Se tea Rae woe Veale oleate acetate aaA ee 161 depth, pollutants are diluted by mixing with air over a thicker layer resulting in lower ground concentrations. the pollutant is approximately uniformly mixed or changes slowly with height up to the top of the mixing taken over land in the early morning during clear skies and winds less than about 10 mph, a ground based inversion is normally found. As this inversion begins to dissipate shortly after sunrise, the vertical temperature fective height of the mixing layer from the readings of When one examines vertical temperature soundings gradient approaches or exceeds the adiabatic lapse rate over an increasingly thicker layer. As a result, the lower- most layer is unstable but is capped by an inversion. Pollutants in this unstable layer from the ground upward are uniformly mixed. Equation (1) describes the concentration of pollutants within such a layerresulting from a continuouspoint source located below the inversion lid Xx ___@ Tu exp { _¥| 5g,3f (1) = concentration gr/m? @ = emission rate of pollutants gr/sec u% = wind speed downwind of source meters/sec x H = mixing depth in meters y = crosswind distance in meters a, = the perpendicular distance in meters from centerline of the plume in the horizontal diree- tion to the point where the concentrationfalls layer, but then falls sharply, we can determine theef- pyrheliometers at twoor three levels. Several techniques for doing this are described below. THE CASE OF UNIFORM MIXING OR CONSTANT DENSITY OF POLLUTANTS Technique for Obtaining Mixing Depths with Two Pyrheliometers and Two Solar Zenith Distances In this method, one pyrheliometer is located at the ground and the other on the rooftop of a building a feet tall. (See Figure 129.) Readings are taken about one hour apart; it is assumed that the mixing depth has changed little during this period. This assumption, of course, is not valid during transitional periods; it may be tolerable during the middle of the day. Theintensity of the solar beam passing from the top of the atmosphere to the top of the building on which the upper pyrheliometer is mounted may be given by f7(i) = [o(h1) exp {—pL(6)}, where ?,(@) = intensity of all band solar radiation at top to 0.61 times the centerline value As the ground based inversion is dissipated and the lower unstable layer increases to the level at which a particularly strong source is located (the chimney top) 6: = the material is brought to the ground in high concentrations and a condition known as fumigation results. Equation (1) may be used under fumigation conditions. Thus, the mixing depth must undergo diurnal variations. Information on the behavior of the mixing depth is useful in forecasting air pollution levels or devising strategies for incident control. Continuous measure- Zo(#:) = areas or during pronounced convection would assist in gaining a fundamentalinsight into those meteorological processes of importance to the air pollution problems. The normal constituents of the atmosphere, oxygen, nitrogen, water vapor, ozone, carbon dioxide, and aerosols attenuate solar radiation. Attenuation is caused to some extent by absorption but to a larger extent by scattering. Measurements of the attenuation of solar radiation within a given layer may be determined by comparing L{#) = ments of the mixing depth over rural as well as urban the radiation readings at the top and bottom of the layer. Three or four pyrheliometers at different. levels would provide information on the variation of attenuation with height, which in turn would be related to the vertical distribution of aerosol. If we assume that in the lower layers above ground (2) T = uw = of building with the sun’s zenith distance Ay solar zenith distance in radians at initial time intensity of normal incidence solar radiation at top of atmosphere with the sun’s zenith distance 6, subscript representing instrument on rooftop of building serving as a platform attenuation coefficient in meters~!, This eoefficient is the sum of the absorption and scattering coefficients. length of solar beam from top of mixing layer to uppermost pyrheliometer with solar zenith distance, 6, . Vor a solar zenith angle of 62, the equation becomes I7(62) = Iq(@2) exp { —pLh(62). (3) Similar equations for pyrheliometers at groundlevel are where Fy((1) = Lo(@1) exp {—p[L(a) + L{61)]} (4) T4(02) = Lo{82) exp { —u[L (62) + L,(62)]} (3) #8 = subscript which refers to the bottom of the building Lq(fi) and L_(62) = length of solar beam from top to bottom of building k