Aerosol Radiative Effects Research Articles

Assessing biases in atmospheric parameters for radiative effects estimation in tropical regions

Many studies highlight the importance of water vapor in tropical atmospheric radiative transfer calculations. However, limited research focuses on understanding the effects of biases in different atmospheric factors, such as pressure, temperature, water vapor, and ozone densities. These biases stem from various sources, including satellites, reanalysis data, and the standard TROPICAL atmosphere. This study explores how these biases influence estimates of two key things: atmospheric radiative forcing (ARF) and heating rate (HR) profiles over the tropical region. The study is undertaken at Gadanki, a rural background site in the tropics. It considers different situations - clear skies and those with aerosols - across different parts of the electromagnetic spectrum and throughout the seasons. Additionally, this study investigates how sensitive ARF and HR profiles are to biases in each of these atmospheric factors within specific spectral ranges. The study results show that the standard TROPICAL atmosphere often differs significantly from real-time measurements, leading to varied effects on radiation. Among the various factors, biases in water vapor density have the most substantial impact on ARF and HR, followed by biases in temperature. Moreover, longwave radiation is more sensitive to water vapor and temperature changes. This study emphasizes the importance of considering the exact state of the atmosphere, especially in tropical regions, when estimating ARF and HR. To improve estimates when real-time data isn't available, the study suggests combining data from the Microwave Limb Sounder (MLS) and the Atmospheric Infrared Sounder (AIRS), and using reanalysis data for broader assessments. It's also important to be cautious when using the TROPICAL atmosphere to analyze the radiative effects of aerosols.

Threefold reduction of modeled uncertainty in direct radiative effects over biomass burning regions by constraining absorbing aerosols.

Absorbing aerosols emitted from biomass burning (BB) greatly affect the radiation balance, cloudiness, and circulation over tropical regions. Assessments of these impacts rely heavily on the modeled aerosol absorption from poorly constrained global models and thus exhibit large uncertainties. By combining the AeroCom model ensemble with satellite and in situ observations, we provide constraints on the aerosol absorption optical depth (AAOD) over the Amazon and Africa. Our approach enables identification of error contributions from emission, lifetime, and MAC (mass absorption coefficient) per model, with MAC and emission dominating the AAOD errors over Amazon and Africa, respectively. In addition to primary emissions, our analysis suggests substantial formation of secondary organic aerosols over the Amazon but not over Africa. Furthermore, we find that differences in direct aerosol radiative effects between models decrease by threefold over the BB source and outflow regions after correcting the identified errors. This highlights the potential to greatly reduce the uncertainty in the most uncertain radiative forcing agent.

Radiative forcing from aerosol–cloud interactions enhanced by large-scale circulation adjustments

The impact of anthropogenic aerosols on clouds is a leading source of uncertainty in estimating the effect of human activity on the climate system. The challenge lies in the scale difference between clouds (~1–10 km) and general circulation and climate (>1,000 km). To address this, we use convection-permitting simulations conducted in a long and narrow domain, to resolve convection while also including a representation of large-scale processes. We examine a set of simulations that include a sea surface temperature gradient—which drives large-scale circulation—and compare these with simulations that include no gradient. We show that the effective radiative forcing due to aerosol–cloud interactions is strongly enhanced by adjustments of large-scale circulation to aerosol. We find that an increase in aerosol concentration suppresses precipitation in shallow-convective regions, which enhances water vapour transport to the portion of the domain dominated by deep convection. The subsequent increase in latent heat release in deep-convective regions strengthens the overturning circulation and surface evaporation. These changes can explain the increase in cloudiness under higher aerosol concentrations and, consequently, the large aerosol radiative effect. This work highlights the fundamental importance of large-scale circulation adjustments in understanding the effective radiative forcing from aerosol–cloud interactions.

Anthropogenic Aerosols Weaken Land–Atmosphere Coupling Over North China

AbstractNorth China, characterized by its strong land–atmosphere coupling, also has a high concentration of atmospheric aerosols from anthropogenic emissions. However, the impact of aerosols on land–atmosphere coupling in this region remains partially unclear. Here, we use Weather Research and Forecasting model coupled with Chemistry (WRF‐Chem) experiments to show that the aerosol radiative effect weakens land–atmosphere coupling in North China. This weakening occurs across all five indexes used to measure different aspects of land–atmosphere coupling. Notably, the weakening is more pronounced for indexes that describe surface coupling compared to the index that characterizes the coupling between the planetary boundary layer (PBL) and clouds. The mechanisms underlying the aerosol influence can be primarily attributed to the reduction of land surface fluxes and their sensitivities to soil moisture, while the weakened entrainment of moisture at the upper boundary of the PBL may also contribute to the effects.

Historical (1960–2014) lightning and LNOx trends and their controlling factors in a chemistry–climate model

Abstract. Lightning can cause natural hazards that result in human and animal injuries and fatalities, infrastructure destruction, and wildfire ignition. Lightning-produced NOx (LNOx), a major NOx (NOx=NO+NO2) source, plays a vital role in atmospheric chemistry and global climate. The Earth has experienced marked global warming and changes in aerosol and aerosol precursor emissions (AeroPEs) since the 1960s. Investigating long-term historical (1960–2014) lightning and LNOx trends can provide important indicators for all lightning-related phenomena and for LNOx effects on atmospheric chemistry and global climate. Understanding how global warming and changes in AeroPEs influence historical lightning and LNOx trends can be helpful in providing a scientific basis for assessing future lightning and LNOx trends. Moreover, global lightning activities' responses to large volcanic eruptions such as the 1991 Pinatubo eruption are not well elucidated and are worth exploring. This study employed the widely used cloud top height lightning scheme (CTH scheme) and the newly developed ice-based ECMWF-McCAUL lightning scheme to investigate historical (1960–2014) lightning and LNOx trends and variations as well as their influencing factors (global warming, increases in AeroPEs, and the Pinatubo eruption) in the framework of the CHASER (MIROC) chemistry–climate model. The results of the sensitivity experiments indicate that both lightning schemes simulated almost flat global mean lightning flash rate anomaly trends during 1960–2014 in CHASER (the Mann–Kendall trend test (significance inferred as 5 %) shows no trend for the ECMWF-McCAUL scheme, but a 0.03 % yr−1 significant increasing trend is detected for the CTH scheme). Moreover, both lightning schemes suggest that past global warming enhances historical trends for global mean lightning density and global LNOx emissions in a positive direction (around 0.03 % yr−1 or 3 % K−1). However, past increases in AeroPEs exert an opposite effect on the lightning and LNOx trends (−0.07 % to −0.04 % yr−1 for lightning and −0.08 % to −0.03 % yr−1 for LNOx) when one considers only the aerosol radiative effects in the cumulus convection scheme. Additionally, effects of past global warming and increases in AeroPEs in lightning trends were found to be heterogeneous across different regions when analyzing lightning trends on the global map. Lastly, this paper is the first of study results suggesting that global lightning activities were markedly suppressed during the first year after the Pinatubo eruption as shown in both lightning schemes (global lightning activities decreased by as much as 18.10 % as simulated by the ECMWF-McCAUL scheme). Based on the simulated suppressed lightning activities after the Pinatubo eruption, the findings also indicate that global LNOx emissions decreased after the 2- to 3-year Pinatubo eruption (1.99 %–8.47 % for the annual percentage reduction). Model intercomparisons of lightning flash rate trends and variations between our study (CHASER) and other Coupled Model Intercomparison Project Phase 6 (CMIP6) models indicate great uncertainties in historical (1960–2014) global lightning trend simulations. Such uncertainties must be investigated further.

Optical Properties and Vertical Distribution of Aerosols Using Polarization Lidar and Sun Photometer over Lanzhou Suburb in Northwest China

To better understand aerosol vertical distribution and radiation effects, the seasonal variation and vertical distribution characteristics of aerosol optical properties were analyzed based on the aerosol extinction coefficient, depolarization ratio and backscatter Ångström exponent derived from the dual-wavelength polarization lidar at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) from December 2009 to November 2012. Combining the CE-318 sun photometer, the microphysical, optical and vertical distribution characteristics of aerosol during a dust process were discussed comprehensively. The results revealed that the vertical profiles of the aerosol extinction coefficient and depolarization ratio clearly had seasonal variation characteristics. The aerosol optical depth (AOD) integrating with the aerosol extinction coefficient within 0–2 km in the spring, summer, autumn and winter accounted for 48%, 45%, 56% and 58% of the total AOD, respectively. The non-spherical feature was most distinctive in the spring, followed by the winter, autumn and summer. The particle size of aerosol in the lower layer was larger than that in the upper layer according to the vertical profile of the backscatter Ångström exponent. The cluster analysis of backward trajectory showed SACOL is dominated by dust aerosol in the spring and the mixtures of dust with anthropogenic pollution in the winter. A dust event in April 2010 was selected and the analysis showed that it mainly came from the high-altitude and long-range transportation from the Taklamakan Desert. During this period, the extinction coefficient increased up to 0.9 km−1, the maximum AOD was 2.21 and the SSA ranged from 0.92 to 0.99. The radiation force in the atmosphere reached 126.15 W/m2. It can be found that the influence of aerosol on the atmospheric radiation effect cannot be ignored.

3D assimilation and radiative impact assessment of aerosol black carbon over the Indian region using aircraft, balloon, ground-based, and multi-satellite observations

Abstract. A three-dimensional (spatial and vertical) gridded data set of black carbon (BC) aerosols has been developed for the first time over the Indian mainland using data from a dense ground-based network, aircraft- and balloon-based measurements from multiple campaigns, and multi-satellite observations, following statistical assimilation techniques. The assimilated data reveal that the satellite products tend to underestimate (overestimate) the aerosol absorption at lower (higher) altitudes with possible climate implications. The regional maps of BC-induced atmospheric heating derived using this data set capture the elevated aerosol heating layers over the Indian region along with the spatial high over the Indo-Gangetic Plain. It is shown that, over most of the Indian region, the incorporation of realistic profiles of aerosol absorption/extinction coefficients and single scattering albedo into the radiative transfer calculations leads to enhanced high-altitude warming. This could strongly influence the upper-tropospheric and lower-stratospheric processes, including the vertical transport of BC to higher altitudes, and thus have larger implications for atmospheric stability than what would be predicted using satellite observations alone. This will have larger implications for atmospheric stability than what would be predicted using satellite observations alone and could strongly influence the upper-tropospheric and lower-stratospheric processes, including increased vertical transport of BC to higher altitudes. The 3D assimilated BC data set will be helpful in reducing the uncertainty in aerosol radiative effects in climate model simulations over the Indian region.

An Aerosol Optical Module With Observation‐Constrained Black Carbon Properties for Global Climate Models

AbstractAtmospheric black carbon (BC) aerosols have been long‐lasting uncertain components in environmental and climate studies. Global climate models (GCMs) potentially overestimate BC absorption efficiency due to a lack of consideration of complex BC microphysical and mixing properties. We extract multiple BC properties from observations and develop an aerosol optical module known as Advanced Black Carbon (ABC) in the framework of the Modal Aerosol Model version 4 (MAM4). The ABC module is implemented in the Community Atmosphere Model version 6 (CAM6) and evaluated by in situ and remote sensing observations. CAM6‐ABC addresses the shortcomings of CAM6‐MAM4 in terms of BC microphysical and mixing properties, particularly their size, mixing state and optical simulations. Sensitivity simulations show that the global BC absorption aerosol optical depth at 550 nm simulated by CAM6‐ABC is reduced by ∼29% compared with that in CAM6‐MAM4. The BC absorption enhancement simulated by CAM6‐ABC is reduced from ∼2.6 of the default MAM4 to ∼1.4, which is closer to the observed values (mostly less than 1.5). With improved BC absorption estimation, the biases of aerosol single‐scattering coalbedo simulations are reduced by 18%–69% compared with global Aerosol Robotic Network observations. Moreover, the globally averaged BC direct radiative effect is reduced from 0.37 to 0.28 W/m2 at the top of the atmosphere. Our new scheme alleviates the overestimation of BC absorption in GCMs by constraining BC microphysical and mixing properties when assessing aerosol radiative and climate effects, and it can be easily implemented in most modal‐based aerosol modules of climate models.

Surface Radiative Forcing as a Climate-Change Indicator in North India due to the Combined Effects of Dust and Biomass Burning

This study estimates the radiative forcing by biomass burning and dust aerosols over the Indian subcontinent, with emphasis on the Indo-Gangetic Plains (IGP) during the period from January 2021 to April 2021, based on multiple satellite and reanalysis datasets. In this respect, we used retrievals from the Moderate Resolution Spectroradiometer (MODIS) and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) system, as well as reanalysis data from the Goddard Earth Observing System, version 5 (GEOS-5), the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2), the Copernicus Atmosphere Monitoring Service (CAMS), and ERA-Interim. According to the MERRA-2 and the CAMS, the highest black carbon (BC) concentrations in January 2021 were 7–8 µg m−3, which were significantly lower than measurements performed in main cities along the IGP, such as Patiala, Delhi, and Kanpur. The meteorological data analysis accompanied by the CALIPSO lidar measurements showed that the vertical distribution of total attenuated backscatter (TAB) could reach altitudes of up to ~4–5 km and could be transported over the central Himalayan region. The spatial-averaged daily aerosol radiative forcing (ARF) values over the Indian subcontinent from January 2021 to April 2021 were found to range from −51.40 to −6.08 W m−2 (mean of −22.02 ± 9.19 W m−2), while on a monthly basis, the ARF values varied widely, from −146.24 to −1.63 W m−2 (mean of −45.56 ± 22.85 W m−2) over different parts of the study region. Furthermore, the spatial-averaged daily BC radiative forcing ranged from −2.23 to −0.35 (−1.01 ± 0.40 W m−2), while it varied from −15.29 to −0.31 W m−2 (−2.46 ± 2.32 W m−2) over different regions of southern Asia, indicating a rather small contribution to the total aerosol radiative effect and a large presence of highly scattering aerosols. Our findings highlight the importance of growing biomass burning, in light of recent climate change and the rapid decline in air quality over North India and the Indian Ocean.

Influence of aerosol on photosynthetically active radiation under haze conditions

Photosynthetically active radiation (PAR, 400-700 nm), as a sub-band in solar shortwave radiation (280-2800 nm), has profoundly affected vegetation primary productivity and biogeochemical cycles of carbon. However, few previous studies focused on the quantitative effects of aerosols on PAR, a sub-band of the shortwave spectrum, during the winter haze period. Based on ground-based remote sensing observations, we have quantitatively investigated the role of aerosol in PAR variation under haze period through radiative transfer simulations at Wuhan, China. The results indicate that direct solar PAR irradiance is attenuated more dramatically and diffuse downward irradiance is enhanced weaklier as aerosol optical depth (AOD) increases in the haze period, which leads to a result of more significant surface aerosol radiative effect (ARE) in PAR spectrum (AREPAR). Simultaneously, the percentage of AREPAR in ARESW spectrum decreases by 3.33%, indicating that PAR is attenuated more than other sub-bands in SW as pollution enhances. Meanwhile, the ARE efficiency (REE) in PAR weakens from -122 W/m2 under the clear-air condition to -80 W/m2 during the severe-pollution period. These phenomena are attributed to stronger forward scattering in PAR due to the hygroscopic growth of fine-mode aerosol particles during haze. The results provides support for further studies on the effect of pollutants on PAR, and then vegetation productivity.

Identifying the natural and anthropogenic drivers of absorbing aerosols using OMI data and HYSPLIT model over South Asia

Aerosols absorption contributes significantly to the total radiative effects of aerosols and so an important component of radiative forcing estimates. Therefore, this study explores the spatiotemporal distribution of ultraviolet aerosol index (UVAI), future trends, potential sources of absorbing aerosols and their relationship with temperature, wind speed, precipitation and total ozone column using Ozone Monitoring Instrument retrieved UVAI and HYSPLIT model over South Asia during October 2004 to March 2022. The mean UVAI within the ranges of 0.56–1.62 are observed over Eastern and Southern Pakistan and Northern India associated with dust and biomass burning aerosols. The interannual variations in UVAI show that the values of UVAI increases from 1.73 to 3.11 during 2018–2021 over the Indo-Gangetic Plain. Contrary to this, UVAI < 0 is observed along the Karakorum and Himalaya range during 2005–2021 indicating presence of non-absorbing aerosols. The interaannual variations in UVAI reveal highest UVAI of 0.64 in December followed by 0.51 in July over South Asia. Seasonally, UVAI shows increasing trend at the rate of 0.9064 DJF−1, 0.3810 JJA−1, 0.2707 SON−1 and 0.0774 MAM−1 over South Asia. A positive correlation of 0.56 is observed between UVAI and wind speed followed by over Southern Pakistan followed 0.43 between UVAI and total ozone column Southern Pakistan and India. The UVAI shows increasing trend at the rate of 0.1409, 0.1124, 0.1224, 0.1015, 0.1242 and 0.2054 per year over Lahore, Karachi, Kanpur, New-Delhi, Varanasi, and Dhaka with maximum UVAI of 5.55, 4.47, 4.51, 4.99, 4.61 and 4.65 respectively during the study period. The anthropogenic productivity analysis reveals that primary industry and secondary industry contributes in lowering UVAI values whereas tertiary industry, energy consumption and gross domestic products increase aerosols loading in South Asia. Moreover, HYSPLIT cluster analysis further reveals the localized and trans-boundary sources of absorbing aerosols over the selected cities.

Benefit of aerosol reduction to winter wheat during China's clean air action: A case study of Henan Province

A strongly declining aerosol radiative effect has been observed in China since 2013 after implementing the clean air action, yet its impact on wheat (Triticum aestivum L.) production remains unclear. We use satellite measures and a biophysical crop model to assess the impact of aerosol-induced radiative perturbations on winter wheat production in the agricultural belt of Henan province from 2013 to 2018. After calibrating parameters with the extended Fourier Amplitude Sensitivity Test (EFAST) and the generalized likelihood uncertainty estimation (GLUE) method, the DSSAT CERES-Wheat model was able to simulate crop biomass and yield more accurately. We found that the aerosol negatively impacted wheat biomass by 21.87% and yield by 22.48% from 2006 to 2018, and the biomass effects from planting to anthesis were more significant compared to anthesis to maturity. Due to the strict clean air action, under all-sky conditions, the surface solar shortwave radiation (SSR) in 2018 increased by about 7.08% over 2006-2013 during the wheat growing seasons. As a result of the improvement of crop photosynthesis, winter wheat biomass and yield increased by an average of 5.46% and 2.9%, respectively. Our findings show that crop carbon uptake and yield will benefit from the clean air action in China, helping to ensure national food and health security.

Study on the characteristics of actinic radiation and direct aerosol radiation effects in the Pearl River Delta region

Combined air pollution prevails in the Pearl River Delta (PRD) region in recent years; large amounts of aerosols have caused a significant attenuation of actinic radiation. Solar radiation is the driving energy for photochemical reactions and plays an important role in the ozone formation. In this study, characteristics of actinic radiation and atmospheric composition in the PRD region have been analyzed based on long-term observation. The radiation transfer model (SBDART) is used to quantitatively estimate the direct radiation effect and actinic radiation effect of aerosol. Results indicate that PM2.5 concentration and atmospheric extinction coefficient have shown a downward trend. Meanwhile, ozone and Single Scattering Albedo (SSA) have increased. The average value of SSA is 0.914 ± 0.041, attaining the highest level in spring, followed by autumn and summer, and the lowest is found in winter. A linear relationship between UVA/UVB of actinic radiation flux and solar shortwave radiation has been revealed. Through the conversion formula proposed in the article, conventional radiation data can be converted into actinic radiation flux and species photolysis rate. The actinic radiation and radiation attenuation caused by aerosols shows an overall downward trend at the surface. In the ultraviolet to visible wavelength range (280–670 nm), the annual average attenuation of aerosol direct radiation and actinic radiation is 61.6 ± 31.8 W/m2 and 120.1 ± 59.7 W/m2, respectively. The radiation attenuation caused by aerosols is the largest in spring, with little difference in summer, autumn and winter, which are 92.9 ± 42.1 W/m2, 53.7 ± 21.3 W/m2, 48.0 ± 13.7 W/m2 and 52.1 ± 15.8 W/m2, respectively. The attenuation of actinic radiation caused by aerosol is the largest in spring, followed by winter, and there is little difference between summer and autumn, which are 174.8 ± 78.4 W/m2, 111.1 ± 33.1 W/m2, 98.3 ± 38.4 W/m2 and 96.7 ± 26.9 W/m2, respectively. The direct radiation effect of aerosol and the actinic radiation effect have opposite trends with increasing height. Below the aerosol scale height, the larger SSA is corresponding with greater radiant flux, and it is opposite above the aerosol scale height. SSA has the same effect on the actinic radiant flux of the entire layer: The larger SSA is in accordance with greater actinic radiant flux.

Predicting Real Refractive Index of Organic Aerosols From Elemental Composition

AbstractAccurate estimates of aerosol refractive index (RI) are critical for modeling aerosol‐radiation interaction, yet this information is limited for ambient organic aerosols, leading to large uncertainties in estimating aerosol radiative effects. We present a semi‐empirical model that predicts the real RI n of organic aerosol material from its widely measured oxygen‐to‐carbon (O:C) and hydrogen‐to‐carbon (H:C) elemental ratios. The model was based on the theoretical framework of Lorenz‐Lorentz equation and trained with n‐values at 589 nm () of 160 pure compounds. The predictions can be expanded to predict n‐values in a wide spectrum between 300 and 1,200 nm. The model was validated with newly measured and literature datasets of n‐values for laboratory secondary organic aerosol (SOA) materials. Uncertainties of predictions for all SOA samples are within 5%. The model suggests that ‐values of organic aerosols may vary within a relatively small range for typical O:C and H:C values observed in the atmosphere.

Extreme Smog Challenge of India Intensified by Increasing Lower Tropospheric Stability

AbstractExtreme smog in India widely impacts air quality in late autumn and winter months. While the links between emissions, air quality and health impacts are well‐recognized, the association of smog and its intensification with climatic trends in the lower troposphere, where aerosol pollution and its radiative effects manifest, are not understood well. Here we use long‐term satellite data to show a significant increase in aerosol exceedances over northern India, resulting in sustained atmospheric warming and surface cooling trends over the last two decades. We find several lines of evidence suggesting these aerosol radiative effects have induced a multidecadal (1980–2019) strengthening of lower tropospheric stability and increase in relative humidity, leading to over fivefold increase in poor visibility days. Given this crucial aerosol‐radiation‐meteorological feedback driving the smog intensification, results from this study would help inform mitigation strategies supporting stronger region‐wide measures, which are critical for solving the smog challenge in India.

Retrieval of aerosol single scattering albedo using joint satellite and surface visibility measurements

Aerosol single scattering albedo (SSA) measuring the ratio of scattering to extinction is a critical parameter in determining aerosol radiative effect. However, the retrieval of SSA from satellite platforms is limited because most existing passive satellite sensors such as Moderate Resolution Imaging Spectroradiometer (MODIS) only provide the measurements of reflected solar radiation at the top of the atmosphere (TOA), which are only capable of retrieving aerosol optical depth (AOD) on the basis of assumed SSA. In this study, we develop a machine learning based algorithm to retrieve SSA using joint surface visibility and satellite radiation measurements. With ancillary information such as meteorological condition, surface visibility can be regarded as a proxy of column AOD. By combining MODIS measured TOA apparent reflectance with ground visibility as well as other auxiliary parameters, we retrieve SSA at over 6000 visibility stations worldwide. The validation results during the period from 2010 to 2019 show high consistency with AERONET retrieved SSA, with daily, monthly, and seasonal retrievals all exhibiting high correlations of 0.68, 0.96, and 0.97, respectively. Daily, monthly, and seasonal RMSE values are 0.044, 0.012, and 0.011, respectively. The mean absolute bias (MAB) of daily retrievals is 0.031 and 62% of the samples fall within the uncertainty interval of ±0.03. Monthly and seasonal MAB values decrease to below 0.01. Over 97% of the monthly and seasonal retrievals are within the error envelope of ±0.03. Our work generates a global aerosol SSA dataset with extensive coverage over land, which can be used for validating and improving climate models and the estimation of aerosol radiative forcing.

Atmospheric aerosol size distribution impacts radiative effects over the Himalayas via modulating aerosol single-scattering albedo

The single-scattering albedo (SSA) of atmospheric aerosols is a key parameter that controls aerosol radiative effects. The variation of SSA is thought to be mainly regulated by aerosol absorption in the Himalayas and South Asia, but observations contradict this idea. In situ field campaigns conducted over two Himalayan sites revealed that SSA was strongly dependent on scattering but weakly correlated with absorption. Observational results combined with the Mie theory further illustrated that SSA was primarily modulated by size distribution rather than absorption. Aerosol Robotic Network (AERONET) data showed similar impacts of size distribution on SSA and that aerosol radiative forcing efficiencies were significantly dependent on SSA. Aerosol size distribution therefore considerably affects radiative forcing by modulating aerosol SSA over the Himalayas. This study highlighted the influence of aerosol size distribution on radiative forcing over the Himalayas, which has important implications for understanding aerosol radiative effects globally.

Distinct Seasonality in Aerosol Responses to Emission Control Over Northern China

AbstractDespite intensive research exploring how aerosols respond to emission control over Northern China, efforts mostly focus on the environmental benefit, especially in winter. Here we found that unlike the most substantial PM2.5 concentration reduction in winter, aerosol optical depth (AOD) declines more than 2 times faster in summer, causing an increase in aerosol radiative effects of 1.2 (5.7)Wm−2 on all‐sky (clear‐sky) conditions over 2013–2019 and largely shaping the climate impact. Low‐level aerosols are shown to be the prime contributor under the synergetic effects of aerosol composition and ambient relative humidity (RH). The dominance of the highly hygroscopic sulfate combined with high RH enables a strong extinction efficiency of the reduced summertime aerosols, while the insignificant AOD decline in wintertime result from the dominance of organic aerosols with weak hygroscopicity, and is further offset by the increased frequencies of extremely high RH. We show the environmental and climatic responses of aerosols to emission control exhibit distinctively different seasonality.

Absorbing Aerosol Optical Properties and Radiative Effects on Near-Surface Photochemistry in East Asia

Absorbing aerosols have significant influences on tropospheric photochemistry and regional climate change. Here, the direct radiative effects of absorbing aerosols at the major AERONET sites in East Asia and corresponding impacts on near-surface photochemical processes were quantified by employing a radiation transfer model. The average annual aerosol optical depth (AOD) of sites in China, Korea, and Japan was 1.15, 1.02 and 0.94, respectively, and the corresponding proportion of absorbing aerosol optical depth (AAOD) was 8.61%, 6.69%, and 6.49%, respectively. The influence of absorbing aerosol on ultraviolet (UV) radiation mainly focused on UV-A band (315–400 nm). Under the influence of such radiative effect, the annual mean near-surface J[NO2] (J[O1D]) of sites in China, Korea, and Japan decreased by 16.95% (22.42%), 9.61% (13.55%), and 9.63% (13.79%), respectively. In Beijing–Tianjin–Hebei (BTH) and Yangtze River Delta (YRD) region, the annual average AOD was 1.48 and 1.29, and the AAOD was 0.14 and 0.13, respectively. The UV radiative forcing caused by aerosols dominated by black carbon (BC-dominated aerosols) on the surface was −3.19 and −2.98 W m−2, respectively, accounting for about 40% of the total aerosol radiative forcing, indicating that the reduction efficiency of BC-dominated aerosols on solar radiation was higher than that of other types of aerosols. The annual mean J[NO2] (J[O1D]) decreased by 14.90% (20.53%) and 13.71% (18.20%) due to the BC-dominated aerosols. The daily maximum photolysis rate usually occurred near noon due to the diurnal variation of solar zenith angle and, thus, the daily average photolysis rate decreased by 2–3% higher than that average during 10:00–14:00.

Direct Radiative Effects of Aerosols on Numerical Weather Forecasts—A Comparison of Two Aerosol Datasets in the NCEP GFS

Abstract This study compares aerosol direct radiative effects on numerical weather forecasts made by the NCEP Global Forecast System (GFS) with two different aerosol datasets, the Optical Properties of Aerosols and Clouds (OPAC) and MERRA-2 aerosol climatologies. The underestimation of aerosol optical depth (AOD) by OPAC over northwest Africa, central to East Africa, the Arabian Peninsula, Southeast Asia, and the Indo-Gangetic Plain, and overestimation in the storm-track regions in both hemispheres are reduced by MERRA-2. Surface downward shortwave (SW) and longwave (LW) fluxes and the top-of-the-atmosphere SW and outgoing LW fluxes from model forecasts are compared with CERES satellite observations. Forecasts made with OPAC aerosols have large radiative flux biases, especially in northwest Africa and the storm-track regions. These biases are also reduced in the forecasts made with MERRA-2 aerosols. The improvements from MERRA-2 are most noticeable in the surface downward SW fluxes. GFS medium-range weather forecasts made with the MERRA-2 aerosols demonstrated slightly improved forecast accuracy of sea level pressure and precipitation over the Indian and East Asian summer monsoon region. A stronger Africa easterly jet is produced, associated with a low pressure over the east Atlantic Ocean and west of northwest Africa. Impacts on large-scale skill scores such as 500-hPa geopotential height anomaly correlation are generally positive in the Northern Hemisphere and the Pacific and North American regions in both the winter and summer seasons.