Pre-existing Particle Surface Area Research Articles

Statistical analysis of 4-year observations of aerosol sizes in a semi-rural continental environment

We have made long-term observations of particle size distributions in a semi-rural continental environment of the United States (Kent, Ohio) over a 4-year period, to investigate the seasonal trend of new particle formation (NPF). NPF events occurred throughout the year, but more frequently during the spring (∼40%) and fall (∼32%) than in the summer (∼19%) and winter (∼17%). The particle nucleation rates were also highest in the spring (with the median and standard deviation value of 6.8 ± 1.3 cm−3 s−1) and lowest in the winter (1.0 ± 0.9 cm−3 s−1). Particle growth rates were highest in the summer (6.4 ± 0.4 nm h−1) and lowest in the winter (4.2 ± 0.3 nm h−1), a trend different from nucleation rates, indicating that different chemical species may be involved in particle nucleation and growth processes. Backward trajectory calculations show that NPF events were often associated with air masses originated from the east–southeast where numerous large size power plants are located, containing high sulfur dioxide (SO2). The NPF events in general had lower relative humidity, lower surface area of pre-existing particles, and higher solar radiation.

Ion‐mediated nucleation in the atmosphere: Key controlling parameters, implications, and look‐up table

Nucleation is an important source of atmospheric particles and ubiquitous ions in the atmosphere have long been known to promote nucleation. An ion‐mediated nucleation (IMN) mechanism based on a kinetic model is supported by recent measurements of the excess charge on freshly nucleated particles and ion cluster evolution during nucleation events. Here we investigate the dependence of steady state IMN rate (JIMN) on key controlling parameters. We find that sulfuric acid vapor concentration, temperature, relative humidity, ionization rate, and surface area of preexisting particles have profound and nonlinear impacts on JIMN. The sensitivities of JIMN to the changes in these key parameters may imply important physical feedback mechanisms involving climate and emission changes, solar variations, nucleation, aerosol number abundance, and aerosol indirect radiative forcing. We also describe a five‐dimensional JIMN look‐up table derived from the most recent version of the IMN model, with the key parameters covering a wide range of atmospheric conditions. With the look‐up table and a multiple‐variable interpolation subroutine, JIMN and the properties of critical clusters can be determined efficiently and accurately under given atmospheric conditions. The look‐up table reduces the computational costs of the IMN rate calculations significantly (by a factor of around 8000) and can be readily incorporated into multidimensional models to study the secondary particle formation via IMN and associated climatic and health effects.

Atmospheric nanoparticle observations in the low free troposphere during upward orographic flows at Izaña Mountain Observatory

Abstract. This study investigates the processes and conditions favouring the formation of nanoparticles (diameter<10 nm) which are frequently observed on high mountains reaching the low free troposphere. This was done through an analysis of a data set collected at Izaña Global Atmospheric Watch Observatory (Canary Islands; 2367 m above sea level). This high mountain supersite is located well above the stratocumulus layer characteristic of the subtropical oceanic tropospheres. At night, when the catabic flow regime is well established, free troposphere aerosols were measured. The development of orographic buoyant upward flows during daylight resulted in an increase of water vapour, SO2 and NOy concentrations. These ascending airflows perturbed the free troposphere and resulted in high concentrations of 3–10 nm particles (N3–10) due to new particle formation. An analysis of the 5-min average time series allowed the identification of two main types of N3–10 event. In Type I events a linear relationship between N3–10 and SO2 was observed (r2 coefficients 0.70–0.95 and a mean slope of 11 cm−3 ppt−1 for 5-min averaged data; SO2 concentrations from tens to hundreds of ppt). These particles seem to be formed during upward transport (probably within or after the outflows of clouds typically located below Izaña). During Type II events, no correlation between SO2 and N3–10 was observed and 3–10 nm particles were formed in-situ at noon and during the afternoon due to the condensation of vapours linked to photochemistry. New particle formation was observed almost every day owing to the favourable conditions associated with the entry of boundary layer air in the low free troposphere, even if SO2 concentrations are rather low at Izaña (tens to hundreds of ppt). The low surface area of pre-existing particles, low temperature and high radiation intensity clearly favoured the formation of nanoparticles. The low surface area of pre-existing particles in the upward flows is furthered by in-cloud particles scavenging in the stratocumulus layer typically located below Izaña. The higher temperature and the presence of coarse Saharan dust particles decrease the efficiency of the new particle formation mechanisms in summer. Thus, the "N3–10 versus SO2" slope (for r2>0.7 cases) was higher in autumn and winter (~15 cm−3 ppt−1 as average) than in summer (2–8 cm−3 ppt−1). These field observations suggest that elevated mounts that reaches the free troposphere may act as source regions for new particles.

High temporal resolution measurements of roadside particle size distributions and their implications for exposure

High time resolution, size segregated studies of ultrafine particle number concentrations (PNC) were performed at two UK roadside locations using a DMS500 particle spectrometer including sampling with and without a thermodenuder. Intercomparisons between the DMS500, an SMPS and a CPC, indicate that the SMPS can underestimate PNC due to the transient nature of excursions in concentrations and the longer scan time of the instrument. Short duration transient periods of high number concentrations made considerable contributions to hourly average PNC. The transient nature of changes in PNC potentially affects the type of exposure experienced in the roadside environment, as well as indoor outdoor exchange. The transient shifts were dominated by Aitken mode particles during high traffic periods, with the Aitken mode comprised of internally mixed particles with a low volatility core (thought to be primarily soot) and a surface of condensed and absorbed material. Transient periods of high nucleation mode PNC were also found, but were relatively more important during night-time and afternoon periods, due to secondary formation processes and lower concentrations of pre-existing particles. The thermodenuder studies indicated the presence of highly volatile liquid droplets in the nucleation mode which were lost on heating. Transient events and complexities in particle mixing state within the ultrafine fraction, coupled with discrepancies between instruments, make the potential adoption of a total number based air quality standard more complicated than first appears. Nucleation mode particle numbers are highly dependant on atmospheric and traffic conditions as well as pre-existing particle surface area and may be toxicologically less significant if they are predominantly liquid droplets. Aitken mode particles have been shown in previous studies to have higher toxicological significance due to their chemical composition. Their hourly average PNC correlated well with PM2.5 and estimated PM1 with lower significance of equivalent correlation of nucleation mode particles.

A review of particle formation events and growth in the atmosphere in the various environments and discussion of mechanistic implications

This review highlights recent observations from a large number of studies investigating formation and growth within different environments and discusses the importance of various mechanisms of particle formation and growth between the different environments. Whilst, several mechanisms for new particle formation which proposed the importance of each mechanism are still the centre of much debate. Proposed nucleation mechanisms include condensation of a binary mixture of sulphuric acid and water; ternary nucleation of sulphuric acid, water and a third molecule, most likely ammonia; ion-induced nucleation; secondary organic aerosol formation involving condensation of low- or non-volatile organic compounds and homogeneous nucleation of iodine oxides. Laboratory and modelling studies have shown these mechanisms can occur in the atmosphere although the contribution depends on the concentrations of precursor compounds present. In addition, atmospheric particle formation events are significantly affected by environmental factors, such as temperature, humidity and the surface area of pre-existing particles, which is also discussed here. One major problem hampering our current understanding is that these new particles are smaller than the lower size detection limit of most instruments and are only observed after some particle growth has occurred. Particles growth occurs through condensation of supersaturated vapours on the surface of the nucleated particles. This requires a lower degree of supersaturation than nucleation and thus condensation of the nucleating species reduces the rate of particle formation. Therefore, it is believed that particle growth often occurs through the condensation of other gases, including organic and inorganic compounds, than those responsible for nucleation. This decoupling of nucleation and growth means that the individual gases responsible for nucleation and growth can be unclear. Since observations of particle formation only occur following growth to observable sizes it is possible that a pool of undetectable particles exist at all times but are only observed following significant condensational growth.

Binary H2SO4‐H2O homogeneous nucleation based on kinetic quasi‐unary nucleation model: Look‐up tables

Recently, we have shown that the binary homogeneous nucleation (BHN) of H2SO4 and H2O can be treated as a quasi‐unary nucleation (QUN) of H2SO4 in the equilibrium with H2O vapor and have developed a kinetic H2SO4‐H2O nucleation model, which simulates the dynamic evolutions of cluster distributions explicitly. In this paper we present improved and updated version of the QUN model, which has been modified through the incorporation of more rigorous thermodynamics of the cluster formation and the effect of the surface area of preexisting particles on nucleation rates. The difference in the model predictions of the improved and original QUN models is not dramatic; however, the improved QUN model is more consistent thermodynamically. The predicted BHN rates based on the QUN model are in good agreement with existing observations within the range of uncertainties. We also study the dependence of kinetic BHN rates on key input parameters and discuss the conditions in which the H2SO4‐H2O homogeneous nucleation in the atmosphere may become significant. The nucleation rates look‐up tables derived from the QUN model, which can be readily used to find the nucleation rates at given conditions, are presented. The look‐up tables cover a wide range of key parameters that can be found in the ambient atmosphere, in the laboratory studies, and in the exhaust plumes. These look‐up tables can be easily incorporated in multidimensional models to predict BHN rates, and they can be utilized directly for the analysis and interpretation of the observed nucleation events.

The Hohenpeissenberg aerosol formation experiment (HAFEX): a long-term study including size-resolved aerosol, H<sub>2</sub>SO<sub>4</sub>, OH, and monoterpenes measurements

Abstract. Ambient aerosol size distributions (>3 nm) and OH, H2SO4, and terpene concentrations were measured from April 1998 to August 2000 at a rural continental site in southern Germany. New particle formation (NPF) events were detected on 18% of all days, typically during midday hours under sunny and dry conditions. The number of newly formed particles correlated significantly with solar irradiance and ambient levels of H2SO4. A pronounced anti-correlatation of NPF events with the pre-existing particle surface area was identified in the cold season, often associated with the advection of dry and relatively clean air masses from southerly directions (Alps). Estimates of the particle formation rate based on observations were around 1 cm-3 s-1, being in agreement with the predictions of ternary homogeneous H2SO4-NH3-H2O nucleation within a few orders of magnitude. The experimentally determined nucleation mode particle growth rates were on average 2.6 nm h-1, with a fraction of 0.7 nm h-1 being attributed to the co-condensation of H2SO4-H2O-NH3. The magnitude of nucleation mode particle growth was neither significantly correlated to H2SO4, nor to the observed particle formation rate. Turn-over rate calculations of measured monoterpenes and aromatic hydrocarbons suggest that especially the oxidation products of monoterpenes have the capacity to contribute to the growth of nucleation mode particles. Although a large number of precursor gases, aerosol and meteorological parameters were measured, the ultimate key factors controlling the occurence of NPF events could not be identified.

Numerical simulation of new particle formation over the northwest Atlantic using the MM5 mesoscale model coupled with sulfur chemistry

The production of sulfuric acid vapor and new particle formation are studied over the northwest Atlantic for two synoptic cases: a “wet” case in April with a large amount of cloud and precipitation in the simulation domain and a relatively “dry” one in March. The Fifth‐Generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5) is utilized coupled on‐line with a simple sulfur chemistry mechanism. Our numerical simulations show that there are three regions where significant binary homogeneous nucleation occurs. One region is in convective outflow where high anthropogenic SO2 concentration transported upward and low air temperature in the free troposphere (FT) favor the binary homogeneous nucleation of sulfuric acid gas and water vapor. The second region is near the top of marine boundary layer (MBL) clouds. The third region is in the clear FT at 500–600 hPa. In the three regions, significant nucleation events are all connected with relative humidity (RH) higher than 60%. For the northwest Atlantic we found that homogeneous nucleation generally occurs from the late morning through the afternoon on the northwest side of our simulation domain where anthropogenic SO2 concentration is high enough to produce sufficient sulfuric acid gas through the oxidation by OH radical. Significant nucleation does not necessarily happen in regions with very low preexisting particle surface area in our simulated cases. Our simulated total aerosol number concentrations in regions with significant new particle formation are generally consistent with field measurements. Nucleation rates are enhanced by NH3 especially in the MBL. Sensitivity tests show that a grid of 10 km (and time step of 30 s) yielded significantly higher nucleation as compared with a grid size of 30 km (and time step of 90 s). The latter grid may be too coarse to resolve the concentration gradients, which likely strongly impact the highly nonlinear homogeneous nucleation system.

Bursts of Aitken mode and ultrafine particles observed at the top of continental boundary layer clouds

Airborne in situ measurements of “bursts” (high number concentrations) of aerosol particles in the Aitken (10–100 nm in diameter) and ultrafine (3–10 nm) modes are described. The bursts were observed at the top of stratiform boundary layer clouds over Germany in winters 1996/1997 and 1997/1998 as well as in summer 1998. The pronounced bursts reached particle number concentrations of several thousand particles cm −3 of air in the Aitken and several tens of thousand particles cm −3 in the ultrafine mode. Two cases are investigated in detail via vertical profile measurements and horizontal flights at cloud top. These measurements reveal that the particle bursts occur in the cloud-free air just between the cloud parcels at cloud top. In one case, a correlation of a distinct particle burst with low pre-existing particle surface area was observed.

From molecular clusters to nanoparticles: Role of ambient ionization in tropospheric aerosol formation

We investigate the role of background ionization, associated mainly with galactic cosmic radiation, in the generation and evolution of ultrafine particles in the marine boundary layer. We follow the entire course of aerosol evolution, from the initial buildup of molecular clusters (charged and uncharged) through their growth into stable nanoparticles. The model used for this purpose is based on a unified collisional (kinetic) mechanism that treats the interactions between vapors, neutral and charged clusters, and particles at all sizes. We show that air ions are likely to play a central role in the formation of new ultrafine particles. The nucleation of aerosols under atmospheric conditions involves a series of competing processes, including molecular aggregation, evaporation, and scavenging by preexisting particles. In this highly sensitive nonlinear system, electrically charged embryos have a competitive advantage over similar neutral embryos. The charged clusters experience enhanced growth and stability as a consequence of electrostatic interactions. Simulations of a major nucleation event observed during the Pacific Exploratory Mission (PEM) Tropics‐A can explain most of the observed features in the ultrafine particle behavior. The key parameters controlling this behavior are the concentrations of precursor vapors and the surface area of preexisting particles, as well as the background ionization rate. We find that systematic variations in ionization levels due to the modulation of galactic cosmic radiation by the solar cycle are sufficient to cause a notable variation in aerosol production. This effect is greatest when the ambient nucleation rate is limited principally by the availability of ions. Hence we conclude that the greatest influence of such ionization is likely to occur in and above the marine boundary layer. While a systematic change in the ultrafine particle production rate is likely to affect the population of cloud condensation nuclei and hence cloud optical properties, the magnitude of the effect cannot be directly inferred from the present analysis, and requires additional analysis based on specific aerosol‐cloud interactions.

Aerosol size distributions in the cloudy atmospheric boundary layer of the North Atlantic Ocean

Measurements are presented of the size distribution and volatility of particles in the cloudy atmospheric boundary layer of the North Atlantic Ocean. The results suggest that particle nucleation of acid sulfates occurs but is neither spatially nor temporally homogeneous. Such nucleation appears to be inversely correlated with preexisting particle surface area. The few measurements obtained in clouds show a different acid sulfate distribution from those in clear air.