Author
Pekka Rantala
Bio: Pekka Rantala is an academic researcher from University of Helsinki. The author has contributed to research in topics: Eddy covariance & Aerosol. The author has an hindex of 18, co-authored 38 publications receiving 1518 citations.
Papers
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TL;DR: Three separate size regimes below 2-nm diameter are identified that build up a physically, chemically, and dynamically consistent framework on atmospheric nucleation—more specifically, aerosol formation via neutral pathways.
Abstract: Atmospheric nucleation is the dominant source of aerosol particles in the global atmosphere and an important player in aerosol climatic effects. The key steps of this process occur in the sub–2-nanometer (nm) size range, in which direct size-segregated observations have not been possible until very recently. Here, we present detailed observations of atmospheric nanoparticles and clusters down to 1-nm mobility diameter. We identified three separate size regimes below 2-nm diameter that build up a physically, chemically, and dynamically consistent framework on atmospheric nucleation—more specifically, aerosol formation via neutral pathways. Our findings emphasize the important role of organic compounds in atmospheric aerosol formation, subsequent aerosol growth, radiative forcing and associated feedbacks between biogenic emissions, clouds, and climate.
890 citations
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TL;DR: In this paper, the authors measured the total loss rate of hydroxyl radicals (OH) caused by all reactive species in air at the Finnish boreal forest station SMEAR II in Hyytiala.
Abstract: . Ambient total OH reactivity was measured at the Finnish boreal forest station SMEAR II in Hyytiala (Latitude 61°51' N; Longitude 24°17' E) in July and August 2010 using the Comparative Reactivity Method (CRM). The CRM – total OH reactivity method – is a direct, in-situ determination of the total loss rate of hydroxyl radicals (OH) caused by all reactive species in air. During the intensive field campaign HUMPPA-COPEC 2010 (Hyytiala United Measurements of Photochemistry and Particles in Air – Comprehensive Organic Precursor Emission and Concentration study) the total OH reactivity was monitored both inside (18 m) and directly above the forest canopy (24 m) for the first time. The comparison between these two total OH reactivity measurements, absolute values and the temporal variation have been analyzed here. Stable boundary layer conditions during night and turbulent mixing in the daytime induced low and high short-term variability, respectively. The impact on total OH reactivity from biogenic emissions and associated photochemical products was measured under "normal" and "stressed" (i.e. prolonged high temperature) conditions. The advection of biomass burning emissions to the site caused a marked change in the total OH reactivity vertical profile. By comparing the OH reactivity contribution from individually measured compounds and the directly measured total OH reactivity, the size of any unaccounted for or "missing" sink can be deduced for various atmospheric influences. For "normal" boreal conditions a missing OH reactivity of 58%, whereas for "stressed" boreal conditions a missing OH reactivity of 89% was determined. Various sources of not quantified OH reactive species are proposed as possible explanation for the high missing OH reactivity.
127 citations
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TL;DR: In this article, the authors evaluated the capability of current state-of-the-art mass spectrometers equipped with different chemical ionization sources to detect the oxidation products formed from α-Pinene ozonolysis under various conditions.
Abstract: . The impact of aerosols on climate and air quality remains poorly understood
due to multiple factors. One of the current limitations is the incomplete
understanding of the contribution of oxygenated products, generated from the
gas-phase oxidation of volatile organic compounds (VOCs), to aerosol
formation. Indeed, atmospheric gaseous chemical processes yield thousands of
(highly) oxygenated species, spanning a wide range of chemical formulas,
functional groups and, consequently, volatilities. While recent mass
spectrometric developments have allowed extensive on-line detection of a
myriad of oxygenated organic species, playing a central role in atmospheric
chemistry, the detailed quantification and characterization of this diverse
group of compounds remains extremely challenging. To address this challenge,
we evaluated the capability of current state-of-the-art mass spectrometers
equipped with different chemical ionization sources to detect the oxidation
products formed from α -Pinene ozonolysis under various conditions.
Five different mass spectrometers were deployed simultaneously for a chamber
study. Two chemical ionization atmospheric pressure interface time-of-flight
mass spectrometers (CI-APi-TOF) with nitrate and amine reagent ion
chemistries and an iodide chemical ionization time-of-flight mass
spectrometer (TOF-CIMS) were used. Additionally, a proton transfer reaction
time-of-flight mass spectrometer (PTR-TOF 8000) and a new “vocus” PTR-TOF
were also deployed. In the current study, we compared around 1000 different
compounds between each of the five instruments, with the aim of determining
which oxygenated VOCs (OVOCs) the different methods were sensitive to and
identifying regions where two or more instruments were able to detect
species with similar molecular formulae. We utilized a large variability in
conditions (including different VOCs, ozone, NOx and OH scavenger
concentrations) in our newly constructed atmospheric simulation chamber for
a comprehensive correlation analysis between all instruments. This analysis,
combined with estimated concentrations for identified molecules in each
instrument, yielded both expected and surprising results. As anticipated
based on earlier studies, the PTR instruments were the only ones able to
measure the precursor VOC, the iodide TOF-CIMS efficiently detected many
semi-volatile organic compounds (SVOCs) with three to five oxygen atoms, and the
nitrate CI-APi-TOF was mainly sensitive to highly oxygenated organic (O > 5) molecules (HOMs). In addition, the vocus showed good
agreement with the iodide TOF-CIMS for the SVOC, including a range of
organonitrates. The amine CI-APi-TOF agreed well with the nitrate CI-APi-TOF
for HOM dimers. However, the loadings in our experiments caused the amine
reagent ion to be considerably depleted, causing nonlinear responses for
monomers. This study explores and highlights both benefits and limitations
of currently available chemical ionization mass spectrometry instrumentation
for characterizing the wide variety of OVOCs in the atmosphere. While
specifically shown for the case of α -Pinene ozonolysis, we expect
our general findings to also be valid for a wide range of other VOC–oxidant
systems. As discussed in this study, no single instrument configuration can
be deemed better or worse than the others, as the optimal instrument for a
particular study ultimately depends on the specific target of the study.
103 citations
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Max Planck Society1, University of Helsinki2, Finnish Meteorological Institute3, University of Science and Technology Houari Boumediene4, Indian Institute of Science5, Brigham Young University–Idaho6, University of Huelva7, Northwestern University8, University of California, San Diego9, Wageningen University and Research Centre10, University of Mainz11
TL;DR: In this paper, the authors describe the background, instrumentation, goals, and the regional influences on the HUMPPA-COPEC intensive field measurement campaign, conducted at the Boreal forest research station SMEAR II (Station for Measuring Ecosystem-Atmosphere Relation) in Hyytiala, Finland from 12 July-12 August 2010.
Abstract: . This paper describes the background, instrumentation, goals, and the regional influences on the HUMPPA-COPEC intensive field measurement campaign, conducted at the Boreal forest research station SMEAR II (Station for Measuring Ecosystem-Atmosphere Relation) in Hyytiala, Finland from 12 July–12 August 2010. The prevailing meteorological conditions during the campaign are examined and contrasted with those of the past six years. Back trajectory analyses show that meteorological conditions at the site in 2010 were characterized by a higher proportion of southerly flow than in the other years studied. As a result the summer of 2010 was anomalously warm and high in ozone making the campaign relevant for the analysis of possible future climates. A comprehensive land use analysis, provided on both 5 and 50 km scales, shows that the main vegetation types surrounding the site on both the regional and local scales are: coniferous forest (Scots pine and/or Norway spruce); mixed forest (Birch and conifers); and woodland scrub (e.g. Willows, Aspen); indicating that the campaign results can be taken as representative of the Boreal forest ecosystem. In addition to the influence of biogenic emissions, the measurement site was occasionally impacted by sources other than vegetation. Specific tracers have been used here to identify the time periods when such sources have impacted the site namely: biomass burning (acetonitrile and CO), urban anthropogenic pollution (pentane and SO2) and the nearby Korkeakoski sawmill (enantiomeric ratio of chiral monoterpenes). None of these sources dominated the study period, allowing the Boreal forest summertime emissions to be assessed and contrasted with various other source signatures.
102 citations
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TL;DR: A new model is developed with near-explicit representation of atmospheric new particle formation (NPF) and HOM formation that accurately depicts highly oxygenated molecule and climate dynamics over Boreal forests.
Abstract: Over Boreal regions, monoterpenes emitted from the forest are the main precursors for secondary organic aerosol (SOA) formation and the primary driver of the growth of new aerosol particles to climatically important cloud condensation nuclei (CCN). Autoxidation of monoterpenes leads to rapid formation of Highly Oxygenated organic Molecules (HOM). We have developed the first model with near-explicit representation of atmospheric new particle formation (NPF) and HOM formation. The model can reproduce the observed NPF, HOM gas-phase composition and SOA formation over the Boreal forest. During the spring, HOM SOA formation increases the CCN concentration by ~10 % and causes a direct aerosol radiative forcing of −0.10 W/m2. In contrast, NPF reduces the number of CCN at updraft velocities < 0.2 m/s, and causes a direct aerosol radiative forcing of +0.15 W/m2. Hence, while HOM SOA contributes to climate cooling, NPF can result in climate warming over the Boreal forest. Forests emit compounds into the atmosphere that are oxidized into highly oxygenated molecules that serve as precursors for cloud condensation nuclei–a process that impacts the climate, but is poorly represented in models. Here the authors create a new model that accurately depicts highly oxygenated molecule and climate dynamics over Boreal forests.
88 citations
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2,261 citations
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TL;DR: It is found that a direct pathway leads from several biogenic VOCs, such as monoterpenes, to the formation of large amounts of extremely low-volatility vapours, helping to explain the discrepancy between the observed atmospheric burden of secondary organic aerosol and that reported by many model studies.
Abstract: Forests emit large quantities of volatile organic compounds (VOCs) to the atmosphere. Their condensable oxidation products can form secondary organic aerosol, a significant and ubiquitous component of atmospheric aerosol, which is known to affect the Earth's radiation balance by scattering solar radiation and by acting as cloud condensation nuclei. The quantitative assessment of such climate effects remains hampered by a number of factors, including an incomplete understanding of how biogenic VOCs contribute to the formation of atmospheric secondary organic aerosol. The growth of newly formed particles from sizes of less than three nanometres up to the sizes of cloud condensation nuclei (about one hundred nanometres) in many continental ecosystems requires abundant, essentially non-volatile organic vapours, but the sources and compositions of such vapours remain unknown. Here we investigate the oxidation of VOCs, in particular the terpene α-pinene, under atmospherically relevant conditions in chamber experiments. We find that a direct pathway leads from several biogenic VOCs, such as monoterpenes, to the formation of large amounts of extremely low-volatility vapours. These vapours form at significant mass yield in the gas phase and condense irreversibly onto aerosol surfaces to produce secondary organic aerosol, helping to explain the discrepancy between the observed atmospheric burden of secondary organic aerosol and that reported by many model studies. We further demonstrate how these low-volatility vapours can enhance, or even dominate, the formation and growth of aerosol particles over forested regions, providing a missing link between biogenic VOCs and their conversion to aerosol particles. Our findings could help to improve assessments of biosphere-aerosol-climate feedback mechanisms, and the air quality and climate effects of biogenic emissions generally.
1,340 citations
01 Dec 2012
Abstract: We upscaled FLUXNET observations of carbon dioxide, water, and energy fluxes to the global scale using the machine learning technique, model tree ensembles (MTE). We trained MTE to predict site-level gross primary productivity (GPP), terrestrial ecosystem respiration (TER), net ecosystem exchange (NEE), latent energy (LE), and sensible heat (H) based on remote sensing indices, climate and meteorological data, and information on land use. We applied the trained MTEs to generate global flux fields at a 0.5 degrees x 0.5 degrees spatial resolution and a monthly temporal resolution from 1982 to 2008. Cross-validation analyses revealed good performance of MTE in predicting among-site flux variability with modeling efficiencies (MEf) between 0.64 and 0.84, except for NEE (MEf = 0.32). Performance was also good for predicting seasonal patterns (MEf between 0.84 and 0.89, except for NEE (0.64)). By comparison, predictions of monthly anomalies were not as strong (MEf between 0.29 and 0.52). Improved accounting of disturbance and lagged environmental effects, along with improved characterization of errors in the training data set, would contribute most to further reducing uncertainties. Our global estimates of LE (158 +/- 7 J x 10(18) yr(-1)), H (164 +/- 15 J x 10(18) yr(-1)), and GPP (119 +/- 6 Pg C yr(-1)) were similar to independent estimates. Our global TER estimate (96 +/- 6 Pg C yr(-1)) was likely underestimated by 5-10%. Hot spot regions of interannual variability in carbon fluxes occurred in semiarid to semihumid regions and were controlled by moisture supply. Overall, GPP was more important to interannual variability in NEE than TER. Our empirically derived fluxes may be used for calibration and evaluation of land surface process models and for exploratory and diagnostic assessments of the biosphere.
948 citations
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Goethe University Frankfurt1, CERN2, University of Helsinki3, Paul Scherrer Institute4, University of Beira Interior5, University of Innsbruck6, Carnegie Mellon University7, California Institute of Technology8, University of Leeds9, University of Eastern Finland10, University of Vienna11, Lebedev Physical Institute12, Finnish Meteorological Institute13, Kyoto University14, Helsinki Institute of Physics15, Stockholm University16, Leibniz Association17
TL;DR: The results show that, in regions of the atmosphere near amine sources, both amines and sulphur dioxide should be considered when assessing the impact of anthropogenic activities on particle formation.
Abstract: Nucleation of aerosol particles from trace atmospheric vapours is thought to provide up to half of global cloud condensation nuclei(1). Aerosols can cause a net cooling of climate by scattering sun ...
738 citations
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TL;DR: Many current topics are covered such as mesoscale meteorology, radar cloud studies and numerical cloud modelling, and topics from the second edition, such as severe storms, precipitation processes and large scale aspects of cloud physics, have been revised.
709 citations