Gas-phase degradation of organic compounds in the troposphere

The present status of knowledge of the gas-phase reactions of selected classes of volatile non-methane organic compounds (NMOCs) [alkanes, alkenes, aromatic hydrocarbons, oxygen-containing NMOCs and nitrogen-containing NMOCs] and their degradation products in the troposphere is briefly discussed. In the troposphere, NMOCs can undergo photolysis, reaction with the hydroxyl (OH) radical during daylight hours, reaction with the nitrate (NO3) radical (primarily during nighttime), reaction with ozone (O-3), and, in certain situations, reaction with the chlorine (Cl) atom. The kinetics and mechanisms of the initial OH radical, NO3 radical and O-3 reactions are reasonably well understood. However, the subsequent reactions of the organic radicals formed after the initial OH radical, NO3 radical and O-3 reactions are in many cases much less well understood, and photolysis of NMOCs under tropospheric conditions is in general a poorly understood area. In the tropospheric degradations of NMOCs, the important intermediate radicals are alkyl or substituted alkyl radicals (Ti), alkyl peroxy or substituted alkyl peroxy radicals (R(O) over dot (2)), and alkoxy or substituted alkoxy radicals (R(O) over dot). While much progress has been made in elucidating the reactions of organic peroxy and alkoxy radicals, the mechanisms of the gas-phase reactions of O-3 With alkenes, and the mechanisms and products of the OH radical-initiated reactions of aromatic hydrocarbons, there are still areas of uncertainty;which impact the ability to accurately model the tropospheric degradations of NMOCs and to predict the products formed. These will be discussed.