Increase In Aerosol Loading Research Articles

Investigation of aerosol effects on a deep convection case in Southeast China with budget analysis of hydrometeor mass and latent heat

It remains an open question which microphysical processes determine the sign of aerosol effects on deep convective rain. This lack of knowledge prevents us from understanding the uncertainties of the aerosol effects on deep convection. We try to answer this question through a cloud-resolving model (CRM) simulation on a summer storm that occurred in Southeast China, by analyzing both column hydrometeor mass and latent heat budget and quantifying the contribution of each microphysical process to the rain variations with aerosol. Our results show that warm-phase microphysical processes have stronger contributions (52% −99%) to the rain variations than ice-phase microphysical processes (1%−48%) when gradually increasing cloud condensation nuclei (CCN) concentration from 125 cm−3 (clean environment) to 16,000 cm−3 (highly polluted environment). Condensation and evaporation of cloud droplets are identified as two crucial microphysical processes, both of which are enhanced with increased aerosol loading. The competition between them (i.e., net condensation) essentially determines the sign of aerosol effects on surface rain. We also explore aerosol effects on updraft velocity from the perspective of buoyancy and latent heat budget, it reveals that the largely enhanced convective updrafts below 7 km are attributed to the increase of condensation heating. While the competition between enhanced condensation heating and suppressed deposition heating restricts the aerosol-induced convective invigoration above 7 km. This study highlights the dominant role of net condensation in the aerosol-imposed invigoration of deep convection under highly polluted environments, suggesting that an explicit treatment of vapor diffusion processes is necessary to simulate aerosol effects on deep convection more accurately.

Spatial distribution of aerosol optical depth over India during COVID-19 lockdown phase-1

The present study analysed the spatial distribution of Aerosol Optical Depth (AOD) over India during the COVID-19 lockdown phase -1 (March 25 to April 15, 2020) using MODIS Terra (MOD04) AOD data (550 nm) during 2001–2020. Air temperature, rainfall, forest fire incidents, and wind patterns were analysed to understand their effect on the distribution of aerosols over India during the lockdown phase-1. Moderate absorption fine aerosol type is predominant but sparsely distributed over India during the study period compared to the reference period indicating the positive influence of the lockdown. Mean AOD has reduced by 9% over India during the lockdown phase-1 compared to the corresponding mean of the past 19 years (2001–2019). About 70% of the states/UTs of India showed a reduction in mean AOD due to restrictions on non-essential economic activities and rainfall occurrence. However, some states showed an increase in aerosol loading over specific pockets despite the restrictions on economic activities (Arunachal Pradesh, Assam, Gujarat, Orissa, Andhra Pradesh, Madhya Pradesh, Chhattisgarh, Maharashtra, Assam, Nagaland, Manipur and Karnataka) because of active forest fire cases. This study would be helpful for planners and policymakers to adopt suitable measures to control the rising concentrations of aerosols over hotspot regions of India.

Modeling Aerosol Effects on Liquid Clouds in the Summertime Arctic

AbstractAn investigation of aerosol effects on the Arctic summer clouds is carried out by using a fully‐coupled version of GEM‐MACH, an online air quality forecast model. Model simulations are carried out for the July 2014 NETCARE field campaign based from Resolute, Nunavut, Canada. For a pristine period, the model simulated thin water clouds, with liquid water path <50 g m−2, at relatively low altitudes (<200 m). The model simulated more and smaller droplets with higher liquid water content (LWC) during a polluted period. Further sensitivity analysis indicates that the increased aerosol loading from long‐range transport during the polluted period is responsible for the increase in CDNC and decreased droplet size, which also led to an increase in cloud liquid water content (LWC), a decrease in precipitation, and increased cloudiness in the area. The model agrees with observation showing that aerosols smaller than 100 nm are commonly activated in the summer Arctic, with even smaller aerosols (<50 nm) being activated during the pristine period. The impact of atmospheric dimethyl sulfide (DMS) upon the Arctic summer cloud microphysics was also investigated. The inclusion of DMS leads to an increase in CDNC and smaller droplet sizes. It also improves the modeled aerosol size distribution considerably compared to the observation, particularly in the size range between 60 and 200 nm. The enhancement of CDNC due to DMS is more significant (e.g., >50%) in the regions with higher DMS(g) mixing ratios, resulting in higher sulphate mass increases in these regions.

Analysing the day/night seasonal and annual changes and trends in land surface temperature and surface urban heat island intensity (SUHII) for Indian cities

• Day/night seasonal and annual changes and trends (2001- 2018) in land surface temperature and surface urban heat island intensity (SUHII) for Indian cities (Lucknow, Kolkata and Pune). • Nighttime LST is a better indicator to study the urban heating effect due to its sensitivity towards urbanization. • Positive trends in mean annual daytime (0.003 to 0.059 °C/year) and nighttime (0.030 to 0.078 °C/year) LST are observed in all cities, except in Pune where urban cooling occurs during daytime. • Nighttime SUHII is positive for all cities with mean annual SUHII ranging from 1.34 to 2.07 °C. Understanding the patterns and possible causes of urban heat islands (UHI) effect due to urbanisation-induced anthropogenic activities is an important area in urban climatic research. Few studies on long-term thermal behaviour of Indian cities are available in the literature. Towards this, the present study investigates the day/night seasonal and annual changes and trends in land surface temperature (LST) and surface urban heat island intensity (SUHII) during the last two decades for three Indian cities (Lucknow, Kolkata and Pune) with distinct urban, physiographic and climatic setting. Moderate Resolution Imaging Spectroradiometer (MODIS) LST data products acquired during daytime and nighttime from 2001 to 2018 are processed and analysed for this purpose. MODIS-based aerosol optical depth (AOD) and normalised difference vegetation index (NDVI) products are used to understand the plausible reasons of change in LST and SUHII patterns. The urban (contiguous built-up) and non-urban (surrounding rural) areas are delineated by applying city clustering algorithm on MODIS Land Cover datasets of 500 m spatial resolution. Mann-Kendall and Seasonal-Kendall tests along with Theil-Sen estimator are used for trend analysis. The findings show that the diurnal temperature range (DTR) has decreased from 2001 to 2018 due to higher increase in nighttime LST as compared to daytime. Nighttime LST is a better indicator to study the urban heating effect due to its sensitivity towards urbanisation. Positive trends in mean annual daytime (0.003 to 0.059 °C/year) and nighttime (0.030 to 0.078 °C/year) LST are observed in all cities, except in Pune where urban cooling occurs during daytime. Increase in aerosol loading (with consequent decrease in air temperature and surface insolation) and vegetative cover over time are attributed as the primary reasons of urban cooling effect in Pune during daytime. Seasonal analysis indicate high warming rates during monsoon and summer seasons, particularly during nighttime (0.062 to 0.1 °C/year). Nighttime SUHII is positive for all cities with mean annual SUHII ranging from 1.34 to 2.07 °C. Statistically significant ( p <0.01) increasing trends are observed in mean annual SUHII (0.009 °C/year to 0.022 °C/year) for Kolkata and Pune. Decreasing trend in SUHII is observed over Lucknow due to higher rate of increase in LST in non-urban area as compared to urban area. The study highlights that geographical, physiographic and climatic setting along with anthropogenic processes in urban and its surrounding non-urban areas are critical towards understanding the causative factors in exacerbating the phenomenon of urban heating.

MODIS high-resolution MAIAC aerosol product: Global validation and analysis

This paper attempt to verify the accuracy of the Multi-Angle Implementation of Atmospheric Correction (MAIAC) aerosol optical depth (AOD) product on a global scale using AOD measurements during 2000–2019 at 332 Aerosol Robotic Network (AERONET) sites. The results indicate that MAIAC AOD retrievals are highly correlated with ground-based AOD measurements. The correlation coefficients (R) are greater than 0.8 at more than 68% of AERONET sites. Meanwhile, the performance of MAIAC AOD retrievals in different climate zones is also investigated. The accuracy of MAIAC AOD retrievals is high (within expected error (EE) = 87.49% and 83.15%) in the regions of tropical rainforest climate and tropical open forest climate. Then, the uncertainties and estimation deviation of MAIAC AOD retrievals in various land-cover-types are analyzed. The results find that the MAIAC AOD retrievals show high correlation with aerosol measurements in the forest (within EE > 95%), while the poor accuracy is always observed in the barren areas (within EE = 62.56%, RMSE = 0.12). The uncertainty of the MAIAC AOD retrievals is not only related to the satellite and solar geometries, but also depends on the aerosol conditions, which become larger with the increase of aerosol loading and Ångstrom exponent (AE). Finally, the spatial variations of AOD values over the world are investigated. High AOD values are mainly distributed in Central Africa, Saudi Arabia, Southern Asia and China.

Modifications in aerosol physical, optical and radiative properties during heavy aerosol events over Dushanbe, Central Asia

The location of Central Asia, almost at the center of the global dust belt region, makes it susceptible for dust events. The studies on atmospheric impact of dust over the region are very limited despite the large area occupied by the region and its proximity to the mountain regions (Tianshan, Hindu Kush-Karakoram-Himalayas, and Tibetan Plateau). In this study, we analyse and explain the modification in aerosols’ physical, optical and radiative properties during various levels of aerosol loading observed over Central Asia utilizing the data collected during 2010–2018 at the AERONET station in Dushanbe, Tajikistan. Aerosol episodes were classified as strong anthropogenic, strong dust and extreme dust. The mean aerosol optical depth (AOD) during these three types of events was observed a factor of ~3, 3.5 and 6.6, respectively, higher than the mean AOD for the period 2010–2018. The corresponding mean fine-mode fraction was 0.94, 0.20 and 0.16, respectively, clearly indicating the dominance of fine-mode anthropogenic aerosol during the first type of events, whereas coarse-mode dust aerosol dominated during the other two types of events. This was corroborated by the relationships among various aerosol parameters (AOD vs. AE, and EAE vs. AAE, SSA and RRI). The mean aerosol radiative forcing (ARF) at the top of the atmosphere (ARFTOA), the bottom of the atmosphere (ARFBOA), and in the atmosphere (ARFATM) were −35 ± 7, −73 ± 16, and 38 ± 17 Wm−2 during strong anthropogenic events, −48 ± 12, −85 ± 24, and 37 ± 15 Wm−2 during strong dust event, and −68 ± 19, −117 ± 38, and 49 ± 21 Wm−2 during extreme dust events. Increase in aerosol loading enhanced the aerosol-induced atmospheric heating rate to 0.5–1.6 K day−1 (strong anthropogenic events), 0.4–1.9 K day−1 (strong dust events) and 0.8–2.7 K day−1 (extreme dust events). The source regions of air masses to Dushanbe during the onset of such events are also identified. Our study contributes to the understanding of dust and anthropogenic aerosols, in particular the extreme events and their disproportionally high radiative impacts over Central Asia.

Sahelian Heat Wave Characterization From Observational Data Sets

AbstractThis paper makes use of spaceborne observational data sets in order to characterize radiative processes involved in spring time heat waves in the Sahel. Spring corresponds to the hottest period of the year, with a high aerosol load, a gradual moistening, and the presence of clouds contributing to greenhouse effect. Heat waves are defined as synoptic events that have a large spatial extent and a duration longer than 3 days. Two types of heat waves are studied: daytime heat waves, detected with the daily maximum temperature and nighttime heat waves, detected with the daily minimum temperature. Daytime heat waves correspond to situations where cloud optical thickness is lower than the climatology and a large number of these situations are also associated with a lower aerosol load and a drier atmosphere. Nighttime heat waves correspond to a moister atmosphere compared to the climatology. In a large fraction of them, an increase in aerosol loading is also observed. This study, only based on observational data sets, highlights the subtle but different radiative balance at play in both types of events.

Two decades of aerosol observations by AATSR, MISR, MODIS and MERRA-2 over India and Indian Ocean

Atmospheric aerosols play key roles in radiation budget, ecosystem dynamics, air quality and cloud microphysics of a region and thus, they greatly influence the global climate, ecosystem and public health. We present the temporal variability of atmospheric aerosols over India and north Indian Ocean (NIO) for the past two decades (2000–2019). Here, the measurements from Moderate Resolution Imaging Spectroradiometer (MODIS) on Terra, Multi-angle Imaging Spectroradiometer (MISR) on Terra, and Advanced Along Track Scanning Radiometer (AATSR) measurements from Envisat, and Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2) reanalysis data are considered. On average, the Indo Gangetic Plain (IGP) shows the largest (0.5–1.2), and NIO and Himalaya show the smallest (0.1 or smaller) Aerosol Optical depth (AOD) values. The peak AOD loading is observed in June–July-August, and IGP has a secondary peak in March owing to the stubble and biomass burning in winter months. In contrast, the peak aerosol loading in the northeast is in winter due to forest and biomass burning during the period there. The inter-annual variability is very small in NIO, Himalaya and Bay of Bengal. The trends estimated from the combined (AATSR, MISR, MODIS and MERRA-2) data show the highest positive trends at the lower IGP and east central regions, about 0.8–1.2/dec, and are statistically significant. This is consistent with the urban activity, industries and dense population there. However, the Desert, northern Himalaya and northern Arabian Sea show insignificant negative trends, from −0.2 to −0.4/dec, as the anthropogenic sources of aerosols are very limited there. The bias estimate shows that most satellite and reanalysis data are in very good agreement at all regions (within 0.1–0.2). Even though the bias in the measurements are considered, the trends estimated are still large enough to be statistically significant. The analyses, therefore, caution the increasing aerosol loading and their plausible climate feedback in these regions. The assessment also demonstrate the potential of synergetic use of multiple-platform measurements for climate system studies.

Cloud ‘shrinking’ and ‘optical thinning’ in the ‘dimming’ period and a subsequent recovery in the ‘brightening’ period over China

There was a dramatic increase in aerosol loading in China after the 1960s due to rapid industrialization, followed by a gradual reduction due to improvements in air quality since the early 2000s. They are deemed to be the main cause of ‘dimming’ and ‘brightening’ in China, respectively. China, therefore, provides an ideal testbed to investigate the multi-decadal evolution of clouds under a background of large variations in the amount of atmospheric aerosols. We used a unique combination of long-term in-situ observational records combined with a radiative transfer model to investigate the changes in clouds and aerosols over the last 60 years (1958–2018) over China. We found during the dimming period, the clouds over China shrunk in area steadily, gradually thinned in terms of optical depth, and thereby blocked less and less solar radiation. This situation reversed during the brightening period. The clouds over China showed a quick expansion in extent and thickening in terms of optical depth, and accordingly the amounts of solar radiation blocked by clouds recovered rapidly. It is observationally demonstrated that high levels of pollution and the associated amount of aerosols cause a suppression of cloud growth and a reduction of optical depth.

Impacts of aerosols produced by biomass burning on the stratocumulus‐to‐cumulus transition in the equatorial Atlantic

AbstractThe impact of aerosols produced by biomass burning on the stratocumulus‐to‐cumulus transition (SCT) in the equatorial Atlantic is studied using satellite‐based and reanalysis data for the month of June. The month of June is highlighted because it represents monsoon onset as well as the largest sea surface temperature gradient in the summer, which is the peak season of tropical African biomass burning. Boundary layer deepening and increasing temperatures put the location of the SCT within the Gulf of Guinea. Satellite retrievals indicate that the bulk of aerosols occur near 1,500 m in altitude, either above or below the boundary layer depending on latitudinal position. Changes in smoke loading over the Gulf of Guinea due to greater transport from southern Africa leads to increases in low‐level cloud cover above cloud decks and decreases when mixed within the boundary layer. Further south, we find significant changes to temperature, cloud top height, tropospheric stability and moisture availability near maximum aerosol loading. In addition, changes in vertical velocity during dirty conditions further reinforce changes in tropospheric stability. These effects combine to shorten the SCT in space during increased aerosol loading episodes.

Measurement report: aerosol hygroscopic properties extended to 600 nm in the urban environment

Abstract. Submicron particles larger than 300 nm dominate the aerosol light extinction and mass concentration in the atmosphere. The water uptake ability of this size range greatly influences the particle mass, visibility degradation, and particle chemistry. However, most previous field measurements on aerosol hygroscopicity are limited within 350 nm. In this study, the size-resolved aerosol hygroscopic properties over an extended size range (50–600 nm) at 85 % relative humidity were investigated in Beijing winter from 27 November 2019 to 14 January 2020 using a humidity tandem differential mobility analyzer (HTDMA) instrument. The corresponding aerosol optical properties were also analyzed using the Mie scattering theory. Results show that the averaged probability distribution of growth factor (GF-PDF) is generally constituted by a more hygroscopic (MH) group and a less hygroscopic (LH) group (including hydrophobic). For the particles larger than 300 nm, there exists a large fraction of LH group particles, resulting in an unexpected low hygroscopicity. During the development of pollution, when particles are gradually aged and accumulated, the bulk hygroscopicity above 300 nm is enhanced significantly by the growth and expansion of the MH group. This result is supported by previous chemical composition analysis, and we give more direct and detailed evidence from the growth factor and mixing-state aspects. Our calculations indicate that the optical contribution of particles larger than 300 nm constitutes about two-thirds of the total aerosol extinction. The large hygroscopic variation in aerosols above 300 nm will influence the light degradation comparably with the increase in aerosol loading in low-visibility haze events. Our studies highlight that the hygroscopic properties above 300 nm are complex and vary greatly with different pollution stages. Therefore more field measurements and investigations need to be done in the future.

Evaluation of multiple surface irradiance-based clear sky detection methods at Xianghe—A heavy polluted site on the North China Plain: 香河站太阳辐射识别晴空方法的评估

Surface irradiance measurements with high temporal resolution can be used to detect clear skies, which is a critical step for further study, such as aerosol and cloud radiative effects. Twenty-one clear-sky detection (CSD) methods are assessed based on five years of 1-min surface irradiance data at Xianghe—a heavily polluted station on the North China Plain. Total-sky imager (TSI) discrimination results corrected by manual checks are used as the benchmark for the evaluation. The performance heavily relies on the criteria adopted by the CSD methods. Those with higher cloudy-sky detection accuracy rates produce lower clear-sky accuracy rates, and vice versa. A general tendency in common among all CSD methods is the detection accuracy deteriorates when aerosol loading increases. Nearly all criteria adopted in CSD methods are too strict to detect clear skies under polluted conditions, which is more severe if clear-sky irradiance is not properly estimated. The mean true positive rate (CSD method correctly detects clear sky) decreases from 45% for aerosol optical depth (AOD) ≤ 0.2% to 6% for AOD > 0.5. The results clearly indicate that CSD methods in a highly polluted region still need further improvements.摘要根据位于华北平原的重污染站点——香河5年的分钟级别地表太阳辐射和人工订正的全天空成像仪数据, 对21种晴空识别 (CSD) 方法进行了评估:晴空识别准确率较高的方法云天识别准确率较低, 反之亦然;由于CSD 方法采用的参数阈值不适用于污染情况, 当气溶胶含量增加时, 识别准确率呈下降趋势.研究结果显示, 利用太阳辐射数据识别晴空的方法在高污染地区使用时需进行改进.

Quantifying Long‐Term Seasonal and Regional Impacts of North American Fire Activity on Continental Boundary Layer Aerosols and Cloud Condensation Nuclei

AbstractAn intimate knowledge of aerosol transport is essential in reducing the uncertainty of the impacts of aerosols on cloud development. Data sets from the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement platform in the Southern Great Plains region (ARM‐SGP) and the National Aeronautics and Space Administration (NASA) Modern‐Era Retrospective Analysis for Research and Applications, version 2 (MERRA‐2), showed seasonal increases in aerosol loading and total carbon concentration during the spring and summer months (2008–2016) which was attributed to fire activity and smoke transport within North America. The monthly mean MERRA‐2 surface carbonaceous aerosol mass concentration and ARM‐SGP total carbon products were strongly correlated (R = 0.82, p &lt; 0.01) along with a moderate correlation with the ARM‐SGP cloud condensation nuclei (NCCN) product (0.5, p ~ 0.1). The monthly mean ARM‐SGP total carbon and NCCN products were strongly correlated (0.7, p ~ 0.01). An additional product denoting fire number and coverage taken from the National Interagency Fire Center (NIFC) showed a moderate correlation with the MERRA‐2 carbonaceous product (0.45, p &lt; 0.01) during the 1981–2016 warm season months (March–September). With respect to meteorological conditions, the correlation between the NIFC fire product and MERRA‐2 850‐hPa isobaric height anomalies was lower (0.26, p ~ 0.13) due to the variability in the frequency, intensity, and number of fires in North America. An observed increase in the isobaric height anomaly during the past decade may lead to frequent synoptic ridging and drier conditions with more fires, thereby potentially impacting cloud/precipitation processes and decreasing air quality.

Distinct aerosol effects on cloud-to-ground lightning in the plateau and basin regions of Sichuan, Southwest China

Abstract. The joint effects of aerosol, thermodynamic, and cloud-related factors on cloud-to-ground lightning in Sichuan were investigated by a comprehensive analysis of ground-based measurements made from 2005 to 2017 in combination with reanalysis data. Data include aerosol optical depth, cloud-to-ground (CG) lightning density, convective available potential energy (CAPE), mid-level relative humidity, lower- to mid-tropospheric vertical wind shear, cloud-base height, total column liquid water (TCLW), and total column ice water (TCIW). Results show that CG lightning density and aerosols are positively correlated in the plateau region and negatively correlated in the basin region. Sulfate aerosols are found to be more strongly associated with lightning than total aerosols, so this study focuses on the role of sulfate aerosols in lightning activity. In the plateau region, the lower aerosol concentration stimulates lightning activity through microphysical effects. Increasing the aerosol loading decreases the cloud droplet size, reducing the cloud droplet collision–coalescence efficiency and inhibiting the warm-rain process. More small cloud droplets are transported above the freezing level to participate in the freezing process, forming more ice particles and releasing more latent heat during the freezing process. Thus, an increase in the aerosol loading increases CAPE, TCLW, and TCIW, stimulating CG lightning in the plateau region. In the basin region, by contrast, the higher concentration of aerosols inhibits lightning activity through the radiative effect. An increase in the aerosol loading reduces the amount of solar radiation reaching the ground, thereby lowering the CAPE. The intensity of convection decreases, resulting in less supercooled water being transported to the freezing level and fewer ice particles forming, thereby increasing the total liquid water content. Thus, an increase in the aerosol loading suppresses the intensity of convective activity and CG lightning in the basin region.

Anthropogenic emissions from South Asia reverses the aerosol indirect effect over the northern Indian Ocean

Atmospheric aerosols play an important role in the formation of warm clouds by acting as efficient cloud condensation nuclei (CCN) and their interactions are believed to cool the Earth-Atmosphere system (‘first indirect effect or Twomey effect’) in a highly uncertain manner compared to the other forcing agents. Here we demonstrate using long-term (2003–2016) satellite observations (NASA’s A-train satellite constellations) over the northern Indian Ocean, that enhanced aerosol loading (due to anthropogenic emissions) can reverse the first indirect effect significantly. In contrast to Twomey effect, a statistically significant increase in cloud effective radius (CER, µm) is observed with respect to an increase in aerosol loading for clouds having low liquid water path (LWP < 75 g m−2) and drier cloud tops. Probable physical mechanisms for this effect are the intense competition for available water vapour due to higher concentrations of anthropogenic aerosols and entrainment of dry air on cloud tops. For such clouds, cloud water content showed a negative response to cloud droplet number concentrations and the estimated intrinsic radiative effect suggest a warming at the Top of the Atmosphere. Although uncertainties exist in quantifying aerosol-cloud interactions (ACI) using satellite observations, present study indicates the physical existence of anti-Twomey effect over the northern Indian Ocean during south Asian outflow.

Atmospheric aerosol and black carbon optical properties and associated radiative forcing under haze conditions

The optical properties and radiative forcing of atmospheric aerosol (ARF) and black carbon (BC) aerosol (BCRF) in ultraviolet (UV), visible (VIS), near-infrared (NIR), and shortwave (SW) spectra were investigated under haze conditions based on the observations of the Aethalometer and sun-sky radiometer and simulations from libRadtran. The results show that the BC concentrations increased greatly from 2.73 μg/m3 under clear-air conditions to 7.95 μg/m3 under severe haze conditions, while BC aerosol optical depth (AOD) increased from 0.025 to 0.092. A high correlation (R2=0.62) was found between BC AOD and absorbing aerosol optical depth (AAOD) derived from the sun-sky radiometer. The BCRF in SW (BCRFSW) varied from -10.20 W/m2 under clear-air conditions to -25.40 W/m2 under severe hazy conditions. However, its fraction in ARF (ARFSW) decreased from 19% to 17% simultaneously, which is mainly related to the decrease of the ratio of BC AOD to AOD. The fraction of ARF in VIS in ARFSW decreased from 56.3% under clear-air conditions to 50.5% under severe haze conditions, while the fraction of BCRF in VIS in BCRFSW was much larger, and increased from 72.9% to 73.8%. The BCRF efficiency (BCRFE) was much larger than ARF efficiency (ARFE), and both of them decreased with the development of haze. The ARFE in SW decreased from -173.84 W/m2 under clear-air conditions to -112.75 W/m2 under severe haze conditions while BCRFESW varied from -482.50 W/m2 to -321.88 W/m2. The decrease of ARFE and BCRFE is related to the increase of aerosol loading and asymmetry factor (ASY). The ASY increased and the forward scattering was enhanced with the development of haze due to the hygroscopic growth of aerosol particles, which reduced the extinction efficiency of aerosols including BC on solar radiation and the cooling effect on the surface.

Ten Years of Aerosol Effects on Single-Layer Overcast Clouds over the US Southern Great Plains and the China Loess Plateau

Using almost 10 years of observations of clouds and aerosols from the US Southern Great Plains (SGP) atmospheric observatory and the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) in China, the impact of aerosols on single-layer overcast clouds over continental land for different regimes were investigated. Atmospheric conditions at the two sites were first compared in an attempt to isolate the influence of aerosols on cloud properties from dynamic and thermodynamic influences. Cloud types and amounts are similar at the two sites. The dominant aerosol types at the SGP and SACOL sites are sulphate and dust, respectively, with greater aerosol optical depths (AODs) and absorption at the SACOL site. Aerosol first indirect effect (FIE) ranges from 0.021 to 0.152 and from −0.078 to 0.047 at the SGP and SACOL sites, respectively, when using the AOD below cloud base as CCN proxy. Although differences exist, the influence of meteorological conditions on the FIE at the two sites is consistent. FIEs are easily detected under descending motion and dry condition. The FIE at the SGP site is larger than that at the SACOL site, which suggests that the cloud albedo effect is more sensitive under relatively cleaner atmospheric conditions and the dominating aerosol at the SACOL site has less hygroscopicity. The radiative forcing of the FIE over the SGP site is −3.2 W m−2 for each 0.05 increment in FIE. Cloud durations generally prolong as aerosol loading increases, which is consistent with the hypothesis of the aerosol second indirect effect. The negative relationship between cloud duration time and aerosol loading when aerosol loading reaches a large value further might suggest a semidirect effect.

Reexamining the Mechanisms of East Asian Summer Monsoon Changes in Response to Non–East Asian Anthropogenic Aerosol Forcing

AbstractThis study examines the mechanisms by which the East Asian summer monsoon (EASM) changes in response to non–East Asian (NEA) anthropogenic aerosol forcing by distinguishing the fast direct atmospheric response and slow ocean-mediated response to forcing using a global aerosol–climate coupled model. The results show that NEA aerosol forcing significantly exacerbates the weakening of the EASM due to local aerosol forcing. The fast response is dominant in the weakening of the EASM and an anomalous precipitation pattern over eastern China resembling the “southern flood and northern drought” pattern in the total response to NEA aerosol forcing. Changes in upper-tropospheric temperature caused by the fast response play a major role in the impact of NEA aerosol forcing on the EASM. Anomalous cooling occurs during summer in the upper troposphere (at ~40°N) over East Asia caused by the fast response. This is due to the combined effects of strong eastward cold advection in the Northern Hemisphere midlatitudes caused by increased aerosol loading in Europe and the resulting change in local meridional heat transport in East Asia. Subsequently, the zonal wind speed changes on either side of the anomalous cooling, and the East Asian subtropical jet shifts equatorward, thereby weakening the EASM. The changes in atmospheric temperature and the local Hadley cell caused by the slow response to NEA aerosol forcing are conducive to strengthening the southwesterly winds over eastern China. Our study suggests the importance of NEA aerosol forcing in driving changes in the EASM on a fast time scale.

Validation and Accuracy Analysis of the Collection 6.1 MODIS Aerosol Optical Depth Over the Westernmost City in China Based on the Sun‐Sky Radiometer Observations From SONET

AbstractToward the Kashi region in northwest of China, the Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) retrievals from Collection 6 (C6) MYD, Collection 6.1 (C6.1) MOD, and C6.1 MYD during 2016–2017 are compared with ground‐based measurements from the Sun‐sky Radiometer Network (SONET), and the first comprehensive evaluation of the Dark Target (DT) and Deep Blue (DB) retrievals with a 10‐km spatial resolution in the latest C6.1 MYD AOD data set during 2016–2018 is presented. In general, C6.1 MYD AOD products (both of DT and DB algorithm) are the most effective in Kashi of the three collections, and there is an overall underestimation of DB AOD, while DT AOD that slightly outperformed DB AOD in Kashi is overestimated on the whole. As to the factors that influence the accuracy of MODIS AOD, for DB algorithm, the overestimations of the surface reflectance and Single Scattering Albedo that DB aerosol model assumed can cause underestimation of DB AOD retrievals over Kashi, while the ones for DT algorithm are opposite. Besides, the coarse dust particles with lower veracity are predominant in Kashi region, which illustrated that the errors of particle size assumption in C6.1 MYD DT and DB algorithms will make large inversion error of MODIS AOD. Moreover, whatever DB or DT algorithm, the accuracy of AOD is diminished as the aerosol loading increases. The more realistic aerosol models and surface characterizations are necessary during the process of generating the MODIS aerosol retrievals in Kashi region.

Atmospheric visibility variation over global land surface during 1973–2012: Influence of meteorological factors and effect of aerosol, cloud on ABL evolution

Atmospheric visibility (Vis) is used to evaluate air pollution over global land surface during 1973–2012. Over global more populated areas with major anthropogenic emissions and hygroscopic aerosol species, weaker surface wind is generally associated with more severe air pollution and Vis degradation, while an increase of relative humidity (RH) generally favors more efficient secondary formation and hygroscopic growth of aerosol and Vis impairment. The allocation of meteorological factor with the major aerosol component over global different areas can influence the formation and variation of aerosol and cloud, and their effects on the atmospheric boundary layer (ABL) evolution and air pollution. Over East Asia, India and Southeast Asia, the air pollution with a growing concern with Vis decrease can be associated to an increase in anthropogenic emission and a synchronous stabilization of ABL (characterized by a weakened, less variable surface wind and a decreased diurnal temperature range, DTR). Both the increase of aerosol loading and cloud cover could (i) increase daily minimum air temperature versus decrease daily maximum air temperature and result in a decrease of DTR, (ii) reduce solar radiation reaching the surface, stabilize the ABL and aggravate air pollution. While Vis increase over Europe could be related to more effective emission control and an intensification of surface wind. Also significantly, a close relationship is identified between RH and DTR; RH explains about 90% of the variance of DTR over Europe in summer; a drier weather is corresponding to a greater surface incoming short-wave flux and a greater DTR.