Tools › Environmental › Air Quality › Air Dispersion Screening › Example
100 m stack of the SCREEN3 guide (without its building): worst-case 1-hour concentration by Gaussian plume screening against the sulfur dioxide standard
| Tool | Air Dispersion Screening |
| Source | U.S. Environmental Protection Agency (1995). SCREEN3 Model User's Guide. EPA-454/B-95-004, Office of Air Quality Planning and Standards, Research Triangle Park, NC. Section 2.4 and Figure 3 (stack parameters and fluxes, p. 22); Figure 5 (flare example, pp. 29–31) and Figure 9 (volume source example, p. 40) for the printed concentrations the method is checked against. |
| Level | Professional |
Problem
A 100 m stack of 2 m inside diameter discharges 100 g/s of sulfur dioxide at 15 m/s and 450 K into air at 293 K in an urban setting with no nearby building. Screen it as EPA's SCREEN3 procedure does: scan stability classes A to F with their 10 m wind speeds, find the maximum 1-hour ground-level concentration between 100 m and 50 km and its distance, convert it to ppm and to longer averaging times, and compare it with the 1-hour standard of 75 ppb.
Inputs to Enter
- Source type stack; emission rate 360 kg/h (100 g/s); stack height 100 m, diameter 2 m, exit velocity 15 m/s; gas 450 K, air 293 K; stack-tip downwash applied
- Urban dispersion coefficients; receptor height 0; terrain height 0; distances 100 m to 50,000 m; anemometer height 10 m; full meteorology
- Sulfur dioxide, 1 hour: molecular weight 64.066 g/mol, standard 196.4 µg/m³ (75 ppb at 25 °C and 760 mm Hg), averaging factor 1, background 0
Answers
- Buoyancy flux F_b = 51.319 m⁴/s³ and momentum flux F_m = 146.500 m⁴/s² (guide p. 22 prints 51.319 and 146.500)
- Governing condition class C with 1.0 m/s at 10 m: wind at stack height 1.585 m/s, final rise 21.425 F_b^0.75 / u_s = 259.20 m, plume height 359.20 m, mixing height 360.2 m
- Maximum 1-hour concentration 251.4 µg/m³ (0.09601 ppm) at 1,289 m, with σ_y = 241.96 m and σ_z = 268.23 m; class maxima A and B 251.1, D 223.3, E 148.5, F 177.7 µg/m³
- Longer averages with EPA's factors: 3 hours 226.3, 8 hours 176.0, 24 hours 100.6, annual 20.11 µg/m³
- Total / standard = 251.4 / 196.4 = 1.280: the screening result exceeds the level, so refined modelling or a lower rate is needed; the rate at which the screening total equals the standard is 78.12 g/s
Notes
The tool's default inputs are this case; press Submit. The stack is the one of the guide's point-source example, but that example includes building downwash, which the tool does not model, so the concentrations here are the tool's own and only the two fluxes are printed values. The method is checked by test_air_dispersion_screening.php (2,009 checks): every printed line of the guide's flare example (maximum 1,461 µg/m³ at 1,046 m) and volume-source example (257.5 µg/m³ at 109 m), an independent numpy / scipy implementation on twenty stacks, and conservation of mass. Choosing Flare or Volume source in the Source Type list loads those two examples. A screening estimate, not a regulatory model run.