Air quality measurements—From rubber bands to tapping the rainbow

Introduction

The 2017 Critical Review (CR) (Hidy et al., 2017) provided an overview of air quality measurement processes and the influence of the results obtained from them on environmental policies. As with all Air & Waste Management Association (A&WMA) CRs, expert discussants have provided different views on Hidy et al. (2017). In the comments summarized below, these discussants and the CR authors note their agreements and disagreements with the CR content and conclusions, provide additional information and perspective, and suggest additional resources for more in-depth study of the topic in the following subsections. The appearance of these discussants as coauthors does not necessarily indicate their agreement with the opinions of other discussants. As a special addendum, the supplemental material contains a more detailed memoir of Dr. Peter K. Mueller, who has dedicated more than his 65-yr continuous A&WMA membership to advancing the technology and use of air quality measurements.

Measurements for permitting and policy—comments by Sara J. Head

Three air monitoring examples emphasize several key points made in the CR. The White River Shale Project (WRSP) was part of a federal oil shale lands leasing program started in the early 1970s (WRSP, 1974). Six tracts, two each in Utah, Colorado, and Wyoming, were offered for lease by the federal government to spur oil shale development. The WRSP leased about 10,000 acres, dubbed tracts Ua and Ub, in the Uintah Basin in eastern Utah near the border with Colorado. Because this program involved leasing public lands, environmental baseline monitoring was required from the lease holder. A comprehensive set of air quality and meteorological parameters was measured from 1976 to 1980, including two air quality monitoring sites and six meteorological stations, some with 30-m towers. There were also some special tracer release measurements to evaluate dispersion models used for permitting the oil shale operation. Data storage options were not as compact as they are now, and at the end of the program there were dozens of boxes of magnetic and paper tapes, as well as hard copy quarterly reports of the validated and processed data. This extensive database seemed like a treasure trove of information, albeit a storage nightmare. The tapes were offered to many agencies, universities, and consulting organizations, but no one would take the boxes and the tapes were eventually disposed of. Some of the quarterly reports remain available on the Bureau of Land Management’s (BLM) Web site (AeroVironment, 1981), but these data are not very usable. As the CR noted, it is unfortunate that the extensive efforts that went into this program and the data collected were for naught.

In the second example, the National Park Service (NPS) intended to identify, obtain, review, and archive air quality and meteorological data monitored in the national parks, for the most part by state and local air agencies near each park. Prior to this concerted effort, the data were not available from a single source. In addition to the existing sites, five new monitoring sites were installed. The program also established a quality assurance/quality control (QA/QC) program to ensure that monitoring data could be relied upon. There was a wide variation in the quality of the data collected by the various state agencies across the United States, which became apparent from comparisons among nearby sites. This example drove home the importance of good QA/QC in data collection efforts. Having oversight and ensuring a consistent QA/QC program is vitally important for background air quality data used for permitting stationary sources near the parks and implementing regulations uniformly.

The third example is related to air monitoring program in Santa Barbara County, California. This county is relatively small in area (3789 square miles) and population (about 440,000 in 2014), but there are six state/local air monitoring stations (SLAMS), five of which monitor criteria pollutants (carbon monixide [CO], ozone [O₃], nitrogen oxides [NO_x], particulate matter (PM) with an aerodynamic diameter ≤10 μm [PM₁₀], and sufur dioxide [SO₂]). Twelve prevention of significant deterioration (PSD) stations are used to determine baseline air quality and the impacts of specific operations. There has been extensive offshore oil and gas development in this area, which is the reason (and provides funding) for this level of monitoring. This example illustrates how monitor siting can have an important effect on policy and permitting. Because most of the monitoring sites are for specific project requirements, the stations tend to be at lower elevations and clustered nearby along the coast. Currently, Santa Barbara County is in attainment of all National Ambient Air Quality Standards (NAAQS) (U.S. Environmental Protection Agency [EPA], 2017a). However, in adjacent San Luis Obispo County, with a smaller area (3616 square miles) and population (about 270,000 in 2014), one of the 10 SLAMS is located at a higher elevation (about 2000 feet) (Air Pollution Control District of San Luis Obispo County [APCDSLOC], 2017). Based on the design value at this site, a portion of that county is designated as nonattainment of the O₃ NAAQS. The nonattainment designation brings with it additional O₃ attainment planning requirements, including rule development and more stringent permitting requirements. Two similar counties have different air quality requirements, seemingly because one county has a monitoring site at a higher elevation (where transported O₃ is found) and the other does not, even though there are similar high elevations (but no monitors).

When taking air quality measurements, keeping data storage and archiving in mind, as well as ensuring good QA, are important considerations. The placement of air monitoring stations can have a substantial impact on policy and permitting requirements.

The dangers of poor data quality—comments by Eric D. Stevenson

The CR discussed important advances in air monitoring technologies and the people, such as Peter Mueller, who contributed to these advances.

Air monitoring is poised to experience significant changes in the near future. New technology will allow more measurements in real time and low-cost sensors can put measurement technologies into the hands of more people. Compositional, spatial, and temporal limitations that have restricted current air monitoring measurements may quickly fade. As these technologies are brought to bear, it is important to remember how good data and good policy decisions are related.

The goal of air monitoring should be to produce enough data of sufficient quality to inform policy makers about the appropriate course of action. More data of poor quality or data collected over time periods that are difficult to relate to health consequences may not be useful if data limitations are not understood or acknowledged. The improper use of that data often takes resources from collecting and understanding measurements that can provide actionable results. Newer technologies should be incorporated into current air monitoring networks such that specific data objectives are identified as appropriate and where new technologies can be applied, while leveraging older technologies currently in use. As the technologies are brought online, it is important to remember that the data should be used to:

• Support actions by facilities, transportation, and land use planners to identify and improve their processes, equipment, and development plans to avoid unnecessary exposures.

• Improve and verify modeling results so that simulations can provide air quality information on a more granular level.

• Improve understanding of the health effects of air pollution.

• Guide the identification, understanding, and improvement of local air quality “hotspots.”

• Demonstrate consistent air quality trends over time.

This list is by no means exhaustive, but context and understanding of what measurements mean will continue to be as important as the collection of the data itself. Air monitoring has come a long way and has led to air quality improvements beneficial to human health. As technologies and methods continue to improve and expand, it is important to appreciate what makes data useful and ensure that, as these advances are brought online, the measurements are used in ways that support actionable responses.

Maintaining consistency with evolving measurement technologies—Comments by John G. Watson and Judith C. Chow

The CR documented the evolution of different air quality measurement systems, but it did not address the issue of transitioning from one technology to another. It is possible that 50 yr from now, environmental managers will look back on costly, air-conditioned structures filled with large pieces of energy-consuming equipment as we look at rubber bands and rainbows today. The challenge of changing measurement systems is how to achieve consistency with the long-term record during and after the transition.

For NAAQS compliance measurements, the EPA has defined Federal Reference Methods (FRM) for each criteria pollutant in the Code of Federal Regulations (CFR; 2016a). For the gases, the measurement principles are ultraviolet fluorescence and pararosaniline (West-Gaeke) for SO₂, nondispersive infrared absorption for CO, and chemiluminescence for O₃ and NO_x. As noted in the CR, PM exhibits greater measurement variability owing to its changing size distribution and composition with location and season, and more emphasis is placed on standardizing characteristics of the sampling inlets, filter media, filter handling, and weighing conditions (Chow, 1995; Watson et al., 2017). Creating a monitor that implements the FRM principles is insufficient for it to attain acceptance as an FRM, as EPA has defined a series of challenges that must be met, documented, and submitted for approval (CFR, 2016b). For gases, these tests involve challenges with known concentrations that span the expected ambient levels, long-term tests of baseline and span variability, responses to interferents added to the test gases, rise and fall times, and field comparisons with FRMs. PM field samplers must use the specified inlet design or go through extensive and costly wind-tunnel testing to gain approval for a new inlet. Other requirements related to flow control, temperature variations, and laboratory conditions allow more flexibility concerning hardware and software as long as they attain the specified requirements. Federal Equivalent Methods (FEM) are those using a measurement principle that differs from the FRM but that return equivalent results. For gases, most of the FRM criteria must still be met, including tests for interferences and collocated sampling with an approved FRM. Most of the criteria gas monitors on EPA’s list of approved compliance monitors are FEMs (EPA, 2017b), as some alternative measurement principles are less costly to implement or there were minor deviations from the FRM criteria.

PM FEMs are divided into three classes:

• Class I is a manual filter method that is essentially the same as the FRM but with minor deviations from the specified design.

• Class II is a manual filter method that applies the FRM principle but has large deviations from the specified design (e.g., a different PM₁₀ or PM_2.5 inlet).

• Class III is a continuous instrument capable of 1-hr PM₁₀ or PM_2.5 measurements as well as a 24-hr value that is comparable to a FRM in various sampling environments.

The difference between Classes I and II is minor, especially for PM_2.5. For example, when limitations of the original WINS-96 impactor used for the FRM PM_2.5 inlet were found (Peters and Vanderpool, 1996; Vanderpool et al., 2001a, 2001b), the Very Sharp Cut Cyclone was developed to replace it (Kenny et al., 2004). The Class III designation depends very much on the testing environments. The Tapered Element Oscillating Microbalance (TEOM) FEM for PM₁₀ heats the sample to 50 °C, which provides results comparable to an FRM in environments with a stable aerosol, such as one dominated by ammonium sulfate, but it evaporates certain species, such as ammonium nitrate, in other environments (Hering et al., 2004). This issue was addressed for the TEOM FEM designation for PM_2.5 monitoring (Grover et al., 2006), but the PM₁₀ FEM is still in use. As a result, it is not unusual to find PM_2.5 > PM₁₀ at sites using TEOMs for both measurements.

There has not been such attention to continuity for noncriteria atmospheric measurements, and again it is the PM component that presents the greatest difficulty. Black carbon (BC), elemental carbon (EC), or soot is the most prominent example, as it has been quantified as a PM indicator for more than 100 yr, but with ever-changing methods. BC measurements have evolved from the original visible smoke plume reading of Ringelmann (Griebling, 1952), through British Smoke (Hill, 1936), then the coefficient of haze (Hemeon et al., 1953), to various versions of the aethalometer (Drinovec et al., 2015; Rosen et al., 1980), and to thermal and optical laboratory methods (Watson et al., 2005). Aside from its potential adverse health effects (Eklund et al., 2014; Grahame et al., 2014), BC is implicated in visibility reduction (Watson, 2002) and global warming (Fiore et al., 2015). It is also becoming evident that brown carbon (BrC) is just as important as BC (EPA, 2012), with different sources and environmental effects. The IMPROVE PM_2.5 speciation network (IMPROVE, 2017) adopted the organic and elemental carbon analysis method of Huntzicker et al. (1982) and has applied it in a consistent manner since 1986. This has allowed for the long-term tracking of EC trends (Chen et al., 2012; Murphy et al., 2011), demonstrating the effectiveness of emission controls, especially on diesel exhaust. The urban Chemical Speciation Network (CSN), on the other hand, applied a different method that was not equivalent to the IMPROVE, and this created difficulties for tracking trends and modeling. EPA’s Clean Air Scientific Advisory Committee (CASAC) examined this incompatibility (Hopke et al., 2005), noting that “There are existing problems of harmonization of data from the IMPROVE and STN (Speciation Trends Network, later included in the CSN) that need to be addressed … it is an appropriate time to change the STN network to what will shortly be the new IMPROVE protocol so that there will be comparability between measurements.” EPA began this transition in 2006, and the IMPROVE and CSN networks carbon concentrations have been compatible since then.

It is inevitable that instrumentation will change with time as older instrumentation wears out and replacement parts are no longer available. More modern instrumentation can also reduce electronic noise and minimize the effects of interferents. However, such changes need to be made with planning and forethought. Some lessons learned from such changes in the past include:

• Thoroughly review and evaluate literature on measurement principles, laboratory testing, known and potential interferents, and field comparisons under a variety of environmental conditions.

• Conduct comparison studies at locations or on samples that span the full spectrum of atmospheric compositions and source mixtures.

• Select a time, preferably the beginning of a year or calendar quarter, when the change is made from one measurement method to another.

• Document the changes and evaluate their potential ramifications for long-term trends.

• Examine the trends and attempt to distinguish real responses to environmental changes from changes in the measurement method.

International efforts—Comments by George M. Hidy

A broader view of measurements comes from looking at international programs characterizing multiscale air quality. Measurements and monitoring have expanded across the world since the 1980s to support regulatory requirements guided by various national and international programs (European Union [EU], 2017; World Health Organization [WHO], 2006). These range from local and regional air monitoring for exposure estimation to global projects aimed at establishing air chemistry and its relation to ecosystem modification and climate change.

As part of the World Meteorological Organization (WMO), a global-scale monitoring program was organized as the Global Atmosphere Watch (GAW) (WMO, 2017a), which has 31 global and 647 regional sites located around the world. The number of sites belies the coverage for chemical species; the stations have varying capabilities, usually covering surface meteorology, with concentration on the optical properties of the troposphere. O₃ is measured at 30 global and 125 regional sites. Measurements of CO, oxides of NO_x, SO₂, and volatile organic compounds (VOCs) vary by site (Schultz et al., 2015). Aerosol measurements focus on light extinction and turbidity, with less emphasis on mass concentration or particle composition. Data from the GAW cooperating sites are provided through the World Data Center for Greenhouse Gases (WMO, 2017b).

Most national regulatory programs for air quality have followed guidelines established by the World Health Organization (WHO), which are similar to the U.S. approach but with different limits on acceptable ambient concentrations (WHO, 2006). Long-term measurements include the U.S. criteria pollutants for gases and particles, and atmospheric deposition, as well as a range of optical and other properties associated with PM. The United States, Canada, and Mexico have extensive monitoring networks (Scheffe et al., 2011). Canadian networks include the National Air Pollution Surveillance Network (NAPS) and the Canadian Air and Precipitation Monitoring Network (CAPMON) (Environment Canada, 2017). The Canadian programs show stations mainly along its southern border, with a few isolated locations further north in remote areas. The Mexican network, Programa Nacional de Monitoreo Atmosferico (PNMA), is concentrated around Mexico City, but it covers other cities and the U.S. border, particularly in the west (Instituto Nacional de Ecologia y Cambio Climatico [INECC], 2017). Routine air quality measurements that support European public health assessments have proceeded with a large multinational network since the 1970s (Kuhlbusch et al., 2004). The principal network of 270 sites across Europe is the European Monitoring and Evaluation Programme (EMEP) (European Environment Agency [EEA], 2017; Guerreiro et al., 2014; Torseth et al., 2012). EMEP includes a subset of sites under the EUROAIRNET organization (EIONET, 2017). Additional sites have been reported through the EUROTRAC initiative (Torseth et al., 2012). EMEP sites include measurements for the European Union standards for SO₂, NO₂, O₃ PM₁₀ (mass and composition), PM_2.5, and meteorological parameters. EUROTRAC was set up in 1986 to provide research experiments on O₃, photo-oxidants, and processes leading to acidity in the air. Two sites in EMEP participate in the GAW initiative. Data are available for EMEP and EUROAIRNET through the Airbase site (EIONET, 2017).

Measurements in East Asia are available for several countries, including India, China, Korea, and Japan. India’s National Air Quality Monitoring Programme (NAQMP) is operated through the Central Pollution Control Board (CPCB; 2017), with 603 sites in 300 cities and towns, 26 states, and 4 territories. Measurements for O₃, SO₂, NO_x, total suspended particulate (TSP; particles with aerodynamic diameters less than ~30 μm), PM₁₀ and PM_2.5 are included at several of the sites.

China’s national Air Reporting System initiated after 2012 includes measurements of the WHO gas and particle concentrations. Large efforts are being made to determine compliance with China’s recently issued PM_2.5 standard (Cao et al., 2013). China’s network is mainly urban with 945 sites in 190 cities (Rohde and Muller, 2015). Plans call for 5000 stations managed by authorities at four levels—state, provincial, city, and county. The China National Environmental Monitoring Center operates 1436 stations that include the WHO pollutants listed above (Zheng, 2017). Air quality monitoring in Korea began at the end of the 1970s; as of 2000, the measurements have covered the U.S. criteria pollutants (Ghim et al., 2002). Measurements of air quality in Japan focused in the 1990s on dry and wet acid deposition, with emphasis on SO₂ and NO_x (Fukushima, 2006). Later Japanese standards and guidelines were adopted following U.S. criteria pollutants and include meteorological parameters. As of 2011, a network of local monitoring sites are in place in Japan (Wakamatsu et al., 2013), mainly on Honshu, and focusing on urban areas such as Tokyo, Osaka-Hyogo, and Aichi-Mie areas.

Australian air quality measurements are provided at the commonwealth level (Department of Environment and Energy [DEE], 2001) following WHO guidelines. With exception of a few remote sites, air quality measurements appear to be delegated to the commonwealth states. The urban areas of Australia have good air quality with respect to WHO standards, and air issues are subordinated to water quality concerns.

The African continent has widely varying commitment to air quality management depending on national development (Ndamitso et al., 2016; Tanimowo, 2000). Issues focus on high dust levels, with increasing interest in SO₂ and NO_x in urban environments. Indoor pollution also is a major issue in Africa, especially when solid fuels are used for cooking and heating (Engelbrecht et al., 2001). The region-wide conditions are best observed by satellite since only three GAW global sites and eight regional sites are located on the continent. South Africa appears to be farthest along in developing a national regulatory framework and research programs (Reason et al., 2006).

The international collection of air quality measurements exhibits a wide range of quality for accuracy and precision. Through national programs that follow WHO or national standards, including the GAW protocol, data quality has improved in recent years. Limited studies (if any) have been organized to compare instrument performance, reference standards, standard operating procedures, and data review protocols. As a result, there is considerable uncertainty in long-term observations, including trends. Inspection of maps of various national monitoring sites indicates spatial limitations in data coverage; nevertheless, there now exists an increasingly useful set of long-term data for air quality management.

A personal historical perspective—comments by Peter K. Mueller

As noted in the CR, the design and implementation of the Clean Air Act (CAA) and its amendments has depended on pollutant measurements and their refinement. The CAA was structured to permit debates on the technical interpretation of measurements, and this has influenced U.S. policy since the 1950s. My experience in environmental chemistry programs illustrates the role of measurements influencing governmental and nongovernmental organization (NGO) policies since the 1960s. These include: (1) the elimination of lead (Pb) from motor vehicle fuels; (2) the role of fine particles (<2.5 µm aerodynamic diameter) in health and visibility impairment; (3) the characterization of regional-scale acidic pollution and its relationships to acid rain and visibility; (4) the development of noncarbonate carbon knowledge; and (5) the design and implementation of second- and third-generation coordinated measurement programs.

In the 1950s, the California State Department Public Health (CSDPH) was concerned about exposure to airborne Pb as an adjunct to broader concerns of urban air pollution, thereby motivating CSDPH’s Air and Industrial Hygiene Laboratory (AIHL) (Mueller, 1967) studies of airborne Pb. Early ambient Pb assessments were based on chemical analysis TSP filter samples with an after-sampling effort to remove and resuspend deposits for separation into respirable size ranges. It became evident that postsampling size segregation biased mass concentrations in favor of coarse particles >10 um. This bias led to conclusions that Pb was not a major problem because the large particles would fall out along roadways, thereby limiting overall human exposures except near traffic. AIHL research after 1959 (Mueller, 1961; Mueller et al., 1964) showed that a large fraction of the Pb in engine exhaust and ambient air were <10 µm, allowing Pb to disperse over urban and surrounding rural areas.

Furthermore, it was known at the time that these smaller particles were more respirable than the >10 µm particles, thereby creating a key pathway for Pb to enter humans (Bates et al., 1966; Landahl, 1950). The lung retention and epidemiological studies led to debates about removing Pb additives from gasoline in the 1960s. Public health protection combined with concern over contamination of after-engine catalyst emission controls resulted in California promulgating the first ambient air quality standard for Pb of 1.5 µg/m³ 30-day average in 1970 (California Air Resources Board [CARB], 2017a), accompanied by agreements with the petroleum and motor vehicle industries for phasing out Pb additives in the 1970s. The EPA did not promulgate a Pb NAAQS until 1978, with a reduction from 1.5 µg/m³ quarterly average to 0.15 µg/m³ rolling average issued in 2008 (EPA, 2008).

In 1959, California adopted the first visibility standard (CARB, 2017b), defined as PM concentrations “…sufficient to reduce visibility to less than three mi[les] when relative humidity is less than 70 percent…” This was amended to 10 miles visual range in 1969. At the time, policy makers understood that visibility impairment was related to airborne particles, mainly in the range of 0.1–10 μm. However, relatively little was known about the details of secondary particle formation compared with primary emissions from combustion and other sources. An ad hoc experiment in Pasadena to study the physicochemical and optical properties of smog aerosols was organized by an informal arrangement between Sheldon Friedlander, Ken Whitby, Bob Charlson, and me (Mueller et al., 1972; Whitby et al., 1972). The study results influenced the research establishment at the California Air Resources Board (CARB) to undertake a statewide experiment, the California Aerosol Characterization Experiment (ACHEX), in 1972–1973 (Hidy, 2011; Hidy et al., 1980). ACHEX intended to measure the properties of aerosol particles for different emission and air chemistry conditions across California, then interpret these results in terms of particle production in photochemical smog versus a range of known urban and rural primary particle emitters. ACHEX identified the role of organic vapor oxidation to produce fine particles. It led to the now widely accepted theory of multimodel mass size distributions having distinct chemical compositions in different size ranges (Hidy et al., 1980). These results, combined with similar research on other regions, justified selection of the PM₁₀ and PM_2.5 mass indicators that were incorporated into the NAAQS (EPA, 1987, 1997).

Early measurements of gaseous SO₂ and particulate sulfate (SO₄) (Altshuller, 1973, 1976) indicated local-scale impacts for SO₂, but more widespread contributions to TSP from SO₄. Tall stacks were used to dilute SO₂ emissions before reaching the ground to minimize local hotspots. The electric utilities used the tall stack rule to build power plants after the 1950s. The power generators declared that regional pollution played no significant role in PM NAAQS exceedances. In the 1970s, it was recognized that the deposition of acid forming species (acid rain) was harming ecosystems (Dochinger and Seliga, 1975). The economic stress of air pollution control on the utilities created major regional pollution measurement initiatives such as the Sulfate Regional Experiment (SURE) (1983; Mueller et al., 1980) and subsequent studies in the eastern and western United States focusing on regional-scale sites (Mueller et al., 1982b; Tombach et al., 1987) along with the interaction of regional-scale measurements and models. These resource-intensive measurement projects characterized the potential sources of regional SO₂, NO_x, O₃, and PM under meteorological conditions that fostered high pollution levels (National Acid Precipitation Assessment Program [NAPAP], 1990). These studies also added knowledge about acid deposition and visibility impairment across the United States that could be attributed to various classes of sources. These regional measurements also led to the abandonment of the tall stack option for ground-level pollution reduction and so-called “supplemental emission control” in which plant operations were curtailed when ground-level monitors showed excessive concentrations.

The results of regional studies sponsored by the private sector and the federal government justified the SO₂ cap and trade rules implemented in the 1990 CAA amendments, which were further extended to NO_x and VOC precursors for regional O₃ in North America (Chameides et al., 2000; Hales, 2003; Russell and Dennis, 2000).

With coworkers at AIHL and elsewhere, we developed experimental methods to quantify noncarbonate PM organic and elemental carbon (OC and EC) and demonstrated that these components were important contributors to PM mass in both urban and rural samples (Fung, 1990; Fung et al., 2002, 1982a; Mueller et al., 1971). This result was confirmed by a number of particle composition studies and resulted in a policy shift to include a multipollutant consideration of health risks (Hidy and Pennell, 2010).

Although long-term carbon dioxide (CO₂) measurements (Keeling, 1970; Keeling et al., 2011) indicated the possibility of atmospheric warming from CO₂ radiation absorption (augmented with radiative scattering and absorption from particles and other radiatively absorbing gases), most potential climate impacts were determined by models. I had the opportunity to work with colleagues at the National Center for Atmospheric Research (NCAR) in 1990 to develop a climate modeling framework that critically compared simulated outcomes with measurements as a means to improve the modeling mechanisms. This initiative sponsored the lease and management of a large supercomputer for modeling experiments (Henderson-Sellers et al., 1995). The project was partially successful in motivating modelers to seek broader use of measurements for climate model improvement (Trenberth, 1992). This need continues today in the climate science community, and it is partially supported with measurements of changes in the cryosphere, as well as sea level rise, ice core chemistry, and other nonatmospheric indicators of warming.

Supplemental data

Supplemental data for this article can be accessed on the publisher’s Website.

Additional information

Notes on contributors

Michael T. Kleinman

Dr. Michael T. Kleinman is professor of environmental toxicology and co-director of the Air Pollution Health Effects Laboratory in the Department of Community and Environmental Medicine, University of California, Irvine, and past chair of the Critical Review Committee.

Sara J. Head

Ms. Sara J. Head, QEP, is a principal scientist at Yorke Engineering, LLC. She is an A&WMA fellow member and past A&WMA President. She also is chair of the Ventura County Air Pollution Control District Advisory Committee.

Eric D. Stevenson

Dr. Eric Stevenson is Director of Meteorology, Measurement, and Rules at the Bay Area Air Quality Management District.

John G. Watson

Dr. John G. Watson is a research professor of the Desert Research Institute, a former chair of the Critical Review Committee, and author of the 2002 Critical Review.

Judith C. Chow

Dr. Judith C. Chow is a research professor at the Desert Research Institute, a former chair of the Critical Review Committee, and author of the 1995 Critical Review.

George. M. Hidy

Dr. George Hidy is a principal of Envair/Aerochem, a former co-editor of the A&WMA’s journal, a former chair of the Critical Review Committee, and author of the 1984 and 2010 Critical Reviews.

Samuel L. Altshuler

Samuel L. Altshuler is an air quality consultant and the current chair of the Critical Review Committee.

Peter K. Mueller

Dr. Peter K. Mueller is a principal of TropoChem and the longest continuing member of the A&WMA (65 yr).