Prede Sky Radiometer Research Articles

Novel use of an adapted ultraviolet double monochromator for measurements of global and direct irradiance, ozone, and aerosol

Abstract. A novel ultraviolet spectrometer has been developed and tested over 10 years at Lauder, New Zealand. The system, UV2, makes alternating measurements of the global and direct UV irradiance and can therefore be used to measure ozone and aerosol optical depth. After an analysis of the stability of UV2, these measurements, along with UV irradiance, are compared to relevant observations made by an additional UV spectrometer (UV4), a Dobson spectrophotometer (no. 072), and two radiometers measuring aerosol optical depth – a Prede sky radiometer and a Middleton Solar radiometer (SP02). UV2 irradiance is shown to be lower than UV4 by between 2.5 % and 3.5 %, with a standard deviation of a similar magnitude. Total column ozone values are shown to agree with Dobson spectrophotometer values with a mean bias of 2.57 Dobson units (DU) and standard deviation of 1.15 DU when using the direct sun measurements. Aerosol optical depth at 400–412 and 500 nm agrees to within 0.015 and is comparable to the difference between the reference radiometers. Further work is needed, particularly in the radiometric calibration at longer wavelengths, in order to determine if this instrument can supersede or enhance measurements made by the Dobson spectrophotometer or the aerosol radiometers.

Development of on-site self-calibration and retrieval methods for sky-radiometer observations of precipitable water vapor

Abstract. The Prede sky radiometer measures direct solar irradiance and the angular distribution of diffuse radiances at the ultraviolet, visible, and near-infrared wavelengths. These data are utilized for the remote sensing of aerosols, water vapor, ozone, and clouds, but the calibration constant, which is the sensor output current of the extraterrestrial solar irradiance at the mean distance between Earth and the Sun, is needed. The aerosol channels, which are the weak gas absorption wavelengths of 340, 380, 400, 500, 675, 870, and 1020 nm, can be calibrated by an on-site self-calibration method, the Improved Langley method. This on-site self-calibration method is useful for the continuous long-term observation of aerosol properties. However, the continuous long-term observation of precipitable water vapor (PWV) by the sky radiometer remains challenging because calibrating the water vapor absorption channel of 940 nm generally relies on the standard Langley (SL) method at limited observation sites (e.g., the Mauna Loa Observatory) and the transfer of the calibration constant by a side-by-side comparison with the reference sky radiometer calibrated by the SL method. In this study, we developed the SKYMAP algorithm, a new on-site method of self-calibrating the water vapor channel of the sky radiometer using diffuse radiances normalized by direct solar irradiance (normalized radiances). Because the sky radiometer measures direct solar irradiance and diffuse radiance using the same sensor, the normalization cancels the calibration constant included in the measurements. The SKYMAP algorithm consists of three steps. First, aerosol optical and microphysical properties are retrieved using direct solar irradiances and normalized radiances at aerosol channels. The aerosol optical properties at the water vapor channel are interpolated from those at aerosol channels. Second, PWV is retrieved using the angular distribution of the normalized radiances at the water vapor channel. Third, the calibration constant at the water vapor channel is estimated from the transmittance of PWV and aerosol optical properties. Intensive sensitivity tests of the SKYMAP algorithm using simulated data of the sky radiometer showed that the calibration constant is retrieved reasonably well for PWV<2 cm, which indicates that the SKYMAP algorithm can calibrate the water vapor channel on-site in dry conditions. Next, the SKYMAP algorithm was applied to actual measurements under the clear-sky and low-PWV (<2 cm) conditions at two sites, Tsukuba and Chiba, Japan, and the annual mean calibration constants at the two sites were determined. The SKYMAP-derived calibration constants were 10.1 % and 3.2 % lower, respectively, than those determined by a side-by-side comparison with the reference sky radiometer. After determining the calibration constant, we obtained PWV from the direct solar irradiances in both the dry and wet seasons. The retrieved PWV values corresponded well to those derived from a global-navigation-satellite-system–global-positioning-system receiver, a microwave radiometer, and an AERONET (Aerosol Robotic Network) sun–sky radiometer at both sites. The correlation coefficients were greater than 0.96. We calculated the bias errors and the root mean square errors by comparing PWV between the DSRAD (direct solar irradiance) algorithm and other instruments. The magnitude of the bias error and the root mean square error were <0.163 and <0.251 cm for PWV<3 cm, respectively. However, our method tended to underestimate PWV in the wet conditions, and the magnitude of the bias error and the root mean square error became large, <0.594 and <0.722 cm for PWV>3 cm, respectively. This problem was mainly due to the overestimation of the aerosol optical thickness before the retrieval of PWV. These results show that the SKYMAP algorithm enables us to observe PWV over the long term, based on its unique on-site self-calibration method.

Ground-based retrievals of aerosol column absorption in the UV spectral region and their implications for GEMS measurements

Quantifying the spectral variation of column aerosol absorption in the ultraviolet (UV) and visible (Vis) wavelengths is required for accurate satellite-based aerosol and trace-gas retrievals. Retrievals of the column-averaged imaginary part of refractive index and single scattering albedo (SSA) in the UV–Vis range have been performed at Yonsei University, Seoul, Korea, since 2016 by combining co-located measurements from the NASA Aerosol Robotic Network (AERONET) Cimel sun-sky photometer, the Ultraviolet Multifilter Rotating Shadowband Radiometer (UV-MFRSR), the SKYNET Prede sky radiometer, and the NASA Pandora sun spectrometer. We investigated the spectral variation of column-averaged imaginary part of refractive index for UV–Vis wavelengths to refine models used in our aerosol retrieval algorithm to process measurements from the upcoming Geostationary Environment Monitoring Satellite (GEMS). The retrieved imaginary part of refractive index for highly absorbing fine pollution particles (BC), dust (DS), and non-absorbing (NA) particles in the selected UV–Vis range (380–440 nm) showed 0–20%, 30%, and 0–40% of spectral dependence, respectively. Retrievals of Ozone Monitoring Instrument (OMI) measurement data using the improved aerosol model showed improved correlation with AERONET data compared to the old algorithm that did not properly account for aerosol absorption effects. These results corroborate the advantage of using local climatology derived from ground-based UV–Vis spectral aerosol absorption measurements for satellite GEMS aerosol retrievals over East Asia. Moreover, this study reveals that spectral variations in the UV column aerosol absorption in East Asia differ from those in other regions.

Estimation of aerosol radiative forcing over an aged-background aerosol feature during advection and non-advection events using a ground-based data obtained from a Prede Skyradiometer observation

Estimation of aerosol radiative forcing (ARF) was performed using a radiative transfer model (Rstar6b) along with physical and optical parameters of aerosols obtained from sky radiometer observation over Indian Astronomical Observatory (IAO), Hanle, Ladakh, during 2008–2010 from dust, anthropogenic, and aged background observing conditions. ARF was estimated at the top of the atmosphere (TOA), in the atmosphere, and at the surface during the three observing conditions. During dust and anthropogenic events, average aerosol optical depth (AOD at 500nm) went up to 0.24 from the aged background observing condition 0.04. Such enhancement of AOD is associated by the combination of desert-dust and anthropogenic aerosols transported from distant sources as noticed from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). Such three types of aerosols are also identified from the observed properties of single scattering albedo (SSA), aerosol asymmetry (AS) parameter, and aerosol size distribution. The estimated ARFs at TOA, at the surface, and in atmosphere are −3.73, −6.82, and 3.40Wm−2, respectively during the dust advection period. On the contrary, the respective ARFs during the aged background observing condition are −1.50, −2.22, and 0.70Wm−2, respectively. A significant difference of spectral AOD is observed during dust, anthropogenic, and aged background observing conditions. Ångström exponent (AE) decreases from 1.05 in the aged background observing condition to 0.40 in the dust event. A significant difference of coarse–fine mode volume distribution is also observed between the dust and the anthropogenic cases. Further, the study reveals high aerosols induced during the dust and the anthropogenic episodes caused warming at atmosphere and cooling at surface which collectively may affect the local atmospheric circulation.

Calibration of a Sky radiometer (Prede) using observations obtained from Hanle and Merak high-altitude stations in Ladakh

The present work was carried out to check the performance of a Prede Sky radiometer and the stability of the solar calibration constant (F0s) at two high altitude pristine stations, Hanle and Merak in Ladakh region during 2008 to 2012. Two procedures for F0s calculation were applied: one based on the normal Langley plot technique, the other based on a modified method that allows determining F0s from measurements of sky irradiance. The instrument measures sun/sky irradiance at five wavelengths in the visible and near infrared regions, and the data were processed using 4.2 version of the Skyrad.Pack software. The inputs of the software are solid view angles (SVA) at the five wavelengths and sky irradiances at different scattering angles normalized by the direct irradiances. Auxiliary sets of data such as surface albedo, atmospheric pressure, and total column ozone of the station are required to run the software. The software has two processing levels. In the first scheme, Aerosol optical depth (AOD) is retrieved along with single scattering albedo (SSA), phase function, and volume size distribution as the products of radiative transfer calculation. In the second scheme, AOD and the other parameters are calculated from Lambert–Beer law after estimating the F0s from the first scheme. On an average, the deviation of SVA during the last five year period was found to be stable with less than 1.0% from the SVA values set during installation. The estimated F0s from normal and improved Langley methods were quite agreed. The estimated F0s show temporal variation and the uncertainties of estimated F0s in the present study were within 2%. Further, uncertainties of AOD and SSA due to ±2% variation in the F0s were within 0.02 and 0.2, respectively. The estimated coefficients of variation (CV) of the determined F0s show a dependence on the observed AOD.

Aerosol Optical Properties Retrieved from a Prede Sky Radiometer over an Urban Site of Beijing, China

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 (α), volumes of different aerosol particle size distributions, and single-scattering albedos (SSA), have been analyzed.

Field measurement of clear-sky solar irradiance in Badain Jaran Desert of Northwestern China

The Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) sponsored and conducted an intensive field campaign on dust aerosols in Badain Jaran Desert of Northwestern China from April 20 to June 20, 2010. A set of state-of-the-art broadband radiometers and sun/sky photometers were deployed along with launched radiosonde. In this paper, we compared the simulated solar irradiances by using the SBDART radiative transfer model with those from the ground-based measurements for 69 selected cases of 7 days. It was shown that the averaged aerosol optical depth at 500nm (AOD500) is 0.18±0.09 with AOD500 less than 0.5 for all cases. The single-scattering albedo and asymmetry factor at 675nm are 0.928±0.035, 0.712±0.023, respectively. The AODs retrieved from the CIMEL sun photometer at various wavelengths agree well with those from the PREDE sky radiometer, and the columnar water vapor contents from CIMEL also agree well with radiosonde observations. In the radiative closure experiment, we used a collocated thermopile pyrgeometer with a shadow and ventilator to correct the thermal dome offset of diffuse irradiance measurement. The mean differences between model and measurements are −9.1Wm−2 (−2.6%) for the direct irradiance, +3.1Wm−2 (+2.8%) for diffuse irradiance, and −6.0Wm−2 (−1.3%) for global irradiance, which indicates an excellent radiative closure. Aerosol shortwave direct radiative forcing (ARF) and radiative heating rate are also investigated. The daily mean ARF ranges from −4.8 to +0.4Wm−2 at the top of the atmosphere, −5.2 to −15.6Wm−2 at the surface, and 5.2 to 10.8Wm−2 in the atmosphere. The corresponding radiative heating rates for the whole atmosphere due to dust aerosols are 0.07, 0.11, 0.14, 0.11, 0.10, 0.08, and 0.07K/day for the 7 selected cloudless days. These solar radiative forcing can be considered as the representative impact of background dust aerosol in Northwestern China.

Instrument calibration and aerosol optical depth validation of the China Aerosol Remote Sensing Network

This paper introduced the calibration of the CE‐318 sunphotometer of the China Aerosol Remote Sensing Network (CARSNET) and the validation of aerosol optical depth (AOD) by AOD module of ASTPWin software compared with the simultaneous measurements of the Aerosol Robotic Network (AERONET)/Photométrie pour le Traitement Opérationnel de Normalization Satellitaire (PHOTONS) and PREDE skyradiometer. The results show that the CARSNET AOD measurements have the same accuracy as the AERONET/PHOTONS. On the basis of a comparison between CARSNET and AERONET, the AODs from CARSNET at 1020, 870, 670, and 440 nm are about 0.03, 0.01, 0.01, and 0.01 larger than those from AERONET, respectively. The aerosol optical properties over Beijing acquired through the CE‐318 sunphotometers of one AERONET/PHOTONS site and two CARSNET sites were analyzed on the basis of 4‐year measurements. It was obvious that the AOD of the Shangdianzi site (rural site) was lower than that of the two urban sites (the Institute of Atmospheric Physics (IAP) site (north urban site) and the Beijing Meteorological Observatory (BJO) site (south urban site)). The AOD of BJO was about 0.05, 0.04, 0.05, and 0.06 larger than that of IAP at 1020, 870, 670, and 440 nm, respectively, indicating that there is more local pollution in the south part of Beijing. The highest AOD was found in summer because of the stagnation planetary boundary layer and transport of pollutants from large pollution centers south of Beijing. The high temperature and relative humidity in summer also favor the production of aerosol precursor and the hygroscopic growth of the existing particles locally, which results in high AOD. In contrast, the lowest AOD at the two urban sites and one rural site in Beijing occurred in winter as the frequent cold air masses help pollutants diffuse easily.

Intercomparison between aerosol optical properties by a PREDE skyradiometer and CIMEL sunphotometer over Beijing, China

Abstract. This study compares the aerosol optical and physical properties simultaneously measured by a SKYNET PREDE skyradiometer and AERONET/PHOTONS CIMEL sunphotometer at a location in Beijing, China. Aerosol optical properties (AOP) including the Aerosol Optical Depth (AOD), Angstrom exponent (α), volume size distribution, single scattering albedo (ω) and the complex refractive index were compared. The difference between the two types of instruments was less than 1.3% for the AOD and less than 4% for the single scattering albedo below the wavelength of 670 nm. There is a difference between the volume size distribution patterns derived from two instruments, which is probably due to difference of measurement protocols and inversion algorithms for the respective instruments. AOP under three distinct weather conditions (background, haze, and dust days) over Beijing were compared by using the retrieved skyradiometer and sunphotometer data combined with MODIS satellite results, pyranometer measurements, PM10 measurements, and backtrajectory analysis. The results show that the significant difference of AOP under background, haze, and dust days over Beijing is probably due to different aerosol components under distinct weather conditions.

Retrievals of Antarctic aerosol characteristics using a Sun‐sky radiometer during the 2001–2002 austral summer campaign

In order to characterize the Antarctic aerosol and to analyze the effect of katabatic winds on the properties of suspended particles, measurements of solar direct and diffuse irradiance were carried out at the Italian Terra Nova Bay station in Antarctica, during the 2001–2002 austral summer campaign. Measurements were performed by the ground‐based PREDE sky radiometer and processed by using the Skyrad inversion code. Aerosol optical thickness at 500 nm was found to vary between 0.01 and 0.02. The volume size distribution curves showed bimodal features with the two modes located within 0.1–0.3 μm and 5–7 μm radius intervals, respectively. The real part of the refractive index characterizing the Antarctic aerosol was found to have a mean value of 1.40. During the katabatic event the analysis indicated that the advection of larger and drier fresh particles, together with the removal of marine suspended particles, caused the decrease in aerosol optical thickness.