Strong atmospheric new particle formation in winter in urban Shanghai, China

Particle size distributions in the range of 1.34– 615 nm were recorded from 25 November 2013 to 25 January 2014 in urban Shanghai, using a combination of one nano condensation nucleus counter system, one nano scanning mobility particle sizer (SMPS), and one longSMPS. Measurements of sulfur dioxide by an SO2 analyzer with pulsed UV fluorescence technique allowed calculation of sulfuric acid proxy. In addition, concentrations of ammonia were recorded with a differential optical absorption spectroscopy. During this 62-day campaign, 13 new particle formation (NPF) events were identified with strong bursts of sub-3 nm particles and subsequent fast growth of newly formed particles. The observed nucleation rate (J1.34), formation rate of 3 nm particles (J3), and condensation sink were 112.4–271.0 cm s, 2.3– 19.2 cm s, and 0.030–0.10 s, respectively. Subsequent cluster/nanoparticle growth (GR) showed a clear size dependence, with average values of GR1.35∼1.39, GR1.39∼1.46, GR1.46∼1.70, GR1.70∼2.39, GR2.39∼7, and GR7∼20 being 1.6± 1.0, 1.4± 2.2, 7.2± 7.1, 9.0± 11.4, 10.9± 9.8, and 11.4± 9.7 nm h, respectively. Correlation between nucleation rate (J1.34) and sulfuric acid proxy indicates that nucleation rate J1.34 was proportional to a 0.65±0.28 power of sulfuric acid proxy, indicating that the nucleation of particles can be explained by the activation theory. Correlation between nucleation rate (J1.34) and gas-phase ammonia suggests that ammonia was associated with NPF events. The calculated sulfuric acid proxy was sufficient to explain the subsequent growth of 1.34–3 nm particles, but its contribution became smaller as the particle size grew. Qualitatively, NPF events in urban Shanghai likely occur on days with low levels of aerosol surface area, meaning the sulfuric acid proxy is only a valid predictor when aerosol surface area is low.