On The Characteristics Of Particle Number Size Distribution And New Particle Formation Events In Beijing

The spatial and temporal variation of particle number size distribution is an indicator of primary and secondary sources of atmospheric aerosols which can reflect the dynamic and chemical processes they were subjected to.New particle formation events are the dominate source of secondary atmospheric aerosol number which can grow to climate relevant sizes.Studies on new particle formation and subsequent particle growth are then important for studying the formation and development of haze in urban Beijing.While NPF produces the seed particles,the growth of these particles is the bridge between nucleation events and their associated atmospheric effects.Hence,it is important to focus on the smallest particles(cluster and nucleation mode particles)as well as their growth in order to assess their effect of on air quality and climate.Here,we conducted long-term online observations on particle number size distributions ranging between 1 and 840 nm.We also monitored trace gases concentration(O₃,NO_x,SO₂...),meteorological conditions as well as component of sub-micrometer aerosol components at western campus of Beijing University of Chemistry Technology(BUCT).First,we assessed the sources of the different particle modes in Beijing.For doing so,we investigated the properties and characteristic of the particle number size distributions as well as their seasonal and diurnal variations.Based on two-year datasets of 6-840 nm particle number size distribution and one-year dataset of 1-3 nm particle number size distribution.We found that on NPF event days,the smallest particles(1–3 nm)were 31000±20000 cm^-3in number concentration while those in the 6-840 nm particle were 16400±8000 cm^-3.On the other hand,on NPF non-event days,the 1-3nm particle number concentration was 21000±30000 cm^-3and the total6-840 nm particle number concentration was 17600±8000 cm^-3.These results show the significant contribution of NPF events to the total aerosol budget in sub 3 nm size fraction.Additionally,the higher concentration of the larger particles could be the reason behind the limited occurrence of NPF in Beijing on non NPF event days.Additionally,we show that NPF event and traffic emissions are the dominant secondary and primary sources of1-100 nm atmospheric particles,respectively.Since the growth of the particles is crucial for reaching climate relevant sizes,thereby affecting the climate,we focused on the growth of the nucleation mode particles.Particles can grow either by gases condensing on their surface,or by coagulating with other present particles.Using the Atmospheric Related Chemistry and Aerosol box(ARCA box)model,we were able to separate the observed growth rates of particles in Beijing into two groups:(1)particle mode diameter growth rates due to coagulation scavenge(GR_coag)and(2)particle growth rates due to vapor condensation(GR_cond).Here,we focused on the factors affecting both GR_coag and GR_condgiven their role in controlling the total particle GR.Based on the analysis of two-year datasets of 6-100 nm particle growth rates,we found:the observed particle growth rates was 3.89 nm·h^-1 on median,which means it takes at least 20 hours for particles to grow from nucleation mode to accumulation mode,possibly exerting influence on haze formation.Interestingly,the observed particle growth rates show no dependence on particle diameter.The GR_coag contributed 28%-69%to the particle GR,and positively correlated with CS,hence,in polluted areas GR_coag has important contribution to the total GR observed especially on more polluted days.On the other hand,GR_cond positively correlated with atmospheric oxidation.GR_cond shows no correlation with CS.Finally,in order to assess the regionality of the NPF events,we compared the occurrence and characteristics of NPF events observed in urban Beijing and at another mountain site in West Beijing.Additionally,we studied the regional characteristics of NPF events in terms of the relative start times of the events and ending diameters of particle growth.This study research highlights the contribution of anthropogenic emissions(here sulfuric acid)to new particle formation and growth as well as the important role of meteorology.It also helps identify the extent by which NPF events extend and evaluate the spatial extent of the sources of secondary aerosols and their growth at early-stage.Here,based on observations in summer 2019 at both locations we found that condensation sink(CS)is the most important factor affecting the occurrence of NPF events at both sites.Air masses indirectly affect the occurrence of NPF events by affecting CS:air masses from north Beijing favoring NPF event most as they are corrected to the lowest CS among all directions.In addition,other factors e.g.solar radiation and SO₂ also affecting occurrence of NPF events at the mountain site by providing enough precursor vapors.Particle formation rates(J₇)were different between the two sites owing to the availability of precursor vapors(here Sulfuric acid):at urban Beijing J₇ was in the range of 3.0-10.0 cm^-3s^-1;and at mountain site in the range of 0.75-3.0 cm^-3s^-1.Particle growth rates(GR)on the other hand were rather similar providing the distinction between the formation and growth processes.At urban Beijing,GR were in the range of 4.8-12.9 nm·h^-1;and at mountain site were in the range of 5.7-10.5nm·h^-1.The extension of NPF event regions upwind Beijing is correlated with population density and the strength and possibility of NPF events in less populated regions is smaller than those at more populated regions.