Christine S. Sloane

Optical properties of aerosols of mixed composition

Abstract A model is developed which relates the reduction in visual air quality to the chemical and physical properties of airborne aerosol particles. The aerosols are considered to be inhomogeneous and to have a mixed chemical composition which varies with particle size. The water content of the aerosol particles is derived from a semi-empirical application of the thermodynamic equations for particle growth. The model is valid for all ranges of relative humidity. The light scattering and absorption coefficients are calculated using Mie theory extended to concentric sphere scatterers. The core of each particle, which is primarily taken to be nonvolatile carbon, is enclosed in a water-soluble exterior. The soluble layer responds to changes in the relative humidity by absorbing (or releasing) water. The model predictions are in excellent agreement with experimental measurements made in Denver in the fall of 1978.

Size-segregated fine particle measurements by chemical species and their impact on visibility impairment in Denver☆

Abstract In the winter of 1987–1988, a field study was undertaken to identify the principal causes of visibility impairment in Denver so that a plan to improve air quality could be designed. One focus of the field study was the measurement of the mass of sulfate, nitrate, organic and elemental carbon in particles as a function of particle size. The analysis of those measurements led to the following conclusions: (1) emissions of ammonia in areas northeast of Denver contribute to the mass of fine particle ammonium nitrate in Denver; (2) soot is much less efficient at scattering light than the other constituents of the Denver brown cloud, and much more efficient at absorbing light; (3) meteorological conditions that produce episodes of visibility impairment in Denver can be distinguished by the associated size distribution of airborne fine particles and (4) a significant fraction of the particle mass of organic carbon measured in the 1987–1988 Denver Brown Cloud Study could have been adsorbed gases.

Optical properties of aerosols—comparison of measurements with model calculations

Abstract Light scattering and absorption by a model aerosol is compared with measurements. The aerosol model is consistent with size distribution measurements by chemical species and with meteorological variables. External and internal mixtures of homogeneous, spherical particles at thermodynamic equilibrium are considered. Calculations with this physical model are contrasted with those of statistical models.

Prediction of ambient light scattering using a physical model responsive to relative humidity: Validation with measurements from detroit

Abstract A model for the prediction of atmospheric light extinction from field measurements of airborne aerosol concentration, chemical speciation, and size distribution has been tested with field data from the Detroit, Michigan area. The amount of light scattered by airborne particulates in ambient conditions is accurately predicted by the model using measurements of accumulation mode sulfate, nitrate, volatile and nonvolatile carbon. Model predictions were compared with simultaneously measured light scattering. The model is unique amongst physical models in its simulation of the effect of relative humidity on visibility reduction and on the light scattering efficiency of specific chemical constituents of the airborne aerosol. A proven physical model offers an advantage over statistical models in that it should prove reliable for extrapolation of ambient conditions well outside conditions currently realized. Within the range of the available data, the physical model is shown to simulate the light scattering as well as statistical models. Calculations with the physical model indicate that on average sulfate compounds are less efficient scatterers of light in ambient settings than statistical analyses would indicate.

Effect of composition on aerosol light scattering efficiencies

Abstract Forecasts of the impact of emissions changes on visibility use light scattering efficiencies—the change in the amount of light scattered with a change in mass of an aerosol constituent. This paper demonstrates how light scattering efficiencies depend on interactions between the aerosol constituents. Calculations are presented for two model aerosols: NH4HSO4 droplets and a ‘typical’ urban aerosol. These calculations demonstrate that the traditional means of predicting visibility impairment for increases in atmospheric emissions is not generally appropriate. Different light scattering efficiencies are needed for distinct meteorological conditions and aerosol compositions thus for typical high and low visibility events.

Visibility trends—I. Methods of analysis

Visibility trends which reflect changes in optical air quality due to air pollution are examined. Two methods of analysis of National Climatic Center visibility data are considered: cumulative percentiles and ridits. Each is applicable to data having nonstandard probability distribution functions. The dependence of trend lines derived from each method of analysis on meteorology is explored by application to mideastern U.S. sites. The most representative data base includes midday observations in the absence of precipitation and nigh relative humidity. A qualitative index of the visibility trend is given by the net percentage change in visibility over the 1948–1978 period obtained from a linear least-squares fit to the trend line. When carefully applied, the 60th cumulative percentile trend line and the mean ridit trend line are in complete concurrence.

Visibility trends—II. Mideastern United States 1948—1978

Trends in visibility in the mideastern United Slates are identified. Yearly and seasonal patterns are extracted from National Weather Service data using cumulative percentile and ridit analyses. The 1948–1978 time period is considered. No persistent pattern in the overall yearly visibility trends applies to the entire region. However, a deterioration of visibility in the 1960s during the summer quarter occurred regionally; it is most pronounced at median sized urban locations which experienced high growth. Those locations also experienced spring quarter declines in visibility. Further deterioration has not occurred in the 1970s. Metropolitan centers and more rural sites have shown improvement in fall and winter visibilities in the 1970s. Similarities between trends in visibility and trends in ambient sulfate concentrations and SO2 emissions are discussed. Possible meteorological influences are noted.

Meteorologically adjusted air quality trends: Visibility

Abstract If trends in air quality are to be interpreted in terms of changes in pollutant emissions, the impact of meteorology on those trends needs to be removed. There is good reason to suspect that changes in the weather in the mideastern U.S. over the last 30 years may have contributed to the observed reduction in visual air quality. This report represents an attempt to extract changes in air quality which were not the result of changes in local meteorology. This analysis focuses on changes in visibility under meteorological conditions ‘typical’ for each locale and season over the period 1948–1981. Trends in visual air quality are summarized in terms of a weighted linear least-squares estimate of the percentage change in visibility over the entire 34 year period within each season and at each of 15 representative sites. Confidence limits associated with these percentage changes are evaluated. This meteorologically-adjusted analysis, which focuses on midrange visibility (60th percentile), is more optimistic than those which do not consider meteorological factors or an error analysis. At the metropolitan sites and in the more rural areas, declines in the summertime visibility appear moderate, and significant improvements in visual air quality are seen in the first and fourth quarters. In contrast, the fast growing, medium-sized urban areas do not show a significant improvement in the fall or winter, and the decline in spring and summer visibility levels at these sites is evident even after adjustment for changes in local meteorology.

Summertime visibility declines: Meteorological influences

Abstract Meteorological factors affecting summertime visibility have been examined with a focus on two periods of time: 1952–1956 and 1971–1975. Summertime visibility in the mideastern United States reached its 30-year peak in the mid-fifties and subsequently declined to its lowest levels in the early seventies. This deterioration has been attributed by many to increased summertime sulfur dioxide emissions. However, the meteorology affecting visibility also varied in a significant manner over that period. Examination of the meteorological record using several statistical methods, among them the run test and autocorrelation analysis, shows conditions adverse to visibility are episodic in nature. A comparison of episodes in the two focal time periods revealed striking differences. The meteorological parameters most affecting visual air quality differed significantly and in a direction conducive to better visibility in the mid-fifties.

The Development of a Model to Examine Source-Receptor Relationships for Visibility on the Colorado Plateau

This paper describes the development and application of the Visibility and Haze in the Western Atmosphere (VISHWA) model to understand the source-receptor relationships that govern chemical species relevant to visibility degradation in the western United States. The model was developed as part of a project referred to as Visibility Assessment for Regional Emission Distributions (VARED), the objective of which is to estimate the contributions of various geographical regions, compounds, and emission sources to light scattering and absorption by particles on the Colorado Plateau. The VISHWA model is a modified version of a comprehensive Eulerian model, known as the Acid Deposition and Oxidant Model.1 The modifications were designed to obtain the computational efficiency required to simulate a one-year period at about 1/25th of real time, and at the same time incorporate mechanistic features relevant to realistic modeling of the fate and transport of visibility degrading species. The modifications included use of a condensed chemical mechanism; incorporation of reactions to simulate the formation of secondary organic particles; and use of a semi-Lagrangian advection scheme to preserve concentration peaks during advection. The model was evaluated with 1992 air quality data from Project MOHAVE (Measurements of Haze and Visual Effects) intensive experiments. An important conclusion of this evaluation is that aqueous-phase oxidation of SO2 to sulfate in nonprecipitating clouds makes a significant contribution to observed sulfate levels during winter as well as summer. Model estimates of ambient sulfate for the winter intensive were within a factor of 2 of the observations for 75% of the values. The corresponding statistic for the summer intensive was 90%. Model estimates of carbon were within a factor of 2 of the limited set of observations.