Dust Abundance Research Articles

Formation of the Active Star-forming Region LHA 120-N 44 Triggered by Tidally Driven Colliding H i Flows

Abstract The second most active site of high-mass star formation next to R136 in the Large Magellanic Cloud (LMC) is N44. We carried out a detailed analysis of H i at 60″ resolution by using the ATCA and Parkes data. We presented decomposition of the H i emission into two velocity components (the L and D components) with a velocity separation of ∼60 km s−1. In addition, we newly defined the I component whose velocity is intermediate between the L and D components. The D component was used to derive the rotation curve of the LMC disk, which is consistent with the stellar rotation curve. Toward the active cluster-forming region of LHA 120-N 44, the three velocity components of H i gas show signatures of dynamical interaction, including bridges and complementary spatial distributions. We hypothesize that the L and D components have been colliding with each other since 5 Myr ago, and the interaction triggered formation of the O and early-B stars ionizing N44. In the hypothesis, the I component is interpreted as decelerated gas in terms of momentum exchange in the collisional interaction of the L and D components. In the N44 region, the Planck submillimeter dust optical depth is correlated with the H i intensity, which is well approximated by a linear regression. We found that the N44 region shows a significantly steeper regression line than in the bar region, indicating less dust abundance in the N44 region, which is ascribed to the tidal interaction between the LMC and the SMC 0.2 Gyr ago.

Classification of aerosols using multiple clustering techniques over Zanjan, Iran, during 2010-2014

A more detailed study and identification of aerosol types can help to better understand the sources and effects of aerosols. In the present study, a number of optical properties of aerosols have been investigated seasonal for discrimination of aerosol types during 2010-2014 over Zanjan, Iran. Also using AERosol RObotic NETwork (AERONET) data, aerosol was classified by multiple clustering techniques. Both fine and coarse modes particles were seen in seasonal averaged of Aerosol Volume Size Distribution (AVSD). Single Scattering Albedo (SSA) variations indicate the presence of scattering aerosol like dust in the spring, summer and fall, and dominance of absorbing type aerosols in the winter. The maximum value of the phase function was observed in the summer and in small scattering angle which can be due to presence of coarse mode dust particles. The scatter plot of Aerosol Optical Depth (AOD) versus Angstrom Exponent (AE) is one of the most effective methods to find aerosol types. Extinction Angstrom exponent (EAE) versus SSA and EAE versus absorption Angstrom exponent (AAE) are other ways to classification of aerosol types. Graphs show abundance of dust in the spring, summer and fall in Zanjan's atmosphere. Also presence of urban/industrial aerosols is in all seasons, especially in the fall and winter. In addition mixed aerosols exist in all seasons. On the other hand, no biomass burning aerosols found in Zanjan's atmosphere.

Evolution of the Dust Composition in Damped Lyα Systems

Abstract We present a method for estimating the relative abundances of refractory elements in the interstellar dust of galaxies hosting damped Lyα (DLA) systems. The method requires gas-phase column densities of volatile and refractory elements, obtained from absorption-line spectroscopy, and interstellar abundances of the same elements, predicted by chemical evolution models of DLA galaxies. We applied this method to the sample of DLA systems with measurements of Mg, Si, S, Fe, and Zn column densities. We find that the dust abundance ratios (Si/Fe)d and (Mg/Fe)d decrease by almost two orders of magnitude in the metallicity range between ≃1/100 solar to roughly solar. This decrease is stronger than the well-known decline of α/Fe ratios with metallicity observed in metal-poor stars and galaxies, suggesting the existence of metallicity-dependent mechanisms of dust production. To cast light on these mechanisms we investigated the contributions of different stellar sources and interstellar processes to the galactic cycle of dust. We find that Type II SNe are important contributors to the dust composition at low metallicity ([Fe/H] < −0.6), whereas dust accretion in the interstellar medium appears to be important at higher metallicities, leading to a gradual rise of iron-rich particles, possibly in metal form. To further investigate the nature of the dust, we introduced an idealized model of dust grains based on a mixture of silicates (pyroxenes and olivines) and an iron-rich constituent. The model reproduces the evolutionary trends and suggests that olivines are dominant in silicates, in line with other studies of interstellar dust composition.

New Constraints on the Nature and Origin of the Leading Arm of the Magellanic Stream

Abstract We present a new precision measurement of gas-phase abundances of S, O, N, Si, Fe, P, Al, Ca as well as molecular hydrogen (H2) in the Leading Arm (region II, LA II) of the Magellanic Stream (MS) toward the Seyfert galaxy NGC 3783. The results are based on high-quality archival ultraviolet/optical/radio data from various different instruments (HST/STIS, FUSE, AAT, GBT, GB140 ft, ATCA). Our study updates previous results from lower-resolution data and provides for the first time a self-consistent component model of the complex multiphase absorber, delivering important constraints on the nature and origin of LA II. We derive a uniform, moderate α abundance in the two main absorber groups at +245 and +190 km s−1 of α/H = 0.30 ± 0.05 solar, a low nitrogen abundance of N/H = 0.05 ± 0.01 solar, and a high dust content with substantial dust depletion values for Si, Fe, Al, and Ca. These α, N, and dust abundances in LA II are similar to those observed in the Small Magellanic Cloud (SMC). From the analysis of the H2 absorption, we determine a high thermal pressure of P/k ≈ 1680 K cm−3 in LA II, in line with the idea that LA II is located in the inner Milky Way halo at a z-height of <20 kpc, where it hydrodynamically interacts with the ambient hot coronal gas. Our study supports a scenario in which LA II stems from the breakup of a metal- and dust-enriched progenitor cloud that was recently (200–500 Myr ago) stripped from the SMC.

Radiative transfer of ionizing radiation through gas and dust: the stellar source case

We present a new dust extension to the Monte Carlo radiative transfer code crash, which enables it to simulate the propagation of ionizing radiation through mixtures of gas and dust. The new code is applied to study the impact of dust absorption on idealized galactic H II regions and on small scale reionization. We find that H II regions are reduced in size by the presence of dust, while their inner temperature and ionization structure remain largely unaffected. In the small scale reionization simulation, dust hardens ionization fronts and delays the overlap of ionized bubbles. This effect is found to depend only weakly on the assumed abundance of dust in underdense regions.

INHALABLE AND RESPIRABLE DUST CONCENTRATION OF SOILED STONE, METAL AND CERAMIC ARTEFACT INSIDE NATIONAL MUSEUM MALAYSIA

Airborne particulate matter is a key indicator of air pollution brought into the air by a variety of natural and human activities. As it can travel in distances and suspended in the atmosphere, it affects human health and museum artefact. The primary idea of this paper is to discuss the mass concentration of airborne particulates matter in terms of inhalable and respirable dust from the stone, metal and ceramic display showcase. The fluctuation of temperature and relative humidity, the museum cleaning activity, the usage of HVAC system and construction activity affect the soiling defect of stone, metal and ceramic artefact. The approach selected to collect primary data for this research is by conducting case study at the National Museum of Malaysia. Personal air sampling was conducted using Casella 7-Holes and Cyclone sampler head. Results revealed high abundances of inhalable dust at stone and ceramic artefact both in Galley A and B. This study is beneficial towards the Jabatan Muzium Malaysia (JMM), Department of Environment (DOE), Ministry of Health (MOH) and Ministry of Natural Resources and Environment (MONRE) by emphasising that reducing air pollution will reduce soiling defect of the museum artefact. Thus, it can reduce government expenses to the museum management towards cleaning procedure for the damaged artefact.

INHALABLE AND RESPIRABLE DUST CONCENTRATION OF SOILED STONE, METAL AND CERAMIC ARTEFACT INSIDE NATIONAL MUSEUM MALAYSIA

Airborne particulate matter is a key indicator of air pollution brought into the air by a variety of natural and human activities. As it can travel in distances and suspended in the atmosphere, it affects human health and museum artefact. The primary idea of this paper is to discuss the mass concentration of airborne particulates matter in terms of inhalable and respirable dust from the stone, metal and ceramic display showcase. The fluctuation of temperature and relative humidity, the museum cleaning activity, the usage of HVAC system and construction activity affect the soiling defect of stone, metal and ceramic artefact. The approach selected to collect primary data for this research is by conducting case study at the National Museum of Malaysia. Personal air sampling was conducted using Casella 7-Holes and Cyclone sampler head. Results revealed high abundances of inhalable dust at stone and ceramic artefact both in Galley A and B. This study is beneficial towards the Jabatan Muzium Malaysia (JMM), Department of Environment (DOE), Ministry of Health (MOH) and Ministry of Natural Resources and Environment (MONRE) by emphasising that reducing air pollution will reduce soiling defect of the museum artefact. Thus, it can reduce government expenses to the museum management towards cleaning procedure for the damaged artefact.

Meteorological pressure at Gale crater from a comparison of REMS/MSL data and MCD modelling: Effect of dust storms

We examine the record of atmospheric pressure in Gale crater measured in-situ by the Rover Environmental Monitoring Station (REMS) instrument (Gómez-Elvira et al., 2012) on the Mars Science Laboratory (MSL) rover over two Martian years. We compare the data with pressure predictions from the Mars Climate Database (MCD) (Forget et al., 1999; Millour et al., 2015) version 5.2, which is a climatological database derived from numerical simulations of the Martian atmosphere produced by a General Circulation Model run over several Martian years. Seasonal and daily trends in pressure data from REMS are well reproduced by the standard climatology of the MCD using its high resolution mode. This high-resolution mode extrapolates pressure values from the nominal model into the altitude of each location using a high-resolution topography model and a fine tuning of the vertical scale height that was chosen to mimic effects of slope winds not directly accounted for in the General Circulation Model on which the MCD is based. Differences between the synthetic MCD pressure data and the REMS measurements are produced by meteorological features that are identified on particular groups of sols and quantified in intensity. We show that regional dust storms outside Gale crater and dust abundance at the crater are important factors in the behaviour of the pressure exciting larger amplitudes on the daily pressure variations and causing most of the largest REMS-MCD differences. We compare the pressure signals with published data of the dust optical depth obtained by the REMS ultraviolet photodiodes and the Mastcam instrument on MSL, and with orbital images of the planet acquired by the MARCI instrument on the Mars Reconnaissance Orbiter (MRO). We show that in some cases regional dust storms induce a characteristic signature in the surface pressure measured by REMS several sols before the dust arrives to Gale crater. We explore the capability of daily pressure measurements to serve as a fast detector of the development of dust storms in the context of the MSL, Insight and Mars 2020 missions.

Small impacts on the giant planet Jupiter

Context. Video observations of Jupiter obtained by amateur astronomers over the past 8 years have shown five flashes of light with durations of 1–2 s, each observed by at least two observers that were geographically separated. The first three of these events occurred on 3 June 2010, 20 August 2010, and 10 September 2012. Previous analyses of their light curves showed that they were caused by the impact of objects of 5–20 m in diameter, depending on their density, with a released energy comparable to superbolides on Earth of the class of the Chelyabinsk airburst. The most recent two flashes on Jupiter were detected on 17 March 2016 and 26 May 2017 and are analyzed here. Aims. We characterize the energy involved together with the masses and sizes of the objects that produced these flashes. The rate of similar impacts on Jupiter provides improved constraints on the total flux of impacts on the planet, which can be compared to the amount of exogenic species detected in the upper atmosphere of Jupiter. Methods. We extracted light curves of the flashes and calculated the masses and sizes of the impacting objects after calibrating each video observation. We also present results from a systematic search of impacts on >72 000 video amateur observations with a customized software that is based on differential photometry of the images. An examination of the number of amateur observations of Jupiter as a function of time over the past years allows us to interpret the statistics of these impact detections. Results. The cumulative flux of small objects (5–20 m or larger) that impact Jupiter is predicted to be low (10–65 impacts per year), and only a fraction of them are potentially observable from Earth (4–25 observable impacts per year in a perfect survey). These numbers imply that many observers are required to efficiently discover Jupiter impacts. Conclusions. We predict that more impacts will be found in the next years, with Jupiter opposition displaced toward summer in the northern hemisphere where most amateur astronomers observe. Objects of this size contribute negligibly to the abundance of exogenous species and dust in the stratosphere of Jupiter when compared with the continuous flux from interplanetary dust particles punctuated by giant impacts. Flashes of a high enough brightness (comparable at their peak to a +3.3 magnitude star) could produce an observable debris field on the planet. We estimate that a continuous search for these impacts might find these events once every 0.4–2.6 yr.

Unseen Dust Emission and Global Dust Abundance: Documenting Dust Emission from the Mojave Desert (USA) by Daily Remote Camera Imagery and Wind‐Erosion Measurements

AbstractA large gap in understanding the effects of atmospheric dust at all spatial scales is uncertainty about how much and whence dust is emitted annually. Direct digital recording of dust emission at high spatial and temporal resolution would together with periodic flux measurements support improved estimates of local‐scale dust flux. Remote camera recording of dust‐emitting settings on and around Soda Lake (Mojave Desert) was conducted every 15 min during daylight between 10 November 2010 and 31 December 2016. Examination of 135,000 images revealed frequent dust events characterized as dust days, 68 per year on average, when dust plumes obscured mountains beyond areas of dust generation. Dust was not observed from examination of satellite retrievals (Moderate Resolution Imaging Spectroradiometer and Geostationary Operational Environmental Satellite System) during six cloudless days of extreme emission. Estimated masses of emitted dust fractions were made from measured horizontal mass flux, sediment particle sizes, and areas of dust generation. Between April 2000 and June 2013, nearly 4 Tg/year of dust (particles < 63 μm) were emitted across the study area. Higher rates (about 7 Tg/year) were estimated following the December 2010 Mojave River flood that deposited sediment across the lake basin. Within the Mojave and Great Basin deserts of western North America, many settings akin to those at Soda Lake similarly emit dust that is rarely detected in satellite retrievals, implying that local and regional dust emissions from western North America are far underestimated and that, by extension to relatively small dust‐source areas across all drylands, global dust emissions may also be greatly underestimated.

Star Formation and Molecular Gas in Extremely Metal-poor Galaxies: Insights from the Thermal Balance in the Neutral Gas

AbstractThe apparent lack of cold molecular gas in blue compact dwarf (BCD) galaxies is at variance with their intense star-formation episode. The CO molecule, often used a tracer of H2 through a conversion function, is selectively photodissociated in dust-poor environments and, as a result, a potentially large fraction of H2 is expected to reside in the so-called CO-dark gas, where it could be traced instead by infrared cooling lines [CI], [CII], and [OI]. Although the fraction of CO-dark gas to total molecular gas is in theory expected to be relatively large in metal-poor galaxies, many uncertainties remain due to the difficulty in identifying the main heating mechanism associated to the cooling lines observed in such galaxies.Investigations of the Herschel Dwarf Galaxy Survey (DGS; Madden et al.2013) show that the heating mechanism in the neutral gas of BCDs cannot be dominated by the photoelectric effect on dust grains below some threshold metallicity due to the low abundance of dust and polycyclic aromatic hydrocarbons, implying that other heating mechanisms need to be invoked, along with a new interpretation of the corresponding infrared line diagnostics. In the study presented here and in Lebouteiller et al. (2017), we use optical and infrared lines to constrain the physical conditions in the HII region + HI region of the BCD I Zw 18 (18 Mpc; ≍2% solar metallicity) within a consistent photoionization and photodissociation model. We show that the HI region is entirely heated by a single ultraluminous X-ray source with important consequences on the applicability of [CII] to trace the star-formation rate and to trace the CO-dark gas. We derive stringent upper limits on the size of H2 clumps that may be detected in the future with JWST and IRAM/NOEMA. We also show that the nature of the X-ray source can be inferred from the corresponding signatures in the ISM. Finally we speculate that star formation may be quenched in extremely metal-poor dwarf galaxies due to X-ray photoionization.

Long-term observations of cloud condensation nuclei over the Amazon rain forest – Part 2: Variability and characteristics of biomass burning, long-range transport, and pristine rain forest aerosols

Abstract. Size-resolved measurements of atmospheric aerosol and cloud condensation nuclei (CCN) concentrations and hygroscopicity were conducted over a full seasonal cycle at the remote Amazon Tall Tower Observatory (ATTO, March 2014–February 2015). In a preceding companion paper, we presented annually and seasonally averaged data and parametrizations (Part 1; Pöhlker et al., 2016a). In the present study (Part 2), we analyze key features and implications of aerosol and CCN properties for the following characteristic atmospheric conditions: Empirically pristine rain forest (PR) conditions, where no influence of pollution was detectable, as observed during parts of the wet season from March to May. The PR episodes are characterized by a bimodal aerosol size distribution (strong Aitken mode with DAit ≈ 70 nm and NAit ≈ 160 cm−3, weak accumulation mode with Dacc ≈ 160 nm and Nacc≈ 90 cm−3), a chemical composition dominated by organic compounds, and relatively low particle hygroscopicity (κAit≈ 0.12, κacc ≈ 0.18). Long-range-transport (LRT) events, which frequently bring Saharan dust, African biomass smoke, and sea spray aerosols into the Amazon Basin, mostly during February to April. The LRT episodes are characterized by a dominant accumulation mode (DAit ≈ 80 nm, NAit ≈ 120 cm−3 vs. Dacc ≈ 180 nm, Nacc ≈ 310 cm−3), an increased abundance of dust and salt, and relatively high hygroscopicity (κAit≈ 0.18, κacc ≈ 0.35). The coarse mode is also significantly enhanced during these events. Biomass burning (BB) conditions characteristic for the Amazonian dry season from August to November. The BB episodes show a very strong accumulation mode (DAit ≈ 70 nm, NAit ≈ 140 cm−3 vs. Dacc ≈ 170 nm, Nacc ≈ 3400 cm−3), very high organic mass fractions (∼ 90 %), and correspondingly low hygroscopicity (κAit≈ 0.14, κacc ≈ 0.17). Mixed-pollution (MPOL) conditions with a superposition of African and Amazonian aerosol emissions during the dry season. During the MPOL episode presented here as a case study, we observed African aerosols with a broad monomodal distribution (D ≈ 130 nm, NCN,10 ≈ 1300 cm−3), with high sulfate mass fractions (∼ 20 %) from volcanic sources and correspondingly high hygroscopicity (κ< 100 nm ≈ 0.14, κ>100nm≈ 0.22), which were periodically mixed with fresh smoke from nearby fires (D ≈ 110 nm, NCN,10 ≈ 2800 cm−3) with an organic-dominated composition and sharply decreased hygroscopicity (κ<150nm≈ 0.10, κ>150nm≈ 0.20). Insights into the aerosol mixing state are provided by particle hygroscopicity (κ) distribution plots, which indicate largely internal mixing for the PR aerosols (narrow κ distribution) and more external mixing for the BB, LRT, and MPOL aerosols (broad κ distributions). The CCN spectra (CCN concentration plotted against water vapor supersaturation) obtained for the different case studies indicate distinctly different regimes of cloud formation and microphysics depending on aerosol properties and meteorological conditions. The measurement results suggest that CCN activation and droplet formation in convective clouds are mostly aerosol-limited under PR and LRT conditions and updraft-limited under BB and MPOL conditions. Normalized CCN efficiency spectra (CCN divided by aerosol number concentration plotted against water vapor supersaturation) and corresponding parameterizations (Gaussian error function fits) provide a basis for further analysis and model studies of aerosol–cloud interactions in the Amazon.

A geochemical perspective on the natural abundance and predominant sources of trace elements in cranberries (Vaccinium oxycoccus) from remote bogs in the Boreal region of northern Alberta, Canada

Trace elements in native cranberry (Vaccinium oxycoccus) were compared with the underlying Sphagnum moss on which it grows, from two remote ombrotrophic (rain-fed) peat bogs in northern Alberta, Canada. The purpose of the comparison was to distinguish between dust inputs to the berries versus plant uptake from the substrate, and to determine the natural abundance of trace elements in native berries. Using Al as an indicator of the abundance of soil-derived mineral particles, the abundance of dust on the surface of the berries is 20 to 29× lower than that of the substrate (moss). Other lithophile elements (V, Cr, Co, Ga, Li and Y) show similar differences between moss and berry. The concentrations of Rb and Ba in berries were similar to moss and Sr within a factor of 3 to 4×, probably reflecting passive uptake of these lithophile elements by the plants, even though they have no known physiological function. Of the micronutrients examined (Mn, Fe, Ni, Cu, Zn and Mo), Cu and Mn were more abundant in berries than moss, Ni and Zn yielded similar concentrations in both whereas Fe followed by Mo showed the greatest concentration difference. For these micronutrients, uptake by the plants through their roots via the substrate (moss and peat) outweighs contributions from atmospheric dusts.In respect to potentially toxic “heavy metals”, Pb concentrations in the moss (BMW, 89 ± 7.3 μg/kg; CMW, 93 ± 27 μg/kg) are below the natural, “background” values reported for ancient layers of Swiss peat from the mid-Holocene (>6000 years old). The Pb concentrations in the berries, however, are 19 to 47× lower than in the underlying moss indicating that Pb in the berries, like Al, is exclusively supplied by dust. Cadmium in the berries is at or above the level found in moss due to active uptake by the plants from the substrate, most likely because of the chemical similarity of this element to Zn. Silver, Sb and Tl in the berries were <LOD, but assuming that they are supplied exclusively by dust inputs, their concentrations in the berries can be estimated from the Al concentrations. Taken together, these data provide an estimate of the natural abundance of these elements in berries, based on measured (Cd, Pb) and calculated (Ag, Sb, Tl) values.

Cosmological simulation with dust formation and destruction

To investigate the evolution of dust in a cosmological volume, we perform hydrodynamic simulations, in which the enrichment of metals and dust is treated self-consistently with star formation and stellar feedback. We consider dust evolution driven by dust production in stellar ejecta, dust destruction by sputtering, grain growth by accretion and coagulation, and grain disruption by shattering, and treat small and large grains separately to trace the grain size distribution. After confirming that our model nicely reproduces the observed relation between dust-to-gas ratio and metallicity for nearby galaxies, we concentrate on the dust abundance over the cosmological volume in this paper. The comoving dust mass density has a peak at redshift $z\sim 1$--2, coincident with the observationally suggested dustiest epoch in the Universe. {In the local Universe}, roughly 10 per cent of the dust is contained in the intergalactic medium (IGM), where only 1/3--1/4 of the dust survives against dust destruction by sputtering. We also show that the dust mass function is roughly reproduced at $\lesssim 10^8$ M$_\odot$, while the massive end still has a discrepancy, which indicates {the necessity of stronger feedback in massive galaxies}. %%The relation showed that accretion is essential for dusty galaxies. In addition, our model broadly reproduces the observed radial profile of dust surface density in the circum-galactic medium (CGM). While our model satisfies the observational constraints for the dust extinction {on cosmological scales}, it predicts that the dust in the CGM and IGM is dominated by large ($> 0.03~\mu$m) grains, which is in tension with the steep reddening curves {observed} in the CGM.

Orbital altitude dust at Mars, its implication and a prototype for its detection

Dust at orbital altitude on Mars could be mainly due to interplanetary source, cometary source or natural satellites of Mars. Recent work and MAVEN observations predict that dust at orbital altitudes could be interplanetary in nature. Since, the source of such particles is not understood unambiguously, it remains a puzzling question to space scientist about what is a source of it. An instrument on future Mars orbiter, which can measure the dust particle parameters like mass, velocity and flux, may be helpful to address such issues.In this article, dust flux at orbital altitudes on Mars is presented in the initial part. To study origin, abundance, distribution, flux and seasonal variation of dust at orbital altitude on Mars, a Mars Orbit Dust Experiment (MODEX) has been proposed and development of its prototype has been initiated at Physical Research Laboratory. Though, earlier dust detectors by other researchers might have been designed using the impact plasma theory, they are not found with thorough explanation explicitly, in the literature. We have presented the impact plasma theory for the first time to explain detector target design. Also, a prototype dust detector, its front end electronics and initial testing results using a pulse laser are reported here. The ions and electrons of impact plasma have been separated using biased electrodes. The results demonstrate the possible use of pulse laser for testing the impact ionization dust detector, prior to calibrating the dust detector using a dust accelerator facility. The detector under development could provide actual dust flux coming to Mars and using the measurements, one can understand the origin, abundance, distribution, flux and seasonal variation of dust.

Dust abundance and grain size in galaxy halos

We investigate the abundance and properties (especially, grain size) of dust in galaxy halos using available observational data in the literature. There are two major sets of data. One is (i) the reddening curves at redshifts z∼1 and 2 derived for Mg ii absorbers, which are assumed to trace the medium in galaxy halos. The other is (ii) the cosmic extinction up to z∼2 mainly traced by distant background quasars. For (i), the observed reddening curves favor a grain radius of a∼0.03μm for silicate, while graphite is not supported because of its strong 2175 Å bump. Using amorphous carbon improves the fit to the reddening curves compared with graphite if the grain radius is a≲0.03μm. For (ii), the cosmic extinction requires η≳10−2 (η is the ratio of the halo dust mass to the stellar mass; the observationally suggested value is η∼10−3) for silicate if a∼0.03μm as suggested by the reddening curve constraint. Thus, for silicate, we do not find any grain radius that satisfies both (i) and (ii) unless the halo dust abundance is much larger than suggested by the observations. For amorphous carbon, in contrast, a wide range of grain radius (a∼0.01−0.3 μm) is accepted by the cosmic extinction; thus, we find that a grain radius range of a∼0.01–0.03 μm is supported by combining (i) and (ii). We also discuss the origin of dust in galaxy halos, focusing on the importance of grain size in the physical mechanism of dust supply to galaxy halos.

The greening of Northwest Indian subcontinent and reduction of dust abundance resulting from Indian summer monsoon revival

The trends of both rainfall and circulation strength of the Indian summer monsoon has been reviving since 2002. Here, using observational data, we demonstrate a statistically significant greening over the Northwest Indian Subcontinent and a consequent decline in dust abundance due to the monsoon revival. The enhanced monsoonal rainfall causes an increase in soil moisture, which results in a significant greening in the Northwest Indian Subcontinent. These increases in rainfall, soil moisture, and vegetation together lead to a substantial reduction of the dust abundance in this region, especially the Thar Desert, as shown by a negative trend in satellite-retrieved aerosol optical depth. The monsoonal rainfall-induced trends in vegetation growth and dust abundance in the Northwest Indian Subcontinent have important implications for agriculture production and air quality given the projected increases and a westward expansion of the global summer monsoon rainfall at the end of this century.

Large Interstellar Polarisation Survey

It is well known that the dust properties of the diffuse interstellar medium exhibit variations towards different sight-lines on a large scale. We have investigated the variability of the dust characteristics on a small scale, and from cloud-to-cloud. We use low-resolution spectro-polarimetric data obtained in the context of the Large Interstellar Polarisation Survey (LIPS) towards 59 sight-lines in the Southern Hemisphere, and we fit these data using a dust model composed of silicate and carbon particles with sizes from the molecular to the sub-micrometre domain. Large (≥6 nm) silicates of prolate shape account for the observed polarisation. For 32 sight-lines we complement our data set with UVES archive high-resolution spectra, which enable us to establish the presence of single-cloud or multiple-clouds towards individual sight-lines. We find that the majority of these 35 sight-lines intersect two or more clouds, while eight of them are dominated by a single absorbing cloud. We confirm several correlations between extinction and parameters of the Serkowski law with dust parameters, but we also find previously undetected correlations between these parameters that are valid only in single-cloud sight-lines. We find that interstellar polarisation from multiple-clouds is smaller than from single-cloud sight-lines, showing that the presence of a second or more clouds depolarises the incoming radiation. We find large variations of the dust characteristics from cloud-to-cloud. However, when we average a sufficiently large number of clouds in single-cloud or multiple-cloud sight-lines, we always retrieve similar mean dust parameters. The typical dust abundances of the single-cloud cases are [C]/[H] = 92 ppm and [Si]/[H] = 20 ppm.

Study of temporal variability and mass closure of PM2.5 and its chemical constituents during weak south-west monsoon

Ambient PM2.5 (n = 42) samples have been collected during a weak South-west (SW) monsoon (July‒September, 2015) from central part of the Indo-Gangetic Plain (IGP; at Kanpur). Organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), water-soluble inorganic species (WSIS) and metals have been measured in this study. Air-mass back trajectory (AMBT) analysis reveals atmospheric transport from three major pathways indicated as Type-1 (n = 24; traversing through western region), Type-2 (n = 8; traversing eastern region) and Type-3 (n = 10; terrestrial air-masses). Total carbonaceous aerosols (TCA), mineral dust (MD) and WSIS in Type-1 air-mass contributed 18.7 ± 8%, 40.3 ± 14.5% and 35.2 ± 13.4%, respectively to the PM2.5 mass. TCA, MD and WSIS mass fractions constituted 27.1 ± 13.2%, 29.1 ± 7.9% and 34.9 ± 10.7% in Type-2 air-mass and 25.2 ± 14.1%, 34.6 ± 14% and 33.3 ± 9.8% in Type-3 air-mass, respectively. Mineral dust percentage fraction and composition (based on Fe/Al ratio) suggests significant differences between Type-1 and Type-2 air-masses. Based on statistical two-tailed t-test, the mineral dust composition of Type-1 versus Type-3 and Type-2 versus Type-3 air-masses look near similar. However, average mass ratios of OC/EC (6.2–6.4), WSOC/OC (≈0.50; exception being difference in Type-2 versus Type-3) and K+BB/OC (0.05–0.07; K+BB: biomass burning derived potassium) look near similar in PM2.5 associated with all the three air-masses; indicating carbonaceous aerosols from well-mixed sources with dominant contribution from biomass burning emission. A significant abundance of organic aerosols and mineral dust in presence of high moisture content and their mixing over IGP has implications to perturbation in regional scale radiative forcing.

Populating H2 and CO in galaxy simulation with dust evolution

There are two major theoretical issues for the star formation law (the relation between the surface densities of molecular gas and star formation rate on a galaxy scale): (i) At low metallicity, it is not obvious that star-forming regions are rich in H$_2$ because the H$_2$ formation rate depends on the dust abundance; and (ii) whether or not CO really traces H$_2$ is uncertain, especially at low metallicity. To clarify these issues, we use a hydrodynamic simulation of an isolated disc galaxy with a spatial resolution of a few tens parsecs. The evolution of dust abundance and grain size distribution is treated consistently with the metal enrichment and the physical state of the interstellar medium. We compute the H$_2$ and CO abundances using a subgrid post-processing model based on the dust abundance and the dissociating radiation field calculated in the simulation. We find that when the metallicity is $\lesssim 0.4$ Z$_\odot$ ($t<1$ Gyr), H$_2$ is not a good tracer of star formation rate because H$_2$-rich regions are limited to dense compact regions. At $Z\gtrsim 0.8$ Z$_\odot$, a tight star formation law is established for both H$_2$ and CO. At old ($t \sim 10$ Gyr) ages, we also find that adopting the so-called MRN grain size distribution with an appropriate dust-to-metal ratio over the entire disc gives reasonable estimates for the H$_2$ and CO abundances. For CO, improving the spatial resolution of the simulation is important while the H$_2$ abundance is not sensitive to sub-resolution structures at $Z\gtrsim 0.4$ Z$_\odot$.