Photochemical Modeling

Exponent’s staff has been involved in photochemical modeling since its inception in the early 1970s. We contributed to the development of two early photochemical grid models, UAM and CALGRID. Since that time, our staff have been continuously been involved in the evolution, development, application, and evaluation of these models. Our involvement extends to the latest generation of photochemical grid models, which includes CAMx and CMAQ. We were also among the first to develop single source photochemical plume models, such as the Reactive Plume Model (RPM) and the Particle Plume Model (PPM).

Photochemical air quality models have been applied to help tackle regional ozone and other secondary air pollutant problems. They are used throughout the world by local, state, and federal air agencies to evaluate regional mitigation and emission control measures and strategies, to assess whether measured concentrations can be explained by emission inventories. The earliest photochemical models emerged in the 1970s to address the emerging, poorly understood air quality problem known at the time as “photochemical smog.”

Modern photochemical grid models are the central parts of larger modeling systems that include other model components. Our scientists are familiar with all of these other models, including:


  • Regional meteorological models to provide 3-D, time-varying meteorological inputs, for example MM5, WRF, and RAP
  • Regional emissions models to provide time and location varying air emissions for all types of emission sources, and for a large number emitted chemical molecular species, for example EIS and SMOKE
  • Land-use models defining the use and surface coverage of the land within the modeling domain
  • Photochemical kinetics modules, that define the chemical reactions between the primary (emitted) and secondary (formed) air pollutant molecular species, such as Carbon-Bond Mechanism versions 5 and 6, and SAPRC
  • Actinic light models that provide time and spatially varying solar ultraviolet radiation fluxes that drive the photochemical reaction system, such as the TUV model
  • Biogenic and geogenic emissions models, including BEIS, MEGAN, and GLOBEIS.

Exponent staff have also designed and/or managed complex local- and regional-scale air pollution measurement studies funded to test the performance of regional photochemical models. These have been conducted in the US and elsewhere in the world. Some of these regional studies include:


  • Gulf of Mexico Air Quality Study (GMAQS)
  • Paso Del Norte Ozone Study (PDNOS)
  • Taiwan Blue Sky Study
  • Southern California Air Transport Study (SOCAT07)
  • Studies at Dugway Proving Ground and Edward Air Force Base

We have also been responsible for many projects that involved all aspects of photochemical dispersion modeling, regional and international emission inventory development, meteorological modeling, air quality planning, emission control strategy evaluation, and air dispersion measurements.

Professionals