Rapid formation of isoprene photo-oxidation products observed in Amazonia

摘要

Isoprene represents the single most important reactive hydrocarbonfor atmospheric chemistry in the tropical atmosphere. It plays a centralrole in global and regional atmospheric chemistry and possible climatefeedbacks. Photo-oxidation of primary hydrocarbons (e.g. isoprene) leads tothe formation of oxygenated VOCs (OVOCs). The evolution of theseintermediates affects the oxidative capacity of the atmosphere (by reactingwith OH) and can contribute to secondary aerosol formation, a poorlyunderstood process. An accurate and quantitative understanding of VOCoxidation processes is needed for model simulations of regional air qualityand global climate. Based on field measurements conducted during theAmazonian Aerosol Characterization Experiment (AMAZE-08) we show that theproduction of certain OVOCs (e.g. hydroxyacetone) from isoprenephoto-oxidation in the lower atmosphere is significantly underpredicted bystandard chemistry schemes. Recently reported fast secondary productioncould explain 50% of the observed discrepancy with the remaining partpossibly produced via a novel primary production channel, which has beenproposed theoretically. The observations of OVOCs are also used to test arecently proposed HO recycling mechanism via degradation of isopreneperoxy radicals. If generalized our observations suggest that promptphotochemical formation of OVOCs and other uncertainties in VOC oxidationschemes could result in uncertainties of modelled OH reactivity, potentiallyexplaining a fraction of the missing OH sink over forests which haspreviously been largely attributed to a missing source of primary biogenicVOCs.