Peroxy radical kinetics and new particle formation

Abstract

Chamber experiments showing “pure biogenic nucleation” have shown an important role for covalently bound organic association products (“dimers”). These form from peroxy-radical (RO₂) cross reactions. Chamber experiments at low-NO_x conditions often have quite high hydrocarbon reactant concentrations and relatively low concentrations of oxygenated volatile organic compounds (OVOCs). This can skew the radical chemistry in chambers relative to the real atmosphere, favoring RO₂ and disfavoring HO₂ radicals. RO₂ cross reaction kinetics are in turn highly uncertain. Here we explore the implications of the RO₂ to HO₂ ratio in chamber experiments as well as the implications of uncertain RO₂ cross reaction kinetics and the potential for added CO to mimic more atmospheric radical conditions. We treat a plausible range of RO₂ rate coefficients under both typical chamber conditions and atmospheric conditions to see how dimerization is affected by high concentrations of OVOCs, and thus lower RO₂ : HO₂ relative to smog chamber experiments. We find that if RO₂ reactions are fast, relatively high yields of low volatility dimers can participate in new particle formation. The results are highly sensitive to both the (uncertain) RO₂ kinetics as well as RO₂ : HO₂, suggesting both that low-NO_x chamber results should be extrapolated to the atmosphere with caution but also that the atmosphere itself may be highly sensitive to the specific (and rich) mixture of organic compounds and thus peroxy radicals.