Showing papers by "Tuukka Petäjä published in 2007"

Toward direct measurement of atmospheric nucleation.

Abstract: Atmospheric aerosol formation is known to occur almost all over the world, and the importance of these particles to climate and air quality has been recognized. Although almost all of the processes driving aerosol formation take place below a particle diameter of 3 nanometers, observations cover only larger particles. We introduce an instrumental setup to measure atmospheric concentrations of both neutral and charged nanometer-sized clusters. By applying the instruments in the field, we come to three important conclusions: (i) A pool of numerous neutral clusters in the sub–3 nanometer size range is continuously present; (ii) the processes initiating atmospheric aerosol formation start from particle sizes of ∼1.5 nanometers; and (iii) neutral nucleation dominates over the ion-induced mechanism, at least in boreal forest conditions.

New particle formation in Beijing, China: Statistical analysis of a 1‐year data set

Abstract: [1] Particle number size distributions between 3 nm and 10 μm were measured in Beijing, China. New particle formation events were observed on around 40% of the measurement days from March 2004 to February 2005 and were generally observed under low relative humidity and sunny conditions. Though occurring during all seasons, new particle formation events had highest frequency in spring and lowest frequency in summer. Events were classified as “clean” or “polluted” groups mainly according to the condensational sink and the local wind. The formation rate range was from 3.3 to 81.4 cm−3 s−1. The growth rate varied from 0.1 to 11.2 nm h−1. The seasonal variation of condensable vapor concentration showed the highest values during summer months due to enhanced photochemical and biological activities as well as stagnant air masses preventing exchange with cleaner air.

Horizontal homogeneity and vertical extent of new particle formation events

Abstract: During the SATURN campaign 2002, new particle formation, i.e. the occurrence of ultrafine particles was investigated simultaneously at four ground-based measurement sites. The maximum distance between the sites was 50 km. Additionally, vertical profiles of aerosol particles from 5–10 nm have been measured by a tethered-balloon-borne system at one of the sites. In general, two different scenarios have been found: (i) new particle formation was measured at all sites nearly in parallel with subsequent particle growth (homogeneous case) and (ii) new particle formation was observed at one to three sites irregularly (inhomogeneous case) where subsequent particle growth was often interrupted. The homogeneous case was connected with stable synoptical conditions, i.e. the region was influenced by a high pressure system. Here, the horizontal extent of the phenomenon has been estimated to be 400 km at maximum. In the vertical dimension, the ultrafine particles are well mixed within the entire boundary layer. In the inhomogeneous case the new particle formation depends mainly on the incoming solar radiation and was often interrupted due the occurrence of clouds. Thus, single point measurements are not representative for a larger region in that case. DOI: 10.1111/j.1600-0889.2007.00260.x

Ion and particle number concentrations and size distributions along the Trans-Siberian railroad

Abstract: Aerosol concentrations and properties in Russia are not well known. There are only few studies published on aerosols in Russia. However, these aerosols can have a major effect on global climate. We measured aerosol particle and air ion number size distributions together with relevant information on meteorological conditions and atmospheric trace gas concentrations in Russia. Our purpose was to get new insight on number concentrations of aerosol particle and air ions in different parts of Russia, and to examine which sources and sinks affected the observed concentrations. During a two-week TROICA-9 expedition between 4 and 18 October 2005, we travelled on the Trans-Siberian railroad from Moscow to Vladivostok and back, conducting measurements constantly along the route. The lowest aerosol particle number concentrations, around 500 cm -3 , were observed at remote sites and the highest concentrations of around 40 000 cm -3 were observed near large industrial towns. The particle number concentration correlated best with nitrogen oxide and carbon monoxide concentrations. Pollutant levels were at their highest in the vicinity of towns, even though important pollution sources such as wood burning and forest fires also existed in rural areas. Concentrations of positive and negative intermediate and large ions were of the same order of magnitude as has been observed in previous studies made in boreal forests. Concentrations of intermediate ions were often low of the order of a few ions cm -3 , but their concentration increased during nucleation, rain and snowfall events. Concentrations of positive and negative cluster ions were sometimes very high, reaching values of about 5000 cm -3 in case of negative ions. We also detected exceptionally high ion production rates of up to 30 s -1 cm -3 due to 222-radon decay. Concentrations of cluster ions correlated quite well with the ion production rate but less so with the ion sink. Two particle formation events were observed, during which the particle growth rates varied between 2.4 and 11.4 nm h -1 . Smaller particles grew slower than the bigger ones.

Charging state of the atmospheric nucleation mode : Implications for separating neutral and ion-induced nucleation

Abstract: [1] One way of getting information about the relative roles of neutral and ion-induced nucleation in the atmosphere is to measure the charging state of aerosol particles, i.e., the extent by which the charged fraction of aerosol particles differs from the corresponding equilibrium charged fraction. By using a theoretical approach, we investigated how the charging state of a growing nucleation mode behaves under different atmospheric conditions. We found that the time evolution of the nuclei charging state is governed by two parameters: the initial nuclei charging state determined by the nucleation mechanism and the parameter K that is directly proportional to the cluster ion concentration and inversely proportional to the nuclei growth rate. We demonstrated that if the value K is larger than a certain threshold value (2–4 nm−1), any information about the initial charging state of the nuclei, and thereby about the nucleation mechanism, will be lost by the time the nuclei grow into the measurement size range (>3 nm). By making a few simplifying assumptions, we derived an analytical expression for the functional dependence of the nuclei charging state as a function of the nuclei size. We demonstrated that the derived expression can usually be fitted into experimental data on nuclei charging states. When the value of K is small enough, the obtained fitting can be extrapolated successfully down to sizes where the nucleation has taken place to obtain information about the relative importance of neutral and ion-induced nucleation.

Micrometeorological observations of a microburst in Southern Finland

Abstract: On the afternoon of 3 July 2004 in Hyytiala (Juupajoki, Finland), convective cells produced a strong downburst causing forest damage. The SMEAR II field station, situated near the damage site, enabled a unique micrometeorological analysis of a microburst with differences above and inside the canopy. At the time of the event, a squall line associated with a cold front was crossing Hyytiala with a reflectivity maximum in the middle of the squall line. A bow echo, rear-inflow notch, and probable mesovortex were observed in radar data. The bow echo moved west-north-west, and its apex travelled just north of Hyytiala. The turbulence data were analysed at two locations above the forest canopy and at one location at sub-canopy. At 1412 EET (Eastern European Time, UTC+2), the horizontal and vertical wind speed increased and the wind veered, reflecting the arrival of a gust front. At the same time, the carbon dioxide concentration increased due to turbulent mixing, the temperature decreased due to cold air flow from aloft and aerosol particle concentration decreased due to rain scavenging. An increase in the number concentration of ultra-fine particles (< 10 nm) was detected, supporting the new particle formation either from cloud outflow or due to rain. Five minutes after the gust front (1417 EET), strong horizontal and downward vertical wind speed gusts occurred with maxima of 22 and 15 m s−1, respectively, reflecting the microburst. The turbulence spectra before, during and after the event were consistent with traditional turbulence spectral theory.

Understanding the formation of biogenic secondary organic aerosol from α-pinene in smog chamber studies: role of organic peroxy radicals

Abstract: . This study focusses on the description of the nucleation process observed during the ozone reaction of the biogenic monoterpene α-pinene in smog chambers. Therefore, a detailed aerosol dynamics model (UHMA) was extended by a tropospheric chemistry module and a detailed description of the first steps of organic nucleation. We assume secondary ozonides to act as nucleation initiating molecules, which are subsequently activated by reactions with organic peroxy radicals (RO2). With this set-up the observed particle size distributions of an exemplary experiment in Valencia was reproduced, when only the long-lived organic compounds like carboxylic acids and carbonyl compounds are detected by the available aerosol size distribution instruments. Our results indicate that fragile or reactive species might get destroyed because of weak bond breakage during the size classification. This assumption would imply a serious detection problem in nucleation studies to be solved.

The Effect of Particle Size, Composition, and Shape on Lidar Backscattering

Abstract: The optical properties of urban aerosol particles are calculated using exact methods at 910 nmwavelength under different assumptions about particle composition and shape. Results show thatspherical particles backscatter light on the average about 40 percent more than the correspondingnonspherical particles, the difference generally being larger for larger particles. In contrast, changingthe particle composition from water to silicate increases backscattering by about a factor of three.Typical values for absorptivity seem to have negligible impact on backscattering. 1 Introduction Accurate modeling of light scattering properties of aerosol particles is essential to conductreliable remote sensing observations. Although it is well known that the scattering propertiesof nonspherical particles can be significantly different from those of spherical particles,assumption of spherical particles is still widely used in remote sensing applications. This canlead to large errors when retrieving aerosol optical properties with lidar inversions, especiallyif dust particles are present (e.g [1]).The aim of this work was to study the effect of particle size, shape, and composition on lidarbackscattering in an urban environment. On one hand, sensitivity studies based on opticalmodelling were conducted to estimate the relative importance of different physical factorsaffecting backscattering of light by aerosol particles. On the other hand, modeledbackscattering values were compared with lidar measurements. To facilitate the comparison,the lidar measurements were conducted in a vicinity of aerosol in situ measurement site.

New Particle Formation in Clean Savannah Environment

Abstract: Southern African savanna background environment lacks previous, continuous long-term combined sub-micrometer aerosol number concentration and gas measurements. We have build a mobile measurement trailer, which contains measurements of aerosol number size distributions 10-840 nm, positive and negative ion size distributions 0.4-40 nm, aerosol mass PM 10 , PM 2.5 , PM 1 , inorganic compound in PM 10 and PM 2.5 , gases like SO 2 , NO, NOx, CO, O 3 and basic meteorology. In addition, we measure periodically VOCs. The trailer is well- protected against thunder storms, electricity breaks and other typical problems. It is connected to internet via GPRS-modem. Preliminary results show that in a clean savanna environment, new particle formation takes place every sunny day, with relatively high nucleation rates and very high growth rates (up to 15 nm/h).

Marine Aerosol and Secondary Particle Formation over the North Atlantic

Abstract: During the summer of 2006 a measurement campaign was conducted on a ship in the North Atlantic. We measured the concentration, size distribution and hygroscopicity of the atmospheric aerosol particles. The size distribution consisted of two separate modes, one peaking at 50 nm and the other at 200 nm. Based on the hygroscopicity measurements, the first mode was internally mixed and had a hygro- scopic growth factor of 1.6-1.7. In the afternoon of 13 June we observed a mode of small particles, which grew during the evening to 30-40 nm. These particles are believed to be secondary particles, which would make this a very rare observation. Further analysis will be made once more supporting data becomes available.

Measurement of the Charging State with an Ion-DMPS to Estimate the Contribution of Ion-induced Nucleation

Abstract: In this work, the Ion-DMPS is presented and the analyzed data from over one year of measurements in Hyytiala, Finland (April 13, 2005-September 21, 2006), is discussed. The Ion-DMPS allows calculation of the charging state of par- ticles taking part in a nucleation event. A physically sound equation that describes that charging state of particles during a nucleation event is presented and fitted to the Ion-DMPS data. The behavior predicted by the equation of the charging state fitted very well to the experimental data obtained from the Ion-DMPS. According to this analysis, an overwhelming majority of the new particle formation events appeared to include at least some participation of ion-induced nucleation. The analysis of a year of data showed that the contribution of ion-induced nucleation varied between 0% and slightly less than 40%. The median ion-induced participa- tion for the new particle formation events analyzed was between 5% and 10%.

Air Ion Measurements at Mace Head on the West Coast of Ireland

Abstract: Coastal nucleation events and behavior of cluster ions were characterized through the measurements of air ion mobility distributions at the Mace Head research station on the west coast of Ireland in 2006 We measured concentrations of cluster ions and charged aerosol particles in the size range of 04–40 nm These measurements allow us to characterize freshly nucleated charged particles with diameter below 3 nm The analysis shows that bursts of intermediate ions (16–7 nm) are a frequent phenomenon in marine coastal environment Nucleation evens occurred during most of the measurement days We classified the nucleation burst events Particle formation and growth events mostly coincided with the presence of low tide Small ions concentrations appear to be strongly dependent on the variations of meteorological parameters including wind speed and direction

Improving Particle Detection Effi ciency of a Condensation Particle Counter by Means of Pulse Height Analysis

Abstract: Pulse height analysis of white light scattered from the droplets formed in the CPC condenser was applied to distinguish between homogeneous and heterogeneous nucleation with high butanol supersaturation. With this method the activation of calibration aerosol particles down to ca. 1.5 nm was clearly observable. Experimental results show that the instrument described here is a powerful tool in atmospheric sub-3 nm aerosol particle measurements.

Investigating the Chemical Composition of Growing Nucleation Mode Particles with CPC Battery

Abstract: Formation and growth of fresh newly formed atmospheric aerosols were investigated using a condensation particle counter battery (CPCB). From the meas- urement data we identified days when the two CPC pairs within the CPCB showed differential signals as well as days when no differences in the signals were detected. This categorization was complemented by nucleation event classification. We found out that growing nucleation mode particles were water-soluble both at 3 and 10 nm. During the measurement period water insoluble and water soluble pollutants were detected by the CPCB system. The measurements for this study were performed at SMEAR II station in Hyytiala, Finland, between March and May 2006.