273 research outputs found
The formation of secondary organic aerosol from the isoprene + OH reaction in the absence of NO<sub>x</sub>
The reaction of isoprene (C<sub>5</sub>H<sub>8</sub>) with hydroxyl radicals has been
studied in the absence of nitrogen oxides (NO<sub>x</sub>) to determine physical
and chemical characteristics of the secondary organic aerosol formed.
Experiments were conducted using a smog chamber operated in a steady-state
mode permitting measurements of moderately low aerosol levels. GC-MS analysis
was conducted to measure methyl butenediols in the gas phase and
polyols in the aerosol phase. Analyses were made to obtain several bulk
aerosol parameters from the reaction including values for the organic mass
to organic carbon ratio, the effective enthalpy of vaporization
(ΔH<sub>vap</sub><sup>eff</sup>), organic peroxide fraction, and the aerosol yield.
<br><br>
The gas phase analysis showed the presence of methacrolein, methyl vinyl
ketone, and four isomers of the methyl butenediols. These gas-phase
compounds may serve as precursors for one or more of several compounds
detected in the aerosol phase including 2-methylglyceric acid, three
2-methyl alkenetriols, and two 2-methyl tetrols. In contrast to most
previous studies, the 2-methyl tetrols (and the 2-methyl alkenetriols) were
found to form in the absence of acidic sulfate aerosol. However, reaction
conditions did not favor the production of HO<sub>2</sub> radicals, thus allowing
RO<sub>2</sub>+RO<sub>2</sub> reactions to proceed more readily than if higher
HO<sub>2</sub> levels had been generated.
<br><br>
SOA/SOC (i.e. OM/OC) was found to average 1.9 in the absence of NO<sub>x</sub>.
The effective enthalpy of vaporization was measured as 38.6 kJ mol<sup>−1</sup>,
consistent with values used previously in modeling studies. The yields in
this work (using an independent technique than used previously) are lower than
those of Kroll et al. (2006) for similar aerosol masses. SOC yields reported
in this work range from 0.5–1.4% for carbon masses between 17 and
49 μgC m<sup>−3</sup>
SOA formation from the atmospheric oxidation of 2-methyl-3-buten-2-ol and its implications for PM<sub>2.5</sub>
The formation of secondary organic aerosol (SOA) generated by irradiating 2-methyl-3-buten-2-ol (MBO) in the presence and/or absence of NO<sub>x</sub>, H<sub>2</sub>O<sub>2</sub>, and/or SO<sub>2</sub> was examined. Experiments were conducted in smog chambers operated in either dynamic or static mode. A filter/denuder sampling system was used for simultaneously collecting gas- and particle-phase products. The structural characterization of gas and particulate products was investigated using BSTFA, BSTFA + PFBHA, and DNPH derivatization techniques followed by GC-MS and liquid chromatography analysis. This analysis showed the occurrence of more than 68 oxygenated organic compounds in the gas and particle phases, 28 of which were tentatively identified. The major components observed include 2,3-dihydroxyisopentanol (DHIP), 2-hydroxy-2-oxoisopentanol, 2,3-dihydroxy-3-methylbutanal, 2,3-dihydroxy-2-methylsuccinic acid, 2-hydroxy-2-methylpropanedioic acid, acetone, glyoxal, methylglyoxal, glycolaldehyde, and formaldehyde. Most of these oxygenated compounds were detected for the first time in this study. <br><br> While measurements of the gas-phase photooxidation products have been made, the focus of this work has been an examination of the particle phase. SOA from some experiments was analyzed for the organic mass to organic carbon ratio (OM/OC), the effective enthalpy of vaporization (ΔH<sub>vap</sub><sup>eff</sup>), and the aerosol yield. Additionally, aerosol size, volume, and number concentrations were measured by a Scanning Mobility Particle Sizer coupled to a Condensation Particle Counter system. The OM/OC ratio was 2.1 in the MBO/H<sub>2</sub>O<sub>2</sub> system. The ΔH<sub>vap</sub><sup>eff</sup> was 41 kJ mol<sup>−1</sup>, a value similar to that of isoprene SOA. The laboratory SOA yield measured in this study was 0.7% in MBO/H<sub>2</sub>O<sub>2</sub> for an aerosol mass of 33 μg m<sup>−3</sup>. Secondary organic aerosol was found to be negligible under conditions with oxides of nitrogen (NO<sub>x</sub>) present. Time profiles and proposed reaction schemes are provided for selected compounds. <br><br> The contribution of SOA products from MBO oxidation to ambient PM<sub>2.5</sub> was investigated by analyzing a series of ambient PM<sub>2.5</sub> samples collected in several places around the United States. In addition to the occurrence of several organic compounds in both field and laboratory samples, DHIP was found to originate only from the oxidation of MBO, and therefore this compound could potentially serve as a tracer for MBO SOA. Initial attempts have been made to quantify the concentrations of DHIP and other compounds based on surrogate compound calibrations. The average concentrations of DHIP in ambient PM<sub>2.5</sub> samples from Duke Forest in North Carolina ranged from zero during cold seasons to approximately 1 ng m<sup>−3</sup> during warm seasons. This appears to be the first time that DHIP has been detected in ambient PM<sub>2.5</sub> samples. The occurrence of several other compounds in both laboratory and field samples suggests that SOA originating from MBO can contribute under selected ambient conditions to the ambient aerosol mainly in areas where MBO emissions are high
Characterization of polar organosulfates in secondary organic aerosol from the unsaturated aldehydes 2-E-pentenal, 2-E-hexenal, and 3-Z-hexenal
We show in the present study that the unsaturated aldehydes 2-E-pentenal, 2-E-hexenal, and 3-Z-hexenal are biogenic volatile organic compound (BVOC) precursors for polar organosulfates with molecular weights (MWs) 230 and 214, which are also present in ambient fine aerosol from a forested site, i.e., K-puszta, Hungary. These results complement those obtained in a previous study showing that the green leaf aldehyde 3-Z-hexenal serves as a precursor for MW 226 organosulfates. Thus, in addition to isoprene, the green leaf volatiles (GLVs) 2-E-hexenal and 3-Z-hexenal, emitted due to plant stress (mechanical wounding or insect attack), and 2-E-pentenal, a photolysis product of 3-Z-hexenal, should be taken into account for secondary organic aerosol and organosulfate formation. Polar organosulfates are of climatic relevance because of their hydrophilic properties and cloud effects. Extensive use was made of organic mass spectrometry (MS) and detailed interpretation of MS data (i.e., ion trap MS and accurate mass measurements) to elucidate the chemical structures of the MW 230, 214 and 170 organosulfates formed from 2-E-pentenal and indirectly from 2-E-hexenal and 3-Z-hexenal. In addition, quantum chemical calculations were performed to explain the different mass spectral behavior of 2,3-dihydroxypentanoic acid sulfate derivatives, where only the isomer with the sulfate group at C-3 results in the loss of SO3. The MW 214 organosulfates formed from 2-E-pentenal are explained by epoxidation of the double bond in the gas phase and sulfation of the epoxy group with sulfuric acid in the particle phase through the same pathway as that proposed for 3-sulfooxy-2-hydroxy-2-methylpropanoic acid from the isoprene-related alpha,beta-unsaturated aldehyde methacrolein in previous work (Lin et al., 2013). The MW 230 organosulfates formed from 2-E-pentenal are tentatively explained by a novel pathway, which bears features of the latter pathway but introduces an additional hydroxyl group at the C-4 position. Evidence is also presented that the MW 214 positional isomer, 2-sulfooxy-3-hydroxypentanoic acid, is unstable and decarboxylates, giving rise to 1-sulfooxy-2-hydroxybutane, a MW 170 organosulfate. Furthermore, evidence is obtained that lactic acid sulfate is generated from 2-E-pentenal. This chemistry could be important on a regional and local scale where GLV emissions such as from grasses and cereal crops are substantial
Influence of aerosol acidity on the chemical composition of secondary organic aerosol from β-caryophyllene
The secondary organic aerosol (SOA) yield of β-caryophyllene photooxidation is enhanced by aerosol acidity. In the present study, the influence of aerosol acidity on the chemical composition of β-caryophyllene SOA is investigated using ultra performance liquid chromatography/electrospray ionization-time-of-flight mass spectrometry (UPLC/ESI-TOFMS). A number of first-, second- and higher-generation gas-phase products having carbonyl and carboxylic acid functional groups are detected in the particle phase. Particle-phase reaction products formed via hydration and organosulfate formation processes are also detected. Increased acidity leads to different effects on the abundance of individual products; significantly, abundances of organosulfates are correlated with aerosol acidity. To our knowledge, this is the first detection of organosulfates and nitrated organosulfates derived from a sesquiterpene. The increase of certain particle-phase reaction products with increased acidity provides chemical evidence to support the acid-enhanced SOA yields. Based on the agreement between the chromatographic retention times and accurate mass measurements of chamber and field samples, three β-caryophyllene products (i.e., β-nocaryophyllon aldehyde, β-hydroxynocaryophyllon aldehyde, and β-dihydroxynocaryophyllon aldehyde) are suggested as chemical tracers for β-caryophyllene SOA. These compounds are detected in both day and night ambient samples collected in downtown Atlanta, GA and rural Yorkville, GA during the 2008 August Mini-Intensive Gas and Aerosol Study (AMIGAS)
Yields and molecular composition of gas-phase and secondary organic aerosol from the photooxidation of the volatile consumer product benzyl alcohol: formation of highly oxygenated and hydroxy nitro-aromatic compounds
Recently, volatile chemical products (VCPs) have been increasingly
recognized as important precursors for secondary organic aerosol (SOA) and
ozone in urban areas. However, their atmospheric chemistry, physical
transformation, and impact on climate, environment, and human health
remain poorly understood. Here, the yields and chemical composition at the
molecular level of gas- and particle-phase products originating from the
photooxidation of one of these VCPs, benzyl alcohol (BnOH), are reported.
The SOA was generated in the presence of seed aerosol from nebulized
ammonium sulfate solution in a 14.5 m3 smog chamber operated in flow
mode. More than 50 organic compounds containing nitrogen and/or up to seven
oxygen atoms were identified by mass spectrometry. While a detailed
non-targeted analysis has been made, our primary focus has been to examine
highly oxygenated and nitro-aromatic compounds. The major components include
ring-opening products with a high oxygen-to-carbon ratio (e.g., malic acid,
tartaric acids, arabic acid, trihydroxy-oxo-pentanoic acids, and pentaric
acid) and ring-retaining products (e.g., benzaldehyde, benzoic acid,
catechol, 3-nitrobenzyl alcohol, 4-nitrocatechol, 2-hydroxy-5-nitrobenzyl
alcohol, 2-nitrophloroglucinol, 3,4-dihydroxy-5-nitrobenzyl alcohol). The
presence of some of these products in the gas and particle phases
simultaneously provides evidence of their gas–particle partitioning. These
oxygenated oxidation products made dominant contributions to the SOA
particle composition in both low- and high-NOx systems. Yields, organic mass
to organic carbon ratio, and proposed reaction schemes for selected
compounds are provided. The aerosol yield was 5.2 % for
BnOH/H2O2 at an SOA concentration of 52.9 µg m−3 and
ranged between 1.7 % and 8.1 % for BnOH / NOx at an SOA concentration of 40.0–119.5 µg m−3.</p
Chemical composition of isoprene SOA under acidic and non-acidic conditions: effect of relative humidity
The effect of acidity and relative humidity on bulk
isoprene aerosol parameters has been investigated in several studies;
however, few measurements have been conducted on individual aerosol
compounds. The focus of this study has been the examination of the effect of
acidity and relative humidity on secondary organic aerosol (SOA) chemical
composition from isoprene photooxidation in the presence of nitrogen oxide
(NOx). A detailed characterization of SOA at the molecular level was also
investigated. Experiments were conducted in a 14.5 m3 smog chamber
operated in flow mode. Based on a detailed analysis of mass spectra obtained
from gas chromatography–mass spectrometry of silylated derivatives in
electron impact and chemical ionization modes, ultra-high performance
liquid chromatography/electrospray ionization/time-of-flight high-resolution
mass spectrometry, and collision-induced dissociation in the negative
ionization modes, we characterized not only typical isoprene products but
also new oxygenated compounds. A series of nitroxy-organosulfates (NOSs) were
tentatively identified on the basis of high-resolution mass spectra. Under
acidic conditions, the major identified compounds include
2-methyltetrols (2MT), 2-methylglyceric acid (2mGA), and 2MT-OS. Other products identified
include epoxydiols, mono- and dicarboxylic acids, other organic sulfates,
and nitroxy- and nitrosoxy-OS. The contribution of SOA products from
isoprene oxidation to PM2.5 was investigated by analyzing ambient
aerosol collected at rural sites in Poland. Methyltetrols, 2mGA, and several
organosulfates and nitroxy-OS were detected in both the field and laboratory
samples. The influence of relative humidity on SOA formation was modest in
non-acidic-seed experiments and stronger under acidic seed aerosol. Total
secondary organic carbon decreased with increasing relative humidity under
both acidic and non-acidic conditions. While the yields of some of the
specific organic compounds decreased with increasing relative humidity,
others varied in an indeterminate manner from changes in the relative
humidity.</p
Microservice Transition and its Granularity Problem: A Systematic Mapping Study
Microservices have gained wide recognition and acceptance in software
industries as an emerging architectural style for autonomic, scalable, and more
reliable computing. The transition to microservices has been highly motivated
by the need for better alignment of technical design decisions with improving
value potentials of architectures. Despite microservices' popularity, research
still lacks disciplined understanding of transition and consensus on the
principles and activities underlying "micro-ing" architectures. In this paper,
we report on a systematic mapping study that consolidates various views,
approaches and activities that commonly assist in the transition to
microservices. The study aims to provide a better understanding of the
transition; it also contributes a working definition of the transition and
technical activities underlying it. We term the transition and technical
activities leading to microservice architectures as microservitization. We then
shed light on a fundamental problem of microservitization: microservice
granularity and reasoning about its adaptation as first-class entities. This
study reviews state-of-the-art and -practice related to reasoning about
microservice granularity; it reviews modelling approaches, aspects considered,
guidelines and processes used to reason about microservice granularity. This
study identifies opportunities for future research and development related to
reasoning about microservice granularity.Comment: 36 pages including references, 6 figures, and 3 table
Re-exposure to the hypobaric hypoxic brain injury of high altitude: plasma S100B levels and the possible effect of acclimatisation on blood–brain barrier dysfunction
B Cells Participate in Thymic Negative Selection of Murine Auto-reactive CD4+ T Cells
It is well documented that thymic epithelial cells participate in the process of negative selection in the thymus. In recent years it was reported that also dendritic cells enter the thymus and contribute to this process, thus allowing for the depletion of thymocytes that are specific to peripherally expressed self-antigens. Here we report that also B cells may take part in the elimination of auto-reactive thymocytes. Using a unique mouse model we show that B cells induce negative selection of self-reactive thymocytes in a process that leads to the deletion of these cells whereas regulatory T cells are spared. These findings have direct implication in autoimmunity, as expression of a myelin antigen by B cells in the thymus renders the mice resistant to autoimmune inflammation of the CNS
Emissions of organic carbon and methane from petroleum and dairy operations in California's San Joaquin Valley
Petroleum and dairy operations are prominent sources of gas-phase organic compounds in California's San Joaquin Valley. It is essential to understand the emissions and air quality impacts of these relatively understudied sources, especially for oil/gas operations in light of increasing US production. Ground site measurements in Bakersfield and regional aircraft measurements of reactive gas-phase organic compounds and methane were part of the CalNex (California Research at the Nexus of Air Quality and Climate Change) project to determine the sources contributing to regional gas-phase organic carbon emissions. Using a combination of near-source and downwind data, we assess the composition and magnitude of emissions, and provide average source profiles. To examine the spatial distribution of emissions in the San Joaquin Valley, we developed a statistical modeling method using ground-based data and the FLEXPART-WRF transport and meteorological model. We present evidence for large sources of paraffinic hydrocarbons from petroleum operations and oxygenated compounds from dairy (and other cattle) operations. In addition to the small straight-chain alkanes typically associated with petroleum operations, we observed a wide range of branched and cyclic alkanes, most of which have limited previous in situ measurements or characterization in petroleum operation emissions. Observed dairy emissions were dominated by ethanol, methanol, acetic acid, and methane. Dairy operations were responsible for the vast majority of methane emissions in the San Joaquin Valley; observations of methane were well correlated with non-vehicular ethanol, and multiple assessments of the spatial distribution of emissions in the San Joaquin Valley highlight the dominance of dairy operations for methane emissions. The petroleum operations source profile was developed using the composition of non-methane hydrocarbons in unrefined natural gas associated with crude oil. The observed source profile is consistent with fugitive emissions of condensate during storage or processing of associated gas following extraction and methane separation. Aircraft observations of concentration hotspots near oil wells and dairies are consistent with the statistical source footprint determined via our FLEXPART-WRF-based modeling method and ground-based data. We quantitatively compared our observations at Bakersfield to the California Air Resources Board emission inventory and find consistency for relative emission rates of reactive organic gases between the aforementioned sources and motor vehicles in the region. We estimate that petroleum and dairy operations each comprised 22% of anthropogenic non-methane organic carbon at Bakersfield and were each responsible for 8–13% of potential precursors to ozone. Yet, their direct impacts as potential secondary organic aerosol (SOA) precursors were estimated to be minor for the source profiles observed in the San Joaquin Valley
- …