Effects Of Anthropogenic Aerosols Research Articles

Understanding aerosol–climate–ecosystem interactions and the implications for terrestrial carbon sink using the Community Earth System Model

Anthropogenic aerosols exert significant effects on the terrestrial ecosystem carbon cycle by causing climate change and altering surface hydrological conditions through radiation and cloud interaction, but such aerosol–climate–ecosystem interactions are difficult to quantify using currently available observations. Here, we used the Community Earth System Model (CESM, version 2.1.3) to quantify the effects of anthropogenic aerosols on terrestrial ecosystem carbon fluxes during the period from 1980 to 2014. The results found that the response of temperature and radiation to anthropogenic aerosols in different regions shows considerable variability in the CESM. Anthropogenic aerosols caused a 0.26 Pg C (0.43%) reduction in cumulative net biome production (NBP) during the period 1980-2014. In Southeast China (SEC) and East United States (EUS), anthropogenic aerosols caused a net reduction in NBP of 0.27 and 0.09 Pg C, respectively. In India (IND) and Europe (EUR), anthropogenic aerosols caused an increase of NBP by 0.07 and 0.14 Pg C, respectively. In addition, the decrease of the carbon sink was mainly dominated by changes in TER and others (40.6%), and the increase of the carbon sink was mainly dominated by changes in gross primary production (GPP) (38.9%). We also found that temperature is the dominant climate variable controlling the carbon cycle with a relative contribution of about 38%. Finally, the carbon flux response at the global scale is significantly lower than at the regional scale, which in turn highlights the importance of the assessment of the regional carbon cycle.

Effects of anthropogenic aerosols on the evolution of Indian summer monsoon

The NCAR Community Earth System Model is used to study the influences of anthropogenic aerosols on the Indian summer monsoon (ISM). We perform two sets of 30-year simulations subject to the prescribed perpetual SST annual cycle. One is triggered by the year 2000 climatology anthropogenic aerosol emissions data over the Indian Peninsula (referred to as AERO), and the other one is by the year 1850 (referred to as CTL). Only aerosol direct effects are included in the experiments. In our results, the transition of ISM in AERO relative to the CTL exhibits a similar ensemble-mean onset date with a larger spread, and more abrupt onset in late spring, and an earlier but more gradual withdrawal in early fall. The aerosols-induced circulation changes feature an upward motion over the northeastern Indian Peninsula and strengthened anticyclonic circulation over the Arabia Sea in the pre-monsoon season, and a northward shift of monsoon flow in the developed monsoon period along with strengthened local meridional circulation over northern India. The strengthened anticyclonic circulation over Arabia Sea caused a 16% increase in natural dust transport from the Middle East in the pre-monsoon season. The elevated aerosol heating over Tibet causes stronger ascending motion in the pre-monsoon period that leads to earlier and more abrupt ISM onset. The earlier monsoon withdrawal is attributed to the aerosol-induced anticyclonic flow within 10°–25°N and cyclonic flow within 0°–10°N over eastern India and Bay of Bengal that resemble the ISM seasonal transition in September.

On the importance of the model representation of organic aerosol in simulations of the direct radiative effect of Siberian biomass burning aerosol in the eastern Arctic

Shortcomings and uncertainties in the model representation of atmospheric transformations (the aging) of organic aerosol (OA) have long been identified as one of the potential sources of considerable uncertainty in OA simulations with both global and regional models. However, the impact of this uncertainty on predictions of radiative and climate effects of both anthropogenic and biomass burning (BB) aerosol yet needs to be understood. This study examines the importance of the model representation of OA for simulating the direct radiative effect (DRE) of Siberian BB aerosol in the eastern Arctic. We employ a regional coupled chemistry-meteorology model and a global fire emission database to simulate the optical properties and DRE of BB aerosol emitted from intense Siberian fires in July 2016 and compare the DRE estimates that were obtained using two alternative representations of Siberian BB OA. One of them is a “default” OA representation that predicts very little secondary OA (SOA), and another involves a simple original OA parameterization that has been developed previously within the volatility basis set (VBS) framework and features a strong production of SOA. The simulations of the aerosol optical properties are evaluated against satellite observations of the aerosol optical depth (AOD) in Siberia and the Arctic as well as against values of the single scattering albedo derived from in situ observations of the aerosol absorption and scattering coefficients at four Arctic sites. While the simulations with the default OA representation are found to strongly underestimate AOD both in Siberia and the eastern Arctic, the use of the VBS parameterization considerably improves the agreement between the AOD simulations and observations in both regions. Simulations of the single scattering albedo are found to be overall rather adequate with both representations. Differences in the OA representations are found to result in major differences in the estimates of the DRE of Siberian BB aerosol in the eastern Arctic. Specifically, although the simulations with both representations predict that the DRE is predominantly negative at the top of the atmosphere (TOA), the magnitude of the mean DRE is found to be more than twice as large (6.0 W m−2) with the VBS parameterization than with the default OA representation (2.8 W m−2). An even larger difference (by a factor of 3.5) is found between the estimates of the DRE over the snow- or ice-covered areas. The different treatments of the BB OA evolution are associated also with considerably different contributions of black and brown carbon to the DRE estimates. Overall, our results indicate that model estimates of the DRE of Siberian BB aerosol in the eastern Arctic are strongly sensitive to the assumptions regarding the evolution of OA in Siberian BB plumes and that the SOA formation in these plumes is one of the major factors determining the magnitude of the radiative effects of Siberian BB aerosol in the real atmosphere.

Evaluation and attribution of historical surface air temperature changes over China simulated by coupled and uncoupled models

The increase in the average surface air temperature anomaly (SATA) over China is higher than the global average. However, the accurate simulation and attribution of regional SATA evolution remain challenging for current global climate models. This study simulates historical SATA variations over China using the coupled (FGOALS-g3) and uncoupled (atmospheric component, GAMIL3) models and examines their possible causes. Results show that both models reproduce the historical SATA variation with higher correlation coefficients (0.735 and 0.782) than many global climate models (0.25–0.56), although they overestimate or underestimate the changes of SATA to some extent in different periods. The results show that the cooling trend during 1941–1970 is well simulated with the coupled model while poorly presented with the uncoupled model; the coupled simulations particularly produce stronger long-term trends than the uncoupled ones during 1870–2014 considering full interaction among the atmosphere, ocean, and sea ice. In contrast, the uncoupled simulations reproduce better decadal and multi-decadal SATA variations owing to the constraints of the observed sea surface temperature (SST), such as the Atlantic multidecadal oscillation, and sea ice cover. Using Detection and Attribution Model Intercomparison Project (DAMIP) experiments, we found that the warming in the early 20th century and the recent 50 years is mainly driven by natural forcings and greenhouse gases (GHGs), whereas the cooling during 1941–1970 is caused by natural factors and anthropogenic aerosols. The cooling effects of anthropogenic aerosols are mainly attributed from the indirect SST-mediated responses through the atmosphere-ocean interactions in the coupled model.

Pollution tracker: Finding industrial sources of aerosol emission in satellite imagery

Abstract The effects of anthropogenic aerosol, solid or liquid particles suspended in the air, are the biggest contributor to uncertainty in current climate perturbations. Heavy industry sites, such as coal power plants and steel manufacturers, large sources of greenhouse gases, also emit large amounts of aerosol in a small area. This makes them ideal places to study aerosol interactions with radiation and clouds. However, existing data sets of heavy industry locations are either not public, or suffer from reporting gaps. Here, we develop a supervised deep learning algorithm to detect unreported industry sites in high-resolution satellite data, using the existing data sets for training. For the pipeline to be viable at global scale, we employ a two-step approach. The first step uses 10 m resolution data, which is scanned for potential industry sites, before using 1.2 m resolution images to confirm or reject detections. On held-out test data, the models perform well, with the lower resolution one reaching up to 94% accuracy. Deployed to a large test region, the first stage model yields many false positive detections. The second stage, higher resolution model shows promising results at filtering these out, while keeping the true positives, improving the precision to 42% overall, so that human review becomes feasible. In the deployment area, we find five new heavy industry sites which were not in the training data. This demonstrates that the approach can be used to complement existing data sets of heavy industry sites.

Effects of anthropogenic aerosols and sea salt aerosols during a summer precipitation event in the Yangtze River Delta

Currently, we know little about the effects of anthropogenic aerosols (AAs) and sea salt aerosols (SSAs) on heavy precipitation over the densely populated coastal areas. In this view, we developed a local aerosol activation parametrization scheme which conforms to the Yangtze River Delta (YRD) characteristics using cloud condensation nucleus (CCN) with supersaturation at 0.4 (CCN0.4) and aerosol optical depth (AOD) data at 550 nm (AOD550) to improve the model performance. Meanwhile, we simulated a heavy precipitation over the YRD during early summer using the WRF-Chem model (version 4.1) with the Morrison microphysics scheme. Increased AAs and SSAs presented opposite responses in droplets concentrations and precipitation intensity. The increased AAs increased the aerosol concentrations, leading to more but smaller droplets; and the cloud droplets mixing ratio Qc was slightly increased by 17.7% due to the weakened auto-conversion from droplets to raindrops, while the rain mixing ratio Qr was decreased by about 5.7%. On the contrary, presence of SSAs increased the effective (EFC) radius of cloud droplets because of the larger particles. Along with increased AAs, due to the influences of decreased Qr and smaller droplets, probability density function (PDF) of the hourly surface rain rates shifted to the smaller values (< 0.5 mm/h), which was caused by lowered latent heat release resulting from the enhanced rain evaporative cooling and weakened condensation rate, weakening the convective intensity and updrafts. Meanwhile, decreased updrafts resulted in fewer cloud droplets being transformed to the upper air, less accretion of droplets by ice crystal and snow, and reduced ice mixing ratio Qi and snow mixing ratio Qs. The averaged surface precipitation accumulated for 24 h decreased by about 7.2%, and the intensity of radar reflectivity was decreased by about 7.0%. In contrast, presence of SSAs enhanced the auto-conversion droplets and weakened the evaporation of raindrops, thus increasing the Qr. In this case, the 24-h accumulated averaged surface rainfall was increased by about 30.1% with the larger radar reflectivity and increased area of large reflectivity values (> 20 dbz). The latent heat release and related microphysical processes were enhanced due to the changes of conversion rate, which, together with the increased Qr and larger droplets, strengthened the precipitation, increased the incidence of heavier precipitation (> 2 mm/h), and enhanced the ice-phase microphysical processes.

Equilibrium climate sensitivity increases with aerosol concentration due to changes in precipitation efficiency

Abstract. How Earth's climate reacts to anthropogenic forcing is one of the most burning questions faced by today's scientific community. A leading source of uncertainty in estimating this sensitivity is related to the response of clouds. Under the canonical climate-change perspective of forcings and feedbacks, the effect of anthropogenic aerosols on clouds is categorized under the forcing component, while the modifications of the radiative properties of clouds due to climate change are considered in the feedback component. Each of these components contributes the largest portion of uncertainty to its relevant category and is largely studied separately from the other. In this paper, using idealized cloud-resolving radiative–convective-equilibrium simulations, with a slab ocean model, we show that aerosol–cloud interactions could affect cloud feedback. Specifically, we show that equilibrium climate sensitivity increases under high aerosol concentration due to an increase in the short-wave cloud feedback. The short-wave cloud feedback is enhanced under high-aerosol conditions due to a stronger increase in the precipitation efficiency with warming, which can be explained by higher sensitivity of the droplet size and the cloud water content to the CO2 concentration rise. These results indicate a possible connection between cloud feedback and aerosol–cloud interactions.

Interaction between different mixing aerosol direct effects and East Asian summer monsoon

Aerosol mixtures, which are still unclear in current knowledge, may cause large uncertainties in aerosol climate effect assessments. To better understand this research gap, a well-developed online coupled regional climate-chemistry model is employed here to investigate the influences of different aerosol mixing states on the direct interactions between aerosols and the East Asian summer monsoon (EASM). The results show that anthropogenic aerosols have high-level loadings with heterogeneous spatial distributions in East Asia. Black carbon aerosol loading accounts for more than 13% of the totals in this region in summer. Thus, different aerosol mixing states cause very different aerosol single scattering albedos, with a variation of 0.27 in East Asia in summer. Consequently, the sign of the aerosol instantaneous direct radiative forcing at the top of the atmosphere is changed, varying from − 0.95 to + 1.50 W/m2 with increasing internal mixing aerosols. The influence of aerosol mixtures on regional climate responses seems to be weaker. The EASM circulation can be enhanced due to the warming effect of anthropogenic aerosols in the lower atmosphere, which further induces considerable aerosol accumulation associated with dynamic field anomaly, decrease in rainfall and so on, despite aerosol mixtures. However, this interaction between aerosols and the EASM will become more obvious if the aerosols are more mixed internally. Additionally, the differences in aerosol-induced EASM anomalies during the strongest and weakest monsoon index years are highly determined by the aerosol mixing states. The results here may further help us better address the environmental and climate change issues in East Asia.

Radiative effects and feedbacks of anthropogenic aerosols on boundary layer meteorology and fine particulate matter during the COVID-19 lockdown over China

The COVID-19 epidemic has exerted significant impacts on human health, social and economic activities, air quality and atmospheric chemistry, and potentially on climate change. In this study, an online coupled regional climate–chemistry–aerosol model (RIEMS-Chem) was applied to explore the direct, indirect, and feedback effects of anthropogenic aerosols on radiation, boundary layer meteorology, and fine particulate matter during the COVID-19 lockdown period from 23 January to 8 April 2020 over China. Model performance was validated against a variety of observations for meteorological variables, PM2.5 and its chemical components, aerosol optical properties, as well as shortwave radiation flux, which demonstrated that RIEMS-Chem was able to reproduce the spatial distribution and temporal variation of the above variables reasonably well. During the study period, direct radiative effect (DRE) of anthropogenic aerosols was stronger than indirect radiative effect (IRE) in most regions north of the Yangtze River, whereas IRE dominated over DRE in the Yangtze River regions and South China. In North China, DRE induced larger changes in meteorology and PM2.5 than those induced by IRE, whereas in South China, the changes by IRE were remarkably larger than those by DRE. Emission reduction alone during the COVID-19 lockdown reduced PM2.5 concentration by approximately 32 % on average over East China. As a result, DRE at the surface was weakened by 15 %, whereas IRE changed little over East China, leading to a decrease in total radiative effect (TRE) by approximately 7 % in terms of domain average. The DRE-induced changes in meteorology and PM2.5 were weakened due to emission reduction, whereas the IRE-induced changes were almost the same between the cases with and without emission reductions. By aerosol radiative and feedback effects, the COVID-19 emission reductions resulted in 0.06 °C and 0.04 °C surface warming, 1.6 and 4.0 μg m−3 PM2.5 decrease, 0.4 and 1.3 mm precipitation increase during the lockdown period in 2020 in terms of domain average over North China and South China, respectively, whereas the lockdown caused negligible changes on average over East Asia.

Effect of Anthropogenic Aerosols on Wheat Production in the Eastern Indo-Gangetic Plain

The Indo Gangetic Plain (IGP) is a food basket of South Asia and is considered a hotspot for air pollution due to persistently high emissions of anthropogenic aerosols. High levels of aerosols in the IGP not only affect the health of people but also the health of the natural system and the climate of the region. Aerosol effects on crop production in the IGP is an emerging area of interest for policymakers and the scientific community due to their possible effect on the food security and livelihood of millions of people in the region. To investigate the effect of anthropogenic aerosols on wheat production in the eastern IGP, we used a calibrated and validated Agricultural Production System Simulator (APSIM) model at nodes in Bangladesh, India and Nepal, 2015–2017. The effects of anthropogenic aerosols on wheat production were examined by running the APSIM model under three conditions: firstly, the condition with anthropogenic aerosols, using the observed meteorological data; secondly, the condition without anthropogenic aerosols, considering only the radiative effect of anthropogenic aerosols (adding the reduced radiation due to anthropogenic aerosols on the observed data); thirdly, the condition without anthropogenic aerosols, considering the radiation as well as temperature effects (by adding the reduced solar radiation and temperature due to anthropogenic aerosols on the observed data). The study revealed that, on average, anthropogenic aerosols reduced the wheat grain yield, biomass yield, and crop evapotranspiration by 11.2–13.5%, 21.2–22%, and 13.5–15%, respectively, when considering the 2015–2017 seasons at the target sites of eastern IGP. The study also showed an average reduction of more than 3.2 kg per capita per annum of wheat production in the eastern IGP due to anthropogenic aerosols, which has a substantial effect on food security in the region. Moreover, the loss of wheat grain yield due to anthropogenic aerosols in the eastern IGP is estimated to be more than 300 million USD per annum during the study period, which indicates a significant effect of anthropogenic aerosols on wheat production in the eastern IGP.

Changes in aerosol loading before, during and after the COVID-19 pandemic outbreak in China: Effects of anthropogenic and natural aerosol

Anthropogenic emissions reduced sharply in the short-term during the coronavirus disease pandemic (COVID-19). As COVID-19 is still ongoing, changes in atmospheric aerosol loading over China and the factors of their variations remain unclear. In this study, we used multi-source satellite observations and reanalysis datasets to synergistically analyze the spring (February–May) evolution of aerosol optical depth (AOD) for multiple aerosol types over Eastern China (EC) before, during and after the COVID-19 lockdown period. Regional meteorological effects and the radiative response were also quantitatively assessed. Compared to the same period before COVID-19 (i.e., in 2019), a total decrease of −14.6 % in tropospheric TROPOMI nitrogen dioxide (NO2) and a decrease of −6.8 % in MODIS AOD were observed over EC during the lockdown period (i.e., in 2020). After the lockdown period (i.e., in 2021), anthropogenic emissions returned to previous levels and there was a slight increase (+2.3 %) in AOD over EC. Moreover, changes in aerosol loading have spatial differences. AOD decreased significantly in the North China Plain (−14.0 %, NCP) and Yangtze River Delta (−9.4 %) regions, where anthropogenic aerosol dominated the aerosol loading. Impacted by strong wildfires in Southeast Asia during the lockdown period, carbonaceous AOD increased by +9.1 % in South China, which partially offset the emission reductions. Extreme dust storms swept through the northern region in the period after COVID-19, with an increase of +23.5 % in NCP and + 42.9 % in Northeast China (NEC) for dust AOD. However, unfavorable meteorological conditions overwhelmed the benefits of emission reductions, resulting in a +20.1 % increase in AOD in NEC during the lockdown period. Furthermore, the downward shortwave radiative flux showed a positive anomaly due to the reduced aerosol loading in the atmosphere during the lockdown period. This study highlights that we can benefit from short-term controls for the improvement of air pollution, but we also need to seriously considered the cross-regional transport of natural aerosol and meteorological drivers.

Using Ice Cores to Evaluate CMIP6 Aerosol Concentrations Over the Historical Era

AbstractThe radiative effect of anthropogenic aerosols is one of the largest uncertainties in Earth's energy budget over the industrial period. This uncertainty is in part due to sparse observations of aerosol concentrations in the pre‐satellite era. To address this lack of measurements, ice cores can be used, which contain the aerosol concentration record. To date, these observations have been under‐utilized for comparison to aerosol concentrations found in state‐of‐the‐art Earth system models (ESMs). Here we compare long term trends in concentrations of sulfate and black carbon (BC) between 15 ice cores and 11 ESMs over nine regions around the world during the period 1850–2000. We find that for sulfate concentration trends model results generally agree with ice core records, whereas for BC concentration the model trends differ from the records. Absolute concentrations of both investigated species are overestimated by the models, probably in part due to representation errors. However, we assume that biases in relative trends are not altered by these errors. Ice cores in the European Alps and Greenland record a maximum BC concentration before 1950, while most ESMs used in this study agree on a post‐1950 maximum. We source this bias to an error in BC emission inventories in Europe. Emission perturbation experiments using NorESM2‐LM support the observed finding that BC concentrations in Northern Greenland ice cores are recording European emissions. Errors in BC emission inventories have implications for all future and past studies where Coupled Model Intercomparison Project Phase 6 historical simulations are compared to observations relevant to aerosol forcing.

Further improvement and evaluation of nudging in the E3SM Atmosphere Model version 1 (EAMv1): simulations of the mean climate, weather events, and anthropogenic aerosol effects

Abstract. A previous study on the use of nudging in E3SM Atmosphere Model version 1 (EAMv1) had an unresolved issue; i.e., a simulation nudged to EAMv1's own meteorology showed non-negligible deviations from the free-running baseline simulation over some of the subtropical marine stratocumulus and trade cumulus regions. Here, we demonstrate that the deviations can be substantially reduced by (1) changing where the nudging tendency is calculated in the time integration loop of a nudged EAM simulation so as to improve consistency with the free-running baseline and (2) increasing the frequency of the constraining data so as to better capture strong sub-diurnal variations. The fact that modification (2) improves the climate representativeness of the nudged simulations has motivated us to investigate whether the use of newer reanalysis products with higher data frequency can help improve nudged hindcast simulations by better capturing the observed weather events. To answer this question, we present simulations conducted at EAMv1's standard horizontal resolution (approximately 1∘) with nudging towards 6-hourly ERA-Interim reanalysis or 6-hourly, 3-hourly, or hourly ERA5 reanalysis. These simulations are evaluated against the climatology of free-running EAMv1 simulations as well as reanalyses, satellite retrievals, and in situ measurements from the Atmospheric Radiation Measurement user facility. For the 1∘ EAMv1 simulations, we recommend using the relocated nudging tendency calculation and the ERA5 reanalysis at 3-hourly or higher frequency. Simulations used for estimating the anthropogenic aerosol effects often use nudging to help discern signal from noise. The sensitivity of such estimates to the configuration of nudging is investigated in EAMv1, again using the standard 1∘ horizontal resolution. We find that, when estimating the global mean effects, the frequency of constraining data has relatively small impacts, while the choice of nudged variables can change the results substantially. The nudging of air temperature (in addition to horizontal winds) has two non-negligible effects. First, when the constraining data come from reanalysis, the nudging-induced mean bias correction can cause significant changes in the simulated clouds and hence substantially different estimates of the aerosol effects. The impact of the mean bias correction on ice cloud formation has been noted in previous studies and is also seen in EAMv1. For applications like ours, where the preferred configurations of nudging are those capable of providing results consistent with the multi-year free-running simulations, the consequence of the mean bias correction is undesirable. The second important impact of temperature nudging is a significant suppression of adjustments to aerosol forcing, which also causes changes in the estimated aerosol effects. This effect can be seen in simulations nudged to either reanalysis or EAM's own meteorology. These results suggest that nudging horizontal winds but not temperature is a better choice for estimating the anthropogenic aerosol effects.

Detection and attribution of extreme precipitation events over the Asian monsoon region

Extreme precipitation events have enormous impacts on the natural and human aspects of most regions. This study presents a detection and attribution analysis of extreme precipitation in the Asian monsoon region (AM) from 1950 to 2014 using the Coupled Model Intercomparison Project Phase 6 (CMIP6) models. The observed positive PI (probability-based index) trends for annual maxima of 5-day (Rx5day) and 1-day (Rx1day) precipitation accumulations are 6.04%/100yr and 10.96%/100yr. The simulated PI trends for Rx5day and Rx1day under greenhouse-gas (GHG) forcing are 9.70%/100yr and 10.86%/100yr, respectively, while the trends are −8.99%/100yr and −7.01%/100yr under the aerosol (AER) forcing. Greenhouse-gas concentrations alone cause extreme precipitation increases, while the offsetting effects of anthropogenic aerosols may result in weaker increasing trends in ALL forcing simulations. The anthropogenic (ANT) signals are detectable, while the natural (NAT) signals could not be distinguished from the noise (internal climate variability) based on the optimal fingerprinting method. GHG forcing is detected for AM's extreme precipitation when GHG, AER, and NAT forcing are all considered. A three-signal analysis confirms that CO2 forcing had a detectable influence on observations, whereas the influence of volcanic and solar-irradiance forcings could not be distinguished. Our results provide evidence that anthropogenic greenhouse gases (mainly CO2) are the prime external factor influencing the increase of AM's extreme precipitation.

Human Influence on Seasonal Precipitation in Europe

Abstract The response of precipitation to global warming is manifest in the strengthening of the hydrological cycle but can be complex on regional scales. Fingerprinting analyses have so far detected the effect of human influence on regional changes of precipitation extremes. Here we examine changes in seasonal precipitation in Europe since the beginning of the twentieth century and use an ensemble of new climate models to assess the role of different climatic forcings, both natural and anthropogenic. We find that human influence gives rise to a characteristic pattern of contrasting trends, with drier seasons in the Mediterranean basin and wetter over the rest of the continent. The trends are stronger in winter and weaker in summer, when drying is more spatially widespread. The anthropogenic signal is dominated by the response to greenhouse gas emissions, but is also weakened, to some extent, by the opposite effect of anthropogenic aerosols. Using a formal fingerprinting attribution methodology, we show here for the first time that the effects of the total anthropogenic forcing, and also of its greenhouse gas component, can be detected in observed changes of winter precipitation. Greenhouse gas emissions are also found to drive an increase in precipitation variability in all seasons. Moreover, the models suggest that human influence alters characteristics of seasonal extremes, with the frequency of high precipitation extremes increasing everywhere except the Mediterranean basin, where low precipitation extremes become more common. Regional attribution information contributes to the scientific basis that can help European citizens build their climate resilience.

Climatic–Environmental Effects of Aerosols and Their Sensitivity to Aerosol Mixing States in East Asia in Winter

To establish the direct climatic and environmental effect of anthropogenic aerosols in East Asia in winter under external, internal, and partial internal mixing (EM, IM and PIM) states, a well-developed regional climate–chemical model RegCCMS is used by carrying out sensitive numerical simulations. Different aerosol mixing states yield different aerosol optical and radiative properties. The regional averaged EM aerosol single scattering albedo is approximately 1.4 times that of IM. The average aerosol effective radiative forcing in the atmosphere ranges from −0.35 to +1.40 W/m2 with increasing internal mixed aerosols. Due to the absorption of black carbon aerosol, lower air temperatures are increased, which likely weakens the EAWM circulations and makes the atmospheric boundary more stable. Consequently, substantial accumulations of aerosols further appear in most regions of China. This type of interaction will be intensified when more aerosols are internally mixed. Overall, the aerosol mixing states may be important for regional air pollution and climate change assessments. The different aerosol mixing states in East Asia in winter will result in a variation from 0.04 to 0.11 K for the averaged lower air temperature anomaly and from approximately 0.45 to 2.98 μg/m3 for the aerosol loading anomaly, respectively, due to the different mixing aerosols.

Effective radiative forcing of anthropogenic aerosols in E3SM version 1: historical changes, causality, decomposition, and parameterization sensitivities

Abstract. The effective radiative forcing of anthropogenic aerosols (ERFaer) is an important measure of the anthropogenic aerosol effects simulated by a global climate model. Here we analyze ERFaer simulated by the E3SM version 1 (E3SMv1) atmospheric model using both century-long free-running atmosphere–land simulations and short nudged simulations. We relate the simulated ERFaer to characteristics of the aerosol composition and optical properties, and we evaluate the relationships between key aerosol and cloud properties. In terms of historical changes from the year 1870 to 2014, our results show that the global mean anthropogenic aerosol burden and optical depth increase during the simulation period as expected, but the regional averages show large differences in the temporal evolution. The largest regional differences are found in the emission-induced evolution of the burden and optical depth of the sulfate aerosol: a strong decreasing trend is seen in the Northern Hemisphere high-latitude region after around 1970, while a continued increase is simulated in the tropics. The relationships between key aerosol and cloud properties (relative changes between pre-industrial and present-day conditions) also show evident changes after 1970, diverging from the linear relationships exhibited for the period of 1870–1969. In addition to the regional differences in the simulated relationships, a reduced sensitivity in cloud droplet number and other cloud properties to aerosol perturbations is seen when the aerosol perturbation is large. Consequently, the global annual mean ERFaer magnitude does not increase after around 1970. The ERFaer in E3SMv1 is relatively large compared to the recently published multi-model estimates; the primary reason is the large indirect aerosol effect (i.e., through aerosol–cloud interactions). Compared to other models, E3SMv1 features large relative changes in the cloud droplet effective radius in response to aerosol perturbations. Large sensitivity is also seen in the liquid cloud optical depth, which is determined by changes in both the effective radius and liquid water path. Aerosol-induced changes in liquid and ice cloud properties in E3SMv1 are found to have a strong correlation, as the evolution of anthropogenic sulfate aerosols affects both the liquid cloud formation and the homogeneous ice nucleation in cirrus clouds (that causes a large effect on longwave ERFaer). As suggested by a previous study, the large ERFaer appears to be one of the reasons why the model cannot reproduce the observed global mean temperature evolution in the second half of the 20th century. Sensitivity simulations are performed to understand which parameterization and/or parameter changes have a large impact on the simulated ERFaer. The ERFaer estimates in E3SMv1 for the shortwave and longwave components are sensitive to the parameterization changes in both liquid and ice cloud processes. When the parameterization of ice cloud processes is modified, the top-of-model forcing changes in the shortwave and longwave components largely offset each other, so the net effect is negligible. This suggests that, to reduce the magnitude of the net ERFaer, it would be more effective to reduce the anthropogenic aerosol effect through liquid or mixed-phase clouds.

Impacts on Meteorological Parameters and Pollutants by Aerosol Direct Radiative Effect Over Tianjin, China

The direct radiative effect (DRE) of anthropogenic aerosols on meteorological elements and pollutants over Tianjin, China, was investigated using a fully coupled meteorology-chemistry model [Weather Research and Forecasting (WRF) model coupled with Chemistry (WRF-Chem)] for the entire year of 2021. The results showed that the ground solar radiation decreased from 208.84 W m−2 to 194.52 W m−2 due to DRE in Tianjin, and the percentage of the reduction was 6.86%. The reduction of ground solar radiation resulted in a 0.90% decrease in temperature, 1.13% decrease in boundary layer height, and 6.01% decrease in visibility, respectively, and a 0.80% increase in relative humidity. Changes in meteorological parameters changed the diffuse conditions of pollutants, leading to an increase of PM2.5. Higher concentration of pollutants leads to stronger DRE, and the percentages of changes caused by DRE on polluted days are higher than those on clean days. The DRE influence on pollutants can be summarized as a weather → aerosol → weather → air quality circulation mechanism. The DRE had more severe influence on meteorological elements and pollutants on foggy days. With the emission reduction of pollutants, the influence of the DRE has declined.

Detection and attribution of the decreasing precipitation and extreme drought 2020 in southeastern China

• Precipitation deficit 2020 over southeastern China results from anomalously persistent WPSH and weak cold air intrusion. • Anthropogenic aerosol effects have decreased precipitation and increased drought occurrences. • Anthropogenic impacts on the intensity, duration, and severity of drought are not detectable. Southeastern China experienced a drying trend and the precipitation deficit in 2020 exerts great stress to the water supply and the environment. This paper analyzed the large-scale climate patterns relating to the precipitation deficit in 2020 and attribute the decreasing precipitation and drought over southeastern China based on observations and large GCM ensembles, including HadGEM3-GA6 and CESM LENS single forcing simulations. Results show that the precipitation deficit in 2020 mostly results from the anomalously persistent WPSH over South China and a lack of cold air intrusion to South China, and it is also associated with the La Niña-like Pacific. Detection analyses show that anthropogenic influences have resulted in decreasing precipitation. The aerosol emissions, including industrial aerosols (AER) and biomass burning aerosols (BMB), have substantially decreased precipitation over southeastern China, while the greenhouse gas emissions (GHG) have slightly increased precipitation. Because the AER and BMB effects of decreasing precipitation are larger than the GHG effects of increasing precipitation, precipitation shows a decreasing trend in recent decades. However, as the aerosols emission from industry and biomass burning decreases, the AER and BMB effects of decreasing precipitation will be reduced. Future precipitation over southeastern China is projected to increase due to the GHG effects of increasing precipitation. There were AER and BMB impacts on occurrences of drought over southeastern China, but the anthropogenic GHG impacts on the intensity, duration, and severity of drought are not detectable.

Anthropogenic Aerosols Effects on Ice Clouds: A Review

Since the ability of anthropogenic aerosols to act as ice nucleation particles has been recognized, the effect of anthropogenic aerosols on ice clouds has attracted increasing attentions. In recent years, some progress has been made in investigating the effects of anthropogenic aerosols on ice clouds. In this paper, we briefly review the study on the impact of anthropogenic aerosols on ice nuclei, properties and radiative forcing of ice clouds. Anthropogenic aerosols can form ice nuclei through homogeneous nucleation and heterogeneous nucleation. Convective strength can modulate the response of ice clouds to anthropogenic aerosols by affecting the nucleation activities. There have been large uncertainties in calculating the radiative forcing of anthropogenic aerosols on ice clouds in climate models. Further studies on the impact of anthropogenic aerosols on ice clouds are imperative to provide better parameterization schemes and reduce the uncertainties of aerosol indirect effects.