More than twenty years ago, a biological regulation of climate was proposed whereby emissions of dimethyl sulphide from oceanic phytoplankton resulted in the formation of aerosol particles that acted ...as cloud condensation nuclei in the marine boundary layer. In this hypothesis--referred to as CLAW--the increase in cloud condensation nuclei led to an increase in cloud albedo with the resulting changes in temperature and radiation initiating a climate feedback altering dimethyl sulphide emissions from phytoplankton. Over the past two decades, observations in the marine boundary layer, laboratory studies and modelling efforts have been conducted seeking evidence for the CLAW hypothesis. The results indicate that a dimethyl sulphide biological control over cloud condensation nuclei probably does not exist and that sources of these nuclei to the marine boundary layer and the response of clouds to changes in aerosol are much more complex than was recognized twenty years ago. These results indicate that it is time to retire the CLAW hypothesis.
Black carbon aerosol plays a unique and important role in Earth's climate system. Black carbon is a type of carbonaceous material with a unique combination of physical properties. This assessment ...provides an evaluation of black‐carbon climate forcing that is comprehensive in its inclusion of all known and relevant processes and that is quantitative in providing best estimates and uncertainties of the main forcing terms: direct solar absorption; influence on liquid, mixed phase, and ice clouds; and deposition on snow and ice. These effects are calculated with climate models, but when possible, they are evaluated with both microphysical measurements and field observations. Predominant sources are combustion related, namely, fossil fuels for transportation, solid fuels for industrial and residential uses, and open burning of biomass. Total global emissions of black carbon using bottom‐up inventory methods are 7500 Gg yr−1 in the year 2000 with an uncertainty range of 2000 to 29000. However, global atmospheric absorption attributable to black carbon is too low in many models and should be increased by a factor of almost 3. After this scaling, the best estimate for the industrial‐era (1750 to 2005) direct radiative forcing of atmospheric black carbon is +0.71 W m−2 with 90% uncertainty bounds of (+0.08, +1.27) W m−2. Total direct forcing by all black carbon sources, without subtracting the preindustrial background, is estimated as +0.88 (+0.17, +1.48) W m−2. Direct radiative forcing alone does not capture important rapid adjustment mechanisms. A framework is described and used for quantifying climate forcings, including rapid adjustments. The best estimate of industrial‐era climate forcing of black carbon through all forcing mechanisms, including clouds and cryosphere forcing, is +1.1 W m−2 with 90% uncertainty bounds of +0.17 to +2.1 W m−2. Thus, there is a very high probability that black carbon emissions, independent of co‐emitted species, have a positive forcing and warm the climate. We estimate that black carbon, with a total climate forcing of +1.1 W m−2, is the second most important human emission in terms of its climate forcing in the present‐day atmosphere; only carbon dioxide is estimated to have a greater forcing. Sources that emit black carbon also emit other short‐lived species that may either cool or warm climate. Climate forcings from co‐emitted species are estimated and used in the framework described herein. When the principal effects of short‐lived co‐emissions, including cooling agents such as sulfur dioxide, are included in net forcing, energy‐related sources (fossil fuel and biofuel) have an industrial‐era climate forcing of +0.22 (−0.50 to +1.08) W m−2 during the first year after emission. For a few of these sources, such as diesel engines and possibly residential biofuels, warming is strong enough that eliminating all short‐lived emissions from these sources would reduce net climate forcing (i.e., produce cooling). When open burning emissions, which emit high levels of organic matter, are included in the total, the best estimate of net industrial‐era climate forcing by all short‐lived species from black‐carbon‐rich sources becomes slightly negative (−0.06 W m−2 with 90% uncertainty bounds of −1.45 to +1.29 W m−2). The uncertainties in net climate forcing from black‐carbon‐rich sources are substantial, largely due to lack of knowledge about cloud interactions with both black carbon and co‐emitted organic carbon. In prioritizing potential black‐carbon mitigation actions, non‐science factors, such as technical feasibility, costs, policy design, and implementation feasibility play important roles. The major sources of black carbon are presently in different stages with regard to the feasibility for near‐term mitigation. This assessment, by evaluating the large number and complexity of the associated physical and radiative processes in black‐carbon climate forcing, sets a baseline from which to improve future climate forcing estimates.
MEG offers dynamic and spectral resolution for resting-state connectivity which is unavailable in fMRI. However, there are a wide range of available network estimation methods for MEG, and little in ...the way of existing guidance on which ones to employ. In this technical note, we investigate the extent to which many popular measures of stationary connectivity are suitable for use in resting-state MEG, localising magnetic sources with a scalar beamformer. We use as empirical criteria that network measures for individual subjects should be repeatable, and that group-level connectivity estimation shows good reproducibility. Using publically-available data from the Human Connectome Project, we test the reliability of 12 network estimation techniques against these criteria. We find that the impact of magnetic field spread or spatial leakage artefact is profound, creates a major confound for many connectivity measures, and can artificially inflate measures of consistency. Among those robust to this effect, we find poor test-retest reliability in phase- or coherence-based metrics such as the phase lag index or the imaginary part of coherency. The most consistent methods for stationary connectivity estimation over all of our tests are simple amplitude envelope correlation and partial correlation measures.
•Comparison of the repeatability of 12 common network estimation methods.•Consistency of estimation tested at group-level, subject-level and between subjects.•Best-performing methods are correlations in band-limited power.•Methods should correct for the effects of spatial leakage between sources.
Thinning of Arctic sea ice gives rise to ice fracturing and leads (areas of open water surrounded by sea ice) that are a potential source of sea salt aerosol. Atmospheric particle inorganic ion ...concentrations, local sea ice conditions, and meteorology at Barrow, AK, from 2006 to 2009, were combined to investigate the dependence of submicron (aerodynamic diameter < 1 µm) and supermicron (aerodynamic diameter 1–10 µm) sea salt mass concentrations on sea ice coverage and wind speed. Consistent with a wind‐dependent source, supermicron sea salt mass concentrations increased in the presence of nearby leads and wind speeds greater than 4 m s−1. Increased supermicron and submicron sea salt chloride depletion was observed for periods of low winds or a lack of nearby open water, consistent with transported sea salt influence. Sea salt aerosol produced from leads has the potential to alter cloud formation, as well as the chemical composition of the Arctic atmosphere and snowpack.
Key Points
Local sea ice coverage and wind speed are controlling factors for Arctic sea salt concentrations
Sea salt aerosol is produced from sea ice leads at wind speeds > 4 m/s
The influence of long‐range transported sea salt aerosol was greatest during periods of lower winds and increased sea ice coverage
We combine in situ measurements of sea salt aerosols (SS) from open ocean cruises and ground-based stations together with aerosol optical depth (AOD) observations from MODIS and AERONET, and the ...GEOS-Chem global chemical transport model to provide new constraints on SS emissions over the world's oceans. We find that the GEOS-Chem model using the Gong (2003) source function overestimates cruise observations of coarse mode SS mass concentrations by factors of 2–3 at high wind speeds over the cold waters of the Southern, North Pacific and North Atlantic Oceans. Furthermore, the model systematically underestimates SS over the warm tropical waters of the Central Pacific, Atlantic, and Indian Oceans. This pattern is confirmed by SS measurements from a global network of 15 island and coastal stations. The model discrepancy at high wind speeds (>6 m s −1) has a clear dependence on sea surface temperature (SST). We use the cruise observations to derive an empirical SS source function depending on both wind speed and SST. Implementing this new source function in GEOS-Chem results in improved agreement with in situ observations, with a decrease in the model bias from +64% to +33% for the cruises and from +32% to −5% for the ground-based sites. We also show that the wind speed-SST source function significantly improves agreement with MODIS and AERONET AOD, and provides an explanation for the high AOD observed over the tropical oceans. With the wind speed-SST formulation, global SS emissions show a small decrease from 5200 Mg yr−1 to 4600 Mg yr−1, while the SS burden decreases from 9.1 to 8.5 mg m−2. The spatial distribution of SS, however, is greatly affected, with the SS burden increasing by 50% in the tropics and decreasing by 40% at mid- and high-latitudes. Our results imply a stronger than expected halogen source from SS in the tropical marine boundary layer. They also imply stronger radiative forcing of SS in the tropics and a larger response of SS emissions to climate change than previously thought.
Submicron particles were collected on board the NOAA R/V Ronald H. Brown during the VAMOS Ocean‐Cloud‐Atmosphere‐Land Study Regional Experiment (VOCALS‐REx) in the southeast Pacific marine boundary ...layer in October and November 2008. The aerosol in this region was characterized by low numbers of particles (150–700 cm−3) that were dominated by sulfate ions at concentrations of 0.9 ± 0.7 μg m−3 and organic mass at 0.6 ± 0.4 μg m−3, with no measurable nitrate and low ammonium ion concentrations. Measurements of submicron organic aerosol functional groups and trace elements show that continental outflow of anthropogenic emissions is the dominant source of organic mass (OM) to the southeast Pacific with an additional, smaller contribution of organic mass from primary marine sources. This continental source is supported by a correlation between OM and radon. Saturated aliphatic C‐CH (alkane) composed 41 ± 27% of OM. Carboxylic acid COOH (32 ± 23% of OM) was observed in single particles internally mixed with ketonic carbonyl, carbonate, and potassium. Organosulfate COSO3 (4 ± 8% of OM) was observed only during the periods of highest organic and sulfate concentrations and lowest ammonium concentrations, consistent with a sulfuric acid epoxide hydrolysis for proposed surrogate compounds (e.g., isoprene oxidation products) or reactive glyoxal uptake mechanisms from laboratory studies. This correlation suggests that in high‐sulfate, low‐ammonium conditions, the formation of organosulfate compounds in the atmosphere contributes a significant fraction of aerosol OM (up to 13% in continental air masses). Organic hydroxyl C‐OH composed 20 ± 12% of OM and up to 50% of remote marine OM and was inversely correlated with radon indicating a marine source. A two‐factor solution of positive matrix factorization (PMF) analysis resulted in one factor dominated by organic hydroxyl (>70% by mass) and one factor dominated by saturated aliphatic C‐CH (alkane) and carboxylic acid (together, 90% by mass), identified as the marine and combustion factors, respectively. Measurements of particle concentrations in the study region compared with concentrations estimated from MODIS aerosol optical depth indicate that continental outflow results in MBL particle concentrations elevated up to 2 times the background level (less than 300 cm−3) away from shore and up to 10 times the background level at the coast. The presence of both coastal fossil fuel combustion and marine sources of oxygenated organic aerosol results in little change in the oxygenated fraction and oxygen to carbon ratio (O/C) along the outflow of the region's dominant organic particle source.
Measuring individual circadian phase is important to diagnose and treat circadian rhythm sleep-wake disorders and circadian misalignment, inform chronotherapy, and advance circadian science. Initial ...findings using blood transcriptomics to predict the circadian phase marker dim-light melatonin onset (DLMO) show promise. Alternatively, there are limited attempts using metabolomics to predict DLMO and no known omics-based biomarkers predict dim-light melatonin offset (DLMOff). We analyzed the human plasma metabolome during adequate and insufficient sleep to predict DLMO and DLMOff using one blood sample. Sixteen (8 male/8 female) healthy participants aged 22.4 ± 4.8 years (mean ± SD) completed an in-laboratory study with 3 baseline days (9 h sleep opportunity/night), followed by a randomized cross-over protocol with 9-h adequate sleep and 5-h insufficient sleep conditions, each lasting 5 days. Blood was collected hourly during the final 24 h of each condition to independently determine DLMO and DLMOff. Blood samples collected every 4 h were analyzed by untargeted metabolomics and were randomly split into training (68%) and test (32%) sets for biomarker analyses. DLMO and DLMOff biomarker models were developed using partial least squares regression in the training set followed by performance assessments using the test set. At baseline, the DLMOff model showed the highest performance (0.91 R2 and 1.1 ± 1.1 h median absolute error ± interquartile range MdAE ± IQR), with significantly (p < 0.01) lower prediction error versus the DLMO model. When all conditions (baseline, 9 h, and 5 h) were included in performance analyses, the DLMO (0.60 R2; 2.2 ± 2.8 h MdAE; 44% of the samples with an error under 2 h) and DLMOff (0.62 R2; 1.8 ± 2.6 h MdAE; 51% of the samples with an error under 2 h) models were not statistically different. These findings show promise for metabolomics-based biomarkers of circadian phase and highlight the need to test biomarkers that predict multiple circadian phase markers under different physiological conditions.
Reliable characterization of particles freshly emitted from the ocean surface requires a sampling method that is able to isolate those particles and prevent them from interacting with ambient gases ...and particles. Here we report measurements of particles directly emitted from the ocean using a newly developed in situ particle generator (Sea Sweep). The Sea Sweep was deployed alongside R/V Atlantis off the coast of California during May of 2010. Bubbles were generated 0.75 m below the ocean surface with stainless steel frits and swept into a hood/vacuum hose to feed a suite of aerosol instrumentation on board the ship. The number size distribution of the directly emitted, nascent particles had a dominant mode at 55–60 nm (dry diameter) and secondary modes at 30–40 nm and 200–300 nm. The nascent aerosol was not volatile at 230°C and was not enriched in SO4=, Ca++, K+, or Mg++above that found in surface seawater. The organic component of the nascent aerosol (7% of the dry submicrometer mass) volatilized at a temperature between 230 and 600°C. The submicrometer organic aerosol characterized by mass spectrometry was dominated by non‐oxygenated hydrocarbons. The nascent aerosol at 50, 100, and 145 nm dry diameter behaved hygroscopically like an internal mixture of sea salt with a small organic component. The CCN/CN activation ratio for 60 nm Sea Sweep particles was near 1 for all supersaturations of 0.3 and higher indicating that all of the particles took up water and grew to cloud drop size. The nascent organic aerosol mass fraction did not increase in regions of higher surface seawater chlorophyll but did show a positive correlation with seawater dimethylsulfide (DMS).
Key Points
The ocean is a source of sub 100nm particles to the atmosphere
Hygroscopically the particles behave like an internal mixture of sea salt/organic
Organic mass fraction did not correlate with chlorophyll
Wintertime ammonium nitrate aerosol pollution is a severe air quality issue affecting both developed and rapidly urbanizing regions from Europe to East Asia. In the United States, it is acute in ...western basins subject to inversions that confine pollutants near the surface. Measurements and modeling of a wintertime pollution episode in Salt Lake Valley, Utah, demonstrate that ammonium nitrate is closely related to photochemical ozone through a common parameter, total odd oxygen, Ox,total. We show that the traditional nitrogen oxide and volatile organic compound (NOx‐VOC) framework for evaluating ozone mitigation strategies also applies to ammonium nitrate. Despite being nitrate‐limited, ammonium nitrate aerosol pollution in Salt Lake Valley is responsive to VOCs control and, counterintuitively, not initially responsive to NOx control. We demonstrate simultaneous nitrate limitation and NOx saturation and suggest this phenomenon may be general. This finding may identify an unrecognized control strategy to address a global public health issue in regions with severe winter aerosol pollution.
Plain Language Summary
Particulate matter (PM) is dangerous to human health and impacts visibility and climate. In the United States, Europe, and Asia, PM is severe in urban areas in the winter when ammonium nitrate, NH4NO3, comprises an appreciable fraction of the total PM mass. A key control strategy is to reduce emissions of the limiting reagent. Using measurements from a recent field campaign in the Salt Lake Valley, Utah, which experiences high PM levels in winter, we demonstrate that emission control strategies can be evaluated using the same framework commonly used to control ozone, another common pollutant that occurs at high levels in urban areas in the summer. We show that initial control of the NOx precursor is ineffective at reducing NH4NO3 aerosol in the Salt Lake Valley, while initial control of volatile organic compounds, which are not a direct precursor for either nitrate or ammonium, is effective due to their influence on oxidation cycles. This finding differs from many mitigation strategies in the western United States and may also be relevant to other regions in Europe and Asia which experience high wintertime PM.
Key Points
Wintertime ammonium nitrate aerosol pollution is closely tied to photochemical ozone production through a common parameter, Ox,total
Box modeling reveals ammonium nitrate formation in the Salt Lake Valley is nitrate‐limited but NOx‐saturated
Mitigation strategies that focus on NOx control in some wintertime‐polluted layers may initially increase ammonium nitrate
Aerosol variations and trends over different land and ocean regions from 1980 to 2009 are analyzed with the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model and observations from ...multiple satellite sensors and available ground-based networks. Excluding time periods with large volcanic influence, aerosol optical depth (AOD) and surface concentration over polluted land regions generally vary with anthropogenic emissions, but the magnitude of this association can be dampened by the presence of natural aerosols, especially dust. Over the 30-year period in this study, the largest reduction in aerosol levels occurs over Europe, where AOD has decreased by 40–60% on average and surface sulfate concentrations have declined by a factor of up to 3–4. In contrast, East Asia and South Asia show AOD increases, but the relatively high level of dust aerosols in Asia reduces the correlation between AOD and pollutant emission trends. Over major dust source regions, model analysis indicates that the change of dust emissions over the Sahara and Sahel has been predominantly driven by the change of near-surface wind speed, but over Central Asia it has been largely influenced by the change of the surface wetness. The decreasing dust trend in the North African dust outflow region of the tropical North Atlantic and the receptor sites of Barbados and Miami is closely associated with an increase of the sea surface temperature in the North Atlantic. This temperature increase may drive the decrease of the wind velocity over North Africa, which reduces the dust emission, and the increase of precipitation over the tropical North Atlantic, which enhances dust removal during transport. Despite significant trends over some major continental source regions, the model-calculated global annual average AOD shows little change over land and ocean in the past three decades, because opposite trends in different land regions cancel each other out in the global average, and changes over large open oceans are negligible. This highlights the necessity for regional-scale assessment of aerosols and their climate impacts, as global-scale average values can obscure important regional changes.
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