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. The contribution of new particle formation (NPF) to cloud condensationnuclei (CCN) number concentration (NCCN) varies largely under differentenvironments and depends on several key factors such as formation rate (J),growth rate (GR), distribution of preexisting particles, and properties ofnew particles during NPF events. This study investigates the contribution ofNPF to the NCCN and its controlling factors based on measurementsconducted at the Heshan supersite, in the Pearl River Delta (PRD) region ofChina during fall 2019. The size-resolved cloud condensation nucleiactivity and size-resolved particle hygroscopicity were measured by a cloudcondensation nuclei counter (CCNc) and a hygroscopic tandem differentialmobility analyzer (HTDMA), respectively, along with a scanning mobilityparticle sizer (SMPS) and a diethylene glycol scanning mobility particlesizer (DEG-SMPS) for particle number size distribution (PNSD). A typical NPFevent on 29 October was chosen to investigate the contribution of theNPF to NCCN under several supersaturation (SS) ratios. Two particleproperties (hygroscopicity and surface tension) affect CCN activation withthe latter being more important in terms of the CCN concentration(NCCN). A lower value of surface tension (i.e., 0.06 N m−1) than thepure-water assumption (0.073 N m−1) could increase the NCCN atSS  =  1.0 % by about 20 % during the nonevent period and by about 40 % during the event. In addition, an earlier peak time corresponding to a lowercritical diameter (D50) was also observed. The results show that highformation rate, growth rate, and low background particle concentration leadto high number concentrations of newly formed particles. The high growthrate was found to have the most significant impact on the NCCN, whichcan be attributed to the fact that a higher growth rate can grow particlesto the CCN size in a shorter time before they are scavenged by preexistingparticles. Two other NPF events (an event on 18 October in thiscampaign and an event on 12 December 2014 in Panyu) were chosen toperform sensitivity tests under different scenarios (growth rate, formationrate, and background particle concentration). The calculated NCCN atSS = 1.0 % on 12 December 2014 was significantly lower than thatfrom the other two events. The event on 12 December was re-simulatedusing the growth rate taken from the event on 18 October whichresulted in similar CCN concentrations between the two events (12 December and 18 October), implying that the growth rate is the majorimpact factor for CCN activation. Our results highlight the importance ofgrowth rate and surface tension when evaluating the contribution of NPF tothe NCCN.

atmospheric chemistry and physics

The important roles of surface tension and growth rate in the contribution of new particle formation (NPF) to cloud condensation nuclei (CCN) number concentration: evidence from field measurements in southern China