It is being suggested that particle-bound or particle-induced reactive oxygen species (ROS), which significantly contribute to the oxidative potential (OP) of aerosol particles, are a promising ...metric linking aerosol compositions to toxicity and adverse health effects. However, accurate ROS quantification remains challenging due to the reactive and short-lived nature of many ROS components and the lack of appropriate analytical methods for a reliable quantification. Consequently, it remains difficult to gauge their impact on human health, especially to identify how aerosol particle sources and atmospheric processes drive particle-bound ROS formation in a real-world urban environment.
Theoretical, laboratory, and chamber studies have shown
fast regeneration of the hydroxyl radical (OH) in the photochemistry of isoprene,
largely due to unimolecular reactions which were previously ...thought not to
be important under atmospheric conditions. Based on early field
measurements, nearly complete regeneration was hypothesized for a wide range
of tropospheric conditions, including areas such as the rainforest where
slow regeneration of OH radicals is expected due to low concentrations of
nitric oxide (NO). In this work the OH regeneration in isoprene
oxidation is directly quantified for the first time through experiments
covering a wide range of atmospherically relevant NO levels (between 0.15
and 2 ppbv – parts per billion by volume) in the atmospheric simulation
chamber SAPHIR. These conditions cover remote areas partially influenced by
anthropogenic NO emissions, giving a regeneration efficiency of OH close to 1, and areas like the Amazonian rainforest with very low NO, resulting in
a surprisingly high regeneration efficiency of 0.5, i.e. a factor of 2 to 3
higher than explainable in the absence of unimolecular reactions. The
measured radical concentrations were compared to model calculations, and the
best agreement was observed when at least 50 % of the total loss of
isoprene peroxy radicals conformers (weighted by their abundance) occurs via
isomerization reactions for NO lower than 0.2 ppbv. For these levels of NO,
up to 50 % of the OH radicals are regenerated from the products of the 1,6
α-hydroxy-hydrogen shift (1,6-H shift) of Z-δ-RO2 radicals through the photolysis of an unsaturated hydroperoxy aldehyde (HPALD)
and/or through the fast aldehydic hydrogen shift (rate constant
∼10 s−1 at 300 K) in di-hydroperoxy carbonyl peroxy
radicals (di-HPCARP-RO2), depending on their relative yield. The
agreement between all measured and modelled trace gases (hydroxyl,
hydroperoxy, and organic peroxy radicals, carbon monoxide, and the sum of
methyl vinyl ketone, methacrolein, and hydroxyl hydroperoxides) is nearly
independent of the adopted yield of HPALD and di-HPCARP-RO2 as both
degrade relatively fast (<1 h), forming the OH radical and CO among
other products. Taking into consideration this and earlier isoprene studies,
considerable uncertainties remain on the distribution of oxygenated products,
which affect radical levels and organic aerosol downwind of unpolluted
isoprene-dominated regions.
The oxidation of nitric oxide to nitrogen dioxide by hydroperoxy (HO2) and organic peroxy radicals (RO2) is responsible for the chemical net ozone production in the troposphere and for the ...regeneration of hydroxyl radicals, the most important oxidant in the atmosphere. In Summer 2014, a field campaign was conducted in the North China Plain, where increasingly severe ozone pollution has been experienced in the last years. Chemical conditions in the campaign were representative for this area. Radical and trace gas concentrations were measured, allowing for calculating the turnover rates of gas-phase radical reactions. Therefore, the importance of heterogeneous HO2 uptake on aerosol could be experimentally determined. HO2 uptake could have suppressed ozone formation at that time because of the competition with gas-phase reactions that produce ozone. The successful reduction of the aerosol load in the North China Plain in the last years could have led to a significant decrease of HO2 loss on particles, so that ozone-forming reactions could have gained importance in the last years. However, the analysis of the measured radical budget in this campaign shows that HO2 aerosol uptake did not impact radical chemistry for chemical conditions in 2014. Therefore, reduced HO2 uptake on aerosol since then is likely not the reason for the increasing number of ozone pollution events in the North China Plain, contradicting conclusions made from model calculations reported in the literature.
Highly oxygenated organic molecules (HOM) are found to play an important role in the formation and growth of secondary organic aerosol (SOA). SOA is an important type of aerosol with significant ...impact on air quality and climate. Compared with the oxidation of volatile organic compounds by ozone (O3) and hydroxyl radical (OH), HOM formation in the oxidation by nitrate radical (NO3), an important oxidant at nighttime and dawn, has received less attention. In this study, HOM formation in the reaction of isoprene with NO3 was investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). A large number of HOM, including monomers (C5), dimers (C10), and trimers (C15), both closed-shell compounds and open-shell peroxy radicals (RO2), were identified and were classified into various series according to their formula. Their formation pathways were proposed based on the peroxy radicals observed and known mechanisms in the literature, which were further constrained by the time profiles of HOM after sequential isoprene addition to differentiate first- and second-generation products. HOM monomers containing one to three N atoms (1–3N-monomers) were formed, starting with NO3 addition to carbon double bond, forming peroxy radicals, followed by autoxidation. 1N-monomers were formed by both the direct reaction of NO3 with isoprene and of NO3 with first-generation products. 2N-monomers (e.g., C5H8N2On(n=7–13), C5H10N2On(n=8–14)) were likely the termination products of C5H9N2On⚫, which was formed by the addition of NO3 to C5-hydroxynitrate (C5H9NO4), a first-generation product containing one carbon double bond. 2N-monomers, which were second-generation products, dominated in monomers and accounted for ∼34 % of all HOM, indicating the important role of second-generation oxidation in HOM formation in the isoprene + NO3 reaction under our experimental conditions. H shift of alkoxy radicals to form peroxy radicals and subsequent autoxidation (“alkoxy–peroxy” pathway) was found to be an important pathway of HOM formation. HOM dimers were mostly formed by the accretion reaction of various HOM monomer RO2 and via the termination reactions of dimer RO2 formed by further reaction of closed-shell dimers with NO3 and possibly by the reaction of C5–RO2 with isoprene. HOM trimers were likely formed by the accretion reaction of dimer RO2 with monomer RO2. The concentrations of different HOM showed distinct time profiles during the reaction, which was linked to their formation pathway. HOM concentrations either showed a typical time profile of first-generation products, second-generation products, or a combination of both, indicating multiple formation pathways and/or multiple isomers. Total HOM molar yield was estimated to be 1.2 %-0.7%+1.3%, which corresponded to a SOA yield of∼3.6 % assuming the molecular weight of C5H9NO6 as the lower limit. This yield suggests that HOM may contribute a significant fraction to SOA yield in the reaction of isoprene with NO3.
Oxidation of volatile organic compounds (VOCs) can lead to the formation of secondary organic aerosol (SOA), a significant component of atmospheric fine particles, which can affect air quality, human ...health, and climate change. However, the current understanding of the formation mechanism of SOA is still incomplete, which is not only due to the complexity of the chemistry but also relates to analytical challenges in SOA precursor detection and quantification. Recent instrumental advances, especially the development of high-resolution time-of-flight chemical ionization mass spectrometry (CIMS), greatly improved both the detection and quantification of low- and extremely low-volatility organic molecules (LVOCs/ELVOCs), which largely facilitated the investigation of SOA formation pathways. However, analyzing and interpreting complex mass spectrometric data remain a challenging task. This necessitates the use of dimension reduction techniques to simplify mass spectrometric data with the purpose of extracting chemical and kinetic information of the investigated system. Here we present an approach to apply fuzzy c-means clustering (FCM) to analyze CIMS data from a chamber experiment, aiming to investigate the gas phase chemistry of the nitrate-radical-initiated oxidation of isoprene.
In this study, the NOx dependence of secondary organic aerosol (SOA) formation from photooxidation of the biogenic volatile organic compound (BVOC) β-pinene was comprehensively investigated in the ...Jülich Plant Atmosphere Chamber. Consistent with the results of previous NOx studies we found increases of SOA yields with increasing NOx at low-NOx conditions (NOx0 < 30 ppb, BVOC0 / NOx0 > 10 ppbC ppb-1). Furthermore, increasing NOx at high-NOx conditions (NOx0 > 30 ppb, BVOC0 / NOx0 ∼ 10 to∼ 2.6 ppbC ppb-1) suppressed the SOA yield. The increase of SOA yield at low-NOx conditions was attributed to an increase of OH concentration, most probably by OH recycling in NO + HO2 → NO2 + OH reaction. Separate measurements without NOx addition but with different OH primary production rates confirmed the OH dependence of SOA yields. After removing the effect of OH concentration on SOA mass growth by keeping the OH concentration constant, SOA yields only decreased with increasing NOx. Measuring the NOx dependence of SOA yields at lower NO / NO2 ratio showed less pronounced increase in both OH concentration and SOA yield. This result was consistent with our assumption of OH recycling by NO and to SOA yields being dependent on OH concentrations. Our results furthermore indicated that NOx dependencies vary for different NOx compositions. A substantial fraction of the NOx-induced decrease of SOA yields at high-NOx conditions was caused by NOx-induced suppression of new particle formation (NPF), which subsequently limits the particle surface where low volatiles condense. This was shown by probing the NOx dependence of SOA formation in the presence of seed particles. After eliminating the effect of NOx-induced suppression of NPF and NOx-induced changes of OH concentrations, the remaining effect of NOx on the SOA yield from β-pinene photooxidation was moderate. Compared to β-pinene, the SOA formation from α-pinene photooxidation was only suppressed by increasing NOx. However, basic mechanisms of the NOx impacts were the same as that of β-pinene.
Isoprene oxidation by nitrate radical (NO3) is a potentially important
source of secondary organic aerosol (SOA). It is suggested that the second
or later-generation products are the more substantial ...contributors to SOA.
However, there are few studies investigating the multi-generation chemistry
of isoprene-NO3 reaction, and information about the volatility of
different isoprene nitrates, which is essential to evaluate their potential
to form SOA and determine their atmospheric fate, is rare. In this work, we
studied the reaction between isoprene and NO3 in the SAPHIR chamber
(Jülich) under near-atmospheric conditions. Various oxidation products
were measured by a high-resolution time-of-flight chemical ionization mass
spectrometer using Br− as the reagent ion. Most of the products
detected are organic nitrates, and they are grouped into monomers (C4
and C5 products) and dimers (C10 products) with 1–3 nitrate
groups according to their chemical composition. Most of the observed
products match expected termination products observed in previous studies,
but some compounds such as monomers and dimers with three nitrogen atoms
were rarely reported in the literature as gas-phase products from isoprene
oxidation by NO3. Possible formation mechanisms for these compounds are
proposed. The multi-generation chemistry of isoprene and NO3 is
characterized by taking advantage of the time behavior of different
products. In addition, the vapor pressures of diverse isoprene nitrates are
calculated by different parametrization methods. An estimation of the vapor
pressure is also derived from their condensation behavior. According to our
results, isoprene monomers belong to intermediate-volatility or
semi-volatile organic compounds and thus have little effect on SOA
formation. In contrast, the dimers are expected to have low or extremely low
volatility, indicating that they are potentially substantial contributors to
SOA. However, the monomers constitute 80 % of the total explained signals
on average, while the dimers contribute less than 2 %, suggesting that the
contribution of isoprene NO3 oxidation to SOA by condensation should be
low under atmospheric conditions. We expect a SOA mass yield of about 5 %
from the wall-loss- and dilution-corrected mass concentrations, assuming that
all of the isoprene dimers in the low- or extremely low-volatility organic
compound (LVOC or ELVOC) range will condense completely.
The coronavirus-19 (COVID-19) pandemic led to government interventions to limit the spread of the disease which are unprecedented in recent history; for example, stay at home orders led to sudden ...decreases in atmospheric emissions from the transportation sector. In this review article, the current understanding of the influence of emission reductions on atmospheric pollutant concentrations and air quality is summarized for nitrogen dioxide (NO2), particulate matter (PM2.5), ozone (O3), ammonia, sulfur dioxide, black carbon, volatile organic compounds, and carbon monoxide (CO). In the first 7 months following the onset of the pandemic, more than 200 papers were accepted by peer-reviewed journals utilizing observations from ground-based and satellite instruments. Only about one-third of this literature incorporates a specific method for meteorological correction or normalization for comparing data from the lockdown period with prior reference observations despite the importance of doing so on the interpretation of results. We use the government stringency index (SI) as an indicator for the severity of lockdown measures and show how key air pollutants change as the SI increases. The observed decrease of NO2 with increasing SI is in general agreement with emission inventories that account for the lockdown. Other compounds such as O3, PM2.5, and CO are also broadly covered. Due to the importance of atmospheric chemistry on O3 and PM2.5 concentrations, their responses may not be linear with respect to primary pollutants. At most sites, we found O3 increased, whereas PM2.5 decreased slightly, with increasing SI. Changes of other compounds are found to be understudied. We highlight future research needs for utilizing the emerging data sets as a preview of a future state of the atmosphere in a world with targeted permanent reductions of emissions. Finally, we emphasize the need to account for the effects of meteorology, emission trends, and atmospheric chemistry when determining the lockdown effects on pollutant concentrations.
Oxidation of volatile organic compounds (VOCs) can lead to the formation of secondary organic aerosol (SOA), a significant component of atmospheric fine particles, which can affect air quality, human ...health, and climate change. However, the current understanding of the formation mechanism of SOA is still incomplete, which is not only due to the complexity of the chemistry but also relates to analytical challenges in SOA precursor detection and quantification. Recent instrumental advances, especially the development of high-resolution time-of-flight chemical ionization mass spectrometry (CIMS), greatly improved both the detection and quantification of low- and extremely low-volatility organic molecules (LVOCs/ELVOCs), which largely facilitated the investigation of SOA formation pathways. However, analyzing and interpreting complex mass spectrometric data remain a challenging task. This necessitates the use of dimension reduction techniques to simplify mass spectrometric data with the purpose of extracting chemical and kinetic information of the investigated system. Here we present an approach to apply fuzzy c-means clustering (FCM) to analyze CIMS data from a chamber experiment, aiming to investigate the gas phase chemistry of the nitrate-radical-initiated oxidation of isoprene.The performance of FCM was evaluated and validated. By applying FCM to measurements, various oxidation products were classified into different groups, based on their chemical and kinetic properties, and the common patterns of their time series were identified, which provided insight into the chemistry of the investigated system. The chemical properties of the clusters are described by elemental ratios and the average carbon oxidation state, and the kinetic behaviors are parameterized with a generation number and effective rate coefficient (describing the average reactivity of a species) using the gamma kinetic parameterization model. In addition, the fuzziness of FCM algorithm provides a possibility for the separation of isomers or different chemical processes that species are involved in, which could be useful for mechanism development. Overall, FCM is a technique that can be applied well to simplify complex mass spectrometric data, and the chemical and kinetic properties derived from clustering can be utilized to understand the reaction system of interest.
This open access title presents atmospheric simulation chambers as effective tools for atmospheric chemistry research. State-of-the-art simulation chambers provide unprecedented opportunities for ...atmospheric scientists to perform experiments that address the most important questions in air quality and climate research. The book covers technical details about chamber preparation and practical guidelines on their usage, while also delivering relevant historical and contextual information. It not only serves as a key publication for knowledge transfer within the simulation chamber research community, but it also provides the global atmospheric science community with a unique resource that outlines best practice for the operation of simulation chambers. The authors summarize the latest advances in chamber interoperability and standard protocols in order to provide the research community and the next generations of scientists with a unique technical reference guide for the use of simulation chambers. The volume will be of great interest to researchers and graduates working in the fields of Atmospheric and Environmental Sciences.
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