George M. Hidy

Summary of the California Aerosol Characterization Experiment.

This report gives a brief summary of the experimental plan and the early results of field observations from the California Aerosol Characterization Experiment (ACHEX). The objectives of the program center on questions of the sources and evolution of aerosols in urban air, as they are superimposed on a natural background, particularly as they relate to visibilitydegradation. The project initially nvolved the use of an elaborately instrumented mobile laboratory combined with a fixed station in San Jose, in Pasadena, and in Riverside, and later incorporated several satellite monitoring stations in the Los Angeles Basin. Intensive field observationswere taken from July 1972 to November 1972 in several urban and nonurban locations between the San Francisco Bay area and the South Coast Basin, covering the Los Angeles area. In a second phase of the ACHEX, aerosols accompanying photochemical smog were studied intensively in the Los Angeles Basin during the period between July and October of 1973. The observations...

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. EPAs 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.

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. EPAs 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.

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

Numerous papers analyze ground-level ozone (O3) trends since the 1980s, but few have linked O3 trends 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. O3 concentrations 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 NO2 concentrations 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 O3 photochemistry. The linear O3–NO2 relationships found from our multispatial scale analysis can be used to extrapolate the rate of change of O3 with projected NOx emission reductions, which suggests that future declines in annual fourth-highest daily average 8-hr maximum O3 concentrations are unlikely to reach 65 ppbv or lower everywhere in the next decade. Measurements do not indicate increased annual reduction rates in (high) O3 concentrations beyond the multidecadal precursor proportionality, since aggressive measures for NOx and VOC reduction are in place and have not produced an accelerated O3 reduction rate beyond that prior to the mid-2000s. Empirically estimated changes in O3 with emissions suggest that O3 is less sensitive to precursor reductions than is found by the CAMx (v. 6.1) photochemical model. Options for increasing the rate of O3 change are limited by photochemical factors, including the increase in NOx sensitivity with time (NMOC/NOx ratio increase), increase in O3 production 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 O3 and precursors based on long term trends in U.S. locations. The results indicate that ground-level O3 concentrations have responded predictably to reductions in VOC and NOx since the 1980s. The analysis reveals linear relations between the highest O3 and NO2 concentrations. 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.

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 earths 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 µm 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 earths 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. Implications: Aerosols in the troposphere incorporate issues of air pollution exposing humans and ecosystems, and global forcing of the climate of the atmosphere. Fundamental physicochemical knowledge of suspended particles of different sizes is essential to addressing these as technical issues across the world. Current literature informs policymakers about the nature of tropospheric aerosols and their global scale spatial and temporal distributions. With this knowledge, cooperative programs for addressing environmental issues across national borders can be constructed with insight into sources and receptors of pollution and natural air chemistry. For the United States the policy-relevant background for particles is better defined from this work. Supplemental Materials: Supplemental materials are available for this paper. Go to the publishers online edition of the Journal of the Air & Waste Management Association for detailed data tables, color graphics, summary dynamics, and additional references.