477 research outputs found
Recommended from our members
Chemical ageing and transformation of diffusivity in semi-solid multi-component organic aerosol particles
Recent experimental evidence underlines the importance of reduced diffusivity in amorphous semi-solid or glassy atmospheric aerosols. This paper investigates the impact of
diffusivity on the ageing of multi-component reactive organic particles representative of atmospheric cooking aerosols. We apply and extend the recently developed KM-SUB
model in a study of a 12-component mixture containing oleic and palmitoleic acids. We demonstrate that changes in the diffusivity may explain the evolution of chemical loss
rates in ageing semi-solid particles, and we resolve surface and bulk processes under transient reaction conditions considering diffusivities altered by oligomerisation. This new model treatment allows prediction of the ageing of mixed organic multi-component
aerosols over atmospherically relevant time scales and conditions. We illustrate the impact of changing diffusivity on the chemical half-life of reactive components in semisolid particles, and we demonstrate how solidification and crust formation at the particle
surface can affect the chemical transformation of organic aerosols
Recommended from our members
Coupling aerosol surface and bulk chemistry with a kinetic double layer model (K2-SUB): oxidation of oleic acid by ozone
We present a kinetic double layer model coupling aerosol surface and bulk chemistry (K2-SUB) based on the PRA framework of gas-particle interactions (Poschl-Rudich-Ammann, 2007). K2-SUB is applied to a popular model system of atmospheric heterogeneous chemistry: the interaction of ozone with oleic acid. We show that our modelling approach allows de-convoluting surface and bulk processes, which has been a controversial topic and remains an important challenge for the understanding and description of atmospheric aerosol transformation. In particular, we demonstrate how a detailed treatment of adsorption and reaction at the surface can be coupled to a description of bulk reaction and transport that is consistent with traditional resistor model formulations.
From literature data we have derived a consistent set of kinetic parameters that characterise mass transport and chemical reaction of ozone at the surface and in the bulk of oleic acid droplets. Due to the wide range of rate coefficients reported from different experimental studies, the exact proportions between surface and bulk reaction rates remain uncertain. Nevertheless, the model results suggest an important role of chemical reaction in the bulk and an approximate upper limit of similar to 10(-11) cm(2) s(-1) for the surface reaction rate coefficient. Sensitivity studies show that the surface accommodation coefficient of the gas-phase reactant has a strong non-linear influence on both surface and bulk chemical reactions. We suggest that K2-SUB may be used to design, interpret and analyse future experiments for better discrimination between surface and bulk processes in the oleic acid-ozone system as well as in other heterogeneous reaction systems of atmospheric relevance
Recommended from our members
Kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB): the influence of interfacial transport and bulk diffusion on the oxidation of oleic acid by ozone
We present a novel kinetic multi-layer model that explicitly resolves mass transport
and chemical reaction at the surface and in the bulk of aerosol particles (KM-SUB).
The model is based on the PRA framework of gasâparticle interactions (Pšoschl et al.,
5 2007), and it includes reversible adsorption, surface reactions and surface-bulk exchange
as well as bulk diffusion and reaction. Unlike earlier models, KM-SUB does
not require simplifying assumptions about steady-state conditions and radial mixing.
The temporal evolution and concentration profiles of volatile and non-volatile species
at the gas-particle interface and in the particle bulk can be modeled along with surface
10 concentrations and gas uptake coefficients.
In this study we explore and exemplify the effects of bulk diffusion on the rate of reactive
gas uptake for a simple reference system, the ozonolysis of oleic acid particles,
in comparison to experimental data and earlier model studies. We demonstrate how
KM-SUB can be used to interpret and analyze experimental data from laboratory stud15
ies, and how the results can be extrapolated to atmospheric conditions. In particular,
we show how interfacial transport and bulk transport, i.e., surface accommodation, bulk
accommodation and bulk diffusion, influence the kinetics of the chemical reaction. Sensitivity
studies suggest that in fine air particulate matter oleic acid and compounds with
similar reactivity against ozone (C=C double bonds) can reach chemical lifetimes of
20 multiple hours only if they are embedded in a (semi-)solid matrix with very low diffusion
coefficients (10â10 cm2 sâ1).
Depending on the complexity of the investigated system, unlimited numbers of
volatile and non-volatile species and chemical reactions can be flexibly added and
treated with KM-SUB. We propose and intend to pursue the application of KM-SUB
25 as a basis for the development of a detailed master mechanism of aerosol chemistry
as well as for the derivation of simplified but realistic parameterizations for large-scale
atmospheric and climate models
Recommended from our members
Exploring NOy chemistry in levitated aqueous aerosol droplets
Chemistry of reactive nitrogen oxides, NOy, is crucial
for our understanding of composition and properties
of the Earthâs atmosphere. The proof-of-principle
experiments demonstrated that we are able to study the
atmospheric fate of nitrogen oxides that has significant
impact on global climate and hydrological cycle, thus
affecting the likelihood of local floods and acid rain
Recommended from our members
Kinetic studies of nitrate radicals: flash photolysis at 193 nm
Nitrate radicals, NO3, were produced for the first time by 193 nm laser flash photolysis of N2O5 and HNO3. Detection was achieved due to NO3âs strong absorption at 622.7 nm confirmed by measurements of the absorption spectrum in the range of 617 to 625 nm using both NO3 precursors. Time-resolved kinetic studies allowed the determination of room temperature rate coefficients for the reactions of NO3 with 2-methylbut-2-ene and NO2 of (1.28 ± 0.11) Ă 10â11 and (8.4 ± 1.2) Ă 10â13 cm3 moleculeâ1 sâ1, respectively. The rate coefficients compare well to previous measurements with alternative techniques suggesting that the reported method is valid and may be applied in follow-up studies. The rate coefficient for 2-methylbut-2-ene is compared to previous measurements and predictions for the alkene as well as the related alkenol. The new data confirm a previously suggested deactivation of the reactive site of the double bond if adjacent to an OH group. A calculated atmospheric lifetime for 2-methylbut-2-ene with respect to NO3-initated oxidation of less than 3 min suggests predominant removal by NO3 in the atmosphere
Recommended from our members
Kinetic multi-layer model of gas-particle interactions in aerosols and clouds (KM-GAP): linking condensation, evaporation and chemical reactions of organics, oxidants and water
We present a novel kinetic multi-layer model for gas-particle interactions in aerosols and clouds (KM-GAP) that treats explicitly all steps of mass transport and chemical reaction of semi-volatile species partitioning between gas phase, particle surface and particle bulk. KM-GAP is based on the PRA model framework (Pöschl-Rudich-Ammann, 2007), and it includes gas phase diffusion, reversible adsorption, surface reactions, bulk diffusion and reaction, as well as condensation, evaporation and heat transfer. The size change of atmospheric particles and the temporal evolution and spatial profile of the concentration of individual chemical species can be modelled along with gas uptake and accommodation coefficients. Depending on the complexity of the investigated system, unlimited numbers of semi-volatile species, chemical reactions, and physical processes can be treated, and the model shall help to bridge gaps in the understanding and quantification of multiphase chemistry and microphysics in atmo- spheric aerosols and clouds.
In this study we demonstrate how KM-GAP can be used to analyze, interpret and design experimental investigations of changes in particle size and chemical composition in response to condensation, evaporation, and chemical reaction. For the condensational growth of water droplets, our kinetic model results provide a direct link between laboratory observations and molecular dynamic simulations, confirming that the accommodation coefficient of water at 270 K is close to unity. Literature data on the evaporation of dioctyl phthalate as a function of particle size and time can be reproduced, and the model results suggest that changes in the experimental conditions like aerosol particle concentration and chamber geometry may influence the evaporation kinetics and can be optimized for eĂ°cient probing of specific physical effects and parameters. With regard to oxidative aging of organic aerosol particles, we illustrate how the formation and evaporation of volatile reaction products like nonanal can cause a decrease in the size of oleic acid particles exposed to ozone
How long does it take to compute the eigenvalues of a random symmetric matrix?
We present the results of an empirical study of the performance of the QR
algorithm (with and without shifts) and the Toda algorithm on random symmetric
matrices. The random matrices are chosen from six ensembles, four of which lie
in the Wigner class. For all three algorithms, we observe a form of
universality for the deflation time statistics for random matrices within the
Wigner class. For these ensembles, the empirical distribution of a normalized
deflation time is found to collapse onto a curve that depends only on the
algorithm, but not on the matrix size or deflation tolerance provided the
matrix size is large enough (see Figure 4, Figure 7 and Figure 10). For the QR
algorithm with the Wilkinson shift, the observed universality is even stronger
and includes certain non-Wigner ensembles. Our experiments also provide a
quantitative statistical picture of the accelerated convergence with shifts.Comment: 20 Figures; Revision includes a treatment of the QR algorithm with
shift
Deep Learning Transient Detection with VERITAS
Ground-based -ray observatories, such as the VERITAS array of imaging
atmospheric Cherenkov telescopes, provide insight into very-high-energy (VHE,
) astrophysical transient events. Examples
include the evaporation of primordial black holes, gamma-ray bursts and flaring
blazars. Identifying such events with a serendipitous location and time of
occurrence is difficult. Thus, employing a robust search method becomes
crucial. An implementation of a transient detection method based on
deep-learning techniques for VERITAS will be presented. This data-driven
approach significantly reduces the dependency on the characterization of the
instrument response and the modelling of the expected transient signal. The
response of the instrument is affected by various factors, such as the
elevation of the source and the night sky background. The study of these
effects allows enhancing the deep learning method with additional parameters to
infer their influences on the data. This improves the performance and stability
for a wide range of observational conditions. We illustrate our method for an
historic flare of the blazar BL Lac that was detected by VERITAS in October
2016. We find a promising performance for the detection of such a flare in
timescales of minutes that compares well with the VERITAS standard analysis.Comment: Proceedings of the 37th International Cosmic Ray Conference (ICRC
2021
Recommended from our members
Oxidation of biogenic and water-soluble compounds in aqueous and organic aerosol droplets by ozone: a kinetic and product analysis approach using laser Raman tweezers
The results of an experimental study into the oxidative degradation of proxies for atmospheric aerosol are presented. We demonstrate that the laser Raman tweezers method can be used successfully to obtain uptake coeffcients for gaseous oxidants on individual aqueous and organic droplets, whilst the size and composition of the droplets is simultaneously followed. A laser tweezers system was used to trap individual droplets containing an unsaturated organic compound in either an aqueous or organic ( alkane) solvent. The droplet was exposed to gas- phase ozone and the reaction kinetics and products followed using Raman spectroscopy. The reactions of three different organic compounds with ozone were studied: fumarate anions, benzoate anions and alpha pinene. The fumarate and benzoate anions in aqueous solution were used to represent components of humic- like substances, HULIS; a alpha- pinene in an alkane solvent was studied as a proxy for biogenic aerosol. The kinetic analysis shows that for these systems the diffusive transport and mass accommodation of ozone is relatively fast, and that liquid- phase di. ffusion and reaction are the rate determining steps. Uptake coe. ffcients, g, were found to be ( 1.1 +/- 0.7) x 10(-5), ( 1.5 +/- 0.7) x 10 (-5) and ( 3.0 - 7.5) x 10 (-3) for the reactions of ozone with the fumarate, benzoate and a- pinene containing droplets, respectively. Liquid- phase bimolecular rate coe. cients for reactions of dissolved ozone molecules with fumarate, benzoate and a- pinene were also obtained: k(fumarate) = ( 2.7 +/- 2) x 10 (5), k(benzoate) = ( 3.5 +/- 3) x 10 (5) and k(alpha-pinene) = ( 1-3) x 10(7) dm(3) mol (-1) s (- 1). The droplet size was found to remain stable over the course of the oxidation process for the HULIS- proxies and for the oxidation of a- pinene in pentadecane. The study of the alpha- pinene/ ozone system is the first using organic seed particles to show that the hygroscopicity of the particle does not increase dramatically over the course of the oxidation. No products were detected by Raman spectroscopy for the reaction of benzoate ions with ozone. One product peak, consistent with aqueous carbonate anions, was observed when following the oxidation of fumarate ions by ozone. Product peaks observed in the reaction of ozone with alpha- pinene suggest the formation of new species containing carbonyl groups
Indoor particulate matter (PM) from cooking in UK students' studio flats and associated intervention strategies: evaluation of cooking methods, PM concentrations and personal exposures using low-cost sensors
- âŠ