Negative Radiative Forcing Research Articles

Insect Herbivory Caused Plant Stress Emissions Increases the Negative Radiative Forcing of Aerosols.

Plant stress in a changing climate is predicted to increase plant volatile organic compound (VOC) emissions and thus can affect the formed secondary organic aerosol (SOA) concentrations, which in turn affect the radiative properties of clouds and aerosol. However, global aerosol‐climate models do not usually consider plant stress induced VOCs in their emission schemes. In this study, we modified the monoterpene emission factors in biogenic emission model to simulate biotic stress caused by insect herbivory on needleleaf evergreen boreal and broadleaf deciduous boreal trees and studied the consequent effects on SOA formation, aerosol‐cloud interactions as well as direct radiative effects of formed SOA. Simulations were done altering the fraction of stressed and healthy trees in the latest version of ECHAM‐HAMMOZ (ECHAM6.3‐HAM2.3‐MOZ1.0) global aerosol‐climate model. Our simulations showed that increasing the extent of stress to the aforementioned tree types, substantially increased the SOA burden especially over the areas where these trees are located. This indicates that increased VOC emissions due to increasing stress enhance the SOA formation via oxidation of VOCs to low VOCs. In addition, cloud droplet number concentration at the cloud top increased with increasing extent of biotic stress. This indicates that as SOA formation increases, it further enhances the number of particles acting as cloud condensation nuclei. The increase in SOA formation also decreased both all‐sky and clear‐sky radiative forcing. This was due to a shift in particle size distributions that enhanced aerosol reflecting and scattering of incoming solar radiation.

The Response of Radiative Forcing to High Spatiotemporally Resolved Land‐Use Change and Transition From 1982 to 2010 in China

AbstractSignificant uncertainties remain in the estimate of radiative forcing (RF) induced by land‐use change (LUC), partially attributable to the lack of reliable LUC data with a high spatiotemporal resolution. We implemented a high spatiotemporally resolved LUC data set in an earth system model (OSCAR) to examine the response of RF to LUC from 1982 to 2010 in China. Results were compared with the RF estimated using a low spatiotemporally resolved LUC inventory employed previously. The updated LUC data set reduces negative RF by −3.8% from 2000 to 2010 due to the changes in surface albedo subject to LU transition. The simulated mean RF driven by CO2 associated with LUC from 1982 to 2010 using a high spatiotemporally resolved LUC data set reached 0.074 W m−2, considerably higher than 0.022 W m−2 of mean RF derived from the low spatiotemporally resolved LUC inventory, implying increasing net RF and more substantial LUC induced warming.

Quantifying negative radiative forcing of non-permanent and permanent soil carbon sinks

Reversibility of soil carbon sinks is a major obstacle in assigning soil carbon sequestration as negative emission technology and it is still unclear how a non-permanent CO2 removal shall be accounted for. In this study, we combine various scenarios of reversible and non-reversible soil carbon sinks with atmospheric CO2 impulse response functions and calculations of the resulting radiative forcing. A time horizon of up to 500 years was considered. Results show that any soil carbon sink generates negative radiative forcing (i.e., cooling) when aggregated over longer time scales. Whereas also non-permanent CO2 removals from the atmosphere provide negative average radiative forcing, their effect is substantially smaller than that of permanent removals of the same magnitude. We show that the average annual soil organic carbon balance over the integrated time window largely determines the average radiative forcing independently of rates of carbon gain or loss and longevity of the sink. This basic principle allows an unbiased assessment, comparison, and rating of mitigation projects that take advantage of soil carbon. The suggested approach is based on quantitative and relatively simple metrics and may therefore support guidance to climate policies and soil carbon markets.

Tropospheric warming over the northern Indian Ocean caused by South Asian anthropogenic aerosols: possible impact on the upper troposphere and lower stratosphere

Abstract. Atmospheric concentrations of South Asian anthropogenic aerosols and their transport play a key role in the regional hydrological cycle. Here, we use the ECHAM6-HAMMOZ chemistry–climate model to show the structure and implications of the transport pathways of these aerosols during spring (March–May). Our simulations indicate that large amounts of anthropogenic aerosols are transported from South Asia to the northern Indian Ocean and western Pacific. These aerosols are then lifted into the upper troposphere and lower stratosphere (UTLS) by the ascending branch of the Hadley circulation, where they enter the westerly jet. They are further transported to the Southern Hemisphere (∼15–30∘ S) and downward (320–340 K) via westerly ducts over the tropical Atlantic (5∘ S–5∘ N, 10–40∘ W) and Pacific (5∘ S–5∘ N, 95–140∘ E). The carbonaceous aerosols are also transported to the Arctic, leading to local heating (0.08–0.3 K per month, an increase by 10 %–60 %). The presence of anthropogenic aerosols causes a negative radiative forcing (RF) at the top of the atmosphere (TOA) (−0.90 ± 0.089 W m−2) and surface (−5.87 ± 0.31 W m−2) and atmospheric warming (+4.96 ± 0.24 W m−2) over South Asia (60–90∘ E, 8–23∘ N), except over the Indo-Gangetic Plain (75–83∘ E, 23–30∘ N), where RF at the TOA is positive (+1.27 ± 0.16 W m−2) due to large concentrations of absorbing aerosols. The carbonaceous aerosols lead to in-atmospheric heating along the aerosol column extending from the boundary layer to the upper troposphere (0.1 to 0.4 K per month, increase by 4 %–60 %) and in the lower stratosphere at 40–90∘ N (0.02 to 0.3 K per month, increase by 10 %–60 %). The increase in tropospheric heating due to aerosols results in an increase in water vapor concentrations, which are then transported from the northern Indian Ocean–western Pacific to the UTLS over 45–45∘ N (increasing water vapor by 1 %–10 %).

Cool Pavements: State of the Art and New Technologies

With growing urban populations, methods of reducing the urban heat island effect have become increasingly important. Cool pavements altering the heat storage of materials used in pavements can lead to lower surface temperatures and reduce the thermal radiation emitted to the atmosphere. Cool pavement technologies utilize various strategies to reduce the temperature of new and existing pavements, including increased albedo, evaporative cooling, and reduced heat conduction. This process of negative radiation forces helps offset the impacts of increasing atmospheric temperatures. This paper presents an extensive analysis of the state of the art of cool pavements. The properties and principles of cool pavements are reviewed, including reflectivity, thermal emittance, heat transfer, thermal capacity, and permeability. The different types, research directions, and applications of reflective pavements are outlined and discussed. Maintenance and restoration technologies of cool pavements are reviewed, including permeable pavements. Results show that cool pavements have significant temperature reduction potential in the urban environment. This research is important for policy actions of the European Union, noting that European and international business stakeholders have recently expressed their interest in new ways of reducing energy consumption through technologically advanced pavements.

Identifying main uncertainties in estimating past and present radiative forcing of peatlands.

Reconstructions of past climate impact, that is, radiative forcing (RF), of peatland carbon (C) dynamics show that immediately after peatland initiation the climate warming effect of CH4 emissions exceeds the cooling effect of CO2 uptake, but thereafter the net effect of most peatlands will move toward cooling, when RF switches from positive to negative. Reconstructing peatland C dynamics necessarily involves uncertainties related to basic assumptions on past CO2 flux, CH4 emission and peatland expansion. We investigated the effect of these uncertainties on the RF of three peatlands, using either apparent C accumulation rates, net C balance (NCB) or NCB plus C loss during fires as basis for CO2 uptake estimate; applying a plausible range for CH4 emission; and assuming linearly interpolated expansion between basal dates or comparatively early or late expansion. When we factored that some C would only be stored temporarily (NCB and NCB+fire), the estimated past cooling effect of CO2 uptake increased, but the present-day RF was affected little. Altering the assumptions behind the reconstructed CO2 flux or expansion patterns caused the RF to peak earlier and advanced the switch from positive to negative RF by several thousand years. Compared with NCB, including fires had only small additional effect on RF lasting less than 1000year. The largest uncertainty in reconstructing peatland RF was associated with CH4 emissions. As shown by the consistently positive RF modelled for one site, and in some cases for the other two, peatlands with high CH4 emissions and low C accumulation rates may have remained climate warming agents since their initiation. Although uncertainties in present-day RF were mainly due to the assumed CH4 emission rates, the uncertainty in lateral expansion still had a significant effect on the present-day RF, highlighting the importance to consider uncertainties in the past peatland C balance in RF reconstructions.

Formation, radiative forcing, and climatic effects of severe regional haze

Abstract. Severe regional haze events, which are characterized by exceedingly high levels of fine particulate matter (PM), occur frequently in many developing countries (such as China and India), with profound implications for human health, weather, and climate. The occurrence of the haze extremes involves a complex interplay between primary emissions, secondary formation, and conducive meteorological conditions, and the relative contributions of the various processes remain unclear. Here we investigated severe regional haze episodes in 2013 over the Northern China Plain (NCP), by evaluating the PM production and the interactions between elevated PM and the planetary boundary layer (PBL). Analysis of the ground-based measurements and satellite observations of PM properties shows nearly synchronized temporal PM variations among the three megacities (Beijing, Baoding, and Shijiazhuang) in this region and a coincidence of the aerosol optical depth (AOD) hotspots with the three megacities during the polluted period. During the clean-to-hazy transition, the measured oxygenated organic aerosol concentration ([OOA]) well correlates with the odd-oxygen concentration ([Ox]=[O3]+[NO2]), and the mean [OOA] / [Ox] ratio in Beijing is much larger than those in other megacities (such as Mexico City and Houston), indicating highly efficient photochemical activity. Simulations using the Weather Research and Forecasting (WRF) model coupled with an explicit aerosol radiative module reveal that strong aerosol–PBL interaction during the polluted period results in a suppressed and stabilized PBL and elevated humidity, triggering a positive feedback to amplify the haze severity at the ground level. Model sensitivity study illustrates the importance of black carbon (BC) in the haze–PBL interaction and the aerosol regional climatic effect, contributing to more than 30 % of the PBL collapse and about half of the positive radiative forcing on the top of the atmosphere. Overall, severe regional haze exhibits strong negative radiative forcing (cooling) of −63 to −88 W m−2 at the surface and strong positive radiative forcing (warming) of 57 to 82 W m−2 in the atmosphere, with a slightly negative net radiative forcing of about −6 W m−2 on the top of the atmosphere. Our work establishes a synthetic view for the dominant regional features during severe haze events, unraveling rapid in situ PM production and inefficient transport, both of which are amplified by atmospheric stagnation. On the other hand, regional transport sufficiently disperses gaseous aerosol precursors (e.g., sulfur dioxide, nitrogen oxides, volatile organic compounds, and ammonia) during the clean period, which subsequently result in rapid in situ PM production via photochemistry during the transition period and via multiphase chemistry during the polluted period. Our findings highlight the co-benefits for reduction in BC emissions, which not only improve local and regional air quality by minimizing air stagnation but also mitigate the global warming by alleviating the positive direct radiative forcing.

Climate Controls on the Interseasonal and Interannual Variability of the Surface Mass and Energy Balances of a Tropical Glacier (Zongo Glacier, Bolivia, 16°S): New Insights From the Multi‐Year Application of a Distributed Energy Balance Model

AbstractThe application of a distributed energy balance model over nine years at an hourly time step to a 20 × 20 m grid cell over Glacier Zongo (Bolivia, 16°S) enabled assessment of the climate factors that control the interseasonal and interannual variability of its surface mass balance. The model was validated by comparing the measured and simulated discharge at the outlet, albedo at the Automatic Weather Station, surface state and annual mass balance both glacier‐wide and as a function of altitude. Analysis of the mean monthly energy fluxes highlighted the importance of the meteorological conditions over October and November on the variability of the annual surface mass balance. Two sensitivity analyses are presented, one of the distribution of precipitation over time which maintains a physical coherence between the different meteorological variables and one of the impact of prolonged periods of intense cloud radiative forcing on the surface mass balance. The distribution of precipitation events over time and their associated amounts are the main drivers of the interannual variability of the surface mass balance via an albedo feedback effect. Additionally, prolonged periods of negative cloud radiative forcing, specifically over the month of November, notably reduce the melt rate.

Effects of heavy grazing on the microclimate of a humid grassland mountain ecosystem: Insights from a biomass removal experiment

In high-altitude Andean grasslands (páramo), overgrazing leads to alterations in both vegetation and microclimate. These alterations need to be identified to devise land management strategies that will preserve and enhance ecosystem processes. To elucidate this issue, we designed an overgrazing experiment: we selected two plots covered with native grass (pajonal), in one of which we mowed to the ground surface. We left the second plot undisturbed to serve as a control. For both plots, we continuously monitored albedo and ancillary energetic components to generate quarterly and yearly comparisons for the following parameters: (a) impacts on albedo and resilience of grass; (b) radiative forcing of albedo; and (c) land surface temperature feedback during the recovery period. In the first quarter following removal, when the soil was covered with light litter, median albedo increased 38.81% (0.16 ± 0.02), then began a gradual decrease, which continued until its full recovery 1.75 years later (0.10 ± 0.01). During the first year of the experiment, a strong mean negative instantaneous radiative forcing was observed (−7.08 ± 6.03 Wm−2), signifying a reduction in net shortwave energy. This forcing returned to normal, pre-intervention conditions (−0.55 ± 0.97 Wm−2) after 1.75 years, equal to the energetic recovery period of the grass. Both the amount (from 133.0 ± 44.72 to 119.67 ± 39.30 Wm−2) and the partitioning (net shortwave decreased 5%; net longwave increased 9.7%) of net energy were altered after removal, evidence of cooling feedback during the recovery period. This feedback indicated that the decrease in albedo (1.25%) or instantaneous radiative forcing (−4.67 Wm−2) resulted in a decrease in land surface temperature of 1 °C. Thus, our overgrazing experiment without soil destruction followed by a natural recovery time has identified the energetic recovery period for grass in the páramos; suggesting the albedo as a good indicator of grass resilience.

Study on acoustic radiation force of an elastic sphere in an off-axial Gaussian beam using localized approximation.

In this paper, the expansion coefficients of the off-axial Gaussian beam are obtained using the localized approximation and the translational addition theorem for spherical wave function. The three-dimensional acoustic radiation force of a sphere positioned in an off-axial Gaussian beam is derived. The axial acoustic radiation force of a rigid sphere is computed to verify the derived expressions. The effect of the position of a polystyrene sphere in an off-axial Gaussian beam on the transverse and axial acoustic radiation forces is studied to explore the changing law of particle acoustic manipulation using a Gaussian beam. The calculated results show that the axial force repels the polystyrene particle away from the center of the beam. However, for the transverse force, there is a negative acoustic radiation force at some positions, which is related to the position of the polystyrene sphere in the Gaussian beam, and the negative transverse forces usually pull the polystyrene particle toward the beam axis. In addition, the numerical simulations based on the finite element method are presented to validate the analytical theory, and the comparison results are in good agreement with each other. The study may provide a theoretical basis for the development of single-beam acoustic tweezers using a Gaussian beam.

Direct Radiative Effect (DRE) of Dust Aerosols on West African and East Asian Monsoon: The Role of Ocean‐Atmosphere Interactions

AbstractThe direct radiative effect (DRE) of dust aerosols on the West African and East Asian monsoons is analyzed using the Community Atmosphere Model version 4 containing the dust aerosol parameterization module with and without the coupling of ocean‐atmosphere processes, respectively. The atmospheric model (AM) and the ocean‐atmosphere coupled model (CM) can both capture the major spatial characteristics of dust aerosols and summer monsoons over West Africa and East Asia. Our results show that the DREs on the West African (WASM) and East Asian summer monsoon (EASM) are more significant in the CM simulations compared to the AM simulations, causing the strengthening of the WASM and weakening of the EASM. The DRE of dust aerosols induces a negative radiative forcing leading to a surface cooling of the downstream ocean of the dust source region when the coupled ocean feedback is added. The distinct DRE of dust aerosols on the WASM and EASM are resulted from the relative locations of the ocean cooling regions to the respective monsoon regions. This study shows the importance of the coupled ocean‐atmosphere processes in the DRE and its influences on the WASM and EASM systems.

Variability of Aerosols and Clouds Over North Indian and Myanmar During the COVID-19 Lockdown Period

The implementation of a nationwide lockdown to curb the spread of COVID-19 disease has reduced the loading of anthropogenic aerosols. However, AOD distribution over South Asia during the lockdown period shows a dipole pattern: reduction over North Indian and enhancement over the Myanmar region. This dipole pattern is evident in some datasets (MODIS, MERRA, and CALIPSO). MODIS fire counts collocated with CALIPSO smoke aerosols show enhancement over Myanmar indicating the contribution from fires. However, over the North India region number of fires during the lockdown period are less compared to climatology. Thus, the observed reduction in AOD is due to fires and anthropogenic sources. Our analysis shows that aerosols originating from biomass burning forms a layer (900–600 hPa) over the Myanmar region that produces atmospheric heating (0–2.8 K/day) that eventually leads to cloud dissipation/burning (negative in-atmospheric cloud radiative forcing ∼ −13 W/m2) and precipitation reduction (−1 to −4 mm) over Myanmar. In contrast, the aerosol reduction over North India favors cloud formation, that is, increase in cloud cover and reduction in specific cloud liquid water content leading to precipitation enhancement, indicating the anti-Twomey effect.

Transport of black carbon from Central and West Asia to the Tibetan Plateau: Seasonality and climate effect

Black carbon (BC) exerts potential effect on climate, especially in the Tibetan Plateau (TP), where the cryosphere and environment are very sensitive to climate change. Although transport of atmospheric BC from South and East Asia to the TP has been comprehensively investigated, transport of BC from Central and West Asia (CWA) to the TP and its climate effect on the region have received little attention and are warrant investigation. Therefore, based on the observation and ERA-Interim reanalysis data, this study investigated transport of atmospheric BC from CWA to the TP, its seasonality and climate effect over the TP using WRF-Chem model. On an annual scale, BC from CWA contributes to 5.8% of total BC over the TP. Seasonally, the contribution rates were 5.1%, 5.9%, 6.2%, and 5.7% in spring, summer, autumn, and winter, respectively. The area-averaged surface radiative forcing over the TP induced by BC from CWA ranged from −0.14 to −0.04 W m−2, with the largest and smallest negative radiative forcing occurring in autumn and winter, respectively. Affected by BC from CWA, the area-averaged surface temperature over the TP increased by 0.033 °C in summer, whereas it decreased by 0.002, 0.005, and 0.001 °C in spring, autumn and winter, respectively. In the atmosphere over the TP, the positive radiative forcing with values of 0.17, 0.20, 0.04, and 0.07 W m−2 were induced by BC from CWA in spring, summer, autumn, and winter, respectively. At the top of the atmosphere over the TP, the calculated radiative forcing associated with BC from CWA were 0.08, 0.14, −0.1, and 0.03 W m−2 in spring, summer, autumn, and winter, respectively. On an annual scale, the radiative forcing in the atmosphere and at the top of the atmosphere over the TP caused by BC from CWA were 0.12 W m−2 and 0.04 W m−2, respectively. This study enriched the theoretical connotation of transboundary transport of BC aerosols to the TP.

Anthropogenic aerosol effects on tropospheric circulation and sea surface temperature (1980–2020): separating the role of zonally asymmetric forcings

Abstract. Anthropogenic aerosols (AAs) induce global and regional tropospheric circulation adjustments due to the radiative energy perturbations. The overall cooling effects of AA, which mask a portion of global warming, have been the subject of many studies but still have large uncertainty. The interhemispheric contrast in AA forcing has also been demonstrated to induce a major shift in atmospheric circulation. However, the zonal redistribution of AA emissions since start of the 20th century, with a notable decline in the Western Hemisphere (North America and Europe) and a continuous increase in the Eastern Hemisphere (South Asia and East Asia), has received less attention. Here we utilize four sets of single-model initial-condition large-ensemble simulations with various combinations of external forcings to quantify the radiative and circulation responses due to the spatial redistribution of AA forcing during 1980–2020. In particular, we focus on the distinct climate responses due to fossil-fuel-related (FF) aerosols emitted from the Western Hemisphere (WH) versus the Eastern Hemisphere (EH). The zonal (west to east) redistribution of FF aerosol emission since the 1980s leads to a weakening negative radiative forcing over the WH mid-to-high latitudes and an enhancing negative radiative forcing over the EH at lower latitudes. Overall, the FF aerosol leads to a northward shift of the Hadley cell and an equatorward shift of the Northern Hemisphere (NH) jet stream. Here, two sets of regional FF simulations (Fix_EastFF1920 and Fix_WestFF1920) are performed to separate the roles of zonally asymmetric aerosol forcings. We find that the WH aerosol forcing, located in the extratropics, dominates the northward shift of the Hadley cell by inducing an interhemispheric imbalance in radiative forcing. On the other hand, the EH aerosol forcing, located closer to the tropics, dominates the equatorward shift of the NH jet stream. The consistent relationship between the jet stream shift and the top-of-atmosphere net solar flux (FSNTOA) gradient suggests that the latter serves as a rule-of-thumb guidance for the expected shift of the NH jet stream. The surface effect of EH aerosol forcing (mainly from low- to midlatitudes) is confined more locally and only induces weak warming over the northeastern Pacific and North Atlantic. In contrast, the WH aerosol reduction leads to a large-scale warming over NH mid-to-high latitudes that largely offsets the cooling over the northeastern Pacific due to EH aerosols. The simulated competing roles of regional aerosol forcings in driving atmospheric circulation and surface temperature responses during the recent decades highlight the importance of considering zonally asymmetric forcings (west to east) and also their meridional locations within the NH (tropical vs. extratropical).

Climate Response to Latitudinal and Altitudinal Distribution of Stratospheric Sulfate Aerosols

AbstractStratospheric aerosol injection is one of the most widely studied solar radiation modification methods to reduce some effects of anthropogenic warming. We perform idealized simulations to analyze climate response to different latitudinal and altitudinal distributions of additional stratospheric sulphate aerosols. We analyze climate response to both large volcanic‐size aerosols that are representative of sulphate aerosols which form when SO2 is injected into the stratosphere by major volcanic eruptions or by proposed stratospheric sulphate aerosol geoengineering option, and small background‐size aerosols which are present in the stratosphere during quiescent periods with no major volcanic eruptions. Our simulations show that under the climate state with doubled atmospheric CO2, for the same total mass, independent of size‐distributions, aerosols concentrated at higher latitudes produce less negative effective radiative forcing but larger surface cooling as a result of larger forcing efficacy. For the same latitudinal distribution, small background‐size aerosols added at lower altitudes cause larger surface cooling as a result of hygroscopic growth that increases scattering of sunlight. In contrast, large volcanic‐size aerosols added at lower altitudes cause smaller surface cooling than at higher altitudes as a result of enhanced stratospheric heating and associated water vapor feedback that enhance absorption of solar and terrestrial radiation. For both background‐size and volcanic‐size aerosols, asymmetric distributions about the equator induce substantial cross‐equatorial energy transport, causing the Intertropical Convergence Zone to move to the warmer hemisphere with less aerosol loading. Our study helps to understand the climate consequences of different spatial patterns and size distributions of stratospheric aerosol perturbation.

Tidal and Nontidal Marsh Restoration: A Trade‐Off Between Carbon Sequestration, Methane Emissions, and Soil Accretion

AbstractSupport for coastal wetland restoration projects that consider carbon (C) storage as a climate mitigation benefit is growing as coastal wetlands are sites of substantial C sequestration. However, the climate footprint of wetland restoration remains controversial as wetlands can also be large sources of methane (CH4). We quantify the vertical fluxes of C in restored fresh and oligohaline nontidal wetlands with managed hydrology and a tidal euhaline marsh in California's San Francisco Bay‐Delta. We combine the use of eddy covariance atmospheric flux measurements with 210Pb‐derived soil C accumulation rates to quantify the C sequestration efficiency of restored wetlands and their associated climate mitigation service. Nontidal managed wetlands were the most efficient in burying C on‐site, with soil C accumulation rates as high as their net atmospheric C uptake (−280 ± 90 and −350 ± 150 g C m−2 yr−1). In contrast, the restored tidal wetland exhibited lower C burial rates over decadal timescales (70 ± 19 g C m−2 yr−1) that accounted for ∼13%–23% of its annual C uptake, suggesting that the remaining fraction is exported via lateral hydrologic flux. From an ecosystem radiative balance perspective, the restored tidal wetland showed a > 10 times higher CO2‐sequestration to CH4‐emission ratio than the nontidal managed wetlands. Thus overall, tidal wetland restoration resulted in a negative radiative forcing (cooling) through increased soil C accumulation, while nontidal wetland restoration led to an early positive forcing (warming) through increased CH4 emissions potentially lasting between 2.1 ± 2.0 to 8 ± 4 decades.

Aerosol physico–optical–radiative characterization and classification during summer over Ny-Ålesund, Arctic

ABSTRACT With an aim to study the year-to-year changes in physical, optical, and radiative properties of polar aerosols, special observation campaigns were conducted during three consecutive summers of 2010, 2011 and 2012 over Ny-Ålesund (78.9°N, 11.9°E, 42 m AMSL), Arctic, employing a ground-based Multi-channel Solar-radiometer (MICROTOPS II Sun- photometer). In the present study, the columnar Aerosol Optical Depth (AOD) at λ = 500 nm and the Angstrom exponent (α) data sets are used to analyse the daily mean AOD and α by using a liner regression (best-fit) method. The results highlight that the columnar aerosol optical depth (mean AOD@500 nm for the study period) is 0.144 ± 0.05. Signature of secondary aerosol formation through marine sources based on variation in two wavelength AODs is also studied. These columnar AOD measurements over the study region are found to be in good agreement within experimental limitations with the synchronous MODIS satellite derived values. The 3 years’ α mean value is found to be 0.797 ± 0.328 within the range of 0.01–1.4. The frequency of occurrence and relative frequency histograms of α exhibited two modes centred at around 1.2 and 0.8. The primary and secondary modes reveal that the marine aerosols in the study region are mixed type, and they contribute about 44%, with the accumulation particles alone contributing about 31%. We found a close correspondence between aerosol optical properties and local meteorological conditions over the experimental location. The aerosol radiative forcing estimations revealed higher negative aerosol radiative forcing (indicating cooling) at Bottom-of-the-Atmosphere (BOA) and positive radiative forcing (indicating warming) in the Atmosphere (ATM) and Top-of-the-Atmosphere (TOA) during 2012 as compared to those obtained during 2010 and 2011, which is consistent with the observed AOD and α. The airmass back-trajectory model analysis also showed intrusion of anthropogenic aerosols from neighbouring regions into the study region, particularly during the year 2012.

Transverse acoustic spin and torque from pure spinning of objects

Acoustic spin has been recently explored for many applications. In particular, transverse acoustic spin was demonstrated for inhomogeneous acoustic fields. In this Letter, we show the emergence of acoustic spin and torque in rotating acoustic objects of the same physical properties as the surrounding, to single out the effects purely due to rotation. The spinning of a cylindrical column of air or water in the same medium possesses intrinsic spin angular momentum, and we study the torque and force it experiences in evanescent acoustic fields. The resulting discontinuity can thus scatter sound in unusual ways, including a negative radiation force, although it has no imaginary part in its parameters.

Non-diffractive acoustic beams produce negative radiation force in certain regions

A method of particle manipulation, one based on the force of acoustic radiation, has drawn wide attention. However, the real concept behind “acoustic tweezers”—negative acoustic radiation force (ARF)—has not been realized in experiments. In this paper, a prediction of a negative ARF generated by the non-diffractive acoustic beam is proposed. Its underlying physical mechanism is also analyzed in detail. On the basis of an analysis of energy flux density, the analytical region of negative radiation produced by the non-diffractive beam is solved completely. Forecast methods based on this solution are proposed that lay the foundation for realizing acoustic tweezers and offer the possibility of designing devices that produce negative ARFs. In addition, the negative propagation of acoustic beams in normal materials is realized, raising a possible alternative means to accomplish acoustic beam control.

The Dependence of Ship‐Polluted Marine Cloud Properties and Radiative Forcing on Background Drop Concentrations

AbstractMarine low clouds of the busy shipping lane in the southeast Atlantic during the springs of 2003–2015 were analyzed to study the dependence of their properties and radiative forcing on the background cloud drop concentrations (Nd‐bg). The overall average cloud radiative effect within the shipping lane was larger by only −1 Wm−2 compared to the adjacent clouds. However, this near‐zero averaged effect was composed of large negative cloud radiative forcing (CRF) for the cleanest (13%) of the cases with Nd‐bg < 50 cm−3, which was almost neutralized by a positive CRF for the 40% of the cases with Nd‐bg > 70 cm−3. A possible explanation for this positive forcing is the cloud burning effect of the black carbon from ship emissions. The negative forcing was composed of 45% for the cloud fraction (Cf) effect and 55% for the albedo effects, which included a small contribution of the liquid water path (LWP) effect. Positive Cf susceptibility to Nd‐core was at maximum for the lowest Nd‐bg and disappeared near 60 cm−3. The albedo susceptibility to Nd‐core reached its theoretical limit of 1/3 for constant LWP when Nd‐bg <40 cm−3, and diminishes to 0.2 when Nd‐bg = 60 cm−3. These susceptibilities represent cause and effect relationships because the differences of aerosols caused by the ship emissions vary at a much smaller scale than meteorology. Globally, under similar environmental conditions, nearly half of the area has Nd < 50 cm−3, thus possessing the indicated large susceptibility of negative radiative forcing to anthropogenic aerosols.