1,721,106 research outputs found
Statistical analysis of the long-range transport of the 2015 Calbuco volcanic plume from ground-based and space-borne observations
Full text linkThis study investigates the influence of the 2015 Calbuco eruption (41.2∘ S, 72.4∘ W; Chile) on the total columnar aerosol optical properties over the Southern Hemisphere. The well-known technic of sun photometry was applied for the investigation of the transport and spatio-temporal evolution of the optical properties of the volcanic plume. The CIMEL sun photometer measurements performed at six South American and three African sites were statistically analysed. This study involves the use of the satellite observations and a back-trajectory model. The passage of the Calbuco plume is statistically detectable in the aerosol optical depth (AOD) observations obtained from sun photometer and MODIS observations. This statistical detection confirms that the majority of the plume was transported over the northeastern parts of South America and reached the South African region 1 week after the eruption. The plume impacted the southern parts of South America to a lesser extent. The highest AOD anomalies were observed over the northeastern parts of South America. Over the South African sites, the AOD anomalies induced by the spread of the plume were quite homogeneously distributed between the east and west coasts. The optical characteristics of the plume near the source region were consistent with an ash-bearing plume. Conversely, sites further from the Calbuco volcano were influenced by ash-free plume. The optical properties discussed in this paper will be used as inputs for numerical models for further investigation of the ageing of the Calbuco plume in a forthcoming stud
Smoke and clouds above the Southeast Atlantic: upcoming field campaigns probe absorbing aerosol’s impact on climate
Full text linkFrom July through October, smoke from biomass-burning (BB) fires on the southern African subcontinent is transported westward through the free troposphere over one of the largest stratocumulus cloud decks on our planet (Fig. 1). BB aerosol (smoke) absorbs shortwave radiation efficiently. This fundamental property implicates smoke within myriad small-scale processes with potential large-scale impacts on climate that are not yet well understood. A coordinated, international team of scientists from the United States, United Kingdom, France, South Africa, and Namibia will provide an unprecedented interrogation of this smoke-and-cloud regime from 2016 to 2018, using multiple aircraft and surface-based instrumentation suites to span much of the breadth of the southeast Atlanti
An overview of mesoscale aerosol processes, comparisons, and validation studies from DRAGON networks
Full text linkOver the past 24 years, the AErosol RObotic
NETwork (AERONET) program has provided highly accu-
rate remote-sensing characterization of aerosol optical and
physical properties for an increasingly extensive geographic
distribution including all continents and many oceanic is-
land and coastal sites. The measurements and retrievals from
the AERONET global network have addressed satellite and
model validation needs very well, but there have been chal-
lenges in making comparisons to similar parameters from
in situ surface and airborne measurements. Additionally,
with improved spatial and temporal satellite remote sens-
ing of aerosols, there is a need for higher spatial-resolution
ground-based remote-sensing networks. An effort to address
these needs resulted in a number of field campaign net-
works called Distributed Regional Aerosol Gridded Observa-
tion Networks (DRAGONs) that were designed to provide a
database for in situ and remote-sensing comparison and anal-
ysis of local to mesoscale variability in aerosol properties.
This paper describes the DRAGON deployments that will
continue to contribute to the growing body of research re-
lated to meso- and microscale aerosol features and processes.
The research presented in this special issue illustrates the di-
versity of topics that has resulted from the application of data
from these network
Global scale variability of the mineral dust long-wave refractive index: a new dataset of in situ measurements for climate modeling and remote sensing
Full text linkModeling the interaction of dust with long-wave (LW) radiation is still a challenge because of the scarcity of information on the complex refractive index of dust from different source regions. In particular, little is known about the variability of the refractive index as a function of the dust mineralogical composition, which depends on the specific emission source, and its size distribution, which is modified during transport. As a consequence, to date, climate models and remote sensing retrievals generally use a spatially invariant and time-constant value for the dust LW refractive index.
In this paper, the variability of the mineral dust LW refractive index as a function of its mineralogical composition and size distribution is explored by in situ measurements in a large smog chamber. Mineral dust aerosols were generated from 19 natural soils from 8 regions: northern Africa, the Sahel, eastern Africa and the Middle East, eastern Asia, North and South America, southern Africa, and Australia. Soil samples were selected from a total of 137 available samples in order to represent the diversity of sources from arid and semi-arid areas worldwide and to account for the heterogeneity of the soil composition at the global scale. Aerosol samples generated from soils were re-suspended in the chamber, where their LW extinction spectra (3–15 µm), size distribution, and mineralogical composition were measured. The generated aerosol exhibits a realistic size distribution and mineralogy, including both the sub- and super-micron fractions, and represents in typical atmospheric proportions the main LW-active minerals, such as clays, quartz, and calcite. The complex refractive index of the aerosol is obtained by an optical inversion based upon the measured extinction spectrum and size distribution.
Results from the present study show that the imaginary LW refractive index (k) of dust varies greatly both in magnitude and spectral shape from sample to sample, reflecting the differences in particle composition. In the 3–15 µm spectral range, k is between ∼ 0.001 and 0.92. The strength of the dust absorption at ∼ 7 and 11.4 µm depends on the amount of calcite within the samples, while the absorption between 8 and 14 µm is determined by the relative abundance of quartz and clays. The imaginary part (k) is observed to vary both from region to region and for varying sources within the same region. Conversely, for the real part (n), which is in the range 0.84–1.94, values are observed to agree for all dust samples across most of the spectrum within the error bars. This implies that while a constant n can be probably assumed for dust from different sources, a varying k should be used both at the global and the regional scale. A linear relationship between the magnitude of the imaginary refractive index at 7.0, 9.2, and 11.4 µm and the mass concentration of calcite and quartz absorbing at these wavelengths was found. We suggest that this may lead to predictive rules to estimate the LW refractive index of dust in specific bands based on an assumed or predicted mineralogical composition, or conversely, to estimate the dust composition from measurements of the LW extinction at specific wavebands.
Based on the results of the present study, we recommend that climate models and remote sensing instruments operating at infrared wavelengths, such as IASI (infrared atmospheric sounder interferometer), use regionally dependent refractive indices rather than generic values. Our observations also suggest that the refractive index of dust in the LW does not change as a result of the loss of coarse particles by gravitational settling, so that constant values of n and k could be assumed close to sources and following transport.
The whole dataset of the dust complex refractive indices presented in this paper is made available to the scientific community in the Supplemen
Complex refractive indices and single-scattering albedo of global dust aerosols in the shortwave spectrum and relationship to size and iron content
Full text linkThe optical properties of airborne mineral dust depend on its mineralogy, size distribution, and shape, and they might vary between different source regions. To date, large differences in refractive index values found in the literature have not been fully explained. In this paper we present a new dataset of complex refractive indices (m=n−ik) and single-scattering albedos (SSAs) for 19 mineral dust aerosols over the 370–950 nm range in dry conditions. Dust aerosols were generated from natural parent soils from eight source regions (northern Africa, Sahel, Middle East, eastern Asia, North and South America, southern Africa, and Australia). They were selected to represent the global-scale variability of the dust mineralogy. Dust was resuspended into a 4.2 m3 smog chamber where its spectral shortwave scattering (βsca) and absorption (βabs) coefficients, number size distribution, and bulk composition were measured. The complex refractive index was estimated by Mie calculations combining optical and size data, while the spectral SSA was directly retrieved from βsca and βabs measurements. Dust is assumed to be spherical in the whole data treatment, which introduces a potential source of uncertainty. Our results show that the imaginary part of the refractive index (k) and the SSA vary widely from sample to sample, with values for k in the range 0.0011 to 0.0088 at 370 nm, 0.0006 to 0.0048 at 520 nm, and 0.0003 to 0.0021 at 950 nm, as well as values for SSA in the range 0.70 to 0.96 at 370 nm, 0.85 to 0.98 at 520 nm, and 0.95 to 0.99 at 950 nm. In contrast, the real part of the refractive index (n) is mostly source (and wavelength) independent, with an average value between 1.48 and 1.55. The sample-to-sample variability in our dataset of k and SSA is mostly related to differences in the dust iron content. In particular, a wavelength-dependent linear relationship is found between the magnitude of k and SSA and the mass concentrations of both iron oxide and total elemental iron, with iron oxide better correlated than total elemental iron with both k and SSA. The value of k was found to be independent of size. When the iron oxide content exceeds 3 %, the SSA linearly decreases with an increasing fraction of coarse particles at short wavelengths (< 600 nm).
Compared to the literature, our values for the real part of the refractive index and SSA are in line with past results, while we found lower values of k compared to most of the literature values currently used in climate models.
We recommend that source-dependent values of the SW spectral refractive index and SSA be used in models and remote sensing retrievals instead of generic values. In particular, the close relationships found between k or SSA and the iron content in dust enable the establishment of predictive rules for spectrally resolved SW absorption based on particle compositio
Fine PM emission factors from residential burning of solid fuels using traditional cast-iron coal stoves
Full text linkResidential burning of solid fuels is a major source of fine particulate matter (PM2.5), which degrades indoor and ambient air quality in low-income settlements. The adverse impact of fine particulate emissions on the environment and human health is well-documented in other countries such as China and India; however, there is need for local studies to report on emission factors from residential burning of solid fuels. An emission factor quantifies the total mass of a pollutant emitted per amount of fuel burned. Emission factor is an input parameter in air quality modelling to forecast a pollutant concentrations over time and when calculating total emissions from a specific source. Local emission factors are central to managing air quality for they give results that are representative of the source compared with international emission factors. Quantifying emissions, understanding household fuel use patterns and interaction with the stove (stove operation behaviour) during a burning event is fundamental when designing emission control strategies. The aim of the study is to quantify fine particulate matter emissions from residential coal burning using systematic field measurements. The objectives of the study are (i) to characterize stove operation behavior effect on the emissions and (ii) to quantify PM2.5 emission factors using field measurements. Isokinetic (2015) and direct (2014) stack sampling tests were done to observe how PM emissions profiles change with stove operation behavior and to quantify PM2.5 emitted per kilogram of fuel burned. Fine PM emission profiles change with stove operation behavior with an emission factor ranging 6.8 g.kg-1 and 13.5 g.kg-1. The study results implies that residential coal burning is a major source of fine particulate matter in the residential area. As demonstrated that stove operation behaviour affect stove to fuel combination emissions; it is therefore suggested that those factors leading to increase emissions should be kept minimum
Intercomparison and assessment of long-term (2004 – 2013) multiple satellite aerosol products over two contrasting sites in South Africa
No full texto build a long-term database and improve the accuracy of the satellite products used for aerosol studies, there is a need to carry out intercomparison and validation of these satellite observations with ground-based measurements. With this objective, we estimated the long-term inter-annual variations and percentage change in trends of aerosol optical depth (AOD) retrieved from MODerate resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging Spectro-Radiometer (MISR) sensors for a 10-year period during 2004–2013 over two distinct sites namely, Skukuza (SKZ; 24.99°S, 31.58°E) and Richards Bay (RBAY; 28.8°S, 21.1°E) in South Africa. The validation performed over SKZ site shows that MISR was better correlated with AErosol RObotic NETwork (AERONET) when compared to Terra and Aqua satellites of MODIS. Later both the MODIS products (Terra and Aqua) were compared on the annual and seasonal basis to derive the relationship between them through scattering plot. The long-term regression analysis performed at these sites shows that the annual trends were decreasing, with the MODIS products underestimating MISR. This is due to difficulties of the MODIS algorithm when dealing with highly complex surface reflectance conditions and aerosol model assumptions. Also, the temporal variations of AOD derived from the two sensors noticed maximum in spring (September/October) and minimum in winter (June). Further, the Ultra-Violet Aerosol Index (UVAI) retrieved from the Ozone Monitoring Instrument (OMI) at the two locations for 9 years (2005–2013) showed a significant increasing trend with a high value of +0.009 yr−1 at SKZ than +0.006 yr−1 at RBAY during the study period, which is due to the transport of dust and smoke particle
Intra-pixel variability in satellite tropospheric NO2 column densities derived from simultaneous space-borne and airborne observations over the South African Highveld
No full textAircraft measurements of NO2 using an imaging differential optical absorption spectrometer (iDOAS) instrument over the South African Highveld region in August 2007 are presented and compared to satellite measurements from OMI and SCIAMACHY. In situ aerosol and trace-gas vertical profile measurements, along with aerosol optical thickness and single-scattering albedo measurements from the Aerosol Robotic Network (AERONET), are used to devise scenarios for a radiative transfer modelling sensitivity study. Uncertainty in the air-mass factor due to variations in the aerosol and NO2 profile shape is constrained and used to calculate vertical column densities (VCDs), which are compared to co-located satellite measurements. The lower spatial resolution of the satellites cannot resolve the detailed plume structures revealed in the aircraft measurements. The airborne DOAS in general measured steeper horizontal gradients and higher peak NO2 vertical column density. Aircraft measurements close to major sources, spatially averaged to the satellite resolution, indicate NO2 column densities more than twice those measured by the satellite. The agreement between the high-resolution aircraft instrument and the satellite instrument improves with distance from the source, this is attributed to horizontal and vertical dispersion of NO2 in the boundary layer. Despite the low spatial resolution, satellite images reveal point sources and plumes that retain their structure for several hundred kilometres downwin
Investigation of the relative fine and coarse mode aerosol loadings and properties in the Southern Arabian Gulf region
No full textThe aerosol chemistry environment of the Arabian Gulf region is extraordinarily complex, with high concentrations of dust aerosols from surrounding deserts mixed with anthropogenic aerosols originating from a large petrochemical industry and pockets of highly urbanized areas. Despite the high levels of aerosols experienced by this region, little research has been done to explore the chemical composition of both the anthropogenic and mineral dust portion of the aerosol burden. The intensive portion of the United Arab Emirates Unified Aerosol Experiment (UAE2), conducted during August and September 2004 was designed in part to resolve the aerosol chemistry through the use of multiple size-segregated aerosol samplers. The coarse mode mass (derived by subtracting the PM2.5 aerosol mass from the PM10 mass) is largely dust at 76% ± 7% of the total coarse mode mass, but is significantly impacted by anthropogenic pollution, primarily sulfate and nitrate. The PM2.5 aerosol mass also contains a large dust burden, at 38% ± 26%, but the anthropogenic component dominates. The total aerosol burden has significant impact not only on the atmosphere, but also the local population, as the air quality levels for both the PM10 and PM2.5 aerosol masses reached unhealthy levels for 24% of the days sample
Emissions management and health exposure: should all power stations be treated equal?
No full textAt the centre of all air quality regulation stands the right of humans to an environment that is not harmful to health and well-being. In many developing countries, including South Africa, coal-fired power station emissions are managed both from an ambient air quality and minimum emissions standpoint. Ambient air quality standards and minimum emissions standards (MES) are often in conflict with one another, as power stations in which vicinity ambient air quality standards are met still have to comply with a blanket set of MES. In developing countries this often leads to the unnecessary incurrence of already constrained financial resources. This study proposes an alternative emissions management strategy where potential human health exposure is used as a decision-making basis on which power station emissions control is founded. The potential human health exposure of population groups to primary particulate matter with a diameter of 10 μm or less (PM10), secondary sulphurous and nitrous particulate matter (PM), sulphur dioxide (SO2) and oxides of nitrogen (NOx) emissions are calculated and compared for 13 power stations in the highveld of South Africa. It was found that the potential human health exposure to individual power stations differ substantially. It is suggested that it makes more sense both from both a human health and fiscal perspective that emissions from coal-fired power stations be managed on an individual power station basis, especially in developing countrie
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