This study presents the results of the Fourth Filter Radiometer Comparison that was held in
Davos, Switzerland, between 28 September and 16 October 2015. Thirty filter
radiometers and spectroradiometers from 12 countries participated including
reference instruments from global aerosol networks. The absolute differences
of all instruments compared to the reference have been based on the World
Meteorological Organization (WMO) criterion defined as follows: “95% of
the measured data has to be within 0.005
Growing recognition of the role of atmospheric aerosols in the determination and modification of the Earth's radiation budget and hydrological cycle through their direct and indirect effects has led to a steady increase of scientific interest in aerosol physical, chemical and optical properties over the last decades (Augustine et al., 2008; Lohmann and Feichter, 2005; Nyeki et al., 2012; Wehrli, 2008). The main parameter related to columnar integrated optical activity of aerosols is their optical depth, which can be derived from ground-based measurements of the attenuation of sunlight but also from modeling of scattered radiation observed from space (e.g., Levy et al., 2013; Sayer et al., 2012; Kahn et al., 2005; Li et al., 2014; Toledano et al., 2011). Aerosol optical depth (AOD) is the single most comprehensive variable to assess the aerosol load of the atmosphere and the most important aerosol-related parameter for radiative forcing studies. This significance is illustrated by the fact that AOD is one of the core aerosol parameters of the World Meteorological Organization (WMO, 2003) Global Atmosphere Watch (GAW) program.
AOD can be derived from the ground with measurements of the spectral
transmission of direct solar radiation by various types of instruments such
as sun-pointing or rotating shadow-band filter radiometers, as well as
spectroradiometers. It can be determined as the difference between the
observed total optical depth and the modeled optical depths of molecular
(Rayleigh) scattering and gaseous absorption, which depend on wavelength.
Since AOD is often a small difference between two larger numbers (mainly the
total optical depth and the Rayleigh scattering), it is very sensitive to
small calibration errors and to a lesser degree to the chosen algorithms for
the modeled components. The main source of error in sun photometry is the use
of incorrectly estimated calibration constants,
Surface-based global networks of AOD measurements, such as the AErosol RObotic NETwork (AERONET) (Holben et al., 1998, 2001), the Global Atmospheric Watch Precision Filter Radiometer network (GAW-PFR) (McArthur et al., 2003; Wehrli, 2005), the SKYradiometer NETwork (SKYNET) (Aoki et al., 2006; Kim et al., 2008), the Bureau of Meteorology AOD Australian network (Mitchell and Forgan, 2003) and the National Oceanic and Atmospheric Administration Earth System Research Laboratory's (NOAA ESRL) Surface Radiation network (SURFRAD) (Augustine et al., 2000) and NOAA ESRL global baseline observatories (Dutton et al., 1994) are used to measure spectral AODs at various locations worldwide. Several AOD intercomparison campaigns with the participation of different instrument types that belong to some of the above networks have taken place as short-term intensive field campaigns and have proven themselves a successful method of relating the methodologies of standards from one network to another (Aoki et al., 2006; Kim et al., 2005; McArthur et al., 2003; Mitchell and Forgan, 2003; Schmid et al., 1999).
Simultaneously, most of the previous AOD comparison studies, including the
first, second and third filter radiometer comparisons (FRC-I, FRC-II and FRC-III), were
conducted under clear atmospheric conditions, which are preferred for
evaluating the differences in instrument calibrations. Results from FRC-I to
III were not published as the intercomparisons were effectively organized on
an ad hoc basis amongst participants of the International
Pyrheliometer Comparisons (IPCs) at the Physikalisch-Meteorologisches
Observatorium Davos, World Radiation Center (PMOD/WRC), Davos, Switzerland.
FRC-I to FRC-III were held for 2 weeks in September–October 2000, 2005 and 2010,
respectively. FRC-II and FRC-III were based on AOD results derived from
simultaneous measurements by each participant according to their standard
protocol and evaluated by their preferred algorithms, including
cloud screening. Recommendations by WMO experts (WMO, 2005) were implemented
as of FRC-II. A large number of radiometers were present during both FRC-II
(14 from 9 countries) and FRC-III (17 from 10 countries). The main
conclusions were as follows: (i) most of the ground-based AOD-measuring instruments
were able to achieve comparable results to within
In this study, we present the results of the Fourth FRC intercomparison campaign in which 30 instruments, from 12 countries, belonging to the above-mentioned global or national networks, participated. Section 2 presents the instrumentation, the location of measurements and the analytical methodology used. Section 3 describes the intercomparison results, while conclusions in Sect. 4 investigate AOD calculation methods and assumptions involved and set the framework within which the homogeneity of networks will be feasible through standardization of instrumentation and procedures in combination with a multi-faceted data quality control and quality assurance system. The whole activity aims to homogenize and harmonize AOD measurements on a global scale. The comparison protocol was formulated according to the WMO recommendations (WMO, 2003, 2005).
The World Optical depth Research and Calibration Center (WORCC) was
established at Davos in 1996 and was assigned the mission by WMO to develop
stable instrumentation and improved methods for calibration and observation
of AOD. These new developments were demonstrated in a global pilot network
(Wehrli, 2008). Toward this goal and concurrent with the 12th International
Pyrheliometer Comparison (IPC-XII), FRC-IV was held.
Representatives for instrumentation belonging to different aerosol optical depth global networks
were invited to participate. The comparison took place on the premises of the
PMOD/WRC from 28 September to 16 October 2015. Thirty filter radiometers and
spectroradiometers from 12 countries participated in this campaign. PMOD/WRC
(46
Average AOD at 500 nm measured by the three reference PFR instruments (WORCC triad) during 5 days with cloud-free sky conditions. Data points represent 1 min measurements.
During FRC-IV, there were 5 days (28–30 September, 1 and 12 October) mainly with sunshine and only very limited presence of clouds. Measurements from these days have been used to compare the participating instruments. During the five intercomparison days, AOD varied from 0.02 up to 0.12 at 500 nm, which can be considered as normal values for the area. Figure 1 shows the AOD variability during the intercomparison days, as measured by the WORCC triad that is defined as the mean of three well-maintained precision filter radiometer (PFR) instruments. Before the start of the campaign, the PFR triad was intercompared with three additional PFR instruments that had performed measurements at stations in Izaña, Tenerife, Spain, (two instruments) and Mauna Loa, Hawaii, United States, for a period of 9 months. The calibration of the particular instruments was based on the Langley calibration technique. During five cloudless days in August–September 2015, the three Langley-calibrated instruments were compared with the three PFR triad instruments. The differences in AOD for all instruments were from 0.2 to 0.5 % or up to 0.0005 in AOD at all wavelengths.
Filter radiometers have been used in meteorology for at least 40 years to measure atmospheric haze or turbidity. Modern sun photometers use dielectric interference filters and silicon photodetectors resembling the filter radiometers used in metrology. The PFRs (Wehrli, 2000) have been designed with emphasis on radiometric stability and a small number of instruments were built for a trial network of AOD measurement sites (Wehrli, 2005).
The participating filter radiometers were either of the direct pointed type, e.g., classic sun photometers, including sky-scanning radiometers used in direct sun mode, or hemispherical rotating shadow-band radiometers. These included the following (see Table 1 for further details).
Nine instruments were of the PFR type (manufactured by PMOD/WRC) that is
used in the GAW AOD network (Wehrli, 2005). The PFR is a classic sun
photometer with four independent channels and a field of view (FoV) of 2.5
Two radiometers were of the Carter–Scott SP02 type (Mitchell and Forgan,
2003), which is similar to the PFR, but has a wider FoV of 5
Three Cimel (CIM) CE318 sun- and sky-scanning radiometers used by AERONET
(Holben et al., 1998) were included; two of them are the CE318-T model, which is the new
standard AERONET instrument with improved performance and which is capable of
performing lunar observations (Barreto et al., 2016). These instruments have
a narrow FoV of 1.2
Four Multi-Filter Rotating Shadowband Radiometer (MFRSRs) (Harrison et al., 1994, 1999) with a hemispheric FoV are used. These measure global horizontal and diffuse
horizontal irradiance (GHI and DHI) in five aerosol channels; the difference in
GHI and DHI divided by the solar-zenith angle is cosine-corrected to provide
calculated direct beam spectral irradiances. The temperature is held near
40
Three Precision Solar Radiometers (PSRs) are used that are direct sun-pointing
spectroradiometers able to measure the spectrum from 300 to 1000 nm with
a wavelength increment of 0.7 nm. The FoV and the full width at half maximum (FWHM) values are 1.5
Three direct sun-pointing POM-2 sky radiometers instruments from Prede Co., Ltd are included.
The instruments have a FoV of 1
Four Solar Spectral Irradiance Meters (SSIMs) from Cofovo Energy Inc are employed. The
instruments measure AOD at six wavelengths with an FoV equal to 2
One Microtops (MIC) handheld aerosol sun photometer from the Solar Light Company was employed.
The instrument measures at five wavelengths between 340 and 936 nm with FoV and
FWHM values equal to 2.5
Historically, instrument comparisons have consisted of bringing a number of instruments together to a single location for a period of several days to several weeks (e.g., Schmid et al., 1999). These types of comparisons are essential in order to try to move the frontiers of instrument and metrology science forward. However, there may be little or no relation between the results of these intensive comparisons and the results from the same instruments when placed in an operational network setting. The comparison that is reported here provides insight into the quality of data output by instruments when attended to following operational protocols, designed by the various data centers, which are responsible for the routine handling of the measurements. Therefore, the results of this comparison should provide an understanding of both the comparability between different networks and the overall data quality of participating networks. However, in addition to these FRC-IV results, homogeneity-related conclusions for different networks are linked with the action of each network towards standardization of calibration and instrumentation and towards the use of standard operational procedures, including data quality control and quality assurance protocols. Given the differences in instrumentation characteristics, calibration strategies (Walker et al., 1987) and processing algorithms used by different networks, the effective equivalence of AOD observations needs to be estimated through intensive observation periods (Schmid et al., 1999) or extensive field comparisons (McArthur et al., 2003; Mitchell and Forgan, 2003) of co-located instruments representing different networks.
It has to be noted that most of the instruments have been installed, maintained and checked by the initial instrument operators that participated in the campaign, with the exception of two Cimel instruments that PMOD WRC staff installed and maintained during the campaign.
Details of sun photometers used during the FRC-IV intercomparison campaign.
AOD is defined as the negative natural logarithm of transmission, normalized
to the vertical path length,
Measurements of solar irradiance were nominally taken each full minute by the participant's data acquisition systems, typically yielding 500 observations per cloudless day. Actual sampling/averaging rates ranged from 15 s to 1 min depending on the instrument. Simultaneous measurements were defined in a time window of 30 s before and after each full CET minute. The raw measurements were evaluated by each participant according to their preferred algorithms, including cloud screening, and were then submitted for comparison. The three Cimel instruments which participated in the campaign measured at different frequencies: (i) one measurement every 3 min, (ii) following the typical AERONET schedule, measurements every 15 min except for the Langley sequence in the morning and evening, in which AOD measurements are more frequent and (iii) measurements every 15 min.
The set of measurements covered wavelengths between 340 and 2200 nm.
Channels at 368
Ångström exponents were derived from optical depths at 500 and 865 nm (29 instruments). Values of atmospheric pressure, precipitable water, relative humidity and temperature readings were made available to all participants by the MeteoSwiss weather station located at PMOD/WRC with a 10 min resolution. Total ozone column content measured with a double Brewer spectroradiometer at PMOD/WRC was available as well. This common auxiliary database was available to all participants in order to avoid AOD-related discrepancies introduced by uncertainties linked with the above-mentioned parameters.
Several of the participating radiometers were calibrated at various sites within a few months prior to FRC-IV. Their performance during this comparison can be used to estimate the homogeneity of AOD observations across weather services, networks or individual measuring sites. For more details about the instrument acronyms, their participation in national or international aerosol networks and their basic calibration technique, see Table S1 in the Supplement.
Each of the instruments that participated in the campaign was calibrated using techniques that are quite well documented in various publications describing the instrument/network calibrations, explained more specifically in the following text.
Number of instruments submitting AOD data for each wavelength during FRC-IV.
During the intercomparison, AOD data delivered by the operators of the
participating radiometers were evaluated using common comparison software.
The comparison was based on AOD results only, as each operator/group used
their own algorithm normally used for standard radiometer operation. The
comparison principles were based on the recommendations formulated during
the WMO experts workshop “Global surface network for long-term
observations of column aerosol optical properties”, held in 2004 in Davos
(WMO, 2005), which called for the following:
at least 1000 data points (1 min data) with AOD at 500 nm between
0.04 and 0.20 a minimum duration of 5 days traceability requiring 95 % uncertainty within
During FRC-IV, weather conditions allowed over 1000 measurements to be made
for most instruments on 5 days, allowing the above-mentioned recommendations
to be fulfilled.
Comparison of the triad (gray points) with the Cimel instruments
(
The intercomparison results presented below are based on AOD values provided by the individual instrument operators compared to the triad. Figure 2 shows an example of this comparison, on a diurnal plot, including various instruments separated into groups of different instrument types, compared to the PFR triad. The majority of the PFRs showed the best performance with absolute AOD differences from the triad ranging in all cases and wavelengths from zero to 0.01. As the measured wavelength increases, the errors are minimized, reaching performance errors close to zero, except for some overestimated outliers for PFR_SE_N35, which were caused by nonsynchronous measurements (timing) for particular periods. Results for the three CIM instruments are almost identical to those of the PFR at 500 and 862 nm (Fig. 2a), while a slight underestimation on the order of 0.01 and 0.005 at the shorter wavelengths 368 and 412 nm (not shown here), respectively, was found. It has to be noted that Cimel AOD values at 412 and 368 nm have been linearly interpolated using the Cimel AOD at 340, 380 and 440 nm and the AEs derived from these three wavelengths. Therefore, part of the difference can be explained by the interpolation-related uncertainties. POM sky radiometers do not measure AOD at 368 and 412 nm. However, comparable results to the CIM and PFR at 862 nm was retrieved, with a slight underestimation, well within the WMO limits, at 500 nm (Fig. 2b), which was not related to the air mass. These results demonstrate the high level of the quality of reference instruments belonging to the GAW-PFR, AERONET and SKYNET networks. The two SPOs, which are similar instruments to the PFRs but with a wider FoV and with no temperature controller, showed good agreement compared to the triad. SPO_AU_1 showed excellent median differences (Fig. 2c). For the SPO_US_1, one of the 5 days of measurements at 500 nm and one of the 5 days at 862 nm gave overestimated values, with excellent agreement on other days and excellent agreement on all days at 500 nm. The overestimates were likely the result of the four FoVs of the SPO not being optimally aligned. During the shipment of the SPO2_US_1 to Davos, the diopter was damaged. It was manually adjusted to its position during FRC-IV without the benefit of a detailed alignment process that is usually conducted to minimize the misalignment of the four independent barrels of the sun photometer. At 368 nm, small SPO_AU_1-calibration-related AOD differences were observed compared to the triad. The four multi-filter radiometer (MFR) instruments showed good agreement for the medians compared to the PFR triad; however, they exhibit larger scatter than the sun-pointing instruments, resulting in a lower precision. McArthur et al. (2003) had previously reported that the MFR-derived AOD does not quite meet the accuracy of the sun-pointing instruments under clean atmospheric conditions. MFR_DE showed an AOD overestimation in various instances that gave results that are outside the WMO-defined AOD limits (Fig. 2d). This small overestimation of the MFR_DE instrument compared to the PFR triad could be due to uncertainties introduced while correcting for their angular response, by the calibration procedure or by incomplete blocking of the diffuser by the shadow band. The MFRSRs that are part of the SURFRAD network (MFR_US2 and MFR_US3) gave a median AOD at 500 nm that is in very good agreement with the PFR triad and is in fact better than some of the other sun-pointing instruments, e.g., Cimel and POM; these two slightly underestimate the AOD at 865 nm but are within the WMO defined limits. Again, these median values of these two MFRs are comparable to the better sun-pointing instruments but give larger scatter. These two MFRs are representative of the SURFRAD network that follows network protocols for calibration and alignment and conducts frequent characterizations of the spectral and angular responses (Augustine et al., 2003; Michalsky et al., 2010). Again, this highlights the high level of the quality of instruments that represent larger networks (GAW-PFR, AERONET, SKYNET and SURFRAD networks).
AOD comparison results at 368
Looking at possible diurnal patterns of the AOD differences shown in Fig. 2,
most of the instruments show relatively constant differences over time (and
air mass). One example of a possible diurnal pattern in the AOD differences
that can be linked with the instrument calibration (as discussed in Cachorro
et al., 2004) is the POM_JP instrument. In that study, differences are
proportional to
Taylor diagrams at the four measuring wavelengths.
Figure 3 shows the comparison results in terms of absolute difference between the triad and the nine individual PFR instruments, three CIM (AERONET) instruments, three POM (SKYNET) instruments, two SPOs, four MFRs, three PSRs, four SSIM instruments and one MIC instrument. The box plots represent the range between the 10th and 90th percentiles, with the in-box dot showing the median and the upper and lower whiskers showing the maximum and minimum error value information that is within 1.5 times the interquartile range of the box edges. The figure shows the good agreement among most of the instruments compared to the reference triad. WMO limits cannot be shown in Fig. 3 as they are air-mass-dependent. However, for FRC-IV these limits were between 0.006 and 0.012 for low solar elevations and local noon, respectively.
PFR AOD comparisons showed that median differences were well within
Overall, the FRC-IV intercomparison results are comparable with the results found by Mitchell and Forgan (2003), Mitchell et al. (2017) and Kim et al. (2008) under low aerosol loading conditions. The magnitude of the instrument's discrepancy could be partly due to the inherently different spectral responses and detector fields of view of each instrument under varying aerosol loadings (Kim et al., 2005). The above results indicate that the pointing instruments provide data of comparable quality. On an observation-by-observation basis, the direct-pointing instruments appear to maintain a difference of lower than 0.01 at nearly all wavelengths in clear stable conditions, equal to or lower than the AOD uncertainty. It is estimated that advances in the following aspects may improve (see Sect. 3.3) agreement at the 0.005 level: (i) instrument pointing, (ii) better determination of the effects of Rayleigh scattering, ozone and other absorbers on the calculation of AOD and (iii) better instrument characterization, especially calibration of the radiometers. Significant improvements in AOD precision and instrument accuracy were obtained upon application of cloud screening.
Concerning additional statistics, we have used Taylor diagrams (Taylor, 2001) in order to evaluate the performance of all instruments at the four measuring wavelengths (Fig. 4). Correlation coefficients (CCs) among the triad and all other instruments were better than 0.9 for all instruments and wavelengths, with the exception of three instruments, only at 865 nm. In the case of the Cimel, PFR and POM, CCs were higher than 0.98 in all cases. The normalized standard deviation in Fig. 4 describes the instrument-measured AOD variability compared to that of the reference (triad). Most of these ratios were well within the 0.8 to 1 area, with the exception of a single PFR instrument, which provided data for only one comparison day.
Overall, statistics at 368, 412 and 500 nm showed an excellent agreement for all instruments, while at 865 nm the instrument scatter within the Taylor diagram space is higher. However, the agreement can still be considered quite good, as seen when examining Fig. 4.
Figure 5 presents the percentage of instruments that lie within the WMO AOD uncertainty criterion. The wavelengths with the lower percentage of instruments within the defined criterion are the nominal 368 and 412 nm channels, while the majority of instruments measure within the defined criterion for the nominal 500 and 865 nm channels (see Table 2). When considering 95 % of measurements, the best results correspond to the 500 nm wavelength followed by the 867 nm wavelength. A main finding is that the lower the wavelength, the lower the reliability, accompanied by the lower percentage of participating/supporting instruments. For a lower percentage of measurements (horizontal axis) the 865 nm wavelength reaches 100 % of participating instruments, which decreases to 83 % at 95 % of data within the WMO limits. The shortest studied wavelength (368 nm) showed that 12 out of 17 instruments were within the WMO criterion, while the remaining five had less than 70 % of the comparison data among the WMO limits.
Percentage of instruments that lie within the WMO criterion
(0.005
The difference in the AE between all participant instruments and the triad is shown in Fig. 6. We have used only the 500/865 nm channels to calculate the AEs in order to have the same calculation principles for all instruments.
Difference in the Ångström exponent between each instrument and the WORCC triad. The boxes represent the 10th and 90th percentiles, while the black lines represent the minimum and maximum values of the distribution excluding the outliers. Outliers (gray dots) are considered to be values outside the 10th and 90th percentiles by 4 times the width of the distribution at a 10 % level. Box colors are only used to differentiate between instruments.
Under low aerosol conditions, a small relative bias in the AOD determination
at 500 and 865 nm can theoretically lead to large deviations in the
calculated AEs. As an example, for AODs of about 0.05 and 0.02 at 500 and
865 nm, respectively, AOD differences of 0.01 and 0.005, respectively, can
lead to AE differences up to
The FRC campaign was a unique opportunity to compare the different cloud-screening algorithms used by each instrument/group. McArthur et al. (2003) reported on instrument/network-related cloud-flagging differences using measurements from a 3-month campaign. The use of such algorithms can lead to significant differences, while the selection of threshold values to filter out the retrievals could lead to large deviations when comparing AOD retrievals from instruments with different cloud-flagging algorithms. For our comparison, we have used one of each of the main types of instruments and compared the number of available retrievals (PFR, POM, SPO, MFR and Cimel instruments). More specifically, we have chosen to examine the instruments of each type with the larger dataset on these 5 days.
The cloud detection algorithm used for the above-mentioned instruments can be summarized as follows.
We have used the tool developed by Heberle et al. (2015) to visualize the coincidence of the instrument datasets that provided 1 min AOD (SPO, MFR, PFR and POM) by plotting Venn diagrams (Fig. 7). Cimel instruments were not included due to the lower AOD-measuring temporal resolution. All instruments only detected cloudless conditions during 25 % of the common measurements. The SPO seems to have the most values that do not appear in common with other instruments (4.9 % solo, and 18 % in common with only one other instrument) and the POM the least (0.1 and 0.8 %). When considering measurements defined as cloudless from at least three out of four instruments, the SPO has the largest number of coincident measurements (69.9 %) followed by the PFR (69.2 %), MFR (59.9 %) and POM (36.3 %). The POM has the smallest dataset, only retrieving AOD from 40 % of all possible (at least one instrument provided cloudless AODs) measurements.
Venn diagram of quality-controlled, clear-sky datasets of SPO, MFR,
PFR and POM data for four cloudless (only very limited presence of clouds)
days.
In order to investigate measurements when only one instrument provides cloud-free minute measurements while all other instruments are marked as
cloud-flagged (as an example in Fig. 7, the SPO has 96 cases/minutes out of a
possible 1944 comparison data/minutes), we calculated an artificial AOD time
series. This was constructed by spline-interpolating the mean AOD of all the
remaining (three) instruments (excluding the Cimel that has a lower temporal
measurement frequency than the rest of the instruments), at the time
intervals for which the fourth instrument (SPO in this example) provides
cloud-free data. It was found that the mean AOD at 500 nm (AOD
In Table 3, we have calculated the score for each instrument, dividing the number of available retrievals by a total of 1944 possible (at least one of the instruments has provided an AOD cloud-free minute value) comparison cases. For Cimel values, for which the measurements are not every minute, we used raw data to count all the recordings and divide the number of cloud-screened data; therefore it is not directly comparable with other instruments. The POM instruments obtained the lowest score in the cloud-screening application, mainly caused by the stringent isolation check added to the adapted Smirnov et al. (2004) algorithm.
Percentage of available cloud-screened AOD data values out of all possible measurements (minutes).
One-minute AOD data on 1 October 2015. Different colors represent the AODs, submitted as cloud-free data. The black line is the mean AOD from the PFR, MFR and SPO for data points when all three instruments provided data. The gray vertical lines represent periods where the PFR, MFR and SPO provided data but the POM characterized them as “cloudy”.
Figure 8 shows AOD measurements at 500 nm for all instruments that were tested for their cloud-flagging algorithms during one single day. As seen in Table 3, the POM instrument seems to cloud-flag various minutes/measurements, while all other instruments/algorithms do not. Such instances are shown in Fig. 8 as gray areas and represent periods when all PFR, SPO and MFR instruments provide AOD (thus they do not “detect” any clouds) while the POM does not provide an AOD. Despite the small instrument-to-instrument differences, the evolution of the AOD during particular periods (gray areas), also described by the mean or artificial AOD, cannot be considered as periods that are affected by clouds. Thus, the POM algorithm is probably too strict compared to the others. In addition, sporadic SPO-related high AOD values after 14:00 UT (at times when no other instrument provides cloudless data) show that during these conditions, the SPO cloud-flagging algorithm was more imprecise.
Slant column optical thickness (right axis – thick lines) and
optical depth differences compared with the triad (left axis dashed lines),
at 500
For the present intercomparison, no common procedures were used for the
removal of gas-phase constituents or Rayleigh scattering; cloud screening,
solar position, timing and calibration methodology were at the discretion
of the network operators. Datasets from each sun photometer network were
corrected for these factors independently. Figure 9 identifies some of the
possible discrepancies that may result when considering NO
One reference day (30 September 2015) was chosen for this comparison
exercise. The slant optical thicknesses of various trace gases and Rayleigh
scattering were obtained from Cimel, PFR, POM and SPO instruments and
individually compared. Furthermore, the respective algorithms for the
calculation of the solar zenith angle and air mass at any given time (as
provided by the responsible scientists of each instrument) were employed.
NO
The analyzed factors result in discrepancies of comparable magnitude at a
wavelength of 500 nm but also illustrate a slightly larger effect due to
differences in the corrections for Rayleigh scattering and water vapor. At
870 nm, the larger discrepancies can be ascribed to different
parametrizations of ozone absorption and Rayleigh scattering. For the case of
the MFR instrument, the effective wavelength of the
“500 nm” filter is about 495.8 nm, which explains the higher Rayleigh optical thickness and the
lower ozone absorption-related value. The deviations between algorithms can
be of either sign and can partially compensate each other in AOD
calculations. Finally, NO
Results from the FRC-IV intercomparison have been presented in this study. Based on the number of instruments and also the participation of reference sun photometers/instruments from various global AOD networks, the campaign could be considered as a successful experiment in assessing the current status of AOD measurement accuracy and precision. The WMO recommendations for AOD comparisons have been adopted for the present campaign and the WORCC PFR triad has been used as a reference.
The absolute differences of all instruments compared to the reference triad
have been reported and are based on the WMO criterion defined as follows: “95 %
of the measured data has to be within 0.005
The statistics from the Taylor diagram analysis revealed the overall accuracy
and homogeneity of the instruments. In particular, the majority of
instruments gave CCs
Ångström exponent calculations using a pair (500 nm and 865 nm) of
wavelengths showed relatively large differences among different instruments.
This was largely related to the uncertainty of this parameter that is linked
with very low AOD uncertainties, under low AOD conditions. AOD differences of
about 0.01 at 500 nm that can be easily related to the instrument
calibration uncertainties can considerably affect such calculations during
low AOD conditions. Hence, this campaign reaffirms that for cases of mean
AOD
Investigating the sources of differences among different instruments, we
compared all parameters included in the AOD retrieval algorithm as provided
by the different participating institutes. All individual differences
(Rayleigh, NO
Different cloud-flagging algorithms can affect the AOD datasets as different instruments/networks use different techniques. During a day with sporadic appearances of high and mid-level clouds (which was deliberately chosen as a “difficult” task for such algorithms), results from different cloud-flagging algorithms limited the AOD comparison datasets between two instruments from 40 to 90 %, depending on the pair of instruments used, compared to the maximum number of cloudless data points calculated by all instruments. In general, using long-term series for determining aerosol climatology at certain locations, cloud screening that is too conservative could lead to the elimination of high AOD local events, while too much conservative cloud screening will introduce biases linked mainly with cirrus clouds. Both approaches will have an impact on aerosol climatology and calculated AOD trends.
In comparison to earlier FRCs (I to III), the latest FRC reported here experienced an increase in both the number of instruments (total of 30) and international participating institutes (12 countries). In addition, analysis at four different wavelengths was performed for the first time. The Cimel/AERONET, PFR/GAW and POM/SKYNET and SPO participating sun photometers showed very good agreement when compared to older intercomparisons. As AOD from algorithm differences was quite small, the results of the comparisons of this instrument group are considered to have been very successful as differences are in most cases well within the calibration and overall instrument AOD uncertainties. The rest of the instruments also showed reasonable agreement with few exceptions. MFR instruments experienced additional uncertainties concerning the diffuser-based measurements. SIM instruments also performed quite well when considering the radiative-transfer-based processing algorithm. In addition, spectral-AOD-retrieving PSR instruments also performed well, especially at the two higher wavelengths. Finally, Microtops AOD data were in most cases within reasonable agreement with the reference triad but additional technical issues such as the handheld-based sun-pointing and the smaller integration time (compared with other instruments) of the direct sun measurement led to enhanced scatter of the results.
Instrument technical features such as differences in the field of view did not play an important role in FRC-IV for the low aerosol load conditions that were encountered. In order to quantify such features and similar issues, intercomparison campaigns have to be organized in moderate to high AOD conditions when forward scattered radiation and circumsolar radiation can play an important role in instruments with different field-of-view entrance optics.
The results of the FRC-IV, which included a large variety of AOD measuring instrumentation via the participation of reference instruments from AERONET Europe, SKYNET, GAW-PFR, SURFRAD and the Australian Aerosol Network, could be considered as a starting point for global AOD harmonization of procedures, recommendations for cloud screening, trace gas corrections and calibration procedures. The ultimate objective is a unified AOD product to be used for long-term aerosol and radiative forcing studies, case studies involving accurate AOD retrievals and satellite-validation-related activities.
Reference triad aerosol optical depth measurements can be provided from PMOD/WRC upon request. Additional instrument data can be requested from the individual instrument principle operators.
The authors declare that they have no conflict of interest.
FRC-IV was organized in the frame of the World Radiation Center–WORCC mandate for the homogenization and harmonization of AOD measurements as defined by WMO-GAW. Authors would like to thank Christian Thomann for his essential and continuous technical support during the FRC-IV campaign. Edited by: Evangelos Gerasopoulos Reviewed by: three anonymous referees