Find in Library

Alkyl nitrates and secondary organic aerosol (SOA) produced during the oxidation of isoprene by nitrate radicals has been observed in the SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber) chamber. A 16 h dark experiment was conducted with temperatures at 289–301 K, and maximum concentrations of 11 ppb isoprene, 62.4 ppb O<sub>3</sub> and 31.1 ppb NO<sub>x</sub>. We find the yield of nitrates is 70±8% from the isoprene + NO<sub>3</sub> reaction, and the yield for secondary dinitrates produced in the reaction of primary isoprene nitrates with NO<sub>3</sub> is 40±20%. We find an effective rate constant for reaction of NO<sub>3</sub> with the group of first generation oxidation products to be 7×10<sup>−14</sup> molecule<sup>−1</sup> cm<sup>3</sup> s<sup>−1</sup>. At the low total organic aerosol concentration in the chamber (max=0.52 μg m<sup>−3</sup>) we observed a mass yield (ΔSOA mass/Δisoprene mass) of 2% for the entire 16 h experiment. However a comparison of the timing of the observed SOA production to a box model simulation of first and second generation oxidation products shows that the yield from the first generation products was <0.7% while the further oxidation of the initial products leads to a yield of 14% (defined as ΔSOA/Δisoprene<sup>2x</sup> where Δisoprene<sup>2x</sup> is the mass of isoprene which reacted twice with NO<sub>3</sub>). The SOA yield of 14% is consistent with equilibrium partitioning of highly functionalized C<sub>5</sub> products of isoprene oxidation.