Peroxy radical kinetics and new particle formation

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-NOx 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-NOx 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.