K. Plauškait˙E, R. Kazlauskait˙E, J. Andriejauskien˙E, and V. Ulevicius

Lithuanian Journal of Physics, Vol. 45, No. 2, pp. 139–147 (2005) PARAMETERIZATION OF NEW PARTICLE FORMATION AND GROWTH AT THE PREILA STATION K. Plauškaite,˙ R. Kazlauskaite,˙ J. Andriejauskiene,˙ and V. Uleviciusˇ Environmental Physics and Chemistry Laboratory, Institute of Physics, Savanori ˛u231, LT-02300 Vilnius, Lithuania E-mail: [email protected] Received 28 February 2005 The aerosol formation and subsequent particle growth in the ambient air have been observed at Preila station located in the coastal/marine environment. Submicron aerosol particle number concentrations and size distributions have been measured using a differential mobility particle sizer (ELAS-5Mc). The studied period covers June, July, and August 1997, June 2000, May 2001, and March, June, July, August, September, and October 2002. In this work, we related the nucleation events to atmospheric circulation and local meteorological parameters. Air mass backward trajectories were used to estimate the prehistory of sampled air. Moreover, trace gas (O3, NOx, and SO2) concentrations were analysed. The observed nucleation mode growth, the source rate of condensable material, and the changes of hygroscopic properties during the nucleation and growth events were analysed. The new particle formation rate, condensation and coagulation sinks were calculated. The growth rates varied between 1.2 and 9.9 nm/h. The formation rate was 0.14 cm−3s−1. Such a low value of the formation rate could explain why in Preila there are so few event days. The median condensation sink was 1.5·10−3 s−1, and the vapour source rate was 8.06·104 cm−3s−1. Keywords: tropospheric aerosol, nucleation, modelling, air mass backward trajectories, meteorological parameters PACS: 92.70.Cp, 92.20Bk, 92.60.-e 1. Introduction that time of the year. Nucleation events were also observed in the outflow regions of convective clouds [4] Bursts of ultrafine particle concentrations have fre- and in regions that had been cleansed by rain [5]. quently been observed at several locations in the world Despite intensive research during several decades, in recent years [1]. It is important to know the pro- the fundamental mechanism that leads to new parti- duction mechanisms responsible for these bursts and cle formation remains uncertain. The binary homo- whether the formed ultrafine particles are able to grow geneous nucleation mechanism of water and sulphuric to sizes of radiatively active aerosol and cloud conden- acid, which has generally been assumed to be the prin- sation nuclei (CCN) that have a substantial effect on cipal mechanism for new particle formation in the at- the Earth’s radiation budget and climate. The number mosphere, is often unable to explain the observed nu- distributions of atmospheric aerosols are determined by cleation rates. Alternatively, ternary homogeneous nu- several physicochemical processes, of which the parti- cleation, also involving ammonia [6], has been sug- cle formation process involving nucleation and conden- gested as a means of accelerating the nucleation pro- sational growth to observable sizes is crucial. cess [7–9]. In order to understand the nucleation process, iden- In this work, we intend to relate the nucleation tification of the atmospheric perturbations predomi- events to atmospheric circulation and local meteoro- nantly responsible for it is required. Generally, the logical parameters. Air mass backward trajectories are probability of new particle formation is connected to used to estimate the prehistory of sampled air. Trace the preexisting particle concentration, proper precur- gas (O3, NOx, and SO2) concentrations are analysed sors, cloudiness, air mass advection, etc. [2]. Nils- together with several meteorological parameters (radi- son et al. [3] related the nucleation events to cold air ation, temperature, relative humidity, wind speed). The mass advection and explained that events occur more observed nucleation mode growth, the source rate of frequently during spring and autumn at larger latitudi- condensable material, and the changes of hygroscopic nal temperature gradients and higher cyclone activity at properties during the nucleation and growth events are c Lithuanian Physical Society, 2005 c Lithuanian Academy of Sciences, 2005 ISSN 1648-8504 140 K. Plauškait˙eet al. / Lithuanian J. Phys. 45, 139–147 (2005) evaluated using equations describing the rate of change Here r is the particle radius, mv is the molecular mass of vapour concentration, aerosol particle number con- of condensable vapour, βm is the transitional correction centration, and particle growth. factor for mass flux, D is the diffusion coefficient, and ρ is the particle density. Equation (3) can be integrated from r to r to obtain 2. Experimental 0 ρ r2 − r2 4 The environmental pollution research station in Prei- C = 0 + − 0.623 λ(r − r ) 0 la (Lithuania) is located on the Curonian Spit, which ∆tDmv 2 3α separates the Curonian Lagoon and the Baltic Sea. The λ + r Curonian Lagoon is a highly eutrophied water body. It + 0.623λ2 ln . (4) λ + r is an enclosed shallow (mean depth 3.7 m) bay, con- 0 nected to the Baltic Sea by the narrow (width 400–600 Here α is the mass accommodation coefficient (i. e., m) Klaipeda˙ strait. At the Preila station the aerosol sticking probability) and λ is the mean free path. Di- particle number concentrations and size distributions in rectly from measurements of the aerosol size distri- the 10–200 nm size range were measured using differ- bution changes and hygroscopicity properties, dr/dt, ential mobility particle sizer (ELAS-5Mc) developed in Sc, Scoag, dN3 nm/dt, N , and the soluble the Environmental Physics and Chemistry Laboratory, nucleation mode fraction can be obtained. Institute of Physics, Lithuania. More information can Using the above equations and the experimental be found in Uleviciusˇ et al. [10]. Meteorological data data, we can also determine J (nucleation rate, or for- were provided by the Atmospheric Pollution Research 1 mation rate, for 1 nm particles), where 1 nm is assumed Laboratory, Institute of Physics, Lithuania. to be the size of a new particle. N1 is the number concentration of 1 nm particles, N3 is the number concen- 3. Theoretical tration of 3 nm particles. J3 is the formation rate of 3 nm particles. In order to obtain J1, we developed a 3.1. Basic equations 5-step derivation of J1. First, an expression for N1 is required: The observed nucleation mode growth, the source rate of condensable material, and the changes of hy- dN1 = J1 − K1N1 − J3 , (5) groscopic properties during the nucleation and growth dt events are analysed using three equations describing the rate of change of vapour concentration, aerosol par- where K1 is the coagulation sink for 1 nm particles. In ticle number concentration, and particle growth. Con- practice, it is the coagulation between 1 nm particles sidering condensable vapour molecules of species X, and larger particles. Then, assuming a steady-state sit- the time dependence of vapour concentration C can be uation, expressed (see also [11]) by J1 = K1N1 + J3 . (6) dC = Q − Sc · C , (1) Second, the link between K N and J can be deter- dt 1 1 3 mined referring to the fact that a fraction of N1 particles where Q is the source rate of the vapour and Sc is its will coagulate before they enter the 3 nm size range. condensation sink on the pre-existing aerosol. The time Coagulation of N1 during the growth from 1 to 3 nm evolution for aerosol number concentration ( ) in size N can be obtained from i class can be presented by dN1,3 dN1,3 dNi = −KN1,3 ⇒ = −Kdt = J − Scoag · N , (2) dt i i dt N1,3 where Ji is the formation rate of particles and Scoag is −Kt ⇒ N1,3 = N1e . (7) the coagulation sink for size i particles. The growth rate can be expressed [12] as Here N1,3 corresponds to the concentration of particles dr m β DC growing from 1 to 3 nm, t is the particle growth time, = v m . (3) dt rρ and K is their coagulation sink. Thus, the fraction of K. Plauškait˙eet al. / Lithuanian J. Phys. 45, 139–147 (2005) 141 −Kt N1 coagulated during the growth is 1 − e , and the Here the transitional correction factor βM can be ex- −Kt fraction of N1 contributing to J3 is e . Therefore, pressed [14] as −Kt K1N1 1 − e Kn+1 ∼ Kt − (8) βM = . (15) = −Kt = e 1 , 4 − 4 − J3 e 0.377Kn + 1 + α 1Kn2 + α 1Kn 3 3 Kt and K1N1 = J3(e − 1). The Knudsen number Kn = λv/r, and the sticking Third, combining the above for J , 1 coefficient α is typically assumed to be unity. In the Kt Kt ≫ ≈ J1 = K1N1 + J3 = J3(e − 1) + J3 = J3e . (9) molecular regime where Kn 1 we have βM 2 3r/(4λv) and Sc ∝ r . On the other hand, in the con- Fourth, as the equation for N3 is tinuum regime where Kn ≪ 1 we get βM = 1 and ∝ dN3 Sc r . However, typical tropospheric aerosol also = J3 − K3N3 , (10) ∝ a dt includes the transitional regime and Sc r , where 1 <a< 2. then it follows that dN3 3.3. Coagulation sink Scoag J3 = + K3N3 . (11) dt The coagulation sink determines how fast the nm- Finally, J1 is given by size aerosol particles are removed due to coagulation. dN The coagulation sink can be determined from J = J eKt = 3 + K N eKt . (12) 1 3 3 3 dt Scoag = KijNj . (16) The time t in equations above corresponds to the par- Xj ticle growth time from 1 to 3 nm, and K is a typical Here Kij is the coagulation coefficient [15, 16], which coagulation sink during the growth, the actual value of in the transitional regime is which is close to K1.

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