ATMOSPHERIC CHEMISTRY OF BIOGENIC HYDROCARBONS - RELEVANCE TO THE AMAZON

The Amazon rainforest, one of the most productive ecosystems of Earth, plays an important role in global atmospheric chemistry. Biogenic emissions from the Amazon include isoprene, terpenes and possibly other reactive organic compounds whose photochemical reactions have a significant impact on atmospheric ozone, carbon monoxide, the hydroxyl radical, precipitation acidity and aerosol formation, among others. In this study, atmospheric oxidation mechanisms are examined for biogenic hydrocarbons that are relevant to known and probable emissions from the Amazon region. Results from recent laboratory studies are used to estimate atmospheric persistence from kinetic data, to outline reaction mechanisms that lead to oxidation products and to assess the atmospheric persistence and fate of the reaction products. Whenever possible, laboratory results are compared to data from field measurements carried out recently in the Amazon and other tropical regions. Common features of atmospheric oxidation mechanisms are outlined first, with focus on the reactions of biogenic hydrocarbons with ozone, with the hydroxyl radical, and with the nitrate radical. For the ozone reaction, formation of OH as a reaction product and its implications are examined; OH yields are given for isoprene, terpenes and other unsaturated compounds. Kinetic data for the ozone and OH reactions are compiled and are used to estimate the atmospheric half-lives of biogenic hydrocarbons under conditions relevant to the Amazon. The Amazon is a major source (13% of global emissions) of isoprene, whose atmospheric chemistry is examined next. Mechanisms are outlined for the O-3, OH and NO3 reactions, Yields of the three major isoprene carbonyl products, formaldehyde, methacrolein (MTA) and methyl vinylketone (MVK) are given for the isoprene-OH and the isoprene-ozone reactions. Further oxidation of MVK and MTA by reactions with OH and with O-3 is discussed. In the presence of oxides of nitrogen, isoprene also leads to the two peroxyacyl nitrates (RC(O)OONO2) PAN (R = CH3) and MPAN (R = CH2 = C(CH3)-). Kinetic data, together with data for hydrocarbons in the atmosphere of the Amazon, are used to show that isoprene plays a major role in OH chemistry, accounting for 71% of OH removal (as compared to only 11% for CO and 5% for methane). Terpenes are discussed next, including monoterpenes (of which only four have been measured in the Amazon) and sesquiterpenes (for which no data exist). The nature and yields of the carbonyl products are compiled for both terpene-ozone and terpene-OH reactions. Reaction mechanisms are outlined. The contribution of terpenes to the global CO budget is compared to that of isoprene. PAN formation and aerosol production from terpenes are discussed. Unsaturated oxygenates (for which no data are available for the Amazon) are examined next. A compilation is given of unsaturated oxygenates that have been recently identified in biogenic emissions. Two categories, namely unsaturated alcohols and unsaturated carbonyls, are discussed in more detail. Reaction mechanisms are outlined; carbonyl products and their yields are listed for the ozone and OH reactions. Also discussed is the formation of peroxyacyl nitrates (PANs) including PPN (R = C2H5) from leaf alcohol and unsaturated PANs such as MPAN from unsaturated carbonyls. The last section of this study focuses on one important category of reaction products, the peroxyacyl nitrates. Their nature and formation yields are given for biogenic precursors including isoprene, terpenes and several unsaturated oxygenates. Atmospheric removal processes for PANs include thermal decomposition, whose rates are given for the relevant compounds studied to date. Other removal processes for PANs in the atmosphere include their reactions with OH and with ozone; reaction rates and mechanisms are discussed for the only compound studied, MPAN. The atmospheric persistence of MPAN is estimated for the three removal pathways of thermal decomposition, reaction with ozone and reaction with OH. Finally, this analysis of the atmospheric chemistry of isoprene, terpenes, unsaturated oxygenates and peroxyacyl nitrates serves as a basis for a brief list of suggestions to carry out additional field and laboratory work that may contribute to improving our understanding of the role, persistence and fate of biogenic hydrocarbons in the atmosphere of the Amazon.