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  • Neefjes, Ivo; Laapas, Mikko; Liu, Yang; Medus, Erika; Miettunen, Elina; Ahonen, Lauri; Quelever, Lauriane; Aalto, Juho; Bäck, Jaana; Kerminen, Veli-Matti; Lampilahti, Janne; Luoma, Krista; Mäki, Mari; Mammarella, Ivan; Petäjä, Tuukka; Räty, Meri; Sarnela, Nina; Ylivinkka, Ilona; Hakala, Simo; Kulmala, Markku; Nieminen, Tuomo; Lintunen, Anna (2022)
    Boreal forests are an important source of trace gases and atmospheric aerosols, as well as a crucial carbon sink. As such, they form a strongly interconnected coupled system with the atmosphere. The SMEAR II station is located in a boreal Scots pine forest in Hyytiala, Finland, and has over 25 years of continuous measurements of atmospheric and ecosystem variables. In this study, we analyse the seasonal variations of trace gases, atmospheric aerosols, greenhouse gases, and meteorological variables, measured at the SMEAR II sta-tion during the past two and a half decades. Several ecosystem and atmospheric variables show seasonal correlations with each other, which suggests seasonal interactions within the climate system that links together ecosystem processes, greenhouse gases, trace gases and atmospheric aerosols. For instance, increased global radiation in summer increases air temperature and consequently affects the plant phenology, which promotes the ecosystem carbon exchange and biogenic volatile organic compound (biogenic VOC) release. This further affects the ambient concentrations of highly oxygenated organic molecules (HOMs) as well as the formation and growth of atmospheric organic aerosols. Organic aerosols subsequently influence aerosol optical properties and, through increased scattering, have the potential to cool the climate. We also discuss the impacts of the warm and dry summers of 2010 and 2018 on the studied variables. For these years, we find a higher-than-average ecosystem primary production especially in June, leading to an increased VOC flux from the forest. The increased VOC flux in turn leads to higher HOM and secondary aerosol concentration in the atmosphere. The latter increases light scattering by atmospheric aero-sol particles and thus leads to climate cooling. The results obtained in this study improve our understanding of how boreal forests respond to climate change.
  • Mazon, Stephany Buenrostro; Kontkanen, Jenni; Manninen, Hanna E.; Nieminen, Tuomo; Kerminen, Veli-Matti; Kulmala, Markku (2016)
    New particle formation (NPF) events are typically observed during daytime when photochemical oxidation takes place. However, nighttime nucleation mode particles have been observed across various locations only sporadically. We present 11 years (2003-2013) of air ion number size distribution data from the SMEAR II station in Hyytiala, Finland, where during a third of the nights a sub-3 nm negative (n = 1324 days) and positive (n = 1174 days) ion events took place. To investigate nocturnal clustering at sizes above the constant small ion pool, we defined cluster events (CE) as a nocturnal event with 2-3 nm ion concentrations reaching 70 cm-3 between 18:00 and 24:00 local time. CE (n = 221 days) were characterized by a rapid, 10-fold increase in the median 2-3 nm ion concentration from the start (similar to 10 cm(-3)) to the event peak (similar to 100 cm(-3)). Furthermore, small and intermediate ions during the CE, NPF events and nonevents were compared: while concentrations of 1.5-2 nm ions were the highest during CE (median 235 cm(-3)), as compared with the NPF events (96 cm(-3)) or the daytime and nighttime nonevents (similar to 20 cm(-3)), 3-7 nm ion concentrations increased notably only during NPF events (median 52 cm(-3)). Specifically, ion concentrations during CE decreased for sizes above-2.4 nm (<10 cm(-3)). In addition, 90% of CE proceeded either a NPF event (55%) or a undefined day (35%), and only 10% of them proceeded a daytime non-event. This study suggests a build-up of 0.9-2.4 nm ion clusters during CE nights (18:00-24:00) that equals or exceeds the ion concentration levels during daytime NPF, but unlike the latter, CE fail to activate and grow clusters > 3 nm in diameter in nighttime Hyytiald.
  • Karl, Matthias; Gross, Allan; Pirjola, Liisa; Leck, Caroline (2011)
  • Xausa, Filippo; Paasonen, Pauli; Makkonen, Risto; Arshinov, Mikhail; Ding, Aijun; Van Der Gon, Hugo Denier; Kerminen, Veli-Matti; Kulmala, Markku (2018)
    Climate models are important tools that are used for generating climate change projections, in which aerosol-climate interactions are one of the main sources of uncertainties. In order to quantify aerosol-radiation and aerosolcloud interactions, detailed input of anthropogenic aerosol number emissions is necessary. However, the anthropogenic aerosol number emissions are usually converted from the corresponding mass emissions in pre-compiled emission inventories through a very simplistic method depending uniquely on chemical composition, particle size and density, which are defined for a few, very wide main source sectors. In this work, the anthropogenic particle number emissions converted from the AeroCom mass in the ECHAM-HAM climate model were replaced with the recently formulated number emissions from the Greenhouse Gas and Air Pollution Interactions and Synergies (GAINS) model. In the GAINS model the emission number size distributions vary, for example, with respect to the fuel and technology. Special attention was paid to accumulation mode particles (particle diameter d(p) > 100 nm) because of (i) their capability of acting as cloud condensation nuclei (CCN), thus forming cloud droplets and affecting Earth's radiation budget, and (ii) their dominant role in forming the coagulation sink and thus limiting the concentration of sub-100 nm particles. In addition, the estimates of anthropogenic CCN formation, and thus the forcing from aerosol-climate interactions, are expected to be affected. Analysis of global particle number concentrations and size distributions reveals that GAINS implementation increases CCN concentration compared with AeroCom, with regional enhancement factors reaching values as high as 10. A comparison between modeled and observed concentrations shows that the increase in number concentration for accumulation mode particles agrees well with measurements, but it leads to a consistent underestimation of both nucleation mode and Aitken mode (d(p) <100 nm) particle number concentrations. This suggests that revisions are needed in the new particle formation and growth schemes currently applied in global modeling frameworks.
  • Kerminen, Veli-Matti; Chen, Xuemeng; Vakkari, Ville; Petäjä, Tuukka; Kulmala, Markku; Bianchi, Federico (2018)
    This review focuses on the observed characteristics of atmospheric new particle formation (NPF) in different environments of the global troposphere. After a short introduction, we will present a theoretical background that discusses the methods used to analyze measurement data on atmospheric NPF and the associated terminology. We will update on our current understanding of regional NPF, i.e. NPF taking simultaneously place over large spatial scales, and complement that with a full review on reported NPF and growth rates during regional NPF events. We will shortly review atmospheric NPF taking place at sub-regional scales. Since the growth of newly-formed particles into larger sizes is of great current interest, we will briefly discuss our observation-based understanding on which gaseous compounds contribute to the growth of newly-formed particles, and what implications this will have on atmospheric cloud condensation nuclei formation. We will finish the review with a summary of our main findings and future outlook that outlines the remaining research questions and needs for additional measurements.
  • Groess, Johannes; Hamed, Amar; Sonntag, Andre; Spindler, Gerald; Manninen, Hanna Elina; Nieminen, Tuomo; Kulmala, Markku; Horrak, Urmas; Plass-Dülmer, Christian; Wiedensohler, Alfred; Birmili, Wolfram (2018)
    This paper revisits the atmospheric new particle formation (NPF) process in the polluted Central European troposphere, focusing on the connection with gas-phase precursors and meteorological parameters. Observations were made at the research station Melpitz (former East Germany) between 2008 and 2011 involving a neutral cluster and air ion spectrometer (NAIS). Particle formation events were classified by a new automated method based on the convolution integral of particle number concentration in the diameter interval 2-20 nm. To study the relevance of gaseous sulfuric acid as a precursor for nucleation, a proxy was derived on the basis of direct measurements during a 1-month campaign in May 2008. As a major result, the number concentration of freshly produced particles correlated significantly with the concentration of sulfur dioxide as the main precursor of sulfuric acid. The condensation sink, a factor potentially inhibiting NPF events, played a subordinate role only. The same held for experimentally determined ammonia concentrations. The analysis of meteorological parameters confirmed the absolute need for solar radiation to induce NPF events and demonstrated the presence of significant turbu-lence during those events. Due to its tight correlation with solar radiation, however, an independent effect of turbulence for NPF could not be established. Based on the diurnal evolution of aerosol, gas-phase, and meteorological parameters near the ground, we further conclude that the particle formation process is likely to start in elevated parts of the boundary layer rather than near ground level.
  • Hao, Liqing; Garmash, Olga; Ehn, Mikael; Miettinen, Pasi; Massoli, Paola; Mikkonen, Santtu; Jokinen, Tuija; Roldin, Pontus; Aalto, Pasi; Yli-Juuti, Taina; Joutsensaari, Jorma; Petäjä, Tuukka; Kulmala, Markku; Lehtinen, Kari E. J.; Worsnop, Douglas R.; Virtanen, Annele (2018)
    Characterizing aerosol chemical composition in response to meteorological changes and atmospheric chemistry is important to gain insights into new particle formation mechanisms. A BAECC (Biogenic Aerosols - Effects on Clouds and Climate) campaign was conducted during the spring 2014 at the SMEAR II station (Station for Measuring Forest Ecosystem-Aerosol Relations) in Finland. The particles were characterized by a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). A PBL (planetary boundary layer) dilution model was developed to assist interpreting the measurement results. Right before nucleation events, the mass concentrations of organic and sulfate aerosol species were both decreased rapidly along with the growth of PBL heights. However, the mass fraction of sulfate aerosol of the total aerosol mass was increased, in contrast to a decrease for the organic mass fraction. Meanwhile, an increase in LVOOA (low-volatility oxygenated organic aerosol) mass fraction of the total organic mass was observed, in distinct comparison to a reduction of SVOOA (semi-volatile OOA) mass fraction. Our results demonstrate that, at the beginning of nucleation events, the observed sulfate aerosol mass was mainly driven by vertical turbulent mixing of sulfate-rich aerosols between the residual layer and the newly formed boundary layer, while the condensation of sulfuric acid (SA) played a minor role in interpreting the measured sulfate mass concentration. For the measured organic aerosols, their temporal profiles were mainly driven by dilution from PBL development, organic aerosol mixing in different boundary layers and/or partitioning of organic vapors, but accurate measurements of organic vapor concentrations and characterization on the spatial aerosol chemical composition are required. In general, the observed aerosol particles by AMS are subjected to joint effects of PBL dilution, atmospheric chemistry and aerosol mixing in different boundary layers. During aerosol growth periods in the nighttime, the mass concentrations of organic aerosols and organic nitrate aerosols were both increased. The increase in SVOOA mass correlated well with the calculated increase in condensed HOMs' (highly oxygenated organic molecules) mass. To our knowledge, our results are the first atmospheric observations showing a connection between increase in SVOOA and condensed HOMs during the nighttime.
  • Zhou, L.; Gierens, R.; Sogachev, A.; Mogensen, D.; Ortega, J.; Smith, J. N.; Harley, P. C.; Prenni, A. J.; Levin, E. J. T.; Turnipseed, A.; Rusanen, A.; Smolander, S.; Guenther, A. B.; Kulmala, Markku; Karl, T.; Boy, M. (2015)
    New particle formation (NPF) is an important atmospheric phenomenon. During an NPF event, particles first form by nucleation and then grow further in size. The growth step is crucial because it controls the number of particles that can become cloud condensation nuclei. Among various physical and chemical processes contributing to particle growth, condensation by organic vapors has been suggested as important. In order to better understand the influence of biogenic emissions on particle growth, we carried out modeling studies of NPF events during the BEACHON-ROCS (Biohydro-atmosphere interactions of Energy, Aerosol, Carbon, H2O, Organics & Nitrogen - Rocky Mountain Organic Carbon Study) campaign at Manitou Experimental Forest Observatory in Colorado, USA. The site is representative of the semi-arid western USA. With the latest Criegee intermediate reaction rates implemented in the chemistry scheme, the model underestimates sulfuric acid concentration by 50 %, suggesting either missing sources of atmospheric sulfuric acid or an overestimated sink term. The results emphasize the contribution from biogenic volatile organic compound emissions to particle growth by demonstrating the effects of the oxidation products of monoterpenes and 2-Methyl-3-buten-2-ol (MBO). Monoterpene oxidation products are shown to influence the nighttime particle loadings significantly, while their concentrations are insufficient to grow the particles during the day. The growth of ultrafine particles in the daytime appears to be closely related to the OH oxidation products of MBO.
  • Lawler, Michael J.; Rissanen, Matti P.; Ehn, Mikael; Mauldin, R. Lee; Sarnela, Nina; Sipilä, Mikko; Smith, James N. (2018)
    New particle formation (NPF) is an important contributor to particle number in many locations, but the chemical drivers for this process are not well understood. Daytime NPF events occur regularly in the springtime Finnish boreal forest and strongly impact aerosol abundance. In April 2014 size-resolved chemical measurements of ambient nanoparticles were made using the Time-of-Flight Thermal Desorption Chemical ionization Mass Spectrometer and we report results from two NPF events. While growth overall was dominated by terpene oxidation products, newly formed 20-70nm particles showed enhancement in apparent alkanoic acids. The events occurred on days with rapid transport of marine air, which correlated with low background aerosol loading and higher gas phase methanesulfonic acid levels. These results are broadly consistent with previous studies on Nordic NPF but indicate that further attention should be given to the sources and role of non-terpenoid organics and the possible contribution of transported marine compounds in this process. Plain Language Summary Clouds are an enormously important part of the climate system because they control the radiation entering and leaving the Earth. Clouds form as water condenses onto small particles called cloud condensation nuclei. These particles can be directly emitted from the Earth's surface, like sea spray, for example, or they can form in the atmosphere out of precursor gases. We have measured the composition of these atmosphere-formed particles to understand better how this process works in the Nordic boreal forest. We found that a diverse mix of processes and molecules are likely involved, possibly including the transport of materials from the ocean. While these results will ultimately lead to a better understanding of ocean-land-cloud interactions, they currently indicate that more work is needed to learn the processes involved.
  • Sahyoun, Maher; Freney, Evelyn; Brito, Joel; Duplissy, Jonathan; Gouhier, Mathieu; Colomb, Aurélie; Dupuy, Regis; Bourianne, Thierry; Nowak, John B.; Yan, Chao; Petäjä, Tuukka; Kulmala, Markku; Schwarzenboeck, Alfons; Planche, Céline; Sellegri, Karine (2019)
    Abstract Volcanic emissions can significantly affect the Earth's radiation budget by emitting aerosol particles and gas-phase species that can result in the new particle formation (NPF). These particles can scatter solar radiation or modify cloud properties, with consequences on health, weather, and climate. To our knowledge, this is the first dedicated study detailing how gas-phase precursors emitted from volcanic plumes can influence the NPF. A series of airborne measurements were performed around the Etna and Stromboli volcanoes within the framework of the CLerVolc and STRAP projects. The ATR-42 aircraft was equipped with a range of instrumentation allowing the measurement of particle number concentration in diameter range above 2.5 nm, and gaseous species to investigate the aerosol dynamics and the processes governing the NPF and their growth within the volcanic plumes. We demonstrate that NPF occurs within the volcanic plumes in the Free Troposphere (FT) and Boundary Layer (BL). Typically, the NPF events were more pronounced in the FT, where the condensational sink was up to two orders of magnitude smaller and the temperature was ~20°C lower than in the BL. Within the passive volcanic plume, the concentration of sulfur dioxide, sulfuric acid, and N2.5 were as high as 92 ppbV, 5.65?108 and 2.4?105 cm?3, respectively. Using these measurements, we propose a new parameterization for NPF rate (J2.5) within the passive volcanic plume in the FT. These results can be incorporated into mesoscale models to better assess the impact of the particle formed by natural processes, i.e. volcanic plumes, on climate.
  • Zaidan, Martha A.; Haapasilta, Ville; Relan, Rishi; Paasonen, Pauli; Kerminen, Veli-Matti; Junninen, Heikki; Kulmala, Markku; Foster, Adam S. (2018)
    Atmospheric new-particle formation (NPF) is a very non-linear process that includes atmospheric chemistry of precursors and clustering physics as well as subsequent growth before NPF can be observed. Thanks to ongoing efforts, now there exists a tremendous amount of atmospheric data, obtained through continuous measurements directly from the atmosphere. This fact makes the analysis by human brains difficult but, on the other hand, enables the usage of modern data science techniques. Here, we calculate and explore the mutual information (MI) between observed NPF events (measured at Hyytiala, Finland) and a wide variety of simultaneously monitored ambient variables: trace gas and aerosol particle concentrations, meteorology, radiation and a few derived quantities. The purpose of the investigations is to identify key factors contributing to the NPF. The applied mutual information method finds that the formation events are strongly linked to sulfuric acid concentration and water content, ultraviolet radiation, condensation sink (CS) and temperature. Previously, these quantities have been well-established to be important players in the phenomenon via dedicated field, laboratory and theoretical research. The novelty of this work is to demonstrate that the same results are now obtained by a data analysis method which operates without supervision and without the need of understanding the physics deeply. This suggests that the method is suitable to be implemented widely in the atmospheric field to discover other interesting phenomena and their relevant variables.
  • Chen, Xuemeng; Virkkula, Aki; Kerminen, Veli-Matti; Manninen, Hanna E.; Busetto, Maurizio; Lanconelli, Christian; Lupi, Angelo; Vitale, Vito; Del Guasta, Massimo; Grigioni, Paolo; Väänänen, Riikka; Duplissy, Ella-Maria; Petäjä, Tuukka; Kulmala, Markku (2017)
    An air ion spectrometer (AIS) was deployed for the first time at the Concordia station at Dome C (75 degrees 06'S, 123 degrees 23'E; 3220 ma.s.l.), Antarctica during the period 22 December 2010-16 November 2011 for measuring the number size distribution of air ions. In this work, we present results obtained from this air ion data set together with aerosol particle and meteorological data. The main processes that modify the number size distribution of air ions during the measurement period at this high-altitude site included new particle formation (NPF, observed on 85 days), wind-induced ion formation (observed on 36 days), and ion production and loss associated with cloud/fog formation (observed on 2 days). For the subset of days when none of these processes seemed to operate, the concentrations of cluster ions (0.9-1.9 nm) exhibited a clear seasonality, with high concentrations in the warm months and low concentrations in the cold. Compared to event-free days, days with NPF were observed with higher cluster ion concentrations. A number of NPF events were observed with restricted growth below 10 nm, which were termed as suppressed NPF. There was another distinct feature, namely a simultaneous presence of two or three separate NPF and subsequent growth events, which were named as multi-mode NPF events. Growth rates (GRs) were determined using two methods: the appearance time method and the mode fitting method. The former method seemed to have advantages in characterizing NPF events with a fast GR, whereas the latter method is more suitable when the GR was slow. The formation rate of 2 nm positive ions (J(2)(+)) was calculated for all the NPF events for which a GR in the 2-3 nm size range could be determined. On average, J(2)(+) was about 0.014 cm(-3) s(-1). The ion production in relation to cloud/fog formation in the size range of 8-42 nm seemed to be a unique feature at Dome C, which has not been reported elsewhere. These ions may, however, either be multiply charged particles but detected as singly charged in the AIS, or be produced inside the instrument, due to the breakage of cloud condensation nuclei (CCN), possibly related to the instrumental behaviour under the extremely cold condition. For the wind-induced ion formation, our observations suggest that the ions originated more likely from atmospheric nucleation of vapours released from the snow than from mechanical charging of shattered snow flakes and ice crystals.
  • Kulmala, Markku; Lappalainen, Hanna K.; Back, Jaana; Laaksonen, An; Nikinmaa, Eero; Riekkola, Marja-Liisa; Vesala, Timo; Viisanen, Yrjo; Aalto, Tuula; Boy, Michael; Dal Maso, Miikka; Ehn, Mikael; Hakola, Hannele; Hari, Pertti; Hartonen, Kari; Hameri, Kaarle; Holtta, Teemu; Junninen, Heikki; Jarvi, Leena; Kurten, Theo; Lauri, Antti; Laurila, Tuomas; Lehtipalo, Katrianne; Lihavainen, Heikki; Lintunen, Anna; Mammarella, Ivan; Manninen, Hanna E.; Petaja, Tuukka; Pihlatie, Mari; Pumpanen, Jukka; Rinne, Janne; Romakkaniemi, Sami; Ruuskanen, Taina; Sipila, Mikko; Sorvari, Sanna; Vehkamaki, Hanna; Virtanen, Annele; Worsnop, Douglas R.; Kerminen, Veli-Matti (2014)
  • Deng, Yange; Kagami, Sara; Ogawa, Shuhei; Kawana, Kaori; Nakayama, Tomoki; Kubodera, Ryo; Adachi, Kouji; Hussein, Tareq; Miyazaki, Yuzo; Mochida, Michihiro (2018)
    The formation of biogenic secondary organic aerosols (BSOAs) in forest environments is potentially important to cloud formation via changes of the cloud condensation nuclei (CCN) activity of aerosols. In this study, the CCN activation of submicrometer aerosols and their chemical compositions and size distributions were measured at a midlatitude forest site in Japan during the summer of 2014 to assess the hygroscopicity of the organic aerosols and their contributions to the local CCN concentrations. The mean number concentrations of the condensation nuclei and CCN at supersaturation (SS) conditions of 0.11-0.80% were 1,238 and 166-740cm(-3), respectively. Organic aerosols and sulfate dominated the submicrometer aerosol mass concentrations. The particle hygroscopicity increased with increases in particle diameters. The hygroscopicity parameter for the organics, (org), was positively correlated with the atomic O to C ratio. The product of (org) and the volume fraction of OA was 0.12, accounting for 38% of the water uptake by aerosol particles. The hygroscopicity parameter of the locally formed fresh BSOA was estimated to be 0.09. The contribution of OA to the CCN number concentration, which was assessed by subtracting the CCN concentration of the hypothetical inorganic aerosols from that of the ambient aerosols, was 50-182cm(-3) for the SS range of 0.11-0.80%. The increase of the CCN number concentrations per 1-g/m(3) increase of the BSOA was 23-299cm(-3) at 0.11-0.80% SS. The contribution of the BSOA to the CCN number concentration can be enhanced by new particle formation. Plain Language Summary Some of the particles suspended in the atmosphere can absorb water vapors around them and act as nuclei to form cloud droplets. These particles are called cloud condensation nuclei (CCN), the quantification of which is important for climate forcing prediction. The ability of a particle to absorb water is referred to as hygroscopicity, which is governed by the chemical composition. Volatile organic vapors emitted by vegetation (i.e., biogenic volatile organic compound) after chemical reactions in the atmosphere can either condense onto existing particles or participate in the formation of new particles and thus change the aerosol chemical composition. The aerosol component originated from biogenic volatile organic compounds, named biogenic secondary organic aerosol (BSOA), is an important constituent of CCN on a global scale. However, the hygroscopicity of BSOA and its contribution to CCN are not understood well. We performed measurements of the hygroscopicity and chemical composition of aerosol particles in a forest in Japan. Based on the observation, we calculated the hygroscopicity of the BSOA formed in the forest and quantified the contribution of the BSOA to the CCN number concentrations. An enhancement of the contribution of BSOA to the CCN number concentrations by new particle formation is suggested, which is an important subject of future studies.
  • Scott, C. E.; Monks, S. A.; Spracklen, D. V.; Arnold, S. R.; Forster, P. M.; Rap, A.; Äijälä, M.; Artaxo, P.; Carslaw, K. S.; Chipperfield, M. P.; Ehn, M.; Gilardoni, S.; Heikkinen, L.; Kulmala, M.; Petäjä, T.; Reddington, C. L. S.; Rizzo, L. V.; Swietlicki, E.; Vignati, E.; Wilson, C. (2018)
    The climate impact of deforestation depends on the relative strength of several biogeochemical and biogeophysical effects. In addition to affecting the exchange of carbon dioxide (CO2) and moisture with the atmosphere and surface albedo, vegetation emits biogenic volatile organic compounds (BVOCs) that alter the formation of short-lived climate forcers (SLCFs), which include aerosol, ozone and methane. Here we show that a scenario of complete global deforestation results in a net positive radiative forcing (RF; 0.12Wm-2) from SLCFs, with the negative RF from decreases in ozone and methane concentrations partially offsetting the positive aerosol RF. Combining RFs due to CO2, surface albedo and SLCFs suggests that global deforestation could cause 0.8 K warming after 100 years, with SLCFs contributing 8% of the effect. However, deforestation as projected by the RCP8.5 scenario leads to zero net RF from SLCF, primarily due to nonlinearities in the aerosol indirect effect.
  • Li, Xiaoxiao; Song, Shaojie; Zhou, Wei; Hao, Jiming; Worsnop, Douglas R.; Jiang, Jingkun (2019)
    Aerosol liquid water (ALW) is ubiquitous in ambient aerosol and plays an important role in the formation of both aerosol organics and inorganics. To investigate the interactions between ALW and aerosol organics during haze formation and evolution, ALW was modelled based on long-term measurement of submicron aerosol composition in different seasons in Beijing. ALW contributed by aerosol inorganics (ALW(inorg)) was modelled by ISORROPIA II, and ALW contributed by organics (ALW(org)) was estimated with kappa-Kohler theory, where the real-time hygroscopicity parameter of the organics (kappa(org)) was calculated from the real-time organic oxygen-to-carbon ratio (O/C). Overall particle hygroscopicity (kappa(total)) was computed by weighting component hygroscopicity parameters based on their volume fractions in the mixture. We found that ALW(org), which is often neglected in traditional ALW modelling, contributes a significant fraction (18 %-32 %) to the total ALW in Beijing. The ALW(org) fraction is largest on the cleanest days when both the organic fraction and kappa(org) are relatively high. The large variation in O/C, from 0.2 to 1.3, indicates the wide variety of organic components. This emphasizes the necessity of using real-time kappa(org), instead of fixed kappa(org), to calculate ALW(org) in Beijing. The significant variation in K org (calculated from O/C), together with highly variable organic or inorganic volume fractions, leads to a wide range of kappa(total) (between 0.20 and 0.45), which has a great impact on water uptake. The variation in organic O/C, or derived K org , was found to be influenced by temperature (T), ALW, and aerosol mass concentrations, among which T and ALW both have promoting effects on O/C. During high-ALW haze episodes, although the organic fraction decreases rapidly, O/C and derived K org increase with the increase in ALW, suggesting the formation of more soluble organics via heterogeneous uptake or aqueous processes. A positive feedback loop is thus formed: during high-ALW episodes, increasing kappa(org), together with decreasing particle organic fraction (or increasing particle inorganic fraction), increases kappa(total), and thus further promotes the ability of particles to uptake water.
  • Kalliokoski, Tuomo; Bäck, Jaana; Boy, Michael; Kulmala, Markku; Kuusinen, Nea; Mäkelä, Annikki; Minkkinen, Kari; Minunno, Francesco; Paasonen, Pauli; Peltoniemi, Mikko; Taipale, Ditte; Valsta, Lauri; Vanhatalo, Anni; Zhou, Luxi; Zhou, Putian; Berninger, Frank (2020)
    The pressure to increase forest and land carbon stocks simultaneously with increasing forest based biomass harvest for energy and materials emphasizes the need for dedicated analyses of impacts and possible trade-offs between these different mitigation options including also forest related biophysical factors, surface albedo and the formation of biogenic Secondary Organic Aerosols (SOA). We analyzed the change in global radiative forcing (RF) due to changes in these climatic agents as affected by the change in state of Finnish forests under increased or decreased harvest scenarios from a baseline. We also included avoided emissions due to wood material and energy substitution. Increasing harvests from baseline (65% of Current Annual Increment) decreased the total carbon sink (carbon in trees, soil and harvested wood products) at least for 50 years. When we coupled this change in carbon with other biosphere responses, surface albedo and aerosols, decreasing harvests from the baseline produced the largest cooling effect during 50 years. Accounting also for the avoided emissions due to increased wood use, the RF responses of the two lowest harvest scenarios were within uncertainty range. Our results show that the effects of forest management on SOA formation should be included in the analyses trying to deduce the net climate impact of forest use. The inclusion of the rarely considered SOA effects enforces the view that the lower the harvest, the more climatic cooling boreal forests provide. These results should act as a caution mark for policy makers who are emphasizing the increased utilization of forest biomass for short-living products and bioenergy as an efficient measure to mitigate climate change.
  • Hong, Juan; Xu, Hanbing; Tan, Haobo; Yin, Changqing; Hao, Liqing; Li, Fei; Cai, Mingfu; Deng, Xuejiao; Wang, Nan; Su, Hang; Cheng, Yafang; Wang, Lin; Petäjä, Tuukka; Kerminen, Veli-Matti (2018)
    Simultaneous measurements of aerosol hygroscopicity and particle-phase chemical composition were performed at a suburban site over the Pearl River Delta region in the late summer of 2016 using a self-assembled hygroscopic tandem differential mobility analyzer (HTDMA) and an Aerodyne quadruple aerosol chemical speciation monitor (ACSM), respectively. The hygroscopic growth factor (HGF) of the Aitken mode (30 nm, 60 nm) and accumulation mode (100 nm, 145 nm) particles were obtained under 90% relative humidity (RH). An external mixture was observed for particles of every size during this study, with a dominant mode of more-hygroscopic (MH) particles, as aged aerosols dominated due to the anthropogenic influence. The HGF of lesshygroscopic (LH) mode particles increased, while their number fractions decreased during the daytime due to a reduced degree of external mixing that probably resulted from the condensation of gaseous species. These LH mode particles in the early morning or late afternoon could be possibly dominated by carbonaceous material emitted from local automobile exhaust during rush hours. During polluted days with air masses flowing mainly from the coastal areas, the chemical composition of aerosols had a clear diurnal variation and a strong correlation with the mean HGF. Closure analysis was carried out between the HTDMA-measured HGF and the ACSM-derived hygroscopicity using various approximations for the hygroscopic growth factor of organic compounds (HGF(org)). Considering the assumptions regarding the differences in the mass fraction of each component between PM1 and 145 nm particles, the hygroscopicity-composition closure was achieved using an HGF(org) of 1.26 for the organic material in the 145 nm particles and a simple linear relationship between the HGForg and the oxidation level inferred from the O : C ratio of the organic material was suggested. Compared with the results from other environments, HGF(org) obtained from our measurements appeared to be less sensitive to the variation of its oxidation level, which is, however, similar to the observations in the urban atmosphere of other megacities in China. This finding suggests that the anthropogenic precursors or the photooxidation mechanisms might differ significantly between the suburban and urban atmosphere in China and those in other background environments. This may lead to different characteristics of the oxidation products in secondary organic aerosols (SOA) and therefore to a different relationship between the HGF(org) and its O : C ratio.
  • Taipale, Ditte; Kerminen, Veli-Matti; Ehn, Mikael; Kulmala, Markku; Niinemets, Ülo (2021)
    Most trees emit volatile organic compounds (VOCs) continuously throughout their life, but the rate of emission and spectrum of emitted VOCs become substantially altered when the trees experience stress. Despite this, models to predict the emissions of VOCs do not account for perturbations caused by biotic plant stress. Considering that such stresses have generally been forecast to increase in both frequency and severity in the future climate, the neglect of stress-induced plant emissions in models might be one of the key obstacles for realistic climate change predictions, since changes in VOC concentrations are known to greatly influence atmospheric aerosol processes. Thus, we constructed a model to study the impact of biotic plant stresses on new particle formation and growth throughout a full growing season. We simulated the influence on aerosol processes caused by herbivory by the European gypsy moth (Lymantria dispar) and autumnal moth (Epirrita autumnata) feeding on pedunculate oak (Quercus robur) and mountain birch (Betula pubescens var. pumila), respectively, and also fungal infections of pedunculate oak and balsam poplar (Populus balsamifera var. suaveolens) by oak powdery mildew (Erysiphe alphitoides) and poplar rust (Melampsora larici-populina), respectively. Our modelling results indicate that all the investigated plant stresses are capable of substantially perturbing both the number and size of aerosol particles in atmospherically relevant conditions, with increases in the amount of newly formed particles by up to about an order of magnitude and additional daily growth of up to almost 50 nm. We also showed that it can be more important to account for biotic plant stresses in models for local and regional predictions of new particle formation and growth during the time of infestation or infection than significant variations in, e.g. leaf area index and temperature and light conditions, which are currently the main parameters controlling predictions of VOC emissions. Our study thus demonstrates that biotic plant stress can be highly atmospherically relevant. To validate our findings, field measurements are urgently needed to quantify the role of stress emissions in atmospheric aerosol processes and for making integration of biotic plant stress emission responses into numerical models for prediction of atmospheric chemistry and physics, including climate change projection models, possible.
  • Dall'Osto, M.; Beddows, D. C. S.; Asmi, A.; Poulain, L.; Hao, L.; Freney, E.; Allan, J. D.; Canagaratna, M.; Crippa, M.; Bianchi, F.; de Leeuw, G.; Eriksson, A.; Swietlicki, E.; Hansson, H. C.; Henzing, J. S.; Granier, C.; Zemankova, K.; Laj, P.; Onasch, T.; Prevot, A.; Putaud, J. P.; Sellegri, K.; Vidal, M.; Virtanen, A.; Simo, R.; Worsnop, D.; O'Dowd, C.; Kulmala, M.; Harrison, Roy M. (2018)
    The formation of new atmospheric particles involves an initial step forming stable clusters less than a nanometre in size (similar to 10 nm). Although at times, the same species can be responsible for both processes, it is thought that more generally each step comprises differing chemical contributors. Here, we present a novel analysis of measurements from a unique multi-station ground-based observing system which reveals new insights into continental-scale patterns associated with new particle formation. Statistical cluster analysis of this unique 2-year multi-station dataset comprising size distribution and chemical composition reveals that across Europe, there are different major seasonal trends depending on geographical location, concomitant with diversity in nucleating species while it seems that the growth phase is dominated by organic aerosol formation. The diversity and seasonality of these events requires an advanced observing system to elucidate the key processes and species driving particle formation, along with detecting continental scale changes in aerosol formation into the future.