Air quality measurements-From rubber bands to tapping the rainbow.

It is axiomatic that good measurements are integral to good public policy for environmental protection. The generalized term for "measurements" includes sampling and quantitation, data integrity, documentation, network design, sponsorship, operations, archiving, and accessing for applications. Each of these components has evolved and advanced over the last 200 years as knowledge of atmospheric chemistry and physics has matured. Air quality was first detected by what people could see and smell in contaminated air. Gaseous pollutants were found to react with certain materials or chemicals, changing the color of dissolved reagents such that their light absorption at selected wavelengths could be related to both the pollutant chemistry and its concentration. Airborne particles have challenged the development of a variety of sensory devices and laboratory assays for characterization of their enormous range of physical and chemical properties. Advanced electronics made possible the sampling, concentration, and detection of gases and particles, both in situ and in laboratory analysis of collected samples. Accurate and precise measurements by these methods have made possible advanced air quality management practices that led to decreasing concentrations over time. New technologies are leading to smaller and cheaper measurement systems that can further expand and enhance current air pollution monitoring networks. IMPLICATIONS: Ambient air quality measurement systems have a large influence on air quality management by determining compliance, tracking trends, elucidating pollutant transport and transformation, and relating concentrations to adverse effects. These systems consist of more than just instrumentation, and involve extensive support efforts for siting, maintenance, calibration, auditing, data validation, data management and access, and data interpretation. These requirements have largely been attained for criteria pollutants regulated by National Ambient Air Quality Standards, but they are rarely attained for nonroutine measurements and research studies.

Precursor reductions and ground-level ozone in the Continental United States.

UNLABELLED: Numerous papers analyze ground-level ozone (O3) trends since the 1980s, but few have linked O3trends with observed changes in nitrogen oxide (NOx) and volatile organic compound (VOC) emissions and ambient concentrations. This analysis of emissions and ambient measurements examines this linkage across the United States on multiple spatial scales from continental to urban. O3concentrations follow the general decreases in both NOx and VOC emissions and ambient concentrations of precursors (nitrogen dioxide, NO2; nonmethane organic compounds, NMOCs). Annual fourth-highest daily peak 8-hr average ozone and annual average or 98th percentile daily maximum hourly NO2concentrations show a statistically significant (p < 0.05) linear fit whose slope is less than 1:1 and intercept is in the 30 to >50 ppbv range. This empirical relationship is consistent with current understanding of O3photochemistry. The linear O3-NO2relationships found from our multispatial scale analysis can be used to extrapolate the rate of change of O3with projected NOx emission reductions, which suggests that future declines in annual fourth-highest daily average 8-hr maximum O3concentrations are unlikely to reach 65 ppbv or lower everywhere in the next decade. Measurements do not indicate increased annual reduction rates in (high) O3concentrations beyond the multidecadal precursor proportionality, since aggressive measures for NOx and VOC reduction are in place and have not produced an accelerated O3reduction rate beyond that prior to the mid-2000s. Empirically estimated changes in O3with emissions suggest that O3is less sensitive to precursor reductions than is found by the CAMx (v. 6.1) photochemical model. Options for increasing the rate of O3change are limited by photochemical factors, including the increase in NOx sensitivity with time (NMOC/NOx ratio increase), increase in O3production efficiency at lower NOx concentrations (higher O3/NOy ratio), and the presence of natural NOx and NMOC precursors and background O3. IMPLICATIONS: This analysis demonstrates empirical relations between O3and precursors based on long term trends in U.S. LOCATIONS: The results indicate that ground-level O3concentrations have responded predictably to reductions in VOC and NOx since the 1980s. The analysis reveals linear relations between the highest O3and NO2concentrations. Extrapolation of the historic trends to the future with expected continued precursor reductions suggest that achieving the 2014 proposed reduction in the U.S. National Ambient Air Quality Standard to a level between 65 and 70 ppbv is unlikely within the next decade. Comparison of measurements with national results from a regulatory photochemical model, CAMx, v. 6.1, suggests that model predictions are more sensitive to emissions changes than the observations would support.

Source attribution of air pollutant concentrations and trends in the southeastern aerosol research and characterization (SEARCH) network.

A new approach for determining the contributions of emission sources to trends in concentrations of particulate matter and gases is developed using the chemical mass balance (CMB) method and the U.S. EPA's National Emission Inventory (NEI). The method extends our earlier analysis by using temporally varying emission profiles and includes accounting of primary and secondary particulate organic carbon with an empirical regression calculation. The model offers a potentially important tool for verifying that annual emission reductions by major source category have yielded changes in ambient pollutant concentrations. Using long-term measurements from well-instrumented monitoring sites, observed trends in ambient pollutant concentrations at urban and rural locations can be attributed to emission changes. Trends apportionment is conducted on 2000-2011 ambient monitoring data from the SEARCH network with NEI emissions data adjusted to improve interinventory consistency. The application accounts for major source category influences in southeastern U.S. regional trends; local anomalies are noted. In the SEARCH region, open burning is important as a source of CO and carbonaceous particles. Improved agreement between predicted and measured particulate carbon is obtained by increasing mobile diesel exhaust and area-source particulate carbon emissions by 1 and 20%, respectively, compared with NEI values. The method is general and is applicable to data from any monitoring site that is instrumented for criteria air pollutants, associated gases, and particle composition.

Tropospheric aerosols: size-differentiated chemistry and large-scale spatial distributions.

Worldwide interest in atmospheric aerosols has emerged since the late 20th century as a part of concerns for air pollution and radiative forcing of the earth's climate. The use of aircraft and balloons for sampling and the use of remote sensing have dramatically expanded knowledge about tropospheric aerosols. Our survey gives an overview of contemporary tropospheric aerosol chemistry based mainly on in situ measurements. It focuses on fine particles less than 1-2.5 microm in diameter. The physical properties of particles by region and altitude are exemplified by particle size distributions, total number and volume concentration, and optical parameters such as extinction coefficient and aerosol optical depth. Particle chemical characterization is size dependent, differentiated by ubiquitous sulfate, and carbon, partially from anthropogenic activity. Large-scale particle distributions extend to intra- and intercontinental proportions involving plumes from population centers to natural disturbances such as dust storms and vegetation fires. In the marine environment, sea salt adds an important component to aerosols. Generally, aerosol components, most of whose sources are at the earth's surface, tend to dilute and decrease in concentration with height, but often show different (layered) profiles depending on meteorological conditions. Key microscopic processes include new particle formation aloft and cloud interactions, both cloud initiation and cloud evaporation. Measurement campaigns aloft are short term, giving snapshots of inherently transient phenomena in the troposphere. Nevertheless, these data, combined with long-term data at the surface and optical depth and transmission observations, yield a unique picture of global tropospheric particle chemistry.

Energy supplies and future engines for land, sea, and air.

The 2012 Critical Review Discussion complements Wilson, (2012), provides pointers to more detailed treatments of different topics and adds additional dimensions to the area of "energy". These include broader aspects of technologies driven by fuel resources and environmental issues, the concept of energy technology innovation, evolution in transportation resources, and complexities of energy policies addressing carbon taxes or carbon trading. National and global energy data bases are identified and evaluated and conversion factors are given to allow their comparability.

Source contributions to atmospheric gases and particulate matter in the southeastern United States.

A new approach for determining the contributions of emission sources to concentrations of particulate matter and gases is developed using the chemical mass balance (CMB) method and the U.S. EPA's National Emission Inventory (NEI). The approach apportions combined gas-phase and condensed-phase concentrations of individual compounds as well as PM(2.5) mass. Because the NEI is used to provide source emission profiles for CMB analysis, the method generates information on the consistency of the NEI with ambient monitoring data. The method also tracks secondary species to primary source emissions, permitting a more complete accounting of the impact of aggregated source types on PM(2.5) mass concentrations. An example application is presented using four years of monitoring data collected at eight sites in the Southeastern Aerosol Research and Characterization (SEARCH) network. Including both primary and secondary species, area sources contributed 2.0-3.7 µg m(-3) (13-26%), point sources contributed 3.0-4.6 µg m(-3) (22-33%), and mobile sources contributed 1.0-6.0 µg m(-3) (9-42%) to mean PM(2.5) mass concentrations. Whereas the NEI generally accounts for the ambient concentrations of gases and particles, certain anomalies are identified, especially related to carbonaceous compounds and dust.

Multipollutant air quality management.

On the basis of a recent NARSTO assessment, this review discusses the factors involved in the implementation of a risk- and results-based multipollutant air quality management strategy applicable to North America. Such a strategy could evolve from current single-pollutant regulatory practices using a series of steps that would seek to minimize risk of exposure for humans and ecosystems while providing for a quantitative evaluation of the effectiveness of the management process. The tools needed to support multipollutant air quality management are summarized. They include application of a formal risk analysis, accounting for atmospheric processes, ambient measurements, emissions characterization, air quality modeling of emissions to ambient concentrations, and characterization of human and ecological responses to ambient pollutant exposure. The new management strategy would expand the current practice of accountability that relates emission reductions and attainment of air quality derived from air quality criteria and standards. Conceptually, achievement of accountability would establish goals optimizing risk reduction associated with pollution management. This expanded approach takes into account the sequence of processes from emissions reduction to resulting changes in ambient concentration. Using ambient concentration as a proxy for exposure, the resulting improvement in human and ecosystem health is estimated. The degree to which this chain of processes and effects can be achieved in current practice is examined in a multipollutant context exemplified by oxidants, as indicated by ozone, particulate matter, and some hazardous air pollutants. Achievement of a multipollutant management strategy will mostly depend on improving knowledge about human and ecosystem response to pollutant exposure.

Remote sensing of particulate pollution from space: have we reached the promised land?

The Critical Review of Hoff and Christopher, along with the discussants, provides an important perspective on the interface between satellite measurement science and air quality observations. A top-down picture of the usefulness of satellite observations in terms of air quality regulatory and technical support requirements can be summarized. The air quality requirements are (1) determination of compliance with the ambient air quality standards, (2) inference of human and ecosystem exposure, (3) identification of intra- and intercontinental events relevant to EE, (4) establishment of trends in ambient concentrations relevant to accountability, (5) regulatory and forecast model applications, and (6) extension of fundamental knowledge relevant to air quality. Each of these topics is important to air quality management, and each has detailed technical issues associated with spatial and temporal resolution, accuracy, and precision, etc. In any case, one can summarize the broad capabilities of measurement systems to address these requirements as listed in Table 1. From this rather superficial summary table, investigators should be encouraged to forward increased interaction between the various measurement communities and to facilitate the utility of a comprehensive portfolio of measurements and adjunct analyses for improved air quality applications. The Critical Review has done much to educate air quality scientists on the possibilities for using satellite remote sensing for various purposes. However, space scientists also need a better education on air quality science. Recently published reviews on PM air quality measurements are available that complement the Hoff-Christopher paper on this topic. The need for greater collaboration of air quality and space scientists is evident in an article published in the July issue of the journal. Al-Hamdan et al. provide an interesting and useful analysis of relationships between surface air quality and space-based satellite AOD to estimate human exposure. They obtain mostly urban PM data from EPA's Air Quality System (AQS), but they neglect the potentially more useful PM2.5 and chemical speciation data from the nonurban Interagency Monitoring of Protected Visual Environments (IMPROVE) and the Southeastern Aerosol Research and Characterization (SEARCH) networks. They correlate PM2.5 mass with optical depth, although visibility assessments show that light extinction is better represented by a weighted sum of PM2.5 sulfate, nitrate, organic carbon, elemental carbon, and soil dust. Their comparison of hourly measurements with filter measurements does not specify the source of the hourly values as TEOM or BAM. Spatial outliers for ground-level measurements are removed to improve the correlation of PM2.5 with AOD, although these "outliers" are probably real values that relate to human exposure or a nearby source effect. The point here is not to overly criticize a good publication that will be highly cited. The intent is to demonstrate the value of air quality and space scientists working together more closely on this topic. This is something the review authors alluded to in their review, but if, as they concluded, the "promised land" has not been reached, then perhaps it is an appropriate time for the atmospheric community to ask, "Can near-term satellite observations play a role in characterizing broad-based (outdoor) exposure to pollutants and consequently influence public health improvement?" and, if so, then, "What comprehensive, integrated system is needed if satellite observations are to be used together with ground-based observations and modeling to continue improving air quality management options?"

Surface-level fine particle mass concentrations: from hemispheric distributions to megacity sources.

Since 1990, basic knowledge of the "chemical climate" of fine particles, has greatly improved from Junge's compilation from the 1960s. A worldwide baseline distribution of fine particle concentrations on a synoptic scale of approximately 1000 km can be estimated at least qualitatively from measurements. A geographical distribution of fine particle characteristics is deduced from a synthesis of a variety of disparate data collected at ground level on all continents, especially in the northern hemisphere. On the average, the regional mass concentrations range from 1 to 80 microg/m3, with the highest concentrations in regions of high population density and industrialization. Fine particles by mass on a continental and hemispheric spatial scale are generally dominated by non-sea salt sulfate (0.2 to approximately 20 microg/m3, or approximately 25%) and organic carbon (0.2-> 10 microg/m3, or approximately 25%), with lesser contributions of ammonium, nitrate, elemental carbon, and elements found in sea salt or soil dust. The crustal and trace metal elements contribute a varied amount to fine particle mass depending on location, with a larger contribution in marine conditions or during certain events such as dust storms or volcanic disturbances. The average distribution of mass concentration and major components depends on the proximity to areal aggregations of sources, most of which are continental in origin, with contributions from sea salt emissions in the marine environment. The highest concentrations generally are within or near very large population and industrial centers, especially in Asia, including parts of China and India, as well as North America and Europe. Natural sources of blowing dust, sea salt, and wildfires contribute to large, intermittent spatial-scale particle loadings beyond these ranges. A sampling of 10 megacities illustrates a range of characteristic particle composition, dependent on local and regional sources. Long-range transport of pollution from spatially aggregated sources over hundreds of kilometers creates persistent regional- and continental-scale gradients of mass concentration, sulfate, and carbon species especially in the northern hemisphere. Data are sparse in the southern hemisphere, especially beyond 45 degrees S, but are generally very low in mass concentrations.

Effects of sulfur dioxide and oxides of nitrogen emission reductions on fine particulate matter mass concentrations: regional comparisons.

Two thermodynamic equilibrium models were applied to estimate changes in mean airborne fine particle (PM2.5) mass concentrations that could result from changes in ambient concentrations of sulfate, nitric acid, or ammonia in the southeastern United States, the midwestern United States, and central California. Pronounced regional differences were found. Southeastern sites exhibited the lowest current mean concentrations of nitrate, and the smallest predicted responses of PM2.5 nitrate and mass concentrations to reductions of nitric acid, which is the principal reaction product of the oxidation of nitrogen dioxide (NO2) and the primary gas-phase precursor of fine particulate nitrate. Weak responses of PM2.5 nitrate and mass concentrations to changes in nitric acid levels occurred even if sulfate concentrations were half of current levels. The midwestern sites showed higher levels of fine particulate nitrate, characterized by cold-season maxima, and were projected to show decreases in overall PM levels following decreases of either sulfate or nitric acid. For some midwestern sites, predicted PM2.5 nitrate concentrations increased as modeled sulfate levels declined, but sulfate reductions always reduced the predicted fine PM mass concentrations; PM2.5 nitrate concentrations became more sensitive to reductions of nitric acid as modeled sulfate concentrations were decreased. The California sites currently have the highest mean concentrations of fine PM nitrate and the lowest mean concentrations of fine PM sulfate. Both the estimated PM2.5 nitrate and fine mass concentrations decreased in response to modeled reductions of nitric acid at all California sites. The results indicate important regional differences in expected PM2.5 mass concentration responses to changes in sulfate and nitrate precursors. Analyses of ambient data, such as described here, can be a key part of weight of evidence (WOE) demonstrations for PM2.5 attainment plans. Acquisition of the data may require special sampling efforts, especially for PM2.5 precursor concentration data.