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A. Venkatram and R. Viskanta

Abstract

A two-stream solar radiation model was combined with a mixed-layer model to study the effects of absorbing aerosols on the thermal structure of the daytime convective boundary layer. A number of simulations were conducted with the model. The results showed that the criterion used in climatic models to determine the cooling or warming effect of aerosols was not readily applicable to micrometeorological scales. It was also found that soil-interface properties were at least as important as aerosol properties in determining aerosol-induced effects. Any conclusions about aerosol effects on the PBL have to be qualified by statements about surface parameters to be meaningful.

An average of the PBL and surface temperatures (θa) is suggested as a physically meaningful indicator of aerosol effects. The results show that aerosols increase θa over surfaces which are relatively wet or have high reflectances; dry, low-albedo surfaces, however, are associated with a decrease in θa in the presence of aerosols.

Another important conclusion of the study is that the soil-PBL system has a built-in mechanism to regulate aerosol-induced heating of the PBL and cooling of the surface. The degree of regulation is dependent on soil-PBL interface properties.

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A. Venkatram and R. Viskanta

Abstract

A one-dimensional transport model was developed to study the effects of radiative participation of elevated pollutant layers. Special features of the model include a turbulent kinetic energy model and a two-stream solar radiation model. Pollutants were assumed to consist of aerosols and pollutant gases. Aerosols were allowed to scatter and absorb energy in the solar spectrum while pollutant gases were assumed to interact only with thermal radiation.

The results of the simulations conducted with the model showed that elevated layers of pollutants could control mixed-layer expansion by modifying the stability of the capping stable layer. Cooling associated with gaseous pollutants generally helped the growth of the mixed layer, while solar heating induced by pollutants hindered mixed-layer growth by creating sharp inversions.

By affecting mixed-layer growth, radiative participation by pollutants also modified pollutant dispersal from the elevated pollutant layer. These results have important implications from the point of air pollution meteorology (especially fumigation) in which it is generally assumed that pollutants are passive.

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R. Viskanta and R. A. Daniel

Abstract

An unsteady two-dimensional transport model has been used to study the short-term effects of elevated pollutant layers on the temperature structure and pollutant dispersion in an urban planetary boundary layer. The effects of radiatively interacting pollutants on radiative transfer have been accounted for and the variation of the physical properties of the soil, the radiation characteristics of the earth's surface, and the urban heat and pollutant emissions along the city were included.

The results of the numerical experiments performed with the model showed that the aerosol-induced heating during daylight hours increases the temperature, while the cooling associated with gaseous pollutants decreases the temperature during the night in the elevated pollutant layer. The net effect of air pollution was to hinder the mixed-layer growth during the day. The results obtained show that the maximum impact of air pollution on the urban PBL is on the evolution of temperature and pollutant concentration profiles and is in agreement with the results of previous investigators.

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R. W. Bergstrom Jr. and R. Viskanta

Abstract

In conjunction with a companion study, this paper discusses the pollutant dispersion as predicted by solving numerically the equations of a transient, one-dimensional transport model. The results show that while the radiative properties of the aerosols have only a slight effect on the daytime pollution concentration; they do lead to decreased instability during the day. The gaseous pollutants, however, play a major role by lifting the elevated stable region thereby reducing pollution buildup. This alteration of an elevated stable region is important since air pollution episodes usually occur when such elevated inversions are present.

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R. W. Bergstrom Jr. and R. Viskanta

Abstract

The short-term effect of radiatively participating pollutants upon the temperature distribution in the boundary layer of the urban atmosphere is predicted. This is accomplished by constructing a mathematical model for atmospheric radiation transfer and one-dimensional mass, momentum and energy transport in the planetary boundary layer. The atmosphere, consisting of gaseous and particulate pollutants as well as natural constituents, is considered to absorb, emit and scatter anisotropically radiant energy. A series of numerical simulations of the thermal structure in the urban atmosphere is performed for summer and winter conditions, with and without an elevated inversion.

The numerical simulations stowed that the aerosol pollutants reduced the solar radiant flux at the surface which in turn lowered its temperature during the day. The additional solar heating due to the pollutants caused the atmosphere to be slightly warmer at higher altitudes. The surface temperatures during the day were slightly higher for non-absorbing aerosols than for aerosols with some absorption. Under the conditions investigated the largest surface temperature reduction was 2C, and the maximum rise of the atmospheric temperature due to the additional solar heating was about 1C after a two-day period.

The gaseous pollutants increased the downward thermal radiation flux and thus raised the surface temperatures at night. The maximum predicted surface temperature increase was about 3C after a two-day simulation. The pollutant gases were shown to enhance the net radiative cooling below a stable region. This additional cooling modified both the nighttime stable layer and the elevated inversion by causing an upward motion of the stable regions. Thus, radiatively participating pollutants were shown to have the potential for changing the thermal structure of an urban atmosphere.

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R. Viskanta, R. W. Bergstrom, and R. O. Johnson

Abstract

Radiative transfer in a polluted urban atmosphere has been studied using a dynamic model. The diurnal nature of radiative transfer for summer conditions has been simulated for an urban area 40 km in extent and the effects of various parameters arising in the problem have been investigated. The results of numerical computations show that air pollution has the potential of playing a major role in the radiative regime of the urban area. Absorption of solar energy by aerosols in realistic models of urban atmosphere are of the same order of magnitude as that due to water vapor. The predicted effect of the air pollution aerosol in the city is to warm the earth-atmosphere system, and the net effect of gaseous pollutant is to warm the surface and cool the planetary boundary layer particularly near the top.

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