Uncertainty analysis of total ozone derived from direct solar irradiance spectra in the presence of unknown spectral deviations

Uncertainty analysis of total ozone derived from direct solar irradiance

Uncertainty analysis of total ozone derived from direct solar irradiance spectra in the presence of unknown spectral deviationsUncertainty analysis of total ozone derived from direct solar irradianceAnna Vaskuri et al.

Anna Vaskuri^{1},Petri Kärhä^{1},Luca Egli^{2},Julian Gröbner^{2},and Erkki Ikonen^{1,3}Anna Vaskuri et al. Anna Vaskuri^{1},Petri Kärhä^{1},Luca Egli^{2},Julian Gröbner^{2},and Erkki Ikonen^{1,3}

^{1}Metrology Research Institute, Aalto University, P.O. Box 15500, 00076 Aalto, Finland

^{2}Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Dorfstrasse 33, 7260 Davos Dorf, Switzerland

^{3}MIKES Metrology, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, 02044 VTT, Finland

Received: 06 Nov 2017 – Discussion started: 05 Feb 2018 – Revised: 17 May 2018 – Accepted: 01 Jun 2018 – Published: 20 Jun 2018

We demonstrate the use of a Monte Carlo model to estimate the uncertainties in total ozone column (TOC) derived from ground-based direct solar spectral
irradiance measurements. The model estimates the effects of possible
systematic spectral deviations in the solar irradiance spectra on the
uncertainties in retrieved TOC. The model is tested with spectral data
measured with three different spectroradiometers at an intercomparison
campaign of the research project “Traceability for atmospheric total column
ozone” at Izaña, Tenerife on 17 September 2016. The TOC values derived at
local noon have expanded uncertainties of 1.3 % (3.6 DU) for a high-end
scanning spectroradiometer, 1.5 % (4.4 DU) for a high-end array
spectroradiometer, and 4.7 % (13.3 DU) for a roughly adopted instrument based on commercially available components and an array spectroradiometer
when correlations are taken into account. When neglecting the effects of
systematic spectral deviations, the uncertainties reduce by a factor of 3.
The TOC results of all devices have good agreement with each other, within
the uncertainties, and with the reference values of the order of 282 DU
during the analysed day, measured with Brewer spectrophotometer #183.

Atmospheric ozone has been defined as an essential climate variable in the
global climate observing system (GCOS-200, 2016) of the World
Meteorological Organization (WMO). Its long-term monitoring is necessary to
document the expected recovery of the ozone layer due to the implementation
of the Montreal Protocol (UNTC, 1987) and its amendments on the
protection of the ozone layer. Atmospheric ozone, first discovered
by Fabry and Buisson (1913), protects humans, the biosphere, and infrastructure
from adverse effects of ultraviolet (UV) radiation by shielding the Earth's
surface from excessive radiation levels (McElroy and Fogal, 2008). Since the
1970s, it is known that human-produced chlorofluorocarbons (CFCs) destroy
atmospheric ozone (Molina and Rowland, 1974) and lead to recurring massive
losses of total ozone in the Antarctic in the form of the ozone hole
(Farman et al., 1985; Solomon et al., 1986). Unprecedented ozone depletion has also been
recently observed in the Arctic (Manney et al., 2011), while in the
midlatitudes, moderate ozone depletion has been
observed (Solomon, 1999). The Montreal Protocol and its amendments have
been successful in reducing the emission of ozone-depleting substances
(Velders et al., 2007). Nevertheless, recent studies give conflicting results
with respect to the observation of the recovery of the ozone layer, and model
projections have shown the recovery to not occur before the middle of the
21st century (Ball et al., 2018; Weber et al., 2018). Therefore, careful monitoring of the
thickness of the ozone layer with uncertainties of 1 % or less is crucial
for verifying the successful implementation of the Montreal Protocol and the
eventual recovery of the ozone layer to pre-1970s levels.

“Traceability for atmospheric total column ozone” (ATMOZ) was a 3-year
project funded partly by the European Metrology Research Programme (EMRP) and
the European Union (ATMOZ project, 2014). The goal of this project was to
produce traceable measurements of total ozone column (TOC) with uncertainties
down to 1 % through a systematic investigation of the radiometric and
spectroscopic aspects of the methodologies used in retrieval. Another
objective of the project was to provide a comprehensive treatment of
uncertainties in all parameters affecting the TOC retrievals using
spectrophotometers. This paper presents an outcome of the work on studying the
uncertainty in TOC obtained from spectral direct solar irradiance
measurements, taking unknown spectral errors explicitly into account.

TOC can be determined from spectral measurements of direct solar UV
irradiance (Huber et al., 1995). We have developed a Monte Carlo (MC)-based
model to estimate the uncertainties in the derived TOC values. One frequently
overlooked problem with uncertainty evaluation is that the spectral data may
hide systematic wavelength-dependent errors due to unknown correlations
(Gardiner et al., 1993; Kärhä et al., 2017b, 2018). Omitting possible correlations may
lead to underestimated uncertainties for derived quantities, since
spectrally varying systematic errors typically produce larger deviations than
uncorrelated noise-like variations that traditional uncertainty estimations
predict. Complete uncertainty budgets for quantities measured are necessary
to understand long-term environmental trends, such as changes in the
stratospheric ozone concentration (e.g. Molina and Rowland, 1974) and solar UV
radiation (e.g. Kerr and McElroy, 1993; McKenzie et al., 2007).

Physically, spectral correlations may originate from lamps or other
light sources used in calibrations. If their temperatures change, e.g. due to
ageing or current setting, a spectral change in the form of Planck's
radiation law is introduced. Non-linearity in the responsivity of a detector
causes systematic differences between high and low measured values. The
introduced spectrally systematic but unknown changes in irradiance may change
the derived TOC values significantly, exceeding the uncertainties calculated,
assuming that the uncertainty in irradiance behaves like noise. The presence
of correlations in measurements can be seen in many ways. For example,
problems have occurred when new ozone absorption cross sections have been
used (Fragkos et al., 2015; Redondas et al., 2014). Derived ozone values may
change significantly because different systematic errors are included in the
different cross sections. Also, TOC estimated from a measured spectrum often
depends on the wavelength region chosen, although the measurement region
should not affect the result much.

In this paper, we introduce a new method for dealing with possible
correlations in spectral irradiance data and analyse uncertainties in ozone
retrievals for three different spectroradiometers used in a recent ATMOZ
intercomparison campaign at Izaña, Tenerife, to demonstrate how it can be
used in practice. One of the instruments is QASUME (Gröbner et al., 2005),
which is the reference UV spectroradiometer at the World Radiation Center
(PMOD/WRC). The second one is an array-based high-quality spectroradiometer
BTS2048-UV-S-WP (BTS) from Gigahertz-Optik (Zuber et al., 2017, 2018),
operated by Physikalisch-Technische Bundesanstalt (PTB). The third one is an
array-based spectroradiometer AvaSpec-ULS2048LTEC (AVODOR) from Avantes,
operated by PMOD/WRC. The field of view of the spectroradiometers has been
limited so that they measure the direct spectral irradiance of the Sun, excluding
most of the indirect radiation from the remainder of the sky.

2 ATMOZ field measurement campaign and instrument description

The ATMOZ project arranged a field measurement campaign (ATMOZ campaign, 2016)
that took place 12–30 September 2016 at the Izaña Atmospheric Observatory,
a high-mountain Global Atmosphere Watch (GAW) station located at an altitude
of 2.36 km above the sea level in Tenerife, Canary Islands, Spain
(28.3090^{∘} N, 16.4990^{∘} W). The measurement campaign was
organized by the Spanish State Meteorological Agency (AEMET) and the PMOD/WRC for
the intercomparison of TOC measured with different participating instruments,
including Dobson and Brewer spectrophotometers and various
spectroradiometers.

The focus of this paper is to study uncertainties in the TOC values derived
from direct solar UV irradiance spectra. Total ozone column TOC is the
vertical ozone profile ${\mathit{\rho}}_{{\mathrm{O}}_{\mathrm{3}}}\phantom{\rule{-0.125em}{0ex}}\phantom{\rule{-0.125em}{0ex}}\left(z\right)$ integrated
over altitude as

from the station altitude z_{0} to the top-of-the-atmosphere altitude
z_{1}. We study the data measured during the day of 17 September 2016 with
three different spectroradiometers using the ozone retrieval algorithm
introduced in Sect. 3. Station pressure was monitored
during the campaign and determined to be 772.8 hPa with a standard
uncertainty of 1.3 hPa. The ozone and temperature profiles were measured
with a sonde during the campaign and examples of them are shown in
Fig. 1.

Figure 1Temperature and ozone profiles as a function of the altitude
measured with a sonde during the ATMOZ field measurement campaign.

Our ozone retrieval method uses one atmospheric layer to reduce computational
complexity. With the single-layer model, the ozone absorption cross section is a
function of the effective temperature, and the relative air mass is a function
of the effective altitude of the ozone layer. Using the vertical ozone
profile ${\mathit{\rho}}_{{\mathrm{O}}_{\mathrm{3}}}\phantom{\rule{-0.125em}{0ex}}\phantom{\rule{-0.125em}{0ex}}\left(z\right)$, the effective altitude
h_{eff}=26 km ± 0.5 km of the ozone layer was estimated by
integration over altitude:

from the station altitude z_{0} to the top-of-the-atmosphere altitude
z_{1}. The corresponding effective temperature T_{eff}=228 K ± 1 K was estimated (Fragkos et al., 2015; Komhyr et al., 1993) as

The uncertainties stated for h_{eff}=26 km ± 0.5 km and
T_{eff}=228 K ± 1 K are standard deviations. The values
and uncertainties are estimated from the measured vertical profiles
(Fig. 1). The profiles are extended to 80 km using
climatology.

The data sets measured with three different spectroradiometers were studied in
this work. These spectroradiometers use different techniques to measure the
spectral distribution of radiation. Monochromator-based spectroradiometers,
such as QASUME, measure one nearly monochromatic wavelength band at a time,
and thus measuring the full spectrum is relatively slow. On the other hand,
they usually have significantly better stray light properties than
array-based spectroradiometers, such as BTS and AVODOR, which image the full
spectrum at once by dispersing the incoming radiation towards a photodiode
array.

The QASUME spectroradiometer collects and guides the incoming radiation with
input optics and a quartz fibre bundle to the entrance slit of a Bentham
DM150 double monochromator (Gröbner et al., 2005). One wavelength at a time is
selected by rotating the two gratings of the double monochromator. Then, the
monochromatic signal is measured with a photomultiplier tube. QASUME is
usually operated in global spectral irradiance mode (Gröbner et al., 2005; Hülsen et al., 2016), but during the campaign it was equipped with a collimator tube
with a full opening angle of 2.5^{∘}, allowing the measurement of
direct solar spectral irradiance (Gröbner et al., 2017). The measurement range
of QASUME during the campaign was limited to 250–500 nm with a step
interval of 0.25 nm, so that one spectrum was measured every 15 min. To
ensure stable outdoor measurements, the double monochromator of QASUME was
mounted inside a temperature-controlled weatherproof box
(Hülsen et al., 2016).

The BTS spectroradiometer utilizes a stationary grating and a back-thinned cooled
CCD array detector, mounted in a Czerny–Turner configuration
(Zuber et al., 2017, 2018). To measure direct solar spectrum, BTS was
equipped with a collimator tube with a full opening angle of 2.8^{∘}
designed by PTB, and it uses an internal filter wheel system with eight
filter positions together with a specific measurement routine to reduce stray
light. BTS was mounted on a solar tracker, EKO STR-32G by EKO Instruments
Co., Ltd., with pointing accuracy better than 0.01^{∘}. A
weatherproof housing with temperature control allows BTS to operate at
ambient temperatures from −25 to 50 ^{∘}C. During the ATMOZ
campaign, the housing temperature of BTS was measured to be stable within
0.1 ^{∘}C (Zuber et al., 2017, 2018). The measurement range of BTS
was 200–430 nm with a step size of 0.2 nm during the campaign. One
spectrum was measured every 45 s.

AVODOR spectroradiometer has a stationary grating and a back-thinned cooled
CCD array detector in a Czerny–Turner configuration. AVODOR measures the
spectrum from 200 to 540 nm with a step size of 0.14 nm in the UV region.
During the ATMOZ campaign, the field of view of AVODOR was limited to
1.5^{∘} by a commercial collimator tube used, J1004-SMA by CMS
Ing.Dr.Schreder GmbH. The spectral range of AVODOR was limited between 295
and 345 nm by a combination of two solar blind filters to reduce stray light
from the visible and infrared parts of the solar spectrum. The solar blind
filters were mounted between the collimator tube and the fibre entrance of
the spectroradiometer. One spectrum was measured every 30 s.

The slit functions of the three spectroradiometers shown in
Fig. 2 were measured with lasers before the measurement
campaign. They are needed when fitting the modelled spectra at the Earth's
surface to the measured spectra. In addition, it is important to note
the different wavelength steps of the data: 0.25 nm for QASUME, 0.2 nm for
BTS, and 0.14 nm for AVODOR. The wavelength steps in the spectral data
affect the magnitude of the uncertainties in TOC created by spectrally random
components. In our full-spectrum TOC retrieval, the number of data points
n, which is smaller with a larger wavelength step interval, affects
uncertainties with a factor of $\mathrm{1}/\sqrt{n}$, as demonstrated in
Kärhä et al. (2017b); Poikonen et al. (2009).

Figure 2Slit functions measured with narrowband lasers for the
spectroradiometers used in the ATMOZ field measurement campaign. The laser wavelengths are
stated in the legend.

Brewer MkIII spectrophotometers used as reference devices for ozone
measurements established by the International Meteorological Organization,
the Commission for Instruments and Methods of Observation (CIMO)
(Redondas et al., 2016), also measure the spectral irradiance at UV region, but
using four narrow wavelength bands at 310.1, 313.5, 316.8, and
320.0 nm (Kipp & Zonen, 2015). The Brewer MkIII instruments solve absolute TOC
values by comparing the logarithms of ratios of count rates between four
wavelength channels, i.e. using the double ratio technique. Determining the
TOC using the double ratio method is invariant for spectral deviations
which have the same relative magnitude at all wavelengths, i.e spectrally
constant deviations.

Our full-spectrum retrieval method performs averaging in the wavelength
domain, whereas the Brewer spectrophotometer does it in the time domain. A Brewer
can measure up to tens of seconds to get millions of photons, so that the
photon noise reduces to a level of 0.1 %. At this low noise level, it is
not critical that only four wavelengths are used. As Brewer instruments are
well-known and widely used, we also compare TOC results obtained using our
full-spectrum retrieval method and the spectroradiometers to those measured
with Brewer #183 during the same day.

In this study, we use an atmospheric ozone retrieval algorithm that is in many
aspects similar to that in the article by Huber et al. (1995). The relationship between
the spectral irradiance E_{s}(λ) at the
Earth's surface and the extraterrestrial solar spectrum E_{ext}(λ) is based on Beer–Lambert–Bouguer absorption law
(Beer, 1852; Bouguer, 1729; Lambert, 1760) as

where ${\mathit{\alpha}}_{{\mathrm{O}}_{\mathrm{3}}}\phantom{\rule{-0.125em}{0ex}}\phantom{\rule{-0.125em}{0ex}}\left(\mathit{\lambda},{T}_{\mathrm{eff}}\right)$ is the ozone absorption cross section at the effective ozone
temperature T_{eff}, ${\mathit{\tau}}_{\mathrm{R}}\left(\mathit{\lambda},{P}_{\mathrm{0}},{z}_{\mathrm{0}},\mathit{\varphi}\right)$ is the Rayleigh scattering optical depth, which depends on the
station pressure P_{0}, the station altitude z_{0}, and the geographic
latitude ϕ. The QASUME-FTS (Fourier transform spectroscopy) data set by Gröbner et al. (2017) was used as the
extraterrestrial spectrum E_{ext}(λ).
Parameter c is a scaling factor set as a free parameter to compensate for
spectrally constant deviations.

The relative air mass of the ozone layer with the Earth's curvature taken into account can be expressed as

where θ is the incident solar zenith angle at the observing site that
is a function of the local time, date, and geographic coordinates
(Meeus, 1998; Reda and Andreas, 2008). Parameter h_{eff} is the altitude of the
ozone layer from the ground, and R is the radius of the Earth. As the ozone
and other molecules creating scattering are distributed at different
altitudes, we calculate the relative air mass factor m_{R} for
Rayleigh scattering at the altitude of 5 km (Gröbner and Kerr, 2001) and
approximate the relative air mass factor of aerosols so that m_{AOD}≈m_{R} (Gröbner et al., 2017). The temperature dependence of
${\mathit{\alpha}}_{{\mathrm{O}}_{\mathrm{3}}}\phantom{\rule{-0.125em}{0ex}}\phantom{\rule{-0.125em}{0ex}}\left(\mathit{\lambda},{T}_{\mathrm{eff}}\right)$ between
203 and 253 K (Weber et al., 2016) was interpolated by a second-degree
polynomial at each wavelength (Paur and Bass, 1985):

We take the Rayleigh scattering optical depth into account using the
state-of-the-art model by Bodhaine et al. (1999). The aerosol optical depth
(AOD) is approximated from the Ångström AOD model
(Ångström, 1964) as

where constant α≈1.4 is the Ångström coefficient at
typical atmospheric conditions and β≥0 is the Ångström
turbidity coefficient.

The model spectrum E_{s}(λ) at the Earth's
surface, convolved by the slit function of the spectroradiometer as shown in
Fig. 2, is fitted with parameters TOC, β, and c
to the measured ground-based spectrum E(λ). The
absolute TOC level obtained depends on the fitting method used. We used a
least squares fitting method (Levenberg, 1944; Marquardt, 1963) with
trust-region optimization with the Matlab function “lsqnonlin”:

where S is the sum of the squared residuals to be minimized, and w(λ_{i}) is the weight for each point measured. Index $i=\mathrm{1},\mathrm{2},\mathrm{\dots},n$ runs over the wavelengths of the spectral measurements.

Figure 3 presents examples of measured and modelled spectra
for the spectroradiometers used in this work. As can be seen, the
signal-to-noise ratios and stray light properties of the devices differ
significantly among different spectroradiometers. All spectra measured with
QASUME are practically noiseless above 10^{−6} W m^{−2} nm^{−1},
resulting in a dynamic range of approximately 4 orders of magnitude. The
dynamic range for BTS is approximately 2 orders of magnitude and less than
2 orders of magnitude for AVODOR.

For QASUME, we use the relative least squares fitting method (RLS)
minimization with $w\left(\mathit{\lambda}\right)=E{\left(\mathit{\lambda}\right)}^{-\mathrm{2}}$, as QASUME does not suffer from stray light and RLS fitting has
been used in the past for monochromator-based spectroradiometers, e.g. by
Huber et al. (1995). These least squares fitting selections are discussed in more
detail in Appendix A.

To minimize the effect of stray light, we use the absolute least squares
fitting method, also known as the ordinary least squares fitting method (OLS), with
w(λ)=1 for BTS and AVODOR, as this selection is
less affected by the lowest irradiance levels where the stray light and noise
are dominant. As shown in Appendix A, using OLS introduces an offset,
approximately 1 % in these measurements, to the retrieved TOC values. We
take this into account by analysing the results at noon that are also less
influenced by stray light when using RLS, and make a correction to the OLS
results:

After this correction, results of all instruments, QASUME, BTS, and AVODOR are comparable.

The shortest fitting wavelength for the spectroradiometers in this work was
selected to be 300 nm, since the typical stray light compensation methods are
not effective below 300 nm (Nevas et al., 2014). The upper fitting wavelength
limit was set to 340 nm with all three spectroradiometers, as the ozone
absorption is not effective above that wavelength. Due to the relatively
large bandwidths of the spectroradiometers (Fig. 2),
calculations before the convolution and the convolution itself were carried
out over a wider range 295–345 nm.

To see whether a global optimum is achieved with our atmospheric ozone
retrieval method, we varied the initial guess values of TOC from 10 to
700 DU, β from 0 to 0.5, and c from 0 to 100. Within the ranges
stated, the free parameters always converged to the same final values
regardless of the initial guess values.

Figure 3Examples of fitting the atmospheric model to the direct ground-based
solar UV spectra between 300 and 340 nm for QASUME (a–b), BTS
(c–d), and AVODOR (e–f). In figures on the left-hand side,
the coloured symbols indicate measured spectra, and the black solid curves
indicate modelled spectra. Figures on the right-hand side show the relative
spectral residuals of the fits. In (a) the abbreviation DR refers
to the dynamic range of QASUME data used in the least squares fitting.

In uncertainty analysis, the combined uncertainties are calculated with the
square sum of the standard deviations of the components; i.e. their variances
are summed up. If correlations of uncertainties are known, they should be
taken into account. This can be carried out with the methods defined in the
Guide to the Expression of Uncertainty in Measurement (JCGM 100, 2008). In
this paper, we do this for all uncertainty components, where the mechanism of
contributing to the uncertainty in TOC is known. However, with some of the
components, we do not exactly know the mechanisms leading to correlations.
With these uncertainties, we estimate the effects that possible correlations
might have using a newly developed MC model described in Kärhä et al. (2017a, b).

In our MC model, possible systematic deviations within uncertainties are reproduced using a cumulative Fourier series

where index $i=\mathrm{1},\mathrm{2},\mathrm{\dots},N$ depicts the order of complexity of the
deviation (Kärhä et al., 2017b); λ_{1} and λ_{2} limit the
wavelength range of the analysis. For calculations before the convolution due
to slit function, this range was set to 295–345 nm. Otherwise, the results
at the ends of the range 300–340 nm would be distorted due to incomplete
convolution. This concerns the uncertainty in the extraterrestrial spectrum
and the ozone absorption cross section. After the convolution, actual fitting
of the modelled spectra to the measured spectra was carried out at
300–340 nm, and this range was also used in the uncertainty analysis of the
components related to the measured spectra. The phase ϕ_{i} of the base
function is an equally distributed MC variable between 0 and 2π. In
addition, f_{0}(λ)=1 is used to account for
constant offset. The weights γ_{i} for the base functions are selected
in an N-dimensional spherical coordinate system (Hicks and Wheeling, 1959) in such
a way that the variance of the final deviation function always equals
unity. In practice, this means that the weights γ_{i} are generated by
scaling the random variables ${Y}_{i}\sim \mathcal{N}\left(\mathrm{0},\mathrm{1}\right)$ as

The complete N-dimensional system requires orthogonal base functions, such
as full periods of sine functions, to allow an arbitrary shape of deviation
function δ(λ) with unity variance. It is
possible to use other orthogonal sets of functions, such as Chebyshev
polynomials instead of sinusoidal base functions, but that would involve more
complicated mathematics. This is discussed in more detail in
Appendix B.

Figure 4First three base functions with unity variance, f_{1} and f_{2}
plotted with the phases ϕ_{i}=π and ${\mathit{\varphi}}_{i}=\mathit{\pi}/\mathrm{2}$.

where u(λ) is the relative standard uncertainty in
the spectrum. The corresponding spectral deviation is applied separately to
other factors of Eq. (4), i.e. the extraterrestrial solar
spectrum, ozone absorption cross section, and Rayleigh scattering.

Variances of the TOC values obtained by varying the weights γ_{i} and
the phase terms ϕ_{i} give the desired uncertainties.
Figure 5a presents how the uncertainty induced by
deviation in spectral irradiance E(λ) (circles)
changes with increasing N. Each standard uncertainty in TOC in
Fig. 5a was estimated from the MC results obtained by
running the TOC retrieval 1000 times so that the phases ϕ_{i} and the
weights γ_{i} of the base functions were independent at each round.
Retrieved TOC deviations resemble the Gaussian distribution when the order of
complexity of the deviation function is N≥2 as illustrated in
Fig. 5b and c. As we can see,
full correlation with the base function f_{0}(λ) at N=0 causes a negligible uncertainty to TOC. The maximum at N=1 gives uncertainty for an unfavourable case of correlations with
base functions f_{0}(λ) and f_{1}(λ). Cases N=80 for QASUME, N=100 for BTS, or N=125 for
AVODOR correspond to the Nyquist criterion ($N=n/\mathrm{2}$) with base functions
and give the uncertainty with no spectral correlations (only random
noise). The obtained TOC value is affected most by spectral distortion that
mimics the spectral shape of the ozone absorption. The first combination of
constant offset and one sinusoidal function with two sign changes within the
region of interest is closest to this extreme.

The ozone absorption cross section ${\mathit{\alpha}}_{{\mathrm{O}}_{\mathrm{3}}}\phantom{\rule{-0.125em}{0ex}}\phantom{\rule{-0.125em}{0ex}}\left(\mathit{\lambda},{T}_{\mathrm{eff}}\right)$ is a direct multiplier of TOC, and thus the
uncertainty in TOC is directly proportional to the deviations in the ozone
absorption cross section. The uncertainty in TOC arising from the spectral
deviation in ${\mathit{\alpha}}_{{\mathrm{O}}_{\mathrm{3}}}\phantom{\rule{-0.125em}{0ex}}\phantom{\rule{-0.125em}{0ex}}\left(\mathit{\lambda},{T}_{\mathrm{eff}}\right)$ is plotted as crosses in Fig. 5a as a function
of increasing N. The ozone absorption cross section, from the
Serdyuchenko–Gorshelev data set (Weber et al., 2016), has a wavelength step
size of 0.01 nm, and thus the standard uncertainty of 0.05 % at the
Nyquist criterion N=2500 is outside the range displayed in
Fig. 5a. Unlike the negligible effect of full
spectral correlation in the spectral irradiance E(λ)
in TOC, full correlation (N=0) in the ozone absorption
cross section produces approximately the same uncertainty as the
unfavourable correlation (N=1). Generally, these results cannot
be known before the analysis is carried out, using a method that does not
have any internal limitation to the shape of the deviation function δ(λ). In some other cases, the uncertainty extreme
appears at higher N values, e.g. N=3, noted for correlated colour
temperature by Kärhä et al. (2017b).

One major problem in uncertainty estimation is that typically many of the
correlations in spectral irradiance data are unknown.
Figure 5a can be used to find limits for the
uncertainties assuming different correlation scenarios. In the analysis
carried out in this paper, we estimate for each uncertainty component which
kind of correlation it most likely has. For this, we divide the correlation
into three categories, full, unfavourable, and
random and estimate fractions on the assortment of these
correlations. “Full” indicates that relative deviation is wavelength
independent, such as with distance setting in spectral irradiance
measurements. “Random” indicates no correlation between spectral
values. As can be seen in Fig. 5a, the uncertainty
caused by noise depends on the Nyquist criterion, which increases with a smaller
number of base functions. “Unfavourable” indicates an unknown
deviation with systematic spectral structure that produces a large deviation
in TOC.

Figure 5(a) Standard uncertainties in TOC at local noon as a
function of the order of complexity N for QASUME spectroradiometer with
1 % deviation in spectral irradiance E(λ) plotted
as circles and 1 % deviation in ozone absorption cross section
${\mathit{\alpha}}_{{\mathrm{O}}_{\mathrm{3}}}\phantom{\rule{-0.125em}{0ex}}\phantom{\rule{-0.125em}{0ex}}\left(\mathit{\lambda},{T}_{\mathrm{eff}}\right)$ plotted as
crosses. The distributions of TOC values arising from 1 % deviation in E(λ) with the order of complexity of N=2 in
(b) and N=80 in (c). The black solid curve denotes
a Gaussian distribution.

Uncertainty budgets of the direct solar spectral irradiance measurements for
QASUME, BTS, and AVODOR are presented in
Tables 1–3. The tables also state
fractions that we estimate for the three correlation types introduced for
each component. The uncertainties due to radiometric calibration include
factors such as the uncertainty in the standard lamp used and the additional
uncertainty due to noise and alignment. QASUME has been validated using
various methods; thus the uncertainty due to calibration is low, at 0.55 %
(Hülsen et al., 2016). For QASUME and BTS, we assume the correlations to be
equally distributed between full correlation, unfavourable
correlation, and random correlation (Kärhä et al., 2018). Spectra
measured with AVODOR are significantly noisier; thus half of the calibration
uncertainty is associated with the random component. Values for the instability of
the calibration lamp are based on long-term monitoring. The lamp irradiances
have been noted to gradually drop, with no significant wavelength structure
within the wavelength region concerned. Non-linearity values are estimations
of the operators of the devices. Non-linearity is typically manifested so
that the responsivity of the device changes gradually from high readings to
low readings. This can cause significant changes to the TOC values; thus we
assume the correlation type to be unfavourable. Uncertainties due to
device stability and temperature dependence are based on long-term
monitoring. The changes have been found to be wavelength independent in
the region concerned; thus full correlation is assumed. Noise is the
average standard deviation of typical measurements at noon over the
wavelength region concerned. The wavelength scales of the devices have been
checked using emission lines of gas discharge lamps. The uncertainty values
given are the estimated standard deviations of the possible remaining errors
after corrections. Wavelength error can introduce a significant change in
TOC, because it introduces an error in the form of the derivative of the
spectral irradiance. Thus, unfavourable correlation is assumed. Most
of the uncertainty components are slightly wavelength dependent, but to
simplify simulations, average uncertainty values are used over the wavelength
range between 300 and 340 nm.

Table 1Uncertainties in the measurement for the QASUME spectroradiometer.

Table 4 presents an uncertainty budget for TOC that
would be obtained with the high-accuracy QASUME spectroradiometer at local
noon. All major uncertainty components are listed and estimated. The
uncertainty components divided into the three correlation types have been
analysed with the new model. The other components in
Table 4 have been solved using traditional MC
modelling because the mechanism for the uncertainty propagating to TOC is
known.

Table 4An example uncertainty budget for QASUME spectroradiometer measured
at local noon on the clear day of 17 September 2016. The last column states
the standard deviations u(TOC) corresponding
to each individual uncertainty component for TOC = 284 DU retrieved from
the QASUME spectrum using the spectral range of 300–340 nm at the solar
zenith angle of 26.35^{∘}. The stated expanded uncertainty, U(TOC)=3.6 DU, was obtained by multiplying the
combined standard uncertainty with a coverage factor k=2.

^{a} Air mass m_{TOC} varies as a function of the altitude of the O_{3} layer. ^{b} Air mass m_{R} varies as a function of the altitude of the Rayleigh scattering layer. ^{c}O_{3} cross section varies as a function of temperature. ^{d} Rayleigh scattering depends on the station pressure.

The uncertainties produced in TOC were obtained separately for all
components by setting other uncertainties to zero. Division of the
correlation to the three categories introduced are stated for each row as
fractions r_{full}, r_{unfav}, and r_{random}. For example, the ground-based spectrum E(λ) is deviated with the three correlation
components as

where ${\mathit{\gamma}}_{i}^{\prime}$ and ${\mathit{\gamma}}_{j}^{\prime \prime}$ are independent MC variables generated
using Eq. (12).

It is worth noting that not all uncertainty components affect the spectrum E(λ) directly but via the exponent of
Eq. (4). Corresponding formulas are used to evaluate the
effect of uncertainties in extraterrestrial solar spectrum, ozone absorption
cross section, and Rayleigh scattering. The last column in
Table 4 states the standard uncertainties produced by
each uncertainty component with the assumed fractions, calculated with an
irradiance spectrum measured at local noon with QASUME (Hülsen et al., 2016).
The expanded uncertainty in the TOC, obtained as the square sum of the
individual components and multiplied with coverage factor k=2, for this
spectral measurement, is 3.6 DU (1.3 %).

The QASUME spectroradiometer has a combined measurement standard uncertainty
of 0.91 % (Hülsen et al., 2016) arising from the uncertainty components
explained in Sect. 4.2. The uncertainty components stated are
typical in solar UV spectral irradiance measurements (Bernhard and Seckmeyer, 1999).
Division of the radiometric calibration uncertainty to equal fractions of
$\mathrm{1}/\sqrt{\mathrm{3}}$ is based on typical spectral correlations noted in
intercomparisons (Kärhä et al., 2018). The lamp data obtained from national
standard laboratories are highly accurate but also typically spectrally
correlated. Due to very low noise, elements such as interpolation functions,
offsets, and slopes are present in the data. When the calibration is
transferred further, uncertainty increases due to noise, and correlations
reduce. We thus assume that in this high accuracy calibration, there are
equal amounts of fully correlated, unfavourably correlated,
and uncorrelated uncertainties.

For E_{ext}(λ), we use QASUME-FTS
(Gröbner et al., 2017). We assume the correlation to be similar to a standard
lamp, thus containing equal fractions of full,
unfavourable, and random correlations. The QASUME-FTS is
provided in air wavelengths with a step size of 0.01 nm. Otherwise, the
wavelength shift due to the vacuum–air interface should be corrected from the
extraterrestrial spectrum.

As the reference ozone absorption cross section, the Serdyuchenko–Gorshelev
data set given in air wavelengths with a step size of 0.01 nm was used with
1.5 % standard uncertainty (Weber et al., 2016). The systematic and random
uncertainties in the Serdyuchenko–Gorshelev data set are given separately
(Weber et al., 2016). We further estimate that the systematic uncertainty
may include equal fractions of fully correlated and unfavourably correlated
uncertainty. Thus, according to that, we use the
fractions of 0.23 for full, 0.23 for unfavourable, and 0.95
for random correlations. Full correlation with a fraction
of 0.23 produces a standard uncertainty of 0.96 DU, unfavourable
correlation with a fraction of 0.23 produces a standard uncertainty of
1.01 DU, and random correlation with a fraction of 0.95 produces a
standard uncertainty of 0.22 DU. Altogether, the ozone cross section causes
an uncertainty of 1.41 DU to TOC and is thus the dominating component in
the uncertainty. If the fractions of correlations are not equally distributed
between full and unfavourable, uncertainty in TOC does not
change significantly from 1.41 DU. Fractions of 0.31 for full, 0
for unfavourable (or fractions of 0 for full, 0.31 for
unfavourable), and 0.95 for random correlations cause an
uncertainty of 1.33 DU (or 1.43 DU) in TOC.

For components a–d in Table 4, the mechanism of
contributing to the uncertainty in TOC is known. We know the standard
uncertainty in the O_{3} layer altitude of 26 km to be u=0.5 km,
so we vary the altitude accordingly and note the variance of the resulting
TOC. Rayleigh scattering and aerosols are set at the altitude of 5 km ±0.5 km, which influences the relative air mass m_{R}≈m_{AOD} (Gröbner et al., 2017). This component causes negligible
uncertainty to TOC. To calculate ${\mathit{\tau}}_{\mathrm{R}}\left(\mathit{\lambda},{P}_{\mathrm{0}},{z}_{\mathrm{0}},\mathit{\varphi}\right)$, we use a model by (Bodhaine et al., 1999) with 0.1 %
uncertainty with equal estimated fractions of correlation types. The
correlation has been obtained by studying how this data deviates from the
model by Nicolet (1984). The ozone and temperature profiles were
measured with a sonde during the campaign and based on the profiles, the
effective altitude of the ozone layer was at 26 km ± 0.5 km at the
effective temperature of 228 K ± 1 K. The effect on TOC was obtained
by randomly varying the altitude using a Gaussian uncertainty distribution.
The same applies to air pressure, which was 772.8 hPa with a standard
uncertainty of 1.3 hPa. The effect of temperature on TOC was obtained by
randomly varying the temperature with its standard uncertainty of 1 K.
Varying the temperature systematically changes the spectral ozone absorption
cross section according to Eq. (6).

Stray light that affects TOC results at large solar zenith angles
(Appendix A) has not been accounted for in the uncertainty
analysis due to a lack of information. Proper correction and estimation of the
uncertainty due to stray light would require the stray light correction
matrix to be measured. The effect of stray light, typical for measurements
with array spectroradiometers, was reduced from the TOC results by fitting BTS
and AVODOR spectra with the OLS method. Then, these TOC values were corrected
to be compatible with the RLS results by Eq. (9). The
correction factor involves a standard uncertainty, which is 0.1 % (0.28 DU)
for BTS and 1.1 % (3.10 DU) for AVODOR.

The calculated TOC values obtained with the three different spectroradiometers
on 17 September 2016 are presented in Fig. 6. Expanded
uncertainties in the TOC values calculated are stated in DU as error bars.
Measurement results of Brewer spectrophotometer #183 used as a reference in
the intercomparison have been included in Fig. 6 as well. Looking
at the absolute values of TOC in Fig. 6, we may conclude that the
results of QASUME and Brewer #183 are in excellent agreement. Also, the TOC
values estimated for BTS and AVODOR are in agreement with Brewer #183 within
uncertainties.

Figure 6Total ozone columns (TOC) derived from the solar UV spectra from
300 nm to 340 nm with expanded uncertainty bars (k=2) for QASUME
indicated as blue circles, BTS indicated as green squares, and AVODOR
indicated as magenta triangles. The TOC measured with Brewer #183 is
plotted as black crosses with grey uncertainty bars (k=2).

The expanded uncertainties in the TOC values obtained with the three
instruments are shown in Fig. 7. The expanded
uncertainties in the TOC data sets at local noon are 3.6 DU (1.3 %) for
the QASUME spectroradiometer, 4.4 DU (1.5 %) for the BTS
spectroradiometer, and 13.3 DU (4.7 %) for the AVODOR spectroradiometer.
The expanded uncertainties stated include the standard deviations of 0.28 DU
for BTS and 3.10 DU for AVODOR, arising from the fitting method.

It is of interest to compare the obtained uncertainties with values assuming
no correlations. If we neglect correlations (i.e. we assume the fractions in
Table 4 to be 0 for the full and unfavourable correlations and 1 for the random correlation, and run
the simulations with the spectrum measured at local noon), we obtain the
expanded uncertainty U_{QASUME}(TOC)=0.9 DU
(0.3 %), U_{BTS}(TOC)=1.1 DU (0.4 %),
and U_{AVODOR}(TOC)=7.7 DU (2.7 %). These
values are on average a factor of 3 lower than the uncertainties accounting
for correlations. This analysis assumes random noise only.

Typical practice in an analysis like this is to add a component introduced
by the standard deviation of the fit to the uncertainty. The standard
uncertainty to be added to u(TOC) because of the
standard deviation of the fit is 0.2 DU with QASUME, 0.7 DU with BTS, and
3.4 DU with AVODOR, raising the corresponding total expanded uncertainties
to 1.0 DU (0.3 %), 1.8 DU (0.6 %), and 10.3 DU (3.6 %). The results
are generally in agreement within these lower uncertainties as well. However,
comparing differences in the TOC results of the different spectroradiometers
does not represent the uncertainty in the absolute TOC scale, since the ozone
retrieval algorithm uses the same extraterrestrial spectrum and ozone
absorption cross section for all the instruments. Changing the
extraterrestrial spectrum or the ozone absorption cross section to another
data set may shift all the TOC values of the instruments beyond the latter
low uncertainties.

Figure 7Expanded uncertainties in the total ozone columns derived
from QASUME, BTS, and AVODOR spectra obtained by the relative least squares
fitting method.

Figure 6 shows that the effect of stray light can be effectively
reduced from the TOC results by using the OLS fitting described in Appendix A
with the correction in Eq. (9). For example, BTS and QASUME
results are in good agreement even at the largest zenith angles. TOC results
of AVODOR are in agreement at noon, but the results before 09:00 and after
17:00 deviate from the other instruments by 10 DU.

In this work, we introduced one possible way to take into account spectral
correlations in the uncertainties in the atmospheric ozone retrieval and
estimated the TOC uncertainties obtained from the spectral data of three
different spectroradiometers, measured at the ATMOZ field measurement
campaign at the Izaña Atmospheric Observatory. It should be noted that the
method proposed has a drawback that the unknown correlations have to be
approximated based on experience. However, the method has merits in
estimating the order of magnitude of possible uncertainties accounting for
correlations. The typical assumption made, that uncertainties are spectrally
uncorrelated, is just an assumption as well and in many cases, it is not valid. The
uncertainty values obtained with the new model are higher than the
uncertainties obtained with the traditional method, which neglects correlations
because some of the major uncertainty components may contain systematic
spectral deviations. These results demonstrate the importance of accounting
for correlations. If their origins and magnitudes are known, they can be
accounted for precisely using methods of JCGM 100 (2008).

The new model uses a similar approach to our previously developed MC
uncertainty model for correlated colour temperature (CCT) (Kärhä et al., 2017b).
In the article, we demonstrated the use of the model for calculating the CCT
of a source resembling standard illuminant A. For standard illuminant A, the case representing
uncertainty with unfavourable correlations in CCT was found at N=3. On the contrary, for the ozone retrieval the deviation at N=1
produces the largest uncertainty, which is in a way trivial compared with
CCT. The use of a set of sine functions as base functions was originally
developed for the more complicated situation of CCT, where it was not known
where the unfavourable uncertainty would be. When we now have
analysed the situation, an uncertainty arising from unfavourable correlations
in the ozone retrieval could be modelled as well, e.g. using a combination of
full spectral deviation, a simple slope, and a parabola as the
deviation function mimicking unfavourable correlations. This is
discussed in more detail in Appendix B.

The new MC method for estimating uncertainties in TOC in the presence of
systematic spectral deviations provides more complete estimations of the
uncertainty budget compared with the traditionally used methods. The TOC
values retrieved from different instrument data were well in agreement within
the uncertainties estimated with the new MC method. Although the TOC results
obtained using different instruments have good agreement, these differences
do not represent the uncertainty in the absolute TOC scale. The TOC
uncertainties we have estimated cover possible offsets in the absolute
TOC scale, arising from the uncertainties in the ozone absorption
cross section and extraterrestrial spectrum that are the dominating
components in the uncertainty budget.

The data sets of Figs. 3 and 6 (for 17 September 2016) are parts
of a larger data set containing the results on all days 12–30 September 2016 of
the comparison campaign. Because more publications will be written on the results
of the comparison campaign, the data cannot yet be uploaded to a public repository. Data used in this article can be provided upon request by email to Anna Vaskuri (anna.vaskuri@aalto.fi).

Appendix A: Selecting the least squares fitting method

Two of the instruments being compared suffer from stray light and noise that
distort the TOC results. In earlier studies, e.g. by Herman et al. (2015), stray
light in array spectroradiometers has been noted to decrease TOC values at
large solar zenith angles, resulting in an inverted U-shape dependence of the
diurnal TOC variation. The effect of stray light can be compensated by
reducing the effect of the short-wavelength tail, either by limiting the
wavelength range or the dynamic range used, or by weighting the results. We
studied various weightings and selection methods for data in order to find an
objective way to perform the ozone retrieval method and to analyse the
results.

Figure A1 shows the TOC results as a function of time analysed
with three different weighting methods for least squares minimization. The
methods include a relative least squares fitting (RLS) in
Eq. (8) with $w\left(\mathit{\lambda}\right)=E{\left(\mathit{\lambda}\right)}^{-\mathrm{2}}$, RLS fitting with the dynamic range (DR) limited to
avoid stray light, and an ordinary least squares fitting method (OLS) with w(λ)=1.

TOC values estimated for QASUME in Fig. A1a have no significant
solar zenith angle dependence, and limiting the dynamic range of RLS fitting
only affects individual TOC values in the early morning and late afternoon
compared with the RLS fitting over the complete spectral range. Using the OLS fitting
method, the diurnal variation of the TOC remains, but the values are
underestimated by a constant factor of 1.013.

TOC values estimated for BTS in Fig. A1b and AVODOR in (c) have
severe dependence on the solar zenith angle when using the RLS fitting method.
The solar zenith angle dependence decreases to approximately half when
limiting the dynamic range to exclude the baseline noise from the fitting.
The best results are obtained by using the OLS minimization, which practically
removes the solar zenith angle dependence for both BTS and AVODOR but
introduces an offset to TOC results. An almost similar offset in TOC results was
noted for QASUME. The OLS method violates the heteroscedasticity of
our data sets, since we know that the absolute spectral uncertainties are not
constant at the studied wavelength range. The only reason to use the OLS
method for array spectroradiometers is to reduce the effect of stray light
from TOC results. Hence, we correct the TOC results obtained with the OLS
method with correction factors estimated from the ratios of TOC values
determined from the local noon spectra using both RLS and OLS methods. The
correction factors were averaged over 10 samples around noon, being 1.006 for
BTS and 1.013 for AVODOR, with standard deviations of 0.1 % (0.28 DU) and
1.1 % (3.10 DU), respectively. This correction makes the TOC results
comparable with devices analysed using the RLS method.

For QASUME, we use RLS minimization with the dynamic range limited to 4
orders of magnitude, as it uses most of the useful undistorted data. This is
also consistent with the earlier methods used with monochromator-based
spectroradiometers, e.g. by Huber et al. (1995).

Figure A1TOC values during the day estimated using different weightings in
the least squares minimization for QASUME (a), BTS (b) and
AVODOR (c). TOC values for Brewer #183 are plotted as black crosses
for comparison. The colour codes and the associated figure legends denote the
weighting and the dynamic range (DR) used.

In the MC uncertainty analysis, it is possible to use other orthogonal sets
of functions instead of sinusoidal functions, such as Chebyshev polynomials
shown in Fig. B1. A Chebyshev polynomial of the first
kind T_{j}(λ) of order j (Kreyszig, 2006,
p. 209) is defined as

where λ_{1} is the short-wavelength limit and λ_{2} is the
long-wavelength limit for the spectra measured. To create arbitrary spectral
deviations with unity variance, each Chebyshev polynomial, except for ${g}_{\mathrm{0}}\left(\mathit{\lambda}\right)={T}_{\mathrm{0}}\left(\mathit{\lambda}\right)=\mathrm{1}$, needs to be
normalized to unity variance as

where σ_{j} is the standard deviation of T_{j}(λ). In practice, Chebyshev polynomials in Fig. B1
can be generated fast using recurrence (e.g. Fateman, 1989) as

where T_{0}(λ)=1, and ${T}_{\mathrm{1}}\left(\mathit{\lambda}\right)=\left(\mathrm{2}\mathit{\lambda}-{\mathit{\lambda}}_{\mathrm{1}}-{\mathit{\lambda}}_{\mathrm{2}}\right)/\left({\mathit{\lambda}}_{\mathrm{2}}-{\mathit{\lambda}}_{\mathrm{1}}\right)$ is a straight line. According to Eq. (B2) the scaling with the
standard deviation is performed after
generating the Chebyshev polynomials.

Each base function of the cumulative Fourier series in
Eq. (10) was formed with sine (odd) and cosine (even)
terms (Kreyszig, 2006, p. 628),

where the phase ϕ_{i} is an equally distributed MC variable between 0
and 2π. Hence, the analysis based on Chebyshev polynomials to be
compatible with the sinusoidal approach, each base function f_{i}(λ) with index i≥1 is formed by the combination of odd
(2i−1) and even (2i) terms as

where the weights cos(ϕ_{i}) and sin(ϕ_{i}) set the variance of f_{i}(λ) to unity.
These base functions f_{i}(λ) can be used with
Eqs. (10) and (13).

Figure B1First five Chebyshev polynomials with unity variance corresponding
to Fig. 4 with sinusoidal base functions.

Figure B2 compares standard uncertainties in TOC obtained by
generating arbitrary spectral deviations using Chebyshev polynomials as base
functions (circles and triangles), formed using
Eq. (B5), with those obtained by using sinusoidal
base functions (crosses). The uncertainties change slightly at the lowest
complexity orders of deviations when sinusoidal base functions are replaced
with Chebyshev polynomials but essentially the results are similar.

Figure B2Standard uncertainties in TOC simulated using a local noon spectrum
of QASUME with Chebyshev polynomials as base functions. The TOC uncertainties
with the input standard deviation of 1 % in the spectral irradiance values
are shown as green circles and the TOC uncertainties with the input standard
deviation of 1 % in the ozone absorption cross section are shown as magenta
triangles. Standard uncertainties simulated with sinusoidal base functions
(black crosses) from Fig. 5a are plotted for
comparison.

Peter Sperfeld from PTB is acknowledged for measuring and providing the BTS
data set. Alberto Redondas and all personnel from Izaña Atmospheric
Research Center, AEMET, Tenerife, Canary Islands, Spain, are acknowledged for
measuring and providing the Brewer #183 data set and the environmental
parameters such as the sonde data. Anna Vaskuri is grateful for the
grant by the Emil Aaltonen Foundation, Finland. This work has been supported
by the European Metrology Research Programme (EMRP) within the joint research
project ENV59 “Traceability for atmospheric total column ozone” (ATMOZ).
The EMRP is jointly funded by the EMRP participating countries within EURAMET
and the European Union.

Edited by: Lok Lamsal
Reviewed by: four anonymous referees

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