Detector Dog Process
This post is intended to answer a couple of questions. The first is, why is an atmospheric chemist and air pollution expert doing dog olfaction work? And the second is what’s actually going on in the process of a dog being able to detect a target odor and follow it to source.
I did the above drawing to help explain this. The drawing is very schematic but I hope it makes the point I wanted to make. This process is complex and most of it has nothing to do with the dog. Most of it is atmospheric chemistry. These processes happen in the same way whether we are talking about odor chemicals releasing from contraband narcotics or pollutants coming from any air pollution source. So that’s why an atmospheric chemist gets involved in dog olfaction issues.
I want to walk through these steps and explain what happens and how those steps influence how a dog can or cannot locate a target odor.
The first step is evaporation. Evaporation is the process of a chemical moving from the liquid phase to the vapor or gas phase. Chemicals that can go from solid to gas also, this is called sublimation. This is key because only chemicals in the vapor phase can be smelled. Many things influence evaporation, some of the most notable are temperature and the surface they are on. Higher temperatures mean more evaporation. Some surfaces are molecularly sticky and tend to make the compounds less likely to evaporate. The range of these influences is pretty large and unique to each individual compound so it is hard to provide a general rule for exactly what will impact any particular chemical.
Quick sidebar for this: vapor phase means it is a gas, dissolved in the other gases of the atmosphere and free to move around and enter a nose. Chemicals are commonly found in one of three phases, gas (or vapor), liquid, or solid. The common example is water, that we can find as ice (solid), liquid water, and water vapor. But pretty much everything else can be found in all three phases. It’s hard to think about something like cocaine or methamphetamine being in those same three physical phases, but they can and are found that way. Like water, they may not evaporate very quickly, but they do evaporate. Chemicals like this are often referred to as “semi-volatile” which means they do readily enter the gas phase, but don’t primarily exist there. Many air pollutants fall into this category.
Step 2 is condensation. What? Things re-condense? Yes, evaporation and condensation are part of the equilibrium process that defines the balance between the liquid and gas phases for a chemical. The reason this is important is that it can sometimes condense somewhere other than where the source was and then evaporate again from that point. This can happen where temperature extremes exist in fairly short distances, such as cold winter days with bright sun where the shaded areas can be much colder than the surrounding.
Step 3 is diffusion. I include this here for completeness, but generally this is not a big influence on the overall process. Diffusion is a slow and small range process, but if I didn’t mention it someone would call me to task for that. So, here it is, but you can almost ignore it.
Step 4 is bulk movement of the air. Think of this as what is controlling a balloon being blown around by the wind. Winds and thermal currents are the main bulk movement processes. This is what carries the odor from the source to wherever it might be detected. The arrows in the drawing are to indicate that this movement is both is the horizontal plane and the vertical plane. So, it is more like a cone of diluted odor. There is no magical boundary on this odor concentration, it’s a gradient. In the simplest case of a single source and wind moving one direction, the highest concentrations will be along the center line, and decreasing concentrations from there. At some point the concentration of the target odor might be below what a dog can smell, but it is still there.
Step 5 is the dilution in the cross wind direction. The lines are to indicate where the cone of detectable odor could be, but it’s not just there. That’s like a single line on a topographic map. In this case, concentration would be higher going toward the midline of the cone of odor.
Step 6 is the detection of the target odor molecule by the dog. In many ways, it’s the handler’s job to get the dog’s nose into place to maximize the chance of detecting the target odor.
Step 7 is the recognition by the dog of its target odor. This is then followed by….
Step 8 is the dog following the concentration gradient to higher and higher concentrations until it finds the source and alerts.
Step 9 is the handler recognizing the alert.
So as you can see the process of how a chemical of odor can get to a dog’s nose and what is needed for the dog to follow that to source is rather complex. But it is also very well understood. This understanding helps us understand the behavior of dogs working odor.
