LAUSR

There was a significant snowfall event in North China from November 23 to 25 in 2015. Considering that most of the bare surface and road dust were covered by snow, the effect of dust and soil could be ignored. Atmospheric particle samples were collected in Zhengzhou, China during a haze event from November 28 to December 4, 2015. To better understand the formation and evolution of this hazy event, the size distribution, particle number, composition of particles and meteorological parameters were measured and analyzed. Results show that the meteorological conditions played an important role in the occurrence and elimination of this event. The hourly fine particle matter (PM2.5) concentration was positively correlated with relative humidity (r = 0.84, p < 0.01) but negatively correlated with wind speed (r = − 0.62, p < 0.01). The particle mass concentrations peaked at the sizes of 1.0 to 1.6 μm on hazy days, however, the total particle number concentrations on hazy days were lower than those on clean days. The median diameter of the number concentration during hazy days was approximately 60 nm, whereas it was 26 nm on clean days. Two new particle formation processes were observed on the clean days both. The proportion of secondary inorganic ions (SO42 −, NO3− and NH4+) on hazy days was higher than that on clean days. The higher NH4+ concentration in this case may be contributed by traffic and coal-power emission. Crustal matter accounted for 2.4% in PM2.5 on hazy days, and it confirmed that the contribution of dust emission source was negligible during this event. The ratios of NO3−/SO42 − ranging from 0.41 to 0.67 indicated the relative importance of stationary combustion. The ratios of OC/EC varied from 2.73 to 3.42 and indicated the presence of secondary organic carbon. Effective haze mitigation should enforce pollutant control measures for primary emission (dust) and secondary aerosol gaseous precursor (NH3, NO2 and SO2).