Reduction In Particulate Matter Concentration Research Articles

Rain-Based Train Washing: A Sustainable Approach to Reduce PM Concentrations in Underground Environments

Fine particle concentrations measured in many underground rail systems around the world consistently exceed those observed at ground level, potentially posing significant implications for human health. While numerous authors have observed these high particle concentrations and analyzed both their atomic compositions and health impacts, few have investigated devices and technologies capable of reducing these high levels in underground environments. In light of these considerations and recognizing the multifaceted challenges associated with maintaining air quality in underground metro systems, the aim of this paper was to evaluate the usefulness and effectiveness of utilizing rainwater for washing trains to abate particulate matter (PM) concentrations in underground rail systems. To achieve this aim, an ad hoc case study was considered: the Naples Metro Line 1 (Italy), which is characterized by 4.5 km in the ground level and 13.5 km underground. A measurement campaign was carried out during storms of strong intensity through PM measuring instruments placed on station platforms along the metro line. Precisely, the trains were washed by the rain in the initial ground level section, and then continued wet within the underground one. The results of this measurement campaign were compared with those of a comparable survey carried out during average clear weather conditions, and the results showed that the train washing produces a significant PM10 concentration reduction of up to about 60% in the underground environment. If confirmed in other experimental settings, these results could lay the groundwork for the introduction of structured washing system devices (e.g., periodically washing trains and/or tunnels) for the reduction of PM concentration in underground metro systems. The present study sought to contribute valuable insights towards sustainable and environmentally conscious approaches to addressing air quality concerns, particularly by harnessing the natural resource of rainwater during specific meteorological events.

Three-Dimensional Numerical Modeling and Analysis for the Municipal Solid-Waste Incineration of the Grate Furnace for Particulate-Matter Generation

A 3D numerical model of the municipal solid waste incineration (MSWI) process was constructed based on a grate furnace with a daily processing capacity of 800 tons. Fluent was used for analyzing key factors affecting the concentration and diffusion level of particulate matter (PM). According to the actual MSWI plant working condition, a 3D model of the incinerator and the waste heat boiler has been constructed under benchmarks. Key factors affecting PM generation were determined by combining mechanistic knowledge and experts’ experience. They were the combustion temperature of solid phase municipal solid waste (MSW), the wall’s PM collision mode, and the second baffle length. Subsequently, the process of resolving the 3D numerical model was delineated. Then, a univariate analysis of the aforementioned 3D model was conducted for the three pivotal factors mentioned above. Conclusively, the effect of the important factors on the number of particles at the outflow of the incinerator was analyzed via orthogonal experiments to obtain the optimal combination. PM concentration initially diminished and then rose with the increased combustion temperature of the solid-phase MSW. Furthermore, a noteworthy reduction in PM concentration was observed when the second baffle length was 12.45–12.95 m. The greatest influence on the PM concentration of the outlet was posed by the wall’s PM collision mode, followed by the second baffle length. The appropriate adjustment of the combustion temperature of the solid-phase MSW, selection of wall materials, and design of the second baffle length were beneficial for diminishing PM concentration and ensuring long-term stable operation of the MSWI process. The combinative optimality of the three key factors was acquired via orthogonal experiments, which proved the subsequent optimal control of PM concentration at the outlet.

Reductions in particulate matter concentrations resulting from air filtration: A randomized sham-controlled crossover study.

One-hundred seventy-two households were recruited from regions with high outdoor air pollution (Fresno and Riverside, CA) to participate in a randomized, sham-controlled, cross-over study to determine the effectiveness of high-efficiency air filtration to reduce indoor particle exposures. In 129households, stand-alone HEPA air cleaners were placed in a bedroom and in the main living area. In 43households, high-efficiency MERV 16 filters were installed in central forced-air heating and cooling systems and the participating households were asked to run the system on a clean-air cycle for 15min per hour. Participating households that completed the study received true air filtration for a year and sham air filtration for a year. Air pollution samples were collected at approximately 6-month intervals, with two measurements in each of the sham and true filtration periods. One week indoor and outdoor time-integrated samples were collected for measurement of PM2.5 , PM10 , and ultrafine particulate matter (UFP) measured as PM0.2 . Reflectance measurements were also made on the PM2.5 filters to estimate black carbon. True filtration significantly improved indoor air quality, with a 48% reduction in the geometric mean indoor PM0.2 and PM2.5 concentrations, and a 31% reduction in PM10 . Geometric mean concentrations of indoor/outdoor reflectance values, indicating fraction of particles of outdoor origin remaining indoors, decreased by 77%. Improvements in particle concentrations were greater with continuously operating stand-alone air cleaners than with intermittent central system filtration. Keeping windows closed and increased utilization of the filtration systems further improved indoor air quality.

The Effect of Urban Green Spaces on Reduction of Particulate Matter Concentration.

In an urban scenario, one of the air pollutants most harmful to human health and environmental is the particulate matter (PM). Considering that urban green areas can contribute to mitigating the effects of PM, this work compares the concentration of PM2.5 in two closer locations in Rio de Janeiro, in order to verify how vegetation cover can actually improve air quality. One is the entrance to the Rebouças Tunnel (RT) and the other is the Rio de Janeiro Botanical Garden (RJBG). For this purpose, PM2.5 samples were taken from September 2017 to March 2018 using a Large Volume Sampler (LVS). The results reveal that RT has a higher concentration of PM2.5 in almost all samples. The RJBG obtains concentrations around 33% less than the other area, suggesting that the presence of urban green areas like the RJBG can reduce PM2.5 levels when compared to places with less vegetation cover, providing better air quality.

The effect of small green walls on reduction of particulate matter concentration in open areas

A major concern in urban areas is the low quality of air, with high levels of particulate matter (PM), consisting of black carbons, volatile organic compounds and various pollutants that are hazardous for the human health and the global environment. Thus, there is an urgent need to decrease air pollution by implementing various short and long-term measures. One of the methods for decreasing air pollution in urban areas is increasing the green infrastructure as plants absorb the particulate matter through their leaves and stems. The initial step in dealing with this problem is raising the public awareness, which is generally low in Skopje and the Balkan region.The aim of the research is to quantify the positive effects on green infrastructure on air pollution and provide research-based inputs that can be used by local governments and decision makers. This paper presents data from continuous measurements on a location in Skopje, provides an assessment of the influence of green zones on air quality in urban areas and correlates it with meteorological factors. This is achieved by using an innovative, low-cost, easy replicable and energy-efficient system, consisted of green wall and stations for monitoring the air quality which are based on wireless sensor network technology.By using statistical tools as Freidman and Mann-Whitney tests, the impact of the relative position of the measurement sensors and the green areas and other objects to the PM concentrations is quantified. The performed analyses confirm that green areas, including green walls, have a high impact in the reduction of PM concentrations in their proximity.The differences in measured values obtained by measurement nodes positioned in relatively small distances are not negligible, thus implying that the relative position of the measurement nodes to the green infrastructure influences the measured PM concentrations. Therefore, the measurement location should be carefully considered for any air quality monitoring system. Measurements with higher spatial granularity should be used for modelling and air quality forecasting purposes.The results in this paper show that the green area mitigates the PM of 2.5 or less micrometers (PM2.5) on average by 25% and PM of 10 or less micrometers (PM10) on average by 37% compared to the neighboring non-green areas. The results show a strong correlation between PM2.5 and PM10. In Skopje, the combination of low temperatures, high humidity and no, or low wind speed lead to high PM concentrations.The presented algorithm compares the statistically obtained data to the reference categories from WHO (from very low to very high, with reference to PM2.5). The described methodology is used to develop a simple decision-making support algorithm for local governments to support their decisions on applying PM mitigation measures.

Two decades of trends in urban particulate matter concentrations across Australia

Australia is a highly developed country with low population density. Capital cities are situated mainly around the coastline and are subjected to different meteorological conditions. This complex set of drivers is expected to result in varying trends in particulate matter (PM) mass concentrations in urban ambient air across the country. Thus, the aim of this study was to determine the long-term trends in PM10 and PM2.5 concentrations in capital cities, and to analyse the factors that influenced such trends. The spatial variability of PM concentrations within the capital cities was first established to identify representative stations. Then trends were determined using the Mann-Kendall trend test, Sen's slope, and the generalised additive model. The results show that, in general, the PM concentrations in Australian cities are relatively low (12.1–21.7 μg m−3 mean daily PM10 and 4.6–8.7 μg m−3 mean daily PM2.5) and within the WHO daily limit 95% of the time. Over the past two decades, very small declines of 8.0 × 10−5–1.1 × 10−3 μg m−3.yr−1 for PM10 and 7.7 × 10−5–2.6 × 10−3 μg m−3.yr−1 for PM2.5 were observed while some stations exhibited increase in concentration based on available data; more stations showed a significant monotonic decline for PM10 than PM2.5. This is attributed to the effectiveness of the implemented emission reduction policies particularly for vehicle exhaust and power generation, given the simultaneous increase in the demand for energy and the number of vehicles over the last two decades. Regarding climate, in the coastal cities of Sydney and Brisbane, high rainfall and strong winds aid in maintaining low PM concentrations despite the significant anthropogenic emissions, while higher PM levels in Darwin can be attributed to its tropical savannah climate, which makes it prone to bushfires and necessitates regular prescribed burnings. PM concentrations increase when exceptional events such as bushfires and dust storms are induced by the extreme climate variability. Further reduction of PM concentrations in Australian cities is unlikely, considering the expanding urbanisation and the changing climate.

On the relationship of anthropogenic air pollution by particulate matter with cancer risk

In the review the author highlights contemporary concepts about the relation between the air pollution by the particulate matter (PM) and human morbidity and mortality due to oncological diseases (OD). The author used materials of the articles indexed in the PubMed and RISC databases. The role of air pollution by PM as a risk factor of carcinogenesis in dependence on size, origin, chemical composition and concentration in air is discussed. PM of road-transport origin contains transitional metals acknowledged as most dangerous and is the result the operational wear of motor transport, road surface, and vehicle emissions. Long-term exposure to PM with an aerodynamic diameter ≤2.5 increases risk of appearance of OD of different localization. Dose-dependent action of PM was established. Reduction in air pollution by PM is accompanied by decrease of premature mortality of population, including from OD, and it can be examined as a modifiable risk factor. The results of the meta-analysis of literature data about the economic damage, caused by morbidity and mortality from OD led to the conclusion that reduction in PM concentration is the most realistic and effective method to decrease these social and economic losses. The accumulated carcinogenic risk provides for the lifelong probability of the development of OD, which requires the active medical examination of workers after the curtailment of work with carcinogens for early diagnostics and treatment of OD. The important preventive measure is to decrease the level of air pollution independently on their initial concentration.

ON THE RELATIONSHIP OF ANTHROPOGENIC AIR POLLUTION BY PARTICULATE MATTER WITH CANCER RISK

In the review the author highlights contemporary concepts about the relation between the air pollution by the particulate matter (PM) and human morbidity and mortality due to oncological diseases (OD). The author used materials of the articles indexed in the PubMed and RISC databases. The role of air pollution by PM as a risk factor of carcinogenesis in dependence on size, origin, chemical composition and concentration in air is discussed. PM of road-transport origin contains transitional metals acknowledged as most dangerous and is the result the operational wear of motor transport, road surface, and vehicle emissions. Long-term exposure to PM with an aerodynamic diameter ≤2.5 increases risk of appearance of OD of different localization. Dose-dependent action of PM was established. Reduction in air pollution by PM is accompanied by decrease of premature mortality of population, including from OD, and it can be examined as a modifiable risk factor. The results of the meta-analysis of literature data about the economic damage, caused by morbidity and mortality from OD led to the conclusion that reduction in PM concentration is the most realistic and effective method to decrease these social and economic losses. The accumulated carcinogenic risk provides for the lifelong probability of the development of OD, which requires the active medical examination of workers after the curtailment of work with carcinogens for early diagnostics and treatment of OD. The important preventive measure is to decrease the level of air pollution independently on their initial concentration.

Particulate matter (PM) emissions and performance of bio-butanol-methanol-gasoline blends coupled with air dilution in SI engines

This study was conducted to investigate PM emissions and performance of SI engines fueled with butanol-methanol-gasoline during air dilution operation. Iso-butanol and methanol were blended with gasoline at volume concentrations of up to 40% (10, 20 vol% iso-butanol and methanol, respectively). The effects of lean mixture on engine performance and PM emissions with blends were investigated to find that the power performance of blended fuels is comparable to that of gasoline. High-proportion blends also showed higher brake thermal efficiency (BTE) than gasoline. The brake specific fuel consumption (BSFC) of blends was also about 10% higher than that of gasoline under stoichiometric conditions; with a lean mixture, a reduction of about 9% in the BSFC of blends was observed. The combustion stability of the blends was reduced relative to that of gasoline. CO emissions from the blends decreased rapidly as the mixture became leaner. The NOx emissions of blended fuels were about 30–50% lower than that of gasoline (lambda >1.15) and decreased rapidly as the mixture grew leaner – a reduction of up to 80% was observed. However, when the excess air ratio was less than 1.15, the NOx emissions were about 30% higher than that of gasoline. The increasing BTE trend of blended fuels was similar to that of gasoline as the load increased. A significant reduction in PM concentration was observed during lean mixture operation, which was further reduced with blended fuels. Reduction of accumulation mode particle was more pronounced for blended fuels relative to nucleation mode particles. The total surface area concentration markedly decreased due to a reduction in particle diameter and total concentration. These results altogether suggest that lean mixture reduces PM emissions of blends and improves BTE; n-butanol/methanol addition also contributed to higher BTE and lower gaseous and PM emissions.

Reduction of particulate matter concentrations by local removal in a building courtyard: Case study for the Delhi American Embassy School

Exposure to particulate matter (PM) is strongly linked to human morbidity and mortality, where higher exposure entails higher all-cause daily mortality and increased long-term risk of cardiopulmonary mortality. The objective of this study is to demonstrate how and to what extent the local removal of PM2.5 can lead to reduced exposure for the children and teachers in the naturally ventilated courtyard of the American Embassy School (AES) high school building in Delhi. The study is performed by computational fluid dynamics (CFD) with the 3D steady Reynolds-averaged Navier-Stokes (RANS) equations in combination with the realizable k-ε turbulence model on a very high resolution grid. First, CFD validation is performed using wind-tunnel experiments of the flow pattern in and above a generic single street canyon. Next, the case study is conducted where four commercially available electrostatic precipitation (ESP) units are installed at different positions inside the courtyard and the resulting performance is evaluated. PM2.5 dispersion is modeled with an Eulerian advection-diffusion equation. It is shown that the best ESP positions yield overall volume-averaged PM2.5 concentration reductions up to 34.1% in the courtyard's corridors, demonstrating the proposed mitigation strategy to be effective. Perspectives for further reduction of the PM concentrations and the related reduction of health risks are discussed.

Short-term impact of PM2.5 on contemporaneous asthma medication use: Behavior and the value of pollution reductions

Asthma ranks among the most costly of chronic diseases, accounting for over $50 billion annually in direct medical expenditures in the United States. At the same time, evidence has accumulated that fine particulate matter pollution can exacerbate asthma symptoms and generate substantial economic costs. To measure these costs, we use a unique nationwide panel dataset tracking asthmatic individuals’ use of rescue medication and their exposure to PM2.5 (particulate matter with an aerodynamic diameter of <2.5 μm) concentration between 2012 and 2017, to estimate the causal relationship between pollution and inhaler use. Our sample consists of individuals using an asthma digital health platform, which relies on a wireless sensor to track the place and time of inhaler use events, as well as regular nonevent location and time indicators. These data provide an accurate measurement of inhaler use and allow spatially and temporally resolute assignment of pollution exposure. Using a high-frequency research design and individual fixed effects, we find that a 1 μg/m3 (12%) increase in weekly exposure to PM2.5 increases weekly inhaler use by 0.82%. We also show that there is seasonal, regional, and income-based heterogeneity in this response. Using our response prediction, and an estimate from the literature on the willingness to pay to avoid asthma symptoms, we show that a nationwide 1 μg/m3 reduction in particulate matter concentration would generate nearly $350 million annually in economic benefits.

Health benefits related to the reduction of PM concentration in ambient air, Silesian Voivodeship, Poland.

Health Impact Assessments (HIA) approach can be executed by calculating the attributable burden of disease. The most common indicators used in the HIA methodology are: premature mortality, morbidity, life-expectancy, and Disability Adjusted Life-Year (DALY). The term Disability Adjusted Life-Years (DALYs) indicates months/years lost due to a premature death or disability. The aim of the study was to present health benefits, expressed in terms of lower total mortality and cardio-respiratory hospitalization rates, due to a decreased particulate matter (PM) concentration in ambient air, in Silesian voivodeship. In this paper, results obtained from the APHEKOM (Improving Knowledge and Communication for Decision Making on Air Pollution and Health in Europe) project, which provided specialized HIA tools, useful for assessing health benefits resulting from reducing air pollution, were used. Both short-term and long-term exposure HIA tools were applied with regard to the appropriate data for Silesian voivodeship. Exposure data were obtained from the Regional Environmental Inspectorate in Katowice, while population and health data were obtained from the Central Statistical Office of Poland or from the Silesian Voivodeship Office, respectively. Health benefits that are related to an improvement of ambient air quality in Silesia region are similar to previous estimates obtained for Kraków city. The reduction of short-term exposure to PM10 by 5 μg/m3 results in a lower number of yearly non-external deaths (2.6-2.75 per 100 000 inhabitants). This effect was also shown to be similar in the city of Zabrze, as well as in the whole Silesia region. The Health Impact Assessments tools developed by the World Health Organization (WHO) can help public health experts make decisions in order to improve the health of populations living in particular regions of Europe.

Study on Particulate Matter of Diesel Engine Using Waste Cooking Oil

Particulate matter (PM) is one of the major pollutants emitted by diesel engine which have adverse effects on human health. Accordingly, many researches have been done to find alternative fuels that are clean and efficient. Biodiesel is preferred as an alternative source for diesel engine which produces lower PM than diesel fuel. However, the manufacturing cost of biodiesel from vegetable oil is expensive. Therefore, using waste cooking oil (WCO) for biodiesel would be more economical and sustainable solution. The characteristics of direct injection diesel engine in term of the PM have been investigated experimentally in this study. The experiments were conducted using single cylinder diesel engine with different speed (1200 rpm, 1500 rpm, 1800 rpm, 2100 rpm, 2400 rpm) at constant load. PM emission of WCO B100 and diesel fuel was compared and the effect of PM components such as soluble organic fraction (SOF) and soot were studied. The result showed WCO B100 reduces the PM emission at all engine speed. Furthermore, both fuels showed highest reduction of PM concentration at moderate engine speed of 1500 rpm.

Particulate matter concentrations in residences: an intervention study evaluating stand-alone filters and air conditioners

This study, a randomized controlled trial, evaluated the effectiveness of free-standing air filters and window air conditioners (ACs) in 126 low-income households of children with asthma. Households were randomized into a control group, a group receiving a free-standing HEPA filter placed in the child's sleeping area, and a group receiving the filter and a window-mounted AC. Indoor air quality (IAQ) was monitored for week-long periods over three to four seasons. High concentrations of particulate matter (PM) and carbon dioxide were frequently seen. When IAQ was monitored, filters reduced PM levels in the child's bedroom by an average of 50%. Filter use varied greatly among households and declined over time, for example, during weeks when pollutants were monitored, filter use was initially high, averaging 84±27%, but dropped to 63±33% in subsequent seasons. In months when households were not visited, use averaged only 34±30%. Filter effectiveness did not vary in homes with central or room ACs. The study shows that measurements over multiple seasons are needed to characterize air quality and filter performance. The effectiveness of interventions using free-standing air filters depends on occupant behavior, and strategies to ensure filter use should be an integral part of interventions. Environmental tobacco smoke (ETS) increased particulate matter (PM) levels by about 14 μg/m3 and was often detected using ETS-specific tracers despite restrictions on smoking in the house as reported on questionnaires administered to caregivers. PM concentrations depended on season, filter usage, relative humidity, air exchange ratios, number of children, outdoor PM levels, sweeping/dusting, and presence of a central air conditioner (AC). Free-standing air filters can be an effective intervention that provides substantial reductions in PM concentrations if the filters are used. However, filter use was variable across the study population and declined over the study duration, and thus strategies are needed to encourage and maintain use of filters. The variability in filter use suggests that exposure misclassification is a potential problem in intervention studies using filters. The installation of a room AC in the bedroom, intended to limit air exchange ratios, along with an air filter, did not lower PM levels more than the filter alone.

Assessment of an Aerosol Treatment To Improve Air Quality in a Swine Concentrated Animal Feeding Operation (CAFO)

Poor air quality within swine concentrated animal feeding operations (CAFOs) poses a threat to workers, the surrounding community, and farm production. Accordingly, the current study was conducted to evaluate a technology for reducing air pollution including particulate matter (PM), viable bacteria, and ammonia within such a facility. The technology consists of an acid-oil-alcohol aerosol applied daily. Its effectiveness was evaluated by comparing air quality from before to after treatment and between treated and untreated sides of a barn separated by an impervious partition. On the untreated side, air quality was typical for a swine CAFO, with mean PM2.5 of 0.28 mg/m3 and PM(TOT) of 1.5 mg/m3. The treatment yielded a reduction in PM concentration of 75-90% from before to after treatment. Effectiveness increased with time, application, and particle size (40% reduction for 1 microm and 90% for >10 microm). Airborne bacteria levels (total bacteria, Enterobacteriaceae, and gram-positive cocci) decreased one logarithmic unit after treatment. In contrast, treatment had no effect on ammonia concentrations. These findings demonstrate the effectiveness of an intervention in yielding exposure and emission reductions.

HEALTH IMPACT OF AMBIENT PARTICULATE AIR POLLUTION IN ITALY

Background The scientific evidence on the health effects of ambient air pollution is substantial. Although several questions remain unanswered, several epidemiological studies have demonstrated the adverse effects of air pollution and characterised dose-response relationships between mortality and pollutants. Among these, the role of particulate matter (PM) has been investigated with regards to its long-term effects on mortality. Due to the ubiquitous nature of PM air pollution, it is likely that its overall impact on human health is large, even if risks are relatively small compared to other environmental risk factors. Large numbers of deaths in urban areas attributable to PM have been estimated in many health impact studies that have been influential in policy making. To further strengthen the case for reduction of PM concentrations, methods that estimate also the impact in terms of life expectancy might be useful. Methods Italian population and all-causes mortality data for 1998 have been used. A life table approach has been followed to calculate the years of life lost (YLL) due to PM pollution, using consevative dose-response risk coefficients (RR = 1.026 per μ10 μg/m3 PM10 increase) estimated in published studies. Linearity of dose-response function is assumed and proportional increases in RR are not age-dependent. Total number of YLL as a function of PM10 levels has been calculated assuming constant rates over time, constant number of births in years after 1998 and null net migration effect. Various scenarios of possible reductions in ambient PM10 concentration have been simulated, starting from a reduction of 5 μg/m3 up to 40 μg/m3. Effects for delays of 5, 10, 20 and 30 years have been calculated, with 95% uncertainty intervals. Results A reduction of 10 μg/m3 in PM10 concentration, with immediate effects, would results in a gain in life expectancy for men of 0.21 years, i.e. about two and a half months (0.17 for women). For all the reduction factors the effect is always higher for men. The total gain for the Italian population is 5.88 millions of life years for men and 5.15 for women. Assuming delays between PM reduction and occurrence of effects, the gains decrease. Gains for larger concentrations reductions are in proportion. Comment In Italy, as in other industrialised countries, reduction of PM concentrations would produce substantial improvement in mortality and life expectancy. Limitations in the methodology and the need to make strong assumptions should be considered-for example uncertainty around the estimates may be larger than reflected by available uncertainty intervals. However health impact assessment type studies have proven informative and effective as a means of communication with the general public and policy makers alike.

Assessing the Health Benefits of Reducing Particulate Matter Air Pollution in the United States

Most Americans are exposed daily to airborne particulate matter (PM), a pollutant regulated by the U.S. Environmental Protection Agency. Current national standards are set for PM10 (particles less than 10 μm in diameter) and new standards have been promulgated for PM2.5 (particles less than 2.5 μm in diameter). Both particle sizes have been associated with mortality and morbidity in studies in the United States and elsewhere and an unambiguously safe level of ambient PM has been difficult to identify. PM10 concentrations have been reduced significantly in U.S. cities over the past two decades and relatively few locations continue to exceed national PM10 standards. However, the new PM2.5 standards will require further reductions in PM concentrations and additional expenditures for emission controls. Information about the health and economic benefits of achieving lower PM concentrations is important because: (1) expected costs of further PM reductions rise after the least-cost options are exhausted, and (2) there is uncertainty about the existence of a threshold safe level for PM. This paper develops and applies a methodology for quantifying the health benefits of potential reductions in ambient PM. Although uncertainties exist about several components of the methodology, the results indicate that the annual nationwide health benefits of achieving the new standards for PM2.5 relative to 1994–1996 ambient concentrations are likely to be between $14 billion and $55 billion annually, with a mean estimate of $32 billion.