To better understand the role of aromatic hydrocarbons in new-particle formation, we measured the particle-phase abundance and volatility of oxidation products following the reaction of aromatic ...hydrocarbons with OH radicals. For this we used thermal desorption in an iodide-adduct Time-of-Flight Chemical-Ionization Mass Spectrometer equipped with a Filter Inlet for Gases and AEROsols (FIGAERO-ToF-CIMS). The particle-phase volatility measurements confirm that oxidation products of toluene and naphthalene can contribute to the initial growth of newly formed particles. Toluene-derived (C7) oxidation products have a similar volatility distribution to that of α-pinene-derived (C10) oxidation products, while naphthalene-derived (C10) oxidation products are much less volatile than those from toluene or α-pinene; they are thus stronger contributors to growth. Rapid progression through multiple generations of oxidation is more pronounced in toluene and naphthalene than in α-pinene, resulting in more oxidation but also favoring functional groups with much lower volatility per added oxygen atom, such as hydroxyl and carboxylic groups instead of hydroperoxide groups. Under conditions typical of polluted urban settings, naphthalene may well contribute to nucleation and the growth of the smallest particles, whereas the more abundant alkyl benzenes may overtake naphthalene once the particles have grown beyond the point where the Kelvin effect strongly influences the condensation driving force.
Abstract
Frequency and intensity of warm and moist air-mass intrusions into the Arctic have increased over the past decades and have been related to sea ice melt. During our year-long expedition in ...the remote central Arctic Ocean, a record-breaking increase in temperature, moisture and downwelling-longwave radiation was observed in mid-April 2020, during an air-mass intrusion carrying air pollutants from northern Eurasia. The two-day intrusion, caused drastic changes in the aerosol size distribution, chemical composition and particle hygroscopicity. Here we show how the intrusion transformed the Arctic from a remote low-particle environment to an area comparable to a central-European urban setting. Additionally, the intrusion resulted in an explosive increase in cloud condensation nuclei, which can have direct effects on Arctic clouds’ radiation, their precipitation patterns, and their lifetime. Thus, unless prompt actions to significantly reduce emissions in the source regions are taken, such intrusion events are expected to continue to affect the Arctic climate.
The interaction between nitrogen monoxide (NO) and organic peroxy radicals (RO
) greatly impacts the formation of highly oxygenated organic molecules (HOM), the key precursors of secondary organic ...aerosols. It has been thought that HOM production can be significantly suppressed by NO even at low concentrations. Here, we perform dedicated experiments focusing on HOM formation from monoterpenes at low NO concentrations (0 - 82 pptv). We demonstrate that such low NO can enhance HOM production by modulating the RO
loss and favoring the formation of alkoxy radicals that can continue to autoxidize through isomerization. These insights suggest that HOM yields from typical boreal forest emissions can vary between 2.5%-6.5%, and HOM formation will not be completely inhibited even at high NO concentrations. Our findings challenge the notion that NO monotonically reduces HOM yields by extending the knowledge of RO
-NO interactions to the low-NO regime. This represents a major advance towards an accurate assessment of HOM budgets, especially in low-NO environments, which prevails in the pre-industrial atmosphere, pristine areas, and the upper boundary layer.
Understanding chemical processes leading to the formation of atmospheric
aerosol particles is crucial to improve our capabilities in predicting the
future climate. However, those mechanisms are still ...inadequately
characterized, especially in polar regions. In this study, we report
observations of neutral and charged aerosol precursor molecules and chemical
cluster composition (qualitatively and quantitatively), as well as air ions
and aerosol particle number concentrations and size distributions from the
Marambio research station (64∘15′ S, 56∘38′ W),
located north of the Antarctic Peninsula. We conducted measurements during
the austral summer, between 15 January and 25 February 2018. The scope of
this study is to characterize new particle formation (NPF) event parameters
and connect our observations of gas-phase compounds with the formation of
secondary aerosols to resolve the nucleation mechanisms at the molecular
scale. NPF occurred on 40 % of measurement days. All NPF events were
observed during days with high solar radiation, mostly with above-freezing
temperatures and with low relative humidity. The averaged formation rate
for 3 nm particles (J3) was 0.686 cm−3 s−1, and the average
particle growth rate (GR3.8–12 nm) was 4.2 nm h−1. Analysis of
neutral aerosol precursor molecules showed measurable concentrations of
iodic acid (IA), sulfuric acid (SA), and methane sulfonic acid (MSA)
throughout the entire measurement period with significant increase in MSA
and SA concentrations during NPF events. We highlight SA as a key
contributor to NPF processes, while IA and MSA likely only contribute
to particle growth. Mechanistically, anion clusters containing ammonia and/or dimethylamine (DMA) and SA were identified, suggesting significant
concentration of ammonia and DMA as well. Those species are likely
contributing to NPF events since SA alone is not sufficient to explain
observed nucleation rates. Here, we provide evidence of the marine origin of
the measured chemical precursors and discuss their potential contribution to
the aerosol phase.
Several studies have investigated new particle formation (NPF) events from
various sites ranging from pristine locations, including forest sites, to
urban areas. However, there is still a dearth of ...studies investigating NPF
processes and subsequent aerosol growth in coastal yet semi-urban sites,
where the tropospheric layer is a concoction of biogenic and anthropogenic
gases and particles. The investigation of factors leading to NPF becomes
extremely complex due to the highly dynamic meteorological conditions at the coastline especially when combined with both continental and oceanic weather conditions. Herein, we engage in a comprehensive study of particle number size distributions and aerosol-forming precursor vapors at the coastal semi-urban site in Helsinki, Finland. The measurement period, 25 June–18 August 2019, was timed with the recurring cyanobacterial summer bloom in the Baltic Sea region and coastal regions of Finland. Our study recorded several regional/local NPF and aerosol burst events during this period. Although the overall anthropogenic influence on sulfuric acid (SA) concentrations was low during the measurement period, we observed that the regional or local NPF events, characterized by SA concentrations on the order of 107 molec.cm-3, occurred mostly when the air mass traveled over the land areas. Interestingly, when the air mass traveled over the Baltic Sea, an area enriched with algae and cyanobacterial blooms, high iodic acid (IA) concentration coincided with an aerosol burst or a spike event at the measurement site. Further, SA-rich bursts were seen when the air mass traveled over the Gulf of Bothnia, enriched with cyanobacterial blooms. The two most important factors affecting aerosol precursor vapor concentrations, and thus the aerosol formation, were speculated to be (1) the type of phytoplankton species and intensity of bloom present in the coastal regions of Finland and the Baltic Sea and (2) the wind direction. During the events, most of the growth of sub-3 nm particles was probably due to SA, rather than IA or methane sulfonic acid (MSA); however much of the particle growth remained unexplained indicative of the strong role of organics in the growth of particles, especially in the 3–7 nm particle size range. Further studies are needed to explore the role of organics in NPF events and the potential influence of cyanobacterial blooms in coastal locations.
Pyruvic acid (CH3C(O)C(O)OH, 2-oxopropanoic acid) is an organic acid of
biogenic origin that plays a crucial role in plant metabolism, is present in
tropospheric air in both gas-phase and ...aerosol-phase, and is implicated in
the formation of secondary organic aerosols (SOAs). Up to now, only a few
field studies have reported mixing ratios of gas-phase pyruvic acid, and its
tropospheric sources and sinks are poorly constrained. We present the first
measurements of gas-phase pyruvic acid in the boreal forest as part of the
IBAIRN (Influence of Biosphere–Atmosphere Interactions on the
Reactive Nitrogen budget) field campaign in Hyytiälä, Finland, in
September 2016. The mean pyruvic acid mixing ratio during IBAIRN was 96 pptv, with a maximum value of 327 pptv. From our measurements we estimated
the overall pyruvic acid source strength and quantified the contributions of
isoprene oxidation and direct emissions from vegetation in this
monoterpene-dominated forested environment. Further, we discuss the
relevance of gas-phase pyruvic acid for atmospheric chemistry by
investigating the impact of its photolysis on acetaldehyde and peroxy
radical production rates. Our results show that, based on our present
understanding of its photochemistry, pyruvic acid is an important source of
acetaldehyde in the boreal environment, exceeding ethane and propane oxidation
by factors of ∼10 and ∼20.
Highly oxygenated organic molecules (HOMs) are important contributors to secondary organic aerosol (SOA) and new-particle formation (NPF) in the boreal atmosphere. This newly discovered class of ...molecules is efficiently formed from atmospheric oxidation of biogenic volatile organic compounds (VOCs), such as monoterpenes, through a process called autoxidation. This process, in which peroxy-radical intermediates isomerize to allow addition of molecular oxygen, is expected to be highly temperature-dependent. Here, we studied the dynamics of HOM formation during α-pinene ozonolysis experiments performed at three different temperatures, 20, 0 and −15 ∘C, in the Aarhus University Research on Aerosol (AURA) chamber. We found that the HOM formation, under our experimental conditions (50 ppb α-pinene and 100 ppb ozone), decreased considerably at lower temperature, with molar yields dropping by around a factor of 50 when experiments were performed at 0 ∘C, compared to 20 ∘C. At −15 ∘C, the HOM signals were already close to the detection limit of the nitrate-based chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer used for measuring gas-phase HOMs. Surprisingly, comparing spectra measured at 0 and 20 ∘C, ratios between HOMs of different oxidation levels, e.g., the typical HOM products C10H14O7, C10H14O9, and C10H14O11, changed considerably less than the total HOM yields. More oxidized species have undergone more isomerization steps; yet, at lower temperature, they did not decrease more than the less oxidized species. One possible explanation is that the primary rate-limiting steps forming these HOMs occur before the products become oxygenated enough to be detected by our CI-APi-TOF (i.e., typically seven or more oxygen atoms). The strong temperature dependence of HOM formation was observed under temperatures highly relevant to the boreal forest, but the exact magnitude of this effect in the atmosphere will be much more complex: the fate of peroxy radicals is a competition between autoxidation (influenced by temperature and VOC type) and bimolecular termination pathways (influenced mainly by concentration of reaction partners). While the temperature influence is likely smaller in the boreal atmosphere than in our chamber, both the magnitude and complexity of this effect clearly deserve more consideration in future studies in order to estimate the ultimate role of HOMs on SOA and NPF under different atmospheric conditions.
Little is known about the effects of subzero temperatures on the formation
of secondary organic aerosol (SOA) from α-pinene. In the current
work, ozone-initiated oxidation of α-pinene at initial
...concentrations of 10 and 50 ppb, respectively, is performed at
temperatures of 20, 0, and −15 ∘C in
the Aarhus University Research on Aerosol (AURA) smog chamber during the Aarhus Chamber Campaign on Highly Oxygenated
Organic Molecules and Aerosols (ACCHA). Herein, we show how temperature influences the
formation and chemical composition of α-pinene-derived SOA with a
specific focus on the formation of organic acids and dimer esters. With
respect to particle formation, the results show significant increase in
particle-formation rates, particle number concentrations, and particle mass
concentrations at low temperatures. In particular, the number concentrations
of sub-10 nm particles were significantly increased at the lower 0 and −15 ∘C temperatures. Temperature also affects
the chemical composition of formed SOA. Here, detailed offline chemical
analyses show that organic acids contribute from 15 % to 30 % by mass,
with highest contributions observed at the lowest temperatures, indicative
of enhanced condensation of these semivolatile species. In comparison, a
total of 30 identified dimer esters were seen to contribute between 4 % and 11 % to the total SOA mass. No significant differences in the chemical
composition (i.e. organic acids and dimer esters) of the α-pinene-derived SOA particles are observed between experiments performed at
10 and 50 ppb initial α-pinene concentrations, thus suggesting a
higher influence of reaction temperature compared to that of α-pinene loading on the SOA chemical composition. Interestingly, the effect
of temperature on the formation of dimer esters differs between the
individual species. The formation of less oxidized dimer esters – with
oxygen-to-carbon ratio (O:C)<0.4 – is shown to increase at low
temperatures, while the formation of the more oxidized species (O:C>0.4) is suppressed, consequently resulting in
temperature-modulated composition of the α-pinene-derived SOA.
Temperature ramping experiments exposing α-pinene-derived SOA to
changing temperatures (heating and cooling) reveal that the chemical
composition of the SOA with respect to dimer esters is governed almost
solely by the temperature at which oxidization started and is insusceptible to
subsequent changes in temperature. Similarly, the resulting SOA mass
concentrations were found to be more influenced by the initial α-pinene oxidation temperatures, thus suggesting that the formation
conditions to a large extent govern the type of SOA formed, rather than the
conditions to which the SOA is later exposed. For the first time, we discuss the relation between the identified dimer
ester and the highly oxygenated organic molecules (HOMs) measured by
chemical ionization–atmospheric pressure interface–time-of-flight mass spectrometer (CI-APi-ToF) during the ACCHA experiments. We propose
that, although very different in chemical structures and O:C ratios, many
dimer esters and HOMs may be linked through similar RO2 reaction
pathways and that dimer esters and HOMs merely represent two different
fates of the RO2 radicals.
In September 2017, we conducted a
proton-transfer-reaction mass-spectrometry (PTR-MS) intercomparison campaign at the CESAR observatory, a rural site in the central Netherlands near the village of ...Cabauw. Nine research groups
deployed a total of 11 instruments covering a wide range of instrument
types and performance. We applied a new calibration method based on fast
injection of a gas standard through a sample loop. This approach allows
calibrations on timescales of seconds, and within a few minutes an automated
sequence can be run allowing one to retrieve diagnostic parameters that indicate
the performance status. We developed a method to retrieve the mass-dependent transmission from the fast calibrations, which is an essential
characteristic of PTR-MS instruments, limiting the potential to calculate
concentrations based on counting statistics and simple reaction kinetics in
the reactor/drift tube. Our measurements show that PTR-MS instruments follow
the simple reaction kinetics if operated in the standard range for pressures
and temperature of the reaction chamber (i.e. 1–4 mbar, 30–120∘,
respectively), as well as a reduced field strength E∕N in the range of 100–160 Td. If
artefacts can be ruled out, it becomes possible to quantify the signals of
uncalibrated organics with accuracies better than ±30 %. The
simple reaction kinetics approach produces less accurate results at E∕N levels
below 100 Td, because significant fractions of primary ions form water
hydronium clusters. Deprotonation through reactive collisions of protonated
organics with water molecules needs to be considered when the collision
energy is a substantial fraction of the exoergicity of the proton transfer
reaction and/or if protonated organics undergo many collisions with water
molecules.
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