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Growth of nucleation mode particles: Source rates of condensable vapour in a smog chamber
Atmospheric Environment 42 (2008) 7405–7411
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Growth of nucleation mode particles: Source rates of condensable vapour in a smog chamber Ari P. Leskinen a, *, Markku Kulmala b, Kari E.J. Lehtinen a, c a
Finnish Meteorological Institute, Kuopio Unit, PO Box 1627, FI-70211 Kuopio, Finland Department of Physics, University of Helsinki, PO Box 64, FI-00014 Helsinki, Finland c Department of Physics, University of Kuopio, PO Box 1627, FI-70211 Kuopio, Finland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 January 2008 Received in revised form 19 May 2008 Accepted 18 June 2008
We investigated the formation and subsequent growth of organic aerosol particles in laboratory conditions by irradiating a mixture of xylene and nitrogen oxides (NOx) with UV light in a 6 m3 Teflon chamber. We used different initial hydrocarbon (HC) and NOx concentrations, with mixing ratios of 3.5–30 (28–240 ppmC/ppm), and monitored the changes in particle size distribution in the range of 7–330 nm. We applied the concept of condensation sink to the measured size distribution data in order to estimate the concentration and source rate of condensable vapours in the chamber. We observed the nucleation and Aitken mode growth rate to be 10.6–18.6 nm h1, which corresponds to a vapour concentration of 1.5–2.4108 cm3, from which our deduced estimation for the source rate was 0.2–2.5107 cm3 s1. These vapour source rates are up to four orders of magnitude higher than the atmospheric values observed in Antarctica and other background stations, up to two orders of magnitude higher than at urban areas of Athens and Marseille, up to 13 times higher than at a coastal site and of the same magnitude, within a factor of five, as in New Delhi, a heavily polluted urban area. In the chamber experiments we observed a strong dependence of the source rate of condensable vapours on the initial NOx concentration. This indicates that oxidation processes play an important role in particle formation and subsequent growth. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Aerosol formation Condensation sink Photooxidation Smog chamber Xylene
1. Introduction The formation and growth of atmospheric aerosols are key processes in the dynamics of atmospheric aerosols (Kulmala et al., 2004a). Atmospheric aerosol formation and their subsequent growth to w100 nm in 1–2 days, takes place frequently in the continental boundary layer, e.g. in Northern Lapland (Komppula et al., 2006), over the remote boreal forest (Ma¨kela¨ et al., 1997; Kulmala et al., 1998) and suburban Helsinki (Va¨keva¨ et al., 2000), over industrialised agricultural regions in Germany (Birmili and Wiedensohler, 2000) and in coastal environments in Europe (O’Dowd * Corresponding author. Tel.: þ358 50 522 9148; fax: þ358 17 162 301. E-mail addresses: ari.leskinen@fmi.fi (A.P. Leskinen), markku. kulmala@helsinki.fi (M. Kulmala), kari.lehtinen@fmi.fi (K.E.J. Lehtinen). 1352-2310/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2008.06.024
et al., 1999). Depending on their source and physical and chemical properties, atmospheric particles influence, e.g. the climate (Ramanathan et al., 2001), human health (Pope and Dockery, 2006) and atmospheric visibility (Sisler and Malm, 1994; Ba¨umer et al., 2008). For quantification and predictive purposes, it is important to understand the underlying processes leading to the formation and growth of atmospheric aerosols in different environments. Observation of the nucleation mode particles is nowadays possible owing to the development of the measurement instruments during the last decades (McMurry, 2000). Also the precursors for the nucleation mode particles can be measured and even nucleation can be studied almost directly (Kulmala et al., 2007). However, further laboratory and field investigations are needed to focus on the particle formation and growth processes in
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various conditions and environments and with different chemical compounds, as, for example, in a boreal environment, organic vapours are shown to be substantial for the observed aerosol formation and growth (Kulmala et al., 2004b; Allan et al., 2006; Tunved et al., 2006). During the past decades smog chamber studies have been extensively performed in order to simulate atmospheric processes of organic compounds and oxidants, such as OH, O3 and NO3, and to investigate the formation of secondary organic compounds (e.g. Kalberer et al., 2004; Paulsen et al., 2006). Smog chambers with controllable conditions can be used, e.g. to produce data for input and verification of chemical and photochemical model calculations (e.g. Finlayson-Pitts and Pitts, 2000). The model calculations can be carried out not only for gaseous reactants and reaction products but also in studies for particle formation and dynamics in the chambers. Xylene is known to have the potential to form ozone and a secondary organic aerosol (SOA) in the presence of nitrogen oxides (NOx) and UV light (e.g. Izumi and Fukuyama, 1990). The SOA yield from xylene/NOx photooxidation is known to depend on the hydrocarbon to NOx (HC to NOx) (Odum et al., 1996; Song et al., 2005) as well as on the hydroxyl radical (OH), propene and carbon monoxide concentrations (Song et al., 2007a), but not on, e.g. gas-phase or aerosol-phase water content (Cocker et al., 2001). In this study, in order to obtain some physical and chemical insight into aerosol formation processes, we used a simple analytical method, based on the concept of condensation sink (Kulmala et al., 2001), to evaluate some previously performed smog chamber experiments (Leskinen et al., 1998), in which we irradiated the mixture of xylene and NOx in a smog chamber with black light. We analysed the size distribution data from the experiments with HC to NOx mixing ratios in the range of 3.5–30 ppm/ ppm (28–240 ppmC/ppm) and calculated the source rate of condensable vapours as a function of time and estimated the minimum secondary organic aerosol yield at the end of each experiment. 2. Experimental set-up We carried out the experiments in a 6 m3 collapsible bag made of 50 mm thick FEP Teflon film and surrounded by an array of 68 pieces of 40-W Sylvania Blacklight 350 UV lamps (Fig. 1) which produce UV irradiation in the wavelength range of 320–400 nm. This kind of chamber is beneficial for UV irradiation studies as the thin Teflon film lets the 320–400 nm UV light through almost completely (penetration 95%) and because the replacement of the sample with clean air is not needed when the sample is drawn out of the chamber. This prevents the gas and particle concentrations in the chamber from being diluted during an experiment. However, the 6 m3 chamber volume and the sampling rate of 10–20 l min1 used in the experiments limits the time span of experiments to 3–5 h as it cannot be sucked completely empty. Also, as the chamber collapses, its surface area to volume ratio increases and wall deposition becomes more effective. We did not characterize the exact wall losses, but in order to avoid problems caused by them we limited the experiment time to 2.5–3 h.
Fig. 1. . Experimental set-up. Solid lines indicate sample lines, dashed lines data connections and circles UV lamps. Kr, neutralizer; DMA, differential mobility analyzer; CPC, condensation particle counter; PC, computer; NOx, NO-NOx analyzer; O3, ozone analyzer; T þ RH, temperature and humidity sensor; DT, datalogger. Not to scale.
Prior to each experiment we flushed the chamber for several hours with dry, filtered air (at room temperature w20 C, relative humidity w3%). The filtered air was free of particles but contained some gaseous trace pollutants. For example, the background NOx concentration, which we measured with a chemiluminescent NO-NOx analyzer (Environnement S.A. Model AC30M), was composed mainly of nitrogen monoxide and was between 0.004 and 0.063 ppm (Table 1). After flushing the chamber, we injected the reactants, xylene vapour and NO2, into the chamber. We heated liquid xylene (a mixture of o-, m- and p-xylenes) in a glass jar through which we led filtered air as a carrier gas. After injecting the xylene we injected gaseous NO2 (99% purity) into the chamber until the NO2 concentration (and the HC to NOx ratio) in the chamber was as desired. During the injection (and throughout the experiments) we used an internal fan which mixed the chamber air in order to equalize the concentrations in the chamber volume. We carried out altogether nine experiments with calculated xylene concentrations of 1.0 ppm (2.5 1013 cm3) or 2.8 ppm (7.0 1013 cm3). These calculated concentrations are based on the ratio of the amount (in mol) of the liquid xylene in the glass jar to that of the air in the chamber, assuming both the xylene vapour and the air as ideal gases. The initial xylene to NOx mixing ratios were in the range of 3.5–30 ppm/ppm (28–240 ppmC/ppm). In addition to the NO and NOx, we monitored the ozone concentration, the temperature and the relative humidity in the chamber with a UV-absorption ozone analyzer (Dasibi Model 1008) and a Humicap sensor (Vaisala HMP 233), respectively, and registered all the data with a DataTaker 50 datalogger. We measured the particle size distribution in the chamber in the size range of 7–330 nm by using a scanning mobility particle sizer (Wang and Flagan, 1990), SMPS, which consisted of a bipolar Kr-85 charger, a TSI Model 3071A differential mobility analyzer (Knutson and Whitby, 1975) and a TSI Model 3022A condensation particle counter. The charger neutralized the aerosol sample into a bipolar charge distribution estimated by Wiedensohler (1988).
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Table 1 Initial values in experiments A–I Experiment
[NO]0 (ppm)
[NO2]0 (ppm)
[HC]0 (ppm)
[HC]0 (1013 cm3)
[HC to NOx]0 (ppm/ppm)
[HC to NOx]0 (ppmC/ppm)
UV lamps (number)
A B C D E F G H I
0.011 0.063 0.031 0.013 0.023 0.007 0.004 0.005 0.024
0.273 0.234 0.529 0.097 0.365 0.136 0.090 0.279 0.275
1.0 2.8 2.8 1.0 2.8 2.8 2.8 2.8 2.8
2.5 7.0 7.0 2.5 7.0 7.0 7.0 7.0 7.0
3.5 9.4 5.0 9.1 7.2 20 30 9.9 9.4
28.0 75.2 40.0 72.8 57.6 160 240 79.2 75.2
68 68 68 68 68 68 68 46 22
[NO]0 is the initial nitrogen monoxide concentration, [NO2]0 is the initial nitrogen dioxide concentration, [HC]0 is the initial hydrocarbon (xylene) concentration, [HC to NOx]0 is the initial hydrocarbon to NOx (NO þ NO2) ratio and UV lamps denotes the number of lamps on.
3. Calculations We analyzed the observed aerosol growth and the source rate of condensable material by using the following equations for the vapour concentration and particle growth (Kulmala et al., 2001). The time dependence of a condensable vapour with concentration C (m3) in air can be presented by (see also Kulmala, 1988; Kulmala et al., 1998)
dC ¼ Q CS C dt
noticing that usually in practise in Eqs. (1) and (4) the vapour concentration C refers to the sum of all condensable vapours. We can obtain the evolution of dp with time, and hence also ddp/dt, through numerical differentiation directly from the size distribution evolutions, enabling a direct estimation of the (total) condensable vapour concentration C by
(1)
where Q (m3 s1) is the source rate and CS (s1) is the condensation sink, which is based on the concept presented by Pirjola et al. (1999). The condensation sink expresses how rapidly molecules will condense on aerosol particles. It depends strongly on the particle size distribution and its inverse is an estimate for the lifetime of the vapour in the atmosphere. We can obtain the condensation sink from measured size distribution by
CS ¼ 2pD
Z
N
X bm;i dp;i Ni dp bm dp n dp ddp ¼ 2pD
0
(2)
i
where D (m2/s) is the diffusion coefficient and the products of the transitional correction factor for mass flux, bm,i, particle diameter, dp,i (m), and number concentration, Ni (m3), are summed over all measured size classes i. We can express the transitional correction factor for mass flux as (Fuchs and Sutugin, 1971)
bm ¼
1 þ Kn 1 þ ð34a þ 0:377 Kn þ 34a Kn2
(3)
where in turn we can express the Knudsen number, Kn ¼ 2l/dp by using the mean free path for air, l (m), and the particle diameter, dp (m). For the mass accommodation coefficient, a, we can use an approximation of unity (Winkler et al., 2004). We can express the rate of change of particle diameter by (Dal Maso et al., 2002)
ddp 4bm mv DC ¼ rdp dt
(4)
where dp (m) is the particle diameter, bm is the transitional correction factor for mass flux, mv (kg) is the molecular mass of the condensable vapour, D (m2 s1) is the diffusion coefficient, and r (kg m3) is the particle density. It is worth
Fig. 2. Particle size distribution as a function of time in experiments A (a) and G (b). Gray scale indicates particle number concentration dN/dlog(dp) and is expressed as cm3.
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using Eq. (4). With C known, we can evaluate dC/dt directly through numerical differentiation. Thus it is possible to estimate also the source rate of condensable vapour Q, by using Eq. (1). By using this method we get both the source rate Q and vapour concentration C as functions of time without the need for unnecessary pseudo-steady state conditions. In the calculations we approximated the diffusion coefficient and molecular mass for the unknown oxidation products with those of xylene: 7.5 106 m2 s1 for the diffusion coefficient and 1.8 1025 kg for the mass of condensing molecules. This approximation is valid and does not make a big error to the calculated values, because the product of the diffusion coefficient and mass of condensing molecules for different compounds is practically almost constant, and the only real parameter is the mass accommodation coefficient (Kulmala et al., 2005). Furthermore, the density of the oxidation products was assumed to be 1.0 103 kg m3, an assumption also used by Song et al. (2005) in their work. The mean free path of air molecules was calculated to be 6.7 108 m in the prevailing conditions (298 K, 1 atm). We calculated the secondary organic aerosol (SOA) yield (Y) by dividing the produced organic aerosol mass DMo (mg/ m3) by the reactive organic gas (ROG) reacted DROG (mg/ m3) (e.g. Odum et al., 1996)
Y ¼
DM o DROG
(5)
However, because we did not measure the concentration of xylene in the gas phase during the experiments and because we do not have knowledge of the chamber wall losses, we were only able to calculate the minimum SOA yields by assuming that all the xylene was consumed in the reactions and no wall losses occurred. 4. Results and discussion We could observe the newly formed particles when they had grown to approximately 15 nm in size and that the particles grew eventually to approximately 100 nm in size, but with different growth rates from one experiment to another, such as, for example, in experiment A (Fig. 2a) with initial HC to NOx ratio of 3.5 ppm/ppm (28 ppmC/ppm) and experiment G (Fig. 2b) with initial HC to NOx ratio of 30 ppm/ppm (240 ppmC/ppm). We also found that the number of UV lamps on affected the growth rate: the more UV irradiation, the faster the particle formation and growth. This is, however, quite obvious and we did not study the effect of UV irradiation level further.
Fig. 3. Geometric mean diameter, (GMD), growth rate (GR), total number and mass concentration (Ntot and mtot, respectively), condensable vapour concentration (C) and its rate of change (dC/dt), condensation sink (CS) and vapour source rate (Q) as a function of time in experiments A (a) and G (b).
A.P. Leskinen et al. / Atmospheric Environment 42 (2008) 7405–7411
We calculated the geometric mean diameter (GMD), marked as dp in Eqs. (2) and (4), and integrated the particle number and mass concentrations over the measured size range of 7–330 nm, to get the total number and mass concentration, denoted as Ntot and mtot, respectively (e.g. experiments A and G in Fig. 3a and b, respectively), excluding the early stage of each experiment when the number concentration was very low and we could not calculate the GMD accurately. Furthermore, we calculated the particle number diameter growth rate (GR), denoted as ddp/dt in Eq. (4), through numerical differentiation from the size distribution data, the vapour concentration, C, by using Eq. (4), the condensation sink, CS, by using Eq. (2), and the vapour source rate, Q, by using Eq. (1), as a function of time (Fig. 3a and b). For comparison between experiments, we calculated the average values (Table 2) for the growth rate, condensation sink, vapour concentration and its source rate for the time period when the vapour concentration was approximately constant (dC/dt ¼ 0 in Fig. 3a and b). Furthermore, we calculated the SOA yield by using Eq. (5). We observed that vapour concentrations of 1.5–2.4 108 cm3 are needed to sustain the growth rates of 10.6–18.6 nm h1. At the same time, the average condensation sink was 1.4–13 102 s1 and the source rate 0.2–2.5 107 cm3 s1 (Table 2). We compared the particle growth rates and the condensation sinks, concentrations and source rate of condensable vapours that we obtained in the smog chamber experiments to the values obtained in the atmosphere during a particle formation events in a Boreal forest in Hyytia¨la¨, Finland (Kulmala et al., 2001), at a coastal site in Mace Head, Ireland (Dal Maso et al., 2002), and other background stations (Aboa in Antarctica and Va¨rrio¨ in Finnish Lapland) as well as in polluted areas of Athens, Greece, of Marseille, France, and of New Delhi, India (Kulmala et al., 2005). The condensation sink values that we obtained in the laboratory experiments were 2–2200 times the values obtained at the background stations (Aboa, Hyytia¨la¨ and Va¨rrio¨), 0.9–40 times those obtained in the polluted urban areas of Athens and Marseille, the same magnitude as (0.2–2.6 times) those obtained in the highly polluted New Delhi (Kulmala et al., 2005), and 7–65 times those obtained at a coastal site (Dal Maso et al., 2002). The calculated particle growth rates and vapour concentrations were of the same magnitude (within a factor of 2) as in New Delhi, 0.1–2.4 times those at
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a coastal site, and 1.4–16 times those in other urban and background measurement sites, excluding Aboa, where the vapour concentration was 4–60 times lower than in the smog chamber. In chamber experiments the formation of new particles is often seen due to oxidation of organic gases prior to formation of organic vapours with low vapour pressure (e.g. Odum et al., 1997). However, the concentrations of organic vapours in chamber experiments are typically at least 1–3 orders of magnitude higher than at field conditions. For example, in our experiments we obtained concentrations that are present only in heavily polluted areas, such as New Delhi. Finally, the source rates obtained by us were 0.1–2.8 times those in New Delhi, 0.4–13 times those in a coastal area, 1.5–300 times those in other urban areas, and 3–30 000 times those in the background stations. The difference was highest between the smog chamber and Aboa. This suggests that our values for the source rate in the smog chamber correspond to those obtained in urban areas where aromatic hydrocarbons and pollutants, such as NOx, are typically present.
Table 2 Calculated values for experiments A–I for the growth rate (GR), condensation sink (CS), vapour concentration (C) source rate (Q), and secondary organic aerosol yield (Y) Experiment GR CS C Q Y (nm h1) (102 s1) (108 cm3) (107 cm3 s1) A B C D E F G H
18.6 10.6 14.7 15.2 14.2 13.7 12.9 16.2
1.72 8.40 9.82 2.93 12.7 3.27 1.45 2.06
2.36 1.47 2.17 1.84 1.93 1.87 1.67 2.34
0.40 1.23 2.15 0.53 2.46 0.61 0.24 0.48
0.0014 0.0030 0.0045 0.0021 0.0046 0.0012 0.0004 0.0009
Fig. 4. Source rate of condensable vapour (Q) as a function of initial NOx concentration (a) and initial HC concentration (b).
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We analysed the differences in growth and source rates with the gas data and observed that the source rate of condensable vapours depends significantly on the initial NOx concentration but seems to be almost independent on the initial xylene concentration (Fig. 4a and b). Of the NOx dependence we can conclude that oxidation processes play an important role in the growth process of organic aerosols. We also observed that the SOA yield is proportional to the initial NOx concentration, but independent on the initial xylene concentration (Fig. 5a and b). Similar observation was made by Song et al. who noticed that the initial NOx level affects the SOA yield from different isomers of xylene (o-, p- and m-xylene) (Song et al., 2007b) and that nitrogen oxide (NO) suppresses SOA formation (Song et al., 2007c). In our studies, we used higher NOx concentrations than Song et al. and did not observe such suppression of SOA formation, which might have been the case with lower NOx concentrations. It is worth noting that in our studies some uncertainty may arise from the fact that we used a mixture of the xylene isomers in our studies instead of the isomers separately.
5. Summary and conclusions In the present study we compared aerosol formation and growth in smog chamber experiments and in atmospheric conditions from the point of view of vapour concentration and source rate. In a smog chamber with high enough concentrations the organic vapours are able to form new aerosol particles. However, in atmospheric conditions the concentrations are much smaller (at least by a factor of 10, typically more than 100), at least in background areas. We observed that the vapour source rates in the smog chamber are up to four orders of magnitude higher than atmospheric values observed in Aboa and other background stations, up to two orders of magnitude higher than at urban areas of Athens and Marseille, up to 13 times higher than at a coastal site and of the same magnitude, within a factor of five, as in New Delhi, a heavily polluted urban area. In the atmosphere organic compounds probably do not have a major role in the nucleation process itself, which is dominated by other nucleation routes, like the sulphuric acid–ammonia–water system (Kulmala et al., 2000) or activation of existing clusters (Kulmala et al., 2006, 2007). However, the organic compounds contribute predominantly to the subsequent condensational growth process, which sets needs for systematic laboratory experiments with pure organic compounds, as we did in this study. Our systematic measurements with different input concentrations of xylene and nitrogen oxides showed a strong dependence of the source rate of condensable vapours and SOA yield on the initial NOx concentration. This indicates that oxidation processes play an important role in particle formation and subsequent growth.
Acknowledgements The experimental studies were funded by the Finnish Funding Agency for Technology and Innovation (Tekes), which is gratefully acknowledged.
References
Fig. 5. Secondary organic aerosol yield (Y) as a function of initial NOx concentration (a) and initial HC concentration (b).
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Observations of ultrafine aerosol particle formation and growth in boreal forest. Geophysical Research Letters 24, 1219–1222. O’Dowd, C., McFiggans, G., Creasey, D.J., Pirjola, L., Hoell, C., Smith, M.H., Allan, B.J., Plane, J.M.C., Heard, D.E., Lee, J.D., Pilling, M.J., Kulmala, M., 1999. On the photochemical production of new particles in the coastal boundary layer. Geophysical Research Letters 26, 1707–1710. Odum, J.R., Hoffman, T., Bowman, F., Collins, D., Flagan, R.C., Seinfeld, J.H., 1996. Gas/particle partitioning and secondary organic aerosol yields. Environmental Science & Technology 30, 2580–2585. Odum, J.R., Jungkamp, T.P.W., Griffin, R.J., Forstner, H.J.L., Flagan, R.C., Seinfeld, J.H., 1997. Aromatics, reformulated gasoline, and atmospheric organic aerosol formation. Environmental Science & Technology 31, 1890–1897. Paulsen, D., Weingartner, E., Rami Alfarra, M., Baltensperger, U., 2006. Volatility measurements of photochemically and nebulizer-generated organic aerosol particles. Journal of Aerosol Science 37, 1025–1051. Pirjola, L., Kulmala, M., Wilck, M., Bischoff, A., Stratmann, F., Otto, E., 1999. Effects of aerosol dynamics on the formation of sulphuric acid aerosols and cloud condensation nuclei. Journal of Aerosol Science 30, 1079–1094. Pope III, C.A., Dockery, D.W., 2006. Health effects of fine particulate air pollution: lines that connect. Journal of Air and Waste Management Association 56, 709–742. Ramanathan, V., Crutzen, P.J., Kiehl, J.T., Rosenfeld, D., 2001. Aerosol, climate, and the hydrological cycle. Science 294, 2119–2124. Sisler, J.F., Malm, W.C., 1994. The relative importance of aerosols to spatial and seasonal trends of impaired visibility in the United States. Atmospheric Environment 28, 851–862. Song, C., Na, K., Cocker III, D.R., 2005. Impact of the hydrocarbon to NOx ratio on secondary organic aerosol formation. Environmental Science & Technology 39, 3143–3149. Song, C., Na, K., Warren, B., Malloy, Q., Cocker III, D.R., 2007a. Impact of propene on secondary organic aerosol formation from m-xylene. Environmental Science & Technology 41, 6990–6995. Song, C., Na, K., Warren, B., Malloy, Q., Cocker III, D.R., 2007b. Secondary organic aerosol formation from the photooxidation of p- and o-xylene. Environmental Science & Technology 41, 7403–7408. Song, C., Na, K., Warren, B., Malloy, Q., Cocker III, D.R., 2007c. Secondary organic aerosol formation from m-xylene in the absence of NOx. Environmental Science & Technology 41, 7409–7416. Tunved, P., Hansson, H.-C., Kerminen, V.-M., Stro¨m, J., Dal Maso, M., Lihavainen, H., Viisanen, Y., Aalto, P.P., Komppula, M., Kulmala, M., 2006. High natural aerosol loading over boreal forests. Science 312, 261–263. Va¨keva¨, M., Ha¨meri, K., Puhakka, T., Nilsson, E.D., Hohti, H., Ma¨kela¨, J.M., 2000. Effects of meteorological processes on aerosol particle size distribution in an urban background area. Journal of Geophysical Research D105, 9807–9821. Wang, S., Flagan, R., 1990. Scanning electrical mobility spectrometer. Aerosol Science and Technology 13, 230–240. Wiedensohler, A., 1988. An approximation of the bipolar charge-distribution for particles in the sub-micron size range. Journal of Aerosol Science 19, 387–389. Winkler, P.M., Vrtala, A., Wagner, P.E., Kulmala, M., Lehtinen, K.E.J., Vesala, T., 2004. Mass and thermal accommodation during gas-liquid condensation of water. Physical Review Letters 93, 075701.
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Growth of nucleation mode particles: Source rates of condensable vapour in a smog chamber Ari P. Leskinen a, *, Markku Kulmala b, Kari E.J. Lehtinen a, c a
Finnish Meteorological Institute, Kuopio Unit, PO Box 1627, FI-70211 Kuopio, Finland Department of Physics, University of Helsinki, PO Box 64, FI-00014 Helsinki, Finland c Department of Physics, University of Kuopio, PO Box 1627, FI-70211 Kuopio, Finland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 January 2008 Received in revised form 19 May 2008 Accepted 18 June 2008
We investigated the formation and subsequent growth of organic aerosol particles in laboratory conditions by irradiating a mixture of xylene and nitrogen oxides (NOx) with UV light in a 6 m3 Teflon chamber. We used different initial hydrocarbon (HC) and NOx concentrations, with mixing ratios of 3.5–30 (28–240 ppmC/ppm), and monitored the changes in particle size distribution in the range of 7–330 nm. We applied the concept of condensation sink to the measured size distribution data in order to estimate the concentration and source rate of condensable vapours in the chamber. We observed the nucleation and Aitken mode growth rate to be 10.6–18.6 nm h1, which corresponds to a vapour concentration of 1.5–2.4108 cm3, from which our deduced estimation for the source rate was 0.2–2.5107 cm3 s1. These vapour source rates are up to four orders of magnitude higher than the atmospheric values observed in Antarctica and other background stations, up to two orders of magnitude higher than at urban areas of Athens and Marseille, up to 13 times higher than at a coastal site and of the same magnitude, within a factor of five, as in New Delhi, a heavily polluted urban area. In the chamber experiments we observed a strong dependence of the source rate of condensable vapours on the initial NOx concentration. This indicates that oxidation processes play an important role in particle formation and subsequent growth. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Aerosol formation Condensation sink Photooxidation Smog chamber Xylene
1. Introduction The formation and growth of atmospheric aerosols are key processes in the dynamics of atmospheric aerosols (Kulmala et al., 2004a). Atmospheric aerosol formation and their subsequent growth to w100 nm in 1–2 days, takes place frequently in the continental boundary layer, e.g. in Northern Lapland (Komppula et al., 2006), over the remote boreal forest (Ma¨kela¨ et al., 1997; Kulmala et al., 1998) and suburban Helsinki (Va¨keva¨ et al., 2000), over industrialised agricultural regions in Germany (Birmili and Wiedensohler, 2000) and in coastal environments in Europe (O’Dowd * Corresponding author. Tel.: þ358 50 522 9148; fax: þ358 17 162 301. E-mail addresses: ari.leskinen@fmi.fi (A.P. Leskinen), markku. kulmala@helsinki.fi (M. Kulmala), kari.lehtinen@fmi.fi (K.E.J. Lehtinen). 1352-2310/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2008.06.024
et al., 1999). Depending on their source and physical and chemical properties, atmospheric particles influence, e.g. the climate (Ramanathan et al., 2001), human health (Pope and Dockery, 2006) and atmospheric visibility (Sisler and Malm, 1994; Ba¨umer et al., 2008). For quantification and predictive purposes, it is important to understand the underlying processes leading to the formation and growth of atmospheric aerosols in different environments. Observation of the nucleation mode particles is nowadays possible owing to the development of the measurement instruments during the last decades (McMurry, 2000). Also the precursors for the nucleation mode particles can be measured and even nucleation can be studied almost directly (Kulmala et al., 2007). However, further laboratory and field investigations are needed to focus on the particle formation and growth processes in
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various conditions and environments and with different chemical compounds, as, for example, in a boreal environment, organic vapours are shown to be substantial for the observed aerosol formation and growth (Kulmala et al., 2004b; Allan et al., 2006; Tunved et al., 2006). During the past decades smog chamber studies have been extensively performed in order to simulate atmospheric processes of organic compounds and oxidants, such as OH, O3 and NO3, and to investigate the formation of secondary organic compounds (e.g. Kalberer et al., 2004; Paulsen et al., 2006). Smog chambers with controllable conditions can be used, e.g. to produce data for input and verification of chemical and photochemical model calculations (e.g. Finlayson-Pitts and Pitts, 2000). The model calculations can be carried out not only for gaseous reactants and reaction products but also in studies for particle formation and dynamics in the chambers. Xylene is known to have the potential to form ozone and a secondary organic aerosol (SOA) in the presence of nitrogen oxides (NOx) and UV light (e.g. Izumi and Fukuyama, 1990). The SOA yield from xylene/NOx photooxidation is known to depend on the hydrocarbon to NOx (HC to NOx) (Odum et al., 1996; Song et al., 2005) as well as on the hydroxyl radical (OH), propene and carbon monoxide concentrations (Song et al., 2007a), but not on, e.g. gas-phase or aerosol-phase water content (Cocker et al., 2001). In this study, in order to obtain some physical and chemical insight into aerosol formation processes, we used a simple analytical method, based on the concept of condensation sink (Kulmala et al., 2001), to evaluate some previously performed smog chamber experiments (Leskinen et al., 1998), in which we irradiated the mixture of xylene and NOx in a smog chamber with black light. We analysed the size distribution data from the experiments with HC to NOx mixing ratios in the range of 3.5–30 ppm/ ppm (28–240 ppmC/ppm) and calculated the source rate of condensable vapours as a function of time and estimated the minimum secondary organic aerosol yield at the end of each experiment. 2. Experimental set-up We carried out the experiments in a 6 m3 collapsible bag made of 50 mm thick FEP Teflon film and surrounded by an array of 68 pieces of 40-W Sylvania Blacklight 350 UV lamps (Fig. 1) which produce UV irradiation in the wavelength range of 320–400 nm. This kind of chamber is beneficial for UV irradiation studies as the thin Teflon film lets the 320–400 nm UV light through almost completely (penetration 95%) and because the replacement of the sample with clean air is not needed when the sample is drawn out of the chamber. This prevents the gas and particle concentrations in the chamber from being diluted during an experiment. However, the 6 m3 chamber volume and the sampling rate of 10–20 l min1 used in the experiments limits the time span of experiments to 3–5 h as it cannot be sucked completely empty. Also, as the chamber collapses, its surface area to volume ratio increases and wall deposition becomes more effective. We did not characterize the exact wall losses, but in order to avoid problems caused by them we limited the experiment time to 2.5–3 h.
Fig. 1. . Experimental set-up. Solid lines indicate sample lines, dashed lines data connections and circles UV lamps. Kr, neutralizer; DMA, differential mobility analyzer; CPC, condensation particle counter; PC, computer; NOx, NO-NOx analyzer; O3, ozone analyzer; T þ RH, temperature and humidity sensor; DT, datalogger. Not to scale.
Prior to each experiment we flushed the chamber for several hours with dry, filtered air (at room temperature w20 C, relative humidity w3%). The filtered air was free of particles but contained some gaseous trace pollutants. For example, the background NOx concentration, which we measured with a chemiluminescent NO-NOx analyzer (Environnement S.A. Model AC30M), was composed mainly of nitrogen monoxide and was between 0.004 and 0.063 ppm (Table 1). After flushing the chamber, we injected the reactants, xylene vapour and NO2, into the chamber. We heated liquid xylene (a mixture of o-, m- and p-xylenes) in a glass jar through which we led filtered air as a carrier gas. After injecting the xylene we injected gaseous NO2 (99% purity) into the chamber until the NO2 concentration (and the HC to NOx ratio) in the chamber was as desired. During the injection (and throughout the experiments) we used an internal fan which mixed the chamber air in order to equalize the concentrations in the chamber volume. We carried out altogether nine experiments with calculated xylene concentrations of 1.0 ppm (2.5 1013 cm3) or 2.8 ppm (7.0 1013 cm3). These calculated concentrations are based on the ratio of the amount (in mol) of the liquid xylene in the glass jar to that of the air in the chamber, assuming both the xylene vapour and the air as ideal gases. The initial xylene to NOx mixing ratios were in the range of 3.5–30 ppm/ppm (28–240 ppmC/ppm). In addition to the NO and NOx, we monitored the ozone concentration, the temperature and the relative humidity in the chamber with a UV-absorption ozone analyzer (Dasibi Model 1008) and a Humicap sensor (Vaisala HMP 233), respectively, and registered all the data with a DataTaker 50 datalogger. We measured the particle size distribution in the chamber in the size range of 7–330 nm by using a scanning mobility particle sizer (Wang and Flagan, 1990), SMPS, which consisted of a bipolar Kr-85 charger, a TSI Model 3071A differential mobility analyzer (Knutson and Whitby, 1975) and a TSI Model 3022A condensation particle counter. The charger neutralized the aerosol sample into a bipolar charge distribution estimated by Wiedensohler (1988).
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Table 1 Initial values in experiments A–I Experiment
[NO]0 (ppm)
[NO2]0 (ppm)
[HC]0 (ppm)
[HC]0 (1013 cm3)
[HC to NOx]0 (ppm/ppm)
[HC to NOx]0 (ppmC/ppm)
UV lamps (number)
A B C D E F G H I
0.011 0.063 0.031 0.013 0.023 0.007 0.004 0.005 0.024
0.273 0.234 0.529 0.097 0.365 0.136 0.090 0.279 0.275
1.0 2.8 2.8 1.0 2.8 2.8 2.8 2.8 2.8
2.5 7.0 7.0 2.5 7.0 7.0 7.0 7.0 7.0
3.5 9.4 5.0 9.1 7.2 20 30 9.9 9.4
28.0 75.2 40.0 72.8 57.6 160 240 79.2 75.2
68 68 68 68 68 68 68 46 22
[NO]0 is the initial nitrogen monoxide concentration, [NO2]0 is the initial nitrogen dioxide concentration, [HC]0 is the initial hydrocarbon (xylene) concentration, [HC to NOx]0 is the initial hydrocarbon to NOx (NO þ NO2) ratio and UV lamps denotes the number of lamps on.
3. Calculations We analyzed the observed aerosol growth and the source rate of condensable material by using the following equations for the vapour concentration and particle growth (Kulmala et al., 2001). The time dependence of a condensable vapour with concentration C (m3) in air can be presented by (see also Kulmala, 1988; Kulmala et al., 1998)
dC ¼ Q CS C dt
noticing that usually in practise in Eqs. (1) and (4) the vapour concentration C refers to the sum of all condensable vapours. We can obtain the evolution of dp with time, and hence also ddp/dt, through numerical differentiation directly from the size distribution evolutions, enabling a direct estimation of the (total) condensable vapour concentration C by
(1)
where Q (m3 s1) is the source rate and CS (s1) is the condensation sink, which is based on the concept presented by Pirjola et al. (1999). The condensation sink expresses how rapidly molecules will condense on aerosol particles. It depends strongly on the particle size distribution and its inverse is an estimate for the lifetime of the vapour in the atmosphere. We can obtain the condensation sink from measured size distribution by
CS ¼ 2pD
Z
N
X bm;i dp;i Ni dp bm dp n dp ddp ¼ 2pD
0
(2)
i
where D (m2/s) is the diffusion coefficient and the products of the transitional correction factor for mass flux, bm,i, particle diameter, dp,i (m), and number concentration, Ni (m3), are summed over all measured size classes i. We can express the transitional correction factor for mass flux as (Fuchs and Sutugin, 1971)
bm ¼
1 þ Kn 1 þ ð34a þ 0:377 Kn þ 34a Kn2
(3)
where in turn we can express the Knudsen number, Kn ¼ 2l/dp by using the mean free path for air, l (m), and the particle diameter, dp (m). For the mass accommodation coefficient, a, we can use an approximation of unity (Winkler et al., 2004). We can express the rate of change of particle diameter by (Dal Maso et al., 2002)
ddp 4bm mv DC ¼ rdp dt
(4)
where dp (m) is the particle diameter, bm is the transitional correction factor for mass flux, mv (kg) is the molecular mass of the condensable vapour, D (m2 s1) is the diffusion coefficient, and r (kg m3) is the particle density. It is worth
Fig. 2. Particle size distribution as a function of time in experiments A (a) and G (b). Gray scale indicates particle number concentration dN/dlog(dp) and is expressed as cm3.
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using Eq. (4). With C known, we can evaluate dC/dt directly through numerical differentiation. Thus it is possible to estimate also the source rate of condensable vapour Q, by using Eq. (1). By using this method we get both the source rate Q and vapour concentration C as functions of time without the need for unnecessary pseudo-steady state conditions. In the calculations we approximated the diffusion coefficient and molecular mass for the unknown oxidation products with those of xylene: 7.5 106 m2 s1 for the diffusion coefficient and 1.8 1025 kg for the mass of condensing molecules. This approximation is valid and does not make a big error to the calculated values, because the product of the diffusion coefficient and mass of condensing molecules for different compounds is practically almost constant, and the only real parameter is the mass accommodation coefficient (Kulmala et al., 2005). Furthermore, the density of the oxidation products was assumed to be 1.0 103 kg m3, an assumption also used by Song et al. (2005) in their work. The mean free path of air molecules was calculated to be 6.7 108 m in the prevailing conditions (298 K, 1 atm). We calculated the secondary organic aerosol (SOA) yield (Y) by dividing the produced organic aerosol mass DMo (mg/ m3) by the reactive organic gas (ROG) reacted DROG (mg/ m3) (e.g. Odum et al., 1996)
Y ¼
DM o DROG
(5)
However, because we did not measure the concentration of xylene in the gas phase during the experiments and because we do not have knowledge of the chamber wall losses, we were only able to calculate the minimum SOA yields by assuming that all the xylene was consumed in the reactions and no wall losses occurred. 4. Results and discussion We could observe the newly formed particles when they had grown to approximately 15 nm in size and that the particles grew eventually to approximately 100 nm in size, but with different growth rates from one experiment to another, such as, for example, in experiment A (Fig. 2a) with initial HC to NOx ratio of 3.5 ppm/ppm (28 ppmC/ppm) and experiment G (Fig. 2b) with initial HC to NOx ratio of 30 ppm/ppm (240 ppmC/ppm). We also found that the number of UV lamps on affected the growth rate: the more UV irradiation, the faster the particle formation and growth. This is, however, quite obvious and we did not study the effect of UV irradiation level further.
Fig. 3. Geometric mean diameter, (GMD), growth rate (GR), total number and mass concentration (Ntot and mtot, respectively), condensable vapour concentration (C) and its rate of change (dC/dt), condensation sink (CS) and vapour source rate (Q) as a function of time in experiments A (a) and G (b).
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We calculated the geometric mean diameter (GMD), marked as dp in Eqs. (2) and (4), and integrated the particle number and mass concentrations over the measured size range of 7–330 nm, to get the total number and mass concentration, denoted as Ntot and mtot, respectively (e.g. experiments A and G in Fig. 3a and b, respectively), excluding the early stage of each experiment when the number concentration was very low and we could not calculate the GMD accurately. Furthermore, we calculated the particle number diameter growth rate (GR), denoted as ddp/dt in Eq. (4), through numerical differentiation from the size distribution data, the vapour concentration, C, by using Eq. (4), the condensation sink, CS, by using Eq. (2), and the vapour source rate, Q, by using Eq. (1), as a function of time (Fig. 3a and b). For comparison between experiments, we calculated the average values (Table 2) for the growth rate, condensation sink, vapour concentration and its source rate for the time period when the vapour concentration was approximately constant (dC/dt ¼ 0 in Fig. 3a and b). Furthermore, we calculated the SOA yield by using Eq. (5). We observed that vapour concentrations of 1.5–2.4 108 cm3 are needed to sustain the growth rates of 10.6–18.6 nm h1. At the same time, the average condensation sink was 1.4–13 102 s1 and the source rate 0.2–2.5 107 cm3 s1 (Table 2). We compared the particle growth rates and the condensation sinks, concentrations and source rate of condensable vapours that we obtained in the smog chamber experiments to the values obtained in the atmosphere during a particle formation events in a Boreal forest in Hyytia¨la¨, Finland (Kulmala et al., 2001), at a coastal site in Mace Head, Ireland (Dal Maso et al., 2002), and other background stations (Aboa in Antarctica and Va¨rrio¨ in Finnish Lapland) as well as in polluted areas of Athens, Greece, of Marseille, France, and of New Delhi, India (Kulmala et al., 2005). The condensation sink values that we obtained in the laboratory experiments were 2–2200 times the values obtained at the background stations (Aboa, Hyytia¨la¨ and Va¨rrio¨), 0.9–40 times those obtained in the polluted urban areas of Athens and Marseille, the same magnitude as (0.2–2.6 times) those obtained in the highly polluted New Delhi (Kulmala et al., 2005), and 7–65 times those obtained at a coastal site (Dal Maso et al., 2002). The calculated particle growth rates and vapour concentrations were of the same magnitude (within a factor of 2) as in New Delhi, 0.1–2.4 times those at
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a coastal site, and 1.4–16 times those in other urban and background measurement sites, excluding Aboa, where the vapour concentration was 4–60 times lower than in the smog chamber. In chamber experiments the formation of new particles is often seen due to oxidation of organic gases prior to formation of organic vapours with low vapour pressure (e.g. Odum et al., 1997). However, the concentrations of organic vapours in chamber experiments are typically at least 1–3 orders of magnitude higher than at field conditions. For example, in our experiments we obtained concentrations that are present only in heavily polluted areas, such as New Delhi. Finally, the source rates obtained by us were 0.1–2.8 times those in New Delhi, 0.4–13 times those in a coastal area, 1.5–300 times those in other urban areas, and 3–30 000 times those in the background stations. The difference was highest between the smog chamber and Aboa. This suggests that our values for the source rate in the smog chamber correspond to those obtained in urban areas where aromatic hydrocarbons and pollutants, such as NOx, are typically present.
Table 2 Calculated values for experiments A–I for the growth rate (GR), condensation sink (CS), vapour concentration (C) source rate (Q), and secondary organic aerosol yield (Y) Experiment GR CS C Q Y (nm h1) (102 s1) (108 cm3) (107 cm3 s1) A B C D E F G H
18.6 10.6 14.7 15.2 14.2 13.7 12.9 16.2
1.72 8.40 9.82 2.93 12.7 3.27 1.45 2.06
2.36 1.47 2.17 1.84 1.93 1.87 1.67 2.34
0.40 1.23 2.15 0.53 2.46 0.61 0.24 0.48
0.0014 0.0030 0.0045 0.0021 0.0046 0.0012 0.0004 0.0009
Fig. 4. Source rate of condensable vapour (Q) as a function of initial NOx concentration (a) and initial HC concentration (b).
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We analysed the differences in growth and source rates with the gas data and observed that the source rate of condensable vapours depends significantly on the initial NOx concentration but seems to be almost independent on the initial xylene concentration (Fig. 4a and b). Of the NOx dependence we can conclude that oxidation processes play an important role in the growth process of organic aerosols. We also observed that the SOA yield is proportional to the initial NOx concentration, but independent on the initial xylene concentration (Fig. 5a and b). Similar observation was made by Song et al. who noticed that the initial NOx level affects the SOA yield from different isomers of xylene (o-, p- and m-xylene) (Song et al., 2007b) and that nitrogen oxide (NO) suppresses SOA formation (Song et al., 2007c). In our studies, we used higher NOx concentrations than Song et al. and did not observe such suppression of SOA formation, which might have been the case with lower NOx concentrations. It is worth noting that in our studies some uncertainty may arise from the fact that we used a mixture of the xylene isomers in our studies instead of the isomers separately.
5. Summary and conclusions In the present study we compared aerosol formation and growth in smog chamber experiments and in atmospheric conditions from the point of view of vapour concentration and source rate. In a smog chamber with high enough concentrations the organic vapours are able to form new aerosol particles. However, in atmospheric conditions the concentrations are much smaller (at least by a factor of 10, typically more than 100), at least in background areas. We observed that the vapour source rates in the smog chamber are up to four orders of magnitude higher than atmospheric values observed in Aboa and other background stations, up to two orders of magnitude higher than at urban areas of Athens and Marseille, up to 13 times higher than at a coastal site and of the same magnitude, within a factor of five, as in New Delhi, a heavily polluted urban area. In the atmosphere organic compounds probably do not have a major role in the nucleation process itself, which is dominated by other nucleation routes, like the sulphuric acid–ammonia–water system (Kulmala et al., 2000) or activation of existing clusters (Kulmala et al., 2006, 2007). However, the organic compounds contribute predominantly to the subsequent condensational growth process, which sets needs for systematic laboratory experiments with pure organic compounds, as we did in this study. Our systematic measurements with different input concentrations of xylene and nitrogen oxides showed a strong dependence of the source rate of condensable vapours and SOA yield on the initial NOx concentration. This indicates that oxidation processes play an important role in particle formation and subsequent growth.
Acknowledgements The experimental studies were funded by the Finnish Funding Agency for Technology and Innovation (Tekes), which is gratefully acknowledged.
References
Fig. 5. Secondary organic aerosol yield (Y) as a function of initial NOx concentration (a) and initial HC concentration (b).
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