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Impact of new particle formation on the concentrations of aerosolsand cloud condensation nuclei around Beijing

H. Matsui,1 M. Koike,1 Y. Kondo,1 N. Takegawa,2 A. Wiedensohler,3 J. D. Fast,4

and R. A. Zaveri4

Received 28 March 2011; revised 14 July 2011; accepted 19 July 2011; published 13 October 2011.

[1] New particle formation (NPF) is one of the most important processes in controlling theconcentrations of aerosols (condensation nuclei, CN) and cloud condensation nuclei(CCN) in the atmosphere. In this study, we introduce a new aerosol model representationwith 20 size bins between 1 nm and 10 mm and activation!type and kinetic nucleationparameterizations into the WRF!chem model (called NPF!explicit WRF!chem). Modelcalculations were conducted in the Beijing region in China for the periods duringthe Campaign of Air Quality Research in Beijing and Surrounding Region 2006(CARE!Beijing 2006) campaign conducted in August and September 2006. Modelcalculations successfully reproduced the timing of NPF and no!NPF days in themeasurements (21 of 26 days). Model calculations also reproduced the subsequent rapidgrowth of new particles with a time scale of half a day. These results suggest that oncea reasonable nucleation rate at a diameter of 1 nm is given, explicit calculations ofcondensation and coagulation processes can reproduce the clear contrast between NPF andno!NPF days as well as further growth up to several tens of nanometers. With thisreasonable representation of the NPF process, we show that NPF contributed 20%–30%of the CN concentrations (>10 nm in diameter) in and around Beijing on average. Wealso show that NPF increases CCN concentrations at higher supersaturations (S > 0.2%),while it decreases them at lower supersaturations (S < 0.1%). This is likely becauseNPF suppresses the increases in both the size and hygroscopicity of preexisting particlesthrough the competition of condensable gases between new particles and preexistingparticles. Sensitivity calculations show that a reduction of primary aerosol emissions, such asblack carbon (BC), would not necessarily decrease CCN concentrations because of anincrease in NPF. Sensitivity calculations also suggest that the reduction ratio of primaryaerosol and SO2 emissions will be key in enhancing or damping the BC mitigation effect.

Citation: Matsui, H., M. Koike, Y. Kondo, N. Takegawa, A. Wiedensohler, J. D. Fast, and R. A. Zaveri (2011), Impact of newparticle formation on the concentrations of aerosols and cloud condensation nuclei around Beijing, J. Geophys. Res., 116,D19208, doi:10.1029/2011JD016025.

1. Introduction

[2] Atmospheric aerosols are considered to play an impor-tant role in the climate of the Earth via absorbing andscattering solar radiation (direct effect) and changing cloudproperties and precipitation processes (indirect effects) [e.g.,Forster et al., 2007; Ramanathan et al., 2001; Lohmann andFeichter, 2005]. They also adversely affect human healthand visibility [e.g., Pope et al., 2002; Somers et al., 2004].Aerosol particles (condensation nuclei, CN) are produced by

two different mechanisms: (1) primary aerosol emission inparticulate form and (2) secondary particle formation throughnew particle formation (NPF) [e.g., Kulmala, 2003]. Aftersufficient growth and change in hygroscopicity of CN bycondensation and coagulation processes, CN can act ascloud condensation nuclei (CCN), which concentrationsare essential to quantify aerosol indirect effects accurately[Lohmann and Feichter, 2005; Dusek et al., 2006].[3] Secondary particle formation has a large impact on

CN and CCN concentrations and ultimately on aerosol indi-rect effects through changes in the aerosol size distribution,mixing state, and hygroscopicity [e.g., Spracklen et al., 2008,2010; Yu et al., 2010; Wang and Penner, 2009; Pierce andAdams, 2009]. Secondary particle formation can be expres-sed by the following two processes [e.g., Kulmala et al.,2000, 2004a; Kulmala, 2003]: (1) cluster formation frommolecules by nucleation and (2) subsequent growth ofclusters to stable new particles, which is generally called

1Department of Earth and Planetary Science, Graduate School ofScience, University of Tokyo, Tokyo, Japan.

2Research Center for Advanced Science and Technology, University ofTokyo, Tokyo, Japan.

3Leibniz Institute for Tropospheric Research, Leipzig, Germany.4Pacific Northwest National Laboratory, Richland, Washington, USA.

Copyright 2011 by the American Geophysical Union.0148!0227/11/2011JD016025

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NPF (Figure 1). The nucleation process is generally pho-tochemically driven and is considered to produce molecularclusters around 1 nm in diameter [Kulmala et al., 2000,2007]. Recent studies suggest that sulfuric acid andammonia (and potentially amines) are the possible speciesdriving cluster formation [Kulmala et al., 2000; Berndtet al., 2005, 2006; Sipilä et al., 2010; Kerminen et al.,2010]. The growth of clusters to aerosols around 3–10 nmcan be described by two competing processes between thecondensation growth of clusters and the coagulation sink ofclusters to preexisting particles [Kerminen and Kulmala,2002; Kerminen et al., 2004; McMurry et al., 2005; Kuanget al., 2008, 2009, 2010; Kulmala and Kerminen, 2008].The balance of these processes is likely to be important forthe occurrence of NPF and to clarify the contribution ofsecondary particle formation to CN and CCN concentrations[Kerminen and Kulmala, 2002; Kerminen et al., 2005;Kulmala et al., 2004a, Kuang et al., 2010]. In addition tosulfuric acid and ammonia, organics could also be veryimportant for the NPF process [e.g., Kulmala et al., 2004b,2004c, 2006a; Kerminen et al., 2004; Ristovski et al., 2010].Organic vapors contribute to the condensation growth ofclusters and new particles, while organic aerosols work as asink of them.[4] There are various theoretical parameterizations to

represent the concentrations of clusters and new particles inmodel calculations, e.g., binary homogeneous nucleation(BHN) [e.g., Vehkamäki et al., 2002], ternary homogeneousnucleation (THN) [e.g., Merikanto et al., 2007], and ion!mediated nucleation (IMN) [e.g., Yu, 2006]. Although theBHN and THN parameterizations have been used in manythree!dimensional modeling studies, they tend to severelyunderestimate particle formation rates, especially within theplanetary boundary layer (PBL) [Lucas and Akimoto, 2006;

Yu and Luo, 2009; Elleman and Covert, 2009; Zhang et al.,2010a]. The role of the IMN mechanism is still unclear andcontroversial [e.g., Kulmala et al., 2007; Yu and Turco,2008; Yu et al., 2008, 2010]. Only recently, some globalmodeling studies [Spracklen et al., 2006, 2010; Makkonenet al., 2009] showed that estimations of CN concentrationswere considerably improved in the PBL when they usedactivated!type (AN) [Kulmala et al., 2006b] or kineticnucleation (KN) parameterizations [McMurry, 1980, 1983;Kulmala et al., 2006b; Laakso et al., 2004; Kuang et al.,2008] by choosing the most suitable scaling factors. Theseparameterizations were empirically derived and describedwith only the first or second powers of sulfuric acid (H2SO4)concentration. Using these parameterizations, Spracklen et al.[2008] and Merikanto et al. [2009] estimated that 3%–20%of CCN at a supersaturation (S) of 0.2% is from secondaryparticle formation at the surface on global average.Merikantoet al. [2010] and Wang and Penner [2009] estimated theimpact of NPF on the aerosol indirect effect and found sub-stantial regional effects of NPF on the first aerosol indirecteffect. They suggested thatwhetherNPF increases or decreasesaerosol indirect effects largely depends on the relative changeof primary particles and SO2 emissions from the preindustrialto the present!day atmosphere.[5] These global modeling studies using the AN and KN

parameterizations showed the importance of NPF in esti-mating CN and CCN concentrations as well as aerosolindirect effects on global and annual scales. However, sincemost comparisons with measurements are made with sea-sonal or annual mean values, detailed comparisons of CNconcentrations and their size distributions with measure-ments are very limited, even though the aerosol size distri-bution is critical in controlling CCN concentrations and thecontribution of nucleation to them. In addition, there are few

Figure 1. Diagram for nucleation and new particle formation processes and their impact on condensa-tion nuclei (CN) and cloud condensation nuclei (CCN) concentrations and aerosol indirect effects. Thered, blue, and green arrows show the competing processes of condensable gases between preexistingparticles and nucleated secondary particles. J* denotes the formation rate of clusters and is defined byequations (1) and (2) in this study.

MATSUI ET AL.: NPF CALCULATION AROUND BEIJING D19208D19208

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Classification of Particles Based on the Size distribution

M. Kulmala et al. / Aerosol Science 35 (2004) 143–176

Fig. 2. Typical particle formation event in Hyyti.al.a boreal forest site on 13th March

1996: particle size distribution data as a surface plot (a), and total particle numberconcentration versus time (b).

Fig. 6. Time series plot of sulphuric acid vapour and 2.7–4 nm particles during a

nucleation event at Idaho Hill, CO on 21 September, 1993 (Weber et al., 1997).

CLARKE ET AL.: PARTICLE PRODUCTION IN THE REMOTE MARINE ATMOSPHERE 16,407

S. Mid Latitudes (Tasmania)

8

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ACE-1 Cloud Outflow---Nucleation ....

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Mid Day /(.'-i:' ."/ H2SO 4, nucleation . .•..-.--.•/.._.'.: [

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Figure 14. A cartoon of the nucleation and cycling of particles in the remote marine troposphere as evidence from the observations made on ACE 1. Clouds both scavenge MBL aerosol and source new particles aloft with meteorology and subsidence linking these mechanisms over large spatial scales (see text).

6

surface areas are below about 5 gm 2 cm -3. Separate evidence for new particle formation in the 3-4 nm range was also directly observed in these regions (P. H. McMurry, personal correspondence, 1997). The fact that the larger UCN have concentrations higher than the UF in most of these regions suggests that growth of nuclei up to 10 gm diameter or larger must occur in a relatively short period (tens of minutes to hours) as the cloud outflow mixes with dry clean air. We note that for these regions of FSSP surface area that are near or below this value, an appreciable contribution to total surface area exists for particle sizes below our "FSSP" lower threshold. Examination of typical RDMA data under these conditions suggest that an additional 50-100% of the FSSP surface area may be present below our FSSP detection limit. During these periods of recent "nucleation events" some of this is new surface area, at times 30%, that results from the nucleation itself. At the same time some of the surface area must be a result of mixing with the environmental air after the onset of nucleation. Hence, we estimate the actual surface area at time of nucleation lies somewhere between the FSSP value and about twice that value.

7. Conclusions

Several flights have been described from the ACE 1 experiment that were focused on aerosol nucleation in the free troposphere. All of these observations were found to be linked directly or by inference to the air that had been recently processed by clouds. The process of nucleation appeared to be active in the region of detrainment and mixing described as cloud outflow and

that growth to detectable sizes of 0.003 gm appeared to be rapid at perhaps a few tens of minutes to hours. Sulfuric acid concentrations also appeared moderately enhanced in these regions with typical concentrations of the order of 107 molecules cm -3. Both particle production and sulfuric acid were found to be more enhanced in cloud outflow regions in noon to early afternoon, apparently as a result of more active photochemistry. Highest new particle concentrations were generally associated with regions of lowest surface areas and water vapor mixing ratios intermediate between moist surface layers and dry regions aloft and above cloud. These observations are in general agreement with previous observations [Perry and Hobbs, 1994] and support their description of cloud processing, although we note that our midlatitude observations occurred at significantly lower altitudes and warmer temperatures(i.e.,-2 ø to-15øC).

Growth and evolution of these particles aloft was evident in the data over periods estimated to be hours to a day or so during which time the smaller nuclei appeared to merge with an Aitken mode usually centered near 0.035-0.060 pm depending upon location and altitude. Flights in postfrontal subsidence revealed smaller nuclei in the MBL both aboard the C-130 and the R/V Discoverer [Bates et al., this issue], indicating that these were likely to have been mixed into the MBL as particles formed aloft by previous convective clouds.

Observations in the tropical free troposphere near Christmas Island supported earlier data that suggested convective cloud processes near the ITCZ resulted in new particle production aloft.

Vertical profiles in this region revealed that smallest monomodal nuclei were present aloft that gradually increased in