Insight into the mechanisms by which sulfur and nitrogen incorporate into nanoparticles during atmospheric new particle formation (NPF) is gained through an assessment of field and laboratory measurements. For both ~1 nm clusters studied in laboratory experiments and 20 nm ambient nanoparticles, sulfuric acid contributes to growth via a barrierless addition process. In contrast, the addition of cation forming nitrogen (i.e. ammonia) may require traversing an activation barrier, which can decrease the rate of addition and influence particle acidity.
