The 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 m.sup.3 smog chamber where its spectral shortwave scattering (β.sub.sca) and absorption (β.sub.abs) 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 β.sub.sca and β.sub.abs 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).
Simulations are performed for the period 2000–2015 by two different regional climate models, ALADIN and RegCM, to quantify the direct and semi-direct radiative effects of biomass-burning aerosols ...(BBAs) in the southeast Atlantic (SEA) region. Different simulations have been performed using strongly absorbing BBAs in accordance with recent in situ observations over the SEA. For the July–August–September (JAS) season, the single scattering albedo (SSA) and total aerosol optical depth (AOD) simulated by the ALADIN and RegCM models are consistent with the MACv2 climatology and MERRA-2 and CAMS-RA reanalyses near the biomass-burning emission sources. However, the above-cloud AOD is slightly underestimated compared to satellite (MODIS and POLDER) data during the transport over the SEA. The direct radiative effect exerted at the continental and oceanic surfaces by BBAs is significant in both models and the radiative effects at the top of the atmosphere indicate a remarkable regional contrast over SEA (in all-sky conditions), with a cooling (warming) north (south) of 10 ∘S, which is in agreement with the recent MACv2 climatology. In addition, the two models indicate that BBAs are responsible for an important shortwave radiative heating of ∼0.5–1 K per day over SEA during JAS with maxima between 2 and 4 km a.m.s.l. (above mean sea level). At these altitudes, BBAs increase air temperature by ∼0.2–0.5 K, with the highest values being co-located with low stratocumulus clouds. Vertical changes in air temperature limit the subsidence of air mass over SEA, creating a cyclonic anomaly. The opposite effect is simulated over the continent due to the increase in lower troposphere stability. The BBA semi-direct effect on the lower troposphere circulation is found to be consistent between the two models. Changes in the cloud fraction are moderate in response to the presence of smoke, and the models differ over the Gulf of Guinea. Finally, the results indicate an important sensitivity of the direct and semi-direct effects to the absorbing properties of BBAs. Over the stratocumulus (Sc) region, DRE varies from +0.94 W m−2 (scattering BBAs) to +3.93 W m−2 (most absorbing BBAs).
Smoke and Clouds above the Southeast Atlantic Zuidema, Paquita; Redemann, Jens; Haywood, James ...
Bulletin of the American Meteorological Society,
07/2016, Letnik:
97, Številka:
7
Journal Article
Recenzirano
Odprti dostop
From 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 Atlantic.
Mineral dust is an important component of the climate system, affecting the
radiation balance, cloud properties, biogeochemical cycles, regional
circulation and precipitation, as well as having ...negative effects on
aviation, solar energy generation and human health. Dust size and composition
has an impact on all these processes. However, changes in dust size
distribution and composition during transport, particularly for coarse
particles, are poorly understood and poorly represented in climate models.
Here we present new in situ airborne observations of dust in the Saharan Air
Layer (SAL) and the marine boundary layer (MBL) at the beginning of its
transatlantic transport pathway, from the AERosol
Properties – Dust (AER-D) fieldwork in August 2015,
within the peak season of North African dust export. This study focuses on
coarse-mode dust properties, including size distribution, mass loading,
shape, composition, refractive indices and optical properties. Size
distributions from 0.1 to 100 µm diameter (d) are presented, fully
incorporating the coarse and giant modes of dust. Within the MBL, mean
effective diameter (deff) and volume median diameter (VMD) were 4.6 and 6.0 µm respectively, giant particles with a mode at
20–30 µm were observed, and composition was dominated by quartz and
alumino-silicates at d > 1 µm. Within the SAL, particles
larger than 20 µm diameter were always present up to 5 km altitude, in
concentrations over 10−5 cm−3, constituting up to 40 % of
total dust mass. Mean deff and VMD were 4.0 and 5.5 µm
respectively. Larger particles were detected in the SAL than can be explained
by sedimentation theory alone. Coarse-mode composition was dominated by
quartz and alumino-silicates; the accumulation mode showed a strong
contribution from sulfate-rich and sea salt particles. In the SAL, measured
single scattering albedos (SSAs) at 550 nm representing d < 2.5 µm were
0.93 to 0.98 (mean 0.97). Optical properties calculated for the full
size distribution (0.1 < d < 100 µm) resulted in lower
SSAs of 0.91–0.98 (mean 0.95) and mass extinction coefficients of 0.27–0.35 m2 g−1 (mean 0.32 m2 g−1). Variability in SSA was mainly
controlled by variability in dust composition (principally iron) rather than
by variations in the size distribution, in contrast with previous observations
over the Sahara where size is the dominant influence. It is important that
models are able to capture the variability and evolution of both dust
composition and size distribution with transport in order to accurately
represent the impacts of dust on climate. These results provide a new SAL
dust dataset, fully representing coarse and giant particles, to aid model
validation and development.
Iceland is a highly active source of natural dust.
Icelandic dust has the potential to directly affect the climate via
dust–radiation interaction and indirectly via dust–cloud interaction,
the ...snow/ice albedo effect and impacts on biogeochemical cycles. The impacts of
Icelandic dust depend on its mineralogical and chemical composition.
However, a lack of data has prevented an accurate assessment of the role of
Icelandic dust in the Earth system. Here, we collected surface sediment
samples from five major Icelandic dust hotspots. Dust aerosols were
generated and suspended in atmospheric chambers, and PM10 and PM20
fractions were collected for further analysis. We found that the dust
samples primarily consist of amorphous basaltic materials ranging from 8 wt %
(from the Hagavatn hotspot) to 60 wt %–90 wt % (other hotspots). Samples
had relatively high total Fe content (10 wt %–13 wt %). Sequential extraction
of Fe to determine its chemical form shows that dithionite Fe (Fe oxides
such as hematite and goethite) and ascorbate Fe (amorphous Fe) contribute
respectively 1 %–6 % and 0.3 %–1.4 % to the total Fe in Icelandic dust. The
magnetite fraction is 7 %–15 % of total Fe and 1 %–2 wt % of PM10,
which is orders of magnitude higher than in mineral dust from northern Africa.
Nevertheless, about 80 %–90% of the Fe is contained in pyroxene and
amorphous glass. The initial Fe solubility (ammonium acetate extraction at
pH 4.7) is from 0.08 % to 0.6 %, which is comparable to low-latitude dust such
as that from northern Africa. The Fe solubility at low pH (i.e. pH 2) is
significantly higher than typical low-latitude dust (up to 30 % at pH 2
after 72 h). Our results revealed the fundamental differences in
composition and mineralogy of Icelandic dust from low-latitude dust. We
attribute these differences to the low degree of chemical weathering, the
basaltic composition of the parent sediments and glacial processes.
Icelandic dust contributes to the atmospheric deposition of soluble Fe and
can impact primary productivity in the North Atlantic Ocean. The distinct
chemical and mineralogical composition, particularly the high magnetite
content (1 wt %–2 wt %), indicates a potentially significant impact of
Icelandic dust on the radiation balance in the subpolar and polar regions.
Modeling 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 Supplement.
The effects of methylglyoxal uptake on the physical and optical properties of aerosol containing amines or ammonium sulfate were determined before and after cloud processing in a temperature- and ...RH-controlled chamber. The formation of brown carbon was observed upon methylglyoxal addition, detected as an increase in water-soluble organic carbon mass absorption coefficients below 370 nm and as a drop in single-scattering albedo at 450 nm. The imaginary refractive index component k 450 reached a maximum value of 0.03 ± 0.009 with aqueous glycine aerosol particles. Browning of solid particles occurred at rates limited by chamber mixing (<1 min), and in liquid particles occurred more gradually, but in all cases occurred much more rapidly than in bulk aqueous studies. Further browning in AS and methylammonium sulfate seeds was triggered by cloud events with chamber lights on, suggesting photosensitized brown carbon formation. Despite these changes in optical aerosol characteristics, increases in dried aerosol mass were rarely observed (<1 μg/m3 in all cases), consistent with previous experiments on methylglyoxal. Under dry, particle-free conditions, methylglyoxal reacted (presumably on chamber walls) with methylamine with a rate constant k = (9 ± 2) × 10–17 cm3 molecule–1 s–1 at 294 K and activation energy E a = 64 ± 37 kJ/mol.
This paper presents new laboratory measurements of the mass absorption efficiency (MAE) between 375 and 850 nm for 12 individual samples of mineral dust from different source areas worldwide and in ...two size classes: PM10. 6 (mass fraction of particles of aerodynamic diameter lower than 10.6 µm) and PM2. 5 (mass fraction of particles of aerodynamic diameter lower than 2.5 µm). The experiments were performed in the CESAM simulation chamber using mineral dust generated from natural parent soils and included optical and gravimetric analyses. The results show that the MAE values are lower for the PM10. 6 mass fraction (range 37–135 × 10−3 m2 g−1 at 375 nm) than for the PM2. 5 (range 95–711 × 10−3 m2 g−1 at 375 nm) and decrease with increasing wavelength as λ−AAE, where the Ångström absorption exponent (AAE) averages between 3.3 and 3.5, regardless of size. The size independence of AAE suggests that, for a given size distribution, the dust composition did not vary with size for this set of samples. Because of its high atmospheric concentration, light absorption by mineral dust can be competitive with black and brown carbon even during atmospheric transport over heavy polluted regions, when dust concentrations are significantly lower than at emission. The AAE values of mineral dust are higher than for black carbon (∼ 1) but in the same range as light-absorbing organic (brown) carbon. As a result, depending on the environment, there can be some ambiguity in apportioning the aerosol absorption optical depth (AAOD) based on spectral dependence, which is relevant to the development of remote sensing of light-absorbing aerosols and their assimilation in climate models. We suggest that the sample-to-sample variability in our dataset of MAE values is related to regional differences in the mineralogical composition of the parent soils. Particularly in the PM2. 5 fraction, we found a strong linear correlation between the dust light-absorption properties and elemental iron rather than the iron oxide fraction, which could ease the application and the validation of climate models that now start to include the representation of the dust composition, as well as for remote sensing of dust absorption in the UV–vis spectral region.
Abstract
Advancing knowledge of the mineralogical composition of dust is key for understanding and predicting its climate and environmental impacts. The variability of dust mineralogy from one source ...to another and its evolution during atmospheric transport is not measured at large scale. In this study we use laboratory measurements to demonstrate that the extinction signature of suspended dust aerosols in the 740 − 1250 cm
−1
atmospheric window can be used to derive dust mineralogy in terms of the main infrared − active minerals, namely quartz, clays, feldspars and calcite. Various spectral signatures in dust extinction enable to distinguish between multiple global sources with changing composition, whereas modifications of the dust extinction spectra with time inform on size − dependent particles mineralogy changes during transport. The present study confirms that spectral and hyperspectral infrared remote sensing observations offer great potential for elucidating the size − segregated mineralogy of airborne dust at regional and global scales.
We present a new characterisation of the three‐dimensional (3D) distribution of dust over the Sahara during summer, exemplified for June 2011. Our approach, called AEROIASI, is based on the ...innovative retrieval of vertical profiles of the dust extinction coefficient from daily cloud‐free hyperspectral Infrared Atmospheric Sounder Interferometer (IASI) satellite observations. AEROIASI observations clearly agree with other widely used measurements (from lidar and radiometers). The 3D characterisation is focused on the dust maximum in June 2011, located in the central Sahara (17–23°N, 1–7°E) and linked to the major atmospheric dynamical drivers associated with the West African Monsoon (WAM) system. AEROIASI shows the near‐surface dust load to be dominated by five major emission events occurring every 3–4 days. These all occur when the study region is under the influence of northward bursts of the WAM and convection‐related cold pools, likely associated with orographic forcing by the Aïr Mountains. During the earliest (June 10) and the dustiest (June 17) cases, northward advection of moisture over the hotspot is favoured by the superposition of cyclonic circulations related to an extratropical disturbance northwest of the Sahara and to the Saharan heat low over Mauritania, respectively. Convection over the hotspot also triggers wave‐like disturbances that travel westwards. The three dustiest events are characterised by elongated dust fronts moving northwards, with a leading edge spanning 200–300 km horizontally and extending from the surface up to 2 km of altitude. Further south, the dust layer progressively elevates to 3.5 km along the slanted isentropes at the interface of the monsoon and the harmattan, increasingly losing contact with the ground. When northerlies blow over the study region, elevated dust layers at 3–5 km are observed, which are transported southwards within the Saharan air layer and westwards along the northern edge of the African easterly jet (after June 13).
We present a new characterisation of the three‐dimensional (3D) distribution of dust over the Sahara during summer, using innovative observations of dust vertical profiles derived from hyperspectral satellite measurements of the Infrared Atmospheric Sounder Interferometer. The 3D characterisation is focused on the dust maximum in June, located in the central Sahara and linked to atmospheric dynamical drivers associated with the West African monsoon system. We show the first observational characterisation of the 3D structure of dust plumes associated with convection‐related cold pools.
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