Research Projects

Project at a Glance

Project Status: complete

Report Published December 2000:

Title: Review and improvement of methods for estimating rates of photolysis in photochemical models. Volumes 1 and 2.

Principal Investigator / Author(s): Vulleumeir, Laurent & Robert Harley

Contractor: LBNL & UC Berkeley

Contract Number: 96-335


Research Program Area: Atmospheric Processes

Topic Areas: Modeling


Abstract:

Photolysis of species such as NO2 and formaldehyde influence significantly the formation of photochemical air pollution. Improved description of spatial and temporal variations in photolysis rates is needed, and improving the description of ultraviolet light intensity is key to improving urban and regional air quality models in this respect. Radiative transfer models were reviewed, and the Tropospheric Ultraviolet-Visible (TUV) model was implemented as a flexible photolysis module for online use within air quality models. Radiation measurements from the 1997 Southern California Ozone Study were used to assess the contributions of aerosols and ozone to variability in atmospheric optical depth. Aerosols were found to account for 90% or more of the observed variability in optical depth.

Sensitivity analysis was combined with an assessment of the likely range of variation for key TUV inputs parameters to estimate their influence on photolysis rate uncertainty. The amount of aerosol and its relative efficiency in scattering versus absorbing UV radiation each were found to lead to ~10% uncertainties in photolysis rate coefficients. The total ozone column (mostly stratospheric ozone) influences photolysis reactions that occur mainly at wavelengths below 300 nm (e.g., ozone and formaldehyde photolysis), but is of negligible influence for reactions that occur over a broad range of UV wavelengths (e.g., NO2).

The photolysis module was used in two versions of the Urban Airshed Model (UAM), and in the SARMAP Air Quality Model (SAQM). For UAM applied to Los Angeles in August 1987, significant increases (up to ~100 ppb) in predicted ozone concentrations were found in localized areas of the eastern part of the South Coast Air Basin. The photolysis module includes both improved treatment of radiative transfer and upgraded absorption cross sections and quantum yields for photolyzing species. When SAQM was applied to central California for August 1990, smaller increases (up to ~ 40 ppb) in predicted ozone concentrations were seen, as expected given the more similar treatment of photolysis.


 

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