Aerosol Optical Properties and Its Radiative Forcing over Yulin, China in 2001 and 2002

The aerosol optical properties and direct radiative forcing over the Mu Us desert of northern China, acquired through a CE318 sunphotometer of the ground-based Aerosol Robotic Network （AERONET）, are analyzed. The seasonal variations in the aerosol optical properties are examined. The effect of meteorological elements （pressure, temperature, water vapor pressure, relative humidity and wind speed） on the aerosol optical properties is also studied. Then, the sources and optical properties under two different cases, a dust event and a pollution event, are compared. The results show that the high aerosol optical depth （AOD） found in Yulin was mostly attributed to the occurrence of dust events in spring from the Mu Us desert and deserts of West China and Mongolia, as well as the impacts of anthropogenic pollutant particles from the middle part of China in the other seasons. The seasonal variation and the probability distribution of the radiative forcing and the radiative forcing efficiency at the surface and the top of the atmosphere are analyzed and regressed using the linear and Gaussian regression methods.

Retrieval of Aerosol Optical Properties over the Beijing Area Using POLDER/PARASOL Satellite Polarization Measurements

Aerosol optical properties over Beijing and Xianghe under several typical weather conditions （clear sky, light haze, heavy pollution and dust storm） are derived from POLDER （POLarization and Directionality of the Earth＇s Reflectances）/PARASOL （Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar） multi-directional, multi-spectral polarized signals using a more reliable retrieval algorithm as proposed in this paper. The results are compared with those of the operational retrieval algorithm of POLDER/PARASOL group and the ground-based AERONET （AErosol RObotic NETwork）/PHOTONS （PHOtometrie pour le Traitement Operational de Normalisation Satellitaire） measurements. It is shown that the aerosol optical parameters derived from the improved algorithm agree well with AERONET/PHOTONS measurement. The retrieval accuracies of aerosol optical thickness （AOT） and effective radius are 0.06 and 0.05 μm respectively, which are close to or better than the required accuracies （0.04 for AOT and 0.1 μm for effective radius） for estimating aerosol direct forcing.

Retrieval of Aerosol Optical Properties over a Vegetation Surface Using Multi-angular, Multi-spectral, and Polarized Data

An algorithm to retrieve aerosol optical properties using multi-angular,multi-spectral,and polarized data without a priori knowledge of the land surface was developed.In the algorithm,the surface polarized reflectance was estimated by eliminating the atmospheric scattering from measured polarized reflectance at 1640 nm.A lookup table （LUT） and an iterative method were adopted in the algorithm to retrieve the aerosol optical thickness （AOT,at 665 nm） and the （A）ngstr（o）m exponent （computed between the AOTs at 665 and 865 nm）.Experiments were performed in Tianjin to verify the algorithm.Data were provided by a newly developed airborne instrument,the Advanced Atmosphere Multi-angle Polarization Radiometer （AMPR）.The AMPR measurements over the target field agreed well with the nearby ground-based sun photometer.An algorithm based on Research Scanning Polarimeter （RSP） measurements was introduced to validate the observational measurements along a flight path over Tianjin.The retrievals were consistent between the two algorithms.The AMPR algorithm shows potential in retrieving aerosol optical properties over a vegetation surface.

Seasonal Variation of Columnar Aerosol Optical Properties in Yangtze River Delta in China

In order to understand the seasonal variation of aerosol optical properties in the Yangtze River Delta,5 years of measurements were conducted during September 2005 to December 2009 at Taihu,China.The monthly averages of aerosol optical depth were commonly 0.6;the maximum seasonal average（0.93） occurred in summer.The magnitude of the Angstr¨om exponent was found to be high throughout the year;the highest values occurred in autumn（1.33） and were the lowest in spring（1.08）.The fine modes of volume size distribution showed the maxima（peaks） at a radius of ～0.15 μm in spring,autumn,and winter;at a radius of ～0.22 μm in summer.The coarse modes showed the maxima（peaks） at a radius of 2.9 μm in spring,summer,and autumn and at a radius of 3.8 μm in winter.The averages of single-scattering albedo were 0.92（spring）,0.92（summer）,0.91（autumn）,and 0.88（winter）.The averages of asymmetry factor were found to be larger in summer than during other seasons;they were taken as 0.66 at 440-1020 nm over Taihu.The real part of the refractive index showed a weak seasonal variation,with averages of 1.48（spring）,1.43（summer）,1.45（autumn）,and 1.48（winter）.The imaginary parts of the refractive index were higher in winter（0.013） than in spring（0.0076）,summer（0.0092）,and autumn（0.0091）,indicating that the atmosphere in the winter had higher absorbtivity.

Analysis and Evaluation of the Global Aerosol Optical Properties Simulated by an Online Aerosol-coupled Non-hydrostatic Icosahedral Atmospheric Model

Aerosol optical properties are simulated using the Spectral Radiation Transport Model I~）r Aerosol Species （SPRINTARS） coupled with the Non-hydrostatic ICosahedral Atmospheric Model （NICAM）. The 3-year global mean all-sky aerosol optical thickness （AOT） at 550 nm, theAngstr/Sm Exponent （AE） based on AOTs at 440 and 870 nm, and the single scattering albedo （SSA） at 550 nm are estimated at 0.123, 0.657 and 0.944, respectively. For each aerosol species, the mean AOT is within the range of the AeroCom models. Both the modeled all-sky and clear-sky results are compared with observations from the Moderate Resolution Imaging Spectroradiometer （MODIS） and the Aerosol Robotic Network （AERONET）. The simulated spatiotemporal distributions of all-sky AOTs can generally reproduce the MODIS retrievals, and the correlation and model skill can be slightly improved using the clear-sky results over most land regions. The differences between clear-sky and all-sky AOTs are larger over polluted regions. Compared with observations from AERONET, the modeled and observed all-sky AOTs and AEs are generally in reasonable agreement, whereas the SSA variation is not well captured. Although the spatiotemporal distributions of all-sky and clear-sky results are similar, the clear-sky results are generally better correlated with the observations. The clear-sky AOT and SSA are generally lower than the all-sky results, especially in those regions where the aerosol chemical composition is contributed to mostly by sulfate aerosol. The modeled clear-sky AE is larger than the all-sky AE over those regions dominated by hydrophilic aerosol, while the＇opposite is found over regions dominated by hydrophobic aerosol.

Accurate measurement of aerosol optical properties using the multilongitudinal mode high-spectral-resolution lidar with self-tuning Mach-Zehnder interferometer

The multilongitudinal mode[MLM]high-spectral-resolution lidar[HSRL]based on the Mach±Zehnder interferometer[MZI]is constructed in Xi’an for accurate measurements of aerosol optical properties.The critical requirement of the optimal match between the free spectral range of MZI and the longitudinal mode interval of the MLM laser is influenced by the laboratory temperature,pressure,and vibration.To realize the optimal separation of aerosol Mie scattering signals and molecular Rayleigh scattering signals excited by the MLM laser,a self-tuning technique to dynamically adjust the optical path difference[OPD]of the MZI is proposed,which utilizes the maximum ratio between the received power of the Mie channel and Rayleigh channel as the criterion of the OPD displacement.The preliminary experiments show the feasibility of the MLM-HSRL with self-tuning MZI and the stable performance in the separation of aerosol Mie scattering signals and molecular Rayleigh scattering signals.

Observation and analysis of near-surface atmospheric aerosol optical properties in urban Beijing

Year-round measurements of the mass concentration and optical properties of fine aerosols （PM2s） from June 2009 to May 2010 at an urban site in Beijing were analyzed. The annual mean values of the PM2.5 mass concentration, absorption coefficient （Ab）, scattering coefficient （Sc） and single scattering albedo （SSA） at 525 nm were 67 ± 66 μg/m^3, 64 ± 62 Mm^-1, 360±405 Mm^-1 and 0.82±0.09, respectively. The bulk mass absorption efficiency and scattering efficiency of the PM2.5 at 525 nm were 0.78 m^2/g and 5.55 m^2]g, respectively. The Ab and Sc showed a similar diurnal variation with a maximum at night and a minimum in the afternoon, whereas SSA displayed an opposite diurnal pattern. Significant increases in the Ab and Sc were observed in pollution episodes caused by the accumulation of pollutants from both local and regional sources under unfavorable weather conditions. Aerosol loadings in dust events increased by several times in the spring, which had limited effects on the Ab and Sc due to the low absorption and scattering efficiency of dust particles. The frequency of haze days was the highest in autumn because of the high aerosol absorption and scattering under unfavorable weather conditions. The daily PM2.5 concentration should be controlled to a level lower than 64 μg/m^3 to prevent the occurrence of haze days according to its exponentially decreased relationship with visibility.

An intensive study on aerosol optical properties and affecting factors in Nanjing,China

The optical properties of aerosol as well as their impacting factors were investigated at a suburb site in Nanjing during autumn from 14 to 28 November 2012. More severe pollution was found together with lower visibility. The average scattering and absorption coefficients（B sca and B abs） were 375.7 ± 209.5 and 41.6 ± 18.7 Mm^（-1）, respectively. Higher ？ngstr？m absorption and scattering exponents were attributed to the presence of more aged aerosol with smaller particles. Relative humidity（RH） was a key factor affecting aerosol extinction. High RH resulted in the impairment of visibility, with hygroscopic growth being independent of the dry extinction coefficient. The hygroscopic growth factor was 1.8 ± 1.2 with RH from 19% to 85%.Light absorption was enhanced by organic carbon（OC）, elemental carbon（EC） and EC coatings,with contributions of 26%, 44% and 75%（532 nm）, respectively. The B sca and B abs increased with increasing N_（100）（number concentration of PM_（2.5）with diameter above 100 nm）, PM_1 surface concentration and PM_（2.5）mass concentration with good correlation.

Characterization of aerosol optical properties, chemical composition and mixing states in the winter season in Shanghai, China

Physical and chemical properties of ambient aerosols at the single particle level were studied in Shanghai from December 22 to 28, 2009. A Cavity-Ring-Down Aerosol Extinction Spectrometer（CRD-AES） and a nephelometer were deployed to measure aerosol light extinction and scattering properties, respectively. An Aerosol Time-of-Flight Mass Spectrometer（ATOFMS）was used to detect single particle sizes and chemical composition. Seven particle types were detected. Air parcels arrived at the sampling site from the vicinity of Shanghai until mid-day of December 25, when they started to originate from North China. The aerosol extinction,scattering, and absorption coefficients all dropped sharply when this cold, clean air arrived.Aerosol particles changed from a highly aged type before this meteorological shift to a relatively fresh type afterwards. The aerosol optical properties were dependent on the wind direction.Aerosols with high extinction coefficient and scattering Angstrom exponent（SAE） were observed when the wind blew from the west and northwest, indicating that they were predominantly fine particles. Nitrate and ammonium correlated most strongly with the change in aerosol optical properties. In the elemental carbon/organic carbon（ECOC） particle type, the diurnal trends of single scattering albedo（SSA） and elemental carbon（EC） signal intensity had a negative correlation. We also found a negative correlation（r =-0.87） between high mass-OC particle number fraction and the SSA in a relatively clean period, suggesting that particulate aromatic components might play an important role in light absorption in urban areas.

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-KarakoramHimalayas,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 (ARF_(TOA)),the bottom of the atmosphere (ARF_(BOA)),and in the atmosphere (ARFATM) were -35±7,-73±16,and38±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-2during 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.

Model Analysis of Influences of Aerosol Mixing State upon Its Optical Properties in East Asia

The air quality model system RAMS （Regional Atmospheric Modeling System）-CMAQ （Models-3 Com- munity Multi-scale Air Quality） coupled with an aerosol optical/radiative module was applied to investigate the impact of different aerosol mixing states （i.e., externally mixed, half externally and half internally mixed, and internally mixed） oil radiative forcing in East Asia. The simulation results show that the aerosol optical depth （AOD） generally increased when the aerosol mixing state changed from externally mixed to internally mixed, while the single scattering albedo （SSA） decreased. Therefore, the scattering and absorption proper- ties of aerosols can be significantly affected by the change of aerosol mixing states. Comparison of simulated and observed SSAs at five AERONET （Aerosol Robotic Network） sites suggests that SSA could be better estimated by considering aerosol particles to be internally mixed. Model analysis indicates that the impact of aerosol mixing state upon aerosol direct radiative forcing （DRF） is complex. Generally, the cooling effect of aerosols over East Asia are enhanced in the northern part of East Asia （Northern China, Korean peninsula, and the surrounding area of Japan） and are reduced in the southern part of East. Asia （Sichuan Basin and Southeast China） by internal mixing process, and the variation range can reach ＋5 W m 2. The analysis shows that the internal mixing between inorganic salt and dust is likely the main reason that the cooling effect strengthens. Conversely, the internal nfixture of anthropogenic aerosols, including sulfate, nitrate, ammonium, black carbon, and organic carbon, could obviously weaken the cooling effect.

A Review of Aerosol Optical Properties and Radiative Effects

Atmospheric aerosols influence the earth＇s radiative balance directly through scattering and absorbing solar radiation,and indirectly through affecting cloud properties.An understanding of aerosol optical properties is fundamental to studies of aerosol effects on climate.Although many such studies have been undertaken,large uncertainties in describing aerosol optical characteristics remain,especially regarding the absorption properties of different aerosols.Aerosol radiative effects are considered as either positive or negative perturbations to the radiation balance,and they include direct,indirect（albedo effect and cloud lifetime effect）,and semi-direct effects.The total direct effect of anthropogenic aerosols is negative（cooling）,although some components may contribute a positive effect（warming）.Both the albedo effect and cloud lifetime effect cool the atmosphere by increasing cloud optical depth and cloud cover,respectively.Absorbing aerosols,such as carbonaceous aerosols and dust,exert a positive forcing at the top of atmosphere and a negative forcing at the surface,and they can directly warm the atmosphere.Internally mixed black carbon aerosols produce a stronger warming effect than externally mixed black carbon particles do.The semidirect effect of absorbing aerosols could amplify this warming effect.Based on observational（ground- and satellite-based） and simulation studies,this paper reviews current progress in research regarding the optical properties and radiative effects of aerosols and also discusses several important issues to be addressed in future studies.

Comparison of Column-Integrated Aerosol Optical and Physical Properties in an Urban and Suburban Site on the North China Plain

The column-integrated optical properties of aerosol in Beijing and Xianghe, two AErosol RObotic NETwork（AERONET）sites situated on the North China Plain（NCP）, are investigated based on Cimel sunphotometer measurements from October2004 to June 2012. The outstanding feature found is that the seasonal medians of aerosol optical depth（AOD） at the two stations are in good agreement. The correlation coefficients and the absolute differences between AOD at the two stations are larger than 0.84 and less than 0.05, respectively. Good agreement in AOD at these two sites（one urban and the other suburban; 70 km apart） indicates that aerosol pollution in the Greater Beijing area is regional in nature. However, we find significant differences in the absorption ？Angstr ¨om Exponent（AAE）, the real and imaginary part of the refractive index, and thereby the single scattering albedo（SSA）, and the difference is seasonally dependent. The feature is found to be more prominent in fall when the fine-mode fraction（FMF） and fine-mode effective radius are significantly different at the two stations,besides the parameters mentioned above. The SSA in Beijing at four wavelengths shows lower values as compared to those in Xianghe, although the difference is not significant in some cases. Significant differences in AAE and fine-mode effective radius indicate that there are differences in aerosol physical and chemical properties in urban and suburban regions on the NCP.

Constraint Inversion Algorithm of Lidar Equationfor Deriving Aerosol Optical Property

A key question of the backward integration algorithm to lidar equation is how to determine the far-endboundary value. This paper develops a Constraint Inversion Algorithm (CIA) for deriving the value andthen the aerosol extinction profile from lidar signals, which uses the ground-level horizontal lidar signals asthe constraint information. The smaller the wavelength is, the more sensitive to the variation of aerosol extinction to backscatter ratio solved by CIA. According to the property an algorithm is further proposed tosimultaneously retrieve the aerosol extinction profile, the size distribution and the imaginary part of its reflective index from the multi-wavelength lidar observations. CIA is tested in the inversion simulations withsatisfactory result.

Recent Progresses in Atmospheric Remote Sensing Research in China——Chinese National Report on Atmospheric Remote Sensing Research in China during 1999-2003

Progresses of atmospheric remote sensing research in China during 1999-2003 are summarily introduced. This research includes: (1) microwave remote sensing of the atmosphere; (2) Lidar remote sensing; (3) remote sensing of aerosol optical properties; and (4) other research related to atmospheric remote sensing, including GPS remote sensing of precipitable water vapor and radiation modei development.

Synergetic analysis of springtime air pollution episodes over Gwangju,Korea

The characteristics of springtime aerosols,including their optical and microphysical properties,were analyzed for the months of March to May of 2009 in Gwangju（35.23°N,126.84°E）,Korea.A high Light Detection and Ranging（LIDAR）-derived aerosol depolarization ratio（δ） of 0.25 ± 0.04 was determined on dust particles during the observation period.The？ngstr？m exponent values of the 440–870 nm wavelength pair（？_（440–870）） and single-scattering albedo at 675 nm（Ω_（675）） measured by a CIMEL sun/sky radiometer were 0.77 ± 0.19 and 0.95 ±0.01,respectively.The elevated dust layers reached a maximum elevation of 4 km above sea level.Anthropogenic/smoke particles that originated from highly populated/industrialized regions could be distinguished by their relatively smaller particle size（？_（440–870） ranged between1.33 and 1.36） and higher light-absorbing（Ω_（675） of 0.92 ± 0.01） characteristics.These aerosols are mostly distributed at altitudes 〈 1.2 km.The root-mean-square deviation（RMSD） between the aerosol optical depth（AOD,τ） derived from LIDAR_（（τ_（LIDAR））） and from the CIMEL sun/sky radiometer_（（τ_（CIMEL））） varied with respect to the surface PM10 concentration.The RMSD between τ_（LIDAR） and τ_（CIMEL） was as low as 13% under lower PM_（10） concentration levels（〈 100 μg/m^3）.In contrast,the RMSD between τ_（LIDAR） and τ_（CIMEL） increased three times（~31%） under high surface PM_（10） concentration levels（〉100 μg/m^3）.These results suggest that the accuracy of τ_（LIDAR） is influenced by specific atmospheric conditions,regardless of its uncertainty.