The ramifications of the conclusions from the SOS 1994/1995 Nashville Middle Tennessee Ozone Study for policies to control ozone on local and regional scales in the Southeastern United States and their applicability to other locales and regions in the United States require additional testing and verification. In addition, these unexpected conclusions force us to consider other factors that may influence the photochemical processing of these compounds. In particular, the role played by particulate matter (PM) in tropospheric chemistry needs to be addressed.
The expansion of our measurements to investigate PM and PM-related processes is timely. Aerosols participate in a variety of chemical and physical processes in the troposphere. On a regional scale, these processes are associated with regional air quality as related to visibility and the effects of fine particles on human health. In this regard, there is a natural synergism between the ozone-related research described previously and the study of processes leading to or involving fine particles. The new Federal air quality regulations force us to recognize that a basic scientific understanding of the chemistry and physics of the atmosphere is prerequisite to the design of effective control strategies for these pollutants and that the concentrations of the pollutants in the atmosphere are often co-dependent because of interacting chemical reactions. With this in mind, we are proposing to expand the SOS measurements program to elucidate:
- how chemical processing on aerosols influences ozone formation;
- how the atmospheric oxidation leading to ozone formation leads to aerosol formation; and
- how atmospheric chemistry influences the growth and/or the chemical composition of aerosols.
This expansion of SOS to include the study of the formation and fate of fine particles in the atmosphere is being led by SOS' affiliated Southern Center for Integrated Study of Secondary Air Pollutants (SCISSAP). The Nashville-99 field study will be closely coordinated with the SCISSAP measurement network. The Field Study measurements will augment those of SCISSAP, providing process-level information on PM formation and distribution and an opportunity to evaluate emerging measurement technologies related to PM.
Measurements are intended to provide a better understanding of the basic chemical, meteorological, and transport processes that determine ozone and fine particle distributions and new information to assist policy- makers in devising optimal ozone and PM management strategies. These studies are encompassed in three broad themes.
Local vs. Regional - Regional Contrasts
The first area of investigation addresses whether ozone or fine particle pollution is a regional or a local problem. The range provided by the WP-3D and the G-1 allows the composition of the atmosphere in a particular location to be placed in a regional perspective. Even though the concentration of ozone and fine particles can be elevated over large areas, it is still an open question how much of either ozone or fine particles are produced locally and how much are produced remotely and then transported to a particular locale.
Although the study will be centered in the Nashville/Middle Tennessee area flights are planned for the Mountain West and the Upper Midwest where meteorological conditions and the mix of ozone and PM-precursor emissions are expected to be significantly different than in the Southeast. These measurements will build on previous NOAA studies in Colorado and the Midwest and the recent BNL study in Phoenix.
Ozone and PM formation in Plumes
The second area of investigation relates to if and how ozone or fine particles observed in a particular location can be attributed to a particular source of precursor compounds located among a complex matrix of precursor sources. An ideal setting to address this question is an isolated urban area set in a rural background with several large point sources of pollution (e.g., fossil fuel burning power plants) imbedded at various distances with a wide range of pollutant emission. Under certain general flow conditions the plumes of the power plants merge with each other and/or the urban plume; in others they don't. Hence, the synergism associated with the interaction of power plants plumes, urban plumes, and the regional background can be investigated under a variety of conditions and in various combinations. As the dimensions of the urban complex grows the magnitude of the problem becomes greater. A second aspect of interaction involves the relation of ozone pollution to fine particle pollution. To date, ozone pollution has been largely treated as a local or sub- regional problem, while fine particle pollution has been viewed as a local problem from the health- effects perspective and a regional problem from the visibility perspective. The co-variation of these pollutants has never been extensively investigated, particularly on the regional scale.
The study of the evolution of the pollutant mix in plumes is particularly useful in the determination of pollution formation kinetics. The advection of plumes into a reasonably uniform background airmass provides a convenient “clock” allowing the quantification of the chemical rates that are critical to model development and evaluation.
Diurnal Cycle in Chemistry and Meteorology
In the overwhelming majority of cases, intensive field studies have targeted the study of daytime chemistry and dynamics. This interest is driven by a recognition of the important role that photochemistry plays in secondary pollutant formation and a concern for the impacts associated with daytime pollution exposures for both humans and plants. The pollutant mix can be significantly affected by non-photochemical reactions that occur at night. The results of the 1995 SOS field study in Nashville highlighted the importance of nighttime mixing processes in the redistribution of pollution throughout the region. This field study offers the opportunity to document the entire diurnal cycle of chemistry and meteorology over one or more complete consecutive diurnal cycles using both surface-based and airborne observations. Information will be developed on processes that are not well described in current models. The measurements will help us understand how rural daytime chemistry, which establishes the residual convective mixed layer above the nocturnal boundary layer, ultimately is coupled to the daytime urban photochemistry that establishes peak ozone and PM levels in urban areas. Similarly, we will learn how daytime urban pollution affects rural air quality on the following day.
In order to address the scientific themes, we propose a research program combining a comprehensive suite of aircraft and ground-based measurements of the chemical and dynamical properties of the PBL and lower free troposphere. The study focuses on six research areas.
Primary PM and PM precursor emissions
The current research-grade ozone precursor inventory for the study area will be expanded to include both natural and anthropogenic sources of PM fine and PM precursors, with particular emphasis on animal and crop-agricultural sources of NH3.
Studies are proposed to characterize the effects of vertical and horizontal transport on the concentrations of ozone and aerosols and their precursors.
Ozone production efficiency
The influence of NOX source strength and ambient VOC distributions on ozone production efficiency (the number of molecules of ozone produced per NOX molecule emitted) will be investigated through studies of urban and power plant plume chemistry.
Characterization of loss processes
Follow-on studies will be conducted to determine the source of the high NOX loss rates observed during the 1995 field experiments. Both mass balance and tracer techniques will be employed.
VOC contribution to ozone and PM formation
The relative role of anthropogenic and biogenic VOCs in ozone and aerosol formation and aerosol composition will be investigated.
Fine particulate matter – formation and characterization
The effect of atmospheric chemistry on the composition and morphology of ambient aerosols will be investigated. The connections between the processes that control ozone and aerosol formation will be studied. The relative importance of primary and secondary aerosols, with their varied sources, to ambient aerosol mass will be determined.
Nighttime chemistry and dynamics
The processes that control the formation and distribution of ozone and aerosols during the daytime and nighttime will be contrasted. A particular effort will be made to characterize the fate of plumes from large power plants that are emitted above the nocturnal boundary layer.