New particle formation (NPF) is the source of over half of the atmosphere's cloud condensation nuclei, thus influencing cloud properties and Earth's energy balance. Unlike in the planetary boundary ...layer, few observations of NPF in the free troposphere exist. We provide observational evidence that at high altitudes, NPF occurs mainly through condensation of highly oxygenated molecules (HOMs), in addition to taking place through sulfuric acid–ammonia nucleation. Neutral nucleation is more than 10 times faster than ion-induced nucleation, and growth rates are size-dependent. NPF is restricted to a time window of 1 to 2 days after contact of the air masses with the planetary boundary layer; this is related to the time needed for oxidation of organic compounds to form HOMs. These findings require improved NPF parameterization in atmospheric models.
The growth of aerosol due to the aqueous phase oxidation of sulfur dioxide by ozone was measured in laboratory-generated clouds created in the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber at the ...European Organization for Nuclear Research (CERN). Experiments were performed at 10 and −10 °C, on acidic (sulfuric acid) and on partially to fully neutralised (ammonium sulfate) seed aerosol. Clouds were generated by performing an adiabatic expansion – pressurising the chamber to 220 hPa above atmospheric pressure, and then rapidly releasing the excess pressure, resulting in a cooling, condensation of water on the aerosol and a cloud lifetime of approximately 6 min. A model was developed to compare the observed aerosol growth with that predicted using oxidation rate constants previously measured in bulk solutions. The model captured the measured aerosol growth very well for experiments performed at 10 and −10 °C, indicating that, in contrast to some previous studies, the oxidation rates of SO2 in a dispersed aqueous system can be well represented by using accepted rate constants, based on bulk measurements. To the best of our knowledge, these are the first laboratory-based measurements of aqueous phase oxidation in a dispersed, super-cooled population of droplets. The measurements are therefore important in confirming that the extrapolation of currently accepted reaction rate constants to temperatures below 0 °C is correct.
Anthropogenic volatile organic compounds (AVOCs) often dominate the urban atmosphere and consist to a large degree of aromatic hydrocarbons (ArHCs), such as benzene, toluene, xylenes, and ...trimethylbenzenes, e.g., from the handling and combustion of fuels. These compounds are important precursors for the formation of secondary organic aerosol. Here we show that the oxidation of aromatics with OH leads to a subsequent autoxidation chain reaction forming highly oxygenated molecules (HOMs) with an O : C ratio of up to 1.09. This is exemplified for five single-ring ArHCs (benzene, toluene, o-/m-/p-xylene, mesitylene (1,3,5-trimethylbenzene) and ethylbenzene), as well as two conjugated polycyclic ArHCs (naphthalene and biphenyl). We report the elemental composition of the HOMs and show the differences in the oxidation patterns of these ArHCs. A potential pathway for the formation of these HOMs from aromatics is presented and discussed. We hypothesize that AVOCs may contribute substantially to new particle formation events that have been detected in urban areas.
The article reviews the existing English‐ and German‐speaking literature on the German works council. Three major research topics are discussed: the ontology and typologies of works councils; their ...current practice and transformation; and their economic outcomes. Although much research has been conducted on the internal functioning of the works council–management relationship, it is clear that we still know little about the determinants of different workplace relations and their outcomes. The article concludes by advocating a reviving research interest in the link between codetermination and political democracy.
Highly oxygenated organic molecules (HOMs) contribute
substantially to the formation and growth of atmospheric aerosol particles,
which affect air quality, human health and Earth's climate. HOMs are ...formed
by rapid, gas-phase autoxidation of volatile organic compounds (VOCs) such
as α-pinene, the most abundant monoterpene in the atmosphere. Due to
their abundance and low volatility, HOMs can play an important role in
new-particle formation (NPF) and the early growth of atmospheric aerosols,
even without any further assistance of other low-volatility compounds such
as sulfuric acid. Both the autoxidation reaction forming HOMs and their
NPF rates are expected to be strongly dependent on
temperature. However, experimental data on both effects are limited.
Dedicated experiments were performed at the CLOUD (Cosmics Leaving OUtdoor
Droplets) chamber at CERN to address this question. In this study, we show
that a decrease in temperature (from +25 to −50 ∘C) results in
a reduced HOM yield and reduced oxidation state of the products, whereas the
NPF rates (J1.7 nm) increase substantially.
Measurements with two different chemical ionization mass spectrometers
(using nitrate and protonated water as reagent ion, respectively) provide
the molecular composition of the gaseous oxidation products, and a
two-dimensional volatility basis set (2D VBS) model provides their volatility
distribution. The HOM yield decreases with temperature from 6.2 % at 25 ∘C to 0.7 % at −50 ∘C. However, there is a strong
reduction of the saturation vapor pressure of each oxidation state as the
temperature is reduced. Overall, the reduction in volatility with
temperature leads to an increase in the nucleation rates by up to 3
orders of magnitude at −50 ∘C compared with 25 ∘C. In
addition, the enhancement of the nucleation rates by ions decreases with
decreasing temperature, since the neutral molecular clusters have increased
stability against evaporation. The resulting data quantify how the interplay
between the temperature-dependent oxidation pathways and the associated
vapor pressures affect biogenic NPF at the molecular
level. Our measurements, therefore, improve our understanding of pure
biogenic NPF for a wide range of tropospheric
temperatures and precursor concentrations.
There are strong indications that particles containing secondary organic aerosol (SOA) exhibit amorphous solid or semi-solid phase states in the atmosphere. This may facilitate heterogeneous ice ...nucleation and thus influence cloud properties. However, experimental ice nucleation studies of biogenic SOA are scarce. Here, we investigated the ice nucleation ability of viscous SOA particles. The SOA particles were produced from the ozone initiated oxidation of α-pinene in an aerosol chamber at temperatures in the range from −38 to −10 °C at 5–15 % relative humidity with respect to water to ensure their formation in a highly viscous phase state, i.e. semi-solid or glassy. The ice nucleation ability of SOA particles with different sizes was investigated with a new continuous flow diffusion chamber. For the first time, we observed heterogeneous ice nucleation of viscous α-pinene SOA for ice saturation ratios between 1.3 and 1.4 significantly below the homogeneous freezing limit. The maximum frozen fractions found at temperatures between −39.0 and −37.2 °C ranged from 6 to 20 % and did not depend on the particle surface area. Global modelling of monoterpene SOA particles suggests that viscous biogenic SOA particles are indeed present in regions where cirrus cloud formation takes place. Hence, they could make up an important contribution to the global ice nucleating particle budget.
Secondary organic aerosol (SOA) yields from the photo-oxidation of α-pinene were investigated in smog chamber (SC) experiments at low (23–29 %) and high (60–69 %) relative humidity (RH), various ...NOx ∕ VOC ratios (0.04–3.8) and with different aerosol seed chemical compositions (acidic to neutralized sulfate-containing or hydrophobic organic). A combination of a scanning mobility particle sizer and an Aerodyne high-resolution time-of-flight aerosol mass spectrometer was used to determine SOA mass concentration and chemical composition. We used a Monte Carlo approach to parameterize smog chamber SOA yields as a function of the condensed phase absorptive mass, which includes the sum of OA and the corresponding bound liquid water content. High RH increased SOA yields by up to 6 times (1.5–6.4) compared to low RH. The yields at low NOx ∕ VOC ratios were in general higher compared to yields at high NOx ∕ VOC ratios. This NOx dependence follows the same trend as seen in previous studies for α-pinene SOA. A novel approach of data evaluation using volatility distributions derived from experimental data served as the basis for thermodynamic phase partitioning calculations of model mixtures in this study. These calculations predict liquid–liquid phase separation into organic-rich and electrolyte phases. At low NOx conditions, equilibrium partitioning between the gas and liquid phases can explain most of the increase in SOA yields observed at high RH, when in addition to the α-pinene photo-oxidation products described in the literature, fragmentation products are added to the model mixtures. This increase is driven by both the increase in the absorptive mass and the solution non-ideality described by the compounds' activity coefficients. In contrast, at high NOx, equilibrium partitioning alone could not explain the strong increase in the yields with RH. This suggests that other processes, e.g. reactive uptake of semi-volatile species into the liquid phase, may occur and be enhanced at higher RH, especially for compounds formed under high NOx conditions, e.g. carbonyls.
Nucleation of atmospheric vapours produces more than half of global cloud
condensation nuclei and so has an important influence on climate. Recent
studies show that monoterpene (C10H16) oxidation ...yields
highly oxygenated products that can nucleate with or without sulfuric acid.
Monoterpenes are emitted mainly by trees, frequently together with isoprene
(C5H8), which has the highest global emission of all organic
vapours. Previous studies have shown that isoprene suppresses new-particle
formation from monoterpenes, but the cause of this suppression is under
debate. Here, in experiments performed under atmospheric conditions in the
CERN CLOUD chamber, we show that isoprene reduces the yield of
highly oxygenated dimers with 19 or 20 carbon atoms – which drive particle
nucleation and early growth – while increasing the production of dimers with
14 or 15 carbon atoms. The dimers (termed C20 and C15,
respectively) are produced by termination reactions between pairs of peroxy
radicals (RO2⚫) arising from monoterpenes or isoprene.
Compared with pure monoterpene conditions, isoprene reduces nucleation rates
at 1.7 nm (depending on the isoprene ∕ monoterpene ratio) and approximately
halves particle growth rates between 1.3 and 3.2 nm. However, above 3.2 nm,
C15 dimers contribute to secondary organic aerosol, and the growth rates
are unaffected by isoprene. We further show that increased hydroxyl radical
(OH⚫) reduces particle formation in our chemical system rather
than enhances it as previously proposed, since it increases isoprene-derived
RO2⚫ radicals that reduce C20 formation.
RO2⚫ termination emerges as the critical step that determines
the highly oxygenated organic molecule (HOM) distribution and the corresponding nucleation capability. Species
that reduce the C20 yield, such as NO, HO2 and as we show
isoprene, can thus effectively reduce biogenic nucleation and early growth.
Therefore the formation rate of organic aerosol in a particular region of
the atmosphere under study will vary according to the precise ambient
conditions.
Under certain conditions, secondary organic aerosol (SOA) particles can exist in the atmosphere in an amorphous solid or semi-solid state. To determine their relevance to processes such as ice ...nucleation or chemistry occurring within particles requires knowledge of the temperature and relative humidity (RH) range for SOA to exist in these states. In the Cosmics Leaving Outdoor Droplets (CLOUD) experiment at The European Organisation for Nuclear Research (CERN), we deployed a new in situ optical method to detect the viscous state of α-pinene SOA particles and measured their transition from the amorphous highly viscous state to states of lower viscosity. The method is based on the depolarising properties of laboratory-produced non-spherical SOA particles and their transformation to non-depolarising spherical particles at relative humidities near the deliquescence point. We found that particles formed and grown in the chamber developed an asymmetric shape through coagulation. A transition to a spherical shape was observed as the RH was increased to between 35 % at −10 °C and 80 % at −38 °C, confirming previous calculations of the viscosity-transition conditions. Consequently, α-pinene SOA particles exist in a viscous state over a wide range of ambient conditions, including the cirrus region of the free troposphere. This has implications for the physical, chemical, and ice-nucleation properties of SOA and SOA-coated particles in the atmosphere.
The ion composition at high altitude (3454 m a.s.l.) was measured with an atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF) during a period of 9 months, from August 2013 to ...April 2014. The negative mass spectra were dominated by the ions of sulfuric, nitric, malonic, and methanesulfonic acid (MSA) as well as SO5−. The most prominent positive ion peaks were from amines. The other cations were mainly organic compounds clustered with a nitrogen-containing ion, which could be either NH4+ or an aminium. Occasionally the positive spectra were characterized by groups of compounds each differing by a methylene group. In the negative spectrum, sulfuric acid was always observed during clear sky conditions following the diurnal cycle of solar irradiation. On many occasions we also saw a high signal of sulfuric acid during nighttime when clusters up to the tetramer were observed. A plausible reason for these events could be evaporation from particles at low relative humidity. A remarkably strong correlation between the signals of SO5− and CH3SO3− was observed for the full measurement period. The presence of these two ions during both the day and the night suggests a non-photochemical channel of formation which is possibly linked to halogen chemistry. Halogenated species, especially Br− and IO3−, were frequently observed in air masses that originated mainly from the Atlantic Ocean and occasionally from continental areas based on back trajectory analyses. We found I2O5 clustered with an ion, a species that was proposed from laboratory and modeling studies. All halogenated ions exhibited an unexpected diurnal behavior with low values during daytime. New particle formation (NPF) events were observed and characterized by (1) highly oxygenated molecules (HOMs) and low sulfuric acid or (2) ammonia–sulfuric acid clusters. We present characteristic spectra for each of these two event types based on 26 nucleation episodes. The mass spectrum of the ammonia–sulfuric acid nucleation event compares very well with laboratory measurements reported from the CLOUD chamber. A source receptor analysis indicates that NPF events at the Jungfraujoch take place within a restricted period of time of 24–48 h after air masses have had contact with the boundary layer. This time frame appears to be crucial to reach an optimal oxidation state and concentration of organic molecules necessary to facilitate nucleation.
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