Isoprene oxidation by nitrate radical: alkyl nitrate and secondary organic aerosol yields

Abstract. 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 O₃ and 31.1 ppb NO_x. We find the yield of nitrates is 70±8% from the isoprene + NO₃ reaction, and the yield for secondary dinitrates produced in the reaction of primary isoprene nitrates with NO₃ is 40±20%. We find an effective rate constant for reaction of NO₃ with the group of first generation oxidation products to be 7×10⁻¹⁴ molecule⁻¹ cm³ s⁻¹. At the low total organic aerosol concentration in the chamber (max=0.52 μg m⁻³) 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^2x where Δisoprene^2x is the mass of isoprene which reacted twice with NO₃). The SOA yield of 14% is consistent with equilibrium partitioning of highly functionalized C₅ products of isoprene oxidation.