Size-resolved effective density of submicron particles during summertime in the rural atmosphere of Beijing, China

Particle density is an important physical property of atmospheric particles. The information on high time-resolution size-resolved particle density is essential for understanding the atmospheric physical and chemical aging processes of aerosols particles. In the present study, a centrifugal particle mass analyzer （CPMA） combined with a differential mobility analyzer （DMA） was deployed to determine the size-resolved effective density of 50 to 350 nm particles at a rural site of Beijing during summer 2016. The measured particle effective densities decreased with increasing particle sizes and ranged from 1.43 to 1.55 g/cm3, on average. The effective particle density distributions were dominated by a mode peaked at around 1.5 g/cm3 for 50 to 350 nm particles. Extra modes with peaks at 1.0, 0.8, and 0.6 g/cm3 for 150, 240, and 350 nm particles, which might be freshly emitted soot particles, were observed during intensive primary emissions episodes. The particle effective densities showed a diurnal variation pattern, with higher values during daytime. A case study showed that the effective density of Aitken mode particles during the new particle formation （NPF） event decreased considerably, indicating the significant contribution of organics to new particle growth.

Characterization of peroxyacetyl nitrate(PAN)under different PM_(2.5) concentration in wintertime at a North China rural site

As a secondary pollutant of photochemical pollution,peroxyacetyl nitrate(PAN)has attracted a close attention.A four-month campaign was conducted at a rural site in North China Plain(NCP)including the measurement of PAN,O_(3),NO_(x),PM_(2.5),oxygenated volatile organic compounds(OVOCs),photolysis rate constants of NO_(2)and O_(3)and meteorological parameters to investigate the wintertime characterization of photochemistry from November 2018 to February 2019.The results showed that the maximum and mean values of PAN were 4.38 and 0.93±0.67 ppbv during the campaign,respectively.The PAN under different PM_(2.5)concentrations from below 75μg/m^(3)up to 250μg/m^(3),showed different diurnal variation and formation rate.In the PM_(2.5)concentration range of above 250μg/m^(3),PAN had the largest daily mean value of 0.64 ppbv and the fastest production rate of 0.33 ppbv/hr.From the perspective of PAN’s production mechanism,the light intensity and precursors concentrations under different PM_(2.5)pollution levels indicated that there were sufficient light intensity and high volatile organic compounds(VOCs)and NO_(x)precursors concentration even under severe pollution level to generate a large amount of PAN.Moreover,the bimodal staggering phenomenon of PAN and PM_(2.5)provided a basis that PAN might aggravate haze through secondary organic aerosols(SOA)formation.