Characterization of particle size distributions during winter haze episodes in urban air

Detail characterization of particle size distribution and its temporal evolution is one of the critical elements towards uncovering mechanisms behind haze formation, yet rarely conducted. To address this deficiency, we conducted comprehensive characterization of particle size distribution during winter in Xi'an, China. Real-time measurements were conducted using a TSI Fast Mobility Particle Sizer Model 3091 (FMPS, from 5.6 to 523 nm) in the Qujiang campus of Xi'an Jiaotong University in the period from December 4th, 2015 to January 8th, 2016. The FMPS readings were adjusted by factors derived from an intercomparison with a TSI Scanning Mobility Particle Sizer (consisting of a TSI DMA 3081 and a CPC 3772, from 15.1 to 850.8 nm). Seven haze episodes and two new particle formation episodes were recorded during the sampling campaign. Two (E1 and E6) of the seven haze episodes are investigated in this study. E1 was an prolonged episode starting from a new particle formation (NPF) episode, followed by low, sustained PM2.5 increase at an average growth rate of 2 mu g m(-3) per hour (from 37 mu g m(-3) to 262 mu g m(-3) within 155 h), while E6 was a short-term haze episode starting from a rapid increase in PM2.5 at an rapid growth rate of 27 mu g m(-3) (from 79 mu g m(-3) to 213 mu g m(-3) within only 5 h). The average total particle number concentrations (PNC) were 3.35 x 10(4) cm(-3), 4.14 x 10(4) cm(-3) and 3.99 x 10(4) cm(-3) during normal days, E1, and E6, respectively, showing an increase in particle number concentration from normal days to haze days (p < .000 for E1 and p < .002 for E6, two-tailed t-test). While statically significant, the magnitude of the increase was not as large as of the increase in PM2.5 concentration. On normal days, the peak in particle number size distribution (PNSD) was centered at smaller particle sizes (around 60-70 nm, computed based on a normal distribution) and shifted towards larger sizes during the night (139 nm at 0:00 and 168 nm at 4:00 am). The diurnal variations of PNSD during E1 and E6 episodes were not as evident as the variations on normal days, with the centers of the major peaks at 179 nm for E1 and 137 nm for E6. It was found that significant changes in PNC and PNSD occurred during the PM (2.5) increase phase of severe haze episodes, but not during the high concentration phase. Since the growth rates of PM2.5 varied during increase phase between E1 and E6, PM2.5 pollution formation mechanisms were different throughout evaluating growth rates as it relates to PM2.5, gaseous pollutants, PNC, PNSD, and meteorological variables in these processes.