Tuesday, May 3, 2022

Downwind Odor

It's called the Rolling Unmasking Effect: "The source is a complex mixture of odorants, yet it is simplified to a single impactful odorant at the receptor downwind. The odor frontal boundary represents the farthest downwind reach of a single compound, while the internal colored ovals represent the boundaries of sequential odor unmasking as the secondary-impact odorants are diluted below their detection/masking concentration levels."

Qualitative Exploration of the ‘Rolling Unmasking Effect’ for Downwind Odor Dispersion from a Model Animal Source. Donald W. Wright et al. International Journal of Environmental Research and Public Health, 2021,18,13085. DOI: 10.3390/ijerph182413085

It's already hard enough to identify odors by their source, but these researchers show us that matching an odor "in the field" to one you think is the source, needs to account for the differentiated dispersal of odorants as the odor plume moves through space.

Just because one odorant scores high on the sniff test --at the source--, doesn't mean that odorant won't be the first to disappear at 10 yards. And just because you smell rotten eggs, doesn't mean that odorant is found in more abundance relative to others at the source, it could be that the rotten egg parts of the smell are better at avoiding dispersal, riding the edge of the odor plume as it emanates from its source. 

Some good terms:
  • rolling unmasking effect
  • downwind odor frontal boundary
  • odorant prioritization 
  • downwind odor impact
  • dynamic dilution olfactormetry

From the paper:

We propose solving environmental odor issues by utilizing troubleshooting techniques developed for the food, beverage, and consumer products industries.

While the composition of environmental odors, as detected by human receptors, carries the potential for extreme complexity, the reality is that there is a high degree of compositional simplification, which typically develops with increasing distance separation from the odor source.

We refer to these two effects as the Rolling Unmasking Effect (i.e., RUE). 

Odors generated from rural and agricultural sources are lowered by "downwind diultion" dispersion strategies, and monitored by dynamic dilution olfactormetry.

There is also broad recognition of a challenge to link specific compounds to resulting downwind odor [10,11]. In one notable example from an odorant prioritization study to the rendering industry [12], just two odorants (trimethylamine (TMA) and dimethylsulfide (DMS)) were identified as the impact-priority odorants downwind of a fish meal processing plant. 

In a more recent study [14,15], these authors were able to identify the specific chemical odorant that is believed primarily responsible for the reported ‘skunky’ odor downwind of dense cannabis-growing operations. ... The compound 3-methyl-2-butene- 1-thiol (i.e., 321 MBT), was the primary source of this ‘skunky’ odor of cannabis [14,15].

This has been shown for p-cresol as a 'sognature' [signature?] downwind odor from confined animal feeding operations (CAFOs), recognizable at a great distance from the source. 

Odors from a large colony of Mexican fee-tailed bats: ammonia, "rat nest",  and "bat cave" or "taco shell", which was dominated by 2-aminoacetophenone, upon approach to the outer ‘odor frontal boundary’; enabled by the decline of odor masking by the quinazoline odorant.

Figure 3. P.T. porcupine encounter in Moody Gardens. (1) Wind direction; (2) odor frontal boundary; (3) approximate secondary (near-source) boundary; (4) investigator's approximate location upon initial encounter and (5) location of outdoor enclosure of the odor source.

PT Porcupine Urine Sampling:

Unfortunately, the panelist (D.W.W.) was unable to confirm the chemical identities of the two character-defining ‘grilled onion’ odorants from the P.T. porcupine environments. Therefore, in a further attempt to identify these unknowns, collaborations with experts in the food flavor/aroma field were engaged.

The near-source smell was perceived as ‘phenolic,’ ‘industrial,’ and ‘foul.’ The dramatic difference in character was particularly surprising considering that only a few paces separated the pleasant 'gilled onion' at the odor frontal boundary and the 'foul' odor deeper into the plume. 

Although the PT porcupine and swine barn sources generate distinctly different odor characteristics at their respective odor boundaries, despite sharing much in common through their VOC emission profiles at the source. 

Focusing on all compounds present at the source often expands the study to include background noise, an unnecessary expenditure if the goal is to reduce downwind environmental odor impact.

One team member did not characterize the odor as 'onion' specifically; instead, it had reminded her of a favorite sauce that her grandmother frequently made. The second team member called the odor character ‘stale onion’.

Post Script:
Here's an odor network for all the odor complaints in southern California circa 2012, via the South Coast Air Quality Management District (SCAQMD)and UCLA post-grad Jane Curren:
Local Odor Vocab

Post Post Script:
Odor wheel for drinking water:
Torrice, M. (adapted from Suffet, M.). The scientists who sniff water. Chem. Eng. News 2017, 95, 16–19.
Suffet, I.H., and P.E. Rosenfeld (2007). The Anatomy of Odour Wheels for Odors of Drinking Water, Wastewater, Compost and the Urban Environment, Water Science and Technology 55(5), 335-344.

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