F. L. Ludwig

An Urban Diffusion Simulation Model For Carbon Monoxide

A relatively simple Gaussian-type diffusion simulation model for calculating urban carbon monoxide (CO) concentrations as a function of local meteorology and the distribution of traffic is described. The model can be used in two ways: (1) in the synoptic mode, in which hourly concentrations at one or many receptor points are calculated from historical or forecast traffic and meteorological data; and (2) in the climatological mode, in which concentration frequency distributions are calculated on the basis of long-term sequences of input data. For model evaluation purposes, an extensive field study involving meteorological and air-quality measurements was conducted during November-December 1970 in San Jose, Calif., which has an automated network to provide traffic data throughout the central business district. Model refinements made on the basis of the data from this experimental program include the addition of a street-canyon submodel to compensate for the important aerodynamic effects of buildings on CO c...

Tropospheric ozone: Concentrations and variabilities in clean remote atmospheres

Abstract Analysis of long-term ozone (O 3 ) data collected at remote sites between latitudes 19°N and 48°N is complemented by aircraft data to support the conclusion that a significant reservoir of ozone is present in the troposphere. Evidence suggests that this O 3 reservoir is predominantly of stratospheric origin and that photochcmical oxidation processes resulting in O 3 production from HCs (including CH 4 ), CO and NO x of natural origin do not contribute significantly to the net O 3 balance in this reservoir. It is found that the predominant source of tropospheric O 3 is due to injections from the stratosphere. The important sinks are O 3 loss at and near the earths surface and to a less certain degree gas phase photolytic destruction. The possibility that natural precursors produce O 3 , at a rate that roughly offsets the gas phase photolytic destruction, can not be ruled out. This tropospheric O 3 shows a distinct seasonal variation, with a maximum in the spring when 1-h O 3 concentrations can approach or exceed 80 parts per billion (ppb). Year to year variations can be important; for example, at Mauna Loa the hourly averaged O 3 concentrations exceeded 80 ppb 1.6% of the days in 1975 and 0% of the days in 1974. No average diurnal variation of O 3 is observed throughout the year within this reservoir. Because O 3 from this reservoir is mixed downward toward the surface under convective atmospheric conditions, the achievement of a yearly 1-h O 3 standard of 80 ppb may be impossible. Analysis of atmospheric data also indicates that the mean residence time of O 3 in the troposphere is long (≈4 months), while that of NO x , is much shorter (0.4–2 days) than the hitherto accepted value of 8–10 days.

Validation and applications of an urban diffusion model for vehicular pollutants

Abstract A practical, multipurpose urban diffusion model (APRAC-1A) has been developed and evaluated for predicting concentrations of inert, vehicle-generated pollutants. The model requires only routinely available meteorological and traffic data to give the following outputs: 1. (1) areal concentration isopleths, 2. (2) sequential hourly point values, 3. (3) frequency distributions. An extensive field experiment was conducted in San Jose to evaluate and refine all components of the model. A street effects submodel was developed to account for the high concentrations and large gradients observed in urban street canyons. Results are presented from a second experimental program conducted in St. Louis to further evaluate the performance of the model. Predicted and observed concentrations of carbon monoxide (CO) in St. Louis differ by root-mean-square values of the order of 3 ppm (parts per million), while frequency distributions of hourly concentrations are given within 2–3 ppm. Along-street observations of CO concentrations show variations generally less than 1 ppm.

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...

An automatic method for determining the mixing depth from lidar observations

Abstract Lidar observations of the atmosphere reveal layers having different concentrations of aerosols. A computer method has been developed to analyze digital records of lidar backscatter in a manner that reveals the presence and heights of significant aerosol layers and the depth of the mixed layer. Boundaries between layers are represented by vertical gradients in the backscatter data. To compute the mixing depth, the backscatter gradients for a given time period (e.g. 10 min) are treated in a two-dimensional height-time array. Positive gradients and negative gradients are organized into groups that are contiguous in space and time. Then an algorithm selects one group as representing the mixing depth, on the basis of the height of each group and the strength of the gradients. Mixing depths determined automatically from the lidar records agreed with subjective estimates of mixing depth (based on data from lidar, radiosondes, and helicopter temperature soundings) 91% of the time for data collected in St. Louis, Missouri during the RAPS program of July and August, 1976. A slightly altered version of the automatic method, applied to lidar observations from the October 1977 AMBIENS experiment in Indiana, was 97% accurate. Real-time application of the method is believed to be feasible.

A Study of Aerosols in Pacific Air Masses

Abstract This paper describes a program of aerosol measurements made in the fall of 1965 at Cape Blanco and Crater Lake, Ore. Aerosol size distributions were determined with a Royco light-scattering type particle counter and with condensation nuclei counters. Size distributions at Crater Lake (2200 m altitude) were found to be well approximated by power law distributions with exponents between 3 and 4. The Cape Blanco data agree well with earlier measurements of sea-spray, aerosol size distributions. A Goetz aerosol centrifuge was used, in conjunction with filter sampling, to determine the size distribution of the chloride- and sulfur-compound fractions of the aerosol. At Crater Lake, sulfur-to-chloride ratios of about 8 were observed and considerable concentrations of other materials could be inferred. The data indicate that significant amounts of sea-spray aerosol did not penetrate upward to altitudes >2000 m. The aerosols identified with these 2000 m tropospheric levels are believed to be aged aerosols...

Simplification of a gaussian puff model for real-time minicomputer use

Abstract Simplifications are described that will substantially reduce the computation time and memory requirements of a Gaussian puff diffusion model without impairing its ability to simulate temporally and spatially changing conditions. Among the more important simplifications are: a variable puff generation rate, merging of closely spaced puffs, nonuniform spatial resolution of the wind field and the use of integers to represent some of the variables. A model using the simplifications was able to simulate the 24-h behavior of emissions from a power plant; concentrations were calculated at 25 points and at 48 different times. The time required on a Data General Nova 1200 minicomputer is about 6 min, but varies with different sequences of meteorological conditions. The memory required is about 24,000 words.

Classification of ozone and weather patterns associated with high ozone concentrations in the san francisco and monterey bay areas

Ozone concentrations exceeded the federal ozone standard on eight days between 1987 and 1992 at an elevation of 335 m in Pinnacles National Monument, the only site to do so in the Monterey Bay Unified Air Pollution Control District (bordering the San Francisco Bay Area to the south) from 1987 through 1989. There are no nearby sources of ozone precursors. Cluster and empirical orthogonal function (EOF) analysis were used to define the spatial patterns of daily ozone maxima associated with Pinnacles violations. Meteorological data were also categorized to determine the combination of conditions that were unique to the Pinnacles violation days. It was concluded from the analyses that the ozone violations at the Pinnacles site represent conditions within or just below the subsidence inversion that is associated with the semipermanent high-pressure system off the west coast. It appears that the ozone is preserved for a considerable period of time in the stable inversion and has traveled long distances. Neither the mechanism by which the ozone is originally trapped, nor the original sources of its precursors are clear from this analysis.

Particle size distribution of urban lead aerosols.

Size distributions of lead aerosols were obtained for 61 test periods in five urban and two remote areas of the United States. The tests were carried out under a variety of weather conditions in downtown, industrial, and residential locations of Los Angeles, the San Francisco Bay area, Chicago, Cincinnati, Philadelphia, and remote sites in Oklahoma and Arizona. Samples were obtained with a modified Goetz Aerosol spectrometer. Various fractions of the collected samples were analyzed by the dithizone method for lead and the size distributions were calculated on the basis of the chemical determinations. Size distributions were generally similar at the several sampling sites covered in the study.

Size distribution of sulfur-containing compounds in urban aerosols.

Abstract An aerosol spectrometer was used to determine the size distribution of sulfur-containing particulate materials in urban areas of northern and southern California. Based on a limited number of summer daytime samples, a typical Menlo Park size distribution had a mass median diameter of about 0.3 μ with upper and lower quartiles at about 0.9 and 0.1 μ, respectively. A typical Pasadena size distribution would be about one-tenth micron larger. The diameters given are for equivalent unit-density spheres of the same settling velocities as the sampled particles.