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

Molecular gas freeze-out in the pre-stellar core L1689B

C17O (J=2-1) observations have been carried out towards the pre-stellar core L1689B. By comparing the relative strengths of the hyperfine components of this line, the emission is shown to be optically thin. This allows accurate CO column densities to be determined and, for reference, this calculation is described in detail. The hydrogen column densities that these measurements imply are substantially smaller than those calculated from SCUBA dust emission data. Furthermore, the C17O column densities are approximately constant across L1689B whereas the SCUBA column densities are peaked towards the centre. The most likely explanation is that CO is depleted from the central regions of L1689B. Simple models of pre-stellar cores with an inner depleted region are compared with the results. This enables the magnitude of the CO depletion to be quantified and also allows the spatial extent of the freeze-out to be firmly established. We estimate that within about 5000 AU of the centre of L1689B, over 90% of the CO has frozen onto grains. This level of depletion can only be achieved after a duration that is at least comparable to the free-fall timescale.Comment: MNRAS letters. 5 pages, 5 figure

Molecular line profiles as diagnostics of protostellar collapse: modelling the `blue asymmetry' in inside-out infall

The evolution of star-forming core analogues undergoing inside-out collapse is studied with a multi-point chemodynamical model which self-consistently computes the abundance distribution of chemical species in the core. For several collapse periods the output chemistry of infall tracer species such as HCO+, CS, and N2H+, is then coupled to an accelerated Lambda-iteration radiative transfer code, which predicts the emerging molecular line profiles using two different input gas/dust temperature distributions. We investigate the sensitivity of the predicted spectral line profiles and line asymmetry ratios to the core temperature distribution, the time-dependent model chemistry, as well as to ad hoc abundance distributions. The line asymmetry is found to be strongly dependent on the adopted chemical abundance distribution. In general, models with a warm central region show higher values of blue asymmetry in optically thick HCO+ and CS lines than models with a starless core temperature profile. We find that in the formal context of Shu-type inside-out infall, and in the absence of rotation or outflows, the relative blue asymmetry of certain HCO+ and CS transitions is a function of time and, subject to the foregoing caveats, can act as a collapse chronometer. The sensitivity of simulated HCO+ line profiles to linear radial variations, subsonic or supersonic, of the internal turbulence field is investigated in the separate case of static cores.Comment: Accepted to MNRAS; 20 pages, 13 fig

The dust temperature distribution in prestellar cores

We have computed the dust temperature distribution to be expected in a pre-protostellar core in the phase prior to the onset of gravitational instability. We have done this in the approximation that the heating of the dust grains is solely due to the attenuated external radiation field and that the core is optically thin to its own radiation. This permits us to consider non spherically symmetric geometries. We predict the intensity distributions of our model cores at millimeter and sub-millimeter wavelengths and compare with observations of the well studied object L1544. We have also developed an analytical approximation for the temperature at the center of spherically symmetric cores and we compare this with the numerical calculations. Our results show (in agreement with Evans et al. 2001) that the temperatures in the nuclei of cores of high visual extinction (> 30 magnitudes) are reduced to values of below ~8 K or roughly half of the surface temperature. This has the consequence that maps at wavelengths shortward of 1.3 mm see predominantly the low density exterior of pre-protostellar cores. It is extremely difficult to deduce the true density distribution from such maps alone. We have computed the intensity distribution expected on the basis of the models of Ciolek & Basu (2000) and compared with the observations of L1544. The agreement is good with a preference for higher inclinations (37 degrees instead of 16) than that adopted by Ciolek & Basu (2000). We find that a simple extension of the analytic approximation allows a reasonably accurate calculation of the dust temperature as a function of radius in cores with density distributions approximating those expected for Bonnor-Ebert spheres and suggest that this may be a useful tool for future calculations of the gas temperature in such cores.Comment: 14 latex pages, 10 ps figures, A&A accepte

Does Infall End Before the Class I Stage?

We have observed HCO+ J=3-2 toward 16 Class I sources and 18 Class 0 sources, many of which were selected from Mardones et al. (1997). Eight sources have profiles significantly skewed to the blue relative to optically thin lines. We suggest six sources as new infall candidates. We find an equal "blue excess" among Class 0 and Class I sources after combining this sample with that of Gregersen et al. (1997). We used a Monte Carlo code to simulate the temporal evolution of line profiles of optically thick lines of HCO+, CS and H2CO in a collapsing cloud and found that HCO+ had the strongest asymmetry at late times. If a blue-peaked line profile implies infall, then the dividing line between the two classes does not trace the end of the infall stage.Comment: 21 pages, 8 figures, accepted by ApJ for April 20, 2000, added acknowledgmen

Social Disorganization and Rural/Urban Crime Rates: A County Level Comparison of Contributing Factors

Social disorganization theory (Shaw & McKay, 1929) is a common explanation for crime. However, few studies have examined its significance for the explanation of crime in rural areas. The current study utilizes county level data from the United States Department of Health and Human Services to examine common characteristics of social disorganization for both rural and urban areas and which of these factors greater contribute to crime rates. The findings are consistent with previous research that finds significant differences between urban and rural areas regarding causes of crime. Overall, this study found that common measures of social disorganization such as income, racial heterogeneity, and migration do play a significant role in predicting the crime rate for both urban and rural areas. However, births and international migration play a significant role only in rural areas

Why do starless cores appear more flattened than protostellar cores?

We evaluate the intrinsic three dimensional shapes of molecular cores, by analysing their projected shapes. We use the recent catalogue of molecular line observations of Jijina et al. and model the data by the method originally devised for elliptical galaxies. Our analysis broadly supports the conclusion of Jones et al. that molecular cores are better represented by triaxial intrinsic shapes (ellipsoids) than biaxial intrinsic shapes (spheroids). However, we find that the best fit to all of the data is obtained with more extreme axial ratios ($1:0.8:0.4$) than those derived by Jones et al. More surprisingly, we find that starless cores have more extreme axial ratios than protostellar cores -- starless cores appear more `flattened'. This is the opposite of what would be expected from modeling the freefall collapse of triaxial ellipsoids. The collapse of starless cores would be expected to proceed most swiftly along the shortest axis - as has been predicted by every modeller since Zel'dovich - which should produce more flattened cores around protostars, the opposite of what is seen.Comment: 7 pages, 3 figure

How effective is the Forestry Commission Scotland's woodland improvement programme--'Woods In and Around Towns' (WIAT)--at improving psychological well-being in deprived urban communities? A quasi-experimental study

Introduction: There is a growing body of evidence that suggests that green spaces may positively influence psychological well-being. This project is designed to take advantage of a natural experiment where planned physical and social interventions to enhance access to natural environments in deprived communities provide an opportunity to prospectively assess impacts on perceived stress and mental well-being.<p></p> Study design and methods: A controlled, prospective study comprising a repeat cross-sectional survey of residents living within 1.5 km of intervention and comparison sites. Three waves of data will be collected: prephysical environment intervention (2013); postphysical environment intervention (2014) and postwoodland promotion social intervention (2015). The primary outcome will be a measure of perceived stress (Perceived Stress Scale) preintervention and postintervention. Secondary, self-report outcomes include: mental well-being (Short Warwick-Edinburgh Mental Well-being Scale), changes in physical activity (IPAQ-short form), health (EuroQoL EQ-5D), perception and use of the woodlands, connectedness to nature (Inclusion of Nature in Self Scale), social cohesion and social capital. An environmental audit will complement the study by evaluating the physical changes in the environment over time and recording any other contextual changes over time. A process evaluation will assess the implementation of the programme. A health economics analysis will assess the cost consequences of each stage of the intervention in relation to the primary and secondary outcomes of the study.<p></p> Ethics and dissemination: Ethical approval has been given by the University of Edinburgh, Edinburgh College of Art Research, Ethics and Knowledge Exchange Committee (ref. 19/06/2012). Findings will be disseminated through peer-reviewed publications, national and international conferences and, at the final stage of the project, through a workshop for those interested in implementing environmental interventions.<p></p&gt

Infall models of Class 0 protostars

We have carried out radiative transfer calculations of infalling, dusty envelopes surrounding embedded protostars to understand the observed properties of the recently identified ``Class 0'' sources. To match the far-infrared peaks in the spectral energy distributions of objects such as the prototype Class 0 source VLA 1623, pure collapse models require mass infall rates \sim10^{-4}\msunyr$^{-1}$. The radial intensity distributions predicted by such infall models are inconsistent with observations of VLA 1623 at sub-mm wavelengths, in agreement with the results of Andre et al. (1993) who found a density profile of $\rho \propto r^{-1/2}$ rather than the expected $\rho \propto r^{-3/2}$ gradient. To resolve this conflict, while still invoking infall to produce the outflow source at the center of VLA 1623, we suggest that the observed sub-mm intensity distribution is the sum of two components: an inner infall zone, plus an outer, more nearly constant-density region. This explanation of the observations requires that roughly half the total mass observed within 2000 AU radius of the source lies in a region external to the infall zone. The column densities for this external region are comparable to those found in the larger Oph A cloud within which VLA 1623 is embedded. The extreme environments of Class 0 sources lead us to suggest an alternative or additional interpretation of these objects: rather than simply concluding with Andre et al. that Class 0 objects only represent the earliest phases of protostellar collapse, and ultimately evolve into older ``Class I'' protostars, we suggest that many Class 0 sources could be the protostars of very dense regions. (Shortened)Comment: 22 pages, including 3 PostScript figures, accepted for publication in The Astrophysical Journa

Radiative Transfer in Prestellar Cores: A Monte Carlo Approach

We use our Monte Carlo radiative transfer code to study non-embedded prestellar cores and cores that are embedded at the centre of a molecular cloud. Our study indicates that the temperature inside embedded cores is lower than in isolated non-embedded cores, and generally less than 12 K, even when the cores are surrounded by an ambient cloud of small visual extinction (Av~5). Our study shows that the best wavelength region to observe embedded cores is between 400 and 500 microns, where the core is quite distinct from the background. We also predict that very sensitive observations (~1-3 MJy/sr) at 170-200 microns can be used to estimate how deeply a core is embedded in its parent molecular cloud. Finally, we present preliminary results of asymmetric models of non-embedded cores.Comment: 8 pages, 15 figures, to appear in the conference proceedings of "Open Issues in Local Star Formation and Early Stellar Evolution", held in Ouro Preto (Brazil), April 5-10, 200