Introduction
Concerning atmospheric physics and chemistry, it is well known that aerosols
play an important role in processes such as the interaction with the solar
radiation and the formation of clouds, which are key to our understanding of
the radiative balance of the Earth–atmosphere system. As pointed out in the
IPCC Fifth Assessment Report , the high spatial and
temporal variability of aerosols, and the different absorbing properties
depending on their type, introduce large uncertainties to radiative
forcing estimations. This makes networks capable of measuring aerosol
properties, over a wide spatial range, in near-real time, of special
importance for the study of climate change. Of course, other research topics,
from satellite validation to the assessment of aerosol-related health issues,
also benefit from the availability of these data sets.
Previous works have already demonstrated the feasibility of using Brewer
spectrophotometers, usually devoted to the measurement of the total ozone
column (TOC) and UV irradiance, to determine the aerosol optical depth (AOD);
see, e.g., , ,
, , ,
, , ,
, , ,
, , ,
, , ,
, , ,
, ,
, and . Although Brewer spectrophotometers can be used to
retrieve AOD at longer wavelengths, in their standard operational mode most
instruments can only produce data in the 300–320 nm range. This is
nevertheless an important wavelength range to study, because the optical
properties of aerosols in the UVB are rather different from those in the
visible and are as of yet not well known seeand references
therein. It is also worth noting that the shortest
wavelength provided by the AOD product of the Aerosol Robotic Network
(AERONET; https://aeronet.gsfc.nasa.gov/), one of the most used sources
for ground-based aerosol data, is 340 nm, which makes Brewer AOD data
in the 300–320 nm range a useful complement.
At the Regional Brewer Calibration Center for Europe (RBCC-E, Izaña
Atmospheric Research Center, Agencia Estatal de Meteorología, Spain;
http://rbcce.aemet.es/), and as part of the activities carried out at
the WMO-CIMO Testbed for Aerosols and Water Vapor Remote Sensing Instruments
(Izaña, Spain), we have implemented an AOD algorithm for the instruments
integrated in EUBREWNET (COST Action ES1207, “a European Brewer Network”;
; http://www.eubrewnet.org/cost1207),
which is comprised of close to 50 Brewer spectrophotometers. Most of these
Brewer instruments operate in Europe and adjacent areas, although some
located farther away, for example in South America and Australia, have also
joined the network. One feature of the AOD algorithm implemented at the
RBCC-E is that all the necessary data for the AOD determination in the 300 to
320 nm wavelength range can be obtained from the standard ozone,
direct sun measurements available in near-real time at EUBREWNET's data server
(http://rbcce.aemet.es/eubrewnet). This data server allows for the
harmonization of network data, providing four ozone product levels (three in
near-real time) with an increasing number of corrections to improve data
quality . It should be noted that EUBREWNET's
data server is currently maintained by the RBCC-E, which itself has operated
without interruption since 2003 under the auspices of the WMO/GAW and the Spanish
Agencia Estatal de Meteorología (http://www.aemet.es/).
Also needed for the determination of the AOD is the data provided by the
calibration of the Brewer instruments. To carry out this task, the RBCC-E
maintains a reference triad of Brewer spectrophotometers at the Izaña
Atmospheric Observatory (IZO, Agencial Estatal de Meteorología, Spain;
http://izana.aemet.es/), located at 2370 ma.s.l. in the
island of Tenerife. Most of the year, the meteorological conditions at IZO
are excellent for the absolute calibration of the Brewer instruments via the
well-known Langley calibration method . The multiple research programs carried out at IZO
provide additional information that helps
to carry out calibrations, such as forecasts of adverse weather
conditions. This absolute calibration is transferred to participating
instruments at international intercomparison campaigns, held in alternate
years at El Arenosillo Atmospheric Observatory (Instituto Nacional de
Técnica Aeroespacial, Huelva, Spain) and the Arosa Lichtklimatisches
Observatorium (MeteoSwiss, Switzerland). For an overview of the last three
campaigns, see , ,
and
.
It should be noted that the RBCC-E provides calibration data for
approximately half the Brewer spectrophotometers integrated in EUBREWNET, and
this paper is focused on these instruments. However, the present
implementation of the AOD algorithm is intended to run directly on
EUBREWNET's data server using any measurements and calibration data available.
This would allow one to extend the applicability of the present implementation of
the AOD algorithm, with minor modifications as needed, to the whole EUBREWNET
network, because any other calibration data can be used in addition to that
supplied by the RBCC-E. This includes calibrations transferred from other
Brewer reference spectrophotometers, such as the one operated by
International Ozone Services (Toronto, Canada; http://www.io3.ca/).
Furthermore, preliminary work on the feasibility of using an Ultraviolet
Precision Filter Radiometer (UVPFR) from the Physikalisch-Meteorologisches
Observatorium Davos and World Radiation Center (Davos, Switzerland;
https://www.pmodwrc.ch/) to calibrate Brewer instruments has also been
carried out .
The present work is organized as follows. The AOD algorithm implemented at
the RBCC-E is described in Sect. . In Sect.
we present results of the calibration of selected Brewers carried out in
2013, and estimate the precision of these instruments for the AOD
determination. Next, we check the stability of the AOD from these Brewer
instruments for the approximately 2-year period between the eighth and
tenth intercomparison campaigns of the RBCC-E, both held at El Arenosillo.
For this, we compare the Brewer AOD with data of collocated Cimel
sun photometers as provided by AERONET. To close Sect. , we
compare the Brewer AOD with the data produced by an UVPFR and derive the
Brewer AOD uncertainty using data acquired during the tenth intercomparison campaign of the RBCC-E. In Sect. we discuss future
improvements of our AOD algorithm, and in Sect. we provide
some closing remarks.
Methodology
We begin this section by providing a short overview of the Brewer
spectrophotometer. Next, we describe the Brewer AOD equation used in the AOD
algorithm implemented at the RBCC-E, placing special emphasis on the origin
of each term. This is followed first by a description of the calibration
procedure, and then by an analytic derivation of the AOD uncertainty within
some simplifications. Finally, we briefly describe the Cimel and UVPFR
instruments.
The Brewer spectrophotometer
The Brewer spectrophotometer was developed in Canada during the 1970s, and
a commercial, automated version became available in the early 1980s.
Currently, it is one of the primary ground-based instruments used to report
TOC data, together with the Dobson spectrometer. The Brewer spectrophotometer
performs measurements of the direct spectral UV irradiances which, through
a well-defined process, are used to calculate the TOC value. In the rest of
this section we highlight the most relevant details for the present work of
the instrument and the measurement process; see for
further information.
The Brewer spectrophotometer measures the direct spectral irradiance in six
channels in the UV (303.2, 306.3, 310.1, 313.5, 316.8, and
320.14 nm), each with approximately a 0.5 nm bandwidth
(resolving power λ/Δλ≈ 600), although that of the
shortest wavelength varies with the Brewer model. The spectral analysis is
achieved by a holographic grating in combination with a slit mask which
selects the channel to be analyzed by a photomultiplier. There are three
types of Brewer instruments currently in use in the EUBREWNET network: the
Mk II and Mk IV models are single monochromators, and the Mk III model is
a double monochromator, a characteristic that reduces stray light in its
measurements .
During direct sun measurements, sunlight enters the instrument through an
inclined quartz window. A right-angle prism directs the incoming light from
the Sun to the optical axis of the instrument. The light subsequently passes
through the fore-optics, which consist of a set of lenses to adequately
focus the beam, an iris diaphragm, and two filter wheels. A ground quartz
diffuser is located on the first filter wheel. The second filter wheel
consists of a set of five neutral density filter attenuators and guarantees
that the detector is working in its linear regime. After passing through the
filter wheels, radiation is then focused onto the entrance slit of the
monochromator.
The Brewer retrieval of the TOC requires instrument characteristics which in
some cases can only be determined by calibration experiments performed at
intercomparison campaigns see, e.g., the GAW reports of the seventh,
eighth, and ninth intercomparison campaigns of the
RBCC-E;.
The instrumental calibration includes all the parameters that affect the
counts measured by the spectrometer, in particular the dead time correction,
temperature coefficients, and filter attenuations. The wavelength calibration
determines the ozone and Rayleigh absorption coefficient. The exact
wavelengths measured by each Brewer spectrophotometer are slightly different
from instrument to instrument. The so-called “dispersion test” is thus used
to determine the exact wavelengths of each instrument and its slit, or
instrumental, functions. An extraterrestrial (calibration) constant is
determined by the Langley method or by comparison with a reference
instrument. The TOC is then finally determined using ratios of measurements
at four wavelengths. In contrast, the individual (absolute) measurements are
used for the determination of the AOD together with calibration parameters
specific to each wavelength, as discussed next.
AOD equation for Brewer spectrophotometers
The attenuation of the direct solar irradiance as it travels through the
Earth's atmosphere is described by the well-known Beer–Lambert–Bouguer
equation see, e.g.,:
I(λ)=I0(λ)e-τ(λ)m,
where I(λ) is the direct solar irradiance of wavelength λ
measured at the ground, I0(λ) is the extraterrestrial (outside the
atmosphere) solar irradiance, τ(λ) is the so-called optical
depth, and m is the optical air mass. Note that, instead of absolute
irradiances, proportional magnitudes can be used, like for example measured
photon rates. The two parameters τ(λ) and m describe
the attenuation of the solar radiation by the different components of the
atmosphere. In the UV range and for cloudless conditions, the main
contributions are produced by the ozone, nitrogen and sulfur dioxides,
Rayleigh molecular scattering, and aerosols. Following previous
authors e.g., we currently do not
consider the contribution of the nitrogen and sulfide dioxides to the optical
depth, which should be rather small in the UV range except at polluted
sites . Under these assumptions, the optical depth
in the UV range can thus be written as
τ(λ)m=τo(λ)mo+τR(λ)mR+τa(λ)ma,
where the subscripts refer to the contributions by ozone (o), Rayleigh (R),
and aerosols (a).
Solving for the aerosol optical depth τa(λ),
Eq. () then becomes
τa(λ)=1malogeI0(λ)-logeI(λ)-τo(λ)mo-τR(λ)mR.
It should be stressed that Eq. () is wavelength dependent and
valid for each wavelength λ measured by the Brewer spectrophotometer
in the UV range. In this work we will consider only the five wavelengths
between 306.3 and 320.1 nm which are measured by all Brewer models.
The wavelength at 303.2 nm has a variable bandwidth which depends on
the Brewer model, and other wavelengths above 320.1 nm are only
routinely measured by Mk IV and V models.
In terms of variables either measured by the Brewer spectrophotometer or
determined by the calibration carried out by the RBCC-E, Eq. ()
can be rewritten as (see Appendix for the corresponding
expression written in the scaled logarithmic space used internally by the
standard Brewer software)
τa(λ)=1malogeI0(λ)-logeI(λ)-Xoko(λ)mo-p1013τR0(λ)mR,
where the variables are as follows:
I0(λ): extraterrestrial counts per second for each wavelength, determined by any of the two calibration
methods described in Sect. .
I(λ): counts per second measured by the Brewer instrument at each wavelength. In addition to the usual
corrections applied to the raw counts in the standard ozone data reduction , we also
apply those described below. This requires the determination of some parameters which are specific to each Brewer
instrument, a process which is carried out during the instrumental calibration performed by the
RBCC-E.
Xo: measured TOC in atm-cm. We currently use the real-time ozone level 1.5 product available at
EUBREWNET's data server. However, instead of the Rayleigh coefficients supplied by default for all Brewer
spectrophotometers, we use specific coefficients for each instrument determined during the RBCC-E calibration. These
coefficients are calculated following the formula of , and this modification in the
Rayleigh contribution lowers the ozone value by approx. 0.003 cm, in agreement with the value reported by
.
ko(λ): ozone absorption coefficients derived from the Bass and Paur cross sections for each
wavelength in cm-1. These coefficients are also determined during the standard ozone calibration performed by
the RBCC-E for each Brewer spectrophotometer seefor further
details.
mo: ozone optical air mass, calculated asmo=1/cosarcsin[ksin(SZA)],where k=6370/(6370+h), h=22 km, and SZA is the solar zenith angle in
degrees.
p: climatological pressure at the observation site, in millibars.
τR0(λ): Rayleigh optical depth at sea level following the formula of
, for each wavelength determined during the RBCC-E calibration
process.
mR: Rayleigh optical air mass, calculated with the same expression as the ozone optical mass but for an
altitude h = 5 km.
ma: aerosol optical air mass, which we approximate with mR. Note that we only consider
measurements up to a maximum optical air mass value of 3.5, so the exact altitude of the aerosol layer has a small effect
on the optical air mass. Thus, for example, in the case of aerosols at sea level, the approximation ma≈mR introduces at most a ∼ 1 % error in the aerosol optical air mass.
As mentioned above, starting from the raw counts measured by the Brewer
instrument, the counts per second used in the AOD calculation are determined
taking into account the effects produced by the dark counts, dead time, and
temperature in the same way as in the ozone data
processing . Further AOD-specific
corrections include the following:
Filter correction, to remove the effect of the different attenuation of each filter used by the Brewer instrument to
avoid the saturation of the photomultiplier. This correction is also applied in the ozone data reduction, but here we
include the wavelength dependence of the attenuation coefficients, as determined during the calibration process.
Note,
however, that we perform our Langley calibration with all the filters separately (see Sect. ), so
that any remaining effect of the different attenuations is taken into account in the calibration
constants.
Internal polarization correction, to correct for the loss of sensitivity of the Brewer due to the polarization
effects produced by its window and grating, mostly noticeable when operating at high solar zenith angles. We use the
correction from the field experiment performed by
.
Correction for the seasonal variation of the Earth–Sun distance, using the eccentricity correction factor of the
Earth's orbit from , as quoted by :E0=1.000110+0.034221cos(Γ)+0.001280sin(Γ)+0.000719cos(2Γ)+0.000077sin(2Γ),where Γ=2π(day number-1)/365.
To these corrected counts per second we also apply the data-quality criteria
defined within EUBREWNET's level 1.5 ozone product (see
http://rbcce.aemet.es/dokuwiki/doku.php for further details):
SD (or cloud) filter, used to remove groups of five measurements with large variability (SD above 2.5 DU)
and thus likely affected by fast-moving clouds.
Optical air mass filter, used to remove measurements taken under conditions of high ozone optical air mass (above
3.5), unreliable due to the fast rising and setting of the Sun in low and mid-latitudes, and affected by stray-light
errors in Mk II and IV instruments .
Mercury lamp test filter, to remove measurements likely affected by a wavelength shift usually produced by
temperature changes in the grating of the Brewer spectrophotometer.
Furthermore, following , we also remove groups of
five AOD measurements for which their SD is greater than 0.02. Together with
the criterion on the SD of the ozone data described above, this ensures that
measurements affected by clouds are removed.
AOD calibration of Brewer instruments
In this section we provide details of two AOD calibration methods for Brewer
spectrophotometers. The Langley plot method is used to calibrate the RBCC-E
reference Brewer spectrophotometer operating at IZO. The calibration transfer
method is then used to calibrate other Brewer instruments operating
simultaneously with the RBCC-E reference Brewer during the intercomparison
campaigns.
Under the stable atmospheric conditions in which Brewer calibrations have to
take place, the total optical depth τ can be considered constant.
Equation () can then be rewritten as a linear equation with the
total optical air mass m as the independent variable and logeI0 as
the intercept:
logeI=-τm+logeI0.
Following the Langley plot method, the determination of the calibration
constant I0 then just requires fitting a linear equation to the data of
a logeI vs. m plot. Note that this equation is valid for each
wavelength and filter position, so that multiple Langley fits are thus
necessary to determine all the calibration constants. We show an example in
Sect. .
In practice, we follow , and apply the
Langley plot method using
logeI+p1013τR0mR=-τ′mo+logeI0,
where the Rayleigh term is considered explicitly, so that the τ′
optical depth now contains the contributions from the ozone and aerosols, as
it also happens with the optical air mass. However, during a large part of
the year the atmospheric conditions at IZO can be considered ideal for the
Langley calibration method, in particular usually featuring a low aerosol
load except in summer months see,
e.g.,. In these conditions,
the largest contribution to τ′ in the UV range is produced by
ozone, and mo can indeed be considered a good approximation for
the optical air mass on the right-hand side of Eq. (). A more
elaborate term for the optical air mass could be used instead, such as the
average weighted by the optical depths proposed
in . However, we have found that in the atmospheric
conditions of IZO and within the optical air mass limits described below,
switching from mo to ma produces differences of
∼ 0.01 % in the calibration constants obtained from the Langley plot,
so we do not expected any combination of the two air masses to introduce any
significant changes.
Following the usual ozone calibration procedure for Brewer
spectrophotometers see, e.g.,, we make separate
Langley plots for each half-day if there are at least 20 observations taken
with the same filter, and consider optical air masses between 1.1 and 3.5.
Finally, we average the calibration constants obtained over a period of
1–2 months, discarding those corresponding to linear regressions with r2
coefficients of determination below 0.995, and/or above/below 1.20 times the
median of the whole ensemble of calibration constants for the whole period.
Performing this Langley calibration procedure at IZO, we can only obtain
calibration constants for just two filter wheel positions (nos. 2 and 3),
leaving another four positions without characterization due to a lack of
measurements at this filter wheel positions. This includes filter wheel
position nos. 0 and 1, which are frequently used at high-latitude sites
because they correspond to lower attenuations (actually, in position no. 0 no
filter is used). To get a more complete calibration for our reference
instrument, another Langley calibration is performed, but with less-demanding
limits – an extended optical air mass range from 1.1 to 5.5, and a more
tolerant value of 0.9 for the r2 coefficient of the linear regression. We have
found that this less-demanding calibration produces results for filter nos. 0
to 3, but at the price of a higher uncertainty. In order to retain the
lower uncertainty of the more-demanding calibration, from the results of the
less-demanding Langley we only use the differences between calibration
constants of different filters. When added to the results for filter
position nos. 2 and 3 of the more-demanding calibration, these differences
allow us to determine calibration constants for filter position nos. 0 and 1.
This is thus the calibration of the reference Brewer spectrophotometer that
we transfer to other Brewer instruments during intercomparison campaigns.
If the Brewer spectrophotometer to be calibrated is operating at the same
place and simultaneously with a reference instrument already producing
reliable AOD values, Eq. () can be solved for the calibration
constant:
logeI0=τarefmR+logeI+Xokomo+p1013τR0mR.
Here, the τaref AOD value is provided by the
reference instrument, and the remaining data are measured by the Brewer being
calibrated. Note that the counts per second I measured by the Brewer being
calibrated include all the corrections described in
Sect. . Equation () is valid for each
simultaneous measurement, with a specific wavelength and filter position. The
complete set of calibration constants I0 can thus be determined solving
this equation for multiple measurements taken under different conditions.
The last days of the intercomparison campaigns of the RBCC-E, after the
Brewer instruments have received maintenance and their ozone calibrations
have been updated or confirmed, provide the necessary timespan to carry out
this calibration transfer procedure. Measurements within 1 min of the
reference instrument are considered simultaneous, and the average of multiple
calibration constants for each wavelength and filter position provides the
final AOD calibration constants.
In Sect. we will show results for selected Brewer
spectrophotometers which took part in both the eighth and tenth
intercomparison campaigns of the RBCC-E, held in the years 2013 and 2015,
respectively, at El Arenosillo Atmospheric Observatory. Brewer #185, the
traveling standard of the IZO triad, was present at both campaigns, and has
been used as a reference to calibrate other participating instruments using the
calibration transfer method just described. The traveling standard of the
RBCC-E itself was calibrated using the Langley plot method, following the
procedure described at the beginning of this section.
Brewer AOD uncertainty
A full analytic derivation of the uncertainty is outside the scope of this
paper. However, we will consider here a simplified model, taking into account
only the three largest contributions found by to
the total uncertainty in the UV range for the UVPFR instrument, whose AOD
algorithm shares similarities with that of the Brewer. We also assume no
correlation between variables, and work within the approximation
ma≈mo≈mR. This latter
approximation is reasonable within the maximum optical air mass value of 3.5
used in the present work, in which case the differences between the various
optical air mass terms is ∼1 % at most. A more careful examination of
the optical air mass is required in other cases;
see .
Taking into account all the above considerations, we write the AOD
uncertainty as
u(τa)=u2(τo)+u2(I0)I02+u2(p)τR0210132,
where each uncertainty u on the right-hand side includes, if necessary,
a factor of 2 to translate from 1σ to 2σ level . These uncertainties arise
from the following:
The ozone optical depth, which has been found by to be the largest contribution in the UV
range for the UVPFR instrument. Ignoring the correlation between variables, the uncertainty of the ozone optical depth can
be approximated by u2(τo)=u2(Xo)ko2+Xo2u2(ko).
Calibration, which is the second-largest contribution according to , and which contributes
u2(I0)/I02 to the total uncertainty.
Pressure, which we keep fixed at a climatological value for each station, thus introducing a term
u2(p)τR02/10132.
For the estimation of the AOD uncertainty, we can assume an average ozone of
340 Dobson units with 1 % uncertainty, values which correspond to Brewer
#185 during the tenth intercomparison campaign of the RBCC-E at El
Arenosillo. Ozone absorption coefficients for the wavelengths between 310 and
320 nm of Brewer #185 range from 2.31 to 0.67 cm-1, with
a 2.1 % uncertainty according to . This
results in a 2σ uncertainty of the ozone optical depth between 0.04
and 0.01. The uncertainty associated with the calibration can be approximated
by the relative SD of the series of calibration constants calculated in the
Langley calibration. In our case, this value is 1 % at the most for all
wavelengths and filters. For the pressure term, the Rayleigh coefficients of
Brewer #185 at sea level are ∼ 1 at all wavelengths, and we will
consider a 1σ uncertainty of 5 hPa.
Within all the approximations considered in this simplified model, the
standard AOD uncertainty at the 95 % level is then 0.04–0.02 in the
310–320 nm wavelength range. An analogue calculation produces 0.06
for the standard uncertainty at 306.3 nm. As we will see in
Sect. , there is fair agreement between these values and
those determined in the Brewer–UVPFR comparison. Regarding previous works,
reported a 2σ uncertainty of
∼ 0.1 for the Brewer AOD in the UVA range, and
recently calculated an uncertainty better than
0.02 for the UVPFR instrument operating close to 320 nm.
Cimel and UVPFR instruments
During the period considered in this work, the Cimel sun photometer model
CE318-N was the standard instrument of AERONET. The sun photometer performs
automatic direct-sun measurements every 15 min at 340, 380, 440, 500, 675,
870, 940, 1020, and 1640 nm nominal wavelengths with a 1.2∘
field of view. The value of the full width at half maximum (FWHM) is
2 nm. Solar extinction measurements are used to derive spectral AOD
and the corresponding Ångström exponent .
The estimated AOD uncertainty is approximately 0.01, increasing up to 0.02 in
the UV wavelengths . Data
acquisition protocols, calibration procedures, and data processing methods
have been extensively described; see, e.g., ,
, and . We use the highest-quality
data set currently available from Cimel sun photometers, the cloud-screened and
quality-assured version 2 level 2.0 product downloaded from the AERONET site
(http://aeronet.gsfc.nasa.gov). We use the shortest wavelength
provided, which is 340 nm, and the 340–440 Ångström exponent
to extrapolate to 320 nm, which is the longest wavelength measured by
the Brewer in its most usual ozone operational mode.
The UVPFR sun photometer is a special version of the Precision Filter
Radiometer (PFR) designed and built at the Physikalisch-Meteorologisches
Observatorium Davos and World Radiation Center (PMOD/WRC) in Davos,
Switzerland. It measures the direct solar irradiance at the four nominal
wavelengths of 305, 311, 318, and 332 nm. The filters and detectors are
operated at a constant temperature of 20 ∘C and are exposed
to solar radiation only during actual measurements. In order to perform
direct sun measurements, the UVPFR is mounted on a solar tracker so that it is
continuously pointing to the Sun. Direct sun measurements are taken each full
minute and the stored signal values are averages of 10 samples for each
channel made over a total duration of 1.25 s. The width of the spectral
response functions is in the order of 1.0–1.3 nm at FWHM. Both
Langley calibrations and AOD retrievals are affected by the finite FWHMs.
Corrections which were used to reduce this influence, together with more
detailed information about the UVPFR, are described by
. Where necessary for our comparisons, the UVPFR
data at the closest wavelengths to those of the Brewer have been interpolated
using the Ångström relationship.
Results
In this section we use data from the eighth and tenth intercomparison campaigns of the RBCC-E, and the period in between, to analyze
the precision of the Brewer AOD data, by checking Brewer–Brewer comparisons (Sect. );
the stability of the Brewer as an AOD-measuring instrument over a 2-year period, by comparing Brewer and
Cimel data (Sect. );
the uncertainty of the Brewer AOD data, by comparing with Cimel and UVPFR instruments (Sect.
and ).
Langley plots for the five Brewer wavelengths between 306.3 and
320.1 nm, for measurements taken with filter nos. 2 (green) and 3 (red)
of Brewer #185 operating at IZO in the morning of 31 May 2013.
Precision and Brewer–Brewer comparison
In this section we discuss the calibration of different Brewer
spectrophotometers in the year 2013, starting with Brewers #183 and #185,
both belonging to the RBCC-E triad based at IZO. These instruments were
independently calibrated at IZO via the Langley procedure described in
Sect. , using data from 1 April to 3 June for Brewer
#183, and from 7 May to 3 June in the case of Brewer #185. Both instruments
were shipped to the eighth intercomparison campaign of the RBCC-E, held in El
Arenosillo (Huelva) in June 2013, marking the end of these date ranges.
Regarding the starting dates, atmospheric conditions at IZO were not
appropriate for the Langley calibration before the beginning of April, and
furthermore Brewer #185 experienced instrumental issues in this month,
leaving us with roughly 2 months of data for the Langley calibration of
Brewer #183 and 1 month for Brewer #185. As an example,
Fig. shows the Langley plots for the five wavelengths
measured by Brewer #185 on one morning. Because we have considered data for
each filter separately, we obtain calibration constants for each wavelength
and filter. The difference between the results for different filters is
∼ 1 % at most, showing that the filter correction applied to the data
(see Sect. ) removes most of the effect produced by the
different attenuation of the different filters.
Summary of the AOD comparison between Brewer #185 and selected
instruments – Brewers #183, #070, #075, #186, #201, and #202, the
first also calibrated by the Langley plot method, and the last five by
transfer from Brewer #185. We show the ID and model of each instrument, the
total number of simultaneous observations within 1 min with Brewer #185,
and, for each nominal Brewer wavelength, the Pearson's correlation
coefficient between the two AOD data sets and the median and SD of their
differences.
Brewer
Correlation, median of differences, SD (1σ) of differences
(Mk)
obs.
306.3 nm
310.1 nm
313.5 nm
316.8 nm
320.1 nm
Calibrated by the Langley method
183 (III)
4695
0.934, 0.0002, 0.0105
0.967, 0.0031, 0.0073
0.975, 0.0033, 0.0064
0.981, 0.0047, 0.0055
0.985, 0.0039, 0.0050
Calibrated by transfer from Brewer #185
070 (IV)
438
0.783, -0.0003, 0.0365
0.948, 0.0000, 0.0145
0.955, 0.0000, 0.0136
0.955, -0.0001, 0.0133
0.955, -0.0001, 0.0133
075 (IV)
303
0.863, -0.0007, 0.0288
0.972, -0.0002, 0.0133
0.972, 0.0000, 0.0134
0.978, -0.0001, 0.0115
0.976, 0.0000, 0.0119
186 (III)
509
0.931, 0.0001, 0.0127
0.960, 0.0001, 0.0092
0.967, 0.0001, 0.0083
0.971, 0.0001, 0.0078
0.973, 0.0000, 0.0075
201 (III)
407
0.907, 0.0001, 0.0106
0.946, 0.0003, 0.0074
0.949, 0.0002, 0.0069
0.956, 0.0001, 0.0064
0.955, 0.0001, 0.0063
202 (III)
464
0.983, 0.0002, 0.0090
0.992, 0.0000, 0.0062
0.993, 0.0001, 0.0057
0.994, 0.0001, 0.0054
0.994, 0.0000, 0.0053
Median
438
0.907, 0.0001, 0.0127
0.960, 0.0000, 0.0092
0.967, 0.0001, 0.0083
0.971, 0.0001, 0.0078
0.973, 0.0000, 0.0075
For the four longest wavelengths, the comparison between the
independently calibrated Brewers #183 and #185 in
Table shows correlation coefficients higher than 0.97,
and biases (provided by the median of the Brewer–Brewer AOD differences) and
SDs lower than 0.005 and 0.007, respectively. For the lowest wavelength at
306.3 nm, the results are slightly worse, with a correlation of 0.94
and a SD of 0.01. The deterioration of the results at the 306.3 nm
wavelength can be explained by the reduction of the signal-to-noise ratio as
the wavelength becomes shorter, a trend we observe in all the
results presented in this work. Still the biases are rather small at all
wavelengths, and both Brewers are of the same model and operate under the
same conditions, so the SDs can be considered to be the precision (or
instrumental repeatability) at the 1σ level, which ranges from 0.01 at
306.3 nm to 0.005 at 320 nm.
recently reported a precision of 0.01 (1σ) for both UVPFR and Brewer
instruments while measuring AOD in the UV. For Cimel instruments measuring
total optical depth, provided a 1σ
precision of better than 0.0025. Note that this latter result corresponds to
the visible range, and the value corresponding to the UV range will likely be
larger, as is also the case with the uncertainty of the Cimel instruments,
which increases from 0.01 in the visible to 0.02 in the UV
range .
The WMO traceability criteria can also be used to
check the quality of the AOD measured by the Brewer instruments. For finite
field-of-view instruments, this criteria requires at least 95 % of the
differences between the measurements of two instruments to be within the
limits
±(0.005+0.010/ma).
AOD differences between observations within 1 min of the
independently calibrated Brewers #183 and #185, plotted as a function of
the aerosol optical air mass. The WMO traceability limits for finite field-of-view instruments (Eq. ) are shown as thick black lines.
Figure shows the differences in AOD for Brewers #183
and #185 as a function of the aerosol optical air mass (which we consider to
be equal to the Rayleigh optical air mass; see Sect. )
together with the WMO traceability limits. The percentages of differences
within the limits range from 73 % at the shortest wavelength to 93 % at
the longest. It has to be noted that the WMO traceability criteria requiring
95 % of the compared data within the limits of Eq. () was
originally defined for selected wavelengths where the absorption of trace
gases is minimal. In this case, UVB AOD differences of 73 to 93 %
fulfilling this criterion can be considered quite good. These values are also
in the same range as those reported in previous studies involving Brewer
instruments. found percentages between 46 and
88 % when comparing independently calibrated Brewer and Cimel instruments
at wavelengths between 306 and 320 nm (extrapolated from
340 nm in the case of the Cimel).
calibrated a Mk IV Brewer with respect to a Cimel instrument at
437 nm, finding that 90 % of the observations were within the WMO
traceability limit once a temperature correction for the Brewer was included.
reported percentages between 85.6 and 97 %
between UVPFR and Brewer photometers operating in the 306–320 nm
range, the Brewer being calibrated using the UVPFR's AOD as reference.
Using the data of the eighth intercomparison campaign of the RBCC-E, held at
El Arenosillo (Huelva, Spain) in June 2013, we have been able to transfer the
Langley calibration of Brewer #185 to five other instruments, namely Brewers
#070 and #186 from Madrid (Spain), #075 from Reading (UK), and #201 from
Tamanrasset (Algeria). Furthermore, we also transferred the same calibration
to Brewer #202 from Kangerlussuaq (Greenland), which was present at IZO in
November 2013. According to the results in Table ,
Brewers #186, #201, and #202 show results similar to Brewer #183, and
thus confirm the precision of 0.01–0.005 for the AOD measurement.
Instruments #070 and #075 are, however, in worse agreement with the
reference, particularly at the shortest measured wavelength. These two Brewer
instruments are Mk IV models, while the others (including #183 and #185)
are Mk III. Very recently, reported that the
polarization correction proposed by might not be
adequate for all Brewer models, and these results might point toward this
direction. Another source of error may be the lack of a correction for the
stray light of the single-grating Mk II and IV Brewer spectrophotometers,
although in previous studies this has been found to be a non-issue for the
AOD determination .
Stability and Brewer–Cimel comparison
In this section we analyze the stability of the Brewer AOD by comparing with
Cimel data over ∼ 2 years, from June 2013 to May 2015. We analyze the
AOD from Brewer spectrophotometers considered in the previous section,
operating at their observation sites. In all cases we compare the Brewer
instruments with collocated Cimel sun photometers, except for the Brewer at
Reading for which the closest Cimel is located ∼ 60 km away at
Chilbolton .
Brewer and Cimel AOD for the 2013–2015 period. AOD series shown in
the (a) panels correspond to daily averages calculated from
Brewer (red) and Cimel (green) observations within 1 min. Daily (blue) and
monthly (magenta) averages of AOD differences are shown in (b). For
the Brewer we use the data for the longest measured wavelength at
320.1 nm, and for the Cimel, the 340 nm AERONET level 2.0
product extrapolated to 320 nm using the 340–440 Ångström
exponent. Note that Brewer #075 operating at Reading is compared to the
Cimel sun photometer operating at Chilbolton. The y axes of the figures on
the left go up to 1 in all cases except Tamanrasset, for which it reaches
a value of 4. Dates on the x axes are shown in DOY/YY format, where DOY is day of year.
Figure summarizes the comparison between the Brewer and
Cimel AOD in the 2013–2015 period. As shown by the AOD series, there is
clear correlation between the data of both instruments, with correlation
coefficients above 0.90. The lowest correlation corresponds to the
Reading/Chilbolton data, which also shows the largest spread of Brewer–Cimel
differences, in part due to the separation between the Brewer and Cimel
sites, and in part due to sporadic data from the Cimel sun photometer. Besides
this instrument, Brewers #070 and #201 show the largest changes. The latter
operates at Tamanrasset under extreme aerosol conditions with very frequent
dust aerosol intrusions, which makes maintenance of the utmost
importance see, e.g.,. Note also that some
maintenance issues that do not produce noticeable errors for the
determination of TOC might affect the AOD, because the former uses ratios of
measurements at different wavelengths, while the latter uses their absolute
values. Regarding Brewer #070, in Sect. it was found to
be one of the instruments in worst agreement with the reference after the
calibration transfer. The better behavior of the collocated Brewer #186 in
these 2-year comparison points to a bad calibration and/or to maintenance
issues as possible reasons for the instability of Brewer #070 AOD data. Of
the remaining instruments, Brewers #183, #185, and #186 remain within the
initial Brewer–Cimel AOD difference for a period of at least
∼ 1.5 years, from June 2013 to November 2014. The rather good stability
of these instruments proves that it is possible to provide quality AOD data
when the instrument maintenance is properly performed. It should be noted
that Brewer intercomparison campaigns are held every 2 years, fairly close to
the 1.5-year stability period mentioned, and provide an opportunity to
verify, provide maintenance, and recalibrate the instruments for AOD operation
if necessary.
Although we expect the extrapolated Cimel data to provide a good and stable
reference for comparison with the Brewer AOD, the uncertainties introduced by the
extrapolation, as well as the change of Brewer AOD calibration over the
2-year period, preclude any precise determination of the Brewer uncertainty
from the data presented in this section. Still, assuming as in
that the biases are a systematic error which
can be corrected (by using, e.g., a different formula of the ozone
absorption coefficients or a different pressure value), the SD of the
Brewer–Cimel difference for the three most stable instruments in
Fig. results in a standard uncertainty at the 95 %
(2σ) level (see, e.g., ) of ∼ 0.05. From this
value and accepting the 2σ uncertainty of 0.02 for the Cimel in the UV
mentioned in Sect. , the uncertainty of the Brewer
would be almost 0.05 at 320 nm. In the next section we compare our
Brewer AOD with that of an UVPFR instrument.
AOD differences, for observations within 1 min at 313.5 nm,
between the Brewer instruments and the UVPFR during the tenth intercomparison campaign of the RBCC-E, plotted as a function of the aerosol optical air
mass. The UVPFR data have been interpolated from those at wavelengths 311.3
and 317.5 nm using the Ångström relationship. The WMO
traceability limits for finite field-of-view instruments
(Eq. ) are shown as thick black lines.
Uncertainty and Brewer–UVPFR comparison
A better experimental determination of the uncertainty can be derived from
the comparison with the UVPFR sun photometer, independently calibrated and
operated by the PMOD/WRC at the tenth intercomparison campaign of the RBCC-E,
held at El Arenosillo in May–June 2015. We present results for 16 of
the Brewer instruments present, including reference Brewer #185 from the IZO
triad. As in the case of the eighth intercomparison campaign, Brewer #185
was calibrated using the Langley plot method at IZO before the campaign, and
this calibration was then transferred to the other Brewer spectrophotometers
using simultaneous data acquired after the instruments had received
maintenance at the campaign.
The plots of the Brewer–UVPFR AOD differences vs. the aerosol optical air
mass in Fig. show that some Brewer instruments, like #044
and #172, largely deviate from the expected 1/ma behavior of
the differences see, e.g.,, while others such as #075 and #117 show a large spread of the data. Both issues might be
related either to problems not solved by the maintenance performed during the
campaign or to effects not fully considered in our AOD determination method,
such as the different polarization corrections required by the different
Brewer models. It should also be noted that our method currently only
includes one data-quality filter specific to AOD, and this may be
insufficient to remove all outliers. Overall, though, Fig.
shows a reasonably good agreement between the Brewer and UVPFR instruments,
with percentages of differences within the WMO traceability limits similar to
those presented in Sect. . AOD differences with respect to
reference Brewer #185 are fairly similar to those shown in
Fig. , although in the comparison with the UVPFR the
differences seem to increase with the optical air mass, something that does
not happen in the Brewer–Brewer comparisons.
Comparison between the AOD data of the Brewer and UVPFR instruments
at the tenth intercomparison campaign of the RBCC-E. We use data from
observations within 1 min and show the same statistics as in
Table .
Brewer
Correlation, median of differences, SD (1σ) of differences
(Mk)
obs.
306.3 nm
310.1 nm
313.5 nm
316.8 nm
320.1 nm
005 (II)
618
0.974, 0.0106, 0.0191
0.989, -0.0005, 0.0154
0.989, 0.0029, 0.0115
0.990, -0.0017, 0.0107
0.993, 0.0005, 0.0088
044 (II)
293
0.966, 0.0114, 0.0240
0.977, -0.0001, 0.0196
0.977, 0.0039, 0.0175
0.980, -0.0010, 0.0154
0.982, 0.0010, 0.0141
070 (IV)
165
0.974, 0.0091, 0.0092
0.989, -0.0035, 0.0064
0.989, -0.0001, 0.0059
0.990, -0.0055, 0.0056
0.991, -0.0021, 0.0052
075 (IV)
1081
0.969, 0.0052, 0.0225
0.975, -0.0073, 0.0200
0.975, -0.0052, 0.0195
0.976, -0.0092, 0.0188
0.977, -0.0069, 0.0181
117 (IV)
810
0.972, 0.0072, 0.0211
0.978, -0.0063, 0.0184
0.978, -0.0028, 0.0181
0.976, -0.0076, 0.0185
0.976, -0.0051, 0.0182
126 (II)
605
0.983, 0.0133, 0.0133
0.987, -0.0002, 0.0114
0.987, 0.0038, 0.0107
0.987, -0.0017, 0.0103
0.988, 0.0009, 0.0097
150 (III)
533
0.960, 0.0137, 0.0242
0.988, -0.0005, 0.0161
0.988, 0.0007, 0.0120
0.986, -0.0052, 0.0115
0.985, -0.0012, 0.0114
158 (III)
366
0.993, 0.0105, 0.0083
0.996, -0.0018, 0.0063
0.996, 0.0023, 0.0061
0.996, -0.0028, 0.0065
0.996, -0.0007, 0.0063
163 (III)
1536
0.993, 0.0080, 0.0091
0.995, -0.0044, 0.0076
0.995, -0.0015, 0.0076
0.994, -0.0067, 0.0081
0.995, -0.0039, 0.0075
172 (III)
371
0.982, 0.0096, 0.0131
0.978, -0.0011, 0.0134
0.978, 0.0031, 0.0136
0.974, -0.0010, 0.0148
0.972, 0.0018, 0.0148
185 (III)
1611
0.988, 0.0087, 0.0095
0.990, -0.0039, 0.0083
0.990, -0.0007, 0.0080
0.989, -0.0061, 0.0083
0.990, -0.0032, 0.0080
186 (III)
416
0.992, 0.0099, 0.0093
0.995, -0.0017, 0.0079
0.995, 0.0023, 0.0071
0.994, -0.0019, 0.0072
0.995, 0.0004, 0.0070
201 (III)
1162
0.979, 0.0090, 0.0135
0.979, -0.0040, 0.0133
0.979, -0.0005, 0.0132
0.975, -0.0057, 0.0144
0.972, -0.0027, 0.0151
202 (III)
764
0.992, 0.0146, 0.0094
0.996, 0.0011, 0.0075
0.996, 0.0043, 0.0076
0.996, -0.0007, 0.0080
0.996, 0.0017, 0.0081
214 (III)
543
0.989, 0.0147, 0.0143
0.995, 0.0014, 0.0145
0.995, 0.0049, 0.0094
0.992, 0.0002, 0.0116
0.983, 0.0023, 0.0168
228 (III)
289
0.976, 0.0095, 0.0159
0.984, -0.0024, 0.0136
0.984, 0.0023, 0.0123
0.985, -0.0029, 0.0120
0.986, -0.0004, 0.0112
Median
574
0.980, 0.0097, 0.0134
0.988, -0.0017, 0.0133
0.988, 0.0023, 0.0111
0.988, -0.0028, 0.0111
0.987, -0.0005, 0.0104
Table summarizes the comparison between the Brewer and
UVPFR data within 1 min at the five standard Brewer ozone wavelengths. As
before, the shortest wavelength shows slightly worse results than the other
four. At 306.3 nm, correlation coefficients are above 0.96, and
biases and SDs below 0.015 and 0.024, respectively. In the range of 310.1 to
320.1 nm, correlations are above 0.97, and biases and SDs are lower
than 0.009 and 0.020, respectively.
The uncertainty of the Brewer AOD can be obtained from the SDs in
Table and the uncertainty of the UVPFR. Care must be taken,
however, to include the effect of terms common to the Brewer and UVPFR AOD.
Among these, the largest according to is the ozone
optical depth τo. Taking into consideration only this term,
the 2σ uncertainty of the Brewer AOD can then be written as
u2(Brewer)=(2σ)2+2u2(τo)-u2(UVPFR).
The SD σ of the Brewer–UVPFR difference for Brewer #185 at
wavelengths between 310.1 and 320.1 nm contributes (2⋅0.01)2
to the total squared uncertainty. For the uncertainty of the ozone optical
depth, we can use the same values calculated in
Sect. . Finally, for the UVPFR,
reports a 2σ uncertainty between 0.04 and
0.02 in this range of wavelengths. All this points to a 2σ uncertainty
between 0.04 and 0.01 for the Brewer AOD in the range of wavelengths from
310.1 to 320.1 nm. For 306.3 nm, the same calculation yields
an uncertainty of 0.05. These values are fairly close to the ones found in
our analytical derivation in Sect. .
Discussion
Despite the generally good results shown in the previous section for our AOD
determination method, there are multiple improvements that could be
introduced in three different aspects: data processing, input data, and
instrumental characterization. Regarding the data processing, note that
besides the ozone-specific filters to mainly remove observations under cloudy
conditions, only one AOD-specific filter is included in our AOD algorithm.
This AOD filter is based on the SD of five consecutive observations and is
expected to remove measurements affected by fast-moving clouds
. Further cloud-detection methods and filters used to
remove measurements under cloudy conditions should be devised and implemented
to improve the quality of the Brewer AOD product. This will require a more
extensive analysis of the AOD data from the whole EUBREWNET network.
Furthermore, optical air mass limits specific to each Brewer model can be
implemented. This would specially benefit Mk III instruments (which are
largely free from stray-light issues) operating at high latitudes.
With regard to the data used as input, first it should be noted that our AOD
is currently produced in real time and as such uses the highest-quality
real-time data available in EUBREWNET, namely the ozone level 1.5 product.
Once the configuration of a Brewer instrument issued in one campaign has been
validated in the next one, a level 2.0 ozone product is produced at
EUBREWNET. We plan to implement an AOD product which will use these validated
ozone values instead of the real-time data. A second point to consider is
that currently the AOD algorithm uses the same pressure value used in the
determination of the ozone, which is the climatological value for the station
where the Brewer operates. A 2σ uncertainty of 10 hPa in the
pressure leads to ∼ 0.01 added uncertainty in the Brewer AOD at the UV
wavelengths, which is approximately half the uncertainty we considered
for the calibration. Using a pressure value measured in situ would be thus
advisable, although this would require adding further hardware and software
infrastructure to the EUBREWNET network. A faster and easier-to-implement
alternative would be to use the pressure data provided by any of the
reanalysis products available, as in AERONET's Version 2 Direct Sun
Algorithm. Likewise, the ozone layer height is currently fixed at
22 km, also the same value used in the default ozone determination
carried out by the Brewer. This could be improved by using a specific value
for each latitude, as in AERONET's algorithm. Finally, note that we currently
use data from wavelengths between 306 and 320 nm because they are
available from the standard ozone measurements. However, it is planned to
implement an AOD-specific measurement routine in selected Brewers of the
EUBREWNET network which will allow one to determine the AOD at longer
wavelengths, including 340 and 354 nm. Note that extending the
wavelength measurement range will likely require changes in our Langley
calibration method, because ozone will stop being the largest contribution to
the extinction. Besides the scientific interest, extending the measurement
range will also allow for a better comparison with data from the AERONET
network and satellite products from, e.g., the Ozone Monitoring Instrument
(OMI).
Regarding the characterization of the Brewer instruments, an important first
point which requires further analysis is the relationship between the TOC and AOD
calibrations, and more specifically which hardware changes in the Brewer
instruments affect one or both retrieval-related calibrations. Going into
more specific details, our results indicate that improvements in the current
polarization correction and taking stray-light effects into consideration
might be necessary. Our polarization correction is applied to data from all
Brewer models taken at solar zenith angles greater than 55∘. However,
reported very recently different
polarization corrections depending on the Brewer model. Furthermore, these
authors have also investigated the polarization at measurement angles below
35∘, finding that a small but non-negligible correction might also be
necessary. Further study is also necessary to characterize the uncertainty
and correction needed by the stray light produced by the scattering on the
single grating of Mk II and IV Brewer spectrophotometers
. Finally, it should be noted that
currently the temperature correction coefficients provided by the RBCC-E for
the ozone calculation are also used in the present implementation of the
Brewer AOD algorithm. These coefficients are relative to the value calculated
for the shortest wavelength, but in the case of the AOD determination they
should be absolute values because data for each wavelength are used
individually. The determination of the absolute temperature coefficients is
currently under study .