A modeling study of the nighttime radical chemistry in the lower continental troposphere 1. Development of a detailed chemical mechanism including nighttime chemistry

We describe a detailed chemical mechanism (140 species, 440 reactions) repr esenting atmospheric chemistry of the lower continental troposphere. We foc us on chemical reactions potentially important for nighttime chemistry. The mechanism is based on the methods and concepts proposed by Carter [1990] f or the development of the Statewide Air Pollution Research Center (SAPRC) m echanism to reduce the number of species and reactions. Peroxy radical chem istry 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 whol e set of reactions has been updated. Representation of peroxy radical chemi stry has been modified to account for variability of organic peroxy radical -peroxy radical reaction rates. New chemical operators and new secondary sp ecies have been added to the mechanism to introduce reactions of alkenes wi th NOB. Reactions of alkenes with O-3 have been largely revised to represen t largest radical yields suggested by recent experimental studies. Chemistr y of three biogenic compounds (isoprene, a-pinene and P-pinene) has been in cluded. The mechanism is coupled to a two-layer box model (including anthro pogenic and biogenic emissions of VOCs and NO, 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 NO, regime. Results from the model are evaluated with typical observe d concentrations in the different environmental cases with a special focus on the representation of radical species concentations, For both daytime an d nighttime, the calculated OH and peroxy radical concentrations are within the range of available observed values although the model tends to overest imate HO2 concentrations in daytime. At night, OH concentrations are found to vary from 1 to 6x10(6) molecules cm(-3) in the urban environment, 5x10(4 ) to 2x10(5) molecules cm(-3) in polluted rural environments and 4x10(4) mo lecules cm-3 in remote environments.