Aerosol Forcing Research Articles

Importance of ice nucleation and precipitation on climate with the Parameterization of Unified Microphysics Across Scales version 1 (PUMASv1)

Abstract. Cloud microphysics is critical for weather and climate prediction. In this work, we document updates and corrections to the cloud microphysical scheme used in the Community Earth System Model (CESM) and other models. These updates include a new nomenclature for the scheme, now called Parameterization of Unified Microphysics Across Scales (PUMAS), and the ability to run the scheme on graphics processing units (GPUs). The main science changes include refactoring an ice number limiter and associated changes to ice nucleation, adding vapor deposition onto snow, and introducing an implicit numerical treatment for sedimentation. We also detail the improvements in computational performance that can be achieved with GPU acceleration. We then show the impact of these scheme changes on the (a) mean state climate, (b) cloud feedback response to warming, and (c) aerosol forcing. We find that corrections are needed to the immersion freezing parameterization and that ice nucleation has important impacts on climate. We also find that the revised scheme produces less cloud liquid and ice but that this can be adjusted by changing the loss process for cloud liquid (autoconversion). Furthermore, there are few discernible effects of the PUMAS changes on cloud feedbacks but some reductions in the magnitude of aerosol–cloud interactions (ACIs). Small cloud feedback changes appear to be related to the implicit sedimentation scheme, with a number of factors affecting ACIs.

UKESM1.1: development and evaluation of an updated configuration of the UK Earth System Model

Abstract. Many Coupled Model Intercomparison Project phase 6 (CMIP6) models have exhibited a substantial cold bias in the global mean surface temperature (GMST) in the latter part of the 20th century. An overly strong negative aerosol forcing has been suggested as a leading contributor to this bias. An updated configuration of UK Earth System Model (UKESM) version 1, UKESM1.1, has been developed with the aim of reducing the historical cold bias in this model. Changes implemented include an improved representation of SO2 dry deposition, along with several other smaller modifications to the aerosol scheme and a retuning of some uncertain parameters of the fully coupled Earth system model. The Diagnostic, Evaluation and Characterization of Klima (DECK) experiments, a six-member historical ensemble and a subset of future scenario simulations are completed. In addition, the total anthropogenic effective radiative forcing (ERF), its components and the effective and transient climate sensitivities are also computed. The UKESM1.1 preindustrial climate is warmer than UKESM1 by up to 0.75 K, and a significant improvement in the historical GMST record is simulated, with the magnitude of the cold bias reduced by over 50 %. The warmer climate increases ocean heat uptake in the Northern Hemisphere oceans and reduces Arctic sea ice, which is in better agreement with observations. Changes to the aerosol and related cloud properties are a driver of the improved GMST simulation despite only a modest reduction in the magnitude of the negative aerosol ERF (which increases by +0.08 W m−2). The total anthropogenic ERF increases from 1.76 W m−2 in UKESM1 to 1.84 W m−2 in UKESM1.1. The effective climate sensitivity (5.27 K) and transient climate response (2.64 K) remain largely unchanged from UKESM1 (5.36 and 2.76 K respectively).

Likely accelerated weakening of Atlantic overturning circulation emerges in optimal salinity fingerprint

The long-term response of the Atlantic meridional overturning circulation (AMOC) to anthropogenic forcing has been difficult to detect from the short direct measurements available due to strong interdecadal variability. Here, we present observational and modeling evidence for a likely accelerated weakening of the AMOC since the 1980s under the combined forcing of anthropogenic greenhouse gases and aerosols. This likely accelerated AMOC weakening signal can be detected in the AMOC fingerprint of salinity pileup remotely in the South Atlantic, but not in the classic warming hole fingerprint locally in the North Atlantic, because the latter is contaminated by the “noise” of interdecadal variability. Our optimal salinity fingerprint retains much of the signal of the long-term AMOC trend response to anthropogenic forcing, while dynamically filtering out shorter climate variability. Given the ongoing anthropogenic forcing, our study indicates a potential further acceleration of AMOC weakening with associated climate impacts in the coming decades.

The influence of dust aerosols on solar radiation and near-surface temperature during a severe duststorm transport episode

In the west of China, a rarely seen black storm, with a high intensity of dust aerosols and a large area of influence, occurred from April 26–29, 2015, for the first time, after more than 30 years. Based on the regional climate model (RegCM version 4.6), combined with Moderate Resolution Imaging Spectroradiometer (MODIS) satellite retrieval, meteorological, and environmental data, this work presents the pollution situation, weather background, and backward trajectory during the black storm process. In addition, we analyzed the temporal–spatial distribution of aerosol optical depth (AOD) and the impacts of dust aerosols on solar radiation and near-surface temperature under this extreme weather condition. We discovered that this black storm process was caused by the surface cold high pressure and frontal transit under the background of the upper-air circulation of “two troughs and two ridges.” The pollutants primarily from Xinjiang and the Central Asia region, along with the airflow, entered northern Xinjiang almost simultaneously with the southwest airflow, piled up along the Tianshan Mountains, and then crossed the mountains into southern Xinjiang. In addition, the areas with high AOD were mostly in the desert regions and basins, whereas the low-value areas were mountainous areas with relatively high altitudes due to the effect of geographical and climatic conditions. The AOD from RegCM 4.6 was generally lower, unlike the MODIS AOD. Moreover, the dust aerosols from this black storm caused a significant decline in net short-wave radiation (NSR) both at the top of the atmosphere (TOA) and surface. The cooling effect was more significant in the region with high AOD. For the areas where the AOD was higher than 0.7, the net short-wave radiative forcing of dust aerosols (ADRF) at the surface was above −70 W•m−2; on the other hand, the cooling effect at the TOA was not as significant as that at the surface, with the ADRFTOA being only about a quarter of the ADRFSUR. The ADRFSUR could reduce the near-surface temperature, and the region with a large temperature drop corresponded to the high-value areas of ADRFSUR/AOD. During this black storm, the near-surface temperature response to dust aerosols reached 0.40°C–2.9°C with a significant temperature drop because of cold air.

Evaluation of E3SM land model snow simulations over the western United States

Abstract. Seasonal snow has crucial impacts on climate, ecosystems, and humans, but it is vulnerable to global warming. The land component (ELM) of the Energy Exascale Earth System Model (E3SM) mechanistically simulates snow processes from accumulation, canopy interception, compaction, and snow aging to melt. Although high-quality field measurements, remote sensing snow products, and data assimilation products with high spatio-temporal resolution are available, there has been no systematic evaluation of the snow properties and phenology in ELM. This study comprehensively evaluates ELM snow simulations over the western United States at 0.125∘ resolution during 2001–2019 using the Snow Telemetry (SNOTEL) in situ networks, MODIS remote sensing products (i.e., MCD43 surface albedo product), the spatially and temporally complete (STC) snow-covered area and grain size (MODSCAG) and MODIS dust and radiative forcing in snow (MODDRFS) products (STC-MODSCAG/STC-MODDRFS), and the snow property inversion from remote sensing (SPIReS) product and two data assimilation products of snow water equivalent and snow depth – i.e., University of Arizona (UA) and SNOw Data Assimilation System (SNODAS). Overall the ELM simulations are consistent with the benchmarking datasets and reproduce the spatio-temporal patterns, interannual variability, and elevation gradients for different snow properties including snow cover fraction (fsno), surface albedo (αsur) over snow cover regions, snow water equivalent (SWE), and snow depth (Dsno). However, there are large biases of fsno with dense forest cover and αsur in the Rocky Mountains and Sierra Nevada in winter, compared to the MODIS products. There are large discrepancies of snow albedo, snow grain size, and light-absorbing particle-induced snow albedo reduction between ELM and the MODIS products, attributed to uncertainties in the aerosol forcing data, snow aging processes in ELM, and remote sensing retrievals. Against UA and SNODAS, ELM has a mean bias of −20.7 mm (−35.9 %) and −20.4 mm (−35.5 %), respectively, for spring, and −13.8 mm (−27.8 %) and −10.2 mm (−22.2 %), respectively, for winter. ELM shows a relatively high correlation with SNOTEL SWE, with mean correlation coefficients of 0.69 but negative mean biases of −122.7 mm. Compared to the snow phenology of STC-MODSCAG and SPIReS, ELM shows delayed snow accumulation onset dates by 17.3 and 12.4 d, earlier snow end dates by 35.5 and 26.8 d, and shorter snow durations by 52.9 and 39.5 d, respectively. This study underscores the need for diagnosing model biases and improving ELM representations of snow properties and snow phenology in mountainous areas for more credible simulation and future projection of mountain snowpack.

Immediate and Long‐Lasting Impacts of the Mt. Pinatubo Eruption on Ocean Oxygen and Carbon Inventories

AbstractLarge volcanic eruptions drive significant climate perturbations through major anomalies in radiative fluxes and the resulting widespread cooling of the surface and upper ocean. Recent studies suggest that these eruptions also drive important variability in air‐sea carbon and oxygen fluxes. By simulating the Earth system using two initial‐condition large ensembles, with and without the aerosol forcing associated with the Mt. Pinatubo eruption in June 1991, we isolate the impact of this volcanic event on physical and biogeochemical properties of the ocean. The Mt. Pinatubo eruption forced significant anomalies in surface fluxes and the ocean interior inventories of heat, oxygen, and carbon. Pinatubo‐driven changes persist for multiple years in the upper ocean and permanently modify the ocean's heat, oxygen, and carbon inventories. Positive anomalies in oxygen concentrations emerge immediately post‐eruption and penetrate into the deep ocean. In contrast, carbon anomalies intensify in the upper ocean over several years post‐eruption, and are largely confined to the upper 150 m. In the tropics and northern high latitudes, the change in oxygen is dominated by surface cooling and subsequent ventilation to mid‐depths, while the carbon anomaly is associated with solubility changes and eruption‐generated El Niño—Southern Oscillation variability. We do not find significant impact of Pinatubo on oxygen or carbon fluxes in the Southern Ocean; but this may be due to Southern Hemisphere aerosol forcing being underestimated in Community Earth System Model 1 simulations.

Uncertainty in Simulating Twentieth Century West African Precipitation Trends: The Role of Anthropogenic Aerosol Emissions

AbstractAnthropogenic aerosol emissions from North America and Europe have strong effects on the decadal variability of the West African monsoon (WAM). Anthropogenic aerosol effective radiative forcing is model dependent, but the impact of such uncertainty on the simulation of long‐term WAM variability is unknown. We use an ensemble of simulations with HadGEM3‐GC3.1 that span the most recent estimates in simulated anthropogenic aerosol effective radiative forcing. We show that uncertainty in anthropogenic aerosol radiative forcing leads to significant uncertainty at simulating multi‐decadal trends in West African precipitation. At the large scale, larger forcing leads to a larger decrease in the interhemispheric temperature gradients, in temperature over both the North Atlantic Ocean and northern Sahara. There are also differences in dynamic changes specific to the WAM (locations of the Saharan heat low and African Easterly Jet, of the strength of the West African westerly jet, and of African Easterly Wave activity). We also assess effects on monsoon precipitation characteristics and temperature. We show that larger aerosol forcing results in a decrease of the number of rainy days and of heavy and extreme precipitation events and warm spells. However, simulated changes in onset and demise dates do not appear to be sensitive to the magnitude of aerosol forcing. Our results demonstrate the importance of reducing the uncertainty in anthropogenic aerosol forcing for understanding and predicting multi‐decadal variability in the WAM.

Reconstructing volcanic radiative forcing since 1990, using a comprehensive emission inventory and spatially resolved sulfur injections from satellite data in a chemistry-climate model

Abstract. This paper presents model simulations of stratospheric aerosols with a focus on explosive volcanic eruptions. Using various (occultation and limb-based) satellite instruments, providing vertical profiles of sulfur dioxide (SO2) and aerosol extinction, we characterized the chemical and radiative influence of volcanic aerosols for the period between 1990 and 2019. We established an improved and extended volcanic SO2 emission inventory that includes more than 500 explosive volcanic eruptions reaching the upper troposphere and the stratosphere. Each perturbation identified was derived from the satellite data and incorporated as a three-dimensional SO2 plume into a chemistry-climate model without the need for additional assumptions about altitude distribution and eruption duration as needed for a “point source” approach. The simultaneous measurements of SO2 and aerosol extinction by up to four satellite instruments enabled a reliable conversion of extinction measurements into injected SO2. In the chemistry-climate model, the SO2 from each individual plume was converted into aerosol particles and their optical properties were determined. Furthermore, the aerosol optical depth (AOD) and the instantaneous radiative forcing on climate were calculated online. Combined with model improvements, the results of the simulations are consistent with the observations of the various satellites. Slight deviations between the observations and model simulations were found for the large volcanic eruption of Pinatubo in 1991 and cases where simultaneous satellite observations were not unique or too sparse. Weak- and medium-strength volcanic eruptions captured in satellite data and the Smithsonian database typically inject about 10 to 50 kt SO2 directly into the upper troposphere/lower stratosphere (UTLS) region or the sulfur species are transported via convection and advection. Our results confirm that these relatively minor eruptions, which occur quite frequently, can nevertheless contribute to the stratospheric aerosol layer and are relevant for the Earth's radiation budget. These minor eruptions cause a total global instantaneous radiative forcing of the order of −0.1 W m−2 at the top of the atmosphere (TOA) compared to a background stratospheric aerosol forcing of about −0.04 W m−2. Medium-strength eruptions injecting about 400 kt SO2 into the stratosphere or accumulation of consecutive smaller eruptions can lead to a total instantaneous forcing of about −0.3 W m−2. We show that it is critical to include the contribution of the extratropical lowermost stratospheric aerosol in the forcing calculations.

Subsurface Ocean Temperature Responses to the Anthropogenic Aerosol Forcing in the North Pacific

AbstractSeparating the climate response to external forcing from internal climate variability is a key challenge. While most previous studies have focused on surface responses, here we examine zonal‐mean patterns of North Pacific subsurface temperature responses. In particular, the changes since 1950 driven by anthropogenic aerosol emissions are found by using a pattern recognition method. Based on the single‐forcing large‐ensemble simulations from two models, we show that aerosol forcing caused a nonmonotonic temporal response and a characteristic zonal‐mean pattern within North Pacific, which is distinct from the pattern associated with internal variability. The aerosol‐forced pattern with the nonmonotonic temporal feature shows a substantial temperature change in subpolar regions and a reversed change on the southern flank of the subtropical gyre. A similar characteristic pattern and nonmonotonic time evolution are extracted from the subsurface observations, which likely reflect the subsurface responses to the aerosol forcing, although differences exist with the simulated responses.

Identification of dust aerosols, their sources, and the effect of soil moisture in Central Asia

Dust aerosols in Central Asia are an important factor in global climate change and attribution studies. Identifying the source of dust in Central Asia is crucial for understanding the ecological environment and climate, locally and globally. In this study, daily dust aerosol data were calculated and extracted for Central Asia from 2003 to 2018. The multi-year trends of dust aerosols were analyzed, dust sources were identified, the characteristics of dust aerosols in dust sources were analyzed, and the influence of soil moisture on sand initiation was explored. The results show that there are distinct seasonal characteristics in the spatial distribution of dust aerosols in Central Asia. The proportion of the area in the zone of high dust aerosols was the greatest in spring. Nearly half of the dust aerosol areas exhibited an increasing trend. A high incidence of dust sources was mainly distributed in the southern Xinjiang region. The trend of change in the dust area first increased and then decreased. With the increase in soil moisture under different wind speed conditions, the aerosols from dust sources all showed an exponentially decreasing trend, and the increase in soil moisture led to an increase in the wind speed threshold of sand initiation. This study provides basic data support for the study of dust aerosols, identifies dust sources, and provides a basis for studying the radiative forcing and climate effects of dust aerosols in Central Asia.

Underestimated Passive Volcanic Sulfur Degassing Implies Overestimated Anthropogenic Aerosol Forcing

AbstractThe Arctic is warming at almost four times the global rate. An estimated sixty percent of greenhouse‐gas‐induced Arctic warming has been offset by anthropogenic aerosols, but the contribution of aerosols to radiative forcing (RF) represents the largest uncertainty in estimating total RF, largely due to unknown preindustrial aerosol abundance. Here, sulfur isotope measurements in a Greenland ice core show that passive volcanic degassing contributes up to 66 ± 10% of preindustrial ice core sulfate in years without major eruptions. A state‐of‐the‐art model indicates passive volcanic sulfur emissions influencing the Arctic are underestimated by up to a factor of three, possibly because many volcanic inventories do not include hydrogen sulfide emissions. Higher preindustrial volcanic sulfur emissions reduce modeled anthropogenic Arctic aerosol cooling by up to a factor of two (+0.11 to +0.29 W m−2), suggesting that underestimating passive volcanic sulfur emissions has significant implications for anthropogenic‐induced Arctic climate change.

Simulation of radiative forcing of smoke aerosol in the Arctic using measurements in the large aerosol chamber of Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences

Simulation of radiative forcing of smoke aerosol in the Arctic using measurements in the large aerosol chamber of Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences

Effects of hygroscopicity on aerosol optical properties and direct radiative forcing in Beijing: Based on two-year observations.

The influence of relative humidity on aerosol properties and the direct radiative forcing of PM10 and PM1 were investigated in Beijing from January 2018 to December 2019. The annual mean scattering hygroscopic growth factor at RH = 80 % [f(80 %)] of PM10 and PM1 were 1.60 ± 0.24 and 1.58 ± 0.22, respectively. The variation of aerosol hygroscopic growth factors of PM10 and PM1 aerosols was similar, which is mainly due to the fact that aerosol scattering in Beijing is dominated by fine particles. The seasonal mean f(80 %) of PM10 from spring to winter were 1.66 ± 0.23, 1.71 ± 0.25, 1.51 ± 0.20, 1.49 ± 0.16, respectively, which were higher in spring and summer, and lower in autumn and winter. The diurnal variation of f(80 %) was relatively higher from 12:00 to 18:00, which could be related to the formation of secondary aerosols by photochemical reactions. f(80 %) shows a strong positive relationship with both the scattering Angström exponent (SAE) and the single scattering albedo (ω0) under dry conditions; therefore, the scattering hygroscopic growth factor could be estimated using these two parameters. The upscatter fraction (β) and single scattering albedo, which are the key aerosol optical properties for the calculation of direct radiative forcing, are also RH-dependent. As RH increases, the upscatter fraction (backscatter fraction) decreases and ω0 increases. The aerosol radiative forcing at RH 80 % was 1.48 times as that in the dry state. The sensitivity experiment showed that the variation in the scattering coefficient with relative humidity had the greatest influence on radiation forcing, followed by β and ω0. The seasonal variation of ΔF(80 %)/ΔF(dry) coincides with that of the aerosol hygroscopic growth factor. Our study suggests that understanding the influence of relative humidity on aerosol properties and direct radiative forcing is important for accurately estimating the radiative forcing of aerosols.

Climate Feedback to Stratospheric Aerosol Forcing: The Key Role of the Pattern Effect

Abstract Volcanic aerosol forcing has previously been found to cause a weak global mean temperature response, as compared with CO2 radiative forcing of equal magnitude: its efficacy is supposedly low, but for reasons that are not fully understood. To investigate this, we perform idealized, time-invariant stratospheric sulfate aerosol forcing simulations with the MPI-ESM-1.2 and compare them with 0.5 × CO2 and 2 × CO2 runs. While the early decades of the aerosol forcing simulations are characterized by strong negative feedback (i.e., low efficacy), the feedback weakens on the decadal to centennial time scale. Although this effect is qualitatively also found in CO2-warming simulations, it is more pronounced for stratospheric aerosol forcing. The strong early and weak late cooling feedbacks compensate, leading to an equilibrium efficacy of approximately 1 in all simulations. The 0.5 × CO2 cooling simulations also exhibit strong feedback changes over time, albeit less than in the idealized aerosol forcing simulations. This suggests that the underlying cause for the feedback change is not exclusively specific to aerosol forcing. One critical region for the feedback differences between simulations with negative and positive radiative forcing is the tropical Indo-Pacific warm-pool region (30°S–30°N, 50°E–160°W). In the first decades of cooling, the temperature change in this region is stronger than the global average, whereas it is stronger outside it for 2 × CO2 warming. In cooling scenarios, this leads to an enhanced activation of the warm-pool region’s strongly negative lapse-rate feedback. Significance Statement Large volcanic eruptions can enhance the scattering aerosol layer in the stratosphere, which leads to a global cooling for a few years. Surprisingly, Earth has been found to cool less from radiative flux perturbations from stratospheric aerosol forcing, in comparison with how much it warms as a result of increases in CO2 concentration. We find that specific surface temperature change patterns after volcanic eruptions cause this effect. The temperature change in the tropical Indian and western Pacific Ocean determines how much global temperature change is needed to regain radiative equilibrium. Our findings contribute to understanding the climate response to volcanic eruptions and are relevant for understanding the mechanisms of climate change due to changes in CO2 concentration.

Relative Roles of Land and Ocean Cooling in Triggering an El Niño Following Tropical Volcanic Eruptions

AbstractPaleoclimate evidence suggests that tropical volcanic eruptions could increase the likelihood of El Niño occurrence. Previous numerical model studies with zonally uniform volcanic aerosol forcing suggested the roles of land cooling‐induced monsoon suppression and ocean cooling‐induced air‐sea interaction in triggering an El Niño following the eruption. Here, we perform targeted sensitivity experiments by confining aerosol forcing over land or ocean only in the fully coupled Community Earth System Model to assess relative roles of these land and ocean cooling. Our results indicate that volcanic aerosol over land, especially over the large landmass of tropical Africa, is more effective in exciting an El Niño than over ocean. The suppressed African monsoon excites Kelvin‐wave westerly wind anomalies over the tropical central Pacific, triggering an El Niño through the Bjerknes feedback. Under the uniform ocean aerosol forcing, the Indian Ocean's fast dipole response induces a slow western Pacific El Niño‐like cooling in the second year.

The Turning Point of the Aerosol Era

AbstractOver the CMIP6 historical period (1850–2014), aerosols provided the largest negative forcing compared to all other climate forcings via their ability to absorb or scatter solar radiation and alter clouds. Aerosols played an important role in counterbalancing warming by greenhouse gases (GHGs). Here we study aerosol forcing trends in the CMIP6 simulations of the NASA Goddard Institute for Space Studies (GISS) ocean‐atmosphere ModelE version 2.1 (GISS‐E2.1‐G) using a fully coupled atmospheric composition configuration, including interactive gas‐phase chemistry, and either an aerosol microphysical (MATRIX) or a mass‐based aerosol (OMA) module. Simulations of the CMIP6 historical period are analyzed as well as four Shared Socioeconomic Pathway (SSP) future scenarios for 2015–2100: SSP1‐2.6, SSP2‐4.5, SSP3‐7.0, and SSP5‐8.5. The main conclusion of this study is that aerosol forcing in the GISS model has reached its turning point, switching from globally increasing to a decreasing trend in the first decade of the 21st century. This result is robust, independent of which aerosol module or SSP scenario is used. Non‐linear aerosol‐cloud interactions dominate as a forcing agent over aerosol‐radiation interactions. Aerosols' ability to counterbalance GHG forcing on the global scale is today at a level comparable to that at the beginning of the last century. In the 1980s, the decade of largest global aerosol loads, aerosols balanced up to 80% of GHG forcing. As a consequence, global warming of the last decades, which is primarily driven by greenhouse gases, has been augmented by the effect of decreasing aerosol cooling in our model. By the end of this century, following the SSP scenarios, aerosols will only counterbalance 0%–20% of GHG forcing, depending on model and on scenario.

Aerosol effects on clouds are concealed by natural cloud heterogeneity and satellite retrieval errors

One major source of uncertainty in the cloud-mediated aerosol forcing arises from the magnitude of the cloud liquid water path (LWP) adjustment to aerosol-cloud interactions, which is poorly constrained by observations. Many of the recent satellite-based studies have observed a decreasing LWP as a function of cloud droplet number concentration (CDNC) as the dominating behavior. Estimating the LWP response to the CDNC changes is a complex task since various confounding factors need to be isolated. However, an important aspect has not been sufficiently considered: the propagation of natural spatial variability and errors in satellite retrievals of cloud optical depth and cloud effective radius to estimates of CDNC and LWP. Here we use satellite and simulated measurements to demonstrate that, because of this propagation, even a positive LWP adjustment is likely to be misinterpreted as negative. This biasing effect therefore leads to an underestimate of the aerosol-cloud-climate cooling and must be properly considered in future studies.

Ocean Emission Pathway and Secondary Formation Mechanism of Aminiums Over the Chinese Marginal Sea

AbstractAmines/aminiums are recognized for their importance in new particle formation, organic nitrogen cycle, and aerosol forcings. Among the complex sources, the ocean origins and generation pathways of aminiums are less understood. During a cruise campaign over the Chinese marginal sea (Bohai Sea), five low‐molecular‐weight aminiums were measured. The strong anthropogenic period (terrestrial transport) and the weak anthropogenic period (dust, clean, and sea fog periods) were identified based on the traceable aerosol chemical species, air masses transport patterns, and marine chlorophyll‐a. Inconsistent spatiotemporal variations of various aminiums were observed. Monomethylaminium (MMAH+), dimethylaminium (DMAH+), ethylaminium (EAH+), and diethylaminium (DEAH+) were more enriched during the weak anthropogenic period than the strong anthropogenic period and especially peaked during the sea fog period. The significant correlation between the four aminiums and methanesulfonic acid suggested their secondary formation pathways. Through further exploring the governing factors, the acid–base neutralization via the aqueous phase processing was identified as a critical process for the secondary transformation of aminiums. In contrast, trimethylaminium (TMAH+) showed moderate correlation with Na+ in the total suspended particles only, suggesting its primary marine sources via enrichment in the sea‐spray aerosol through bubble bursting. Based on the step‐wise correlation analysis, the sea surface organic nitrogenous reservoir was found as the dominant contributor to the measured aminiums over the Bohai Sea, while the role of marine phytoplankton emission in the formation of aminiums was limited.

Large uncertainty in future warming due to aerosol forcing

Large uncertainty in future warming due to aerosol forcing

Assessing effective radiative forcing from aerosol–cloud interactions over the global ocean

How clouds respond to anthropogenic sulfate aerosols is one of the largest sources of uncertainty in the radiative forcing of climate over the industrial era. This uncertainty limits our ability to predict equilibrium climate sensitivity (ECS)-the equilibrium global warming following a doubling of atmospheric CO2. Here, we use satellite observations to quantify relationships between sulfate aerosols and low-level clouds while carefully controlling for meteorology. We then combine the relationships with estimates of the change in sulfate concentration since about 1850 to constrain the associated radiative forcing. We estimate that the cloud-mediated radiative forcing from anthropogenic sulfate aerosols is [Formula: see text] W m-2 over the global ocean (95% confidence). This constraint implies that ECS is likely between 2.9 and 4.5 K (66% confidence). Our results indicate that aerosol forcing is less uncertain and ECS is probably larger than the ranges proposed by recent climate assessments.