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 analyses 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 inorganic sulfurous compounds: 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 the odorous compounds, 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 odor complaints. In comparison, ammonia has a relatively higher odor detectability threshold.

Industrial sources that commonly cause odor impacts include (but are not limited to): wastewater treatment facilities and geothermal energy wells (primarily inorganic odors); pet food manufacturers and oil field operations (primarily organic odors); and landfills and material recovery operations (both inorganic and organic odors). Mass emissions of odorous gases are quantified using appropriate emissions estimation techniques such as source tests, material balance, published emission factors, or engineering calculations. These emissions estimates serve as a key input for odor analysis conducted as part of air quality assessments.

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 account for the effects of meteorological conditions (including turbulence structure and scaling concepts), source release characteristics, and impacts of 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 screening-type 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 resulting impacts are then compared 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 findings and results are documented in the Air Quality Technical Report.