Anal. Chem. 2010, 82, 7871–7878
New Particle Formation and Growth in the Troposphere† Bryan R. Bzdek and Murray V. Johnston University of Delaware
Atmospheric aerosols have deleterious effects on visibility, human health, and global climate. With respect to global climate, the dominant effect is cooling, either directly through the scattering of incoming solar radiation or indirectly by serving as cloud condensation nuclei (CCN) that increase cloud albedo and influence precipitation patterns.1,2 The current level of scientific understanding of these effects on climate is relatively low, especially compared to the heating caused by greenhouse gases.3 An improved understanding of these processes, especially the indirect effect, is essential to increase the accuracy of predicting future changes in climate. The opening figure summarizes the formation of CCN and their impact on Earth’s energy balance. Airborne particles serve as CCN if they grow into cloud droplets when exposed to an air mass supersaturated with water vapor. Though growth is dependent on particle size, composition, and the degree of supersaturation, most particles above ∼100 nm in diameter are thought to be able to serve as CCN.4 Airborne particles in the size range relevant to CCN can be divided into two main categories: primary particles and secondary particles arising from new particle formation (NPF) processes. Primary particles are emitted directly into the atmosphere by natural processes such as sea spray and volcanic action or by anthropogenic processes such as combustion. Primary particles that are too small to serve directly as CCN may grow into the relevant size range by condensation of secondary chemical species such as sulfate, nitrate, and organic compounds that have been partially oxidized in the atmosphere. For many years, it was thought that primary particles were the predominant source of CCN; however, NPF recently has been recognized as a † Part of the special issue “Atmospheric Analysis as Related to Climate Change”.
10.1021/ac100856j 2010 American Chemical Society Published on Web 07/16/2010
ROBERT GATES
The formation and growth of nanoparticles in the atmosphere has important implications for human health and global climate. Understanding the chemistry behind these processes requires development and deployment of innovative analytical methods.
potentially significant contributor to CCN.5-10 NPF occurs when gas phase species in the atmosphere come together to form new particles in the low nanometer size range that subsequently grow into the CCN size range. A recent estimate suggests that 45% of global CCN are derived from NPF.10 However, there are substantial uncertainties associated with the rates of both primary particle emission and NPF, and these uncertainties lead to an even greater uncertainty in the contribution of CCN to global climate.11 For this reason, the relationship between NPF and climate is a rapidly evolving area of study. Analytical Chemistry, Vol. 82, No. 19, October 1, 2010
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Figure 1. Particle size distribution during new particle formation in Pittsburgh, Pennsylvania on 12 September 2002.16
NEW PARTICLE FORMATION NPF is a global phenomenon, with ∼35% of CCN arising from NPF in the free troposphere and ∼10% from NPF in the boundary layer, mostly over land.10 NPF frequently occurs over the continental boundary layer in regional events that extend hundreds of kilometers.12,13 Localized NPF occurs in urban and industrial plumes, as well as in coastal marine locations. Particle size distributions during NPF events are usually obtained with a scanning mobility particle sizer (SMPS). The SMPS consists of a differential mobility analyzer (DMA), which size selects singly charged particles based on electrical mobility diameter (defined as the diameter of a unit density sphere having the same electrical mobility as the particle of interest), and a condensation particle counter (CPC), which grows particles in a supersaturated vapor to a size that permits individual particles to be detected by light scattering.14 With this combination, the lower size limit for efficient detection of particles is typically 3 nm diameter. Therefore, NPF events are arbitrarily defined by the abrupt appearance of g3 nm diameter particles followed by rapid growth.13 In this article, sub-3 nm diameter particles are referred to as molecular clusters, whereas larger particles are referred to as nanoparticles. NPF is illustrated in Figure 1, in which the SMPS-measured particle size distribution is plotted as a function of time on a sunny day in Pittsburgh, Pennsylvania.15,16 Early in the morning, particle formation is observed. These particles continue to grow the remainder of the day, with many reaching the CCN size range. The shape of the plot in Figure 1 represents an “ideal” case in which NPF occurs quickly over a wide spatial region and is followed by rapid growth. The shape can be distorted if, for example, inhomogeneous air parcels mix or particle formation is fast but growth is slow. Almost all NPF events reported in the scientific literature have been observed during the daytime, suggesting that photochemistry plays an important role in the process.12,13 Recent technological advances have made it possible to mobility analyze and detect sub-3 nm diameter clusters.17 These measurements have shown that a relatively stable pool of clusters in the 1-2 nm diameter size range exists in ambient air at a number concentration of 103-104 cm-3. Though the composition of these clusters has not yet been determined, the likely chemical components can be inferred from knowledge of condensable species in the gas phase. In most ambient observations, the nucleation rate is strongly correlated with the sulfuric acid vapor concentration, and nucleation appears to be a kinetically controlled process in which the critical cluster contains two sulfuric acid molecules.18 However, the precise constituents of the critical cluster are unclear because several 7872
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mechanisms for cluster formation and growth are possible. The predominant cluster formation mechanism is believed to be a ternary process involving sulfuric acid, water, and ammonia.19-22 Other possible mechanisms include sulfuric acid-water binary nucleation,19,23-25 ion-induced nucleation,26 nucleation via halogen species,27 and condensation of organic vapors.28 Recent fieldwork suggests that carboxylic acids and organic bases (e.g., amines) may participate in cluster growth.29,30 The relatively constant ambient concentration of these clusters means that formation and loss rates are similar. NPF events such as the one in Figure 1 can occur by activation of the existing cluster pool17 or by rapid formation of new clusters during activation.31 Activation requires that the growth rate associated with condensation of gas phase species exceeds the loss rate due to processes such as evaporation or scavenging onto pre-existing particles. These conditions are most often met when 1) the concentrations of condensable gases rapidly increase, for example by photochemical processes during the daytime, and/ or 2) the pre-existing particle concentration is low, which both favors condensation of gases onto newly formed nanoparticles over larger (and higher surface area) pre-existing particles and decreases the probability of nanoparticle scavenging. Growth rates during NPF events are initially on the order of 1-20 nm/hour, and as a result, particles can reach a size appropriate for CCN activity within one or a few days.9 If the particle survives this time period, then it contributes to CCN activity. An analysis of several NPF events suggests that 1-20% of the particles in an event eventually become CCN, which increases the CCN concentration above its pre-existing value by a factor of 2-9, with a mean enhancement factor of 3.8.9 ANALYTICAL CHALLENGES ASSOCIATED WITH STUDYING NPF Our understanding of NPF and its impact on CCN concentration are based mostly on measurements of particle size distribution with time. Though these measurements show us how NPF proceeds, they give no information on the chemical mechanisms by which nucleation and growth occur. Understanding chemical mechanisms requires the development of analytical methods capable of detecting and characterizing the small amount of matter in these particles. In this section, we illustrate the difficulties associated with chemical analysis of ambient clusters and nanoparticles by analogy with well-known analytical methods. Because a high degree of molecular specificity must be determined for an exceedingly small sample size, our discussion focuses on mass spectrometry (MS). Indeed, the most successful approaches
described in the literature for ambient nanoparticle chemical analysis involve MS. Ambient particles in the 1-10 nm diameter size range are similar in dimension to individual biomolecules such as peptides or proteins. It is informative to compare the analysis of each by sampling through an atmospheric pressure inlet into a mass spectrometer. A highly sensitive analysis by electrospray ionization (ESI) might involve spraying a 1 nM solution at a submicroliter per minute flow rate. Such a source has the potential to produce 1013 particles (individual biomolecules) per second into the air being sampled through the inlet. In contrast, formation rates for the sub-3 nm diameter clusters are typically
