Atmospheric chemistry of VOCs and NOx

The present status of knowledge of the gas-phase reactions of inorganic Ox, HOx and NOx species and of selected classes of volatile organic compounds (VOCs) [alkanes, alkenes, aromatic hydrocarbons, oxygen-containing VOCs and nitrogen-containing VOCs] and their degradation products in the troposphere is discussed. There is now a good qualitative and, in a number of areas, quantitative understanding of the tropospheric chemistry of NOx and VOCs involved in the photochemical formation of ozone. During the past five years much progress has been made in elucidating the reactions of alkoxy radicals, the mechanisms of the gas-phase reactions of O3 with alkenes, and the mechanisms and products of the OH radical-initiated reactions of aromatic hydrocarbons, and further progress is expected. However, there are still areas of uncertainty which impact the ability to accurately model the formation of ozone in urban, rural and regional areas, and these include a need for: rate constants and mechanisms of the reactions of organic peroxy (RȮ2) radicals with NO, NO3 radicals, HO2 radicals and other RȮ2 radicals; organic nitrate yields from the reactions of RȮ2 radicals with NO, preferably as a function of temperature and pressure; the reaction rates of alkoxy radicals for decomposition, isomerization, and reaction with O2, especially for alkoxy radicals other than those formed from alkanes and alkenes; the detailed mechanisms of the reactions of O3 with alkenes and VOCs containing >CC< bonds; the mechanisms and products of the reactions of OH-aromatic adducts with O2 and NO2; the tropospheric chemistry of many oxygenated VOCs formed as first-generation products of VOC photooxidations; and a quantitative understanding of the reaction sequences leading to products which gas/particle partition and lead to secondary organic aerosol formation.