Aitken Mode Particles Research Articles

The characterisation of pollution aerosol in a changing photochemical environment

Abstract. Measurements are presented from a sampling location 50 km downwind of Greater London, UK, to investigate the timescales required for the atmospheric transformations of aerosol in urban emissions plumes in the context of photochemical age based on the benzene to toluene ratio. It is shown that particles at or around 100 nm in diameter exhibit the greatest systematic variability in chemical properties, and thus hygroscopic properties, on a timescale of 1–2 days. The smaller Aitken mode and larger accumulation mode particles exhibit less variability on these timescales, which we propose is as a result of their different residence times in the atmosphere. The larger accumulation particles have been in the atmosphere longer than the 100 nm particles and their chemistry and hygroscopic properties have been integrated over several days and potentially over several source regions. In contrast, the smaller Aitken mode particles show little systematic variability with photochemical age because their atmospheric lifetimes are short, thus chemical changes and hence changes in water affinity have not had time to occur. Increases in the particle diameter of up to 40% are observed at 90% relative humidity in the accumulation mode from the uptake of water as the particles become increasingly soluble in nature.

Accumulation mode aerosol, pockets of open cells, and particle nucleation in the remote subtropical Pacific marine boundary layer

We analyze a marine boundary layer cloud field encountered during the second research flight of the second Dynamics and Chemistry of Marine Stratocumulus Experiment. The cloud field is distinguished by the presence of pockets of open cells. Differences between the pockets and the surrounding stratocumulus clouds are studied utilizing in situ and satellite data. The pockets are characterized as regions where cloud radar echo tops are unusually variable, accumulation mode aerosol concentrations are low, and Aitken mode particles with a mode diameter at 0.02 μm dominate aerosol number concentration. The Aitken mode particles are thought to be generated by a nucleation event which occurred within the marine boundary layer. The low accumulation mode concentrations associated with the pockets are proposed to be necessary for their maintenance.

Observations of urban airborne particle number concentrations during rush-hour conditions: analysis of the number based size distributions and modal parameters

A summertime study of the number concentration and the size distribution of combustion derived nanometre sized particles (termed nanoparticles) from diesel and spark-ignition (SI) engine emissions were made under rush-hour and free-flow traffic conditions at an urban roadside location in Leeds, UK in July 2003. The measured total particle number concentrations (N(TOTAL)) were of the order 1.8 x 10(4) to 3.4 x 10(4) cm(-3), and tended to follow the diurnal traffic flow patterns. The N(TOTAL) was dominated by particles < or =100 nm in diameter which accounted for between 89-93% of the measured particle number. By use of a log-normal fitting procedure, the modal parameters of the number based particle size distribution of urban airborne particulates were derived from the roadside measurements. Four component modes were identified. Two nucleation modes were found, with a smaller, more minor, mode composed principally of sub-11 nm particles, believed to be derived from particles formed from the nucleation of gaseous species in the atmosphere. A second mode, much larger in terms of number, was composed of particles within the size range of 10-20 nm. This second mode was believed to be principally derived from the condensation of the unburned fuel and lube oil (the solvent organic fraction or SOF) as it cooled on leaving the engine exhaust. Third and fourth modes were noted within the size ranges of 28-65 nm and 100-160 nm, respectively. The third mode was believed to be representative of internally mixed Aitken mode particles composed of a soot/ash core with an adsorbed layer of readily volatilisable material. The fourth mode was believed to be composed of chemically aged, secondary particles. The larger nucleation and Aitken modes accounted for between 80-90% of the measured N(TOTAL), and the particles in these modes were believed to be derived from SI and diesel engine emissions. The overall size distribution, particularly in modes II-IV, was observed to be strongly related to the number of primary particle emissions, with larger count median diameters observed under conditions where low numbers of primary soot based particles were present.

A method for detecting the presence of organic fraction in nucleation mode sized particles

Abstract. New particle formation and growth has a very important role in many climate processes. However, the overall knowlegde of the chemical composition of atmospheric nucleation mode (particle diameter, d&lt;20 nm) and the lower end of Aitken mode particles (d≤50 nm) is still insufficient. In this work, we have applied the UFO-TDMA (ultrafine organic tandem differential mobility analyzer) method to shed light on the presence of an organic fraction in the nucleation mode size class in different atmospheric environments. The basic principle of the organic fraction detection is based on our laboratory UFO-TDMA measurements with organic and inorganic compounds. Our laboratory measurements indicate that the usefulness of the UFO-TDMA in the field experiments would arise especially from the fact that atmospherically the most relevant inorganic compounds do not grow in subsaturated ethanol vapor, when particle size is 10 nm in diameter and saturation ratio is about 86% or below it. Furthermore, internally mixed particles composed of ammonium bisulfate and sulfuric acid with sulfuric acid mass fraction ≤33% show no growth at 85% saturation ratio. In contrast, 10 nm particles composed of various oxidized organic compounds of atmospheric relevance are able to grow in those conditions. These discoveries indicate that it is possible to detect the presence of organics in atmospheric nucleation mode sized particles using the UFO-TDMA method. In the future, the UFO-TDMA is expected to be an important aid to describe the composition of atmospheric newly-formed particles.

Spatial and temporal distribution of atmospheric aerosols in the lowermost troposphere over the Amazonian tropical rainforest

Abstract. We present measurements of aerosol physico-chemical properties below 5 km altitude over the tropical rain forest and the marine boundary layer (MBL) obtained during the LBA-CLAIRE 1998 project. The MBL aerosol size distribution some 50-100km of the coast of French Guyana and Suriname showed a bi-modal shape typical of aged and cloud processed aerosol. The average particle number density in the MBL was 383cm-3. The daytime mixed layer height over the rain forest for undisturbed conditions was estimated to be between 1200-1500m. During the morning hours the height of the mixed layer increased by 144-180mh-1. The median daytime aerosol number density in the mixed layer increased from 450cm-3 in the morning to almost 800cm-3 in the late afternoon. The evolution of the aerosol size distribution in the daytime mixed layer over the rain forest showed two distinct patterns. Between dawn and midday, the Aitken mode particle concentrations increased, whereas later during the day, a sharp increase of the accumulation mode aerosol number densities was observed, resulting in a doubling of the morning accumulation mode concentrations from 150cm-3 to 300cm-3. Potential sources of the Aitken mode particles are discussed here including the rapid growth of ultrafine aerosol particles formed aloft and subsequently entrained into the mixed layer, as well as the contribution of emissions from the tropical vegetation to Aitken mode number densities. The observed increase of the accumulation mode aerosol number densities is attributed to the combined effect of: the direct emissions of primary biogenic particles from the rain forest and aerosol in-cloud processing by shallow convective clouds. Based on the similarities among the number densities, the size distributions and the composition of the aerosol in the MBL and the nocturnal residual layer we propose that the air originating in the MBL is transported above the nocturnal mixed layer up to 300-400km inland over the rain forest by night without significant processing.

Size-resolved soluble volume fractions of submicrometer particles in air masses of different character

As a contribution to the joint research project FEBUKO, hygroscopic properties of atmospheric Aitken and accumulation mode particles were measured in the Thueringer Wald, Germany, using a hygroscopicity-tandem differential mobility analyser (H-TDMA). The hygroscopic growth of particles with initial dry diameters of D p = 50 , 150, and 250 nm at 90% relative humidity was used to calculate average distributions of the soluble volume fraction with respect to the hygroscopic growth of ammonium sulphate. The application of this parameterisation procedure was tested by analysing the dataset with respect to the dependence of the soluble volume fraction on particle size and air masses character. With increasing dry particle size, the fraction of particles containing high soluble volume fractions was found to increase. The number of accumulation mode particles in marine air masses passing the sampling site having a large soluble volume fraction was significantly higher than in air masses of more continental character. For particles with D p = 50 nm , no air mass dependence of the soluble volume fraction was found. In marine air masses, particles with D p = 150 and 250 nm are assumed to undergo similar evolution processes, whereas in continental air masses this seemed to be the case for particles with D p = 50 nm and 150 nm.

Measurements of cloud droplet activation of aerosol particles at a clean subarctic background site

Two years of continuous aerosol particle size distribution measurements provided the basis for this cloud droplet activation study. The cloud droplet activation of aerosol particles was studied in Pallas, a clean background site in northern Finland. A slightly different approach compared with traditional methods is presented by measuring simultaneously the cloud interstitial particle size spectrum and a nearby out‐of‐cloud particle size spectrum. The main advantage of this approach is that one can determine the activated fraction of different‐size particles over the whole submicron‐size range and that a large number of cloud activation events can be analyzed. The number of cloud droplet activation days peaked in late autumn and was lowest in summer. The annual variation of “cloud droplet activation” events followed the annual pattern of low clouds (those below 1000 m). A relation was found between the total particle number concentration outside the cloud and the number of activated particles. A larger particle concentration led to a higher number of activated particles, a lower activation percent, and a larger activation diameter (D50). D50 was on average 80 nm and varied from 50 to 128 nm. The average fraction of activated particles during the cloud events was 47% and varied from 9 to 86%. The cleaner and colder air masses from the northern Atlantic or the Arctic Ocean had, on average, 15 nm lower D50 than the more polluted air masses from south and east containing more particles. The annual variation of the number of the activated particles and other variables were also closely related to the annual variation of particle concentration (high in summer and low in winter). On average 87 and 30% of accumulation and Aitken mode particles, respectively, were activated. Aitken mode particles were observed to cover up to 55% of the total number of activated particles (i.e., number of formed droplets), which demonstrates that they have a significant effect on cloud droplet activation and must be taken into account when estimating the aerosol indirect climate effects.

Effects of SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; oxidation on ambient aerosol growth in water and ethanol vapours

Abstract. Hygroscopicity (i.e. water vapour affinity) of atmospheric aerosol particles is one of the key factors in defining their impacts on climate. Condensation of sulphuric acid onto less hygroscopic particles is expected to increase their hygrocopicity and hence their cloud condensation nuclei formation potential. In this study, differences in the hygroscopic and ethanol uptake properties of ultrafine aerosol particles in the Arctic air masses with a different exposure to anthropogenic sulfur pollution were examined. The main discovery was that Aitken mode particles having been exposed to polluted air were more hygroscopic and less soluble to ethanol than after transport in clean air. This aging process was attributed to sulphur dioxide oxidation and subsequent condensation during the transport of these particle to our measurement site. The hygroscopicity of nucleation mode aerosol particles, on the other hand, was approximately the same in all the cases, being indicative of a relatively similar chemical composition despite the differences in air mass transport routes. These particles had also been produced closer to the observation site typically 3–8 h prior to sampling. Apparently, these particles did not have an opportunity to accumulate sulphuric acid on their way to the site, but instead their chemical composition (hygroscopicity and ethanol solubility) resembled that of particles produced in the local or semi-regional ambient conditions.

Measurements in a highly polluted Asian mega city: observations of aerosol number size distribution, modal parameters and nucleation events

Abstract. Diurnal variation of number size distribution (particle size 3-800nm) and modal parameters (geometric standard deviation, geometric mean diameter and modal aerosol particle concentration) in a highly polluted urban environment was investigated during October and November 2002 in New Delhi, India. Continuous monitoring for more than two weeks with the time resolution of 10min was conducted using a Differential Mobility Particle Sizer (twin DMPS). The results indicated clear increase in Aitken mode (25-100nm) particles during traffic peak hours, but towards the evenings there were more Aitken mode particles compared to the mornings. Also high concentrations of accumulation mode particles (&gt;100nm) were detected in the evenings only. In the evenings, biomass/refuse burning and cooking are possible sources beside the traffic. We have also shown that nucleation events are possible in this kind of atmosphere even though as clear nucleation events as observed in rural sites could not be detected. The formation rate of 3nm particles (J3) of the observed events varied from 3.3 to 13.9cm-3s-1 and the growth rate varied from 11.6 to 18.1nmh-1 showing rapid growth and high formation rate, which seems to be typical in urban areas.

Biogenic particles in the surface microlayer and overlaying atmosphere in the central Arctic Ocean during summer

Transmission electron microscopy photographs of airborne particles are compared with those of particles found inthe surface microlayer of the open water between ice floes in the central Arctic Ocean in summer. The similarity inmorphology, physical properties, X-ray spectra and a chemical reaction of the numerous aggregates and their buildingblocks predominantly smaller than 70 nm diameter, and of bacteria and other micro-organisms found in both, stronglysuggests that the airborne particles were ejected from the water by bursting bubbles. The shape of the size distribution ofaggregates in the air is very similar to that in the water, each with a well-defined Aitken mode but shifted towards smallersizes. Diffuse electron-transparent material joining and surrounding the heat resistant and non-hygroscopic particulatesin both the air and water is shown to have properties consistent with the exopolymer secretions (EPS) of microalgae andbacteria in the water. EPS are highly surface-active, highly hydrated molecules that can spontaneously assemble intogels. They are broken down by ultraviolet light or acidification. These properties provide an explanation for the differentresistance to dehydration of bacteria from air and water samples when subjected to a vacuum, and the apparent absenceof sea salt on airborne bacteria and aggregates. The difference in size distribution between the air and water samples isalso explained. The role of EPS and particulate matter from the open lead surface microlayer in the production of theairborne Aitken mode particles and cloud condensation nuclei is examined and concluded to be very important.

Growth Properties of Sub-Micron Aerosols during Cold Season in India

The growth properties of submicron aerosols were investigated on the basis of particle number size distributions measured in field experiments. An analysis of condensation sink, growth rate, concentration of condensable vapors and their source rate, and real and apparent nucleation rates as a function of particle sizes and their number concentrations in the submicron range are presented to quantify their effects during nucleation events. Higher number concentrations of newly formed particles of 0.013, 0.024, 0.075, and 0.133 m around 0900 h indicate that photochemistry plays an important role in their formation, and that they grow principally by gas-to-particle conversion. The magnitudes of nucleation parameterare indicative of the number concentration of newly formed particles by nucleation and their subsequent growth. The estimated value ofis around 10 nm for Aitken mode particles (0.013 and 0.024 µm) and 6 nm for accumulation mode particles (Dp0.1 m). The formation rate of 0.013 and 0.024 m particles is 2.33 cm -3 s -1 and of intermediate particles (0.075 and 0.133 m) is 1.33 cm -3 s -1 . The growth rate of 0.013 and 0.024 m particles is 0.3 and 1 nm h -1 and the rate for 0.075 and 0.133 m particles is 8 and 21 nm h -1 , indicating that high formation rate and rapid growth are generally found in semi-urban and urban areas. These results may be of interest for future study of nucleation processes in different environments.

Aerosol number to volume ratios in Southwest Portugal during ACE-2

Past studies have indicated that long-term averages of the aerosol number to volume ratios (defined as the number of particles larger than a certain diameter divided by the particle volume over some range less than 1 μm) show little variability over the Atlantic. This work presents number to volume ratios (R) measured during the ACE-2 experiment onthe land-based Sagres field site located in Southwest Portugal. The values of R measured in Sagres compare reasonably well with previous measurements over the Atlantic. The main emphasis of this work is therefore to investigate moreclosely possible reasons for the observed stability of the number to volume ratio. Aerosol number size distributions measured in Sagres are parametrized by the sum of two log-normal distributions fitted to the accumulation and to the Aitken mode. The main factor that limits the variability of R is that the parameters of these log-normal distributionsare not always independent but show some covariance. In polluted air mass types correlations between parameters of the Aitken and accumulation mode are mostly responsible for stabilizing R. In marine air mass types the variabilityof R is reduced by an inverse relationship between the accumulation-mode mean diameter and standard deviation, consistent with condensational processes and cloud processing working on the aerosol. However, despite this reduction, the variability of R in marine air mass types is still considerable and R is linearly dependent on the number concentration of particles larger than 90 nm. This partly due to a tail of Aitken-mode particles extending to sizes larger than90 nm.

Application of the VH‐TDMA technique to coastal ambient aerosols

A newly developed VH‐TDMA has been used for the first time to measure the volatile fractions and post volatilization hygroscopic growth factors of ambient aerosols in the coastal marine and urban environments. The results are compared with comparable data for laboratory generated aerosols of known composition. Measurements conducted on coastal Aitken mode particles showed volatilization behavior similar to laboratory generated aerosols composed of methane sulfonic acid and ammonium sulfate. Measurements conducted on 60 nm particles during nucleation events contained a greater fraction of material with similar volatility to ammonium sulfate than was found at other times. These particles were hygroscopic but less so than pure ammonium sulfate. Measurements conducted in the Brisbane central business district during sea breeze conditions show similar behavior to the coastal aerosol, but with additional low volatility species. This aerosol may originate from urban sources or from marine particles acquiring additional secondary aerosol species during transport.

Death of nucleation and Aitken mode particles: observations at extreme atmospheric conditions and their theoretical explanation

Death of nucleation and Aitken mode particles: observations at extreme atmospheric conditions and their theoretical explanation

Aerosol number to volume ratios in Southwest Portugal during ACE-2

Past studies have indicated that long-term averages of the aerosol number to volume ratios (defined as the number ofparticles larger than a certain diameter divided by the particle volume over some range less than 1 μm) show littlevariability over the Atlantic. This work presents number to volume ratios (R) measured during the ACE-2 experiment onthe land-based Sagres field site located in Southwest Portugal. The values of R measured in Sagres compare reasonablywell with previous measurements over the Atlantic. The main emphasis of this work is therefore to investigate moreclosely possible reasons for the observed stability of the number to volume ratio. Aerosol number size distributionsmeasured in Sagres are parametrized by the sum of two log-normal distributions fitted to the accumulation and to theAitken mode. The main factor that limits the variability of R is that the parameters of these log-normal distributionsare not always independent but show some covariance. In polluted air mass types correlations between parameters ofthe Aitken and accumulation mode are mostly responsible for stabilizing R. In marine air mass types the variabilityof R is reduced by an inverse relationship between the accumulation-mode mean diameter and standard deviation, consistent with condensational processes and cloud processing working on the aerosol. However, despite this reduction, the variability of R in marine air mass types is still considerable and R is linearly dependent on the number concentrationof particles larger than 90 nm. This partly due to a tail of Aitken-mode particles extending to sizes larger than90 nm.

Number size distributions and concentrations of the continental summer aerosols in Queen Maud Land, Antarctica

Total particle number concentrations and particle size distributions in the diameter range 3–800 nm were measured in continental Antarctica during the austral summers 2000 and 2001. Observed total particle number concentrations varied between about 200 and 2000 cm−3, with on average higher concentrations in marine/coastal air masses than in continental air masses. Concentrations in excess of about 300–400 cm−3 were attributable to the presence of small (&lt;20 nm) nucleation mode particles. Measured size distributions were fitted with two to four lognormal modes. All the spectra displayed an accumulation mode peaking at about 70–150 nm and an Aitken mode peaking at about 30–50 nm. The average mean diameter of the accumulation mode was much lower in continental air masses, whereas that of the Aitken mode was almost the same for the two air mass types. A nucleation mode was present in more than a half of the measured spectra, and occasionally two nucleation modes could be identified. Evidence on recent new particle formation with subsequent particle growth up to about 40 nm was observed in marine/coastal air masses only. Our measurements indicate that these recently formed particles contribute significantly to the overall particle budget of the Antarctic boundary layer, and they can be also a source of climatically important Aitken and accumulation mode particles.

Meridional distributions of aerosol particle number concentrations in the upper troposphere and lower stratosphere obtained by Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container (CARIBIC) flights

Number concentrations of submicrometer aerosol particles at altitudes of 8.5–11.3 km were measured along a single flight route between Germany (∼50°N, ∼10°E) and the Indic (∼5°N, ∼80°E) over 3 years using a commercial aircraft platform (project Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container (CARIBIC)). During 41 intercontinental flights (∼380 flight hours), more than one million individual particle concentration measurements were made, yielding a comprehensive and unique aerosol data set. Using these data, the first meridional and seasonal probability distributions for ultrafine particles and for Aitken mode plus accumulation mode particles in the upper troposphere (UT) and lower stratosphere (LS) were derived. High particle number concentrations were observed in summer at tropical latitudes over the Arabian Sea and at midlatitudes over Europe, in contrast to lower values in the subtropics over the Middle East. This distribution primarily reflects the vertical transport pattern of the atmosphere. In winter, number concentrations were generally lower. The Intertropical Convergence Zone (ITCZ) was identified as a region of pronounced particle formation, but particle nucleation also occurred in the upper troposphere at midlatitudes. At tropical latitudes, convective transport and photochemistry appear to be the main driving forces for particle formation in the free troposphere. Consistent with these observations, integral length scales calculated from the aerosol data ranged from 6 to 12 km. Compared to particle concentrations in the midlatitudinal upper troposphere, those in the lowermost stratosphere were lower in winter, but equal or higher in summer. This seasonality is likely caused by stronger in‐mixing of upper tropospheric air in summer.

Modelling of the influence of aerosol processes for the dispersion of vehicular exhaust plumes in street environment

We have analysed the influence of various aerosol dynamical processes and plume dilution on the properties of vehicular exhaust particulate matter. A monodisperse aerosol process model MONO32 with four size modes was applied for evaluating the number concentration, size distribution and chemical composition of the particles during a time period of 25 s, after the particles were emitted from the exhaust pipe. The model takes into account aerosol dynamics and gas-phase chemistry including emissions of gases and particles, chemical reactions in the gas phase, dry deposition of particles and gases, homogenous binary H 2SO 4–H 2O or ternary H 2SO 4–H 2O–NH 3 nucleation, multicomponent condensation of H 2SO 4, H 2O and some organic vapour onto the particles, inter- and intra-mode coagulation of particles, and plume dilution with the background air. The dilution parameters are calculated by a simple plume model. Numerical computations were performed assuming specific vehicular exhausts under an urban environment, characteristic for the Helsinki Metropolitan Area, in summer. The results showed that condensation of an insoluble organic vapour is important under the selected conditions, if its concentration exceeds a threshold value of 10 10 or 10 11 cm –3 for the Aitken and accumulation mode particles, respectively. The condensation or evaporation of water can also be an important process; however, its influence is strongly dependent on the hygroscopicity of particles. The effect of coagulation is substantial only, if the dilution of the exhaust plume is neglected. After the computational time of 25 s, the particulate population reaches so-called quasi-equilibrium state, i.e., most of the particulate matter transformation processes have already taken place within this time period.

Number size distributions and concentrations of marine aerosols: Observations during a cruise between the English Channel and the coast of Antarctica

Submicron aerosol number size distributions were measured on board the research ship Akademic Fedorov during a cruise from the English Channel to the coast of Antarctica. The observed sized spectra were fitted with three lognormal modes (the nucleation, Aitken and accumulation modes), and the data were classified according to calculated air mass back trajectories. The total particle number concentrations were mostly &lt;1000 cm−3 in marine air masses and between about 1000 and 10000 cm−3 in continentally influenced air masses. Most of the eastern midlatitude Atlantic was affected by European pollution with high concentrations of both nucleation and Aitken mode particles. Another pollution peak, caused probably by biomass burning in Africa, was seen at about 10 N. Marine air masses showed distinctive latitudinal changes. Over the tropical Atlantic no nucleation mode could be seen, and the total particle number concentration remained usually below 500 cm−3. In the midlatitude and high‐latitude Atlantic several episodes of elevated particle concentrations (&lt;1000 cm−3) caused by either the nucleation or Aitken mode were observed. The geometric mean diameter of the accumulation mode decreased gradually toward the higher southern latitudes with a more rapid decline close to Antarctica. Our observations indicate further that there is no simple and universal relationship between the total particle number concentration, the submicron aerosol volume (or mass), and the number of particles able to act as cloud condensation nuclei over the southern hemispheric oceans.

Hygroscopic properties of nucleation mode and Aitken mode particles during nucleation bursts and in background air on the west coast of Ireland

The hygroscopic and cloud condensation nuclei (CCN) properties of submicrometer atmospheric aerosol particles were investigated using an Ultrafine Tandem Differential Mobility Analyzer (UF‐TDMA) and a CCN counter at the Mace Head Monitoring Station on the west coast of Ireland during the New Particle Formation and Fate in the Coastal Environment (PARFORCE) field campaign in September 1998 and June 1999. These measurements give indirect in situ information on the composition and state of mixing of the aerosol particles. The UF‐TDMA was used for monitoring of hygroscopic diameter growth factors of aerosol particles with dry mobility diameters 8–20 nm when taken from dry state to a controlled humid environment (RH 90%). The CCN counter was used to study the activation of aerosol particles when exposed to supersaturated conditions (dry diameters of 15–150 nm). It was seen that in clean marine air masses during the observed particle formation events, the newly formed nucleation mode particles (8 and 10 nm) most often had low growth factors (between 1.0 and 1.1) resulting from low solubility. This indicates that the nucleation mode particles consist of nonsoluble or weakly soluble species, possible accompanied by a small soluble part; however, when nucleation mode particles were observed outside the event periods, the growth factors were higher (about 1.3–1.4). In contrast, the 20 nm particles usually clearly belonged to the Aitken mode (based on number size distribution measurements) and had hygroscopic properties similar to some common salts (growth factors 1.4–1.5).