A Monte-Carlo Simulation of the Ozone Attainment Process

A Monte-Carlo simulation of the approach to attainment of the National Ambient Air Quality Standard for ozone has been performed for the California Bay Area Air Quality Management District. Four compliance tests together with different design values are used in the simulation. The results show that the present compliance test requiring a zero-percent chance of violation and the design value represented by the fourth highest value in three years makes both the standard and the control requirement much more stringent than generally assumed. In fact, to attain the standard on a long-term basis would require annual means and annual second-highest values that are close to those of the rural background ozone. The simulation also shows that by taking into account the fluctuation of ozone concentrations in the compliance test, such as a t test, and by using a design value consistent with the test, a standard defined in terms of the three-year mean of the annual second-highest values not only is more consistent with the currently- perceived stringency of the present standard, but may also be attainable with a more reasonable control requirement.     

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.     

Demonstrating Attainment of the Air Quality Standards: Integration of Observations and Model Predictions into the Probabilistic Framework

ABSTRACT

This paper introduces an integrated observational-modeling approach to transform the deterministic nature of attainment demonstrations of the National Ambient Air Quality Standard (NAAQS) into the probabilistic framework. While the methods presented here can be used to address any air quality standard that is based on extreme values, this paper focuses on the application to the 1-hr and 8-hr NAAQS for ozone. Extreme value statistics and resampling techniques are applied to estimate the probability of exceeding the NAAQS for both 1-hr and 8-hr ozone concentrations. Within the integrated observation-modeling analysis approach, we show that the model-to-model differences in the predicted responses to emission reductions are smaller than the model-to-model differences in predicted absolute ozone concentrations. We illustrate that the emission reductions stemming from a real-world emission control strategy would substantially reduce the probability of exceeding the NAAQS over a large portion of the eastern United States, especially for the 8-hr average ozone concentrations.     

An Operational Assessment of the Application of the Relative Reduction Factors in the Demonstration of Attainment of the 8-Hr Ozone National Ambient Air Quality Standard

Abstract

The U.S. Environmental Protection Agency in 1997 revised the 1-hr ozone (O₃) National Ambient Air Quality Standard (NAAQS) to one based on an 8-hr average, resulting in potential nonattainment status for substantial portions of the eastern United States. The regulatory process provides for the development of a state implementation plan that includes a demonstration that the projected future O₃ concentrations will be at or below the NAAQS based on photochemical modeling and analytical techniques.

In this study, four photochemical modeling systems, based on two photochemical models, Community Model for Air Quality and the Comprehensive Air Quality Model with extensions, and two emissions processing models, Sparse Matrix Optimization Kernel for Emissions and Emissions Modeling System, were applied to the eastern United States, with emphasis on the northeastern Ozone Transport Region in terms of their response to oxides of nitrogen and volatile organic carbon-focused controls on the estimated design values. With the 8-hr O₃ NAAQS set as a bright-line test, it was found that a given area could be termed as being in or out of attainment of the NAAQS depending upon the modeling system. This suggests the need to provide an estimate of model-to-model uncertainty in the relative reduction factor (RRF) for a better understanding of the uncertainty in projecting the status of an area's attainment. Results indicate that the model-to-model differences considered in this study introduce an uncertainty of the future estimated design value of ～3–5 ppb.     

Demonstrating attainment of the air quality standards: integration of observations and model predictions into the probabilistic framework

This paper introduces an integrated observational-modeling approach to transform the deterministic nature of attainment demonstrations of the National Ambient Air Quality Standard (NAAQS) into the probabilistic framework. While the methods presented here can be used to address any air quality standard that is based on extreme values, this paper focuses on the application to the 1-hr and 8-hr NAAQS for ozone. Extreme value statistics and resampling techniques are applied to estimate the probability of exceeding the NAAQS for both 1-hr and 8-hr ozone concentrations. Within the integrated observation-modeling analysis approach, we show that the model-to-model differences in the predicted responses to emission reductions are smaller than the model-to-model differences in predicted absolute ozone concentrations. We illustrate that the emission reductions stemming from a real-world emission control strategy would substantially reduce the probability of exceeding the NAAQS over a large portion of the eastern United States, especially for the 8-hr average ozone concentrations.     

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.     

Composition and secondary formation of fine particulate matter in the Salt Lake Valley: Winter 2009

Under the National Ambient Air Quality Standards (NAAQS), put in place as a result of the Clean Air Amendments of 1990, three regions in the state of Utah are in violation of the NAAQS for PM₁₀ and PM_2.5 (Salt Lake County, Ogden City, and Utah County). These regions are susceptible to strong inversions that can persist for days to weeks. This meteorology, coupled with the metropolitan nature of these regions, contributes to its violation of the NAAQS for PM during the winter. During January–February 2009, 1-hr averaged concentrations of PM_10-2.5, PM_2.5, NO_x, NO₂, NO, O₃, CO, and NH₃ were measured. Particulate-phase nitrate, nitrite, and sulfate and gas-phase HONO, HNO₃, and SO₂ were also measured on a 1-hr average basis. The results indicate that ammonium nitrate averages 40% of the total PM_2.5 mass in the absence of inversions and up to 69% during strong inversions. Also, the formation of ammonium nitrate is nitric acid limited. Overall, the lower boundary layer in the Salt Lake Valley appears to be oxidant and volatile organic carbon (VOC) limited with respect to ozone formation. The most effective way to reduce ammonium nitrate secondary particle formation during the inversions period is to reduce NO_x emissions. However, a decrease in NO_x will increase ozone concentrations. A better definition of the complete ozone isopleths would better inform this decision.

Implications: Monitoring of air pollution constituents in Salt Lake City, UT, during periods in which PM_2.5 concentrations exceeded the NAAQS, reveals that secondary aerosol formation for this region is NO_x limited. Therefore, NO_x emissions should be targeted in order to reduce secondary particle formation and PM_2.5. Data also indicate that the highest concentrations of sulfur dioxide are associated with winds from the north-northwest, the location of several small refineries.     

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.     

Ozone flux to vegetation and its relationship to plant response and ambient air quality standards

The National Ambient Air Quality Standard (NAAQS) for ozone is based on occurrences of the maximum 8 h average ambient ozone concentration. However, biologists have recommended a cumulative ozone exposure parameter to protect vegetation. In this paper we propose a third alternative which uses quantifiable flux-based numerical parameters as a replacement for cumulative ambient parameters. Herein we discuss the concept of ozone flux as it relates to plant response and the NAAQS, and document information needed before a flux-based ozone NAAQS for vegetation can be implemented. Additional research is needed in techniques for determining plant uptake and in the quantification of plant defensive mechanisms to ozone. Models which include feedback mechanisms should be developed to relate ozone flux, loading, and detoxification with photosynthesis and plant productivity.     

An extended approach to calculate the ozone relative response factors used in the attainment demonstration for the National Ambient Air Quality Standards

With the promulgation of the National Ambient Air Quality Standards (NAAQS or standard) for 8-hr ozone (O₃), the U.S. Environmental Protection Agency (EPA) issued modeling guidance that advocated the use of results from photochemical air quality models in a relative sense. In doing so, the EPA provided guidance on how to calculate relative response factors (RRFs) that can project current design value (DV) mixing ratios into the future for the purpose of determining the attainment status with respect to the O₃ standard. The RRFs recommended by the EPA represent the average response of the photochemical model over a broad range of O₃ mixing ratios above a specified cutoff threshold. However, it is known that O₃ response to emission reductions of limiting precursors (i.e., NO_x and/or VOC) is greater on days with higher O₃ mixing ratios compared to days with lower mixing ratios. In this study, we present a segmented RRF concept termed band-RRF, which takes into account the different model responses at different O₃ mixing ratios. The new band-RRF concept is demonstrated in the San Joaquin Valley (SJV) region of California for the 1-hr and 8-hr O₃ standards. The 1-hr O₃ analysis is relevant to work done in support of the SJV O₃ State Implementation Plan (SIP) submitted to the EPA in 2013. The 8-hr example for the future year of 2019 is presented for illustrative purposes only. Further work will be conducted with attainment deadline of 2032 as part of upcoming SIPs for the 0.075 parts per million (ppm) 8-hr O₃ standard. The applicability of the band-RRF concept to the particulate matter (PM_2.5) standards is also discussed.

Implications:Results of photochemical models are used in regulatory applications in a relative sense using relative response factors (RRFs), which represent the impacts of emissions reductions over a wide range of ozone (O₃) values. It is possible to extend the concept of RRFs to account for the fact that higher O₃ mixing ratios (both 1-hr and 8-hr) respond more to emissions controls of limiting precursors than do lower O₃ mixing ratios. We demonstrate this extended concept, termed band-RRF, for the 1-hr and 8-hr O₃ National Ambient Air Quality Standard (NAAQS or standard) in the San Joaquin Valley of California. This extension can also be made applicable to the 24-hr PM_2.5 and annual PM_2.5 standards.     

Effects of Nighttime Chemistry upon the Transport of Ozone and Ozone Precursors

It has been recognized for several years that ozone in rural areas can exceed the National Ambient Air Quality Standard (NAAQS) for photochemical oxidant whirh was 0.08 ppm for one hour, not to be exceeded more than once per year. During the summer of 1973, the NAAQS was exceeded from 15 to 37% of the time at four rural monitoring sites in Maryland, Pennsylvania, Ohio, and West Virginia.¹ This is a greater violation rate than is found in many urban areas. Dimitriades and Altshuller² have enumerated four possible sources for this rural ozone: (a) transport from urban areas, (b) local photochemical generation from urban ozone precursors, (c) local photochemical generation from precursors of rural origin which may be man-made or natural, and (d) injection of stratospheric ozone into the rural area. This paper considers the chemistry pertinent to the first two of these possible sources of rural ozone, namely the long distance (overnight) transport of ozone and ozone precursors.     

Impact of biogenic emission uncertainties on the simulated response of ozone and fine particulate matter to anthropogenic emission reductions

The role of emissions of volatile organic compounds and nitric oxide from biogenic sources is becoming increasingly important in regulatory air quality modeling as levels of anthropogenic emissions continue to decrease and stricter health-based air quality standards are being adopted. However, considerable uncertainties still exist in the current estimation methodologies for biogenic emissions. The impact of these uncertainties on ozone and fine particulate matter (PM2.5) levels for the eastern United States was studied, focusing on biogenic emissions estimates from two commonly used biogenic emission models, the Model of Emissions of Gases and Aerosols from Nature (MEGAN) and the Biogenic Emissions Inventory System (BEIS). Photochemical grid modeling simulations were performed for two scenarios: one reflecting present day conditions and the other reflecting a hypothetical future year with reductions in emissions of anthropogenic oxides of nitrogen (NOx). For ozone, the use of MEGAN emissions resulted in a higher ozone response to hypothetical anthropogenic NOx emission reductions compared with BEIS. Applying the current U.S. Environmental Protection Agency guidance on regulatory air quality modeling in conjunction with typical maximum ozone concentrations, the differences in estimated future year ozone design values (DVF) stemming from differences in biogenic emissions estimates were on the order of 4 parts per billion (ppb), corresponding to approximately 5% of the daily maximum 8-hr ozone National Ambient Air Quality Standard (NAAQS) of 75 ppb. For PM2.5, the differences were 0.1-0.25 microg/m3 in the summer total organic mass component of DVFs, corresponding to approximately 1-2% of the value of the annual PM2.5 NAAQS of 15 microg/m3. Spatial variations in the ozone and PM2.5 differences also reveal that the impacts of different biogenic emission estimates on ozone and PM2.5 levels are dependent on ambient levels of anthropogenic emissions.     

Projected ozone trends and changes in the ozone-precursor relationship in the South Coast Air Basin in response to varying reductions of precursor emissions

This study examined the effects of varying future reductions in emissions of oxides of nitrogen (NO_x) and volatile organic compounds (VOC) on the location and magnitude of peak ozone levels within California’s South Coast Air Basin (SoCAB or Basin). As ozone formation is currently VOC-limited in the Basin, model simulations with 2030 baseline emissions (?61% for NO_x and ?32% for VOC from 2008) predict 10–20% higher peak ozone levels (i.e., NO_x disbenefit) in the western and central SoCAB compared with the 2008 base simulation. With additional NO_x reductions of 50% beyond the 2030 baseline emissions (?81% from 2008), the predicted ozone levels are reduced by about 15% in the eastern SoCAB but remain comparable to 2008 levels in the western and central Basin. The Basin maximum ozone site shifts westward to more populated areas of the Basin and will result potentially in greater population-weighted exposure to ozone with even a relatively small shortfall in the required NO_x reductions unless accompanied by additional VOC reductions beyond 2030 baseline levels. Once committed to a NO_x-focused control strategy, NO_x reductions exceeding 90% from 2008 levels will be necessary to attain the ozone National Ambient Air Quality Standards (NAAQS). The findings from this study and other recent work that the current VOC emission estimates are underestimated by about 50% suggest that greater future VOC reductions will be necessary to reach the projected 2030 baseline emissions. Increasing the base year VOC emissions by a factor of 1.5 result in higher 2008 baseline ozone predictions, lower relative response factors, and about 20% lower projected design values. If correct, these findings have important implications for the total and optimum mix of VOC and NO_x emission reductions that will be required to attain the ozone NAAQS in the SoCAB.

Implications: Results of this study indicate that ozone levels in the western and central SoCAB would remain the same or increase with even a relatively small shortfall in the projected NO_x reductions under planned NO_x-focused controls. This possibility, therefore, warrants a rigorous analysis of the costs and effects of varying reductions of VOC and NO_x on the formation and combined health impacts of ozone and secondary particles. Given the nonlinearity of ozone formation, such analyses should include the implications of gradually increasing global background ozone concentrations and the Basin’s topography and meteorology on the practical limits of alternative emission control strategies.     

Chemical Composition of Fine Particles in the Tennessee Valley Region

ABSTRACT

Fine particles in the atmosphere have elicited new national ambient air quality standards (NAAQS) because of their potential role in health effects and visibility-reducing haze. Since April 1997, Tennessee Valley Authority (TVA) has measured fine particles (PM_2.5) in the Tennessee Valley region using prototype Federal Reference Method (FRM) samplers, and results indicate that the new NAAQS annual standard will be difficult to meet in this region. The composition of many of these fine particle samples has been determined using analytical methods for elements, soluble ions, and organic and elemental carbon. The results indicate that about one-third of the measured mass is SO₄ ^-2, one-third is organic aerosol, and the remainder is other materials. The fraction of SO₄ ^-2 is highest at rural sites and during summer conditions, with greater proportions of organic aerosol in urban areas throughout the year. Additional measurements of fine particle mass and composition have been made to obtain the short-term variability of fine mass as it pertains to human exposure. Measurements to account for semi-volatile constituents of fine mass (nitrates, semi-volatile organics) indicate that the FRM may significantly under-measure organic constituents. The potentially controllable anthropogenic fraction of organic aerosols is still largely unknown.     

Demonstrating attainment in Atlanta using urban airshed model simulations: impact of boundary conditions and alternative forms of the NAAQS

Urban Airshed Model-Version IV (UAM-IV) simulations on 7–8 July, 1988 for the Atlanta, Georgia, nonattainment area are used to investigate how recent changes in the National Ambient Air Quality Standard (NAAQS) and changes in boundary concentrations may affect attempts to comply with the standard through local emissions reductions. According to model results, the recently promulgated 8 h NAAQS at a level of 0.08 ppmv will require larger emission reductions to comply with the standard than those that are necessary to comply with the previous 1 h/0.12 ppmv NAAQS. Regardless of the form of the NAAQS or the magnitude of the concentrations of O₃ and its precursors at the model domain boundary, UAM-IV simulations for Atlanta predict that NO_x (NO+NO₂) emission reductions are more effective than volatile organic compound reductions in mitigating O₃ pollution. Moreover, the simulations indicate that NO_x emission reductions greater than 60–75% would be required to demonstrate attainment under either form of the standard, even if boundary concentrations of O₃ and its precursors were substantially reduced. Further research is necessary to determine if this weak response to emission controls is truly representative of the real atmosphere, or is a result of the meteorological conditions specific to this episode, or is an artifact of the UAM-IV model or its inputs.     

Characterization of summertime coarse particulate matter in the Desert Southwest—Arizona,USA

A year-long study was conducted in Pinal County, AZ, to characterize coarse (2.5 – 10 μm aerodynamic diameter, AD) and fine (< 2.5 μm AD) particulate matter (PMc and PMf, respectively) to further understand spatial and temporal variations in ambient PM concentrations and composition in rural, arid environments. Measurements of PMc and PMf mass, ions, elements, and carbon concentrations at one-in-six day resolution were obtained at three sites within the region. Results from the summer of 2009 and specifically the local monsoon period are presented.

The summer monsoon season (July – September) and associated rain and/or high wind events, has historically had the largest number of PM₁₀ NAAQS exceedances within a year. Rain events served to clean the atmosphere, decreasing PMc concentrations resulting in a more uniform spatial gradient among the sites. The monsoon period also is characterized by high wind events, increasing PMc mass concentrations, possibly due to increased local wind-driven soil erosion or transport. Two PM₁₀ NAAQS exceedances at the urban monitoring site were explained by high wind events and can likely be excluded from PM₁₀ compliance calculations as exceptional events. At the more rural Cowtown site, PM₁₀ NAAQS exceedances were more frequent, likely due to the impact from local dust sources.

PM mass concentrations at the Cowtown site were typically higher than at the Pinal County Housing and Casa Grande sites. Crustal material was equal to 52-63% of the PMc mass concentration on average. High concentrations of phosphate and organic carbon found at the rural Cowtown were associated with local cattle feeding operations. A relatively high correlation between PMc and PMf (R²?=?0.63) indicated that the lower tail of the coarse particle fraction often impacts the fine particle fraction, increasing the PMf concentrations. Therefore, reductions in PMc sources will likely also reduce PMf concentrations, which also are near the value of the 24-hr PM_2.5 NAAQS.

Implications: In the desert southwest, summer monsoons are often associated with above average PM₁₀ (<10 μm AD) mass concentrations. Competing influences of monsoon rain and wind events showed that rain suppresses ambient concentrations while high wind increase them. In this region, the PMc fraction dominates PM₁₀ and crustal sources contribute 52-63% to local PMc mass concentrations on average. Cattle feedlot emissions are also an important source and a unique chemical signature was identified for this source. Observations suggest monsoon wind events alone cannot explain PM₁₀ NAAQS exceedances, thus requiring these values to remain in compliance calculations rather than being removed as exceptional wind events.     

Expected ozone benefits of reducing nitrogen oxide (NOx) emissions from coal-fired electricity generating units in the eastern United States

On hot summer days in the eastern United States, electricity demand rises, mainly because of increased use of air conditioning. Power plants must provide this additional energy, emitting additional pollutants when meteorological conditions are primed for poor air quality. To evaluate the impact of summertime NO_x emissions from coal-fired electricity generating units (EGUs) on surface ozone formation, we performed a series of sensitivity modeling forecast scenarios utilizing EPA 2018 version 6.0 emissions (2011 base year) and CMAQ v5.0.2. Coal-fired EGU NO_x emissions were adjusted to match the lowest NO_x rates observed during the ozone seasons (April 1–October 31) of 2005–2012 (Scenario A), where ozone decreased by 3–4 ppb in affected areas. When compared to the highest emissions rates during the same time period (Scenario B), ozone increased ～4–7 ppb. NO_x emission rates adjusted to match the observed rates from 2011 (Scenario C) increased ozone by ～4–5 ppb. Finally in Scenario D, the impact of additional NO_x reductions was determined by assuming installation of selective catalytic reduction (SCR) controls on all units lacking postcombustion controls; this decreased ozone by an additional 2–4 ppb relative to Scenario A. Following the announcement of a stricter 8-hour ozone standard, this analysis outlines a strategy that would help bring coastal areas in the mid-Atlantic region closer to attainment, and would also provide profound benefits for upwind states where most of the regional EGU NO_x originates, even if additional capital investments are not made (Scenario A).

Implications: With the 8-hr maximum ozone National Ambient Air Quality Standard (NAAQS) decreasing from 75 to 70 ppb, modeling results indicate that use of postcombustion controls on coal-fired power plants in 2018 could help keep regions in attainment. By operating already existing nitrogen oxide (NO_x) removal devices to their full potential, ozone could be significantly curtailed, achieving ozone reductions by up to 5 ppb in areas around the source of emission and immediately downwind. Ozone improvements are also significant (1–2 ppb) for areas affected by cross-state transport, especially Mid-Atlantic coast regions that had struggled to meet the 75 ppb standard.     

Responses of Desert Annual Plants to Ozone and Water Stress in an in situ Experiment

Desert winter annual plants: Camissonia claviformis, C. hirtella, Caulanthus cooperi, Chaneactis carphoclinia, C. stevioides, Cryptantha angustifolia, C. pterocarya, Erodium cicutarium, Festuca octoflora, Lupinus concinnus, Oenothera californica, Plantago insularis, Platystemon californica, Salvia columbariae, Thelypodium lasiophyllum, and Thysanocarpus curvipes growing on irrigated and non-irrigated plots were exposed in situ to elevated levels of ozone dispensed from an open air exposure system. Plants were exposed intermittently to a gradient of ozone of concentrations ranging between 44 and 133 ppb (nL L^?1) for 35 h over a total of 216 h. Only three species were injured by ozone at the highest ozone concentrations. Leaf injury to C. claviformis—2 percent total foliar injury (TFI), C. hirtella—1 percent TFI, and Erodium cicutarium—2 percent TFI, developed at the highest ozone concentrations. Leaf injury to these species was similar on the irrigated and nonirrigated plots. Leaf water potential and stomatal conductance significantly decreased in C. claviformis, and C. hirtella due to water stress but not ozone. Similar trend for net photosynthesis was also determined. The highest water potential and stomatal conductance values as well as the largest differences in water potential between irrigated and non-irrigated plants were found in the morning.     

Empirical Relationships between Observed Ozone Concentrations and Geographical Areas with Concentrations Likely to Be above 120 ppb

The extent to which existing ozone monitoring data can be used to represent concentrations outside the area immediately surrounding a monitoring station is investigated. The results should be of Interest to those who wish to define areas within which standards are likely to have been exceeded. Relationships are established between the observed second-maximum hour-average ozone concentrations and the area within which It is possible to state with reasonable probability that National Ambient Air Quality Standards (NAAQS) have or have not been exceeded. All pairs of SAROAD stations (Storage and Retrieval of Aerometric Data) separated by 500 km or less which had nearly complete annual data sets were considered for the years 1974 through 1977 to determine the probability that the NAAQS would be exceeded at one station of the pair, given station separation and the observed second-maximum hourly ozone concentration at the other station. The resulting relationship was applied to SAROAD data for 1977, and circles were drawn around each SAROAD monitoring site to show the area within which it is 90% probable that the 120 ppb NAAQS was exceeded during 1977.     

Estimation of Ambient PM2.5 Concentrations in Maryland and Verification by Measured Values

Abstract

In 1997, Maryland had no available ambient Federal Reference Method data on particulate matter less than 2.5 μm in aerodynamic diameter (PM_2.5), but did have annual ambient data for PM smaller than 10 μm (PM₁₀) at 24 sites. The PM₁₀ data were analyzed in conjunction with local annual and seasonal zip-code-level emission inventories and with speciated PM_2.5 data from four nearby monitors in the IMPROVE network (located in the national parks, wildlife refuges, and wilderness areas) in an effort to estimate annual average and seasonal high PM_2.5 concentrations at the 24 PM₁₀ monitor sites operating from 1992 to 1996. All seasonal high concentrations were estimated to be below the 24-hr PM_2.5 National Ambient Air Quality Standards (NAAQS) at the sites operating in Maryland between 1992 and 1996. The estimates also indicated that 12 monitor sites might exceed the 3-year annual average PM_2.5 NAAQS of 15 ug/m³, but Maryland’s air quality shows signs that it has been improving since 1992. The estimates also were compared with actual measurements after the PM_2.5 monitor network was installed. The estimates were adequate for describing the chemical composition of the PM_2.5, forecasting compliance status with the 24-hr and annual standards, and determining the spatial variations in PM_2.5 across central Maryland.