The effect of sulfur dioxide on particle formation and growth by ozonolysis of three monoterpenes (α‐pinene, β‐pinene, and limonene) and isoprene was investigated in the presence of monodisperse ammonium sulfate seed particles and an OH scavenger in a flow tube under dry conditions. Without sulfur dioxide, new particle formation was not observed, and seed particle growth was consistent with condensation of low‐volatility oxidation products produced from each organic precursor. With sulfur dioxide, new particle formation was observed from every precursor studied, consistent with sulfuric acid formation by reaction of sulfur dioxide with stabilized Criegee Intermediates. The presence of sulfur dioxide did not significantly affect seed particle growth rates from α‐pinene and limonene ozonolysis, although chemical composition measurements revealed the presence of organosulfates in the particles following SO2 exposure. Contrarily, the growth of seeds by β‐pinene and isoprene ozonolysis was considerably enhanced by sulfur dioxide, and chemical composition measurements revealed that the enhanced growth was not due to additional organic material, suggesting that inorganic sulfate was likely responsible. The results suggest that a previously unconsidered particle‐phase pathway to growth activated by sulfur dioxide may alter production of cloud condensation nuclei over regions with significant SO2‐alkene interactions.

Figure S1. Growth experiment using 60 nm ammonium sulfate seed particles.Figure S2. Growth experiment using 80 nm ammonium sulfate seed particles.

Nitrogen-containing organic species such as imines and imidazoles can be formed by aqueous reactions of carbonyl-containing compounds in the presence of ammonia. In the work described here, these reactions are studied in airborne aqueous nanodroplets containing ammonium sulfate and glyoxal, methylglyoxal, or glycolaldehyde using a combination of online and offline mass spectrometry. N/C ratios attributed to the organic fraction of the particles (N/C) produced from glyoxal and methylglyoxal were quantified across a wide relative humidity (RH) range. As the RH was lowered, glyoxal was found to increase N/C, attributed to "salting-in" with increasing solute concentration, while methylglyoxal led to a decrease in N/C, attributed to "salting-out". Glycolaldehyde was found to evaporate from the droplets rather than react in the aqueous phase and did not form particulate-phase organic matter from aerosol drying under any of the conditions studied. The results are discussed in the context of ambient nanoparticle composition measurements and suggest that aqueous chemistry may significantly impact nanoparticle composition and growth during new particle formation in locations where emissions of water-soluble dicarbonyls are high, such as the eastern United States.

Abstract. New particle formation (NPF) consists of two steps: nucleation and subsequent growth. At present, chemical and physical mechanisms that govern these two processes are not well understood. Here, we report initial results obtained from the TANGENT (Tandem Aerosol Nucleation and Growth Environment Tube) experiments. The TANGENT apparatus enables us to study these two processes independently. The present study focuses on the effects of temperature on sulfuric acid nucleation and further growth. Our results show that lower temperatures enhance both the nucleation and growth rate. However, under temperatures below 268 K the effects of temperature on the nucleation rate become less significant and the nucleation rate becomes less dependent on relative humidity, indicating that particle formation in the conditions of our flow tube takes place via barrierless nucleation at lower temperatures. We also examined the growth of newly formed particles under differing temperature conditions for nucleation and further growth. Our results show that newly nucleated clusters formed at low temperatures can indeed survive evaporation and grow in a warmer environment in the presence of SO2 and ozone and potentially other contaminant vapors. These results imply that some heterogeneous reactions involving nanoparticles affect nucleation and growth of newly formed particles.