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Particle size is one of the key factors influencing how aerosol particles affect their climate and health effects. Therefore, a better understanding of particle size distributions from various sources is crucial. In urban environments, aerosols are produced in a large number of varying processes and conditions. This study intended to develop the source apportionment of urban aerosols by utilising a novel approach to positive matrix factorisation (PMF). The particle source profiles were detected in particle number size distribution data measured simultaneously in a street canyon and at a nearby urban background station between February 2015 and June 2019 in Helsinki, southern Finland. The novelty of the method is combining the data from both sites and finding profiles for the unified data. Five aerosol sources were found. Four of them were detected at both of the stations: slightly aged traffic (TRA2), secondary combustion aerosol (SCA), secondary aerosol (SecA), and long-range-transported aerosol (LRT). One of the sources, fresh traffic (TRA1) was only detected at a street canyon. The factors were identified based on available auxiliary data. Additionally, the trends of the found factors were studied, and statistically significant decreasing trends were found for TRA1 and SecA. A statistically significant increasing trend was found for TRA2. This work implies that traffic-related aerosols remain important in urban environments and that aerosol sources can be detected using only particle number size distribution data as input in the PMF method. [ABSTRACT FROM AUTHOR]

Particle size is one of the key parameters of aerosol particles affecting their climate and health effects. Therefore, a better understanding of the particle size distributions from different sources is crucial. In urban environments, aerosols are produced in a large number of varying processes, and conditions. This study aims to increase the knowledge of urban aerosol sources using a novel approach to positive matrix factorization (PMF). The particle source profiles are detected in particle number size distribution data measured simultaneously at nearby a street canyon and an urban background station between February 2015 and June 2019 in Helsinki southern Finland. The data is combined into one file so that the data from both stations has the same timestamps. Then PMF finds profiles for the unified data. A total of five different aerosol sources were found. Four of them were detected at both stations: slightly aged traffic (TRA2), secondary combustion aerosol (SCA), secondary aerosol (SecA), and long-range transported aerosol (LRT). One of the sources, fresh traffic was only detected at a street canyon. The factors were identified based on available auxiliary data. This work implies that traffic-related aerosols remain important in urban environments and that aerosol sources can be detected by using only particle number size distribution data as input in the positive matrix factorization method. [ABSTRACT FROM AUTHOR]

Particle size is one of the key parameters of aerosol particles affecting their climate and health effects. Therefore, a better understanding of the particle size distributions from different sources is crucial. In urban environments, aerosols are produced in a large number of varying processes, and conditions. This study aims to increase the knowledge of urban aerosol sources using a novel approach to positive matrix factorization (PMF). The particle source profiles are detected in particle number size distribution data measured simultaneously at nearby a street canyon and an urban background station between February 2015 and June 2019 in Helsinki southern Finland. The data is combined into one file so that the data from both stations has the same timestamps. Then PMF finds profiles for the unified data. A total of five different aerosol sources were found. Four of them were detected at both of the stations: slightly aged traffic (TRA2), secondary combustion aerosol (SCA), secondary aerosol (SecA), and long-range transported aerosol (LRT). One of the sources, fresh traffic was only detected at a street canyon. The factors were identified based on available auxiliary data. This work implies that trafficrelated aerosols remain important in urban environments and that aerosol sources can be detected by using only particle number size distribution data as input in the positive matrix factorization method. [ABSTRACT FROM AUTHOR]

Atmospheric aerosols play an important role in air pollution. Aerosol particle chemical composition is highly variable depending on the season, hour of the day, day of the week, meteorology, and the location of the measurement site. Long measurement periods and high time-resolved data are required in order to achieve statistically relevant amount of data for assessing those variations and evaluate pollution episodes. In this study, we present continuous atmospheric PM1 (particulate matter < 1 µm) concentration and composition measurements at an urban street canyon site located in Helsinki, Finland. The study was performed for four and a half years (2015–2019) and involved highly time-resolved measurements by taking advantage of a suite of online state-of-art instruments such as Aerosol Chemical Speciation Monitor (ACSM), MultiAngle Absorption Photometer (MAAP), Differential Mobility Particle Sizer (DMPS) and Aethalometer (AE). PM1 consisted mostly of organics, with mean mass concentrations of 2.89 µg m-3 (53 % of PM1) followed by inorganic species (1.56 µg m-3, 29 %) and equivalent black carbon (eBC, 0.97 µg m-3, 18 %). A trend analysis revealed a decrease in BC from fossil fuel (BCFF), organics and nitrate over the studied years. Clear seasonal and/or diurnal variations were found for the measured atmospheric PM1 constituents. Particle number and mass size distributions over different seasons revealed the possible influence of secondary organic aerosols (SOA) during summer and the dominance of ultrafine traffic aerosols during winter. The seasonality of measured constituents impacted also on particle´s coating and absorptive properties. The investigation of pollution episodes observed at the site showed that a large fraction of aerosol particle mass was comprised of inorganic species during long-range transport, while during local episodes eBC and organics prevailed together with elevated particle number concentration. Overall, the results enlightened the variability of PM1 concentration and composition in a Nordic traffic site, and its implications on urban air quality. Considering the effects of PM mitigation policies in Northern Europe in the last decades, the results obtained in this study may be considered as illustrative of probable future air quality challenges in countries currently adopting similar environmental regulations. [ABSTRACT FROM AUTHOR]

New particle formation (NPF) events occur almost everywhere in the world and can play an important role as a particle source. The frequency and characteristics of NPF events vary spatially and this variability is yet to be fully understood. In the present study, long term particle size distribution datasets (minimum of three years) from thirteen sites of various land uses and climates from across Europe were studied and NPF events, deriving from secondary formation and not traffic related nucleation, were extracted and analysed. The frequency of NPF events was consistently found to be higher at rural background sites, while the growth and formation rates of newly formed particles were higher at roadsides, underlining the importance of the abundance of condensable compounds of anthropogenic origin found there. The growth rate was higher in summer at all rural background sites studied. The urban background sites presented the highest uncertainty due to greater variability compared to the other two types of site. The origin of incoming air masses and the specific conditions associated with them greatly affect the characteristics of NPF events. In general, cleaner air masses present higher probability for NPF events, while the more polluted ones show higher growth rates. However, different patterns of NPF events were found even at sites in close proximity (< 200 km) due to the different local conditions at each site. Region-wide events were also studied and were found to be associated with the same conditions as local events, although some variability was found which was associated with the different seasonality of the events at two neighbouring sites. NPF events were responsible for an increase in the number concentration of ultrafine particles of more than 400% at rural background sites on the day of their occurrence. The degree of enhancement was less at urban sites due to the increased contribution of other sources within the urban environment. It is evident that, while some variables (such as solar radiation intensity, relative humidity or the concentrations of specific pollutants) appear to have a similar influence on NPF events across all sites, it is impossible to predict the characteristics of NPF events at a site using just these variables, due to the crucial role of local conditions. [ABSTRACT FROM AUTHOR]

Although new particle formation (NPF) events have been studied extensively for some decades, the mechanisms that drive their occurrence and development are yet to be fully elucidated. Laboratory studies have done much to elucidate the molecular processes involved in nucleation, but this knowledge has yet to be linked to NPF events in the atmosphere, except at very clean air sites. There is great difficulty in successful application of the results from laboratory studies to real atmospheric conditions, due to the diversity of atmospheric conditions and observations found, as NPF events occur almost everywhere in the world without following a clearly defined trend of frequency, seasonality, atmospheric conditions or event development. The present study seeks common features in nucleation events by applying a binned linear regression over an extensive dataset from 16 sites of various types (rural and urban backgrounds as well as roadsides) in Europe. A clear positive relation is found between the solar radiation intensity, temperature and atmospheric pressure with the frequency of NPF events, while relative humidity presents a negative relation with NPF event frequency. Wind speed presents a less consistent relationship which appears to be heavily affected by local conditions. While some meteorological variables appear to have a crucial effect on the occurrence and characteristics of NPF events, especially at rural sites, it appears that their role becomes less marked when at higher values. The analysis of chemical composition data presents interesting results. Concentrations of almost all chemical compounds studied (apart from O3) and the Condensation Sink (CS) have a negative relation with NPF event probability, though areas with higher average concentrations of SO2 had higher NPF event probability. Particulate Organic Carbon (OC), Volatile Organic Compounds (VOCs) and particulate phase sulphate consistently had a positive relation with the growth rate of the newly formed particles. As with some meteorological variables, it appears that at increased concentrations of pollutants or the CS, their influence upon NPF probability is reduced. [ABSTRACT FROM AUTHOR]

Long term statistics of atmospheric aerosol and especially cloud scavenging were studied at the Puijo measurement station in Kuopio, Finland, during October 2010-November 2014. Aerosol size distributions, scattering coefficients at three different wavelengths (450 nm, 550 nm, and 700 nm), and absorption coefficient at wavelength 637 nm were measured with a special inlet system to sample interstitial and total aerosol in clouds. On average, accumulation mode particle concentration was found to be temperature dependent with lowest average concentrations of 200 cm-3 around 0 °C increasing to more than 800 cm-3 for temperatures higher than 20 °C. From the in-cloud measurements, both scattering and absorbing material scavenging efficiencies were observed to have slightly increasing temperature dependence. At 0 °C the efficiencies of scattering and absorbing matter were 0.85 and 0.55 with slopes of 0.005 °C-1 and 0.003 °C-1, respectively. Additionally, scavenging efficiencies were studied as a function of the diameter at which half of the particles are activated into cloud droplets. This analysis indicated that the is a higher fraction of absorbing material, typically black carbon, in smaller sizes so that at least 20-30 % of interstitial particles within clouds consist of absorbing material. In addition, the PM1-inlet revealed that approximately 20 % of absorbing material was observed to reside in particles with ambient diameter larger than ~ 1 µm at relative humidity below 90 %. Similarly, 40 % of scattering material was seen to be in particles larger than 1 µm. Altogether, this dataset provides information on size dependent aerosol composition that can be applied in evaluating how well large-scale aerosol models reproduce aerosol composition, especially with respect to scavenging in stratus clouds. [ABSTRACT FROM AUTHOR]

Lung deposited surface area (LDSA) is a relatively new metric that has been argued to be more accurate at predicting health effects from aerosol exposure. For typical atmospheric aerosol, the LDSA concentration depends mainly on the concentration of ultrafine particles (e.g. vehicular exhaust emissions and residential wood combustion) and therefore optical methods cannot be used to measure and quantify it. The objective of this study was to investigate and describe typical characteristics of LDSA under different urban environments and evaluate how a diffusion charging-based Pegasor AQ Urban sensor (Pegasor Ltd., Finland) can be used as an alternative to optical sensors when assessing local combustion emissions and respective LDSA concentrations. Long-term (12 months) sensor measurements of LDSA were carried out at three distinctly different measurement sites (four sensor nodes) in the Helsinki metropolitan area, Finland. The sites were affected mainly by vehicular exhaust emission (street canyon and urban background stations) and by residential wood combustion (two detached housing area stations). The results showed that the accuracy of the AQ Urban was good (R2 = 0.90) for the measurement of LDSA when compared to differential mobility particle sizer. The mean concentrations of LDSA were more than twice as high at the street canyon (mean 22 µm2 cm−3) site when compared to the urban background site (mean 9.4 µm2 cm−3). In the detached housing area, the mean concentrations were 12 µm2 cm−3, and wood combustion typically caused high LDSA peaks in the evenings. High correlations and similar diurnal cycles were observed for the LDSA and black carbon at street canyon and urban background stations. The utilization of a small-scale sensor network (four nodes) showed that the cross-station variability in hourly LDSA concentrations was significant in every site, even within the same detached housing area (distance between the two sites ∼670 m). [ABSTRACT FROM AUTHOR]

Four new ground-based atmospheric monitoring stations in Finland were examined for local and large-scale signals in carbon dioxide mole fraction, and the results were compared with the corresponding values obtained from WMO/GAW site Pallas, northern Finland and NOAA/ESRL marine boundary layer reference time series. The measurements were filtered with knowledge of local weather and air composition. Periods of a presumably well-mixed boundary layer and relatively pollutant-free air were close to the Pallas time series in mole fraction, particularly in the winter. Wintertime mole fractions were 5-10 ppm higher than the signal in the marine boundary layer at all stations. The seasonal amplitude was 18-24 ppm, and diurnal amplitude was 0-3 ppm in winter and 3-20 ppm in the summer months. All stations, with the possible exception of the urban site in Helsinki, showed potential for observing a large-scale carbon dioxide signal. [ABSTRACT FROM AUTHOR]

The Puijo measurement station has produced continuous data on aerosol-cloud interactions since June 2006. The station is located on the top floor of an observation lower in a semi-urban environment near the town of Kuopio in central Finland. The top of the tower (306 m a.s.1.) has been detected to be in cloud approximately 10% of the time. We analysed continuous weather, particle size distribution and cloud droplet size distribution measurements. The effects of local pollutant sources and air mass origin on aerosol-cloud interaction were examined in detail. We were able to find clear evidence of file aerosol indirect effects at the Puijo site. There is a positive correlation between cloud droplet number concentration and particle number concentration. Higher cloud droplet concentration led to a smaller average cloud droplet size. Furthermore, the ratio of cloud droplet number concentration to accumulation mode particle number concentration is smaller when the particle number concentration is higher. Results from ore trajectory analysis indicated that at our site marine air masses had higher particle concentrations and the continental aerosols are more effective in acting as cloud condensation nuclei than marine aerosols, probably due to their larger mean size. We could also distinguish the effect of local pollutant sources. [ABSTRACT FROM AUTHOR]

We introduce a new measurement station that we established in 2005 in an observation tower at Puijo in Kuopio. At Puijo we measure several meteorological parameters, aerosol and cloud droplet size distribution, aerosol optical properties and trace gas concentrations. We summarize the research activities at the station during its three year history and present overall results. We compare the results from Puijo with those measured at the ground level in Kuopio and at the Finnish background stations. We also characterize the measured parameters according to the wind direction and air mass origins, based on trajectory analysis, for the effects of local and remote sources. Our conclusion is that the Puijo tower is a very good place to gather experimental data on cloud formation and aerosol-cloud interaction. In addition to cloud experiments, we would suggest the Puijo measurement station for studies of particle formation, which we also observed frequently. [ABSTRACT FROM AUTHOR]