Measurement Of Size-resolved Density Of Atmospheric Submicron Aerosols At Shangdianzi In Beijing

Density is one of the important physical properties of atmospheric aerosols. The study of aerosol density can provide reference for understanding of aerosol chemical compositions and evaluation of aerosol forming process, which also has important significance to study the deposition efficiency of aerosol particles in atmosphere and the human body. In this study, Differential Mobility Analyzer(DMA)- Aerosol Particle Mass Analyzer(APM)- Condensation Particle Counter(CPC) system has been used to achieve an observation of effective density of 50 ~ 350 nm submicron aerosols at Shangdianzi Regional Atmospheric Background station from August to September and December in 2015. Based on these observations, the distribution and variation of effective density of atmospheric aerosols in summer and winter at Shangdianzi were analyzed, as well as the change of the density during new particle formation events or polluted episodes.The effective density showed an increase trend with diameter in the size range of 50-350 nm distribution data, ranging from 0.96 ~ 1.65 g / cm~3 in summer and 0.98 ~ 1.55 g / cm~3 in winter.,reflecting differences in the chemical composition of particle. Combined with simultaneous measurement of chemical composition of submicron aerosols, it can be seen that the change of effective density is mainly related to the increase of the content of secondary inorganic components, such as ammonium sulfate, ammonium nitrate and so on.There are significant diurnal variations in size-resolved density of atmospheric aerosols in winter. Overall effective particle density of 50 ~ 300 nm in the morning showed the first increase in the trend, at 12: 00 ~ 14:00 has reached a peak, then gradually decreased, which mainly related to the traffic emission and coal-fired heating.The average effective density of aerosol with different particle sizes has slight seasonal variation. In winter he average effective density of each particle size was slightly lower than in summer, which probably due to a large number of low density of organic aerosols from winter coal-fired emissions.The proportion of organic aerosols was higher in winter than in summer, which leads to the effective density is slightly lower.The average effective density 50, 80 and 100 nm particle size particles in new particle formation events was slightly higher, and there was no significant change in average effective density of the other sizes in the new particle formation period. It may be due to the sulfuric acid vapor involved in particle nucleation and growth process.The effective density of 50~200 nm during the winter period was slightly higher than that of the pollution period, while the effective density of 300 nm particles was slightly lower, mainly related to the internal chemical composition. The effective density of 100 nm and 200 nm has a gradually decreasing trend with the increasing of mass concentration of PM1, which is consistent with the low effective density in the pollution episode.According to the analysis of the Aerosol mass spectrometer(AMS)data in the same period, the organic matter in PM1 accounted for 37 % during the winter. By fitting the effective density of 100~300 nm particles with Org/PM1, we conclude that the effective density of 100~300 nm particles decreases gradually with the increase of the proportion of organic matter. When the proportion of organic matter in PM1 reached 60%, the effective density of 100~200 nm particles was close to 1.2 g/cm~3. We can deduce that low effective density is related to the high content of organic matter in a particle.