4-21. It is convenient to know the distance yp where the concentration has dropped to p percent of its value on the plume axis. Consider the horizontal spread in pollutant concentration at ground level. Making use of Equation (4-9), prove that the value of yp at any distance x is found simply from the expression, yp=[2y2ln(100/p)]1/2 4-21. It is convenient to know the distance yp where the concentration has dropped to p percent of its value on the plume axis. Consider the horizontal spread in pollutant concentration at ground level. Making use of Equation (4-9), prove that the value of yp at any distance x is found simply from the expression, yp=[2y2ln(100/p)]1/2