We describe a detailed chemical mechanism (140 species, 440 reactions) representing atmospheric chemistry of the lower continental troposphere. We focus on chemical reactions potentially important for nighttime chemistry. The mechanism is based on the methods and concepts proposed by Carter <1990> for the development of the Statewide Air Pollution Research Center (SAPRC) mechanism to reduce the number of species and reactions. Peroxy radical chemistry is represented with chemical operators and secondary VOCs are lumped into a limited number of surrogate species. To account for new experimental data provided since the first formulation of the SAPRC mechanism, the whole set of reactions has been updated. Representation of peroxy radical chemistry has been modified to account for variability of organic peroxy radical-peroxy radical reaction rates. New chemical operators and new secondary species have been added to the mechanism to introduce reactions of alkenes with NO3. Reactions of alkenes with O3 have been largely revised to represent largest radical yields suggested by recent experimental studies. Chemistry of three biogenic compounds (isoprene, α-pinene and β-pinene) has been included. The mechanism is coupled to a two-layer box model (including anthropogenic and biogenic emissions of VOCs and NOx and dry deposition) in order to represent different chemical environments (from urban to rural cases). Comparisons with the SAPRC-99 mechanism show good agreements especially for high NOx regime. Results from the model are evaluated with typical observed concentrations in the different environmental cases with a special focus on the representation of radical species concentrations. For both daytime and nighttime, the calculated OH and peroxy radical concentrations are within the range of available observed values although the model tends to overestimate HO2 concentrations in daytime. At night, OH concentrations are found to vary from 1 to 6¿106 molecules cm-3 in the urban environment, 5¿104 to 2¿105 molecules cm-3 in polluted rural environments and 4¿106 molecules cm-3 in remote environments. ¿ 2001 American Geophysical Union