1995 Southern Oxidants Study
"Photochemical oxidants are a class of highly reactive chemicals produced in the earth's atmosphere. In the stratosphere, O3 [Ozone], the most abundant of the photochemical oxidants, protects human, plant, and animal life from the harmful effects of ultraviolet light from the sun. In the lower atmosphere, paradoxically, O3 and other photochemical oxidants can have adverse effects on plants, animals, and human health. [. . .] Even though these chemical compounds are normally present only in very low concentrations, their health, economic, and ecological impact can be substantial." -The State of the Southern Oxidants Study (SOS)
The Southern Oxidants Study (SOS) is a strategic alliance of research scientists, engineers, and air quality managers from universities, federal and state governments, industry, and public interest groups formed in 1989 to address the unresolved technical and scientific issues surrounding ozone pollution and ozone pollution management.
The ETL Lidar Division contributed to this alliance during the 1995 Nashville Study through precision measurement and mapping of ozone over the study area with the airborne differential-absorbption lidar (DIAL).
Urban Stagnation Study
Conditions that lead to the highest hourly photochemical ozone (O3) concentrations for a season in an urban area are light-wind conditions, in which pollutants accumulate near the source region. The very highest concentrations are expected during the lightest winds, i.e., during stagnation conditions. During the 1995 S.O.S. Nashville campaign, a 3-day stagnation episode occurred during mid July. Scientists obtained a rich dataset, including several very revealing flights from the ETL airborne O3 differential-absorption lidar (DIAL).
Key scientific questions included:
Data from the airborne O3 DIAL system allow us to address these issues. The data provide vertical cross sections of O3 concentration along the flight path of the aircraft. Intercomparison flights have shown that the O3 concentrations measured by the lidar agree with in situ aircraft measurements by 4 ppbv near the airplane and ~10 ppbv near the earth's surface (i.e., farther from the airplane).
Figure A. Vertical cross sections of O3 concentration and aerosol backscatter for the NW-SE flight legs passing over Nashville for the afternoon flights on 12 July. (i) O3 concentrations (ppbV) for the first pass (1220-1248 CDT, (ii) aerosol backscatter (10-5 m-1 sr-1) for the first pass, and (iii) O3 concentration for the second pass (1405-1433 CDT).
Power Plant Plume Study
The role of airborne lidar in power plant plume studies
To investigate chemical processes in power plant plumes, airborne in situ sensors are typically the instruments of choice because of their ability to detect a multitude of chemical species with high accuracy. However, coverage of the plume area is limited, since the plume is usually only transected at a single height. This means that often assumptions must be made about plume properties when the in situ data are interpreted. An airborne lidar, on the other hand, can map out the cross section of the plume during a single overpass yielding information on plume size, shape, and the vertical distribution of certain pollutants in the plume. Thus, an airborne lidar can provide crucial information on power plant plumes properties which, in concert with in situ measurements, can help further our understanding of power plant plume chemistry and its impact on air quality.
We used NOAA/ETL's airborne ozone and aerosol lidar to investigate power plant plumes in the Nashville area during the 1995 SOS campaign. The downward-looking lidar was typically flown at an altitude of around 3000 m above sea level crossing the power plant plume at multiple distances downwind of the plant. The aerosol backscatter and ozone data gathered while flying across the plume give a detailed picture of the vertical and lateral extent of the plume. Also, the plume location with respect to the top of the boundary layer can be determined because the lidar aerosol returns provide information on the boundary layer height. In addition, we used the lidar ozone data to calculate ozone production rates in the plume as a function of distance downwind.
- The evolution of a power plant plume depends strongly on the local meteorological conditions. In one case study, the plume was spread quickly across the entire depth of the boundary layer due to strong vertical mixing but did not reach into the overlying free troposphere. The plume was shaped symmetrically and had a clearly defined, single core (Fig. 1). In another case, part of the power plant plume had penetrated the inversion capping the boundary layer shortly after release from the stack. Subject to different meteorological conditions the two portions of the plume above and below the boundary layer top evolved differently as they advected downwind resulting in a very irregular shape and a near breakup of the plume (Fig. 2).
- We found that close to the power plant ozone was destroyed creating an ozone deficit by which the plume can be readily identified. This is a well known effect caused by titration of ozone in the presence of elevated NO levels. Farther downwind, where ozone is formed in the plume due to reaction of NOx with ozone precursor gases, we observed ozone production rates of up to 4 ppbV/h. The latter confirms that NOx-rich power plant plumes have the potential of raising local ozone levels significantly over the course of a day.
Questions and Answers about Ozone Pollution (PDF)
1995 Nashville/Middle Tennessee Ozone Study (PDF)
AlvarezII, R.J., C.J. Senff, R.M. Hardesty, D.D. Parrish, W.T. Luke, T.B. Watson, P.H. Daum, and N. Gillani, 1998: Comparisons of airborne lidar measurements of ozone with airborne in situ measurements during the 1995 Southern Oxidants Study, J. Geophys. Res., 103, 31,155 - 31,171.
Banta, R.M., C.J. Senff, A.B. White, M. Trainer, R.T. McNider, R.J. Valente, S.D. Mayor, R.J. Alvarez, R.M. Hardesty, D. Parrish, and F.C. Fehsenfeld, 1998: Daytime buildup and nighttime transport of urban ozone in the boundary layer during a stagnation episode, J. Geophys. Res., 103, 22,519 - 22,544.
Senff, C.J., R.M. Hardesty, R.J. AlvarezII, and S.D. Mayor, 1998: Airborne lidar characterization of power plant plumes during the 1995 Southern Oxidants Study, J. Geophys. Res., 103, 31,173 - 31,189.