Aerosol Complex Refractive Index Research Articles

Inversion of Near-Surface Aerosol Equivalent Complex Refractive Index Based on Aethalometer, Micro-Pulse Lidar and Portable Optical Particle Profiler

In order to investigate the equivalent complex refractive index of atmospheric aerosols near the Earth’s surface, we conducted measurements in the Hefei region from March to April 2022. These measurements utilized a micro-pulse lidar, an Aethalometer, and a Portable Optical Particle Profiler. These measurements encompassed aerosol particle size distribution as well as standard meteorological parameters including temperature, humidity, atmospheric pressure, and wind speed. Subsequently, this dataset was employed to develop an optimization algorithm for retrieving the equivalent complex refractive indices of near-surface aerosols. The methodology relies on lookup tables containing data for extinction efficiency and absorption efficiency factors. It operates on the premise of aerosol property stability within a defined time frame, utilizing measured extinction and absorption coefficients as simultaneous constraints during this period to inversely derive both the real and imaginary parts of the aerosol complex refractive index. Results from the simulation analysis reveal that the newly optimized retrieval algorithm, which relies on lookup tables, exhibits reduced sensitivity to instrument errors when compared to single-point constraint algorithms. This enhancement results in a more efficient and dependable approach for retrieving the aerosol complex refractive index. Empirical inversion and simulation studies were carried out to determine the aerosol equivalent complex refractive index in the Hefei region, utilizing measured data. This inversion process yielded an average complex refractive index of 1.48-i0.017 for aerosols in the Hefei region throughout the experimental period. Correlation analysis unveiled a positive association between the real part of the aerosol complex refractive index and the single-scattering albedo (SSA), whereas the imaginary part displayed a linear negative correlation with the SSA. The mathematical relationship between the real part and the SSA is y=0.19x+0.62, and the corresponding relationship between the imaginary part and the SSA is y=−5.3x+0.99. This research offers a novel method for the retrieval of the aerosol equivalent complex refractive index.

Estimation of Aerosol Complex Refractive Index over a tropical atmosphere using a synergy of in-situ measurements

Aerosol complex refractive index (ACRI) is a key micro-physical parameter used for modeling their radiative effects. In this study, an approach is developed to estimate the ACRI using a combination of surface-based measurements (2008–2015) from the Aerodynamic Particle Sizer (APS), the integrating Nephelometer, and the Aethalometer over a tropical rural location in the southern peninsular India. Around 98% of the real part of the estimated ACRI varied in the range of 1.3–1.8 while 99% of the imaginary part was found to vary in the range of 0.01–0.2 at 550 nm. The estimated imaginary parts are found to be high during pre-monsoon season suggesting the dominance of the increased absorption type aerosols. This is in line with the high concentration of black carbon aerosol observed during the pre-monsoon. The spectral variation of estimated ACRI is also studied and finally the significance of refractive index of surface aerosols in columnar properties is analysed by using the columnar single scattering albedo (SSA) and aerosol optical depth (AOD) observations from Sky radiometer. During winter season, the contribution to the total columnar scattering and absorption are mainly from surface or below the atmospheric boundary layer aerosols, whereas in monsoon season 80–90% contribution is from aerosols above.

Refractive index confidence explorer (RICE): A tool for propagating uncertainties through complex refractive index retrievals from aerosol particles

Accurate and precise retrievals of aerosol complex refractive indices (m) are essential to constraining the direct radiative effect of atmospheric aerosols. Despite this, there is no generally acce...

Determination of the refractive index of ambient aerosols

Abstract The refractive index of ambient aerosols is one of the most important parameters indicating the scattering and absorption properties of aerosols. We proposed a new method for retrieving the refractive index (RI) of ambient particles. The main advantage of our method is that it assimilates the single particle mixing states measured by a single-particle soot photometer, when compared to the traditional optical method of retrieving the ambient aerosol RI. This method was validated by good consistency between the determined RI with this method and retrieved RI with the method of Zhao et al. (2019c) using datasets from field measurements conducted in East China in June of 2018. The results show that the real part of the refractive index of the black carbon (BC)-free particles ranged between 1.37 and 1.51 and this value changed little across different tested wavelengths. The mean complex refractive index for the refractory BC was 1.67 ± 0.67i at 525 nm. The mean imaginary parts of the other non-BC components were 0.019 and 0.023 at 450 nm and 370 nm respectively. Brown carbon contributed to 5%, 13% and 29% of the ambient aerosol light absorption at 525 nm, 450 nm and 370 nm respectively in East China. This study provides the ability to determine the ambient aerosol complex refractive index and these data can be used in models to reduce the uncertainties in estimating aerosol radiative forcing.

The Optical Properties of Aerosols at the Summit of Mount Tai in May and June and the Retrieval of the Complex Refractive Index

To study the optical properties of background atmospheric aerosols in East China, we carried out observations of the physical, chemical and optical properties of atmospheric aerosols at the summit of Mount Tai (Mt. Tai, 1533.7 m above sea level) from 13 May to 25 June 2017. The results show that the average scattering coefficient ( σ sca , 550 ) at 550 nm of the aerosols at the summit of Mt. Tai is 40.3 Mm−1, and the average absorption coefficient ( σ abs , 550 ) at 550 nm is 16.0 Mm−1. The complex refractive index of aerosols is a key parameter for aerosol retrieval and modeling. There are few studies on the equivalent complex refractive index of aerosol in the Taishan area. We calculated the aerosol equivalent complex refractive index using the observed aerosol scattering coefficients, absorption coefficients and particle size distribution data, providing more data support for future modeling in this region. The real part (n) of the complex refractive index at 550 nm of aerosol ranges from 1.31 to 1.98 (mostly under 1.50), with an average value of 1.38, while the imaginary part (k) ranges from 0.014 to 0.251 (less than 0.10 for over 95% samples), with an average value of 0.040. The analysis of the n and k of the aerosol average complex refractive index shows that the scattering properties of the aerosols at the summit of Mt. Tai are relatively weak and the absorption properties are relatively strong when compared with those of other kinds of aerosols. The k of the aerosol complex refractive index at the summit of Mt. Tai has strong correlations with the wind speed, temperature, as revealed by the correlation analysis.

Optimized profile retrievals of aerosol microphysical properties from simulated spaceborne multiwavelength Lidar

This work is an expanded study of one previously published on retrievals of aerosol microphysical properties from space-borne multiwavelength lidar measurements. The earlier studies and this one were done in the framework of the NASA Aerosol-Clouds-Ecosystems (now the Aerosol Clouds Convection and Precipitation) NASA mission. The focus here is on the capabilities of a simulated spaceborne multiwavelength lidar system for retrieving aerosol complex refractive index (m = mr + imi) and spectral single scattering albedo (SSA(λ)), although other bulk parameters such as effective (reff) radius and particle volume (V) and surface (S) concentrations are also studied. The novelty presented here is the use of recently published, case-dependent optimized-constraints on the microphysical retrievals using three backscattering coefficients (β) at 355, 532 and 1064 nm and two extinction coefficients (α) at 355 and 532 nm, typically known as the stand-alone 3β + 2α lidar inversion. Case-dependent optimized-constraints (CDOC) limit the ranges of refractive index, both real (mr) and imaginary (mi) parts, and of radii that are permitted in the retrievals. Such constraints are selected directly from the 3β + 2α measurements through an analysis of the relationship between spectral dependence of aerosol extinction-to-backscatter ratios (LR) and the Ångström exponent of extinction. The analyses presented here for different sets of size distributions and refractive indices reveal that the direct determination of CDOC are only feasible for cases where the uncertainties in the input optical data are less than 15%. For the same simulated spaceborne system and yield than in Whiteman et al., (2018), we demonstrated that the use of CDOC as essential for the retrievals of refractive index and also largely improved retrieval of bulk parameters. A discussion of the global representativeness of CDOC is presented using simulated lidar data from a 24 h satellite track using GEOS model output to initialize the lidar simulator. We found that CDOC are representative of many aerosol mixtures in spite of some outliers (e.g. highly hydrated particles) associated with the assumptions of bimodal size distributions and of the same refractive index for fine and coarse modes. Moreover, sensitivity tests performed using synthetic data reveal that retrievals of imaginary refractive index (mi) and SSA are extremely sensitive to β(355).

Complex refractive index of volcanic ash aerosol in the infrared, visible, and ultraviolet.

Very fine silicate-rich volcanic ash, generated by explosive volcanic eruptions, can efficiently be traced downwind with infrared satellite sounders. Their measurements can also be used to derive physical parameters, such as optical depths and effective radii. However, one of the key requirements for accurate retrievals is a good knowledge of the complex refractive index (CRI) of the ash under investigation. In the past, the vast majority of the studies used the CRIs from Pollack et al. [Icarus19, 372 (1973)ICRSA50019-103510.1016/0019-1035(73)90115-2], which are based on measurements of thin slices of volcanic rock, and therefore are not representative for airborne volcanic ash particles. Here, we report measurements of the CRI of volcanic ash in suspension, generated from samples collected from recent high-impact eruptions in Chile (Puyehue-Cordón Caulle, Calbuco, and Chaitén), Iceland (Eyjafjallajökull and Grímsvötn), and Italy (Etna). The samples cover a wide range of ${{\rm SiO}_2}$SiO2 content (46% to 76%) as confirmed by an X-ray fluorescence analysis. In the experimental setup, volcanic ash was suspended in nitrogen through mechanical agitation. Extinction spectra were recorded in the infrared, visible, and ultraviolet spectral regions. The particle size distribution within the airflow was also recorded. An iterative algorithm allowed us to obtain fully consistent CRIs for the six samples, compatible with the observed extinction spectra and the Kramers-Krönig relations. While a good agreement is found with other recently reported CRIs in the UV/Vis, larger differences are found in the longwave infrared spectral region.

Capabilities and limitations of the single-particle extinction and scattering method for estimating the complex refractive index and size-distribution of spherical and non-spherical submicron particles

Nondestructive inline measurements of submicron-sized particles and gas bubbles are required in many fields of study, including environmental, industrial, and biomedical research. The single particle extinction and scattering (SPES) method is a promising technique for this purpose because of its sensitivity to both the complex refractive index and the size of individual particles. Herein an original modification of the SPES instrument that suppresses measurement bias and background noise is presented. In addition, relevant theory and a practical algorithm for data analysis are presented. The inverse scattering problem of estimating the physical characteristics of a particle from SPES data using Bayesian theory for model selection is formulated, and the performance of the algorithm using simulated data for spherical and non-spherical particles is evaluated. The results suggest that a cluster of SPES data points for an ensemble of particles contains sufficient information to retrieve the complex refractive index and volume equivalent size-distribution of the particles, regardless of particle shape. The Bayesian inversion method also allows incorporation of informative data from other analytical methods (e.g., electron microscope, upstream size-sorting device) through the model and parameter priors. As practical examples, the SPES method was used to constrain the complex refractive index and size distribution of water-insoluble aerosols contained in rainwater. The results suggest that the SPES method itself or its use in combination with other conventional analytical methods can quantify the complex refractive index and size distribution of almost any type of submicron particle (including gas bubbles) in environmental, industrial, and biomedical samples.

Characterization of optically effective complex refractive index of black carbon composite aerosols

Black carbon aerosols, always existing as complicated mixtures of various species, have important impacts on the global and regional climate, whereas their effective aerosol complex refractive index (ACRI) is a must for modeling their radiative effects. Retrieval of optically effective ACRI from accurately calculated scattering and absorption properties of an internal-mixed aerosol model with BC and dust coated with sulfate, roughly representing ambient aged multicomponent BC particles, are performed. The sensitivities of retrieved optically effective ACRI to particle size distribution and composition ratio are explored, and the popular volume weighted average (VWA) method and Bruggeman effective medium (BEM) theory are also considered for comparison. The optically effective ACRIs depicts dramatic variation for monodisperse size parameter larger than π, as opposed to almost a constant for size parameter less than π, whereas the VWA and BEM can hardly provide acceptable effective ACRIs for coarse BC composite particles. The retrieved optically effective ACRI are sensitive to both size distribution and composition ratio, and becomes more sensitive to particle size distribution with aerosols containing more BC content. As BC volume content reaches ~4%, the VWA overestimates the imaginary part of effective ACRI with significant bias when compared with the optically effective ACRI, indicating overestimated aerosol absorption in most atmospheric environments in aerosol-climate models employing the VWA. We suggest that the optically effective ACRI, rather than the ACRI given by the VWA, should be applied to account for coarse BC-containing particles in the state-of-the-art aerosol-climate models.

Estimation and extraction of the aerosol complex refractive index based on Stokes vector measurements.

We present a way to estimate the aerosol complex refractive indices (ACRI) based on a real-time Stokes vector measurement technology. First, we introduce how to simultaneously get the multi-polarization signals of multi-scattering angles. Then we demonstrate the numerical inversion to retrieve an ACRI based on an iterative Mie algorithm. Meanwhile, we show the experimental results of several types of aerosol samples. Our optimal estimation of ACRI inversion shows a good agreement with the references, which confirms the feasibility and then implies a good prospect of multidimensional polarization characterization applied in the future aerosol recognition, especially suitable for near-spherical scatters.

Does optically effective complex refractive index of internal-mixed aerosols have a physically-based meaning?

It is documented that some unreasonable low real parts of aerosol complex refractive index (ACRI) are retrieved from measured particle optical properties, and whether the optically effective ACRI has a physically-based sense is still an open question. Numerical determination of the optically effective ACRI from accurately calculated scattering and absorption properties of polydisperse coated BC particles by the core-shell Mie, representing some internal-mixed aged BC particles under polluted urban environments, is performed with a Mie theory based data analysis scheme. After getting rid of retrieval uncertainties (such as altered shapes), the look-up tables without limiting ACRI ranges based on known particle compositions could theoretically result in some unreasonable extreme real parts of optically effective ACRI of coarse coated BC with some shell/core ratios. Based on limited look-up tables with aerosol compositions considered, the retrieved imaginary parts of optically effective ACRIs of coarse coated BC, showing smooth as a function of shell/core ratio, are significantly lower than those approximated by the volume weighted average (VWA) method by a factor of nearly 3. Although the VWA shows acceptable performances for coated BC in the Aitken or accumulation modes, it could overestimate the absorption of coarse coated BC by a factor of ~2 for large shell/core ratio. This may be one of the reasons why modelled aerosol optical depth is 20% larger than observed. Our study indicates that the optically effective ACRI of internal-mixed BC particles does have a physically-based meaning like real ACRI, unless limited ACRI look-up tables considering aerosol compositions are applied for retrieval. We suggest that the optically effective ACRI, rather than the ACRI given by the VWA, should be considered for coarse internal-mixed particles in the state-of-the-art aerosol-climate models.

Optically effective complex refractive index of coated black carbon aerosols: from numerical aspects

Abstract. Aerosol complex refractive index (ACRI) is an important microphysical parameter used for the studies of modeling their radiative effects. With considerable uncertainties related to retrieval based on observations, a numerical study is a powerful method, if not the only one, to provide a better and more accurate understanding of retrieved optically effective ACRIs of aged black carbon (BC) particles. Numerical investigations of the optically effective ACRIs of polydisperse coated BC aggregates retrieved from their accurate scattering and absorption properties, which are calculated by the multiple-sphere T-matrix method (MSTM), without overall particle shape variations during retrieval, are carried out. The aim of this study is to evaluate the effects of aerosol microphysics, including shell∕core ratio Dp∕Dc, BC geometry, BC position inside coating, and size distribution, on retrieved optically effective ACRIs of coated BC particles. At odds with expectations, retrieved optically effective ACRIs of coated BC particles in coarse mode are not merely impacted by their chemical compositions and shell∕core ratio, being highly complicated functions of particle microphysics. However, in accumulation mode, the coated BC optically effective ACRI is dominantly influenced by particle chemical compositions and the shell∕core ratio. The popular volume-weighted average (VWA) method and effective medium theory (EMT) provide acceptable ACRI results for coated BC in accumulation mode, and the resulting uncertainties in particle scattering and absorption are both less than approximately 10 %. For coarse coated BC, the VWA and EMT, nevertheless, produce dramatically higher imaginary parts than those of optically effective ACRIs, significantly overestimating particle absorption by a factor of nearly 2 for heavily coated BC with a large BC fractal dimension or BC close to the coating boundary. Using the VWA could introduce significant overestimation in aged BC absorption analysis studies, and this may be one of the reasons why modeled aerosol optical depth is 20 % larger than observed, since it is widely employed in the state-of-the-art aerosol–climate models. We propose a simple new ACRI parameterization for fully coated BC with Dp/Dc≥2.0 in coarse mode, which can serve as a guide for the improvement of ACRIs of heavily coated BC, and its scattering and absorption errors are reduced by a factor of nearly 2 compared to the VWA. Our study indicates that a reliable estimate of the radiative effects of aged BC particles in coarse mode would require accounting for the optically effective ACRI, rather than the ACRI given by the VWA, in aerosol–climate models.

Retrievals of aerosol single scattering albedo by multiwavelength lidar measurements: Evaluations with NASA Langley HSRL-2 during discover-AQ field campaigns

This work focuses on the study and evaluation of the retrievals of aerosol complex refractive index (m = mr + imi) and single scattering albedo (SSA) from the inversion of multi-wavelength lidar measurements, particularly of three backscattering coefficients (β) at 355, 532 and 1064 nm and two extinction coefficients (α) at 355 and 532 nm, typically known as the stand-alone 3β + 2α lidar inversion. The focus is on the well-known regularization technique for spherical particles. It is well known that constraints in the range of refractive indices allowed in the inversion are essential, both for the real (mr) and imaginary (mi) parts, due to the under-determined nature of the problem. Usually these constraints are fixed for a given set of inversions. Using a large database of AERONET retrievals, correlations between retrieved mr and mi are observed and those correlations together with results from the GOCART model are used to define optimized, case-dependent, constraints in the stand-alone 3β + 2α lidar inversion. For each inversion performed, the optimized constraints are computed from the 3β + 2α data using a-priori information of extinction-to-backscattered ratio (LR) and the Angstrom exponent computed with α at 355 and 532 nm. The stand-alone 3β + 2α lidar inversion with optimized, case-dependent, constraints is applied to airborne NASA LaRC HSRL-2 experimental measurements during DISCOVER-AQ. The optimized constraints selected from the measured 3β + 2α are compared with the typing classification based on additional multiwavelength depolarization measurements, showing consistency between aerosol size and absorption range and aerosol typing. Evaluations of the SSA retrieved by the stand-alone 3β + 2α lidar inversion with optimized constraints are done by comparisons with correlative airborne in-situ measured SSA. The agreement between both methodologies is satisfactory for most aerosol types as differences are within the uncertainties of each methodology.

Retrieval of Atmospheric Fine Particulate Density Based on Merging Particle Size Distribution Measurements: Multi‐instrument Observation and Quality Control at Shouxian

AbstractDensity of fine mode particles is an important physical parameter. However, its measurement and validation in the actual atmosphere are still difficult. In this study, the measurements at Shouxian, Anhui province, China (32.56°N, 116.78°E, elevation 22.7 m) in winter, are used for density retrieval and quality control. The effective density and number size distribution of fine mode aerosol can be obtained in real time, by using the scanning mobility particle sizer and aerodynamic particle sizer. During the experiment, the average density is about 1.66 ± 0.15 g/cm3 with the uncertainty of 0.19 g/cm3 calculated from the residuals in the overlap region of scanning mobility and aerodynamic particle sizers. A multi‐instrument measurement and quality control scheme is proposed to identify large measurement uncertainties. The extinction and scattering coefficients calculated from the merged fine particle number size distributions by Mie theory are comparable with the measurements from cavity‐enhanced albedometer. The differences between the calculated and observed scattering and extinction coefficients are less than 2%. Meanwhile, the deduced aerosol complex refractive index n and k are obtained by minimizing the sum of squared residuals of extinction and scattering coefficients and comparable with the values retrieved from the Sun‐sky radiometer CE318 measurements. The correlation coefficient on n is 0.72. However, the correlation on k is not optimized, explained by the influence of anthropogenic aerosol particles with strong absorption near ground and the poor accuracy of k. Compared with independent observations, a good agreement between the optical parameters is also obtained.

Simultaneous determination of aerosol optical depth and exponent of the Junge power law from MODIS shortwave infrared bands over Qinghai Lake.

The water-leaving radiances for shortwave infrared (SWIR) channels can be negligible, and these channels also contain information on aerosol particle size. Therefore, the satellite-based data of SWIR channels can be used to estimate aerosol particle size over inland waters [Appl. Opt.39, 887 (2000)APOPAI0003-693510.1364/AO.39.000887]. Supposing the actual atmospheric aerosol size distribution is based on the Junge power law, in this paper an iterative algorithm is used to simultaneously determine the aerosol optical depth (AOD) and the exponent of the Junge power law from Aqua MODIS L1B reflectance data of channels 1.64μm and 2.13μm over Qinghai Lake. Whether using the constant or variable aerosol complex refractive index (ACRI), the retrieved exponent of the Junge power law is always larger than the product value. Supposing the product values are accurate, for the constant ACRI, there are 68.91% and 25.48% pixels of acceptable retrieval AOD and the exponent of the Junge power-law value, respectively. Likewise, there are 71.63% and 43.75% pixels for variable ACRI. Compared with the retrieval error under constant ACRI, there are 58.65% and 98.72% pixels, with a smaller AOD and Junge power-law index retrieval error under variable ACRIs, respectively. In addition, the precision of the AOD retrieved with variable ACRI is improved when the AOD product is less than 0.17. However, under the current environment with frequent aerosol particle pollution, the same ACRI for the ten wavelengths can achieve results with equivalent accuracy compared with variable ACRI.

Evolution of the Complex Refractive Index of Secondary Organic Aerosols during Atmospheric Aging

The wavelength-dependence of the complex refractive indices (RI) in the visible spectral range of secondary organic aerosols (SOA) are rarely studied, and the evolution of the RI with atmospheric aging is largely unknown. In this study, we applied a novel white light-broadband cavity enhanced spectroscopy to measure the changes in the RI (400-650 nm) of β-pinene and p-xylene SOA produced and aged in an oxidation flow reactor, simulating daytime aging under NO x-free conditions. It was found that these SOA are not absorbing in the visible range, and that the real part of the RI, n, shows a slight spectral dependence in the visible range. With increased OH exposure, n first increased and then decreased, possibly due to an increase in aerosol density and chemical mean polarizability for SOA produced at low OH exposures, and a decrease in chemical mean polarizability for SOA produced at high OH exposures, respectively. A simple radiative forcing calculation suggests that atmospheric aging can introduce more than 40% uncertainty due to the changes in the RI for aged SOA.

Effect of Oxidation Process on Complex Refractive Index of Secondary Organic Aerosol Generated from Isoprene.

Oxidation of isoprene by hydroxyl radical (OH), ozone (O3), or nitrate radical (NO3) leads to the formation of secondary organic aerosol (SOA) in the atmosphere. This SOA contributes to the radiation balance by scattering and absorbing solar radiation. In this study, the effect of oxidation processes on the wavelength-dependent complex refractive index (RI) of SOA generated from isoprene was examined. Oxidation conditions did not have a large effect on magnitude and wavelength dependence of the real part of the RI. In the case of SOA generated in the presence of sulfur dioxide (SO2), significant light absorption at short visible and ultraviolet wavelengths with the imaginary part of the RI, up to 0.011 at 375 nm, was observed during oxidation with OH. However, smaller and negligible values were observed during oxidation with O3 and NO3, respectively. Moreover, in the absence of SO2, light absorption was not observed regardless of the oxidation process. There was an empirical correlation between the imaginary part of the RI and the average degree of unsaturation of organic molecules. The results obtained herein demonstrate that oxidation processes should be considered for estimating the radiative effect of isoprene-derived SOA.

典型地区大气气溶胶谱分布和复折射率特征研究

典型地区大气气溶胶谱分布和复折射率特征研究

UV–Vis–IR spectral complex refractive indices and optical properties of brown carbon aerosol from biomass burning

Constraining the complex refractive indices, optical properties and size of brown carbon (BrC) aerosols is a vital endeavor for improving climate models and satellite retrieval algorithms. Smoldering wildfires are the largest source of primary BrC, and fuel parameters such as moisture content, source depth, geographic origin, and fuel packing density could influence the properties of the emitted aerosol. We measured in situ spectral (375–1047 nm) optical properties of BrC aerosols emitted from smoldering combustion of Boreal and Indonesian peatlands across a range of these fuel parameters. Inverse Lorenz–Mie algorithms used these optical measurements along with simultaneously measured particle size distributions to retrieve the aerosol complex refractive indices (m = n + iκ). Our results show that the real part n is constrained between 1.5 and 1.7 with no obvious functionality in wavelength (λ), moisture content, source depth, or geographic origin. With increasing λ from 375 to 532 nm, κ decreased from 0.014 to 0.003, with corresponding increase in single scattering albedo (SSA) from 0.93 to 0.99. The spectral variability of κ follows the Kramers-Kronig dispersion relation for a damped harmonic oscillator. For λ ≥ 532 nm, both κ and SSA showed no spectral dependency. We discuss differences between this study and previous work. The imaginary part κ was sensitive to changes in FPD, and we hypothesize mechanisms that might help explain this observation.

Retrieval of Brown Carbon based on the aerosol complex refractive indices in the winter of Wuhan

The impact of Brown Carbon (BrC) to aerosol light absorption has been paid more attention recently and there are a large number of studies showing that the influence of BrC on radiative forcing should not be ignored. BrC also acts as an important component of haze pollution which is occurring frequently in Wuhan, China. Therefore, it is essential to estimate their optical properties, composition, and mass concentration. Considering most haze pollution happens during the coldest time, we retrieved BrC columnar content during winter in Wuhan for the first time. Our method bases on the fact that BrC showed the strong spectral dependence on UV-light absorption. Using this method, we found that BrC makes up the small proportions of total aerosol volume (less than 10%). In the winter of 2011, we retrieved the daily-averaged columnar-integrated mass concentration of BrC on clear day is 4.353 mg/m2 while that of haze day is 12.750 mg/m2. According to the sensitivity study, we found that the results highly relied on the assumed aerosol refractive index. To reduce the uncertainty of this approach, we need to gain a better understanding of the temporal variability of the radiation absorbing components of these aerosols in the future.