Simple proxies for estimating the concentrations of monoterpenes and their oxidation products at a boreal forest site

The oxidation products of monoterpenes likely have a crucial role in the formation and growth of aerosol particles in boreal forests. However, the continuous measurements of monoterpene concentrations are usually not available on decadal timescales, and the direct measurements of the concentrations of monoterpene oxidation product have so far been scarce. In this study we developed proxies for the concentrations of monoterpenes and their oxidation products at a boreal forest site in Hyytiala, southern Finland. For deriving the proxies we used the monoterpene concentration measured with a proton transfer reaction mass spectrometer (PTR-MS) during 2006-2013. Our proxies for the monoterpene concentration take into account the temperature-controlled emissions from the forest ecosystem, the dilution caused by the mixing within the boundary layer and different oxidation processes. All the versions of our proxies captured the seasonal variation of the monoterpene concentration, the typical proxy-to-measurements ratios being between 0.8 and 1.3 in summer and between 0.6 and 2.6 in winter. In addition, the proxies were able to describe the diurnal variation of the monoterpene concentration rather well, especially in summer months. By utilizing one of the proxies, we calculated the concentration of oxidation products of monoterpenes by considering their production in the oxidation and their loss due to condensation on aerosol particles. The concentration of oxidation products was found to have a clear seasonal cycle, with a maximum in summer and a minimum in winter. The concentration of oxidation products was lowest in the morning or around noon and highest in the evening. In the future, our proxies for the monoterpene concentration and their oxidation products can be used, for example, in the analysis of new particle formation and growth in boreal environments.Peer reviewe

Long-term measurements of volatile organic compounds highlight the importance of sesquiterpenes for the atmospheric chemistry of a boreal forest

The concentrations of terpenoids (isoprene; monoterpenes, MTs; and sesquiterpenes, SQTs) and oxygenated volatile organic compounds (OVOCs; i.e. aldehydes, alcohols, acetates and volatile organic acids, VOAs) were investigated during 2 years at a boreal forest site in Hyytiala, Finland, using in situ gas chromatograph mass spectrometers (GC-MSs). Seasonal and diurnal variations of terpenoid and OVOC concentrations as well as their relationship with meteorological factors were studied. Of the VOCs examined, C-2-C-7 unbranched VOAs showed the highest concentrations, mainly due to their low reactivity. Of the terpenoids, MTs showed the highest concentrations at the site, but seven different highly reactive SQTs were also detected. The monthly and daily mean concentrations of most terpenoids, aldehydes and VOAs were highly dependent on the temperature. The highest exponential correlation with temperature was found for a SQT (beta-caryophyllene) in summer. The diurnal variations in the concentrations could be explained by sources, sinks and vertical mixing. The diurnal variations in MT concentrations were strongly affected by vertical mixing. Based on the temperature correlations and mixing layer height (MLH), simple proxies were developed for estimating the MT and SQT concentrations. To estimate the importance of different compound groups and compounds in local atmospheric chemistry, reactivity with main oxidants (hydroxyl radical, OH; nitrate radical, NO3; and ozone, O-3) and production rates of oxidation products (OxPRs) were calculated. The MTs dominated OH and NO3 radical chemistry, but the SQTs greatly impacted O-3 chemistry, even though the concentrations of SQT were 30 times lower than the MT concentrations. SQTs were also the most important for the production of oxidation products. Since the SQTs show high secondary organic aerosol (SOA) yields, the results clearly indicate the importance of SQTs for local SOA production.Peer reviewe

Long-term analysis of clear-sky new particle formation events and nonevents in Hyytiälä

New particle formation (NPF) events have been observed all around the world and are known to be a major source of atmospheric aerosol particles. Here we combine 20 years of observations in a boreal forest at the SMEAR II station (Station for Measuring Ecosystem-Atmosphere Relations) in Hyytiala, Finland, by building on previously accumulated knowledge and by focusing on clear-sky (non-cloudy) conditions. We first investigated the effect of cloudiness on NPF and then compared the NPF event and nonevent days during clear-sky conditions. In this comparison we considered, for example, the effects of calculated particle formation rates, condensation sink, trace gas concentrations and various meteorological quantities in discriminating NPF events from nonevents. The formation rate of 1.5 nm particles was calculated by using proxies for gaseous sulfuric acid and oxidized products of low volatile organic compounds, together with an empirical nucleation rate coefficient. As expected, our results indicate an increase in the frequency of NPF events under clear-sky conditions in comparison to cloudy ones. Also, focusing on clear-sky conditions enabled us to find a clear separation of many variables related to NPF. For instance, oxidized organic vapors showed a higher concentration during the clear-sky NPF event days, whereas the condensation sink (CS) and some trace gases had higher concentrations during the nonevent days. The calculated formation rate of 3 nm particles showed a notable difference between the NPF event and nonevent days during clear-sky conditions, especially in winter and spring. For springtime, we are able to find a threshold equation for the combined values of ambient temperature and CS, (CS (s(-1)) > -3.091 x 10(-5) x T (in Kelvin) + 0.0120), above which practically no clear-sky NPF event could be observed. Finally, we present a probability distribution for the frequency of NPF events at a specific CS and temperature.Peer reviewe

Comprehensive analysis of particle growth rates from nucleation mode to cloud condensation nuclei in boreal forest

Growth of aerosol particles to sizes at which they can act as cloud condensation nuclei (CCN) is a crucial factor in estimating the current and future impacts of aerosol-cloud-climate interactions. Growth rates (GRs) are typically determined for particles with diameters (d(P)) smaller than 40 nm immediately after a regional new particle formation (NPF) event. These growth rates are often taken as representatives for the particle growth to CCN sizes (d(P) > 50-100 nm). In modelling frameworks, the concentration of the condensable vapours causing the growth is typically calculated with steady state assumptions, where the condensation sink (CS) is the only loss term for the vapours. Additionally, the growth to CCN sizes is represented with the condensation of extremely low-volatility vapours and gas-particle partitioning of semi-volatile vapours. Here, we use a novel automatic method to determine growth rates from below 10 nm to hundreds of nanometres from a 20-year-long particle size distribution (PSD) data set in boreal forest. With this method, we are able to detect growth rates also at times other than immediately after a NPF event. We show that the GR increases with an increasing oxidation rate of monoterpenes, which is closely coupled with the ambient temperature. Based on our analysis, the oxidation reactions of monoterpenes with ozone, hydroxyl radical and nitrate radical all are capable of producing vapours that contribute to the particle growth in the studied size ranges. We find that GR increases with particle diameter, resulting in up to 3-fold increases in GRs for particles with d(P) similar to 100 nm in comparison to those with d(P) similar to 10 nm. We use a single particle model to show that this increase in GR can be explained with aerosol-phase reactions, in which semi-volatile vapours form non-volatile dimers. Finally, our analysis reveals that the GR of particles with d(P) <100 nm is not limited by the condensation sink, even though the GR of larger particles is. Our findings suggest that in the boreal continental environment, the formation of CCN from NPF or sub-100 nm emissions is more effective than previously thought and that the formation of CCN is not as strongly self-limiting a process as the previous estimates have suggested.Peer reviewe

Sesquiterpenes dominate monoterpenes in northern wetland emissions

We have studied biogenic volatile organic compound (BVOC) emissions and their ambient concentrations at a sub-Arctic wetland (Lompolojankka, Finland), which is an open, nutrient-rich sedge fen and a part of the Pallas-Sodankyla Global Atmosphere Watch (GAW) station. Measurements were conducted during the growing season in 2018 using an in situ thermal-desorption-gaschromatograph-mass-spectrometer (TD-GC-MS). Earlier studies have shown that isoprene is emitted from boreal wetlands, and it also turned out to be the most abundant compound in the current study. Monoterpene (MT) emissions were generally less than 10 % of the isoprene emissions (mean isoprene emission over the growing season, 44 mu g M-2 h(-1)), but sesquiterpene (SQT) emissions were higher than MT emissions all the time. The main MTs emitted were alpha-pinene, 1,8-cineol, myrcene, limonene and 3 Delta-carene. Of SQTs cadinene, beta-cadinene and alpha-farnesene had the major contribution. During early growing season the SQT/MT emission rate ratio was similar to 10, but it became smaller as summer proceeded, being only similar to 3 in July. Isoprene, MT and SQT emissions were exponentially dependent on temperature (correlation coefficients (R-2) 0.75, 0.66 and 0.52, respectively). Isoprene emission rates were also found to be exponentially correlated with the gross primary production of CO2 (R-2 = 0.85 in July). Even with the higher emissions from the wetland, ambient air concentrations of isoprene were on average > 100, > 10 and > 6 times lower than MT concentrations in May, June and July, respectively. This indicates that wetland was not the only source affecting atmospheric concentrations at the site, but surrounding coniferous forests, which are high MT emitters, contribute as well. Daily mean MT concentrations had high negative exponential correlation (R-2 = 0.96) with daily mean ozone concentrations indicating that vegetation emissions can be a significant chemical sink of ozone in this sub-Arctic area.Peer reviewe

Ambient observations of dimers from terpene oxidation in the gas phase : Implications for new particle formation and growth

We present ambient observations of dimeric monoterpene oxidation products (C16-20HyO6-9) in gas and particle phases in the boreal forest in Finland in spring 2013 and 2014, detected with a chemical ionization mass spectrometer with a filter inlet for gases and aerosols employing acetate and iodide as reagent ions. These are among the first online dual-phase observations of such dimers in the atmosphere. Estimated saturation concentrations of 10(-15) to 10(-6)mu gm(-3) (based on observed thermal desorptions and group-contribution methods) and measured gas-phase concentrations of 10(-3) to 10(-2)mu gm(-3) (similar to 10(6)-10(7)moleculescm(-3)) corroborate a gas-phase formation mechanism. Regular new particle formation (NPF) events allowed insights into the potential role dimers may play for atmospheric NPF and growth. The observationally constrained Model for Acid-Base chemistry in NAnoparticle Growth indicates a contribution of similar to 5% to early stage particle growth from the similar to 60 gaseous dimer compounds. Plain Language Summary Atmospheric aerosol particles influence climate and air quality. We present new insights into how emissions of volatile organic compounds from trees are transformed in the atmosphere to contribute to the formation and growth of aerosol particles. We detected for the first time over a forest, a group of organic molecules, known to grow particles, in the gas phase at levels far higher than expected. Previous measurements had only measured them in the particles. This finding provides guidance on how models of aerosol formation and growth should describe their appearance and fate in the atmosphere.Peer reviewe

Rapid deposition of oxidized biogenic compounds to a temperate forest

We report fluxes and dry deposition velocities for 16 atmospheric compounds above a southeastern United States forest, including: hydrogen peroxide (H_2O_2), nitric acid (HNO_3), hydrogen cyanide (HCN), hydroxymethyl hydroperoxide, peroxyacetic acid, organic hydroxy nitrates, and other multifunctional species derived from the oxidation of isoprene and monoterpenes. The data suggest that dry deposition is the dominant daytime sink for small, saturated oxygenates. Greater than 6 wt %C emitted as isoprene by the forest was returned by dry deposition of its oxidized products. Peroxides account for a large fraction of the oxidant flux, possibly eclipsing ozone in more pristine regions. The measured organic nitrates comprise a sizable portion (15%) of the oxidized nitrogen input into the canopy, with HNO_3 making up the balance. We observe that water-soluble compounds (e.g., strong acids and hydroperoxides) deposit with low surface resistance whereas compounds with moderate solubility (e.g., organic nitrates and hydroxycarbonyls) or poor solubility (e.g., HCN) exhibited reduced uptake at the surface of plants. To first order, the relative deposition velocities of water-soluble compounds are constrained by their molecular diffusivity. From resistance modeling, we infer a substantial emission flux of formic acid at the canopy level (∼1 nmol m^(−2)⋅s^(−1)). GEOS−Chem, a widely used atmospheric chemical transport model, currently underestimates dry deposition for most molecules studied in this work. Reconciling GEOS−Chem deposition velocities with observations resulted in up to a 45% decrease in the simulated surface concentration of trace gases

Atmospheric organic vapors in two European pine forests measured by a Vocus PTR-TOF : insights into monoterpene and sesquiterpene oxidation processes

Atmospheric organic vapors play essential roles in the formation of secondary organic aerosol. Source identification of these vapors is thus fundamental to understanding their emission sources and chemical evolution in the atmosphere and their further impact on air quality and climate change. In this study, a Vocus proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF) was deployed in two forested environments, the Landes forest in southern France and the boreal forest in southern Finland, to measure atmospheric organic vapors, including both volatile organic compounds (VOCs) and their oxidation products. For the first time, we performed binned positive matrix factorization (binPMF) analysis on the complex mass spectra acquired with the Vocus PTR-TOF and identified various emission sources as well as oxidation processes in the atmosphere. Based on separate analysis of low- and high-mass ranges, 15 PMF factors in the Landes forest and nine PMF factors in the Finnish boreal forest were resolved, showing a high similarity between the two sites. Particularly, terpenes and various terpene reaction products were separated into individual PMF factors with varying oxidation degrees, such as lightly oxidized compounds from both monoterpene and sesquiterpene oxidation, monoterpene-derived organic nitrates, and monoterpene more oxidized compounds. Factors representing monoterpenes dominated the biogenic VOCs in both forests, with lower contributions from the isoprene factors and sesquiterpene factors. Factors of the lightly oxidized products, more oxidized products, and organic nitrates of monoterpenes/sesquiterpenes accounted for 8 %-12% of the measured gas-phase organic vapors in the two forests. Based on the interpretation of the results relating to oxidation processes, further insights were gained regarding monoterpene and sesquiterpene reactions. For example, a strong relative humidity (RH) dependence was found for the behavior of sesquiterpene lightly oxidized compounds. High concentrations of these compounds only occur at high RH; yet similar behavior was not observed for monoterpene oxidation products.Peer reviewe

Long-term trends and large-scale spatial variability of new particle formation in continental boundary layer

Atmospheric aerosol particles affect our lives in various ways. One of the largest sources of aerosols is their formation in the atmosphere via nucleation from precursor vapours. These new particle formation (NPF) events have been observed to occur globally. The climatic effects of secondary aerosol are among the largest uncertainties limiting our scientific understanding of future and past climate changes. Characterizing the processes controlling the formation and growth of aerosols is crucial in order to estimate their effects on air quality, human health, and eventually, global climate. The work presented in this thesis is based on measurements of aerosol formation under various conditions at 12 field sites across Europe, as well as long-term, comprehensive observations of aerosol properties at a boreal forest site in Finland. The results show that new particle formation is a frequent phenomenon at both clean background and polluted Central European sites. On the sites where measurements of both charged and neutral particles were performed, the total particle formation rates at 2 nm size varied in the range 0.9 32 cm-3 s-1, whereas the charged particle formation rates were on the order of 0.05 0.16 cm-3 s-1. Further evidence of the minor role of ions in continental boundary layer particle formation was obtained from the long-term observations, which did not show any connection between variation in the galactic- cosmic-ray-induced ionization and new particle formation. In confirmation of earlier observations, sulphuric acid was found to be closely connected to particle formation rates, but to have only a minor role in their subsequent growth. At the boreal forest site, a long-term decreasing trend in the concentrations of sulphuric acid and aerosol particles was observed, whereas the particle formation and growth rates had a slight increasing trend. Finally, a proxy for the global distribution of nucleation-mode particle concentrations based on satellite data was developed. The results of this thesis give tools to quantify the source rate of atmospheric aerosol from new particle formation in various environments. The characteristics of particle formation determined in this thesis can be used in global models to evaluate the climatic effects of NPF events. The results support previous observations of atmospheric NPF, and give significant new results in particular about the role of ions in NPF in various environments, and the long-term changes in NPF.Maapallon ilmakehässä on lukuisten kaasujen lisäksi vaihtelevia määriä pienhiukkasia, joiden koko ulottuu muutamien nanometrien molekyyliklustereista aina kymmeniin mikrometreihin saakka. Pienhiukkasten pitoisuudet, koko ja koostumus vaihtelevat merkittävästi eri ympäristöjen välillä. Ilmakehän pienhiukkasilla on paitsi paikallisia vaikutuksia ilmanlaatuun ja ihmisten terveyteen, myös suuri rooli maapallon ilmastolle. Ne absorboivat ja sirottavat suoraan auringosta tulevaa säteilyä sekä asettuessaan lumi- ja jääpinnoille voivat muuttaa niiden heijastavuutta. Pienhiukkasten määrällä ja ominaisuuksilla on vaikutusta pilvien muodostumiseen ja siten epäsuorasti pilvien elinikään ja niiden heijastamaan auringon säteilyn määrään. Hiukkasten kokonaisvaikutuksen maapallon ilmastoon arvioidaan olevan viilentävä ja siten nykyistä ilmastonmuutosta hidastava. Tähän arvioon liittyy kuitenkin suuri epävarmuus johtuen pienhiukkasiin liittyvien lukuisten prosessien ja palautemekanismien puutteellisesta ymmärtämisestä. Yksi merkittävä pienhiukkasten lähde on niiden synty ilmakehän höyryistä nukleaation eli kaasu-hiukkas muuntuman kautta. Tässä väitöskirjatyössä on tutkittu ilmakehän pienhiukkasmuodostusta eri puolilla Eurooppaa tehtyjen mittausten avulla. Mittausten avulla saatiin tietoa hiukkasmuodostuksen yleisyyteen vaikuttavista tekijöistä, hiukkasten muodostumis- ja kasvunopeuksista sekä hiukkasten sähkövarauksen vaikutuksesta niiden muodostumiseen. Keski-Suomessa sijaitsevalla Hyytiälän mittausasemalla tehtyjen pitkäkestoisten pienhiukkasmittausten perusteella pystyttiin arvioimaan hiukkasmuodostuksen pitkän aikavälin muutoksia ja tähän vaikuttavia tekijöitä. Lisäksi kehitettiin satelliittimittauksia hyödyntävä yksinkertainen malli hiukkasmuodostuksen maapallonlaajuisen esiintyvyyden ja merkittävyyden arviointiin. Kaikki tämä tieto auttaa kehittämään ilmastomalleja entistä tarkemmiksi ja pienentämään niiden antamien ennusteiden epävarmuutta

Biogenic New Particle Formation : Field Observations and Chamber Experiments

New particle formation (NPF) is an atmospheric phenomenon, observed in many environments globally, and it contributes to a major fraction of the global aerosol number budget thereby affecting both climate and human health. In this thesis, we investigate the mechanisms behind NPF in the boreal forest environment and analyze the long-term behavior of the variables associated with the occurrence of this phenomenon. In order to improve the classification of atmospheric NPF events, especially when considering the increasing number of measurement campaigns and stations, we developed an automatic framework to classify NPF events based on the 2–4 nm ion and 7–25 nm aerosol particle concentrations in the atmosphere. This approach categorizes days into four defined classes: Regional NPF events, transported NPF events, ion bursts and non-events. For regional NPF events, the approach additionally determined the precise period (start and end-time) during which the event occurred. We show that, in the boreal forest, NPF events tend to occur under clear sky conditions with low condensation sinks and moderate temperatures. Using chamber simulations, we further investigated the mechanisms of new particle formation and growth in the boreal forest environment. While sulfuric acid is known to be the driver of NPF, we found that pure biogenic NPF is possible in the absence of sulfuric acid, and that the nucleation is mediated by dimers of highly oxygenated monoterpene oxidation products. We also found that anthropogenic vapors, such as NOx, attenuate the particle formation and growth by modifying the chemical composition of highly oxygenated molecules (HOMs) necessary for nucleation and growth. In the present-day-time atmosphere, we found that highly oxygenated molecules (HOMs) govern ion-induced new particle formation in the boreal forest when the ratio of biogenic HOMs to H2SO4 is greater than 30. Our results show that non-nitrate HOM dimers mediate ion-induced nucleation not only during daytime but also during night-time. In the absence of H2SO4, we observed pure biogenic ion-induced clustering mediated by non-nitrate HOM dimers and trimers; however, these clusters did not grow past 6 nm due to insufficient photochemistry needed for producing condensable vapors that would ensure cluster survival