Browsing by Subject "NO2"

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  • Molteni, Ugo; Simon, Mario; Heinritzi, Martin; Hoyle, Christopher R.; Bernhammer, Anne-Kathrin; Bianchi, Federico; Breitenlechner, Martin; Brilke, Sophia; Dias, António; Duplissy, Jonathan; Frege, Carla; Gordon, Hamish; Heyn, Claudia; Jokinen, Tuija; Kürten, Andreas; Lehtipalo, Katrianne; Makhmutov, Vladimir; Petäjä, Tuukka; Pieber, Simone M.; Praplan, Arnaud P.; Schobesberger, Siegfried; Steiner, Gerhard; Stozhkov, Yuri; Tomé, António; Tröstl, Jasmin; Wagner, Andrea C.; Wagner, Robert; Williamson, Christina; Yan, Chao; Baltensperger, Urs; Curtius, Joachim; Donahue, Neil M.; Hansel, Armin; Kirkby, Jasper; Kulmala, Markku; Worsnop, Douglas R.; Dommen, Josef (2019)
    Terpenes are emitted by vegetation, and their oxidation in the atmosphere is an important source of secondary organic aerosol (SOA). A part of this oxidation can proceed through an autoxidation process, yielding highly oxygenated organic molecules (HOMs) with low saturation vapor pressure. They can therefore contribute, even in the absence of sulfuric acid, to new particle formation (NPF). The understanding of the autoxidation mechanism and its kinetics is still far from complete. Here, we present a mechanistic and kinetic analysis of mass spectrometry data from α-pinene (AP) ozonolysis experiments performed during the CLOUD 8 campaign at CERN. We grouped HOMs in classes according to their identified chemical composition and investigated the relative changes of these groups and their components as a function of the reagent concentration. We determined reaction rate constants for the different HOM peroxy radical reaction pathways. The accretion reaction between HOM peroxy radicals was found to be extremely fast. We developed a pseudo-mechanism for HOM formation and added it to the AP oxidation scheme of the Master Chemical Mechanism (MCM). With this extended model, the observed concentrations and trends in HOM formation were successfully simulated.
  • Yli-Pelkonen, Vesa Johannes; Viippola, Juho Viljami; Kotze, David Johannes; Setälä, Heikki Martti (2017)
    Trees are believed to improve air quality, thus providing an important ecosystem service for urban inhabitants. However, empirical evidence on the beneficial effects of urban vegetation on air quality at the local level and in boreal climatic regions is scarce. We studied the influence of greenbelt-type forest patches on NO2 levels (i) in front of, (ii) inside and (iii) behind greenbelts next to major roads in the Helsinki Metropolitan Area, Finland, during summer and winter using passive collectors. Concentrations of NO2 were significantly higher in front of greenbelts compared to road sides without greenbelts. The more trees there were inside greenbelts the higher the NO2 level in front of greenbelts, likely due to the formation of a recirculation zone of air flow in front of greenbelts. Similarly, NO2 levels were higher inside greenbelts than in open areas without them, likely due to reduced air flow inside greenbelts. NO2 levels behind greenbelts were similar to those detected at the same distance from the road but without greenbelts. Our results suggest that, regardless of season, roadside greenbelts of mostly broadleaf trees do not reduce NO2 levels in near-road environments, but can result in higher NO2 levels in front of and inside greenbelts.
  • Kouki, Kerttu (Helsingfors universitet, 2017)
    Tutkimukseni tavoitteena oli selvittää, miten kasviperäisten hiukkasten määrä muuttuu Amazonilla ilmaston lämmetessä ja miten se vaikuttaa Amazonin ilmastoon. Hallitustenvälisen ilmastonmuutospaneelin (IPCC) mukaan merkittävimmät epävarmuudet ilmastonmuutoksessa liittyvät aerosoleihin, ja luonnolliset aerosolit aiheuttavat suuremman epävarmuuden ilmastoon kuin antropogeeniset aerosolit. Amazonin sademetsä on erityisen sopiva kasviperäisten hiukkasten tutkimiseen ja tutkimuskysymykseni kannalta kiinnostava, sillä etenkin sadekaudella valtaosa aerosoleista on kasviperäisiä. Tutkimusaineistona käytettiin satelliittien keräämiä havaintoja, joiden avulla määritettiin lämpötila (LST, engl. land surface temperature), aerosolien määrä ilmakehässä (AOD, engl. aerosol optical depth) sekä eri pienhiukkaslähteitä. Samat lähteet, jotka tuottavat pienhiukkasia ilmakehään, päästävät sinne myös hivenkaasuja. Erilaiset lähteet tuottavat erilaisia pienhiukkasia ja kaasuja, joten yhdistämällä havaintoja pienhiukkasista ja hivenkaasuista voidaan niiden lähteet selvittää luotettavammin. LST ja AOD määritettiin AATSR:n (Advanced Along-Track Scanning Radiometer) havaintojen avulla. Pienhiukkaslähteiden tunnistamiseen käytettiin OMI:n (Ozone Monitoring Instrument) keräämiä havaintoja typpidioksidista (NO2) ja formaldehydistä (HCHO) sekä AIRS:n (Atmospheric Infrared Sounder) havaintoja hiilimonoksidista (CO). Tulosteni mukaan pienhiukkasten määrä vaihtelee Amazonilla vuoden aikana varsin paljon: sadekaudella hiukkasten määrä on hyvin vähäinen, kun taas kuivalla kaudella määrä kasvaa moninkertaiseksi laajojen metsäpalojen seurauksena. Voisi olettaa, että lämpimämpinä aikoina myös metsäpalot lisääntyisivät, mutta tulosten mukaan palokaudella pienhiukkasten määrä pienenee lämpötilan noustessa. Suuri osa paloista on kuitenkin ihmisen sytyttämiä, joten myös ihmistoiminnalla on merkittävä vaikutus palokauden hiukkasiin. Hiilimonoksidia ja formaldehydiä muodostuu sadekaudella pääosin kasviperäisistä lähteistä, ja erityisesti hiilimonoksidin määrän havaittiin korreloivan positiivisesti lämpötilan kanssa, mikä viittaa kasviperäisten hiukkasten määrän kasvuun lämpötilan noustessa. Sadekaudella suurin osa hiukkasista on kasviperäisiä ja silloin AOD:n lämpötilariippuvuus on 0,008 ± 0,015 K-1, joten kasviperäisten hiukkasten suora säteilyvaikutus on siten –0,22 ± 0,40 Wm-2K-1 pilvettömälle taivaalla ja –0,08 ± 0,16 Wm-2K-1, kun pilvien osuus on 60 % koko taivaasta. Lämpötilan noustessa kasviperäiset hiukkaset siis todennäköisesti aiheuttavat negatiivisen säteilypakotteen ja siten hillitsevät ilmaston lämpenemistä. Toisaalta tulokseni kuitenkin osoittavat, että metsäpalot ovat hiukkasten merkittävin lähde Amazonilla, sillä metsäpalojen yhteydessä esiintyy merkittävästi luonnollista tasoa enemmän hiukkasia. Metsäpaloista syntyneet hiukkaset todennäköisesti määrittelevätkin AOD:n muutokset myös tulevaisuudessa.
  • Karhu, Juho; Hieta, Tuomas; Manoocheri, Farshid; Vainio, Markku; Ikonen, Erkki (2021)
    A high-sensitivity light-emitting diode (LED)-based photoacoustic NO2 sensor is demonstrated. Sensitive photoacoustic gas sensors based on incoherent light sources are typically limited by background noise and drifts due to a strong signal generated by light absorbed at the photoacoustic cell walls. Here, we reach a sub-ppb detection limit and excellent stability using cantilever-enhanced photoacoustic detection and perform a two-channel relative measurement. A white-light LED is used as a light source, and the spectrum is divided into two wavelength channels with a dichroic filter. The photoacoustic signals generated by the two wavelength channels are measured simultaneously and used to solve the NO2 concentration. The background signal is highly correlated between the two channels, and its variations are suppressed in the relative measurement. A noise level below 1 ppb is reached with an averaging time of 70 s. This is, to the best of our knowledge, the first time a sub-ppb detection limit is demonstrated with an LED-based photoacoustic NO2 sensor. As LEDs are available at a wide selection of emission wavelengths, the results show great potential for development of cost-effective and sensitive detectors for a variety of other trace gasses as well.
  • Viippola, Juho Viljami; Whitlow, Thomas; Zhao, Wenlin; Yli-Pelkonen, Vesa Johannes; Mikola, Juha Tapio; Pouyat, Richard; Setälä, Heikki Martti (2018)
    It is often stated that plants remove air pollutants from the urban atmosphere with their large leaf area, thus providing benefits − i.e. ecosystem services − for citizens. However, empirical evidence showing that local-scale air quality is uniformly improved by urban forests is scarce. We studied the influence of conifer-dominated peri-urban forests on the springtime levels of NO2 and particle pollution at different distances from roads, using passive samplers and high time resolution particle counters in a northern climate in Finland. Passive samplers provided average values over a one month period, while active particle counters provided real time measurements of air pollution to mimic human inhalation frequency. NO2 concentrations were slightly higher in forests than in adjacent open areas, while passive particle measurements showed the opposite trend. Active particle monitoring campaigns showed no systematic forest effect for PM2.5, but larger particles were reduced in the forest, corroborating the passive sampling result. Attenuation rates of the mean values of the studied pollutants did not differ between the forest and open habitats. However, high time resolution particle data revealed a distance effect that was apparent only in the forest transect: peak events at the forest edge were higher, while peaks furthest from the road were lower compared to the open transect. Furthermore, the magnitude of PM2.5 peak events was distinctly higher at forest edge than equivalent distance in the open area. Vegetation characteristics, such as canopy cover and tree density, did not explain differences in pollutant levels in majority of cases. Our results imply that evergreen-dominated forests near roads can slightly worsen local air quality regarding NO2 and PM2.5 in northern climates, but that coarser particle pollution can be reduced by such forest vegetation. It seems that the potential of roadside vegetation to mitigate air pollution is largely determined by the vegetation effects on airflow.
  • Yli-Pelkonen, Vesa Johannes; Scott, Anna A.; Viippola, Juho Viljami; Setälä, Heikki Martti (2017)
    Trees and other vegetation absorb and capture air pollutants, leading to the common perception that they, and trees in particular, can improve air quality in cities and provide an important ecosystem service for urban inhabitants. Yet, there has been a lack of empirical evidence showing this at the local scale with different plant configurations and climatic regions. We studied the impact of urban park and forest vegetation on the levels of nitrogen dioxide (NO2) and ground-level ozone (O3) while controlling for temperature during early summer (May) using passive samplers in Baltimore, USA. Concentrations of O3 were significantly lower in tree-covered habitats than in adjacent open habitats, but concentrations of NO2 did not differ significantly between tree-covered and open habitats. Higher temperatures resulted in higher pollutant concentrations and NO2 and O3 concentration were negatively correlated with each other. Our results suggest that the role of trees in reducing NO2 concentrations in urban parks and forests in the Mid-Atlantic USA is minor, but that the presence of tree-cover can result in lower O3 levels compared to similar open areas. Our results further suggest that actions aiming at local air pollution mitigation should consider local variability in vegetation, climate, micro-climate, and traffic conditions.
  • Yli-Pelkonen, Vesa Johannes; Scott, Anna A.; Viippola, Juho Viljami; Setälä, Heikki Martti (2017)
    Trees and other vegetation absorb and capture air pollutants, leading to the common perception that they, and trees in particular, can improve air quality in cities and provide an important ecosystem service for urban inhabitants. Yet, there has been a lack of empirical evidence showing this at the local scale with different plant configurations and climatic regions. We studied the impact of urban park and forest vegetation on the levels of nitrogen dioxide (NO2) and ground-level ozone (O3) while controlling for temperature during early summer (May) using passive samplers in Baltimore, USA. Concentrations of O3 were significantly lower in tree-covered habitats than in adjacent open habitats, but concentrations of NO2 did not differ significantly between tree-covered and open habitats. Higher temperatures resulted in higher pollutant concentrations and NO2 and O3 concentration were negatively correlated with each other. Our results suggest that the role of trees in reducing NO2 concentrations in urban parks and forests in the Mid-Atlantic USA is minor, but that the presence of tree-cover can result in lower O3 levels compared to similar open areas. Our results further suggest that actions aiming at local air pollution mitigation should consider local variability in vegetation, climate, micro-climate, and traffic conditions.