Role In Radiative Forcing Research Articles

Stable Carbon Isotope Signatures of Carbonaceous Aerosol Endmembers in the Tibetan Plateau.

Carbonaceous aerosols play an important role in radiative forcing in the remote and climate-sensitive Tibetan Plateau (TP). However, the sources of carbonaceous aerosols to the TP remain poorly defined, in part due to the lack of regionally relevant data about the sources of carbonaceous aerosols. To address this knowledge gap, we present the first comprehensive analysis of the δ13C signatures of carbonaceous aerosol endmembers local to the TP, encompassing total carbon, water-insoluble particle carbon, and elemental carbon originating from fossil fuel combustion, biomass combustion, and topsoil. The δ13C signatures of these local carbonaceous endmembers differ from components collected in other regions of the world. For instance, fossil fuel-derived aerosols from the TP were 13C-depleted relative to fossil fuel-derived aerosols reported in other regions, while biomass fuel-derived aerosols from the TP were 13C-enriched relative to biomass fuel-derived aerosols reported in other regions. The δ13C values of fine-particle topsoil in the TP were related to regional variations in vegetation type. These findings enhance our understanding of the unique features of carbonaceous aerosols in the TP and aid in accurate source apportionment and environmental assessments of carbonaceous aerosols in this climate-sensitive region.

Technical note: Unsupervised classification of ozone profiles in UKESM1

Abstract. The vertical distribution of ozone in the atmosphere, which features complex spatial and temporal variability set by a balance of production, loss, and advection, is relevant for both surface air pollution and climate via its role in radiative forcing. At present, the way in which regions of coherent ozone structure are defined relies on somewhat arbitrarily drawn boundaries. Here we consider a more general, data-driven method for defining coherent regimes of ozone structure. We apply an unsupervised classification technique called Gaussian mixture modeling (GMM), which represents the underlying distribution of ozone profiles as a linear combination of multi-dimensional Gaussian functions. In doing so, GMM identifies coherent groups or subpopulations of the ozone profile distribution. As a proof-of-concept study, we apply GMM to ozone profiles from three subsets of the UKESM1 coupled climate model runs carried out for CMIP6: specifically, the seasonal mean of a historical subset (2009–2014) and two subsets from two different future climate projections (i.e., SSP1-2.6 and SSP5-8.5). Despite not being given any spatiotemporal information, GMM identifies several spatially coherent regions of ozone structure. Using a combination of statistical guidance and post hoc judgment, we select a six-class representation of global ozone, consisting of two tropical classes and four mid-to-high-latitude classes. The tropical classes feature a relatively high-altitude tropopause, while the higher-latitude classes feature a lower-altitude tropopause and low values of tropospheric ozone, as expected based on broad patterns observed in the atmosphere. Both of the future projections feature lower ozone concentrations at 850 hPa than the historical benchmark, with signatures of ozone hole recovery. We find that the area occupied by the tropical classes is expanded in both future projections, which are most prominent during austral summer. Our results suggest that GMM may be a useful method for identifying coherent ozone regimes, particularly in the context of model analysis.

A new method for the determination of imidazole-like brown carbons using gas chromatography-mass spectrometry

Imidazole-like compounds were important components of brown carbon, which played a significant role in radiative forcing in the atmosphere. In this study, a new detection method of imidazole-like compounds in the atmosphere was developed using gas chromatography-mass spectrometry (GC-MS) due to its high resolution and sensitivity. Seven imidazole-like compounds, including imidazole (IM), 4 (5)-methylimidazole (4MEI), 2-ethylimidazole (2EI), 2,4-dimethylimidazole (2,4-DMI), 2-methylimidazole-4-carbaldehyde (2MI4C), 2,2-biimidazole (BI) and 2-phenylimidazole (2PI) were performed using the GC-MS method. Acetonitrile, pyridine, anhydrous ethanol and isobutyl chloroformate were used to conduct the derivatization of imidazole-like compounds before GC-MS analysis. Optimal amounts of 90 μL acetonitrile, 60 μL pyridine, 200 μL anhydrous ethanol and 70 μL isobutyl chloroformate ensured the derivatization of the 7 imidazole-like compounds. The pH value influenced imidazole-like compounds extraction from the aqueous phase. The result showed that the pH value for the extraction of 7 imidazole-like compounds was adjusted to be 8.0. Limits of detection and quantification for the 7 imidazole-like compounds were 0.0553–0.8914 and 0.2370–1.9373 μg/mL, respectively, using this method. The spiked recoveries for the 7 imidazole-like compounds ranged from 58.84% to 160.99%. The method was further applied to measure imidazole-like compounds from aqueous reactions in the experiment study and atmospheric aerosols in the field study. IM, BI, 4MEI and 2,4-DMI were observed in the aqueous reaction of carbonyls with reduced nitrogen. Furthermore, for the first time in the atmospheric particles quantitative IM and 4MEI, China's northern coast. The developed method would help us better understand the source and evolution of imidazole-like brown carbon aerosols in the atmosphere.

Revisiting the Absorption Spectra of Polycyclic Aromatic Hydrocarbons over Porto (Portugal) by TD-DFT Calculations

Brown carbon is a type of strong light-absorbing carbonaceous aerosol associated with radiative forcing. Nevertheless, the difficulty in correlating the chemical composition of brown carbon with its light absorption properties impairs the proper elucidation of its role in radiative forcing. Here, we have used a time-dependent density functional theory (TD-DFT)-based procedure to revisit the “real-world” absorption spectra of polycyclic aromatic hydrocarbons (PAHs) over the city of Porto, in Portugal, while correcting the spectra for their quantity in PM10 particulate matter. Our aim is to, by comparing these new results with those obtained previously regarding PM2.5 data, evaluate the role of different groupings of particulate matter in the light absorption of brown carbon. The results indicate that irrespective of the absorption spectra corresponding to their PM10 or PM2.5 data, the studied PAHs should contribute to radiative forcing by light absorption at UVA and (sub)visible wavelengths. However, the identity of the individual PAH species that contribute the most for the considered wavelengths can be quite different. Thus, different groupings of particulate matter appear to provide distinct contributions to light absorption and radiative forcing over the same location, even when considering the same class of molecular compounds.

Modeling the nocturnal/diurnal and seasonal real world absorption spectra of polycyclic aromatic hydrocarbons and their derivatives in two Chinese polluted cities

Brown carbon (BrC), an organic aerosol, plays an important role in radiative forcing. Polycyclic aromatic hydrocarbons (PAHs) and their derivatives such as oxygenated and nitrated polycyclic aromatic hydrocarbons (OPAHs & NPAHs) are the major constituents of BrC and are persistent environmental pollutants. Our strategy here is to utilize time dependent-density functional theory (TD-DFT) to model the absorption spectra of PAHs and their derivatives in two Chinese industrial sites: Qingcheng district (site A) and Longtang town of Qingyuan (site B). These data are corrected for “Real-world” experimental concentrations of PAHs over these cities. For the first time, nocturnal/diurnal and seasonal variations of PAHs are being simultaneously studied under a theoretical framework. These findings show that the absorptions at site A and B take place mainly due to PAHs while OPAHs and NPAHs have negligible contribution. The site A is highly affected by climate forcing caused by these PAHs. The absorption in winter is higher as compared to that of in summer. Our theoretical modeling approach remarkably identifies the most relevant PAHs for climate forcing in both Chinese regions.

An overestimation of light absorption of brown carbon in ambient particles caused by using filters with large pore size

As an important component of carbonaceous particles, organic carbon (OC) plays a significant role in radiative forcing in the atmosphere. Recently, the warming effect of light-absorbing OC has been emphasized. Water-soluble organic carbon (WSOC) is commonly used as a surrogate to investigate the light absorption of OC. Thus far, filters with 0.45 μm (PS1) and 0.20 μm pore sizes (PS2) are both used to investigate the light absorption of WSOC, which may cause large divergent results. In this study, we found that the light absorption ability of WSOC treated with PS1 was higher than that of PS2 due to the extinction of suspended particles (e.g., black carbon) with particle size between 0.20 μm and 0.45 μm, although the concentrations of WSOC treated with PS1 and PS2 were very close. This phenomenon was more remarkable at visible wavelengths, resulting in an overestimation of the warming effect of WSOC by 9%–22% for aerosol samples treated by PS1, with the highest values occurring in samples heavily influenced by fossil fuel burning emissions. An overestimation of WSOC light absorption treated by PS1 occurred in the investigated ambient aerosol samples from three sites, so it may be a general phenomenon that also exists in other regions of the world. Therefore, to achieve the actual solar radiative forcing of OC in the atmosphere, it is recommended to use PS2 in the future, and reported data of WSOC treated by PS1 should be re-evaluated.

Physicochemical Mixing State of Sea Spray Aerosols: Morphologies Exhibit Size Dependence

Sea spray aerosols (SSA) play an important role in radiative forcing by directly scattering solar radiation and indirectly by acting as cloud condensation or ice nuclei. These climate-relevant aero...

A new method for extraction of methanol-soluble brown carbon: Implications for investigation of its light absorption ability

As an important component of organic carbon (OC), brown carbon (BrC) plays a significant role in radiative forcing in the atmosphere. Water-insoluble OC (WIOC) generally has higher light absorption ability than water-soluble OC (WSOC). The mass absorption cross-section (MAC) of WIOC is normally investigated by dissolving OC in methanol. However, all the current methods have shortcomings due to neglecting the methanol insoluble particulate carbon that is detached from the filter and suspended in methanol extracts, which results in MAC uncertainties of the methanol-soluble BrC and its climate warming estimation. In this study, by investigating typical biomass combustion sourced aerosols from the Tibetan Plateau and ambient aerosols from rural and urban areas in China, we evaluated the light absorption of extractable OC fraction for the existing methods. Moreover, a new method was developed to overcome the methanol insoluble particulate carbon detachment problem to achieve more reliable MAC values. We found that OC can be dissolved in methanol in a short time (e.g., 1 h) and ultrasonic treatment and long-term soaking do not significantly increase the extractable OC fraction. Additionally, we proved that methanol insoluble particulate carbon detachment in methanol does exist in previous methods, causing overestimation of the BrC mass extracted by methanol and thus the underestimation of MAC values. We therefore recommend the newly developed extraction method in this study to be utilized in future related studies to quantitatively obtain the light absorption property of methanol-soluble BrC.

An Improved Absorption Ångström Exponent (AAE)-Based Method for Evaluating the Contribution of Light Absorption from Brown Carbon with a High-Time Resolution

ABSTRACT While brown carbon (BrC) might play a substantially important role in radiative forcing, an estimation of its light absorption contribution with high-time resolution is still challenging. In this study, a multi-wavelength (370–950 nm) Aethalometer was applied to obtain the wavelength dependent light absorption coefficient (σabs) of aerosols both before and after being heated to 250°C. An improved absorption Angstrom exponent (AAE)-based method was developed to evaluate the contribution of BrC to light absorption at a wavelength of 370 nm (σabs,BrC/σabs,370nm). The σabs,BC at 370 nm was determined from the field measured AAE values for the wavelengths from 880 to 950 nm with a one-hour resolution. The simultaneous measurements of heated aerosols help confirm the negligible influence of BrC on the σabs values across the range of 880–950 nm. Meanwhile, σabs,BrC/σabs,370nm was also estimated with previously reported methods by assuming that the AAE was equal to 1 (Method I) as well as a new approach based on the light absorption enhancement (Method II). While the estimated σabs,BrC/σabs,370nm based on our developed method and Method I is highly correlated (r2 = 0.78), the difference could be as large as > 20% on average. The obtained mean σabs,BrC/σabs,370nm was negative with Method II, indicating the net production of BrC when the aerosols were heated. The difference between the values for σabs,BrC/σabs,370nm obtained by our developed method and by Method II was ~40% on average and much higher (> 50%) during the noon hour, when secondary organic aerosols and sulfate were abundant. We propose that it is more suitable to use an AAE around 0.7 for “pure” BC to evaluate the contribution of BrC to light absorption in the PRD region. The developed method thus helps improve our understanding of the light absorption and climate forcing of BrC.

Causes and Probability of Occurrence of Extreme Precipitation Events like Chennai 2015

Abstract Unprecedented high-intensity flooding induced by extreme precipitation was reported over Chennai in India during November–December of 2015, which led to extensive damage to human life and property. It is of utmost importance to determine the odds of occurrence of such extreme floods in the future, and the related climate phenomena, for planning and mitigation purposes. Here, a suite of simulations from GFDL high-resolution coupled climate models are used to investigate the odds of occurrence of extreme floods induced by extreme precipitation over Chennai and the role of radiative forcing and/or large-scale SST forcing in enhancing the probability of such events in the future. The climate of twentieth-century experiments with large ensembles suggest that the radiative forcing may not enhance the probability of extreme floods over Chennai. Doubling of CO2 experiments also fails to show evidence for an increase of such events in a global warming scenario. Further, this study explores the role of SST forcing from the Indian and Pacific Oceans on the odds of occurrence of Chennai-like floods. Neither El Niño nor La Niña enhances the probability of extreme floods over Chennai. However, a warm Bay of Bengal tends to increase the odds of occurrence of extreme Chennai-like floods. In order to trigger a Chennai like-flood, a conducive weather event, such as a tropical depression over the Bay of Bengal with strong transport of moisture from a moist atmosphere over the warm Bay, is necessary for the intense precipitation.

Extreme North America Winter Storm Season of 2013/14: Roles of Radiative Forcing and the Global Warming Hiatus

Extreme North America Winter Storm Season of 2013/14: Roles of Radiative Forcing and the Global Warming Hiatus

Extreme North America Winter Storm Season of 2013/14: Roles of Radiative Forcing and the Global Warming Hiatus

Extreme North America Winter Storm Season of 2013/14: Roles of Radiative Forcing and the Global Warming Hiatus

The Relative Influence of Tropical Sea Surface Temperatures and Radiative Forcing on the Amundsen Sea Low

Abstract Recent studies suggest that warming trends across West Antarctica and the Antarctic Peninsula and sea ice loss in the adjacent Amundsen and Bellingshausen Seas are linked to changes in the regional atmospheric circulation, represented by the Amundsen Sea low (ASL). Importantly, changes in the ASL have similarly been tied to forcing from the tropics. Here, several model simulations from the Community Atmosphere Model, version 4, are investigated in order to understand the relative roles of tropical sea surface temperature variability and radiative forcing on the variations in trends in the ASL. In comparing across the simulations, it is observed that the addition of time-varying extratropical SSTs and sea ice conditions in general have a much smaller impact on the ASL than tropical SSTs or radiative forcing. Tropical forcing alone explains much of the climatological variability and extreme intensities of the ASL (both strong and weak relative central pressures). The role of radiative forcing is best observed in the ASL trends, with this simulation leading to a marked deepening of the ASL and pressures across the Southern Hemisphere that is consistent with atmospheric reanalysis in austral summer. In austral winter, the simulation with radiative forcing produces stronger trends than observed in reanalysis data, perhaps reflecting the need to couple to an ocean–ice model in order to more realistically simulate the ASL changes. Together, the results suggest that models need to include both the effects from tropical SST variations and radiative forcing when understanding historic and future variations in the ASL.

Exploring the influence of ancient and historic megaherbivore extirpations on the global methane budget

Globally, large-bodied wild mammals are in peril. Because "megamammals" have a disproportionate influence on vegetation, trophic interactions, and ecosystem function, declining populations are of considerable conservation concern. However, this is not new; trophic downgrading occurred in the past, including the African rinderpest epizootic of the 1890s, the massive Great Plains bison kill-off in the 1860s, and the terminal Pleistocene extinction of megafauna. Examining the consequences of these earlier events yields insights into contemporary ecosystem function. Here, we focus on changes in methane emissions, produced as a byproduct of enteric fermentation by herbivores. Although methane is ∼ 200 times less abundant than carbon dioxide in the atmosphere, the greater efficiency of methane in trapping radiation leads to a significant role in radiative forcing of climate. Using global datasets of late Quaternary mammals, domestic livestock, and human population from the United Nations as well as literature sources, we develop a series of allometric regressions relating mammal body mass to population density and CH4 production, which allows estimation of methane production by wild and domestic herbivores for each historic or ancient time period. We find the extirpation of megaherbivores reduced global enteric emissions between 2.2-69.6 Tg CH4 y(-1) during the various time periods, representing a decrease of 0.8-34.8% of the overall inputs to tropospheric input. Our analyses suggest that large-bodied mammals have a greater influence on methane emissions than previously appreciated and, further, that changes in the source pool from herbivores can influence global biogeochemical cycles and, potentially, climate.

Stress, strain, and flow produced by a vibrator in, or on the surface of, a soft solid

The field of biomedical ultrasound is greatly indebted to Edwin Carstensen for important contributions, without number, that he and his associates have made to understanding of the subject. These contributions have dealt not only with linear and nonlinear propagation of ultrasound in biological materials, but also with effects produced by ultrasound through various thermal and nonthermal mechanisms. Such understanding is important for advancing benefits and minimizing risks in applications of ultrasound. Many therapeutic applications discussed in the literature utilize focused beams of megahertz frequency, while others employ lower frequencies. In this talk, some findings will be presented from experiments in which a vibrating source of frequency in the range 20–90 kHz is brought into contact with the surface of a soft viscoelastic solid and information is obtained on resulting fields of strain. Under some conditions the mechanical properties of the solid are altered by an exposure, in an apparent change of internal structure, and the change is maintained (recorded) until erased by a later sonication. The role of radiation force and other mechanisms in producing such effects will be discussed, as well as their possible relevance to ultrasonic angioplasty and other applications.