Fine Mode Effective Radius Research Articles

Climatological‐Scale Analysis of Intensive and Semi‐intensive Aerosol Parameters Derived From AERONET Retrievals Over the Arctic

AbstractWe investigated the climatological‐scale, monthly binned, seasonal variation of AERONET/Dubovik retrievals across six stations in the North American and European Arctic (multiyear sampling periods ranging from 8 to 17 years). A robust, spring‐to‐summer (StoS) increase in the radius of the peak of the fine mode (FM) component of the particle size distribution (PSD) was observed for five of the six stations. The FM aerosol optical depth (AOD) and the FM effective radius at the individual stations showed, respectively, a negligible to moderate StoS decrease and a significant increase. This was interpreted as a trade‐off between the waning influence of smaller FM Arctic haze aerosols and the increasing influence of large FM smoke particles. A springtime, pan‐Arctic PSD peak in the 1.3 μm coarse mode (CM) bin was attributed to Asian dust. It was suggested that the increase in amplitude of a second (4–7 μm) CM peak from July to August at the low‐elevation coastal sites was influenced by wind‐induced sea salt. The CM AOD went through a StoS decrease attributed to the decreasing amplitude of the 1.3 μm peak. A significant StoS CM effective radius increase was ascribed to the decreasing influence of the 1.3 μm peak. StoS FM fraction increases were largely due to the decrease of the CM AOD (decreasing influence of springtime Asian dust). This extensive and intensive climatology of remotely sensed, bimodal properties will, we believe, provide an important reference for future measurements and modeling of Arctic aerosols.

Aerosol retrievals from different polarimeters during the ACEPOL campaign using a common retrieval algorithm

Abstract. In this paper, we present aerosol retrieval results from the ACEPOL (Aerosol Characterization from Polarimeter and Lidar) campaign, which was a joint initiative between NASA and SRON – the Netherlands Institute for Space Research. The campaign took place in October–November 2017 over the western part of the United States. During ACEPOL six different instruments were deployed on the NASA ER-2 high-altitude aircraft, including four multi-angle polarimeters (MAPs): SPEX airborne, the Airborne Hyper Angular Rainbow Polarimeter (AirHARP), the Airborne Multi-angle SpectroPolarimetric Imager (AirMSPI), and the Research Scanning Polarimeter (RSP). Also, two lidars participated: the High Spectral Resolution Lidar-2 (HSRL-2) and the Cloud Physics Lidar (CPL). Flights were conducted mainly for scenes with low aerosol load over land, but some cases with higher AOD were also observed. We perform aerosol retrievals from SPEX airborne, RSP (410–865 nm range only), and AirMSPI using the SRON aerosol retrieval algorithm and compare the results against AERONET (AErosol RObotic NETwork) and HSRL-2 measurements (for SPEX airborne and RSP). All three MAPs compare well against AERONET for the aerosol optical depth (AOD), with a mean absolute error (MAE) between 0.014 and 0.024 at 440 nm. For the fine-mode effective radius the MAE ranges between 0.021 and 0.028 µm. For the comparison with HSRL-2 we focus on a day with low AOD (0.02–0.14 at 532 nm) over the California Central Valley, Arizona, and Nevada (26 October) as well as a flight with high AOD (including measurements with AOD>1.0 at 532 nm) over a prescribed forest fire in Arizona (9 November). For the day with low AOD the MAEs in AOD (at 532 nm) with HSRL-2 are 0.014 and 0.022 for SPEX and RSP, respectively, showing the capability of MAPs to provide accurate AOD retrievals for the challenging case of low AOD over land. For the retrievals over the smoke plume a reasonable agreement in AOD between the MAPs and HSRL-2 was also found (MAE 0.088 and 0.079 for SPEX and RSP, respectively), despite the fact that the comparison is hampered by large spatial variability in AOD throughout the smoke plume. A good comparison is also found between the MAPs and HSRL-2 for the aerosol depolarization ratio (a measure of particle sphericity), with an MAE of 0.023 and 0.016 for SPEX and RSP, respectively. Finally, SPEX and RSP agree very well for the retrieved microphysical and optical properties of the smoke plume.

Comparation of aerosol optical properties and associated radiative effects of air pollution events between summer and winter: A case study in January and July 2014 over Wuhan, Central China

Differences in aerosol optical properties and associated radiative effects at ultraviolet (UV), visible (VIS), near-infrared (NIR), and shortwave (SW) spectra during haze periods between winter and summer of 2014 in Wuhan were compared in this study. With the development of haze, the average aerosol optical depth increased from 0.371 to 0.769 in winter and from 0.602 to 1.163 in summer. A positive correlation was found between absorbing aerosol optical depth and black carbon, only in winter. The mean fine mode effective radius of aerosols increased from 0.128 μm to 0.166 μm in winter and from 0.179 μm to 0.229 μm in summer, due to hygroscopic growth under high humidity conditions during haze periods. The fine mode particles were more dominant in summer than in winter. The single scattering albedo (SSA) increased significantly in both seasons as a result of the emergence of numerous non-absorbent aerosols when haze occurred. The aerosol radiative effect (ARE) increased from −73.46 W/m2 to −115.38 W/m2 (−96.01 W/m2 to −135.67 W/m2) at the earth's surface, and from −15.09 W/m2 to −32.91 W/m2 (−20.49 W/m2 to −49.13 W/m2) at the top of the atmosphere because of the change from clear air conditions to haze periods in winter (summer). Unusual increases in aerosol radiative effect efficiency (AREE) from −40.78 W/m2 to −43.57 W/m2 and AREE fraction of UV in SW spectrum from 1.9% to 3.3% were found at the top of atmosphere in winter. This exception was probably due to dust aerosols with iron components, which had a strong UV absorption ability. The AREE dependence showed a high dependence on particle size in winter but a greater dependence on the SSA in summer. Back trajectory analysis with active fire areas from Moderate Resolution Imaging Spectroradiometer indicated that the black carbon originated from the North in summer, while active fires in winter were rare.

Column-integrated Aerosol Optical Properties Determined Using Ground-based Sun Photometry Measurements in the Hangzhou Region

To investigate the optical properties of aerosols in the Hangzhou region (Hangzhou, Tonglu, Jiande, and Chun'an), the aerosol optical depth (AOD), Ångström exponent (AE), single scattering albedo (SSA), and aerosol size distribution (ASD) were measured using CIMEL sun-photometers in 2012. The results showed that the annual average values of AOD440nm in Hangzhou, Tonglu, Jiande, and Chun'an were 0.94±0.16, 0.84±0.17, 0.82±0.22, and 0.71±0.20, respectively. The values generally decreased from the northeast to the southwest, and represented one of highest AOD districts in the Yangtze River Delta, China. The annual average values of AE440-870nm were 1.24±0.12, 1.19±0.17, 1.06±0.04, and 1.04±0.10, respectively, indicating that particles with small average effective radii were predominant. The relatively lower AE values in March and April were generally attributed to the long-range transport of dust aerosols from Northwest China. Obvious diurnal variations in the AOD were found in Hangzhou, Tonglu, and Jiande, but not in Chun'an. An average fine-mode effective radius of~0.15 μm was observed in spring, autumn, and winter, while a value of~0.25 μm was observed in summer, in conjunction with aerosol hygroscopic growth. An average coarse-mode effective radius of~2.94 μm was observed in summer, autumn, and winter, which was higher than the value in spring. The annual average values of SSA440nm were 0.91±0.01, 0.92±0.03, 0.92±0.02, 0.93±0.02, respectively, indicating that the particles had relatively strong to moderate absorption. Characterization of the aerosol types showed the predominance of biomass burning and urban industrial type aerosols in Hangzhou, while mixed type aerosols were observed in Tonglu, Jiande, and Chun'an.

Combined neural network/Phillips–Tikhonov approach to aerosol retrievals over land from the NASA Research Scanning Polarimeter

Abstract. In this paper, an algorithm for the retrieval of aerosol and land surface properties from airborne spectropolarimetric measurements – combining neural networks and an iterative scheme based on Phillips–Tikhonov regularization – is described. The algorithm – which is an extension of a scheme previously designed for ground-based retrievals – is applied to measurements from the Research Scanning Polarimeter (RSP) on board the NASA ER-2 aircraft. A neural network, trained on a large data set of synthetic measurements, is applied to perform aerosol retrievals from real RSP data, and the neural network retrievals are subsequently used as a first guess for the Phillips–Tikhonov retrieval. The resulting algorithm appears capable of accurately retrieving aerosol optical thickness, fine-mode effective radius and aerosol layer height from RSP data. Among the advantages of using a neural network as initial guess for an iterative algorithm are a decrease in processing time and an increase in the number of converging retrievals.

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 October 2004 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 Angstrom 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.

Column aerosol optical properties and aerosol radiative forcing during a serious haze-fog month over North China Plain in 2013 based on ground-based sunphotometer measurements

Abstract. In January 2013, North China Plain experienced several serious haze events. Cimel sunphotometer measurements at seven sites over rural, suburban and urban regions of North China Plain from 1 to 30 January 2013 were used to further our understanding of spatial-temporal variation of aerosol optical parameters and aerosol radiative forcing (ARF). It was found that Aerosol Optical Depth at 500 nm (AOD500 nm) during non-pollution periods at all stations was lower than 0.30 and increased significantly to greater than 1.00 as pollution events developed. The Angstrom exponent (Alpha) was larger than 0.80 for all stations most of the time. AOD500 nm averages increased from north to south during both polluted and non-polluted periods on the three urban sites in Beijing. The fine mode AOD during pollution periods is about a factor of 2.5 times larger than that during the non-pollution period at urban sites but a factor of 5.0 at suburban and rural sites. The fine mode fraction of AOD675 nm was higher than 80% for all sites during January 2013. The absorption AOD675 nm at rural sites was only about 0.01 during pollution periods, while ~0.03–0.07 and 0.01–0.03 during pollution and non-pollution periods at other sites, respectively. Single scattering albedo varied between 0.87 and 0.95 during January 2013 over North China Plain. The size distribution showed an obvious tri-peak pattern during the most serious period. The fine mode effective radius in the pollution period was about 0.01–0.08 μm larger than during non-pollution periods, while the coarse mode radius in pollution periods was about 0.06–0.38 μm less than that during non-pollution periods. The total, fine and coarse mode particle volumes varied by about 0.06–0.34 μm3, 0.03–0.23 μm3, and 0.03–0.10 μm3, respectively, throughout January 2013. During the most intense period (1–16 January), ARF at the surface exceeded −50 W m−2, −180 W m−2, and −200 W m−2 at rural, suburban, and urban sites, respectively. The ARF readings at the top of the atmosphere were approximately −30 W m−2 in rural and −40–60 W m−2 in urban areas. Positive ARF at the top of the atmosphere at the Huimin suburban site was found to be different from others as a result of the high surface albedo due to snow cover.

Properties of Sarychev sulphate aerosols over the Arctic

Aerosols from the Sarychev Peak volcano entered the Arctic region less than a week after the strongest SO2eruption on June 15 and 16, 2009 and had, by the first week in July, spread out over the entire Arctic region. These predominantly stratospheric aerosols were determined to be sub‐micron in size and inferred to be composed of sulphates produced from the condensation of SO2gases emitted during the eruption. Average (500 nm) Sarychev‐induced stratospheric optical depths (SOD) over the Polar Environmental Atmospheric Research Laboratory (PEARL) at Eureka (Nunavut, Canada) were found to be between 0.03 and 0.05 during the months of July and August, 2009. This estimate, derived from sunphotometry and integrated lidar backscatter profiles was consistent with averages derived from lidar estimates over Ny‐Ålesund (Spitsbergen). The Sarychev SOD e‐folding time at Eureka, deduced from lidar profiles, was found to be approximately 4 months relative to a regression start date of July 27. These profiles initially revealed the presence of multiple Sarychev plumes between the tropopause and about 17 km altitude. After about two months, the complex vertical plume structures had collapsed into fewer, more homogeneous plumes located near the tropopause. It was found that the noisy character of daytime backscatter returns induced an artifactual minimum in the temporal, pan‐Arctic, CALIOP SOD response to Sarychev sulphates. A depolarization ratio discrimination criterion was used to separate the CALIOP stratospheric layer class into a low depolarization subclass which was more representative of Sarychev sulphates. Post‐SAT (post Sarychev Arrival Time) retrievals of the fine mode effective radius (reff,f) and the logarithmic standard deviation for two Eureka sites and Thule (Greenland) were all close to 0.25 μm and 1.6 respectively. The stratospheric analogue to the columnar reff,f average was estimated to be reff,f(+) = 0.29 μm for Eureka data. Stratospheric, Raman lidar retrievals at Ny‐Ålesund, yielded a post‐SAT average of reff,f(+) = 0.27 μm. These results are ∼50% larger than the background stratospheric‐aerosol value. They are also about a factor of two larger than modeling values used in recent publications or about a factor of five larger in terms of (per particle) backscatter cross section.

Analysis of sub-micron parameters derived from multi-altitude and multi-spectral AOD measurements acquired during the 2009 PAM-ARCMIP airborne campaign

Abstract A series of Arctic sunphotometry flights were analyzed in terms of their multi-altitude, sub-micron (fine mode) information content. A spectral deconvolution algorithm (SDA) and a fine mode curvature algorithm (FMC) were applied to extract fine mode and coarse mode optical depth as well as the effective radius of the fine mode as a function of altitude. The fine mode optical depth was differentiated as a function of altitude to retrieve vertical profiles of fine mode extinction coefficient. These optical results were compared with volumetric altitude profiles of fine mode particle size distribution acquired by a UHSAS (Ultra High Sensitivity Aerosol Spectrometer). The results showed that layer-averaged extinction cross sections derived from the ratio of fine mode optical depth to integrated UHSAS number density were significantly larger than extinction cross sections derived from the application of Mie theory to the UHSAS particle size distributions. Vertical profiles of extinction coefficients derived from altitude derivatives of the fine mode optical depth profiles showed some correlation with the UHSAS extinction coefficients. Profiles of the fine mode effective radius retrieved from the sunphotometry data were generally of similar magnitude to columnar averages of the UHSAS radii but no significant trend with altitude could be detected. An example was given of a high altitude smoke plume whose presence eliminated any hope of correlating the volumetric sampling information with the sunphotometer profiles. This is simply a statement of the obvious; that the atmospheric state must be stable in order to make such comparisons.

Comparison of in situ and columnar aerosol spectral measurements during TexAQS-GoMACCS 2006: testing parameterizations for estimating aerosol fine mode properties

Abstract. During the 2006 Texas Air Quality Study and Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS-GoMACCS 2006), the optical, chemical and microphysical properties of atmospheric aerosols were measured on multiple mobile platforms and at ground based stations. In situ measurements of the aerosol light extinction coefficient (σep) were performed by two multi-wavelength cavity ring-down (CRD) instruments, one located on board the NOAA R/V Ronald H. Brown (RHB) and the other located at the University of Houston, Moody Tower (UHMT). An AERONET sunphotometer was also located at the UHMT to measure the columnar aerosol optical depth (AOD). The σep data were used to extract the extinction Ångström exponent (åep), a measure of the wavelength dependence of σep. There was general agreement between the åep (and to a lesser degree σep) measurements by the two spatially separated CRD instruments during multi-day periods, suggesting a regional scale consistency of the sampled aerosols. Two spectral models are applied to the σep and AOD data to extract the fine mode fraction of extinction (η) and the fine mode effective radius (Reff,f). These two parameters are robust measures of the fine mode contribution to total extinction and the fine mode size distribution, respectively. The results of the analysis are compared to Reff,f values extracted using AERONET V2 retrievals and calculated from in situ particle size measurements on the RHB and at UHMT. During a time period when fine mode aerosols dominated the extinction over a large area extending from Houston/Galveston Bay and out into the Gulf of Mexico, the various methods for obtaining Reff,f agree qualitatively (showing the same temporal trend) and quantitatively (pooled standard deviation = 28 nm).

Characterization of atmospheric aerosols using MFRSR measurements

Measurements by multifilter rotating shadowband radiometers (MFRSRs) constitute a valuable global data set with contributions from hundreds of instruments deployed worldwide. The geographical coverage of MFRSR networks is complementary to that of AERONET and often provides better spatial density of measurement sites, especially in the United States. We describe our recently updated analysis algorithm for MFRSR data that allows partitioning of the spectral aerosol optical depth (AOD) into fine and coarse mode AOD and retrieval of the fine mode effective radius. Our recent sensitivity study demonstrated that for a typical measurement accuracy 0.01 of AOD, the trade‐offs between the spectral aerosol extinction and NO2 absorption in the visible range effectively prevent unambiguous retrieval of NO2 column from MFRSR data and may also bias aerosol size distribution retrievals. This has prompted us to adopt a new retrieval approach, which utilizes climatological NO2 (based on SCIAMACHY satellite retrievals) and uses column ozone from TOMS measurements. The performance of this new approach was evaluated using the long‐term data set from the Southern Great Plains (SGP) site operated by the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program. We present a detailed intercomparison of total, fine, and coarse mode AOD and fine mode effective radius between two MFRSRs located at the SGP's Central Facility and with the correlative AERONET Sun‐sky inversion results (Version 2) derived from a collocated CIMEL Sun photometer. The comparison between two MFRSRs demonstrated good consistency of both the measurements and the analysis. Agreement with AERONET inversions is remarkably good, in that differences in AOD components do not exceed the expected measurement accuracy of 0.01, while the retrieved values of fine mode effective radius show no relative bias and only 0.03 μm random error (standard deviation of the differences). We show that if only data with large enough AOD (more than 0.06 at 870 nm) are selected, this error is reduced by a factor of two, becoming about 10% of a typical fine mode effective radius value.

Angstrom exponent and bimodal aerosol size distributions

Power laws have long been used to describe the spectral dependence of aerosol extinction, and the wavelength exponent of the aerosol extinction law is commonly referred to as the Angstrom exponent. The Angstrom exponent is often used as a qualitative indicator of aerosol particle size, with values greater than 2 indicating small particles associated with combustion byproducts, and values less than 1 indicating large particles like sea salt and dust. In this study, we investigate the relationship between the Angstrom exponent and the mode parameters of bimodal aerosol size distributions using Mie theory calculations and Aerosol Robotic Network (AERONET) retrievals. We find that Angstrom exponents based upon seven wavelengths (0.34, 0.38, 0.44, 0.5, 0.67, 0.87, and 1.02 μm) are sensitive to the volume fraction of aerosols with radii less then 0.6 μm but not to the fine mode effective radius. The Angstrom exponent is also known to vary with wavelength, which is commonly referred to as curvature; we show how the spectral curvature can provide additional information about aerosol size distributions for intermediate values of the Angstrom exponent. Curvature also has a significant effect on the conclusions that can be drawn about two‐wavelength Angstrom exponents; long wavelengths (0.67, 0.87 μm) are sensitive to fine mode volume fraction of aerosols but not fine mode effective radius, while short wavelengths (0.38, 0.44 μm) are sensitive to the fine mode effective radius but not the fine mode volume fraction.

Separation of fine and coarse aerosol modes in MFRSR data sets

A new MFRSR data analysis algorithm is presented. Our earlier algorithm assumed a monomodal aerosol size distribution, while the new algorithm allows us to partition the aerosol optical thickness into fine and coarse aerosol modes. In addition, we retrieve the fine mode effective radius and Angstrom exponent. A bimodal gamma distribution is used to describe the aerosol particle size distribution. The algorithm has been tested using a multi‐year data set from the local MFRSR network at the DOE Atmospheric Radiation Measurement (ARM) Program site in Southern Great Plains (SGP). Our retrieved aerosol optical thicknesses (total, fine, and coarse) are compared with the corresponding AERONET almucantar retrieval results derived from a CIMEL sunphotometer co‐located with the MFRSR at the SGP Central Facility. A constrained variant of the algorithm (zero NO2 column values) has been used to define the range of physically justified values of the fine mode effective radius, and for comparison with AERONET particle size retrievals. We use the multiple MFRSR measurements obtained during the year 2000 at the SGP Extended Facilities to examine geographical and seasonal variability of aerosol properties. A correspondence has been found between the geographical variation in the fine mode particle size and aerosol composition (nitrates versus sulfates) as measured by National Atmospheric Deposition Program. We similarly find good correspondence between our retrieved aerosol sizes and the PM2.5 to PM10 ratios obtained from EPA AirData monitoring. Examination of the data from the SGP Central Facility obtained for the period 1993–1997 reveals a decreasing trend in coarse mode aerosol optical thickness during the 1993–1995 period, consistent with the decay of the stratospheric aerosol following the 1991 eruption of Mt. Pinatubo. In contrast, fine mode optical thickness exhibits only seasonal variability with summer maxima during this period.

Optical properties of boreal forest fire smoke derived from Sun photometry

Aerosol optical properties derived fromSun photometry were investigated in terms of climatological trends at twoSun photometer sites significantly affected by western Canadian boreal forest fire smoke and in terms of a 2‐week series of smoke events observed at stations near and distant from boreal forest fires. Aerosol optical depth (τa) statistics for Waskesiu, Saskatchewan, and Thompson, Manitoba, were analyzed for summer data acquired between 1994 and 1999. A significant correlation between the geometric mean and the forest fire frequency indices (hot spots) was found; on the average, 80% of summertime optical depth variation in western Canada can be linked to forest fire sources. The average geometric mean and geometric standard deviation at 500 nm was observed to be 0.074 and 1.7 for the clearest, relatively smoke‐free summer and 0.23 and 3.0 for the summer most influenced by smoke. A systematic decrease of fine mode Angstrom exponent (αf) was noted (dαf/dlog τa∼ −0.6). This decrease roughly corresponds to an increase in the fine mode effective radius (reff) from 0.09 to 0.15 μm and an abundance (A) to size rate increase near 2.0 (dlogA/dlogreff). A 1998 series of forest fire events was tracked using TOMS, AVHRR, and GOES imagery, back trajectories, and data from six Sun photometer sites in Canada and eastern United States. The results showed rates of decrease of αfwith increasing τawhich were similar to the climatological data. An analysis in terms of source to station distance showed a decrease in αfand an increase inreffwith increasing distance. This observation was coherent with previous observations on the particle growth effects of aging.

Raman lidar measurements of the aerosol extinction‐to‐backscatter ratio over the Southern Great Plains

We derive profiles of the aerosol extinction‐to‐backscatter ratio, Sa, at 355 nm using aerosol extinction and backscatter profiles measured during 1998 and 1999 by the operational Raman lidar at the Department of Energy Atmospheric Radiation Measurement Program Southern Great Plains site in north central Oklahoma. Data from this Raman/Rayleigh‐Mie lidar, which measures Raman scattering from nitrogen as well as the combined molecular (Rayleigh) and aerosol (Mie) scattering at the laser wavelength, are used to derive aerosol extinction and backscattering independently as a function of altitude. Because this lidar operates at 355 nm, where molecular backscattering is comparable to aerosol backscattering, Sa retrievals are generally limited to conditions where aerosol extinction at 355 nm is >0.03 km−1. The mean value of Sa at 355 nm derived for this period was 68 sr with a standard deviation of 12 sr. Sa was generally about 5–10 sr higher during high aerosol optical thickness (AOT) (>0.3) conditions than during low AOT (<0.1). A similar increase in Sa was found when the relative humidity increased from 30 to 80%. Large (>15%) variations in the vertical profile of Sa occurred about 30% of the time, which implies that significant variability in the vertical distribution of the aerosol size distribution, shape, and/or composition often occurs. The Raman lidar measurements of Sa were compared with estimates of particle size and refractive index derived from an algorithm that uses ground‐based Sun photometer measurements of Sun and sky radiance. For 17 cases of coincident Raman lidar and Sun and sky radiance measurements, Sa was linearly correlated with the aerosol fine mode effective radius and the volume ratio of fine/coarse particles.