This is a well written paper on the sources and growth of ultrafine particles in urban air.  Chemical composition measurements assist the identification of UFP sources, which include both primary emissions and new particle formation.  Aqueous/heterogeneous growth of preexisting particles is also indicated from chemical composition data.  Measured particle growth rates (3-50 nm) during new particle formation are consistent with theoretical growth rates estimated from condensation of gas-phase sulfuric acid and low-volatility organic compounds. 

Specific comments:

In the next to the last sentence of Section 3.3, the authors estimate the times needed for sub-3 nm particles to grow to above 50 and 100 nm.  The authors do not explicitly state how these times are calculated, but I am assuming that they are based on the measured and/or estimated 3-50 nm growth rates.  If this is the case, then the times they give to grow above 50 nm are accurate, but the times they give to grow above 100 nm are likely to be a substantial overestimate.  Figure S12 shows that the CHON/S composition factor associated with aqueous/heterogeneous chemistry becomes very large above 50 nm, roughly equaling that of the CHO-rich factor which presumably includes (but is not limited to) condensation of low volatility organics.  Therefore, it is likely that particle growth due to aqueous/heterogeneous chemistry equals or exceeds particle growth due to condensation alone.  If both growth channels are taken into account, the actual time to grow to 100 nm is likely to be much shorter than what the authors report.  I encourage the authors to consider this possibility (or to clarify how the times were calculated) and discuss appropriately in the body of the paper. 

A related question:  What is the approximate survival probability of a sub-3 nm particle growing to 50 or 100 nm without being removed by a typical loss process?  It seems the authors dataset is robust enough to provide an estimate.