Agricultural dust emissions have been estimated to contribute around 20% to the global dust burden. In contrast to dusts from arid source regions, the ice-nucleating abilities of which have been ...relatively well studied, soil dusts from fertile sources often contain a substantial fraction of organic matter. Using an experimental methodology which is sensitive to a wide range of ice nucleation efficiencies, we have characterised the immersion mode ice-nucleating activities of dusts (d < 11 μm) extracted from fertile soils collected at four locations around England. By controlling droplet sizes, which ranged in volume from 10−12 to 10−6 L (concentration = 0.02 to 0.1 wt% dust), we have been able to determine the ice nucleation behaviour of soil dust particles at temperatures ranging from 267 K (−6 °C) down to the homogeneous limit of freezing at about 237 K (−36 °C). At temperatures above 258 K (−15 °C) we find that the ice-nucleating activity of soil dusts is diminished by heat treatment or digestion with hydrogen peroxide, suggesting that a major fraction of the ice nuclei stems from biogenic components in the soil. However, below 258 K, we find that the ice active site densities tend towards those expected from the mineral components in the soils, suggesting that the inorganic fraction of soil dusts, in particular the K-feldspar fraction, becomes increasingly important in the initiation of the ice phase at lower temperatures. We conclude that dusts from agricultural activities could contribute significantly to atmospheric IN concentrations, if such dusts exhibit similar activities to those observed in the current laboratory study.
The seasonal cycle in Arctic aerosol is typified by high concentrations of large aged anthropogenic particles transported from lower latitudes in the late Arctic winter and early spring followed by a ...sharp transition to low concentrations of locally sourced smaller particles in the summer. However, multi-model assessments show that many models fail to simulate a realistic cycle. Here, we use a global aerosol microphysics model (GLOMAP) and surface-level aerosol observations to understand how wet scavenging processes control the seasonal variation in Arctic black carbon (BC) and sulphate aerosol. We show that the transition from high wintertime concentrations to low concentrations in the summer is controlled by the transition from ice-phase cloud scavenging to the much more efficient warm cloud scavenging in the late spring troposphere. This seasonal cycle is amplified further by the appearance of warm drizzling cloud in the late spring and summer boundary layer. Implementing these processes in GLOMAP greatly improves the agreement between the model and observations at the three Arctic ground-stations Alert, Barrow and Zeppelin Mountain on Svalbard. The SO4 model-observation correlation coefficient (R) increases from: −0.33 to 0.71 at Alert (82.5° N), from −0.16 to 0.70 at Point Barrow (71.0° N) and from −0.42 to 0.40 at Zeppelin Mountain (78° N). The BC model-observation correlation coefficient increases from −0.68 to 0.72 at Alert and from −0.42 to 0.44 at Barrow. Observations at three marginal Arctic sites (Janiskoski, Oulanka and Karasjok) indicate a far weaker aerosol seasonal cycle, which we show is consistent with the much smaller seasonal change in the frequency of ice clouds compared to higher latitude sites. Our results suggest that the seasonal cycle in Arctic aerosol is driven by temperature-dependent scavenging processes that may be susceptible to modification in a future climate.
ABSTRACT
Plant acclimation to freezing temperatures is very complex. Many temperate plants increase in freezing tolerance upon exposure to a period of low but non‐freezing temperatures, an adaptive ...process known as cold acclimation. This acclimation phenomenon has encouraged investigations of physiological, biochemical, and molecular changes that are associated with the development of freezing tolerance. Although many biochemical and gene‐expression changes occur during cold acclimation, few have been unequivocally demonstrated to contribute to the development of freezing tolerance. However, in the last few years, exciting new progress has been made through the use of mutational analysis and molecular genetic approaches. We now recognize that several interacting signal pathways are activated to bring about cold acclimation and ensure the winter survival of plants. The challenge for the future is to understand these pathways at a mechanistic level. Facile map‐based cloning in Arabidopsis and techniques (such as DNA micro‐arrays) for transcript profiling will provide the tools needed for this task.
Loss of summertime Arctic sea ice will lead to a large increase in the emission of aerosols and precursor gases from the ocean surface. It has been suggested that these enhanced emissions will exert ...substantial aerosol radiative forcings, dominated by the indirect effect of aerosol on clouds. Here, we investigate the potential for these indirect forcings using a global aerosol microphysics model evaluated against aerosol observations from the Arctic Summer Cloud Ocean Study (ASCOS) campaign to examine the response of Arctic cloud condensation nuclei (CCN) to sea-ice retreat. In response to a complete loss of summer ice, we find that north of 70° N emission fluxes of sea salt, marine primary organic aerosol (OA) and dimethyl sulfide increase by a factor of ~ 10, ~ 4 and ~ 15 respectively. However, the CCN response is weak, with negative changes over the central Arctic Ocean. The weak response is due to the efficient scavenging of aerosol by extensive drizzling stratocumulus clouds. In the scavenging-dominated Arctic environment, the production of condensable vapour from oxidation of dimethyl sulfide grows particles to sizes where they can be scavenged. This loss is not sufficiently compensated by new particle formation, due to the suppression of nucleation by the large condensation sink resulting from sea-salt and primary OA emissions. Thus, our results suggest that increased aerosol emissions will not cause a climate feedback through changes in cloud microphysical and radiative properties.
The role of dust as a source of bioavailable phosphorus (Bio‐P) is quantified using a new parameterization for apatite dissolution in combination with global soil data maps and a global aerosol ...transport model. Mineral dust provides 31.2 Gg‐P/year of Bio‐P to the oceans, with 14.3 Gg‐P/year from labile P present in the dust, and an additional 16.9 Gg‐P/year from acid dissolution of apatite in the atmosphere, representing an increase of 120%. The North Atlantic, northwest Pacific, and Mediterranean Sea are identified as important sites of Bio‐P deposition from mineral dust. The acid dissolution process increases the fraction of total‐P that is bioavailable from ~10% globally from the labile pool to 18% in the Atlantic Ocean, 42% in the Pacific Ocean, and 20% in the Indian Ocean, with an ocean global mean value of 22%. Strong seasonal variations, especially in the North Pacific, northwest Atlantic, and Indian Ocean, are driven by large‐scale meteorology and pollution sources from industrial and biomass‐burning regions. Globally constant values of total‐P content and bioavailable fraction used previously do not capture the simulated variability. We find particular sensitivity to the representation of particle‐to‐particle variability of apatite, which supplies Bio‐P through acid‐dissolution, and calcium carbonate, which helps to buffer the dissolution process. A modest 10% external mixing results in an increase of Bio‐P deposition by 18%. The total Bio‐P calculated here (31.2 Gg‐P/year) represents a minimum compared to previous estimates due to the relatively low total‐P in the global soil map used.
Plain Language Summary
Phosphorus (P) is an essential requirement for life. Natural sources of P on land are from rock weathering and fertilizers. By contrast over the open ocean, the major source of P is from falling dust. However, less than 10% of the P in dust is automatically available to phytoplankton for growth, a percentage we call bioavailable‐P. Therefore, changes to the supply of bioavailable‐P to oceans can have considerable impacts on marine ecosystems and the global carbon cycle. Previous work shows acid processes in the atmosphere can convert nonbioavailable minerals to bioavailable‐P. In our previous study we found a simple relationship between acid in the atmosphere and bioavailable‐P formed. Here we use this new relationship, together with global soil data maps on the amount and type of P in dust and a global aerosol transport model, which predicts where dust and acid interact. We calculate how much and where acid‐modified dust ends up in the ocean. We show atmospheric acid processing of dust is particularly important in the Mediterranean Sea, North Atlantic Ocean, northwest Pacific Ocean, and the Indian Ocean. As a result, atmospheric acid pollution increases the amount of oceanic plant growth and reduces the quantity of atmospheric anthropogenic carbon dioxide.
Key Points
New simple parameterization for production of bioavailable P from acid dissolution of mineral dust incorporated into a global aerosol model
Inclusion of acid dissolution increases atmospheric flux of bioavailable P from dust to oceans by 120% and drives dust P bioavailability
Major increases in bioavailable P from atmospheric acid processes occur in Mediterranean Sea, North Atlantic, NW Pacific, and Indian Ocean
The retreat of Arctic sea ice has led to renewed calls to exploit Arctic shipping routes. The diversion of ship traffic through the Arctic will shorten shipping routes and possibly reduce global ...shipping emissions. However, deposition of black carbon (BC) aerosol emitted by additional Arctic ships could cause a reduction in the albedo of snow and ice, accelerating snowmelt and sea ice loss. Here we use recently compiled Arctic shipping emission inventories for 2004 and 2050 together with a global aerosol model to quantify the contribution of future Arctic shipping to high‐latitude BC deposition. Our results show that Arctic shipping in 2050 will contribute less than 1% to the total BC deposition north of 60°N due to the much greater relative contribution of BC transported from non‐shipping sources at lower latitudes. We suggest that regulation of the Arctic shipping industry will be an insufficient control on high‐latitude BC deposition.
Key Points
Contribution of Arctic shipping to high‐latitude BC deposition less than 1%
Extra‐Arctic sources contribute much greater Arctic BC mass than local shipping
Regulation of Arctic shipping unlikely to control high‐latitude BC deposition
Aerosols and their effect on the radiative properties of clouds are one of the largest sources of uncertainty in calculations of the Earth's energy budget. Here the sensitivity of aerosol‐cloud ...albedo effect forcing to 31 aerosol parameters is quantified. Sensitivities are compared over three periods; 1850–2008, 1978–2008, and 1998–2008. Despite declining global anthropogenic SO2 emissions during 1978–2008, a cancelation of regional positive and negative forcings leads to a near‐zero global mean cloud albedo effect forcing. In contrast to existing negative estimates, our results suggest that the aerosol‐cloud albedo effect was likely positive (0.006 to 0.028Wm−2) in the recent decade, making it harder to explain the temperature hiatus as a forced response. Proportional contributions to forcing variance from aerosol processes and natural and anthropogenic emissions are found to be period dependent. To better constrain forcing estimates, the processes that dominate uncertainty on the timescale of interest must be better understood.
Key Points
Forcing sensitivity to aerosol parameters is strongly period dependentUnderstanding near‐future climate is limited if a single period is consideredIn recent decades, parametric uncertainty is smaller than model diversity
We show that Iceland contributes episodically to the regional atmospheric ice-nucleating particle population.
Ice-nucleating particles (INPs) have the potential to remove much of the liquid water in ...climatically important mid- to high-latitude shallow supercooled clouds, markedly reducing their albedo. The INP sources at these latitudes are very poorly defined, but it is known that there are substantial dust sources across the high latitudes, such as Iceland. Here, we show that Icelandic dust emissions are sporadically an important source of INPs at mid to high latitudes by combining ice-nucleating active site density measurements of aircraft-collected Icelandic dust samples with a global aerosol model. Because Iceland is only one of many high-latitude dust sources, we anticipate that the combined effect of all these sources may strongly contribute to the INP population in the mid- and high-latitude northern hemisphere. This is important because these emissions are directly relevant for the cloud-phase climate feedback and because high-latitude dust emissions are expected to increase in a warmer climate.
Abstract
The largest uncertainty in the historical radiative forcing of climate is caused by changes in aerosol particles due to anthropogenic activity. Sophisticated aerosol microphysics processes ...have been included in many climate models in an effort to reduce the uncertainty. However, the models are very challenging to evaluate and constrain because they require extensive in situ measurements of the particle size distribution, number concentration, and chemical composition that are not available from global satellite observations. The Global Aerosol Synthesis and Science Project (GASSP) aims to improve the robustness of global aerosol models by combining new methodologies for quantifying model uncertainty, to create an extensive global dataset of aerosol in situ microphysical and chemical measurements, and to develop new ways to assess the uncertainty associated with comparing sparse point measurements with low-resolution models. GASSP has assembled over 45,000 hours of measurements from ships and aircraft as well as data from over 350 ground stations. The measurements have been harmonized into a standardized format that is easily used by modelers and nonspecialist users. Available measurements are extensive, but they are biased to polluted regions of the Northern Hemisphere, leaving large pristine regions and many continental areas poorly sampled. The aerosol radiative forcing uncertainty can be reduced using a rigorous model–data synthesis approach. Nevertheless, our research highlights significant remaining challenges because of the difficulty of constraining many interwoven model uncertainties simultaneously. Although the physical realism of global aerosol models still needs to be improved, the uncertainty in aerosol radiative forcing will be reduced most effectively by systematically and rigorously constraining the models using extensive syntheses of measurements.
Tropospheric aerosol radiative forcing has persisted for many years as one of the major causes of uncertainty in global climate model simulations. To sample the range of plausible aerosol and ...atmospheric states and perform robust statistical analyses of the radiative forcing, it is important to account for the combined effects of many sources of model uncertainty, which is rarely done due to the high computational cost. This paper describes the designs of two ensembles of the Met Office Hadley Centre Global Environment Model‐U.K. Chemistry and Aerosol global climate model and provides the first analyses of the uncertainties in aerosol radiative forcing and their causes. The first ensemble was designed to comprehensively sample uncertainty in the aerosol state, while the other samples additional uncertainties in the physical model related to clouds, humidity, and radiation, thereby allowing an analysis of uncertainty in the aerosol effective radiative forcing. Each ensemble consists of around 200 simulations of the preindustrial and present‐day atmospheres. The uncertainty in aerosol radiative forcing in our ensembles is comparable to the range of estimates from multimodel intercomparison projects. The mean aerosol effective radiative forcing is −1.45 W/m2 (credible interval of −2.07 to −0.81 W/m2), which encompasses but is more negative than the −1.17 W/m2 in the 2013 Atmospheric Chemistry and Climate Model Intercomparison Project and −0.90 W/m2 in the Intergovernmental Panel on Climate Change Fifth Assessment Report. The ensembles can be used to reduce aerosol radiative forcing uncertainty by challenging them with multiple measurements as well as to isolate potential causes of multimodel differences.
Plain Language Summary
Atmospheric aerosol particles such as dust, pollutants, and smoke interfere with light from the Sun and modify the properties of clouds and thereby affect Earth's climate. However, the effect that aerosols have on climate is one of the major causes of uncertainty in global climate model simulations. We performed a large number of climate model simulations (called an ensemble), with many parts of the model slightly varied, in order to understand the complex behavior of the model and to explore the causes of uncertainty in model outputs. This paper describes the designs of two climate model ensembles and provides the first analyses of the causes of model uncertainty. The first ensemble was designed to comprehensively understand the behavior of aerosols in the atmosphere, while the other includes more general uncertainties in atmospheric processes that can affect aerosols. Each ensemble consists of around 200 simulations. The ranges of the aerosol climate effect in our ensembles are comparable to the ranges of previous estimates from studies that analyzed multiple climate models. These ensembles can be used to reduce uncertainty in how aerosols affect climate by comparing with satellite and ground‐based measurements.
Key Points
Two ensembles of atmospheric simulations were performed perturbing aerosol and physical parameters under different model setups
Thousands of Gaussian process emulators sampling parameter spaces enabled statistical analyses of model's parametric uncertainty.
Fully explored parametric uncertainty of aerosol radiative forcing in a model was found to be comparable to that in multimodel studies
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