Kinetic (<i>T</i> = 201–298 K) and Equilibrium (<i>T</i> = 320–420 K) Measurements of the C₃H₅ + O₂ ⇆ C₃H₅O₂ Reaction

The kinetics and equilibrium of the allyl radical reaction with molecular oxygen have been studied in direct measurements using temperature-controlled tubular flow reactor coupled to a laser photolysis/photoionization mass spectrometer. In low-temperature experiments (<i>T</i> = 201–298 K), association kinetics were observed, and the measured time-resolved C₃H₅ radical signals decayed exponentially to the signal background. In this range, the determined rate coefficients exhibited a negative temperature dependence and were observed to depend on the carrier-gas (He) pressure {<i>p</i> = 0.4–36 Torr, [He] = (1.7–118.0) × 10¹⁶ cm^–3}. The bimolecular rate coefficients obtained vary in the range (0.88–11.6) × 10^–13 cm³ s^–1. In higher-temperature experiments (<i>T</i> = 320–420 K), the C₃H₅ radical signal did not decay to the signal background, indicating equilibration of the reaction. By measuring the radical decay rate under these conditions as a function of temperature and following typical second- and third-law procedures, plotting the resulting ln <i>K</i>_p values versus 1/<i>T</i> in a modified van’t Hoff plot, the thermochemical parameters of the reaction were extracted. The second-law treatment resulted in values of Δ<i>H</i>₂₉₈^° = −78.3 ± 1.1 kJ mol^–1 and Δ<i>S</i>₂₉₈^° = −129.9 ± 3.1 J mol^–1 K^–1, with the uncertainties given as one standard error. When results from a previous investigation were taken into account and the third-law method was applied, the reaction enthalpy was determined as Δ<i>H</i>₂₉₈^° = −75.6 ± 2.3 kJ mol^–1

Efficient Isoprene Secondary Organic Aerosol Formation from a Non-IEPOX Pathway

With a large global emission rate and high reactivity, isoprene has a profound effect upon atmospheric chemistry and composition. The atmospheric pathways by which isoprene converts to secondary organic aerosol (SOA) and how anthropogenic pollutants such as nitrogen oxides and sulfur affect this process are subjects of intense research because particles affect Earth’s climate and local air quality. In the absence of both nitrogen oxides and reactive aqueous seed particles, we measure SOA mass yields from isoprene photochemical oxidation of up to 15%, which are factors of 2 or more higher than those typically used in coupled chemistry climate models. SOA yield is initially constant with the addition of increasing amounts of nitric oxide (NO) but then sharply decreases for input concentrations above 50 ppbv. Online measurements of aerosol molecular composition show that the fate of second-generation RO₂ radicals is key to understanding the efficient SOA formation and the NO_<i>x</i>-dependent yields described here and in the literature. These insights allow for improved quantitative estimates of SOA formation in the preindustrial atmosphere and in biogenic-rich regions with limited anthropogenic impacts and suggest that a more-complex representation of NO_<i>x</i>-dependent SOA yields may be important in models