A new lumped structure photochemical mechanism for large‐scale applications

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Abstract
The lumped‐structure approach for condensing organic chemical mechanisms is attractive, since it yields fewer species and reactions and reduces computational costs. This paper leads through the development of a new lumped‐structure mechanism, largely based on the widely used Carbon Bond Mechanism (CBM‐IV) developed by Gery et al. [1989]. The new mechanism called CBM‐Z extends the original framework to function properly at larger spatial and longer timescales. The major modifications in the mechanism include revised inorganic chemistry; explicit treatment of the lesser reactive paraffins, methane and ethane; revised parameterizations of the reactive paraffin, olefin, and aromatic reactions; inclusion of alkyl and acyl peroxy radical interactions and their reactions with NO3; inclusion of organic nitrates and hydroperoxides; and refined isoprene chemistry based on the condensed one‐product mechanism of Carter [1996]. CBM‐Z was successfully evaluated along with the CBM‐IV, a partially revised CBM‐IV, and a revised Regional Acid Deposition Model (RADM2) mechanism [Stockwell et al., 1990; Kirchner and Stockwell, 1996] using the low NOx and volatile organic compound concentration smog chamber experiments of Simonaitis et al. [1997]. Box model versions of the four mechanisms were also evaluated under a variety of hypothetical urban and rural scenarios for a period of 30 days. Results from CBM‐Z and revised RADM2 were found to be within ±20% of each other, while CBM‐IV and revised CBM‐IV results deviated significantly by up to 50–95%. Sensitivity tests were performed to elucidate the effects of some of the new features added in CBM‐Z. Relative computational memory and time requirements of these mechanisms are also discussed.