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. ¿ 1999 American Geophysical Union