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 Hyytiälä, 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, C2–C7 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 (β-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, O3) and production rates of oxidation products (OxPRs) were calculated. The MTs dominated OH and NO3 radical chemistry, but the SQTs greatly impacted O3 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.
Atmospheric aerosols formed by nucleation of vapors affect radiative forcing and therefore climate. However, the underlying mechanisms of nucleation remain unclear, particularly the involvement of ...organic compounds. Here, we present high-resolution mass spectra of ion clusters observed during new particle formation experiments performed at the Cosmics Leaving Outdoor Droplets chamber at the European Organization for Nuclear Research. The experiments involved sulfuric acid vapor and different stabilizing species, including ammonia and dimethylamine, as well as oxidation products of pinanediol, a surrogate for organic vapors formed from monoterpenes. A striking resemblance is revealed between the mass spectra from the chamber experiments with oxidized organics and ambient data obtained during new particle formation events at the Hyytiälä boreal forest research station. We observe that large oxidized organic compounds, arising from the oxidation of monoterpenes, cluster directly with single sulfuric acid molecules and then form growing clusters of one to three sulfuric acid molecules plus one to four oxidized organics. Most of these organic compounds retain 10 carbon atoms, and some of them are remarkably highly oxidized (oxygen-to-carbon ratios up to 1.2). The average degree of oxygenation of the organic compounds decreases while the clusters are growing. Our measurements therefore connect oxidized organics directly, and in detail, with the very first steps of new particle formation and their growth between 1 and 2 nm in a controlled environment. Thus, they confirm that oxidized organics are involved in both the formation and growth of particles under ambient conditions.
For atmospheric sulfuric acid (SA) concentrations the presence of dimethylamine (DMA) at mixing ratios of several parts per trillion by volume can explain observed boundary layer new particle ...formation rates. However, the concentration and molecular composition of the neutral (uncharged) clusters have not been reported so far due to the lack of suitable instrumentation. Here we report on experiments from the Cosmics Leaving Outdoor Droplets chamber at the European Organization for Nuclear Research revealing the formation of neutral particles containing up to 14 SA and 16 DMA molecules, corresponding to a mobility diameter of about 2 nm, under atmospherically relevant conditions. These measurements bridge the gap between the molecular and particle perspectives of nucleation, revealing the fundamental processes involved in particle formation and growth. The neutral clusters are found to form at or close to the kinetic limit where particle formation is limited only by the collision rate of SA molecules. Even though the neutral particles are stable against evaporation from the SA dimer onward, the formation rates of particles at 1.7-nm size, which contain about 10 SA molecules, are up to 4 orders of magnitude smaller compared with those of the dimer due to coagulation and wall loss of particles before they reach 1.7 nm in diameter. This demonstrates that neither the atmospheric particle formation rate nor its dependence on SA can simply be interpreted in terms of cluster evaporation or the molecular composition of a critical nucleus.
Significance A significant fraction of atmospheric aerosols is formed from the condensation of low-volatility vapors. These newly formed particles can grow, become seeds for cloud particles, and influence climate. New particle formation in the planetary boundary layer generally proceeds via the neutral channel. However, unambiguous identification of neutral nucleating clusters has so far not been possible under atmospherically relevant conditions. We explored the system of sulfuric acid, water, and dimethylamine in a well-controlled laboratory experiment and measured the time-resolved concentrations of neutral clusters. Clusters containing up to 14 sulfuric acid and 16 dimethylamine molecules were observed. Our results demonstrate that a cluster containing as few as two sulfuric acid and one or two dimethylamine molecules is already stable against evaporation.
The Multiple Chamber Aerosol Chemical Aging Study (MUCHACHAS) tested the hypothesis that hydroxyl radical (OH) aging significantly increases the concentration of first-generation biogenic secondary ...organic aerosol (SOA). OH is the dominant atmospheric oxidant, and MUCHACHAS employed environmental chambers of very different designs, using multiple OH sources to explore a range of chemical conditions and potential sources of systematic error. We isolated the effect of OH aging, confirming our hypothesis while observing corresponding changes in SOA properties. The mass increases are consistent with an existing gap between global SOA sources and those predicted in models, and can be described by a mechanism suitable for implementation in those models.
Biogenic volatile organic compounds (BVOCs) are continuously emitted from terrestrial vegetation into the atmosphere and react with various atmospheric oxidants, with ozone being an important one. ...The reaction between BVOCs and ozone can lead to low volatile organic compounds, other pollutants, and the formation of secondary organic aerosols. To understand the chemical and physical processes taking place in the atmosphere, a complete picture of the BVOCs emitted is necessary. However, the large pool of BVOCs present makes it difficult to detect every compound. The total ozone reactivity method can help understand the ozone reactive potential of all BVOCs emitted into the atmosphere and also help determine whether current analytical techniques can measure the total BVOC budget.
We have studied biogenic volatile
organic compound (BVOC) emissions and their ambient
concentrations at a sub-Arctic wetland (Lompolojänkkä, Finland),
which is an open, nutrient-rich sedge fen and a ...part of the
Pallas-Sodankylä Global Atmosphere Watch (GAW) station. Measurements
were conducted during the growing season in 2018 using an in situ thermal-desorption–gas-chromatograph–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 µg m−2 h−1), but sesquiterpene (SQT) emissions were higher than MT
emissions all the time. The main MTs emitted were α-pinene,
1,8-cineol, myrcene, limonene and 3Δ-carene. Of SQTs cadinene,
β-cadinene and α-farnesene had the major contribution. During early growing season the SQT∕MT emission rate ratio was ∼10, but
it became smaller as summer proceeded, being only ∼3 in July.
Isoprene, MT and SQT emissions were exponentially dependent on temperature
(correlation coefficients (R2) 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 (R2=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 (R2=0.96) with daily mean
ozone concentrations indicating that vegetation emissions can be a
significant chemical sink of ozone in this sub-Arctic area.
The reversible partitioning of glyoxal was studied in simulation chamber experiments for the first time by time-resolved measurements of gas-phase and particle-phase concentrations in ...sulfate-containing aerosols. Two complementary methods for the measurement of glyoxal particle-phase concentrations are compared: (1) an offline method utilizing filter sampling of chamber aerosols followed by HPLC-MS/MS analysis and (2) positive matrix factorization (PMF) analysis of aerosol mass spectrometer (AMS) data. Ammonium sulfate (AS) and internally mixed ammonium sulfate/fulvic acid (AS/FA) seed aerosols both show an exponential increase of effective Henry’s law coefficients (K H,eff) with AS concentration (c AS, in mol kg–1 aerosol liquid water, m = molality) and sulfate ionic strength, I(SO4 2–) (m). A modified Setschenow plot confirmed that “salting-in” of glyoxal is responsible for the increased partitioning. The salting constant for glyoxal in AS is K S CHOCHO = (−0.24 ± 0.02) m –1, and found to be independent of the presence of FA. The reversible glyoxal uptake can be described by two distinct reservoirs for monomers and higher molecular weight species filling up at characteristic time constants. These time constants are τ1 ≈ 102 s and τ2 ≈ 104 s at c AS < 12 m, and about 1–2 orders of magnitude slower at higher c AS, suggesting that glyoxal uptake is kinetically limited at high salt concentrations.
Biogenic volatile organic compounds (BVOCs) emitted by the forests are known to have strong impacts in the atmosphere. However, lots of missing reactivity is found, especially in the forest air. ...Therefore better characterization of sources and identification/quantification of unknown reactive compounds is needed. While isoprene and monoterpene (MT) emissions of boreal needle trees have been studied quite intensively, there is much less knowledge on the emissions of boreal deciduous trees and emissions of larger terpenes and oxygenated volatile organic compounds (OVOCs). Here we quantified the downy birch (Betula pubescens) leaf emissions of terpenes, oxygenated terpenes and green leaf volatiles (GLVs) at the SMEAR II boreal forest site using in situ gas chromatographs with mass spectrometers.
Atmospheric new-particle formation affects climate and is one of the least understood atmospheric aerosol processes. The complexity and variability of the atmosphere has hindered elucidation of the ...fundamental mechanism of new-particle formation from gaseous precursors. We show, in experiments performed with the CLOUD (Cosmics Leaving Outdoor Droplets) chamber at CERN, that sulfuric acid and oxidized organic vapors at atmospheric concentrations reproduce particle nucleation rates observed in the lower atmosphere. The experiments reveal a nucleation mechanism involving the formation of clusters containing sulfuric acid and oxidized organic molecules from the very first step. Inclusion of this mechanism in a global aerosol model yields a photochemically and biologically driven seasonal cycle of particle concentrations in the continental boundary layer, in good agreement with observations.
In forested area, a large fraction of total hydroxyl radical (OH) reactivity remains unaccounted for. Very few studies
have looked at the variations in total OH reactivity from biogenic emissions. In ...the present study, we investigate
the total OH reactivity from three common boreal tree species (Scots pine, Norway spruce, and downy birch) by comparing
it with the calculated reactivity from the chemically identified emissions. Total OH reactivity was measured using the
comparative reactivity method (CRM), and the chemical composition of the emissions was quantified with two gas
chromatographs coupled with mass spectrometers (GC–MSs). Dynamic branch enclosures were used, and emissions from one
branch of a tree at the time were measured by periodically rotating between them. Results show that birch had the highest values of total OH reactivity of the emissions (TOHRE), while pine had the
lowest. The main drivers for the known reactivity of pine and spruce were monoterpenes and sesquiterpenes. Birch
emissions were dominated by sesquiterpenes, but monoterpenes and green leaf volatiles (GLVs) were present as well.
However, calculated reactivity values remained low, leading to the highest missing fraction of reactivity (>96 %),
while pine and spruce had similar missing reactivity fractions between 56 % and 82 % (higher in the spring and
decreasing as the summer proceeded). The high average values were driven by low-reactivity periods, and the fraction of
missing reactivity got smaller for pine and spruce when the TOHRE values increased.
Important exceptions were identified for periods when the emission profiles changed from terpenes to GLVs, a family of
compounds containing a backbone of six carbon atoms with various functionalities (e.g. alcohols, aldehydes, esters) that
indicate that the plant is suffering from stress. Then, very high TOHRE values were measured, and the missing fraction
remained high. This study found a different trend in the missing OHRE fraction of the Norway spruce from spring to autumn compared to
one previous study (Nölscher et al., 2013), which indicates that additional studies are required to fully
understand the complexity of biogenic reactive emissions. Future studies of boreal trees in situ should be conducted
to confirm the findings presented.
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