A new lumped structure photochemical mechanism for large‐scale applications

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 NO 3 ; 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 NO x 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.