[Show abstract][Hide abstract] ABSTRACT: Distributions and optical characteristics of aerosols were continuously observed with a polarization- sensitive (532 nm), Mie-scattering (532 and 1064 nm) and Raman-scattering (607 nm) lidar and a sky radiometer in Phimai, Thailand. Polarization lidar measurements indicated that high concentration plumes of spherical aerosols considered as biomass burning smoke were often observed in the dry season. Plumes of non-spherical aerosols considered as long-range transported soil dust from Africa, the Middle East, or Northeast Asia were occasionally observed. Furthermore, low-concentration non- spherical aerosols were almost always observed in the atmospheric mixing layer. Extinction coefficient profiles of spherical aerosols and non-spherical dust exhibited different diurnal variations, and spherical aerosols including smoke were distributed in higher altitudes in the mixing layer and residual layer. The difference can be explained by hygroscopic growth of smoke particles and buoyancy of the smoke. Analysis of seasonal variations of optical properties derived from the Raman lidar and the sky radiometer confirmed that the lidar ratio, aerosol optical depth, and Angstrom exponent were higher in the dry season (October–May) and lower in the wet season (June–September). The single scattering albedo was lower in the dry season. These seasonal variations are explained by frequent biomass burning in the dry season consistent with previous studies in Southeast Asian region. At the same time, the present work confirmed that soil dust was a major aerosol component in Phimai, Thailand.
Environmental Research Letters 06/2015; 10(6):065003. DOI:10.1088/1748-9326/10/6/065003 · 3.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The surface albedo of two large cities in Japan was measured using a pyranometer mounted on a helicopter to avoid the bidirectional reflectance distribution. The daytime albedo was 0.12 in the cities, which was less than that of a nearby forest (0.16). The albedo was dependent on building structure in the cities; the albedo was lower in areas with more buildings, and decreased as the aspect ratio of street canyons increased. There are two reasons for this dependency: the multiple reflection of radiation in the building canopy, as has been shown in many previous studies, and the sparse vegetation in urban areas. These two factors concurrently determine the albedo in a real city, where the vegetation amount decreases as the plan roof ratio increases.
[Show abstract][Hide abstract] ABSTRACT: Seasonal characteristics of aerosol optical properties (AOP) in SKYNET Hefei site are studied using a sky-radiometer from March 2007 to May 2013. The aerosol optical depth (AOD), Angstrom exponent (AE), volume size distributions, single scattering albedo (SSA), refractive index and asymmetry factor (ASY) of aerosols are simultaneously retrieved using the SKYRAD.pack version 4.2 software. During the study period, the AOD varied seasonally, with the maximum value of 1.02 ± 0.42 at 500 nm occurring in the summer and the highest AOD (1.13 ± 0.42) occurred in June due to stagnant climate conditions and accumulation of polluted aerosols before the East Asian summer monsoon. The variation in AE showed a different pattern, with the minimum (0.97 ± 0.28) and maximum values (1.30 ± 0.22) occurring during the spring and fall seasons, respectively. The relatively low value of AE in spring is related to the emission of Asian dust events. The aerosol volume size distributions can be expressed by the tri-modal patterns for each season, consisting of a fine mode with R < 0.6um, a coarse mode with R > 2.5um and a middle mode located between them. The real part of the refractive index increased with wavelength (380-870 nm) while the imaginary part of the refractive index decreased for all seasons except for the summer. The seasonal mean values of SSA were 0.97 ± 0.02(summer), 0.95 ± 0.03(spring), 0.93 ± 0.04(autumn), and 0.91 ± 0.04(winter) at 380 nm indicating more absorbing aerosol in the autumn and winter months. Furthermore, aerosol properties were greatly modified by condensation growth as evidenced by the positive dependencies of AOD, SSA and ASY on relative humidity.
[Show abstract][Hide abstract] ABSTRACT: SKYNET is an international research network of ground-based Prede sky radiometers for the observation and monitoring of aerosol-cloud-radiation interactions in the atmosphere. The algorithm developed by SKYNET is SKYRAD.pack, which can be used to process the measurement data of Prede instruments. In this study, the latest SKYRAD.pack software (Version 5.0) has been used to retrieve the aerosol optical properties measured by a SKYNET Prede sky radiometer over an urban site of Beijing, China. Continuous data have been processed over a two-year period, and inversion products, including aerosol optical depth (AOD), Angstrom exponent (alpha), volumes of different aerosol particle size distributions, and single-scattering albedos (SSA), have been analyzed. AOD values were found to vary from 0.11 (5th percentile) to 1.14 (95th percentile) with a median of 0.34 at 500 nm, and the maximum and minimum seasonal a values in Beijing were 1.05 +/- 0.36 in summer and 0.82 +/- 0.39 in spring. SSA values are higher in summer and spring with a similar value of 0.96 +/- 0.03, but lower in winter with a value of 0.93 +/- 0.04 at 500 nm. Aerosol particles in Beijing clearly demonstrated bimodal size distributions throughout the year: there were coarser particles in spring and finer particles in summer. The alpha values increased with AOD, indicating that fine particles play an important role in the optical properties of aerosols in Beijing. Dust type aerosol occurrence accounted for 4.1%, 5.1%, 0.5%, and 1.2% of all measurements data in spring, summer, autumn, and winter, respectively, according to the dust criteria threshold (alpha < 0.47 and SSA(400 nm) - SSA(1020 nm) < 0.018).
Journal of the Meteorological Society of Japan 05/2014; 92A:17-31. DOI:10.2151/jmsj.2014-A02 · 1.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sun-sky radiometers are instruments created for aerosol study, but they can measure in the water vapour absorption band allowing the estimation of columnar water vapour in clear sky simultaneously with aerosol characteristics, with high temporal resolution. A new methodology is presented for estimating calibration parameters (i.e. characteristic parameters of the atmospheric transmittance and solar calibration constant) directly from the sun-sky radiometer measurements. The methodology is based on the hypothesis that characteristic parameters of the atmospheric transmittance are dependent on vertical profiles of pressure, temperature and moisture occurring at each site of measurement. To obtain the parameters from the proposed methodology some seasonal independent measurements of columnar water vapour taken over a large range of solar zenith angle simultaneously with the sun-sky radiometer measurements, are needed. In this work high time resolution columnar water vapour measurements by GPS were used as independent data set, but also the case when such measurements are not available was considered by developing the surface humidity method (SHM). This methodology makes it possible to retrieve the needed independent data set of columnar water vapour using the standard surface meteorological observations (temperature, pressure and relative humidity) more readily available. The time pattern of columnar water vapour from sun-sky radiometer retrieved using both the methodologies was compared with simultaneous measurements from microwave radiometer, radiosondings and GPS. Water vapour from sun-sky radiometer, obtained using GPS independent measurements, was characterized by an error varying from 1% up to 5%, whereas water vapour from SHM showed an error from 1% up to 11%, depending on the local columnar water occurring at the site during the year. These errors were estimated by comparing water vapour series from sun-sky radiometer against measurements taken by GPS at a nearby station. The accordance between retrievals from sun-sky radiometer and simultaneous measurements from the other instruments was found always within the error both in the case of SHM and of the GPS independent data set. Water vapour obtained using characteristic parameters of the atmospheric transmittance dependent on water vapour was also compared against GPS retrievals, showing a clear improvement with respect to the case when these parameters are kept fixed.
[Show abstract][Hide abstract] ABSTRACT: Ground-based network of cloud measurements is presently limited and there exists uncertainty in the cloud microphysical parameters derived from ground-based measurements. Bias in the i-skyradiometer derived cloud optical depth (τ c) and droplet effective radius (R eff) and the importance of these parameters in the parameterization of clouds in climate models have made us intend to develop a possible method for improving these parameters. A new combination method, which uses zenith sky transmittance and surface radiation measurements, has been proposed in the present study to improve the retrievals. The i-skyradiometer derived parameters τ c and R eff have been provided as a first guess to a radiative transfer model (SBDART) and a new retrieval algorithm has been implemented to obtain the best combination of τ c and R eff having minimum bias (-0.09 and -2.5) between the simulated global and diffuse fluxes at the surface with the collocated surface radiation measurements. The new retrieval method has improved τ c and R eff values compared to those derived using the transmittance only method and are in good agreement with the MODIS satellite retrievals. The study therefore suggests a possible improvement of the i-skyradiometer derived cloud parameters using observed radiation fluxes and a radiative transfer model.
Advances in Meteorology 02/2014; 2014(2014)(849279):8. DOI:10.1155/2014/849279 · 0.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  We analyzed data observed at Fukue-jima (32.752°N, 128.682°E), the downwind of the East Asian desert and urban areas, during the spring season (March, April, and May) of 2008 - 2011 aiming to understand the light-absorption capacity of Asian dust aerosols, which is a topic of controversy. The observational data showed the decreasing tendency of single scattering albedo, SSA (the ratio of scattering to extinction) with the decrease of Ångström exponent, ALPHA (a parameter describing the wavelength dependence of aerosol optical thickness; larger the sizes of dominant aerosols, smaller the ALPHA and vice versa) and the increase of yellow sand index, YSI (the ratio of dust aerosol optical thickness to total aerosol optical thickness), suggesting the important role of coarse-mode dust aerosols on observed low SSA values. The observational data further indicated that the low values of SSA during strong dust events were less likely due to the effect of only strong light-absorbing carbonaceous aerosols, such as black carbon (BC), indicating the association of aerosol size distribution on modulating SSA. Such observational results are justified by numerical calculations showing that aerosol size distribution can be the key factor on modulating SSA even without any change in relative amount of light-absorbing aerosol as well as total aerosol optical thickness. Therefore, the observed low SSAs in the downwind regions during dust events could be partially due to the dominance of coarse-mode aerosols over fine-mode aerosols, which are usual in dust events, along with the effect of mixed light-absorbing aerosols. The study further suggests that such effect of aerosol size distribution on SSA can be one of the important reasons for the low SSAs of dust aerosols in the source region as reported by some studies, if coarse-mode aerosols dominate fine- mode aerosols.
[Show abstract][Hide abstract] ABSTRACT: Absorption aerosol optical depth (AAOD) measurements made by sun-sky photometers are currently the only constraint available for estimates of the global radiative forcing of black carbon (BC), but their validation studies are limited. In this paper, we report the first attempt to compare AAODs derived from single-particle soot photometer (SP2) and ground-based sun-sky photometer (sky radiometer, SKYNET) measurements. During the Aerosol Radiative Forcing in East Asia (A-FORCE) experiments, BC size distribution and mixing state vertical profiles were measured using an SP2 on board a research aircraft near the Fukue Observatory (32.8 degrees N, 128.7 degrees E) over the East China Sea in spring 2009 and late winter 2013. The aerosol extinction coefficients (b(ext)) and single scattering albedo (SSA) at 500 nm were calculated based on aerosol size distribution and detailed BC mixing state information. The calculated aerosol optical depth (AOD) agreed well with the sky radiometer measurements (26%) when dust loadings were low (lidar-derived nonspherical particle contribution to AOD less than 20%). However, under these low-dust conditions, the AAODs obtained from sky radiometer measurements were only half of the in situ estimates. When dust loadings were high, the sky radiometer measurements showed systematically higher AAODs even when all coarse particles were assumed to be dust for in situ measurements. These results indicate that there are considerable uncertainties in AAOD measurements. Uncertainties in the BC refractive index, optical calculations from in situ data, and sky radiometer retrieval analyses are discussed.
[Show abstract][Hide abstract] ABSTRACT: A yellow dust event with moderate strength was observed on 9 April 2012 at Sendai in North part of Japan. Backward trajectory calculations with NOAA HYSPLIT showed the complex flow of aerosols into the North Japan. The sharp edge of this dust cloud was recognized by the data taken at several observatories, and the dust cloud conducted by low pressure system had heterogeneous structure, therefore very complicated interaction among aerosols may occur. Mie scattering lidar data was used to reproduce the radiative effect caused by this yellow dust event at Sendai with radiative transfer model. The results estimated every 15 minutes of radiative forcing at the top of the atmosphere and at the bottom of the atmosphere. The results show the slight warming effects < 6.5 W/m2 during night time both at the top and the bottom of the atmosphere, and during day time the large cooling effects < 150 W/m2 at the bottom and < 60W/m2 at the top of the atmosphere.
Proceedings of SPIE - The International Society for Optical Engineering 12/2013; DOI:10.1117/12.977300 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  A recently developed new particle formation (NPF)-explicit version of the Weather Research and Forecasting Chemistry (WRF-chem) model can explicitly calculate the growth and sink of nucleated clusters with 20 aerosol size bins from 1 nm to 10 µm in diameter. In this study, the model was used to investigate spatial and temporal variations in NPF event frequency and the concentrations of aerosols (condensation nuclei, CN) and cloud condensation nuclei (CCN) within the boundary layer in East Asia in spring 2009. We found a distinct north-south contrast in the NPF frequency and mechanism in East Asia. NPF occurred mainly during limited periods over certain regions between 30° and 45°N (northeast China, Korea, and Japan, including regions around the active volcanoes Miyakejima and Sakurajima). In these latitudes, NPF was suppressed by high concentrations of preexisting particles under stagnant air conditions associated with high-pressure systems, although nucleation occurred more extensively during most of the simulation period. In contrast, south of 30°N, nucleation and NPF were both infrequent because of low SO2 emissions and H2SO4 concentrations. The period-averaged NPF frequency at 30°–45°N was three times that at 20°–30°N. This north-south contrast in NPF frequency was validated by surface measurements in outflow regions of East Asia. The simulated period- and domain-averaged contribution of secondary particles was estimated to be 44% for CN (>10 nm) and 26% for CCN at a supersaturation of 1.0%, though the contribution was highly sensitive to the amount and size distribution of primary aerosol emissions and the rate coefficient of the nucleation parameterization.
[Show abstract][Hide abstract] ABSTRACT: Potential improvements of aerosols algorithms for future climate-oriented satellites such as the coming Global Change Observation Mission Climate/Second generation Global Imager (GCOM-C/SGLI) are discussed based on a validation study of three years’ (2008–2010) daily aerosols properties, that is, the aerosol optical thickness (AOT) and the Ångström exponent (AE) retrieved from two MODIS algorithms. The ground-truth data used for this validation study are aerosols measurements from 3 SKYNET ground sites. The results obtained show a good agreement between the ground-truth data AOT and that of one of the satellites’ algorithms, then a systematic overestimation (around 0.2) by the other satellites’ algorithm. The examination of the AE shows a clear underestimation (by around 0.2–0.3) by both satellites’ algorithms. The uncertainties explaining these ground-satellites’ algorithms discrepancies are examined: the cloud contamination affects differently the aerosols properties (AOT and AE) of both satellites’ algorithms due to the retrieval scale differences between these algorithms. The deviation of the real part of the refractive index values assumed by the satellites’ algorithms from that of the ground tends to decrease the accuracy of the AOT of both satellites’ algorithms. The asymmetry factor (AF) of the ground tends to increase the AE ground-satellites discrepancies as well.
Advances in Meteorology 08/2013; 2013. DOI:10.1155/2013/508064 · 0.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Radiative forcing caused by cirrus clouds and aerosols were computed using the data obtained with lidar installed at the Fukue Island observatory. The MTSAT data were used to discriminate the existence of cirrus clouds. Small fluctuation of cirrus cloud radiative forcing was estimated, and the effects caused by aerosols were also computed around Fukue Island from the end of 2005 to 2010.
[Show abstract][Hide abstract] ABSTRACT: Time-resolved data for size distribution, number concentration and chemical composition of atmospheric aerosols were taken from March 9 to 16, 2012 during Fukue Site Atmospheric Aerosol Observation Campaign (128.7 E, 32.8 N) organized by ASEPH (Impacts of Aerosols in East Asia on Plants and Human Health) project. A scanning mobility particle sizer was employed to continuously measure the particle size distribution from 14 nm to 670 nm and the total number concentration of particles (>3 nm) was measured by a condensation particle counter. Very large amount of polluted air mass transportation (PM2.5 > 65 mg m-3) was observed on March 11, associated with a sudden increase in particle number concentration having a mobility diameter from 100 to 300 nm. The transported air mass contained anthropogenic pollutants such as sulfate (>20 mg m-3), nitrate (>15 mg m-3) and black carbon (>3 mg m-3). Typical new particle formation and growth events were also observed on March 12e13. The onset of particle formation and nucleation bursts was identified at around noon and it is considered that these were nucleated species near the island area. It was found that some new particle formation events were associated with the long range transportation of polluted air mass from East Asian region.
[Show abstract][Hide abstract] ABSTRACT: In order to reduce uncertainty in the estimation of Direct Aerosol
Radiative Forcing (DARF), it is important to improve the estimation of
the single scattering albedo (SSA). In this study, we propose a new data
processing method to improve SSA retrievals for the SKYNET sky
radiometer network, which is one of the growing number of networks of
sun-sky photometers, such as NASA AERONET and others. There are several
reports that SSA values from SKYNET have a bias compared to those from
AERONET, which is regarded to be the most accurate due to its rigorous
calibration routines and data quality and cloud screening algorithms. We
investigated possible causes of errors in SSA that might explain the
known biases through sensitivity experiments using a numerical model,
and also using real data at the SKYNET sites at Pune (18.616°
N/73.800° E) in India and Beijing (39.586° N/116.229° E) in
China. Sensitivity experiments showed that an uncertainty of the order
of ±0.03 in the SSA value can be caused by a possible error in
the ground surface albedo or solid view angle assumed for each
observation site. Another candidate for possible error in the SSA was
found in cirrus contamination generated by imperfect cloud screening in
the SKYNET data processing. Therefore, we developed a new data quality
control method to get rid of low quality or cloud contamination data,
and we applied this method to the real observation data at the Pune site
in SKYNET. After applying this method to the observation data, we were
able to screen out a large amount of cirrus-contaminated data and to
reduce the deviation in the SSA value from that of AERONET. We then
estimated DARF using data screened by our new method. The result showed
that the method significantly reduced the difference of 5 W
m-2 that existed between the SKYNET and AERONET values of
DARF before screening. The present study also suggests the necessity of
preparing suitable a priori information on the distribution of coarse
particles ranging in radius between 10 μm and 30 μm for the
analysis of heavily dust-laden atmospheric cases.
[Show abstract][Hide abstract] ABSTRACT: The fast Radiative Transfer for Television Infrared Observation
Satellite (TIROS) Operational Vertical Sounder (RTTOV) (Version 9.3)
model was used for simulating the effect of East Asian dust on top of
atmosphere radiances. The size distribution of Asian dust was retrieved
from nine years of sky radiometer measurements at Dunhunag located in
the east of Taklimakan desert of China. The default surface emissivity
in RTTOV was replaced by the geographically and monthly varying data
from University of Wisconsin (UW)/Cooperative Institute for
Meteorological Satellite Studies (CIMSS) infrared surface spectral
emissivities. For a given size distribution and surface emissivity, the
effects of three refractive indices of Optical Properties of Aerosols
and Clouds (OPAC) mineral aerosol, dust-like aerosol by Volz, and High
Resolution Transmission (HITRAN) quartz were examined. Results indicate
that the specification of surface emissivity using geographically and
monthly varying UW/CIMSS data significantly improved the performance of
the simulation of AIRS brightness temperature (TB) difference
(BTD) between window channels, in comparison to the results from the use
of default emissivity value of 0.98 in the RTTOV model, i.e., increase
of the correlation coefficient from 0.1 to 0.83 for BTD between 8.9
μm and 11 μm, and from 0.31 to 0.61 for BTD between 3.8 μm and
11 μm. On the other hand, the use of Asian dust size distributions
contributed to a general reduction of radiance biases over
dust-sensitive window bands. A further improvement of the TB
simulations has been made by considering the Volz refractive index,
suggesting that hyperspectral infrared remote sensing of Asian dust can
be improved using the proper optical properties of the dust and surface
Journal of Geophysical Research Atmospheres 05/2012; 117(D9):9211-. DOI:10.1029/2012JD017466 · 3.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The spectral direct and diffuse irradiances observed by a radiometer with a horizontal surface detector have been frequently used to study aerosol optical parameters, such as aerosol optical thickness (taer) and single scattering albedo (v). Such radiometers more or less lack an ideal cosine response. Generally, either the cosine error of observed diffuse irradiance was corrected by assuming an isotropic distribution of sky radiance or it was neglected in the past studies. This study presents an algorithm to retrieve taer and v from direct and diffuse irradiances observed by a radiometer with a nonideal cosine response characteristic by taking into account the cosine errors of observed irradiances in detail. The proposed algorithm considers the anisotropic distribution of sky radiance while correcting the cosine error of observed diffuse irradiance. This algorithm can also be used to calculate the cosine error correction factor of diffuse irradiance. The results show that the aerosol optical parameters and the aerosol direct effect (aerosol radiative forcing and the heating rate) can be heavily affected by the cosine errors of observed direct and diffuse irradiances. The study further shows that assuming the isotropic distribution of sky radiance while correcting the cosine error of observed diffuse irradiance can affect the retrieved v at small and large solar zenith angles; thus, the estimated aerosol direct effect can be quantitatively affected. Because of the cosine errors, this study found the actual values of diffuse irradiances at different wavelengths were underestimated by around 5%-11%.
Journal of Atmospheric and Oceanic Technology 05/2012; 29(5):683-696. DOI:10.1175/JTECH-D-11-00111.1 · 1.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The potential performance of aerosols products of future satellites such
as that of the Global Change Observation Mission-Climate/Second
Generation GLobal Imager (GCOM-C/SGLI) and, the regional climate
uncertainties due to the accuracy of these products are discussed, based
on analyses of actually existing data from comparable (to the
GCOM-C/SGLI) satellites (Terra- and Aqua-MODIS). The analyses conducted
aim at validating three years (2008-2010) of satellites' daily aerosols
products from two algorithms against ground-truth measurements of three
SKYNET (radiation observation network) sites in the East Asian Pacific
region and the impacts of the satellite-ground discrepancies on the
proper determination of climate variations. The aerosols properties
examined are the aerosols optical thickness (AOT) and the
Ångström exponent (AE). Results of these analyses show that
the satellite derived AOT values are either in good agreement or
systematically overestimated (by an average value of 0.2) compared with
ground-truth data. The satellite AE values, an indication of the size of
the particles, though mostly underestimated, behave variably between the
two particle-modes generally found in the aerosol load present in the
region. In the coarse mode particles, the satellite retrieved AE are
overestimated, while in the fine mode particles, they are
underestimated. The role of the approximations generally used in the
aerosols retrieval models, on the enhancement or reduction of
satellite-ground differences and the consequences on the regional
climate uncertainty reveal a relatively complex mixture of the
contamination of adjacent clouds to the aerosols, the aerosol shape and
size distribution pattern and, the refractive index parameter.