Aerosol optical properties at the Alpine station of Saint-Christophe, northern Italy are measured with a POM-02 sky radiometer. Diffuse radiances from the sky and direct sun irradiances are processed with SKYRAD and SUNRAD algorithms, respectively, providing consistent results. The aerosol optical depth at the station is generally very low (0.10 at a wavelength of 500 nm); however, spikes (up to 0.75) can occur due to the transport of Saharan dust and smoke clouds moving on an intercontinental scale, according to NAAPS and HYSPLIT models. In addition, the aerosol microphysical properties are analyzed by means of the Ångström exponent and its spectral variation, which allows discrimination between different types of particles and the assessment of possible contamination by clouds. Influence by local meteorology and anthropogenic emissions is investigated, providing useful information to understand the pollution dynamics in mountainous regions. Finally, we highlight some algorithmic and instrumental issues that are still poorly understood.
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), Ångström exponent (α), 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 α 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 α 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 (α < 0.47 and SSA400 nm - SSA1020 nm < 0.018).
The Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) project initiated an intensive field experiment on dust aerosols in Dunhuang from April 1 to June 12, 2012. Using sky radiometer measurements and conducting model simulations, we investigated the dust aerosol characteristics and its shortwave radiative impact on the regional climate. The daily averaged optical features of the aerosols markedly varied throughout the study period. High aerosol loading and predominantly coarse particulates were observed in the spring of 2012 ascribed to the influence of prevalent dust storm. The single scattering albedo at 500 nm (SSA500) varied from 0.91 to 0.97 on dusty days and from 0.86 to 0.91 on dust-free days, indicating that the dust aerosols sourced from northwest China were not strongly absorbing. Surface radiation quantities estimated by the radiative transfer model excellently agreed with ground-based and satellite observations, with correlation coefficients exceeding 0.990 and mean differences ranging from -3.9 to 17.0 W m-2. The daily mean aerosol shortwave direct radiative forcing (ARF) values were largely negative at the surface (-79.4 to -3.2 W m-2) and moderately positive in the atmosphere (2.2-25.1 W m-2), indicating strong cooling at the surface and moderate atmospheric warming. The monthly averaged ARFEs (ARFs per unit aerosol optical depth at 500 nm (AOD500)) at the surface were (-73.9 ± 11.6) W m-2, (-67.4 ± 8.3) W m-2, and (-74.4 ± 5.4) W m-2 in April, May, and June, respectively (overall average of (-70.8 ± 7.9) W m-2), comparable to previously obtained values in East Asia and India domains. The relations between the diurnal ARFs at the surface and top of the atmosphere (TOA) and the AOD500 indicate that aerosol composition remained relatively stable at Dunhuang during the spring of 2012. The ARF at the TOA was positive for SSA500 less than 0.85 or when the imaginary part at 500 nm exceeded 0.015.
In this study, characteristics of atmospheric phenomena such as haze and yellow sand (Kosa) events were investigated in terms of aerosols by using sky radiometers, Light Detection and Ranging (LIDAR), and optical particle counter (OPC) observations at Fukue-jima and Amami-Oshima Islands from 2003 to 2004. As a result of the data analyses, we determined that aerosol properties such as loading, light absorptivity, particle size, non-sphericity, and vertical distribution showed specific features both in the atmospheric column and near the surface, depending on the atmospheric phenomena compared with normal atmospheric conditions. A specific case was clearly confirmed: the influence of limited light absorptivity dominated even during a Kosa event. In this study, it was confirmed that even if each ground-based instrument observed the phenomena with different ranges for the atmospheric column, lower layer, and surface, the retrieved aerosol properties were consistent. We demonstrated that the combined use of state-of-the-art instrumentation to detect aerosols is useful for quantitatively characterizing the atmospheric phenomena.
Scattering and absorption coefficients, measured with an integrating nephelometer and absorption photometer, respectively, are often used to characterize aerosols. This study developed a method for retrieving the single scattering properties of an aerosol from multiwavelength scattering and absorption data and evaluated its performance and accuracy using simulation data based on Optical Properties of Aerosols and Clouds (OPAC) models. This statistical retrieval method retrieves the volume-size distribution and the complex refractive index simultaneously, which reconstruct the measured values taking into account the angular truncation and non-ideality of the light source. These values are then used to more accurately estimate single scattering properties (scattering, absorption, and extinction coefficients: single scattering albedo (SSA); and the asymmetry factor). With no systematic (bias) error, the root mean square error (RMSE) of the measured scattering coefficient was 4.42 × 10-5 to 6.61 × 10-5 m-1, whereas the RMSE of the retrieved scattering coefficient was 9.48 × 10-6 to 1.09 × 10-5 m-1. The RMSE was thus reduced to about 20 %. The RMSE of the SSA calculated directly from the measured values was 0.014-0.021, and that of the SSA calculated from retrieved values was 0.002, corresponding to a relative error of 0.2 %. In each case, the error in the SSA calculated from the retrieved scattering and extinction coefficients was less than the SSA calculated from the measured values. Sensitivity tests of the systematic (bias) error of absorption and scattering coefficients in SSA retrieval demonstrated that, with a 10 % systematic error, the maximum difference between the true value and the retrieved SSA exceeded 0.02 for a small SSA, but with a systematic error of 3 % or 5 %, the maximum difference was small. Therefore, a systematic error of less than 5 % is desirable. The retrieved volume size distribution and complex refractive index were qualitatively similar to the original values.
To investigate aerosol optical properties, the Meteorological Research Institute has been continuously measuring scattering and absorption coefficients since January 2002 by using an integrating nephelometer and one- and three-wavelength absorption photometers in dry air conditions at Tsukuba, Japan. We used these optical data to investigate trends of aerosol properties and climatology from 2002 to 2013. The results showed that most aerosol characteristics had seasonal variation and decreasing or increasing trends significant at the 95 % confidence level. From 2002 to 2013, the extinction coefficient at 550 nm and absorption coefficient at 530 nm had statistically significant decreases of -1.5 × 10-6 and -5.4 × 10-7 m-1 year-1, respectively. In the same period, the scattering coefficient showed a non-significant decrease of -8.8 × 10-7 m-1 year-1. The single scattering albedo (SSA) at 550 nm had a significant increasing trend of 7.4 × 10-3 year-1. Asymmetry factors did not show a significant trend. The increasing trend in the extinction Ångström exponent was significant, whereas the trend in the effective radius was not significant. The increasing trend of 2.1 × 10-2 year-1 in the absorption Ångström exponent from 2006 to 2013 was significant. This tendency suggests a compositional change of light-absorbing aerosol. Frequency distributions of aerosol properties were investigated during 2006-2012. In this period, absorption coefficients were measured by the three-wavelength absorption photometer. The most frequent values of the extinction coefficient at 550 nm, the absorption coefficient at 530 nm, and the SSA at 550nm were 25 × 10-6, 3.0 × 10-6 m-1, and 0.905, respectively. The analysis using the extinction Ångström exponent showed that aerosol characteristics were dependent on the extinction Ångström exponent. The aerosol characteristics estimated from optical data were consistent with those derived from radiometer data. Therefore, ground-based monitoring of aerosol optical properties is useful for monitoring aerosol characteristics and interpreting variations in the surface radiation budget.
The effects of dust aerosol particles on the properties of clouds over East Asia and the Sahara are studied using moderate resolution imaging spectroradiometer observations from the Terra and Aqua satellites and simulation results of the chemical weather forecast system (CFORS) model. Dust-contaminated clouds are detected using the brightness temperature difference (BTD) method, in which dust or dust-bearing clouds are detected when the BTD between the 11 µm and 12 µm channels in the window region is negative, and by the CFORS model’s dust vertical profile. Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation data are also used to assess the above procedures. For the Sahara region, no obvious changes to effective particle radius (Re) are attributed to dust aerosols. However, for East Asia, dust aerosols are found to change the cloud microphysical properties, decreasing Re by 12 % and increasing the cloud optical depth by 27 % and the liquid water path by 9 %. Re is found to be negatively correlated with sulfate concentration by CFORS in dust-bearing clouds, but not in dust-free clouds, over East Asia. These findings indicate that mineral dust can act as effective cloud condensation nuclei in environments polluted by water-soluble aerosols such as sulfates. We also estimate the indirect radiative effect of dust aerosols in the East Asia region and determine, by radiative transfer calculation, that the net shortwave radiative flux at the top of the atmosphere decreases and the absorption of shortwave radiation by the atmosphere increases with changes in cloud properties due to dust aerosols.
Clouds strongly influence downward longwave radiative flux and affect the radiation budget at the surface. We evaluated the cloud radiative effect in both absolute and relative terms on downward longwave radiation at the surface; we considered variations in the cloud radiative effect with changes in cloud amount, precipitable water, and cloud base height, as measured by eight stations of the Baseline Surface Radiation Network. The downward longwave radiation predicted by a radiative transfer model agreed well with observations. The cloud radiative forcing and contribution ranged from -21 to 92 W m-2 and from -6 % to 38 %, respectively. The cloud effect shows a positive correlation to the shortwave diffusivity index (an index of cloud amount) and a negative correlation to precipitable water amount. The absolute effect values are small, depending on site conditions, but the relative effect values are larger under dry conditions than under humid conditions. Under humid conditions, the effect of the shortwave diffusivity index is very small. Under dry and cold conditions, such as those found in polar regions, negative values of cloud radiative contribution appear frequently because clouds absorb the emissions from temperature inversion layers. In comparison with prior research that used the A-Train satellite product, the present study shows a wider distribution and a larger maximum value for cloud forcing from amount of water vapor. Cloud effect has a roughly negative relationship with cloud base height, but a positive correlation with cloud base height occurs under low clouds at Tateno, which is located on the Pacific Ocean side of Japan. This correlation is because of the unusual relationship between cloud base height and cloud effect at Tateno during the summer and winter seasons. These results describe small-scale and near-surface variations in cloud effect, which are difficult to detect by satellite measurements.
Clouds affect shortwave irradiance and the energy budget at the Earth’s surface. We investigated the potential radiative forcing in China by applying simple linear regression analysis on pyranometer measurements and data from the International Satellite Cloud Climatology Project (ISCCP). The potential radiative forcing is a measure of sensitivity of the surface shortwave irradiance to atmospheric parameters (here, cloud amount and cloud optical thickness). The negative correlation between the potential radiative forcing of cloud amount and cloud optical thickness strengthens with an increase in the cloud optical thickness up to about 8, after which it weakens. In contrast, the correlation coefficient between the cloud optical thickness and its potential radiative forcing varies widely for a low optical thickness and converges to a small negative value with an increase in the optical thickness; this is particularly true for data from the South and East regions. The potential radiative forcings obtained by analyzing pyranometer observation data are consistent with those calculated from ISCCP data in the previous study.
The estimation of the distribution of global anthropogenic heat release (AHR) from 1992 to 2009 was obtained by applying Defense Meteorological Satellite Program (DMSP)/Operational Linescan System (OLS) satellite data. The results indicate that global AHR was geographically concentrated, essentially correlating to economic activities. The anthropogenic heat flux concentrated in the economically developed areas, such as East Asia, Europe, and Eastern North America, reached a level high enough to influence regional climate. In contrast, the anthropogenic heat flux in vast areas, such as Africa, Central and North Asia, and South America, is very small. With the increases in global population and economic development, an increase in AHR was easily found. The model results show that AHR has a significant impact on surface temperature and that it is able to affect global atmospheric circulation, leading to a 1-2 K increase in the high-latitude areas of Eurasia and North America. The results show that AHR is able to affect global climate despite being limited to a region. Although the influence to global warming by AHR is not as large as greenhouse gases, such as carbon dioxide, on a global scale, AHR is an important factor in global climate change that should not be ignored.
A cloud screening method employing two successive procedures of variability test and coarse mode test was developed, aiming at better elimination of cloud-contaminated data in the sky radiometer retrievals. The performance of the new cloud screening method was evaluated by examining statistical features with cloud coverage observations and lidar measurements. The variability test appeared to effectively eliminate data contaminated by relatively thick low-level clouds, whereas the coarse mode test appeared to eliminate data likely contaminated by thin cirrus-type clouds. Overall, the new method was considered to improve the current Sky Radiometer Network (SKYNET) data. The cloud screening method was then applied to dust detection from sky radiometer measurements. The detection performance was evaluated using surface synoptic observations (SYNOP) dust reports and the yellow sand index from NIES lidar measurements. It was shown that the new method helped to detect dust, effectively eliminating cloud-contaminated signals that were similar to those of the dust.
The sky radiometer is a ground-based instrument implemented to investigate the characteristics of not only aerosols but also clouds and water vapor. This study is the first attempt to bring this instrument in use to estimate the columnar ozone concentration (U). A method and preliminary results related to the estimation of U using the 315-nm-channel data of the sky radiometer are presented. The proposed method consists of calculating the calibration constant for direct intensity at 315 nm wavelength, F0(315 nm), usingin situ observation data, which is an alternative of the traditional Langley method, and estimating U. The temporal values of U at Chiba, Japan for the period of January-April, 2013 were estimated, and they were compared with values observed by the ozone monitoring instrument (OMI). The agreement was satisfactory during the initial observation period; however, the values from the sky radiometer were observed to be gradually overestimated with time. The study suggests that the temporal change of F0 (315 nm) is the important factor to be considered while estimating U values for long-term observations and that F0 (315 nm) must be periodically determined. The study further discusses the uncertainties associated with the estimated U due to the uncertainties in the values of input parameters.
Sky radiometer data are used to retrieve aerosol optical properties and to estimate the effect of aerosols on solar irradiance at the Earth’s surface. The Prede POM-02 is equipped with a 940-nm channel corresponding to a primary water vapor absorption band in the near infrared region, but 940-nm data have been underutilized. Atmospheric water vapor is an important factor in determining the surface radiation budget. To retrieve the columnar precipitable water vapor amount from sun-sky radiometers, the 940-nm channel was calibrated using the Langley method, which accounts for gas absorption. The relation between column water vapor and atmospheric transmittance at 940 nm was determined using simulation data, and the results were used to retrieve column water vapor. This method was applied to data collected at Tsukuba, Japan, in 2011 and compared with global positioning system receiver (GPS), microwave radiometer, and radiosonde water vapor retrievals. The highest correlations were found between GPS and radiosonde observations. A comparison of POM-02 and GPS results showed a bias error of 0.09 g cm-2; the root mean square error was 0.179 g cm-2; and r = 0.996. The transmittance of the 940-nm channel was theoretically determined in this study. Therefore, the accuracy of column water vapor retrieval depends on the accuracy of transmittance calculation model.