Atmospheric Nucleation Events Research Articles

Ion Mobility-Mass Spectrometry of Iodine Pentoxide-Iodic Acid Hybrid Cluster Anions in Dry and Humidified Atmospheres.

Nanometer-scale clusters form from vapor-phase precursors and can subsequently grow into nanoparticles during atmospheric nucleation events. A particularly interesting set of clusters relevant to nucleation is hybrid iodine pentoxide-iodic acid clusters of the form (I2O5) x(HIO3) y as these clusters have been observed in coastal region nucleation events in anomalously high concentrations. To better understand their properties, we utilized ion mobility-mass spectrometry to probe the structures of cluster anions of the form (I2O5) x(HIO3) y(IOα)- ( x = 0-7, y = 0-1, α = 1-3), similar to those observed in coastal nucleation events. We show that (I2O5) x(HIO3) y(IOα)- clusters are relatively stable against dissociation during mass spectrometric measurement, as compared to other clusters observed in nucleation events over continental sites, and that at atmospherically relevant relative humidity levels (65% and less) clusters can become sufficiently hydrated to facilitate complete conversion of iodine pentoxide to iodic acid but that water sorption beyond this level is limited, indicating that the clusters do not persist as nanometer-scale droplets in the ambient.

Estimating atmospheric nucleation rates from size distribution measurements: Analytical equations for the case of size dependent growth rates

While laboratory and field measurements indicate that atmospheric nucleation most likely initiates with the formation clusters of ~1.0–1.5nm diameter, most atmospheric observations to date measure only particles larger than 3–10nm in size. Because of this, several analytical formulations have been developed to estimate the real nucleation rate at the initial cluster size from the “apparent” nucleation rate at the measured larger sizes. All previous analytical formulations have assumed a constant particle growth rate below the instrument detection limit; however, recent atmospheric measurements have shown that the growth rate is often strongly size dependent. This study presents new analytical equations to connect the real and “apparent” nucleation rates in two special cases, i.e. when the cluster growth rate follows a (1) linear, or (2) power-law dependence on the particle size. The accuracy of these equations is tested with an ensemble of numerical model simulations of new particle formation events. Both new formulations are capable of estimating the nucleation rate at 1.5nm fairly accurately (largest normalised mean bias −1.4% for the power-law and −23% for the linear events). We find, however, that the power law formulation gives a more accurate estimate of the nucleation rate even for a majority of the events with linear growth rate dependence. Further analysis indicates that previous studies of atmospheric nucleation events, which have assumed a constant cluster growth rate, may have clearly underestimated the real nucleation rate.

An amorphous solid state of biogenic secondary organic aerosol particles

Secondary organic aerosol (SOA) particles are formed in the atmosphere from condensable oxidation products of anthropogenic and biogenic volatile organic compounds (VOCs). On a global scale, biogenic VOCs account for about 90% of VOC emissions and of SOA formation (90 billion kilograms of carbon per year). SOA particles can scatter radiation and act as cloud condensation or ice nuclei, and thereby influence the Earth's radiation balance and climate. They consist of a myriad of different compounds with varying physicochemical properties, and little information is available on the phase state of SOA particles. Gas-particle partitioning models usually assume that SOA particles are liquid, but here we present experimental evidence that they can be solid under ambient conditions. We investigated biogenic SOA particles formed from oxidation products of VOCs in plant chamber experiments and in boreal forests within a few hours after atmospheric nucleation events. On the basis of observed particle bouncing in an aerosol impactor and of electron microscopy we conclude that biogenic SOA particles can adopt an amorphous solid-most probably glassy-state. This amorphous solid state should provoke a rethinking of SOA processes because it may influence the partitioning of semi-volatile compounds, reduce the rate of heterogeneous chemical reactions, affect the particles' ability to accommodate water and act as cloud condensation or ice nuclei, and change the atmospheric lifetime of the particles. Thus, the results of this study challenge traditional views of the kinetics and thermodynamics of SOA formation and transformation in the atmosphere and their implications for air quality and climate.

Changes in the production rate of secondary aerosol particles in Central Europe in view of decreasing SO<sub>2</sub> emissions between 1996 and 2006

Abstract. In anthropogenically influenced atmospheres, sulphur dioxide (SO2) is the main precursor of gaseous sulphuric acid (H2SO4), which in turn is a main precursor for atmospheric particle nucleation. As a result of socio-economic changes, East Germany has seen a dramatic decrease in anthropogenic SO2 emissions between 1989 and present, as documented by routine air quality measurements in many locations. We have attempted to evaluate the influence of changing SO2 concentrations on the frequency and intensity of new particle formation (NPF) using two different data sets (1996–1997; 2003–2006) of experimental particle number size distributions (diameter range 3–750 nm) from the atmospheric research station Melpitz near Leipzig, Germany. Between the two periods SO2 concentrations decreased by 65% on average, while the frequency of NPF events dropped by 45%. Meanwhile, the average formation rate of 3 nm particles decreased by 68% on average. The trends were statistically significant and therefore suggest a connection between the availability of anthropogenic SO2 and freshly formed new particles. In contrast to the decrease in new particle formation, we found an increase in the mean growth rate of freshly nucleated particles (+22%), suggesting that particle nucleation and subsequent growth into larger sizes are delineated with respect to their precursor species. Using three basic parameters, the condensation sink for H2SO4, the SO2 concentration, and the global radiation intensity, we were able to define the characteristic range of atmospheric conditions under which particle formation events take place at the Melpitz site. While the decrease in the concentrations and formation rates of the new particles was rather evident, no similar decrease was found with respect to the generation of cloud condensation nuclei (CCN; particle diameter >100 nm) as a result of atmospheric nucleation events. On the contrary, the production of CCN following nucleation events appears to have increased by tens of percents. Our aerosol dynamics model simulations suggest that such an increase can be caused by the increased particle growth rate.

The role of VOC oxidation products in continental new particle formation

Abstract. Aerosol physical and chemical properties and trace gas concentrations were measured during the QUEST field campaign in March–April 2003, in Hyytiälä, Finland. Our aim was to understand the role of oxidation products of VOC's such as mono- and sesquiterpenes in atmospheric nucleation events. Particle chemical compositions were measured using the Aerodyne Aerosol Mass Spectrometer, and chemical compositions of aerosol samples collected with low-pressure impactors and a high volume sampler were analysed using a number of techniques. The results indicate that during and after new particle formation, all particles larger than 50 nm in diameter contained similar organic substances that are likely to be mono- and sesquiterpene oxidation products. The oxidation products identified in the high volume samples were shown to be mostly aldehydes. In order to study the composition of particles in the 10–50 nm range, we made use of Tandem Differential Mobility Analyzer results. We found that during nucleation events, both 10 and 50 nm particle growth factors due to uptake of ethanol vapour correlate strongly with gas-phase monoterpene oxidation product (MTOP) concentrations, indicating that the organic constituents of particles smaller than 50 nm in diameter are at least partly similar to those of larger particles. We furthermore showed that particle growth rates during the nucleation events are correlated with the gas-phase MTOP concentrations. This indicates that VOC oxidation products may have a key role in determining the spatial and temporal features of the nucleation events. This conclusion was supported by our aircraft measurements of new 3–10 nm particle concentrations, which showed that the nucleation event on 28 March 2003, started at the ground layer, i.e. near the VOC source, and evolved together with the mixed layer. Furthermore, no new particle formation was detected upwind away from the forest, above the frozen Gulf of Bothnia.

Estimating nucleation rates from apparent particle formation rates and vice versa: Revised formulation of the Kerminen–Kulmala equation

Connections between observed particle formation rates (typically at diameter 3 nm or larger) and the actual nucleation rates have important applications in atmospheric science. First, nucleation theories can be evaluated and second, semi-empirical particle formation rates can be developed for large scale models that neglect the cumbersome initial steps of formation and growth. Kerminen and Kulmala, by estimating the particle formation rate, nucleation mode growth rate and scavenging rate onto background particles (coagulation sink) from measured size distribution evolution, derived a simple yet rather accurate formula for this purpose [Kerminen V.-M., Kulmala, M. (2002). Analytical formulae connecting the “real” and the “apparent” 25 nucleation rate and the nuclei number concentration for atmospheric nucleation events, Journal of Aerosol Science 33, 609–622]. The present work reformulates the original theory in a way that two drawbacks are eliminated: (1) the original expression was derived using a slightly inaccurate coagulation sink dependence on particle size and (2) was based on knowing the condensation sink which requires knowledge of the condensing vapors.

‘How to find bananas in the atmospheric aerosol’: new approach for analyzing atmospheric nucleation and growth events

‘How to find bananas in the atmospheric aerosol’: new approach for analyzing atmospheric nucleation and growth events

‘How to find bananas in the atmospheric aerosol’: new approach for analyzing atmospheric nucleation and growth events

We have devised a new search algorithm for secondary particle formation events, or ‘nucleation events’ in data sets of atmospheric particle size distributions. The search algorithm is simple and based on the investigation of 18 integral parameters of the particle size distribution, three of which were found to be most relevant for identifying nucleation events. The algorithm is tested using long-term size distribution data sets of high-size resolution observed at Melpitz, Hohenpeissenberg, and Leipzig, Germany, and Beijing, China, thereby covering a wide range of clean and polluted conditions. By specifying the particular training sets, the method can be used by other researchers with different data sets or different research goals. The same search approach could be applied to identify and analyze other systematic changes in size distribution such as during frontal passages or sand storms. As an example application of the new algorithm, the 50 strongest nucleation events (‘bananas’) at each of the four sites are analyzed statistically in terms of average changes of integral parameters of the particle size distribution.

Hygroscopicity and volatility of 4–10 nm particles during summertime atmospheric nucleation events in urban Atlanta

Continuous measurements of hygroscopicity and volatility of atmospheric aerosol particles of 4–10 nm diameter were conducted with a nanometer tandem differential mobility analyzer (Nano TDMA) during the Aerosol Nucleation and Real‐time Characterization Experiment (ANARChE), which took place in Atlanta in July and August 2002. In the Nano TDMA measurements, particles were exposed to either a high humidity (∼90% RH) or an elevated temperature (∼100°C) downstream of the first differential mobility analyzer (DMA) and were then resized by the second DMA to determine the change in size due to water uptake or evaporation. There were several days when nucleation occurred and high concentrations of sub‐20 nm particles were observed, and during those events, particles of 4–10 nm diameter were very hygroscopic and nonvolatile. These observations, together with parallel Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) measurements of sub‐20 nm particle composition, suggest that the particles were mostly composed of ammoniated sulfates. This finding strongly supports the hypothesis that the nucleation and subsequent growth of nanoparticles were driven by reactions involving sulfuric acid and ammonia during this study.

Nanoparticle formation by ozonolysis of inducible plant volatiles

Abstract. We present the first laboratory experiments of aerosol formation from oxidation of volatile organic species emitted by living plants, a process which for half a century has been known to take place in the atmosphere. We have treated white cabbage plants with methyl jasmonate in order to induce the production of monoterpenes and certain less-volatile sesqui- and homoterpenes. Ozone was introduced into the growth chamber in which the plants were placed, and the subsequent aerosol formation and growth of aerosols were monitored by measuring the particle size distributions continuously during the experiments. Our observations show similar particle formation rates as in the atmosphere but much higher growth rates. The results indicate that the concentrations of nonvolatile oxidation products of plant released precursors needed to induce the nucleation are roughly an order-of-magnitude higher than their concentrations during atmospheric nucleation events. Our results therefore suggest that if oxidized organics are involved in atmospheric nucleation events, their role is to participate in the growth of pre-existing molecular clusters rather than to form such clusters through homogeneous or ion-induced nucleation.

Cloud condensation nucleus production from nucleation events at a highly polluted region

Atmospheric nucleation events have received increasing attention as a potentially important source of aerosol particles affecting climate and human health. Here, we report 2.5 years of continuous nucleation observations from San Pietro Capofiume, Italy (44°39′N, 11°37′W). We show that nucleation events occur frequently in the Po Valley region, even though the region is rather polluted with high pre‐existing particle concentrations. The nucleation events are often very intensive, and the newly formed particles can grow to sizes as large as 100–200 nm in diameter within a few hours. Our calculations of the enhancement of particle number concentrations in different size ranges and comparison with steady‐state and regional emission estimates indicate that the nucleation events produce a significant fraction of particles larger than 100 nm, and thus constitute an important source of cloud condensation nuclei (CCN).

Observations of particle formation and growth in a mountainous forest region in central Europe

In the summers of 2001 and 2002, particle formation and growth were observed at a forest ecosystem research site in the “Fichtelgebirge” mountain range in NE Bavaria, Germany. Atmospheric nucleation events, identified through detailed analysis of the time evolution of submicron particle size distributions, differed considerably from nonevent days with respect to ultrafine particle concentrations and meteorological parameters. Particle diameter growth rates, as quantified through the geometric mean diameter development of the 3–60 nm particle fraction, ranged from 2.2 to 5.7 nm h−1. While H2SO4 concentrations typically explained less than 10% of the observed growth rates, a significant fraction of particle growth may be explained through condensation of organic vapors from α‐pinene oxidation. On several days, cocondensation of H2SO4 and α‐pinene oxidation products was sufficient to fully explain the observed growth dynamics. While BVOC emissions from the tree vegetation may contribute to particle formation, the forest also acts as an effective sink for particles. This is reflected in the dominance of particle deposition to the forest over emission, with the strongest deposition fluxes occurring during particle formation events.

Nucleation rate and vapor concentration estimations using a least squares aerosol dynamics method

A novel method for the determination of particle formation rates and condensable vapor concentrations for atmospheric nucleation events is presented that is based on combining least squares optimization with aerosol dynamics. Tests of the method with simulated events show that the method is able to capture the temporal behavior of formation and growth with reasonable accuracy. Example analyses have been performed for observed particle formation bursts in Hyytiälä, Finland. The results agree with previous analyses that are, in principle, based on analyzing size distribution contour plots by eye. However, the presented method also reveals detailed temporal behavior of the important processes, which is an important improvement compared to the previous methods.

Analytical formulae connecting the “real” and the “apparent” nucleation rate and the nuclei number concentration for atmospheric nucleation events

A simple yet relatively accurate analytical formula has been derived between the “real” atmospheric nucleation rate and the rate at which the resulting clusters, or nuclei, appear at some larger size (the “apparent” nucleation rate) as a result of their growth by condensation. In addition, another analytical formula was derived that connects the nucleation rate and the total nuclei concentration in a desired size range. The derived formulae are applicable to situations in which there are no major fluctuations in the pre-existing particle size distribution or in the concentration of vapours responsible for the nuclei growth, and in which the total nuclei number concentration remains sufficiently low (<10 5–10 6 nuclei cm −3) to prevent effective self-coagulation between the nuclei. With the help of these formulae, an explicit nucleation scheme can be included into an atmospheric model without the requirement that the modelled particle diameter range must be extended down to one nanometer. In field measurements only the “apparent” nucleation rate can currently be determined. The derived formulae provide a means to convert this rate to a “real” nucleation rate, making it possible to test more rigorously the viability of different nucleation theories under atmospheric conditions.

How significantly does coagulational scavenging limit atmospheric particle production?

A model study was performed to investigate the initial fate of nanometer‐size nuclei formed in the atmosphere by homogeneous nucleation. Two important results were obtained: (1) coagulational scavenging into preexisting aerosol particles is an extremely strong sink for nuclei formed recently in the atmosphere, and (2) the growth of fresh nuclei to detectable sizes and above requires in most cases the presence of nonvolatile condensable vapors other than H2SO4(g). These vapors are probably secondary organics, and their total required concentration level ranges from &lt;107 up to 108 molecules cm−3 depending on the preexisting particle population. The fact that newly formed particles are scavenged significantly or even totally before reaching detectable sizes has important consequences when interpreting atmospheric measurements. First, atmospheric nucleation events are probably much more frequent than what has been believed based on earlier observations, and second, the nucleation rates derived from atmospheric observations are likely to be substantially lower than the rates at which new particles are really formed by nucleation. Since newly formed nuclei will be scavenged away unless they grow sufficiently fast, there must be some lower limit for the nuclei growth rate below which the nucleation event will not be recorded using the current measurement techniques. The mimimum growth rate was estimated to be below 1 nm h−1 in “clean” environments and of the order of several nm h−1 in “polluted” environments. This requirement may explain why observed atmospheric particle formation events are accompanied frequently by the growth of new particles to sizes typical of the Aitken mode.