We investigate the relative importance of climate change (CC) and anthropogenic land cover change (ALCC) for the dust emissions and burden changes between the late nineteenth century and today. For ...this purpose, the climate‐aerosol model ECHAM6‐HAM2 is complemented by a new scheme to derive potential dust sources at runtime using the vegetation cover provided by the land component JSBACH of ECHAM6. Dust emissions are computed online using information from the ECHAM6 atmospheric component. This allows us to account for changes in land cover and climate interactively and to distinguish between emissions from natural and agricultural dust sources. For today's climate we find that nearly 10% of dust particles are emitted from agricultural areas. According to our simulations, global annual dust emissions have increased by 25% between the late nineteenth century and today (e.g., from 729 Tg/a to 912 Tg/a). Globally, CC and ALCC (e.g., agricultural expansion) have both contributed to this change (56% and 40%, respectively). There are however large regional differences. For example, change in dust emissions in Africa are clearly dominated by CC. Global dust burden have increased by 24.5% since the late nineteenth century, which results in a clear‐sky radiative forcing at top of the atmosphere of −0.14 W/m2. Based on these findings, we recommend that both climate changes and anthropogenic land cover changes should be considered when investigating long‐term changes in dust emissions.
Key Points
Potential dust sources are computed online in a global climate‐aerosol modelFor today's climate 10% of dust particles are emitted from agricultural areasChanges in climate and land use contributed equally to changes in dust burden
The spatial distribution and properties of submicron organic aerosol (OA)
are among the key sources of uncertainty in our understanding of aerosol
effects on climate. Uncertainties are particularly ...large over remote regions
of the free troposphere and Southern Ocean, where very few data have been
available and where OA predictions from AeroCom Phase II global models span 2 to 3 orders of magnitude, greatly exceeding the model spread over
source regions. The (nearly) pole-to-pole vertical distribution of
non-refractory aerosols was measured with an aerosol mass spectrometer
onboard the NASA DC-8 aircraft as part of the Atmospheric Tomography (ATom)
mission during the Northern Hemisphere summer (August 2016) and winter
(February 2017). This study presents the first extensive characterization of
OA mass concentrations and their level of oxidation in the remote
atmosphere. OA and sulfate are the major contributors by mass to submicron
aerosols in the remote troposphere, together with sea salt in the marine
boundary layer. Sulfate was dominant in the lower stratosphere. OA
concentrations have a strong seasonal and zonal variability, with the
highest levels measured in the lower troposphere in the summer and over the
regions influenced by biomass burning from Africa (up to 10 µg sm−3). Lower concentrations (∼0.1–0.3 µg sm−3)
are observed in the northern middle and high latitudes and very low
concentrations (<0.1 µg sm−3) in the southern middle and
high latitudes. The ATom dataset is used to evaluate predictions of eight
current global chemistry models that implement a variety of commonly used
representations of OA sources and chemistry, as well as of the AeroCom-II
ensemble. The current model ensemble captures the average vertical and
spatial distribution of measured OA concentrations, and the spread of the
individual models remains within a factor of 5. These results are
significantly improved over the AeroCom-II model ensemble, which shows large
overestimations over these regions. However, some of the improved agreement
with observations occurs for the wrong reasons, as models have the tendency
to greatly overestimate the primary OA fraction and underestimate the
secondary fraction. Measured OA in the remote free troposphere is highly
oxygenated, with organic aerosol to organic carbon (OA ∕ OC) ratios of
∼2.2–2.8, and is 30 %–60 % more oxygenated than in current
models, which can lead to significant errors in OA concentrations. The
model–measurement comparisons presented here support the concept of a more
dynamic OA system as proposed by Hodzic et al. (2016), with enhanced removal
of primary OA and a stronger production of secondary OA in global models
needed to provide better agreement with observations.
Radiative effects of absorbing black carbon and mineral dust aerosols are estimated from global aerosol climate model simulations with fixed sea surface temperatures as a boundary condition. ...Semi-direct effects are approximated as the residual between the total direct radiative effect and the instantaneous direct radiative effect of the simulated absorbing aerosol species. No distinction is made for aerosols from natural and anthropogenic sources. Results for global average are highly uncertain due to high model variability, but consistent with previous estimates. The global average results for black carbon aerosol semi-direct effects are small due to cancellation of regionally positive or negative effects, and may be positive or negative overall, depending on the model setup. The presence of mineral dust aerosol above dark surfaces and below a layer containing black carbon aerosol may enhance the reflectivity and act to enhance the positive radiative effect of black carbon aerosol. When mineral dust is absent the semi-direct effect at the top-of-atmosphere of black carbon aerosol from both anthropogenic and natural sources is −0.03 Wm−2, while averaging to +0.09 Wm−2 if dust is included.
We introduce and evaluate aerosol simulations with the global aerosol–climate model ECHAM6.3–HAM2.3, which is the aerosol component of the fully coupled aerosol–chemistry–climate model ECHAM–HAMMOZ. ...Both the host atmospheric climate model ECHAM6.3 and the aerosol model HAM2.3 were updated from previous versions. The updated version of the HAM aerosol model contains improved parameterizations of aerosol processes such as cloud activation, as well as updated emission fields for anthropogenic aerosol species and modifications in the online computation of sea salt and mineral dust aerosol emissions. Aerosol results from nudged and free-running simulations for the 10-year period 2003 to 2012 are compared to various measurements of aerosol properties. While there are regional deviations between the model and observations, the model performs well overall in terms of aerosol optical thickness, but may underestimate coarse-mode aerosol concentrations to some extent so that the modeled particles are smaller than indicated by the observations. Sulfate aerosol measurements in the US and Europe are reproduced well by the model, while carbonaceous aerosol species are biased low. Both mineral dust and sea salt aerosol concentrations are improved compared to previous versions of ECHAM–HAM. The evaluation of the simulated aerosol distributions serves as a basis for the suitability of the model for simulating aerosol–climate interactions in a changing climate.
This study analyzes six frontal dust storms in the Middle East during the cold period (October–March), aiming to examine the atmospheric circulation patterns and force dynamics that triggered the ...fronts and the associated (pre- or post-frontal) dust storms. Cold troughs mostly located over Turkey, Syria and north Iraq played a major role in the front propagation at the surface, while cyclonic conditions and strong winds facilitated the dust storms. The presence of an upper-atmosphere (300 hPa) sub-tropical jet stream traversing from Egypt to Iran constitutes also a dynamic force accompanying the frontal dust storms. Moderate-Resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) observations are used to monitor the spatial and vertical extent of the dust storms, while model (Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), Copernicus Atmospheric Monitoring Service (CAMS), Regional Climate Model-4 (RegCM4)) simulations are also analyzed. The WRF-Chem outputs were in better agreement with the MODIS observations compared to those of CAMS and RegCM4. The fronts were identified by WRF-Chem simulations via gradients in the potential temperature and sudden changes of wind direction in vertical cross-sections. Overall, the uncertainties in the simulations and the remarkable differences between the model outputs indicate that modelling of dust storms in the Middle East is really challenging due to the complex terrain, incorrect representation of the dust sources and soil/surface characteristics, and uncertainties in simulating the wind speed/direction and meteorological dynamics. Given the potential threat by dust storms, more attention should be directed to the dust model development in this region.
Since iron is an important micronutrient, deposition of iron in mineral aerosols can impact the carbon cycle and atmospheric CO2. This paper reviews our current understanding of the global dust cycle ...and identifies future research needs. The global distribution of desert dust is estimated from a combination of observations of dust from in situ concentration, optical depth, and deposition data; observations from satellite; and global atmospheric models. The anthropogenically influenced portion of atmospheric desert dust flux is thought to be smaller than the natural portion, but is difficult to quantify due to the poorly understood response of desert dust to changes in climate, land use, and water use. The iron content of aerosols is thought to vary by a factor of 2, while the uncertainty in dust deposition is at least a factor of 10 in some regions due to the high spatial and temporal variability and limited observations. Importantly, we have a limited understanding of the processes by which relatively insoluble soil iron (typically ∼0.5% is soluble) becomes more soluble (1–80%) during atmospheric transport, but these processes could be impacted by anthropogenic emissions of sulfur or organic acids. In order to understand how humans will impact future iron deposition to the oceans, we need to improve our understanding of: iron deposition to remote oceans, iron chemistry in aerosols, how desert dust sources will respond to climate change, and how humans will impact the transport of bioavailable fraction of iron to the oceans.
This paper reviews our knowledge of the measurement and modeling of mineral dust emissions to the atmosphere, its transport and deposition to the ocean, the release of iron from the dust into ...seawater, and the possible impact of that nutrient on marine biogeochemistry and climate. Of particular concern is our poor understanding of the mechanisms and quantities of dust deposition as well as the extent of iron solubilization from the dust once it enters the ocean. Model estimates of dust deposition in remote oceanic regions vary by more than a factor of 10. The fraction of the iron in dust that is available for use by marine phytoplankton is still highly uncertain. There is an urgent need for a long-term marine atmospheric surface measurement network, spread across all oceans. Because the southern ocean is characterized by large areas with high nitrate but low chlorophyll surface concentrations, that region is particularly sensitive to the input of dust and iron. Data from this region would be valuable, particularly at sites downwind from known dust source areas in South America, Australia, and South Africa. Coordinated field experiments involving both atmospheric and marine measurements are recommended to address the complex and interlinked processes and role of dust/Fe fertilization on marine biogeochemistry and climate.
Aerosol particles can contribute to the Arctic amplification (AA) by direct and indirect radiative effects.
Specifically, black carbon (BC) in the atmosphere, and when deposited on snow and sea ice, ...has a positive warming effect on the top-of-atmosphere (TOA) radiation balance during the polar day.
Current climate models, however, are still struggling to reproduce Arctic aerosol conditions.
We present an evaluation study with the global aerosol-climate model ECHAM6.3-HAM2.3 to examine emission-related uncertainties in the BC distribution and the direct radiative effect of BC.
The model results are comprehensively compared against the latest ground and airborne aerosol observations for the period 2005–2017, with a focus on BC.
Four different setups of air pollution emissions are tested.
The simulations in general match well with the observed amount and temporal variability in near-surface BC in the Arctic.
Using actual daily instead of fixed biomass burning emissions is crucial for reproducing individual pollution events but has only a small influence on the seasonal cycle of BC.
Compared with commonly used fixed anthropogenic emissions for the year 2000, an up-to-date inventory with transient air pollution emissions results in up to a 30 % higher annual BC burden locally.
This causes a higher annual mean all-sky net direct radiative effect of BC of over 0.1 W m−2 at the top of the atmosphere over the Arctic region (60–90∘ N), being locally more than 0.2 W m−2 over the eastern Arctic Ocean.
We estimate BC in the Arctic as leading to an annual net gain of 0.5 W m−2 averaged over the Arctic region but to a local gain of up to 0.8 W m−2 by the direct radiative effect of atmospheric BC plus the effect by the BC-in-snow albedo reduction.
Long-range transport is identified as one of the main sources of uncertainties for ECHAM6.3-HAM2.3, leading to an overestimation of BC in atmospheric layers above 500 hPa, especially in summer.
This is related to a misrepresentation in wet removal in one identified case at least, which was observed during the ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) summer aircraft campaign.
Overall, the current model version has significantly improved since previous intercomparison studies and now performs better than the multi-model average in the Aerosol Comparisons between Observation and Models (AEROCOM) initiative in terms of the spatial and temporal distribution of Arctic BC.
The global aerosol–climate model ECHAM6.3–HAM2.3 (E63H23)
as well as the previous model versions ECHAM5.5–HAM2.0 (E55H20) and ECHAM6.1–HAM2.2
(E61H22) are evaluated using global observational ...datasets for clouds and
precipitation. In E63H23, the amount of low clouds, the liquid and ice water path, and
cloud radiative effects are more realistic than in previous model versions.
E63H23 has a more physically based aerosol activation scheme, improvements
in the cloud cover scheme, changes in the detrainment of convective clouds,
changes in the sticking efficiency for the accretion of ice crystals by snow,
consistent ice crystal shapes throughout the model, and changes in mixed-phase
freezing; an inconsistency in ice crystal number concentration (ICNC) in
cirrus clouds was also removed. Common biases in ECHAM and in E63H23 (and in
previous ECHAM–HAM versions) are a cloud amount in stratocumulus
regions that is too low and deep convective clouds over the Atlantic and Pacific oceans
that form too close to the continents (while tropical land precipitation is
underestimated). There are indications that ICNCs are overestimated in
E63H23. Since clouds are important for effective radiative forcing due to
aerosol–radiation and aerosol–cloud interactions (ERFari+aci) and
equilibrium climate sensitivity (ECS), differences in ERFari+aci
and ECS between the model versions were also analyzed. ERFari+aci is weaker
in E63H23 (−1.0 W m−2) than in E61H22 (−1.2 W m−2) (or E55H20;
−1.1 W m−2). This is caused by the weaker shortwave ERFari+aci
(a new aerosol activation scheme and sea salt emission parameterization in
E63H23, more realistic simulation of cloud water) overcompensating for the
weaker longwave ERFari+aci (removal of an inconsistency in ICNC in
cirrus clouds in E61H22). The decrease in ECS in E63H23 (2.5 K) compared to E61H22 (2.8 K) is due to
changes in the entrainment rate for shallow convection (affecting the cloud
amount feedback) and a stronger cloud phase feedback. Experiments with minimum cloud droplet number concentrations (CDNCmin) of
40 cm−3 or 10 cm−3 show that a higher value of CDNCmin reduces
ERFari+aci as well as ECS in E63H23.
Aerosol‐cloud interactions are an important source of uncertainty in current climate models. To understand and quantify the influence of ice‐nucleating particles in cloud glaciation, it is crucial to ...have a reliable estimation of the hemispheric and seasonal contrast in cloud top phase, which is believed to result from the higher dust aerosol loading in boreal spring. For this reason, we locate and quantify these contrasts by combining three different A‐Train cloud‐phase products for the period 2007–2010. These products rely on a spaceborne lidar, a lidar‐radar synergy, and a radiometer‐polarimeter synergy. We show that the cloud‐phase from the product combination is more reliable and that the estimation of the hemispheric and seasonal contrast has a lower error compared to the individual products. To quantify the contrast in cloud‐phase, we use the hemispheric difference in ice cloud frequency normalized by the liquid cloud frequency in the southern hemisphere between −42 °C and 0 °C. In the midlatitudes, from −15 to −30 °C, the hemispheric contrasts increase with decreasing temperature. At −30 °C, the hemispheric contrast varies from 29% to 39% for the individual cloud‐phase products and from 52% to 73% for the product combination. Similarly, in the northern hemisphere, we assess the seasonal contrast between spring and fall normalized by the liquid cloud frequency during fall. At −30 °C, the seasonal contrast ranges from 21% to 39% for the individual cloud‐phase products and from 54% to 75% for the product combination.
Plain Language Summary
The influence of atmospheric particles on clouds is one of the main unknowns in climate predictions. Particularly, the cloud glaciation process and its dependence on desert dust and soot particles are not well‐understood. To better understand the differences in cloud glaciation between hemispheres, we counted liquid and ice cloud tops, as observed from four different satellites, during 4 years. Combining these observations, we could confirm a higher frequency of ice cloud tops during spring in the northern hemisphere. We found that the contrast between hemispheres is higher than previously thought. These results will help to improve our understanding of cloud glaciation processes, which can be valuable for future climate predictions and for understanding the impact of aerosols on radiation and precipitation.
Key Points
A satellite product ensemble was used to locate and quantify the hemispheric and seasonal contrast in cloud top thermodynamic phase
At −30 °C, half of the liquid cloud tops observed in the southern hemisphere would glaciate in the northern hemisphere
The new product ensemble is more reliable than the individual products and suggests a previous underestimation of the cloud‐phase contrasts
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