A 3-D chemistry-transport model has been applied to the Mexico City metropolitan area to investigate the origin of elevated levels of non-fossil (NF) carbonaceous aerosols observed in this highly ...urbanized region. High time resolution measurements of the fine aerosol concentration and composition, and 12 or 24 h integrated 14C measurements of aerosol modern carbon have been performed in and near Mexico City during the March 2006 MILAGRO field experiment. The non-fossil carbon fraction (fNF), which is lower than the measured modern fraction (fM) due to the elevated 14C in the atmosphere caused by nuclear bomb testing, is estimated from the measured fM and the source-dependent information on modern carbon enrichment. The fNF contained in PM1 total carbon analyzed by a US team (fNFTC) ranged from 0.37 to 0.67 at the downtown location, and from 0.50 to 0.86 at the suburban site. Substantially lower values (i.e. 0.24–0.49) were found for PM10 filters downtown by an independent set of measurements (Swiss team), which are inconsistent with the modeled and known differences between the size ranges, suggesting higher than expected uncertainties in the measurement techniques of 14C. An increase in the non-fossil organic carbon (OC) fraction (fNFOC) by 0.10–0.15 was observed for both sets of filters during periods with enhanced wildfire activity in comparison to periods when fires were suppressed by rain, which is consistent with the wildfire impacts estimated with other methods. Model results show that the relatively high fraction of non-fossil carbon found in Mexico City seems to arise from the combination in about equal proportions of regional biogenic SOA, biomass burning POA and SOA, as well as non-fossil urban POA and SOA. Predicted spatial and temporal variations for fNFOCare similar to those in the measurements between the urban vs. suburban sites, and high-fire vs. low-fire periods. The absolute modeled values of fNFOC are consistent with the Swiss dataset but lower than the US dataset. Resolving the 14C measurement discrepancies is necessary for further progress in model evaluation. The model simulations that included secondary organic aerosol (SOA) formation from semi-volatile and intermediate volatility (S/IVOC) vapors showed improved closure for the total OA mass compared to simulations which only included SOA from VOCs, providing a more realistic basis to evaluate the fNF predictions. fNFOC urban sources of modern carbon are important in reducing or removing the difference in fNF between model and measurements, even though they are often neglected on the interpretation of 14C datasets. An underprediction of biomass burning POA by the model during some mornings also explains a part of the model-measurement differences. The fNF of urban POA and SOA precursors is an important parameter that needs to be better constrained by measurements. Performing faster (≤3 h) 14C measurements in future campaigns is critical to further progress in this area. To our knowledge this is the first time that radiocarbon measurements are used together with aerosol mass spectrometer (AMS) organic components to assess the performance of a regional model for organic aerosols.
The GoAmazon 2014/5 field campaign took place in Manaus, Brazil, and allowed the investigation of the interaction between background-level biogenic air masses and anthropogenic plumes.
We present in ...this work a box model built to simulate the impact of urban chemistry on biogenic secondary organic aerosol (SOA) formation and composition.
An organic chemistry mechanism is generated with the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) to simulate the explicit oxidation of biogenic and anthropogenic compounds.
A parameterization is also included to account for the reactive uptake of isoprene oxidation products on aqueous particles.
The biogenic emissions estimated from existing emission inventories had to be reduced to match measurements.
The model is able to reproduce ozone and NOx for clean and polluted situations.
The explicit model is able to reproduce background case SOA mass concentrations but does not capture the enhancement observed in the urban plume.
The oxidation of biogenic compounds is the major contributor to SOA mass.
A volatility basis set (VBS) parameterization applied to the same cases obtains better results than GECKO-A for predicting SOA mass in the box model.
The explicit mechanism may be missing SOA-formation processes related to the oxidation of monoterpenes that could be implicitly accounted for in the VBS parameterization.
Oxidized phospholipids (OxPLs) containing enzymatically or non-enzymatically oxidized fatty acids (oxylipins) are increasingly recognized as lipid mediators involved in pathogenesis of diseases. ...Further understanding of structure-activity relationship and molecular mechanisms activated by OxPLs is hampered by the complexity of synthesis of individual molecular species. Although dozens of individual free oxylipins are commercially available, their attachment to the phospholipid scaffold requires relatively harsh conditions during activation of carboxy-group, which may lead to decomposition of unstable oxylipins. Furthermore, additional protection-deprotection steps are required for oxylipins containing hydroxy-groups. In this work we describe synthesis of OxPLs containing oxylipins bound at the sn-2-position via an amide-bond that is characteristic of sphingophospholipids. Activation of oxylipins and attachment to the phospholipid scaffold are performed under mild conditions and characterized by high yield. Hydroxy-groups of oxylipins do not interfere with reactions and therefore no protection/deprotection steps are needed. In order to prevent oxylipin migration, a fatty acid residue at the sn-1 was bound through an alkyl bond, which is a common bond present in a large proportion of naturally occurring phospholipids. An additional advantage of combining alkyl and amide bonds in a single phospholipid molecule is that both types of bonds are phospholipase A1/A2-resistant, which may be expected to improve biological stability of OxPLs and thus simplify analysis of their effects. As proof of principle, several alkyl-amide oxidized phosphatidylcholines (OxPCs) containing either linear or prostane ring oxylipins have been synthesized. Importantly, we show here that alkyl-amide-OxPCs demonstrated biological activities similar to those of di-acyl-OxPCs. Alkyl-amide-OxPCs inhibited pro-inflammatory action of LPS and increased endothelial cellular barrier in vitro and in mouse models. The effects of alkyl-amide and di-acyl-OxPCs developed in a similar range of concentrations. We hypothesize that alkyl-amide-OxPLs may become a useful tool for deeper analysis of the structure-activity relationship of OxPLs.
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•Simplified synthesis of alkyl-amide oxidized phospholipids (OxPLs) is described.•Alkyl-amide OxPLs inhibit action of LPS and enhance lung endothelial barrier.•Alkyl-amide OxPLs act similarly to diacyl-OxPLs in vitro and in vivo.•Alkyl-amide OxPLs are a practical tool for analysis of the biological role of OxPLs.
The in-plane shear and compressive properties of unidirectional (UD) HTS40/977-2 carbon fibre-toughened resin (CF/TR) laminates are investigated. Scanning Electron microscopy (SEM) and optical ...microscopy are used to reveal the failure mechanisms developed during compression. It is found that damage initiates by fibre microbuckling (a fibre instability failure mode) which then is followed by yielding of the matrix to form a fibre kink band zone that leads to final fracture. Analytical models are briefly reviewed and a graphical method, based on the shear response of the composite system, is described in order to estimate the UD compressive strength. Predictions for the HTS40/977-2 system are compared to experimental measurements and to data of five other unidirectional carbon fibre reinforced polymer (CFRP) composites that are currently used in aerospace and other structural applications. It is shown that the estimated values are in a good agreement with the measured results.
KORUS-AQ was an international cooperative air quality field study in South Korea that measured local and remote sources of air pollution affecting the Korean Peninsula during May–June 2016. Some of ...the largest aerosol mass concentrations were measured during a Chinese haze transport event (24 May). Air quality forecasts using the WRF-Chem model with aerosol optical depth (AOD) data assimilation captured AOD during this pollution episode but overpredicted surface particulate matter concentrations in South Korea, especially PM2.5, often by a factor of 2 or larger. Analysis revealed multiple sources of model deficiency related to the calculation of optical properties from aerosol mass that explain these discrepancies. Using in situ observations of aerosol size and composition as inputs to the optical properties calculations showed that using a low-resolution size bin representation (four bins) underestimates the efficiency with which aerosols scatter and absorb light (mass extinction efficiency). Besides using finer-resolution size bins (8–16 bins), it was also necessary to increase the refractive indices and hygroscopicity of select aerosol species within the range of values reported in the literature to achieve better consistency with measured values of the mass extinction efficiency (6.7 m2 g−1 observed average) and light-scattering enhancement factor (f(RH)) due to aerosol hygroscopic growth (2.2 observed average). Furthermore, an evaluation of the optical properties obtained using modeled aerosol properties revealed the inability of sectional and modal aerosol representations in WRF-Chem to properly reproduce the observed size distribution, with the models displaying a much wider accumulation mode. Other model deficiencies included an underestimate of organic aerosol density (1.0 g cm−3 in the model vs. observed average of 1.5 g cm−3) and an overprediction of the fractional contribution of submicron inorganic aerosols other than sulfate, ammonium, nitrate, chloride, and sodium corresponding to mostly dust (17 %–28 % modeled vs. 12 % estimated from observations). These results illustrate the complexity of achieving an accurate model representation of optical properties and provide potential solutions that are relevant to multiple disciplines and applications such as air quality forecasts, health impact assessments, climate projections, solar power forecasts, and aerosol data assimilation.
Improvement of the interfacial properties of composites consisting of poly(3-hydroxybutyrate) and flax fibres was provided by addition of 4,4
′-thiodiphenol (TDP) at various concentrations up to 10% ...v/v. The additive TDP is known to form hydrogen bonds with many functional groups. The flax fibres were initially treated with TDP before being used to prepare the composites. The effects of the TDP on the functional groups of the fibres and the matrix were monitored using Fourier transform infrared spectroscopy and it was confirmed that hydrogen bonding occurred in the composites, which consequently brought about advantageous changes in the dynamic flexural properties. The thermal stability determined by thermogravimetry showed a favourable shift of degradation temperatures to higher values and the thermal properties of the matrix determined from differential scanning calorimetry were also affected. With the aid of scanning electron microscopy and polarised optical microscopy, the composites were shown to exhibit improvement in the fibre–matrix bonding and the dynamic mechanical properties of the matrix changed from brittle to ductile as the TDP content increased.
Secondary organic aerosols (SOA) are formed from oxidation of hundreds of volatile organic compounds (VOCs) emitted from anthropogenic and natural sources. Accurate predictions of this chemistry are ...key for air quality and climate studies due to the large contribution of organic aerosols to submicron aerosol mass. Currently, only explicit models, such as the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO‐A), can fully represent the chemical processing of thousands of organic species. However, their extreme computational cost prohibits their use in current chemistry‐climate models, which rely on simplified empirical parameterizations to predict SOA concentrations. This study demonstrates that machine learning can accurately emulate SOA formation from an explicit chemistry model with an approximate error of 2%–8%, up to five days for several precursors and for potentially up to one month for recurrent neural network models, and with 100 to 100,000 times speedup over GECKO‐A, making it computationally useable in a chemistry‐climate model. We generated the training data using thousands of GECKO‐A box simulations sampled from a broad range of initial environmental conditions, and focused on three representative SOA precursors: the oxidation by OH of two anthropogenic (toluene, dodecane), and the oxidation by O3 of one biogenic VOC (α‐pinene). We compare several neural models and quantify their underlying uncertainty and robustness. These are promising results, suggesting that neural network models could be applied to predict SOA in chemistry‐climate models, limited however to the range of environmental conditions that were considered in the training datasets.
Plain Language Summary
Detailed and accurate representation of organic aerosol chemistry is needed to predict the effect of atmospheric aerosols formed from natural and anthropogenic sources on both human health and climate. Ideally, these complex representations of chemistry would be directly included within state‐of‐the‐art weather and climate models to get a fully coupled system with meteorological and climatological feedback all over the globe. However, we are many years away from having the computational power needed to run such fully coupled large‐scale simulations due to the complexity of organic chemistry, which involves hundreds of thousands of organic gaseous and particle species and chemical reactions. As a potential solution, we test an approach that uses a neural network to mimic the solution of an explicit representation of organic chemistry which would be computationally feasible to link with current air quality and climate models.
Key Points
Incorporation of explicit organic chemistry into 3D chemistry‐simulations requires emulation
We developed two types of neural network emulators for the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere chemistry model
The emulators produced accurate and stable simulations for three precursor species
Under the Air Quality Model Evaluation International Initiative, Phase 2 (AQMEII-2), three online-coupled air quality model simulations, with six different configurations, are analyzed for their ...performance, inter-model agreement, and responses to emission and meteorological changes between 2006 and 2010. In this Part I paper, we focus on evaluating O3 and PM2.5 indicator-based analyses, which are important in the development of applicable control strategies of O3 and PM2.5 pollution in different regions worldwide. The O3 indicators agree on widespread NOx-limited and localized VOC-limited conditions in the U.S. The NOy and O3/NOy indicators overpredict the extent of the VOC-limited chemistry in southeast U.S., but are more robust than the H2O2/HNO3, HCHO/NOy, and HCHO/NO2 indicators at the surface, which exhibit relatively more inter-model variability. The column HCHO/NO2 indicator is underpredicted in the O3 and non-O3 seasons, but there is regional variability. For surface PM2.5 indicators, there is good inter-model agreement for the degree of sulfate neutralization; however there are systematic underpredictions in the southeast U.S. There is relatively poor inter-model agreement for the less robust adjusted gas ratio indicator, which is largely overpredicted in the summer and both under- and overpredicted in winter in the southeast U.S. There is good inter-model agreement for the O3 indicator sensitivities, indicating a predominant shift to more NOx-limited conditions in 2010 relative to 2006. There is less agreement for PM2.5 indicator sensitivities, which are less robust, while indicating shifts to either regime due to different responses of aerosol treatments to changes in emissions and meteorology.
•A multi-model evaluation of O3 and PM2.5 indicators is presented for North America.•Widespread NOx-limited regimes during May–September, and localized VOC-limited.•Overprediction in the extent of VOC-limited chemistry in southeast U.S.•NOy and O3/NOy are the most robust O3 indicators.•PM2.5 indicators are less robust than O3 indicators.
Secondary organic aerosol derived from isoprene
epoxydiols (IEPOX-SOA) is thought to contribute the dominant fraction of
total isoprene SOA, but the current volatility-based lumped SOA
...parameterizations are not appropriate to represent the reactive uptake of
IEPOX onto acidified aerosols. A full explicit modeling of this chemistry
is however computationally expensive owing to the many species and reactions
tracked, which makes it difficult to include it in chemistry–climate models
for long-term studies. Here we present three simplified parameterizations
(version 1.0) for IEPOX-SOA simulation, based on an approximate
analytical/fitting solution of the IEPOX-SOA yield and formation timescale.
The yield and timescale can then be directly calculated using the global
model fields of oxidants, NO, aerosol pH and other key properties, and dry
deposition rates. The advantage of the proposed parameterizations is that
they do not require the simulation of the intermediates while retaining the
key physicochemical dependencies. We have implemented the new
parameterizations into the GEOS-Chem v11-02-rc chemical transport model,
which has two empirical treatments for isoprene SOA (the volatility-basis-set, VBS, approach and a fixed 3 % yield parameterization), and compared
all of them to the case with detailed fully explicit chemistry. The best
parameterization (PAR3) captures the global tropospheric burden of IEPOX-SOA
and its spatiotemporal distribution (R2=0.94) vs. those
simulated by the full chemistry, while being more computationally efficient
(∼5 times faster), and accurately captures the response to
changes in NOx and SO2 emissions. On the other hand, the constant
3 % yield that is now the default in GEOS-Chem deviates strongly (R2=0.66), as does the VBS (R2=0.47, 49 % underestimation), with
neither parameterization capturing the response to emission changes. With
the advent of new mass spectrometry instrumentation, many detailed SOA
mechanisms are being developed, which will challenge global and especially
climate models with their computational cost. The methods developed in this
study can be applied to other SOA pathways, which can allow including
accurate SOA simulations in climate and global modeling studies in the
future.
Aerosol sulfate is a major component of submicron particulate matter
(PM1). Sulfate can be present as inorganic (mainly ammonium sulfate,
AS) or organosulfate (OS). Although OS is thought to be a ...smaller fraction
of total sulfate in most cases, recent literature argues that this may not
be the case in more polluted environments. Aerodyne aerosol mass
spectrometers (AMSs) measure total submicron sulfate, but it has been
difficult to apportion AS vs. OS as the detected ion fragments are similar.
Recently, two new methods have been proposed to quantify OS separately from
AS with AMS data. We use observations collected during several airborne
field campaigns covering a wide range of sources and air mass ages (spanning
the continental US, marine remote troposphere, and Korea) and targeted
laboratory experiments to investigate the performance and validity of the
proposed OS methods. Four chemical regimes are defined to categorize the
factors impacting sulfate fragmentation. In polluted areas with high
ammonium nitrate concentrations and in remote areas with high aerosol
acidity, the decomposition and fragmentation of sulfate in the AMS is
influenced by multiple complex effects, and estimation of OS does not seem
possible with current methods. In regions with lower acidity (pH > 0) and ammonium nitrate (fraction of total mass < 0.3), the proposed
OS methods might be more reliable, although application of these methods
often produced nonsensical results. However, the fragmentation of ambient
neutralized sulfate varies somewhat within studies, adding uncertainty,
possibly due to variations in the effect of organics. Under highly acidic
conditions (when calculated pH < 0 and ammonium balance < 0.65), sulfate fragment ratios show a clear relationship with acidity. The
measured ammonium balance (and to a lesser extent, the
HySOx+ / SOx+ AMS ratio) is a promising indicator of rapid estimation of aerosol pH < 0, including when gas-phase
NH3 and HNO3 are not available. These results allow an improved
understanding of important intensive properties of ambient aerosols.
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