Atmospheric Turbulence and Odor Fluctuations: The Inconvenient Physics

The human nose perceives odor during the span of an inhalation of 3 to 5 seconds. Our sense of smell discerns odor variation on the order of several seconds. Odor management is, in part, predicated on odor data collection: discrete samples or odor measurements in ambient air. However attempts at ambient odor quantification are often performed and their results interpreted without considering the profound effects of atmospheric turbulence upon odor and odor fluctuations.Earth’s atmospheric boundary layer is that region of air we breathe and which has its temperature, moisture, wind velocity, and turbulent nature influenced by land surface on which we reside. This turbulence results from: daytime solar-induced thermal (vertical) convection that ceases with sundown; wind velocity (speed and direction) variations; and land surface variations (buildings, vegetation, hills, obstacles, etc.). Because of the velocity fluctuations characteristic of turbulence, a series of discrete ambient odor measurements taken at the same location on a several seconds or longer interval will exhibit a variance with the same or a greater order of magnitude than that of the measured values.In order to demonstrate the magnitude of concentration fluctuations under turbulent conditions, several monitoring campaigns were conducted at (1) a MSW landfill and (2) a WWTP.The first field test was conducted at 11 distinct locations at the landfill with a 3-FID TVA-1000 (Flame Ionization Detector Toxic Vapor Analyzer) apparatus that enabled simultaneous sampling of VOC at 4’’, 5’ and 15’ above-ground levels on a 10 second interval. The landfill monitoring locations were at the perimeter during daytime conditions with a well-established mechanical and thermal turbulence regime. The landfill is located on relatively flat terrain with a 50-meter elevation difference over a 2 km2 area. The source emissions’ variation occurs over a much longer time-scale than that of the local atmospheric conditions. In this scenario, one expects that higher VOC source concentration will imply a proportional increase in odor concentration.The second set of field test was performed at a municipal wastewater treatment plant (WWTP) using an OdoWatch electronic nose system to monitor a primary clarifier and a digester. Both odor monitors were located at 5’ feet elevation above the ground near the source. The WWTP is located within a relatively flat suburban terrain.The measurements at both field sites showed substantial concentration fluctuations. For the landfill, the VOC concentrations fluctuated between zero and 10 to 200 ppm at 5’ and 15’ elevations and from 5 to 400 ppm at 4’’ above the ground. The typical fluctuation frequency was less than one minute with amplitude up to 1200% of the average. Similar patterns were obtained at the WWTP for stable and turbulent atmospheric conditions.This paper provides a literature review of studies on fluctuations of concentration induced by atmospheric turbulence at ground level, information about the field demonstrations, results obtained and data interpretation. This paper concludes with a discussion of the inconvenient technical implications for discrete ambient odor sampling and measurement, field-olfactometry, odor panel field observation, dispersion modeling, and regulatory compliance.