Service Area

Air Quality Sciences

Drawing upon a team of experts with diverse backgrounds and areas of expertise, Gradient offers a wide range of air quality services.  We use regulatory- and industry-standard models to estimate emissions and predict air concentrations from a variety of industries and source types.  Modeling can consider both regulatory compliance assessments and specialized complex scenarios, including impacts from fugitive sources, explosions, leaks, and chemical evaporation.  We also make use of monitored air quality datasets to more fully understand local conditions, and can assist in developing site-specific monitoring and sampling plans for both indoor and outdoor environments.

Gradient’s air quality experts work hand-in-hand with our risk assessors to provide estimates of indoor and outdoor air pollutant exposures that are used as inputs for site-specific risk assessments.  Our air quality experts also routinely conduct exposure assessments for indoor and outdoor air pollutants and can provide testimony on air quality issues.  Our technical skills include:

  • Air dispersion modeling, including industry-standard models such as AERMOD, SCICHEM, HYSPLIT, and CALPUFF
  • Photochemical modeling
  • Dense gas modeling
  • Computational fluid dynamics (CFD)
  • Meteorological data analysis
  • Visibility degradation and analysis
  • Source emission characterization


  • Design of air sampling plans
  • Accidental releases
  • Odor modeling and evaluation
  • Fugitive dust emissions and modeling
  • Occupational air evaluations
  • Indoor air modeling and exposure simulation studies
  • Vapor intrusion
  • Air permitting modeling and support
  • Climate change air quality impacts
  • Remediation impacts

Sample Projects

Air Quality Impacts and Potential Health Risks of Power Plant Soot

Gradient assessed potential air quality impacts and health risks associated with particulate matter (PM) and trace metal constituents, including thallium and arsenic, emitted during upset events and normal operating conditions at a large coal-fired power plant.  We reconstructed emissions and performed air dispersion modeling for short-term soot events, identifying maximum impact areas.  We conducted toxicological assessments for potential worst-case exposures from the soot events.

Hydrogen Sulfide Exposure Near a Midwestern Concentrated Animal Feeding Operation

Gradient evaluated the release of hydrogen sulfide (H2S) from barns and a waste lagoon at a large Midwestern hog farm.  We conducted air monitoring to develop H2S emissions factors for several barns, a waste lagoon, and land application of liquid manure.  We then modeled H2S concentrations downwind of the farm and compared the modeled values with published health benchmarks.

Air Modeling of Power Plant Sulfur Dioxide Impacts in a Nonattainment Area

Gradient used both AERMOD and CALPUFF to predict maximum one-hour sulfur dioxide (SO2) concentrations at the SO2 nonattainment monitor for two coal-fired power plants, one located in the nonattainment area and one located outside the nonattainment area.  We compared findings for the two power plants, as well as for the two different models, and examined the correlation between specific hour-to-hour modeled versus measured SO2 concentrations.

Assessment of PM2.5 Air Quality and Potential Health Risks Posed by Urban Building Construction

Gradient helped design a particulate matter (PM) air monitoring network and evaluated air quality data collected in the vicinity of a large construction project at a major university.  We assessed potential health risks to children at a nearby daycare center from both long- and short-term PM2.5 exposures associated with construction-related activities and communicated these results to faculty, parents, and daycare center staff.

Indoor Air Exposures from a Commercial Printer

To assess potential exposures and health risks associated with operation of a commercial printer and its soybean oil-based ink, Gradient developed testing protocols and oversaw implementation of an indoor air exposure study, which included measurements for a comprehensive set of test analytes (mineral oil, carbon black, glycerol, aldehydes, volatile organic compounds [VOCs], and semi-volatile organic compounds [SVOCs]).  Gradient conducted a toxicological assessment for analytes found in the study and evaluated the likelihood for potential health risks associated with air emissions from the printer.

Assessment of Ozone Exposure and Health Risks Using Two US EPA Models

Gradient developed ozone exposure and risk assessments using the US EPA’s Air Pollutants Exposure (APEX) and Environmental Benefits Mapping and Analysis Program (BenMAP) models.  Using APEX, we conducted a series of sensitivity analyses to assess how alternative model inputs impact exposure and risk assessment results.  For BenMAP, we evaluated the impacts of using alternative model inputs, including air quality inputs and health effects functions, on ozone mortality estimates.

Our Team