The main idea is to honor the ground as a boundary when modeling how concentration changes with height. In the vertical Gaussian distribution, the term with z minus H comes from the real plume source at height H above the ground. The term with z plus H comes from a mirror (image) source located at height −H below the ground. Adding these two contributions using the method of images effectively models the ground’s influence on the plume by reflecting it across the surface. This reflection changes how concentrations near the ground behave and helps reproduce the near-ground concentration pattern without having to solve a more complex boundary-layer problem. It’s not about deposition, chemistry, or wind shear; it’s about capturing the ground boundary’s effect through reflection.

The time period during which contaminants actually enter the air from the source is described as the source duration. This is the active emission window—when the release is happening and the plume starts forming and may be transported by the wind. Plume rise refers to the initial upward motion of the released material, not how long the release lasts. Evaporation time isn’t about releasing the contaminant into the air at all; it’s about how long a liquid takes to volatilize before it becomes airborne. Release duration is a similar idea, but in SAChE the standard term used for the active emission period is source duration.

In this system, atmospheric stability is described by discrete categories that reflect how turbulent the air is and how well pollutants mix. The category that corresponds to a moderate level of instability indicates there’s noticeable turbulence and vertical mixing, but not as intense as the very unstable category. This means pollutants will disperse more than in neutral or slightly stable conditions, but less than under very unstable conditions. So, the idea is that this label represents a middle ground of instability with moderate mixing, not the extremes of stability or instability. The other descriptions don’t fit: very stable conditions come from the stable end of the scale, highly unstable with strong mixing is the very unstable end, and stability isn’t defined by wind speed alone. Also, these categories aren’t meant to be averaged, but are distinct states of atmospheric stability.

This term is the vertical Gaussian distribution that describes how the concentration from an emission located at height H is spread and diminishes with height difference. It forms the vertical shape factor of the plume, showing how concentration at height z falls off as the distance from the source height (z − H) increases, scaled by the vertical dispersion parameter σ_z. The larger σ_z is, the more the plume spread in the vertical, and the slower the concentration drops with height. If ground reflection is included, you’d also add a second similar term with (z + H) to represent particles that reflect off the ground and arrive at height z from an image source below the ground. That makes the total vertical profile the sum of the direct contribution and the reflected one. This exponential is not a wind speed correction, which would enter differently in the solution, nor a standalone vertical diffusive adjustment. It specifically encodes the direct vertical distribution of the plume from the source height.