Secondary organic aerosol (SOA) accounts for a dominant fraction of the submicron atmospheric particle mass, but knowledge of the formation, composition and climate effects of SOA is incomplete and ...limits our understanding of overall aerosol effects in the atmosphere. Organic oligomers were discovered as dominant components in SOA over a decade ago in laboratory experiments and have since been proposed to play a dominant role in many aerosol processes. However, it remains unclear whether oligomers are relevant under ambient atmospheric conditions because they are often not clearly observed in field samples. Here we resolve this long-standing discrepancy by showing that elevated SOA mass is one of the key drivers of oligomer formation in the ambient atmosphere and laboratory experiments. We show for the first time that a specific organic compound class in aerosols, oligomers, is strongly correlated with cloud condensation nuclei (CCN) activities of SOA particles. These findings might have important implications for future climate scenarios where increased temperatures cause higher biogenic volatile organic compound (VOC) emissions, which in turn lead to higher SOA mass formation and significant changes in SOA composition. Such processes would need to be considered in climate models for a realistic representation of future aerosol-climate-biosphere feedbacks.
Intensive measurements of submicron aerosol particles and their chemical composition were performed with an Aerosol Chemical Speciation Monitor (ACSM) at the Cabauw Experimental Site for Atmospheric ...Research (CESAR) in Cabauw, the Netherlands, sampling at 5 m height above ground. The campaign lasted nearly 1 year from July 2012 to June 2013 as part of the EU-FP7-ACTRIS project (Q-ACSM Network). Including equivalent black carbon an average particulate mass concentration of 9.50 µg m−3 was obtained during the whole campaign with dominant contributions from ammonium nitrate (45 %), organic aerosol (OA, 29 %), and ammonium sulfate (19 %). There were 12 exceedances of the World Health Organization (WHO) PM2.5 daily mean limit (25 µg m−3) observed at this rural site using PM1 instrumentation only. Ammonium nitrate and OA represented the largest contributors to total particulate matter during periods of exceedance. Source apportionment of OA was performed season-wise by positive matrix factorization (PMF) using the multilinear engine 2 (ME-2) controlled via the source finder (SoFi). Primary organic aerosols were attributed mainly to traffic (8–16 % contribution to total OA, averaged season-wise) and biomass burning (0–23 %). Secondary organic aerosols (SOAs, 61–84 %) dominated the organic fraction during the whole campaign, particularly on days with high mass loadings. A SOA factor which is attributed to humic-like substances (HULIS) was identified as a highly oxidized background aerosol in Cabauw. This shows the importance of atmospheric aging processes for aerosol concentration at this rural site. Due to the large secondary fraction, the reduction of particulate mass at this rural site is challenging on a local scale.
Although anthropogenic emissions have decreased during the last 2 decades, air pollution is still problematic in Europe. This study analyzes the air quality in Europe using simulations by EURopean ...Air pollution Dispersion—Inverse Model for the year 2016 with updated emissions in view of the annual guideline levels for particulate matter (PM2.5), nitrogen dioxide (NO2), and ozone (O3) released in 2021 by the World Health Organization (WHO). Three different emission scenarios are applied, including a scenario for the committed emission reductions from the European Union (EU), a scenario including additional reductions to specifically mitigate PM2.5, and a scenario in which all anthropogenic emissions are eliminated. Model results show that in Europe, the concentrations of PM2.5 and NO2 exceed the annual WHO guideline levels by up to a factor of 5.6 and 5.2, respectively, in the main polluted regions and by up to a factor of 2 for O3 in Southern Europe. High concentrations of PM2.5 and O3 are homogeneously distributed across Europe with 99% and 100% of the European population exposed to concentrations above the WHO guideline levels, respectively. NO2 concentrations above the annual WHO guideline level are primarily found in populated areas, affecting 323 million inhabitants in 2016. Although the emission scenario designed to mitigate PM2.5 shows a decrease of the highest annual mean concentrations of PM2.5 from 28 µgm−3 to 12 µgm−3, 527 million European inhabitants remain affected by PM2.5 annual mean concentrations above the WHO guideline level. Seasonal mean O3 concentrations after eliminating all anthropogenic emissions (between 60 and 82 µgm−3) are found to be above the WHO guideline level for the entire European continent. The mortality attributable to air pollution is reduced by 47% in the emission scenario for committed emissions by the EU. In the more aggressive scenario designed to mitigate PM2.5, the mortality is reduced by 72%. The study reveals that the emission scenarios and, therefore, the reduction in premature deaths are subject to sectoral emission reductions between 41% and 79%.
This study explores the differences in performance and results by various versions of the ISORROPIA thermodynamic module implemented within the ECHAM/MESSy Atmospheric Chemistry (EMAC) model. Three ...different versions of the module were used, ISORROPIA II v1, ISORROPIA II v2.3, and ISORROPIA-lite. First, ISORROPIA II v2.3 replaced ISORROPIA II v1 in EMAC to improve pH predictions close to neutral conditions. The newly developed ISORROPIA-lite has been added to EMAC alongside ISORROPIA II v2.3. ISORROPIA-lite is more computationally efficient and assumes that atmospheric aerosols exist always as supersaturated aqueous (metastable) solutions, while ISORROPIA II includes the option to allow for the formation of solid salts at low RH conditions (stable state). The predictions of EMAC by employing all three aerosol thermodynamic models were compared to each other and evaluated against surface measurements from three regional observational networks in the polluted Northern Hemisphere (Interagency Monitoring of Protected Visual Environments (IMPROVE), European Monitoring and Evaluation Programme (EMEP), and Acid Deposition Monitoring Network of East Asia (EANET)). The differences between ISORROPIA II v2.3 and ISORROPIA-lite were minimal in all comparisons with the normalized mean absolute difference for the concentrations of all major aerosol components being less than 11 % even when different phase state assumptions were used. The most notable differences were lower aerosol concentrations predicted by ISORROPIA-lite in regions with relative humidity in the range of 20 % to 60 % compared to the predictions of ISORROPIA II v2.3 in stable mode. The comparison against observations yielded satisfactory agreement especially over the USA and Europe but higher deviations over East Asia, where the overprediction of EMAC for nitrate was as high as 4 µg m−3 (∼20 %). The mean annual aerosol pH predicted by ISORROPIA-lite was on average less than a unit lower than ISORROPIA II v2.3 in stable mode, mainly for coarse-mode aerosols over the Middle East. The use of ISORROPIA-lite accelerated EMAC by nearly 5 % compared to the use of ISORROPIA II v2.3 even if the aerosol thermodynamic calculations consume a relatively small fraction of the EMAC computational time. ISORROPIA-lite can therefore be a reliable and computationally efficient alternative to the previous thermodynamic module in EMAC.
The photo-oxidation of myrcene, a monoterpene species emitted by plants, was
investigated at atmospheric conditions in the outdoor simulation chamber
SAPHIR (Simulation of Atmospheric
PHotochemistry ...In a Large Reaction Chamber). The chemical structure of myrcene consists of one moiety that is a
conjugated π system (similar to isoprene) and another moiety that is a
triple-substituted olefinic unit (similar to 2-methyl-2-butene). Hydrogen
shift reactions of organic peroxy radicals (RO2) formed in the
reaction of isoprene with atmospheric OH radicals are known to be of
importance for the regeneration of OH. Structure–activity relationships
(SARs) suggest that similar hydrogen shift reactions like in isoprene may apply
to the isoprenyl part of RO2 radicals formed during the OH oxidation
of myrcene. In addition, SAR predicts further isomerization reactions that
would be competitive with bimolecular RO2 reactions for chemical
conditions that are typical for forested environments with low concentrations
of nitric oxide. Assuming that OH peroxy radicals can rapidly
interconvert by addition and elimination of O2 like in isoprene, bulk
isomerization rate constants of 0.21 and 0.097 s−1
(T=298 K) for the three isomers resulting from the 3′-OH and
1-OH addition, respectively, can be derived from SAR. Measurements of
radicals and trace gases in the experiments allowed us to calculate radical
production and destruction rates, which are expected to be balanced. The largest
discrepancies between production and destruction rates were found for
RO2. Additional loss of organic peroxy radicals due to isomerization
reactions could explain the observed discrepancies. The uncertainty of the
total radical (ROx=OH+HO2+RO2) production rates was high
due to the uncertainty in the yield of radicals from myrcene
ozonolysis. However, results indicate that radical production can only be
balanced if the reaction rate constant of the reaction between hydroperoxy
(HO2) and RO2 radicals derived from myrcene is lower (0.9 to
1.6×10-11 cm3 s−1) than predicted by SAR. Another
explanation of the discrepancies would be that a significant fraction of
products (yield: 0.3 to 0.6) from these reactions include OH and
HO2 radicals instead of radical-terminating organic
peroxides. Experiments also allowed us to determine the yields of organic
oxidation products acetone (yield: 0.45±0.08) and formaldehyde (yield:
0.35±0.08). Acetone and formaldehyde are produced from different oxidation
pathways, so that yields of these compounds reflect the branching ratios of
the initial OH addition to myrcene. Yields determined in the
experiments are consistent with branching ratios expected from SAR. The yield
of organic nitrate was determined from the gas-phase budget analysis of
reactive oxidized nitrogen in the chamber, giving a value of 0.13±0.03. In
addition, the reaction rate constant for myrcene + OH was determined
from the measured myrcene concentration, yielding a value of (2.3±0.3)×10-10 cm3 s−1.
Condensation of carboxylic acids on mineral particles leads to coatings and impacts the particles' potential to act as cloud condensation nuclei (CCN). To determine how the CCN activity of mineral ...particles is impacted by carboxylic acid coatings, the CCN activities of CaCO.sub.3 particles and CaCO.sub.3 particles with oleic acid and malonic acid coatings were compared in this study. The results revealed that small amounts of oleic acid coating (volume fraction (vf) ⤠4.3 %) decreased the CCN activity of CaCO.sub.3 particles, while more oleic acid coating (vf ⥠16 %) increased the CCN activity of CaCO.sub.3 particles. This phenomenon has not been reported before. In contrast, the CCN activity of CaCO.sub.3 particles coated with malonic acid increased with the thickness of the malonic acid coating (vf = 0.4-40 %). Even the smallest amounts of malonic acid coating (vf = 0.4 %) significantly enhanced the CCN activity of CaCO.sub.3 particles from κ = 0.0028±0.0001 to κ = 0.0123±0.0005. This indicates that a small amount of water-soluble organic acid coating may significantly enhance the CCN activity of mineral particles. The presence of water vapor during the coating process with malonic acid additionally increased the CCN activity of the coated CaCO.sub.3 particles, probably because more CaCO.sub.3 reacts with malonic acid when sufficient water is available.
The reactions of biogenic volatile organic compounds (BVOC) with the nitrate radicals (NO
) are major night-time sources of organic nitrates and secondary organic aerosols (SOA) in regions influenced ...by BVOC and anthropogenic emissions. In this study, the formation of gas-phase highly oxygenated organic molecules-organic nitrates (HOM-ON) from NO
-initiated oxidation of a representative monoterpene, β-pinene, was investigated in the SAPHIR chamber (Simulation of Atmosphere PHotochemistry In a large Reaction chamber). Six monomer (C = 7-10, N = 1-2, O = 6-16) and five accretion product (C = 17-20, N = 2-4, O = 9-22) families were identified and further classified into first- or second-generation products based on their temporal behavior. The time lag observed in the peak concentrations between peroxy radicals containing odd and even number of oxygen atoms, as well as between radicals and their corresponding termination products, provided constraints on the HOM-ON formation mechanism. The HOM-ON formation can be explained by unimolecular or bimolecular reactions of peroxy radicals. A dominant portion of carbonylnitrates in HOM-ON was detected, highlighting the significance of unimolecular termination reactions by intramolecular H-shift for the formation of HOM-ON. A mean molar yield of HOM-ON was estimated to be 4.8% (-2.6%/+5.6%), suggesting significant HOM-ON contributions to the SOA formation.
We compared observations of aerosol particle formation and growth in
different parts of the planetary boundary layer at two different
environments that have frequent new particle formation (NPF) ...events. In
summer 2012 we had a campaign in Po Valley, Italy (urban background), and in
spring 2013 a similar campaign took place in Hyytiälä, Finland
(rural background). Our study consists of three case studies of airborne and
ground-based measurements of ion and particle size distribution from
∼1 nm. The airborne measurements were performed using a
Zeppelin inside the boundary layer up to 1000 m altitude. Our observations
show the onset of regional NPF and the subsequent growth of the aerosol
particles happening almost uniformly inside the mixed layer (ML) in both
locations. However, in Hyytiälä we noticed local enhancement in the
intensity of NPF caused by mesoscale boundary layer (BL) dynamics. Additionally, our
observations indicate that in Hyytiälä NPF was probably also taking
place above the ML. In Po Valley we observed NPF that was limited to a
specific air mass.
In this work, we used a Zeppelin NT equipped with six sensor setups, each composed of four different low-cost electrochemical sensors (ECSs) to measure nitrogen oxides (NO and NO2), carbon monoxide, ...and Ox (NO2+O3) in Germany. Additionally, a MIRO MGA laser absorption spectrometer was installed as a reference device for in-flight evaluation of the ECSs. We report not only the influence of temperature on the NO and NO2 sensor outputs but also find a shorter timescale (1 s) dependence of the sensors on the relative humidity gradient. To account for these dependencies, we developed a correction method that is independent of the reference instrument. After applying this correction to all individual sensors, we compare the sensor setups with each other and to the reference device. For the intercomparison of all six setups, we find good agreements with R2≥0.8 but different precisions for each sensor in the range from 1.45 to 6.32 ppb (parts per billion). The comparison to the reference device results in an R2 of 0.88 and a slope of 0.92 for NOx (NO+NO2). Furthermore, the average noise (1σ) of the NO and NO2 sensors reduces significantly from 6.25 and 7.1 to 1.95 and 3.32 ppb, respectively. Finally, we highlight the potential use of ECSs in airborne applications by identifying different pollution sources related to industrial and traffic emissions during multiple commercial and targeted Zeppelin flights in spring 2020. These results are a first milestone towards the quality-assured use of low-cost sensors in airborne settings without a reference device, e.g., on unmanned aerial vehicles (UAVs).
The presence of organic coatings on aerosols may have important consequences to the atmospheric chemistry, in particular to the N2O5 heterogeneous hydrolysis. This is demonstrated by recent ...experiments which show that the uptake of N2O5 by aqueous aerosols is slowed considerably when an organic coating consisting of monoterpene oxidation products is added on the particles. To treat the mechanisms behind the suppression, an extension of the resistor model, which has been widely applied in investigation of the heterogeneous uptake by aerosols, was derived. The extension accounts for dissolution, diffusion, and chemical reactions in a multilayered organic coating, and it provides a parametrization for the heterogeneous uptake by organic-coated aerosols that can be applied in large-scale models. Moreover, the framework was applied to interpret the findings regarding the decreased uptake of N2O5 by the organic-coated aerosols. Performed calculations suggested that the reaction rate constant of N2O5 in the coating is decreased by 3−5 orders of magnitude, in addition to which the product of the solubility of N2O5 and its diffusion coefficient in the coating is reduced more than an order of magnitude compared to the corresponding value for the aqueous phase. The results suggest also that the accommodation coefficient of N2O5 to such coatings is no more than a factor of 2 smaller than that to pure water surfaces. Finally, the relevance of the results to the atmospheric N2O5 heterogeneous hydrolysis is discussed and implications to planning further laboratory studies focusing on secondary organic aerosol formation are pointed out.
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