Measurements of nonvolatile size distribution and its link to traffic soot in urban Shanghai.

Measurements of particle size distribution and size-resolved particle volatility were conducted using a volatility tandem differential mobility analyzers (V-TDMA) in the urban area of Shanghai during wintertime in January 2014. The nonvolatile mode particles with VSF exceeding 0.85 were always externally mixed with more-volatile mode particles. The average VSF ranged from 0.58 to 0.65 for 100-400nm particles, increasing with the increase of particle size. On average, the nonvolatile mode contributed 15-20% of number fraction for 50-400nm particles. Due to their hydrophobic nature, the nonvolatile particles were not easily removed by wet deposition. The concentrations of the nonvolatile mode particles and NOx were well correlated, indicating that the nonvolatile mode particles were mostly attributed to be fresh traffic soot. The diurnal variations in ensemble VSF and number fraction of nonvolatile mode particles exhibited two peaks in clean days, corresponding to morning and evening rush hours. The VSF distributions of 50nm particles were similar during a transition between haze to clean periods whereas in the accumulation mode range, the number fraction of more-volatile mode and the amount of volatile materials in the more-volatile mode particles during haze periods are considerably larger than those in clean periods, indicating different contribution from transported sources.