²⁻-S)m⁻³, 0.157 mg (SO₄²⁻-S)l⁻¹ and 0.04 mg (NO₃⁻-N)ℓ⁻¹.