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Title: Experimental particle formation rates spanning tropospheric sulfuric acid and ammonia abundances, ion production rates, and temperatures: Ternary NH 3 Particle Formation Rates
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  1. Abstract

    New particle formation (NPF) has been observed at various locations, but NPF does not occur in isoprene‐dominant forests. Recent laboratory studies were conducted to understand the role of isoprene in biogenic NPF, and these studies show that isoprene can suppress biogenic NPF, with contradicting theories. To reconcile these discrepancies, we conducted flow tube experiments of biogenic nucleation under a wide range of isoprene over monoterpene carbon ratios (R) and oxidant conditions (OH vs. ozone). Our results show isoprene either suppresses or enhances biogenic NPF, depending onRand oxidation regimes, demonstrating the synergetic effects of isoprene and HOx(OH and HO2) on biogenic NPF. Whereas the suppression of NPF by isoprene is due to the product suppression effects of monoterpene dimers (C20), RO2 + HO2termination reactions also play important roles in suppressing the dimer formation, another likely process to suppress NPF in the atmosphere.

     
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  2. null (Ed.)
    Chamber experiments showing “pure biogenic nucleation” have shown an important role for covalently bound organic association products (“dimers”). These form from peroxy-radical (RO 2 ) 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 RO 2 and disfavoring HO 2 radicals. RO 2 cross reaction kinetics are in turn highly uncertain. Here we explore the implications of the RO 2 to HO 2 ratio in chamber experiments as well as the implications of uncertain RO 2 cross reaction kinetics and the potential for added CO to mimic more atmospheric radical conditions. We treat a plausible range of RO 2 rate coefficients under both typical chamber conditions and atmospheric conditions to see how dimerization is affected by high concentrations of OVOCs, and thus lower RO 2  : HO 2 relative to smog chamber experiments. We find that if RO 2 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) RO 2 kinetics as well as RO 2  : HO 2 , 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. 
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