Particle particulars

Key intermediates and their roles in secondary organic aerosol formation from isoprene have been elucidated.

After its biogenic emission into the atmosphere, isoprene (2-methyl-1,3-butadiene) reacts to produce secondary organic aerosols (SOAs) that create 'haze' and affect climate. The reaction steps that lead from isoprene to SOAs, however, are not fully understood. Now, John Seinfeld and colleagues from the California Institute of Technology in Pasadena have studied isoprene oxidation under atmospherically relevant conditions and found previously unidentified reactive intermediates with key roles in SOA formation¹.

Acidified sulfate aerosols are known to enhance isoprene SOA formation under low-NO_x conditions but it is not clear how. Seinfeld and colleagues studied the products created during the oxidation of isoprene using mass spectrometry techniques. They observed that these sulfate aerosols take up epoxydiols (IEPOX), which are second-generation isoprene oxidation products. Their subsequent particle-phase reactions were responsible for enhanced SOA formation. They also studied SOA formation from isoprene under high NO_x conditions. Most SOA is known to derive from the first-generation oxidation product methacrolein (MCAR) and Seinfeld and co-workers observed that the next step is through methacryloylperoxynitrate (MPAN).

Current models that include known SOA precursors differ from measurements of the atmosphere. The reactions of IEPOX and MPAN observed by Seinfeld and colleagues, however, could account for a major portion of this discrepancy.