Ilmakehän pienhiukkasten kemiallisen koostumuksen määrittäminen käyttäen suodatin- ja jatkuvatoimisia mittauksia

In order to evaluate the influence of ambient aerosol particles on cloud formation, climate and human health, detailed information about the concentration and composition of ambient aerosol particles is needed. The dura-tion of aerosol formation, growth and removal processes in the atmosphere range from minutes to hours, which highlights the need for high-time-resolution data in order to understand the underlying processes. This thesis focuses on characterization of ambient levels, size distributions and sources of water-soluble organic carbon (WSOC) in ambient aerosols. The results show that in the location of this study typically 50-60 % of organic carbon in fine particles is water-soluble. The amount of WSOC was observed to increase as aerosols age, likely due to further oxidation of organic compounds. In the boreal region the main sources of WSOC were biomass burning during the winter and secondary aerosol formation during the summer. WSOC was mainly attributed to a fine particle mode between 0.1 - 1 μm, although different size distributions were measured for different sources. The WSOC concentrations and size distributions had a clear seasonal variation. Another main focus of this thesis was to test and further develop the high-time-resolution methods for chemical characterization of ambient aerosol particles. The concentrations of the main chemical components (ions, OC, EC) of ambient aerosol particles were measured online during a year-long intensive measurement campaign conducted on the SMEAR III station in Southern Finland. The results were compared to the results of traditional filter collections in order to study sampling artifacts and limitations related to each method. To achieve better a time resolution for the WSOC and ion measurements, a particle-into-liquid sampler (PILS) was coupled with a total organic carbon analyzer (TOC) and two ion chromatographs (IC). The PILS-TOC-IC provided important data about diurnal variations and short-time plumes, which cannot be resolved from the filter samples. In summary, the measurements made for this thesis provide new information on the concentrations, size distribu-tions and sources of WSOC in ambient aerosol particles in the boreal region. The analytical and collection me-thods needed for the online characterization of aerosol chemical composition were further developed in order to provide more reliable high-time-resolution measurements.Ilmakehän aerosolit sisältävät pienhiukkasia, joiden koko, pitoisuus sekä kemiallinen koostumus täytyy tuntea, jotta niiden vaikutuksia pilvien muodostumiseen, ilmastoon ja terveyteen voidaan arvioida. Uusien aerosolihiukkasten synty-, kasvu- ja poistumisprosessit ovat nopeita, kestoltaan usein vain muutamista minuuteista tunteihin, minkä takia kemiallisen koostumuksen määrittämiseen tarvitaan jatkuvatoimisia, hyvän aikaresoluution tarjoavia laitteita. Tämä työ keskittyy aerosolihiukkasten vesiliukoisen orgaanisen hiilen pitoisuuden, kokojakaumien sekä lähteiden tarkasteluun Pohjois-Euroopan boreaalisella vyöhykkeellä. Vesiliukoisen orgaanisen hiilen päälähteet olivat biomassan palamisessa syntyneet hiukkaset talvella ja sekundääristen aerosolihiukkasten muodostuminen kesällä. Helsingissä SMEAR III -mittausasemalla tehdyissä mittauksissa pienhiukkasissa (Dp < 1μm) vesiliukoisen orgaanisen hiilen osuus oli keskimäärin 50 60 % orgaanisesta hiilestä. Vesiliukoisten yhdisteiden määrän havaittiin kasvavan aerosolin iän kasvaessa johtuen todennäköisesti orgaanisten yhdisteiden hapettumisesta ja siten muuntumisesta vesiliukoisemmiksi. Vesiliukoisen hiilen kokojakaumissa valtaosa vesiliukoisesta hiilestä oli akkumulaatiomoodissa, 0.1-1 μm:n kokoisissa hiukkasissa. Mitatut vesiliukoisen orgaanisen hiilen kokojakaumat olivat erilaisia eri lähteistä peräisin oleville hiukkasille. Vesiliukoisen orgaanisen hiilen määrässä ja kokojakaumissa havaittiin myös selkeää vuodenaikaisvaihtelua. Toisena päätavoitteena tässä työssä oli testata ja edelleen kehittää jatkuvatoimisia reaaliaikaisia mittausmenetelmiä aerosolien kemiallisen koostumuksen määrittämiseksi. Aerosolien pääkomponenttien (epäorgaaniset ionit, orgaaninen ja epäorgaaninen hiili) pitoisuuksia mitattiin reaaliaikaisesti vuoden ajan SMEAR III -asemalla Etelä-Suomessa. Uutta on se, että koostumuksen muutokset tunnetaan tarkasti ajan funktiona, jolloin päästölähteiden selvittäminen on oleellisesti helpompaa kuin aiemmin. Yhdistämällä PILS-keräin ionikromatografiin ja orgaanisen hiilen kokonaispitoisuutta mittaavaan laitteistoon mahdollistettiin ionien ja vesiliukoisen orgaanisen hiilen hiukkaspitoisuuksien samanaikainen määritys hyvällä aikaresoluutiolla, sekä saatiin uutta tietoa vesiliukoisen hiilen ja ionien vuorokausivaihtelusta ja lyhytkestoisista pitoisuuden muutoksista. Tässä tutkimuksessa tuotettiin uutta tietoa vesiliukoisen orgaanisen hiilen pitoisuuksista, kokojakaumista ja lähteistä Pohjois-Euroopassa. Näytteenkeräys- ja analyysimenetelmiä kehitettiin edelleen jatkuvatoimiseksi, jolloin lähes reaaliaikaiset aerosolien kemiallisen koostumuksen mittaukset tulivat mahdolliseksi

Cantilever-enhanced photoacoustic measurement of light-absorbing aerosols

Photoacoustic detection is a sensitive method for measurement of light-absorbing particles directly in the aerosol phase. In this article, we demonstrate a new sensitive technique for photoacoustic aerosol absorption measurements using a cantilever microphone for the detection of the photoacoustic signal. Compared to conventional diaphragm microphones, a cantilever offers increased sensitivity by up to two orders of magnitude. The measurement setup uses a photoacoustic cell from Gasera PA201 gas measurement system, which we have adapted for aerosol measurements. Here we reached a noise level of 0.013 Mm(-1) (one standard deviation) with a sampling time of 20 s, using a simple single-pass design without a need for a resonant acoustic cell. The sampling time includes 10 s signal averaging time and 10 s sample exchange, since the photoacoustic cell is designed for closed cell operation. We demonstrate the method in measurements of size-selected nigrosin particles and ambient black carbon. Due to the exceptional sensitivity, the technique shows great potential for applications where low detection limits are required, for example size-selected absorption measurements and black carbon detection in ultra clean environments.Peer reviewe

Laboratory evaluation of particle-size selectivity of optical low-cost particulate matter sensors

Low-cost particulate matter (PM) sensors have been under investigation as it has been hypothesized that the use of low-cost and easy-to-use sensors could allow cost-efficient extension of the currently sparse measurement coverage. While the majority of the existing literature highlights that low-cost sensors can indeed be a valuable addition to the list of commonly used measurement tools, it often reiterates that the risk of sensor misuse is still high and that the data obtained from the sensors are only representative of the specific site and its ambient conditions. This implies that there are underlying reasons for inaccuracies in sensor measurements that have yet to be characterized. The objective of this study is to investigate the particle-size selectivity of low-cost sensors. Evaluated sensors were Plantower PMS5003, Nova SDS011, Sensirion SPS30, Sharp GP2Y1010AU0F, Shinyei PPD42NS, and Omron B5W-LD0101. The investigation of size selectivity was carried out in the laboratory using a novel reference aerosol generation system capable of steadily producing monodisperse particles of different sizes (from similar to 0.55 to 8.4 mu m) on-line. The results of the study show that none of the low-cost sensors adhered to the detection ranges declared by the manufacturers; moreover, cursory comparison to a mid-cost aerosol size spectrometer (Grimm 1.108, 2020) indicates that the sensors can only achieve independent responses for one or two size bins, whereas the spectrometer can sufficiently characterize particles with 15 different size bins. These observations provide insight into and evidence of the notion that particle-size selectivity has an essential role in the analysis of the sources of errors in sensors.Peer reviewe

Seasonal and diurnal changes in inorganic ions, carbonaceous matter and mass in ambient aerosol particles in an urban, background area

Concentration and composition of the fine particulate matter (PM) was measured using various online methods for 13 months in an urban, background area in Helsinki, Finland. Seasonal differences were found for ions and carbonaceous compounds. Biomass burning was found to increase inorganic ion and elemental carbon (EC) concentrations in winter, whereas organic carbon (OC) contribution was highest during summer due to secondary aerosol formation. Diurnal cycles, with maxima between 06:00 and 09:00, were recorded for EC and nitrate due to traffic emissions. In addition, the concentrations measured with the online and offline PM sampling devices were compared using regression analysis. In general, a good agreement (r(2) = 0.60-0.95) was found. During the year-long measurements, on average 65% of PM2.5 was identified by submicron chemical analyses (ions, OC, EC). As compared with filter measurements, the high resolution measurements provided important data on short pollution plumes and diurnal changes.Peer reviewe

Determinants of spatial variability of air pollutant concentrations in a street canyon network measured using a mobile laboratory and a drone

Urban air pollutant concentrations are highly variable both in space and time. In order to understand these variabilities high-resolution measurements of air pollutants are needed. Here we present results of a mobile laboratory and a drone measurements made within a street-canyon network in Helsinki, Finland, in summer and winter 2017. The mobile lab-oratory measured the total number concentration (N) and lung-deposited surface area (LDSA) of aerosol particles, and the concentrations of black carbon, nitric oxide (NOx) and ozone (O3). The drone measured the vertical profile of LDSA. The main aims were to examine the spatial variability of air pollutants in a wide street canyon and its immediate surroundings, and find the controlling environmental variables for the observed variability's.The highest concentrations with the most temporal variability were measured at the main street canyon when the mo-bile laboratory was moving with the traffic fleet for all air pollutants except O3. The street canyon concentration levels were more affected by traffic rates whereas on surrounding areas, meteorological conditions dominated. Both the mean flow and turbulence were important, the latter particularly for smaller aerosol particles through LDSA and N. The formation of concentration hotspots in the street network were mostly controlled by mechanical processes but in winter thermal processes became also important for aerosol particles. LDSA showed large variability in the profile shape, and surface and background concentrations. The expected exponential decay functions worked better in well -mixed conditions in summer compared to winter. We derived equation for the vertical decay which was mostly con-trolled by the air temperature. Mean wind dominated the profile shape over both thermal and mechanical turbulence. This study is among the first experimental studies to demonstrate the importance of high-resolution measurements in understanding urban pollutant variability in detail.Peer reviewe

Concentration variation of gaseous and particulate pollutants in the helsinki city centre — observations from a two-year campaign from 2013–2015

The main chemical composition of PM1 (total organics, black carbon, sulphate, nitrate and ammonium), mass concentrations of PM2.5 and PM2.5–10 and concentration of specific trace gases were measured in a high temporal resolution from May 2013 to April 2015 in the city centre of Helsinki, Finland. On average, the concentrations of PM2.5 and PM2.5–10 were 9.1 µg m–3 and 16 µg m–3, respectively, during a two-year campaign. PM1 consisted mostly of organics (60%), followed by sulphate (12%), black carbon (11%), nitrate (9.8%) and ammonium (6.5%). The particle and gas data were combined with the meteorological data in order to obtain information on how local meteorology affects concentrations of air pollutants. Two meteorological parameters that mostly affected the pollutant concentrations were the wind speed and temperature, while sulphate and PM2.5–10 were also impacted by the relative humidity. The highest concentrations of the measured PM1 components were observed when the wind was calm or the temperature was either very cold or very warm. PM2.5–10 concentrations were at the highest during calm or very windy conditions, due to local street and construction dust. The seasonal-and diurnal-varying mixing height did not seem to affect markedly the concentrations of pollutants. Overall, air quality in terms of the aerosol mass was governed by three different main pollution sources in the Helsinki city centre: 1) local sources, of which traffic-related emissions were the most important; 2) long-range or regional transport of pollutants; and 3) local sources of organic aerosol. © 2019, Finnish Environment Institute. All rights reserved.Peer reviewe

Using monosaccharide anhydrides to estimate the impact of wood combustion on fine particles in the Helsinki Metropolitan Area

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Spatiotemporal variation and trends in equivalent black carbon in the Helsinki metropolitan area in Finland

In this study, we present results from 12 years of black carbon (BC) measurements at 14 sites around the Helsinki metropolitan area (HMA) and at one background site outside the HMA. The main local sources of BC in the HMA are traffic and residential wood combustion in fire-places and sauna stoves. All BC measurements were conducted optically, and therefore we refer to the measured BC as equivalent BC (eBC). Measurement stations were located in different environments that represented traffic environment, detached housing area, urban background, and regional background. The measurements of eBC were conducted from 2007 through 2018; however, the times and the lengths of the time series varied at each site. The largest annual mean eBC concentrations were measured at the traffic sites (from 0.67 to 2.64 mu g m(-3)) and the lowest at the regional background sites (from 0.16 to 0.48 mu g m(-3)). The annual mean eBC concentrations at the detached housing and urban background sites varied from 0.64 to 0.80 mu g m(-3) and from 0.42 to 0.68 mu g m(-3), respectively. The clearest seasonal variation was observed at the detached housing sites where residential wood combustion increased the eBC concentrations during the cold season. Diurnal variation in eBC concentration in different urban environments depended clearly on the local sources that were traffic and residential wood combustion. The dependency was not as clear for the typically measured air quality parameters, which were here NOx concentration and mass concentration of particles smaller that 2.5 mu m in diameter (PM2.5). At four sites which had at least a 4-year-long time series available, the eBC concentrations had statistically significant decreasing trends that varied from -10.4 % yr(-1) to -5.9 % yr(-1). Compared to trends determined at urban and regional background sites, the absolute trends decreased fastest at traffic sites, especially during the morning rush hour. Relative long-term trends in eBC and NOx were similar, and their concentrations decreased more rapidly than that of PM2.5. The results indicated that especially emissions from traffic have decreased in the HMA during the last decade. This shows that air pollution control, new emission standards, and a newer fleet of vehicles had an effect on air quality.Peer reviewe