Formation and growth rates of ultrafine atmospheric particles: a review of observations
Introduction
Aerosol particles are ubiquitous in the Earth's atmosphere and influence our quality of life in many different ways. In urban environments, aerosol particles can affect human health through their inhalation (Wichmann & Peters, 2000; Stieb, Judek, & Burnett, 2002). In a global troposphere, and particularly downwind from major pollution sources, aerosol particles are thought to contribute to climate change patterns (Stott et al., 2000; Ramanathan, Crutzen, Kiehl, & Rosenfeld, 2001; Yu, Saxena, & Zhao, 2001; Menon, Del Genio, Koch, & Tselioudis, 2002). Understanding these effects requires detailed information on how aerosol particles enter the atmosphere and how they are transformed there before being removed by dry or wet deposition. Key processes in this respect are the formation of new atmospheric particles and their subsequent growth to larger sizes.
Aitken (1897) was the first to report evidence for new particle formation in the atmosphere. However, quantitative measurements of aerosol formation and growth rates have required the recent developments in instrumentation for measuring size distributions down to sizes as small as in diameter (McMurry, 2000a). We refer to the 3– particles as the “nucleation mode” (called sometimes also the ultrafine mode), since nucleation and growth from gaseous precursors leads to the formation of such very small particles. Other particle modes that have been previously documented are the Aitken nuclei (20–), accumulation (90–) and coarse (particles in diameter) modes.
Many studies conducted in the free troposphere, and especially near clouds and close to the tropopause, have detected large numbers of very small, 3– diameter aerosol particles (e.g. Hoffman, 1993; Perry & Hobbs, 1994; Hoppel, Frick, Fitzgerald, & Larson, 1994; Clarke et al., 1998b; Clarke, Kapustin, Eisele, Weber, & McMurry, 1999a; Clarke et al., 1999b; Nyeki et al., 1999; Keil & Wendisch, 2001; Weber et al., 2001b; Twohy et al., 2002). In the continental boundary layer, there are frequent observations of recent nucleation events, i.e. the formation of ultrafine particles detected at a few nm, accompanied by the subsequent growth of these particles to within the next 1–2 days. Such observations span from the northernmost sub-arctic Lapland to the remote boreal forest (Kulmala, Toivonen, Mäkelä, & Laaksonen, 1998; Mäkelä et al., 1997) to suburban Helsinki (Väkevä et al., 2000), to urban Atlanta, Pittsburgh and St. Louis (Woo, Chen, Pui, & McMurry, 2001; Stanier, Khlystov, & Pandis, 2002; Shi and Qian, 2003), to industrialised agricultural regions in Germany (Birmili & Wiedensohler, 2000a; Birmili et al., 2003) and also to coastal environments around Europe (O'Dowd et al., 1999). Nucleation has been observed with monitors on mountains (Weber, McMurry, Eisele, & Tanner, 1995; Weber, Marti, McMurry, Eisele, Tanner, & Jefferson 1996, Weber, Marti, McMurry, Eisele, Tanner, & Jefferson 1997), and evidence for the role of biogenic emissions in aerosol formation has also been reported (Kavouras, Mihalopoulos, & Stephanou, 1998; Weber et al., 1998). A limitation of most observations is that measurements were either made at a fixed point (ground), or on platforms not necessarily moving along with the same air parcel. Observations of new particle formation may therefore be biased by spatial variations of constituents in different air parcels.
A variety of different nucleation mechanisms have been proposed for the atmosphere. The most widely studied ones are the binary water–sulphuric acid nucleation (e.g. Kulmala & Laaksonen, 1990), ternary water–sulphuric acid–ammonia nucleation (Kulmala et al., 2000c) and ion-induced nucleation (Yu & Turco, 2000). A technique is available for measuring sulphuric acid vapour, and such measurements have been reported for a few nucleation studies. Techniques for measuring ammonia with high time resolution at ppt levels are now becoming available, but measurements of ammonia during nucleation events are rare (e.g. Berresheim et al., 2002). Organic vapours could, in principle, participate in nucleation, but nucleation mechanisms that involve organics have not yet been identified. It appears very likely, however, that organics contribute to growth of nucleated particles (O'Dowd, Aalto, Hämeri, Kulmala, & Hoffmann, 2002b). In practise it is very important to investigate nucleation and growth processes separately, since different species can participate in these processes.
In this review we summarise recent observations of particle formation and growth. Altogether these measurements span a broad range of both geographical locations and ambient conditions. Where possible, we report the formation rate of particles, because is the current minimum detectable size. Some studies involved the use of instruments with a minimum detectable size that is larger than . In such cases we estimated particle formation rates at the minimum detectable size. Growth rates can also be determined from measured nucleation mode size distributions.
There are several studies in which there is clear evidence on aerosol formation but no quantitative estimation of particle production rates is possible (e.g. Aitken, 1897). An ideal situation in this regard is when continuous size distribution measurements of particles are available. This is the case at the SMEAR II station in Finland (Kulmala et al., 2001) and at several U.S.E.P.A supersites, including those in Atlanta (Woo et al., 2001), Pittsburgh (Stanier et al., 2002) and St. Louis (Shi and Qian, 2003). Such data enable the determination of both particle formation and growth rates.
Section snippets
On observations
In this study we review more than 100 publications that report observations of ultrafine particles in the atmosphere. The studies included are presented in Table 1, from which one can see the number of each paper (to be used later), the authors, and the location (latitude, longitude, name of the place) and the measurement time period. A global map showing the measurement locations is presented in Fig. 1. As can be seen, measurements have been performed all over the world, even though Europe and
Instrumentation
Studies of atmospheric particle formation and growth require measurements of nucleation mode particles . Simultaneous measurements of nucleating gases can provide further insights into mechanisms. Here, we give a brief summary of the relevant methods, their characteristics, and limitations. For more detailed and historical aspects of aerosol measurement technology, the reader is referred to the rich body of literature on the subject (e.g. McMurry 2000a, McMurry 2000b; Flagan, 1998).
Estimation of the particle formation and growth rates
Critical clusters formed by atmospheric nucleation events cannot yet be measured quantitatively due to instrumental limitations. Only one measurement of clusters during nucleation events has been reported, and it showed that clusters were present when 2.7– particles were detected (Weber et al., 1995). More work on the distribution and composition of such clusters is needed to refine our understanding of atmospheric nucleation.
Because critical clusters cannot yet be measured, we are unable to
Discussion
The annual variations in growth rates during regional nucleation events in the Hyytiälä forest (Mäkelä et al., 2000), rural Hohenpeissenberg (Birmili et al., 2003), and urban St. Louis (Shi and Qian, 2003) are shown in Fig. 4. Note that in all locations the growth rates during the summer range from 4 to . The growth rates during the winter are considerably lower (0.5–). The GR data in Table 2 show qualitatively that the rates are significantly lower at the poles than at
Summary and conclusions
The formation and growth of new aerosol particles is of growing interest due to their climatic and health effects. The question “How and under what conditions does new particle formation occur?” has exercised the minds of meteorologists and physicists since the time of John Aitken, who in the late 1880s built the first apparatus to measure the number of dust and fog particles. However, only during the last 10 years has the measurement technology developed to such a level that size distributions
Acknowledgements
The authors are grateful to several groups collecting and publishing the data. Particularly we would like to thank Dr. E.D. Nilsson, Mrs. S.-P- Malvikko, Mr. M. Savimaa, Dr. I.K. Koponen and Mr. Pasi Aalto for their help. We acknowledge financial support by the Academy of Finland (Project No. 47668). WB acknowledges support by EU Marie Curie grant EKV4-CT-2000-50002. PHM acknowledges support from the U.S. DOE through Grant No. DE-FG02-98R62556 and EPA through Grant No. R829620, and the
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