William Viezee

Stratospheric ozone in the lower troposphere —I. Presentation and interpretation of aircraft measurements

Abstract The United States has instituted a number of air pollution regulations to control ozone, including an air quality standard for oxidants of 120 ppb for one hour. Accordingly, there is considerable interest in determining the magnitude of the natural (i.e. nonanthropogenic) component of ozone concentration near the ground, much of which is generally believed to come from the stratosphere. Toward this end, an extensive program of aircraft measurements of tropospheric ozone originating from the stratosphere was carried out over the Central U.S. in spring and fall 1978. On 10 of these flights, the vertical structure of stratospheric ozone intrusions was well mapped by aircraft penetrations at several altitudes extending between 2 and 8 km above sea level (ASL) in the southern portions of tropospheric low-pressure troughs. The field measurements show that stratospheric ozone intrusions into the troposphere occur more frequently than earlier studies had indicated. Ozone intrusions were found in virtually every trough, regardless of intensity, within which suitable measurements were taken. A close relationship was found between: 1. (1) maximum ozone concentrations in the intrusion; and 2. (2) trough intensity as characterized by maximum wind speed at 300 mb (approximately 10 km ASL). The intrusions typically are characterized by peak ozone concentrations at higher altitudes (6–8 km ASL) in the range of 240–400 ppb, diminishing to 100–200 ppb at lower altitudes as mixing with surrounding air occurs. Measured concentrations during spring were almost twice as high as those measured during fall, but the intrusion structures were very similar during both seasons. The data show that stratospheric ozone intrusions are typically 100–300 km wide in the crosswind direction, are several hundreds of kilometers long, and can be tracked down at least as far as the top of the atmospheric boundary layer (about 2 km ASL). Possible mechanisms for downward transport within the boundary layer include normal convective mixing, organized convection associated with cloud and precipitation processes, and organized downward motion within frontal zones.

Stratospheric ozone in the lower troposphere. II: Assessment of downward flux and ground-level impact

Abstract Aircraft measurements of four stratospheric O3-intrusion events (two during spring and two during fall) are used in conjunction with concurrent meteorological analyses to estimate the downward 03 flux in the upper and lower troposphere. The aircraft measurements used are among those reported earlier in Part I of this paper (Johnson and Viezee, 1981). The calculated upper tropospheric fluxes for the four cases show good agreement with earlier estimates by Danielsen and Mohnen (1977) of O3 fluxes associated with tropopause folding events. The estimated lower-tropospheric O3 fluxes for the two spring events and for one fall event suggest that more than half of the O3 mass injected into the upper troposphere by these stratospheric intrusions is probably mixed and diluted into the troposphere above 700 mb (3 km ASL). Large, direct impacts of stratospheric O3 intrusions at ground-level are thus unlikely. A review and analysis of the limited number of published observations of high O3 in stratospheric intrusions, and of anomalously high O3 at ground level attributed to stratospheric intrusions, also suggests that direct ground-level impacts may be infrequent (less than 1 per cent of the time), and most likely are associated with O3 concentrations (v/v) of 100 ppb or less. Additional observational studies are required to conclusively quantify the ground-level impact of stratospheric O3.

The impact of stratospheric ozone on tropospheric air quality

A background of ozone (O3), principally of stratospheric origin, is present in the lower free troposphere. Typical mean O3 levels of 50 ppb, 40 ppb, and 30 ppb are encountered here in spring, summer, and fall, respectively. Maximum hourly O3 concentrations which are twice these mean values can be expected. Ozone from the free troposphere is routinely brought down to ground level under turbulent atmospheric conditions. Deep and rapid Intrusions of stratospheric air into the lower troposphere are associated with low-pressure troughs and occur regularly. In the mid troposphere, O3 levels as high as 300 ppb are found within these intrusions. Observational data showing these intrusions, containing high O3 concentrations, to directly reach ground level are currently lacking. Over the United States, an intrusion was present aloft on 8 9% of the days in 1978. The frequency, however, is somewhat reduced in summer and a northward movement is evident. During 1978, no intrusion occurred south of 30°N between June and...

Measurement of volatile organic chemicals at selected sites in California

Abstract Urban air concentrations of 24 selected volatile organic chemicals that may be potentially hazardous to human health and environment were measured during field experiments conducted at two California locations, at Houston, TX, and at Denver, CO. Chemicals measured included chlorofluorocarbons, halomethanes, haloethanes, halopropanes, chloroethylenes and aromatic hydrocarbons. With emphasis on California sites, data from these studies are analysed and interpreted with respect to variabilities in ambient air concentrations, diurnal changes, relation to prevailing meteorology, sources and trends. Except in a few instances, mean concentrations are typically between 0 and 5 ppb. Significant variabilities in atmospheric concentrations associated with intense sources and adverse meteorological conditions are shown to exist. In addition to short-term variability, there is evidence of systematic diurnal and seasonal trends. In some instances it is possible to detect declining trends (e.g. ethylene dibromide and chloroethylenes) resulting from the effectiveness of control strategies.

Enhancement of PAN abundance in the Pacific marine air upon contact with selected surfaces

Peroxyacetyl nitrate (PAN) concentrations in the marine air of the eastern Pacific Ocean were found to increase by a factor of 1–6 (average 3.2 ± 1.4) over their normal values when the ambient air was stored in an all-glass environment for a period of 1–2 min prior to analysis. This 20–170 ppt enhancement in PAN levels appeared to vary diurnally. No strong evidence for latitudinal dependence was found. The phenomenon of surface PAN enhancement in the marine air could not be duplicated with samples of highly polluted city air or with laboratory-generated PAN/air mixtures. The mechanisms by which this additional PAN is generated in marine air are not known, but some speculations are presented. Additional systematic studies are required to identify the cause(s) of this “apparent PAN” formation.

Airborne Lidar Tracking of Fluorescent Tracers for Atmospheric Transport and Diffusion Studies

Development and validation of transport models for the study of regional acid deposition require improved observations of pollutant transport and dispersion processes. No suitable method for air-parcel tracking along nonconstant density surfaces is available. The feasibility of using an airborne lidar system to observe atmospheric transport and dispersion of fluorescent-dye-particle (FDP) tracers was demonstrated for various meteorological conditions and FDP-release scenarios in the general area of the Cross-Appalachian Tracer Experiment (CAPTEX) during October 1983. This paper presents some of the results obtained on six case studies, each of which illustrates a unique application of the technique.

South Central Coast Cooperative Aerometric Monitoring Program (SCCCAMP)

Abstract The SCCCAMP field measurement program, conducted 3 September to 7 October 1985, is the most comprehensive mesoscale photochemical study of its type ever undertaken. The study area encompasses 2 × 104 km2 of coastal and interior south-central California including the Santa Barbara Channel. A review of earlier experimental and analytical studies in the area is followed by the organizational framework and planning for this cooperative program. The experimental design and measurement systems are described. Existing ground-based meteorological and air pollution networks were supplemented by additional surface aerometric stations, dual Doppler radars, rawinsondes, and a network of Doppler acoustic profilers. Airborne measurement platforms included one dual-channel lidar, three aerometric sampling aircraft, and a meteorological research aircraft. Gas tracer tests included 4-h releases of three perfluorocarbon gas tracers. Tracer measurements were made over two-day periods at 50 surface locations and alo...