One of the California Environmental Quality Act (CEQA) air quality significance criteria is whether a project would “create objectionable odors affecting a substantial number of people.” For an existing source, odor complaints to the local Air District may prompt an odor analysis. For a proposed source, odor analysis may assist in project design with the aim of forestalling possible future complaints.
Most odorous substances can be classified as either inorganic gases or organic vapors. The principal odorous gases that can be emitted from industrial processes are sulfurous: hydrogen sulfide, carbonyl sulfide, carbon disulfide, and mercaptans. To a lesser extent, various organic vapors, ammonia, and formaldehyde can also be emitted in odor-inducing amounts. Of these, hydrogen sulfide – known as the “rotten egg” odor – has the lowest detectability threshold. In other words, even a very low concentration of hydrogen sulfide can be detected by most people and possibly result in complaints. In comparison, ammonia has a somewhat high odor detectability threshold.
Industrial sources that commonly cause odor impacts include wastewater treatment facilities, landfills and material recovery operations, pet food manufacturers, and oil field operations. Mass emissions of odorous gases are quantified using appropriate emissions estimation techniques such as source tests, material balance, published emission factors, or engineering calculations.
Utilizing mass emission rates, downwind concentrations of odorous compounds can be estimated using air dispersion models such as the U.S. Environmental Protection Agency’s (EPA’s) AERMOD modeling system (computer software). Models such as AERMOD use mathematical functions to characterize the atmospheric processes that disperse pollutants emitted by a source. Dispersion algorithms in AERMOD include calculations based on planetary boundary layer turbulence structure and scaling concepts, treatment of either surface or elevated sources, and characterization of either simple or complex terrain. For small and/or simple sources where AERMOD is not necessary, the EPA’s AERSCREEN model can be used for a more economical analysis.
Based on mass emission rates, release parameters, terrain characteristics, and meteorological inputs, ambient pollutant concentrations are calculated by the dispersion model at identified downwind receptor locations. The results are then used to determine impacts relative to published odor thresholds or other criteria identified by the CEQA Lead Agency to evaluate the potential for odor complaints to the local Air District and documented in the Air Quality Technical Report.