[show abstract][hide abstract] ABSTRACT: The continuous measurements of aerosol particle deposition velocity have been performed from January 2004 to January 2005 using a REA technique with dynamic deadband. We measured aerosol particle deposition velocity in the size range of 10–150 nanometer with 5–10 nanometer steps using differential mobility analyser for sizing. We were able to measure two size classes simultaneously. One size class was changed at one month intervals, another we kept constant at 30 nm to investigate the effect of seasonal and meteorological variation on deposition velocity. We found that the 80–100 nanometer size particles had the lowest deposition velocity, about 0.4 cm s−1. Deposition velocity increased with decreasing or increasing particle diameter from 80–100 nanometer size. We also found that deposition velocity increases as a function of friction velocity.
Tellus B 06/2007; 59(3):381 - 386. · 3.20 Impact Factor
[show abstract][hide abstract] ABSTRACT: Atmospheric aerosol particle formation is frequently observed throughout the atmosphere, but despite various attempts of explanation, the processes behind it remain unclear. In this study data mining techniques were used to find the key parameters needed for atmospheric aerosol particle formation to occur. A dataset of 8 years of 80 variables collected at the boreal forest station (SMEAR II) in Southern Finland was used, incorporating variables such as radiation, humidity, SO<sub>2</sub>, ozone and present aerosol surface area. This data was analyzed using clustering and classification methods. The aim of this approach was to gain new parameters independent of any subjective interpretation. This resulted in two key parameters, relative humidity and preexisting aerosol particle surface (condensation sink), capable in explaining 88% of the nucleation events. The inclusion of any further parameters did not improve the results notably. Using these two variables it was possible to derive a nucleation probability function. Interestingly, the two most important variables are related to mechanisms that prevent the nucleation from starting and particles from growing, while parameters related to initiation of particle formation seemed to be less important. Nucleation occurs only with low relative humidity and condensation sink values. One possible explanation for the effect of high water content is that it prevents biogenic hydrocarbon ozonolysis reactions from producing sufficient amounts of low volatility compounds, which might be able to nucleate. Unfortunately the most important biogenic hydrocarbon compound emissions were not available for this study. Another effect of water vapour may be due to its linkage to cloudiness which may prevent the formation of nucleating and/or condensing vapours. A high number of preexisting particles will act as a sink for condensable vapours that otherwise would have been able to form sufficient supersaturation and initiate the nucleation process.
ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2005; · 5.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: Size-segregated chemical aerosol analysis of a total 5 integrated samples has been performed for the atmospheric aerosol during events of new particle formation. The experiments were conducted during the BIOFOR 3 measurement campaign at a boreal forest site in southern Finland in spring 1999. Aerosol samples collected by a cascade low-pressure impactor were taken selectively to distinguish particle formation event aerosol from non-event aerosol. The division into “event” and “non-event” cases was done “in situ” at field, based on the on-line submicron number size distribution. The results on the chemical ionic composition of the particles show only small differences between the event and non-event sample sets. The event samples show lower concentrations of total sulfate and ammonium as well as light dicarboxylic acids such as oxalate, malonate and succinate. In the event samples, nucleation mode particle MSA (methanesulphonic acid) was found to be present exceeding the concentrations found in the non-event samples, but at larger particle sizes the sample sets contained rather similar concentrations of MSA. The most significant difference between the event and non-event sets was found for dimethylammonium, ionic component of dimethylamine ((CH3)2NH), which seems to be present in the particle phase during the particle formation periods and/or during the subsequent particle growth. The absolute event sample dimethylamine concentrations were more than 30-fold greater than the non-event concentrations in the accumulation mode size range. On the other hand, the non-event back-up filter stage for sub-30 nm particles contained more dimethylamine than the event samples. This fractionation is probably a condensation artifact of the impactor sampling. A simple mass balance estimate is performed to evaluate the quality and consistency of the results for the overall mass concentration.
Tellus B 03/2003; 53(4):380 - 393. · 3.20 Impact Factor
[show abstract][hide abstract] ABSTRACT: In order to be able to detect annual and diurnal variation the nucleation, Aitken and accumulation mode aerosol number concentrations were investigated. In this study particulate matter and aerosol number concentrations based on three years (1999–2001) measurements in four different places, Helsinki, Hyytiälä, Pallas and Värriö in Finland were utilized. Mean PM10 concentration was found to be in urban, urban background, disturbed rural and rural conditions 18.7, 15.3, 10.2 and . PM2.5 concentrations in urban, urban background and rural conditions were 9.6, 8.2 and . PM1 concentration in rural area were . Total number concentrations between 10 and in urban, rural and arctic background were 16660, 2110 and about , respectively. Annual cycle of different particle sizes was also investigated. Particle mass was found to have maximum during the spring and autumn. Nucleation mode particle concentrations had their maximum both in urban and rural conditions in spring and autumn. For Aitken mode a similar variation was found. Accumulation mode particles were found to have the highest concentrations during summer and the lowest concentrations during winter. In urban conditions local sources drive over natural variability, but a natural cycle can still be seen. Diurnal variation was found to be associated with traffic in urban conditions. In rural conditions, diurnal variation of nucleation mode particles were connected to new particle formation. It was also found, that there is no correlation between ultrafine and larger particles.
[show abstract][hide abstract] ABSTRACT: Biogenic volatile organic compound (VOC) concentrations in ambient air were measured in Central Finland from April 2000 to April 2002. The concentrations of isoprene increased in May and declined in September, but the concentrations of its oxidation products, methacrolein and methyl vinyl ketone, remained at about 100 ppt level also after that suggesting they might have anthropogenic sources as well. The winter and summer mean midday concentrations of monoterpenes were 97 and 250 pptv, respectively. The winter concentrations of monoterpenes were unexpectedly high, but they can be explained by small, but existing, emissions from at least Picea abies and also by the much longer atmospheric lifetime of monoterpenes in winter than in summer. The hydroxyl and nitrate radical concentrations in Central Finland were calculated for the year 2000 using an observationally constrained photochemical box model. Using the calculated hydroxyl radical concentrations the atmospheric lifetime of α-pinene with respect to the OH reaction was estimated to be 50 times longer in December than in July. The nitrate and ozone reactions extend the atmospheric lifetime of α-pinene to 3–4 fold in winter in comparison with the summer lifetime.The mean afternoon (12–4 p.m.) emission potentials (at 30°C according to Guenther et al., J. Geophys. Res. 98 (1993) 12609) were 0.4, 0.2, 1.4, 0.7, 0.4, and 0.2 μg g−1dry weight h−1 in January, April, May, June, July, and October, respectively. In summer the VOC emissions also included isoprene and sesquiterpenes. The highest isoprene emission potential was observed in June (1.3 μg g−1dry weight h−1, (at T=30°C and PPFD=1000 μmol m−2 s−1 according to Guenther et al.) and the highest sesquiterpene emission potential in July (0.6 μg g−1dry weight h−1, scaled to 30°C similarly to the monoterpenes).
[show abstract][hide abstract] ABSTRACT: The diffusive sampling method was evaluated for measuring benzene, toluene, ethylbenzene, xylenes, styrene, propylbenzene, ethyltoluenes, trimethylbenzenes and methyl tert-butyl ether (MTBE) in the urban air of Helsinki, Finland. Concentrations were measured in 2-week periods at four different sites during the year 2000. Tube type adsorbent tubes were pre-packed with Carbopack-B (60/80). Analysis was conducted using thermal desorption and gas chromatograph coupled to a mass spectrometer. In different seasons, during five diffusive sampling periods, parallel measurements were conducted using pumped and online sampling. The compared techniques agreed reasonably well for other compounds than trimethylbenzenes. Based on comparisons, diffusive uptake rates for ethyltoluenes, styrene, propylbenzene and MTBE were determined, and for trimethylbenzenes, uptake rates were revised. The concentrations of aromatic compounds in Helsinki metropolitan area were also compared to the concentrations of a rural, forested site in Central Finland.
Science of The Total Environment 11/2002; 298(1-3):55-64. · 3.26 Impact Factor
[show abstract][hide abstract] ABSTRACT: Atmospheric aerosol particle formation is frequently observed in various environments. Yet, despite numerous studies, processes behind these so called nucleation events remain unclear. In this work we describe the use of data mining techniques to detect factors influencing particle formation. These techniques are applied to a dataset of eight years of 80 variables collected at the boreal forest station (SMEAR II) in Southern Finland, including air pollutant, weather, gas and particle measurements. In a previous study classification methods have been used together with feature selection in order to understand what causes nucleation. Each day was classi- fied as an event day, when a nucleation event occurs, or as a nonevent day, and looking at which features were selected gives us information on which factors are important for the aerosol formation process. This way it was possible to identify two key variables, relative humidity and preexisting aerosol particle surface (condensation sink), capable of explaining 88% of the nucleation events. Using these two variables a nucleation probability function could be derived. In this paper this nucleation probability function has been tested on data collected from other sites, Varri ¨ o in Northern Lapland and Aspvreten in Sweden. We show that in the extreme conditions in V¨ arri ¨ o the nucleation parameter does not work, whereas in Aspvreten the two key variables can be used to identify nucleation events, though the nucleation parameter has to be adjusted slightly. The two key variables are related to mechanisms that prevent nucleation. One reason for the domination of preventive mechanisms could be the existence of more than one mechanism causing nucleation. Another intriguing phenomenon, pos- sibly related to this, is the temporal variation of nucleation events. We have investigated temporal phenomena in nucleation by using classification methods in a sliding window.We discuss some aspects of this approach and present some results obtained.