Question

σ_{y and} σ_{z are dispersion coefficients,  vertical and horizontal standard deviation of a plume} - these are used to find maximum concentration

For stability class C, the ratio of σ_y/σ_z is independent of distance. Explain the physical significance of this.

Answer 1

Plume Dispersion Coefficients, sigmay  sy  and  sigmaz  sz

sy = horizontal dispersion coefficient    sz = vertical dispersion coefficient

These sy and sz dispersion coefficients (sometimes called standard deviations) have units of meters and correspond to an air pollutant sampling time of 10 minutes.  The dispersion coefficients are function of the atmospheric stability class and the downwind distance x from the air pollutant emission source.  The magnitude of the sy and sz dispersion coefficients can be estimated using the equations reported by D. O. Martin (1976) “The Change of Concentration Standard Deviations with Distance”, J. Air Pollution Control Assoc. Vol 24 p 832.

sz = cxd + f         sy = axb

Pasquill (1961) used the strength of incoming sunlight (i.e., insolation) and cloud cover to represent atmospheric stability, and divided the possible meteorological conditions into six categories, A, B, C, D, E, and F, where A is the most unstable environment and F is the most stable.

For stability class C, the ratio of σy/σz is independent of distance upto 1 Km from stack.

Approximate values of sz as a function of downwind distance x in km are shown in the table below:

Downwind		sz	Values	(meters)
Distance   x  (km)	A	B	C	D	E	F
0.1	14	11	7	5	4	2
0.2	29	20	14	8	6	4
0.4	72	40	26	15	11	7
0.7	215	73	43	24	17	11
1.0	455	110	61	32	21	14
2.0	1950	230	115	50	34	22
4.0		500	220	77	49	31
7.0		780	360	109	66	39
10.0		1350	510	135	79	46
20.0		2900	950	205	110	60

The sz value for class C stability is upto 61 which is less than 100 it means it is independent of distance for upto 1Km of stack.