Three inputs decide most dispersion results

Atmospheric stability alone changes the picture. With the screening equations behind our home-page model, an 18 g/s source at 70 m effective height and wind at 4 m/s, neutral class D puts the peak ground-level concentration about 1.3 km downwind at roughly 98 µg/m³. Stable class F lowers the peak to about 28 µg/m³ and moves it more than 7 km away. Receptors that look clear under one assumption sit near the maximum under another.

The meteorological record

US regulatory practice sets the bar explicitly. Appendix W to 40 CFR Part 51 requires five years of representative National Weather Service data, at least one year of site-specific data, or at least three years of prognostic data, and prefers site-specific data where one to five years exist.¹ Site-specific records must be at least 90 percent complete in each quarter, with substitution limited to 10 percent of hours, or 876 in a year.²

Many jurisdictions in the Gulf and India do not codify this. Citing the US requirement where local rules are silent gives the adviser a reference point and shows the choice was deliberate.

Source parameters

Stack height, internal diameter, exit velocity and exit temperature set plume rise, and plume rise sets where the plume reaches the ground. List each with its origin: vendor data sheet, design basis or measurement.

Buildings and terrain

Building downwash can bring a plume to the ground within a few hundred metres of a short stack. Elevated terrain can put receptors in contact with a plume that a flat-terrain run never brings down. State which building dimensions and which terrain dataset the model used.