ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-2803-2016Estimates of free-tropospheric NO2 and HCHO mixing ratios derived from
high-altitude mountain MAX-DOAS observations at midlatitudes and in the
tropicsSchreierStefan F.schreier@iup.physik.uni-bremen.deRichterAndreashttps://orcid.org/0000-0003-3339-212XWittrockFolkardBurrowsJohn P.https://orcid.org/0000-0003-1547-8130Institute of Environmental Physics, University of Bremen, Bremen, GermanyStefan F. Schreier (schreier@iup.physik.uni-bremen.de)4March2016165280328179October201512November201511February201624February2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/2803/2016/acp-16-2803-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/2803/2016/acp-16-2803-2016.pdf
In this study, mixing ratios of NO2 (XNO2) and HCHO (XHCHO)
in the free troposphere are derived from two multi-axis differential optical
absorption spectroscopy (MAX-DOAS) data sets collected at Zugspitze
(2650 m a.s.l., Germany) and Pico Espejo (4765 m a.s.l., Venezuela). The estimation
of NO2 and HCHO mixing ratios is based on the modified geometrical
approach, which assumes a single-scattering geometry and a scattering point
altitude close to the instrument altitude. Firstly, the horizontal optical
path length (hOPL) is obtained from O4 differential slant column
densities (DSCDs) in the horizontal (0∘) and vertical
(90∘) viewing directions. Secondly, XNO2 and XHCHO are
estimated from the NO2 and HCHO DSCDs at the 0∘ and
90∘ viewing directions and averaged along the obtained hOPLs. As
the MAX-DOAS instrument was performing measurements in the ultraviolet
region, wavelength ranges of 346–372 and 338–357 nm are selected for the
DOAS analysis to retrieve NO2 and HCHO DSCDs, respectively. In order to
compare the measured O4 DSCDs and moreover to perform some sensitivity
tests, the radiative transfer model SCIATRAN with adapted altitude settings
for mountainous terrain is operated to simulate synthetic spectra, on which
the DOAS analysis is also applied. The overall agreement between measured
and synthetic O4 DSCDs is better for the higher Pico Espejo station
than for Zugspitze. Further sensitivity analysis shows that a change in
surface albedo (from 0.05 to 0.7) can influence the O4 DSCDs, with a
larger absolute difference observed for the horizontal viewing direction.
Consequently, the hOPL can vary by about 5 % throughout the season, for
example when winter snow cover fully disappears in summer. Typical values of
hOPLs during clear-sky conditions are 19 km (14 km) at Zugspitze and 34 km
(26.5 km) at Pico Espejo when using the 346–372 (338–357 nm) fitting
window. The estimated monthly values of XNO2 (XHCHO), averaged
over these hOPLs during clear-sky conditions, are in the range of 60–100 ppt
(500–950 ppt) at Zugspitze and 8.5–15.5 ppt (255–385 ppt) at Pico Espejo.
Interestingly, multi-year-averaged monthly means of XNO2 and XHCHO
increase towards the end of the dry season at the Pico Espejo site,
suggesting that both trace gases are frequently lifted above the boundary
layer as a result of South American biomass burning.
Introduction
Tropospheric nitrogen oxides (NOx= NO + NO2) are released
from various human activities (e.g., the burning of oil, coal, gas, and
wood). Natural sources of NOx include lightning, wildfires, and
microbial activity in soils (Lee et al., 1997). Nitric oxide (NO) is the
predominant part of NOx released from these sources, but it is quickly
converted to nitrogen dioxide (NO2) by reaction with ozone (O3).
The major part of NOx emissions remains in the boundary layer (BL).
However, in addition to NOx from lightning, significant amounts of
NOx from surface sources occasionally reach the free troposphere (FT)
due to meteorology, orographic uplift of BL air masses, or pyro-convection.
Formaldehyde (HCHO) is the most abundant carbonyl in the atmosphere and a
valuable tracer of volatile organic compound (VOC) sources as it is produced
from the oxidation of VOCs. Major sources of HCHO include the oxidation of
methane (CH4) (Lowe and Schmidt, 1983) and non-methane hydrocarbons
(NMHCs) (Fried et al., 2003). While HCHO formation in the remote marine
atmosphere is dominated by CH4 oxidation (Weller et al., 2000), the
major precursor of HCHO in the continental BL is isoprene (Guenther et al.,
2006). Further sources of HCHO are direct emissions from biomass burning
(Andreae and Merlet, 2001), anthropogenic combustion processes (Anderson et
al., 1996), and vegetation (Seco et al., 2007).
As NO2 and HCHO are good indicators of tropospheric pollution,
extensive ground-based measurement networks have been exploited in order to
monitor their amounts and distributions (WMO, 2010). While ground-based in
situ measurements represent rather surface and/or local amounts of NO2
and HCHO, remote-sensing techniques such as the multi-axis differential
optical absorption spectroscopy (MAX-DOAS) have the potential to derive both
horizontal and vertical distributions of these trace gases in the immediate
vicinity of the station (Hönninger et al., 2004; Wittrock et al., 2004).
There exist some records of NO2 in the free troposphere from aircraft
in situ measurements (Singh et al., 2009) and balloon-borne soundings
(Kritten et al., 2010). However, these observations are sparse and have mainly
been performed during short field campaigns. Recently, global free-tropospheric
NO2 mixing ratios have been obtained from satellite data using the
cloud-slicing technique (Choi et al., 2014). While the overall order of
magnitude of satellite-derived values is similar to those taken from
aircraft in situ measurements, the agreement of individual comparisons is
not always good. Data records of HCHO in the free troposphere are also rare
and deduced at different seasons, regions, and altitudes (Schuster et al.,
1990; Arlander et al., 1995; Singh et al., 2001).
Although the number of ground-based in situ and remote-sensing instruments
with which to observe tropospheric pollution has increased in recent years, the
level of knowledge concerning representative NO2 and HCHO amounts in
the FT is still rather low. There are two reasons for this:
firstly, ground-based in situ measurements performed at high-altitude sites
are influenced by air masses that are lifted along the mountain slope to the
station (Reidmiller et al., 2010) and, thus, often represent rather
(polluted) BL air. Secondly, MAX-DOAS measurements have been mainly focused
on the BL, where the vast majority of tropospheric pollution is found.
Only very recently has a novel approach to estimate NO2 mixing ratios in
the FT from MAX-DOAS measurements been proposed (Gomez et al., 2014). In that
study, the retrieval of O4 and NO2 differential slant column
densities (DSCDs) was performed in the visible part of the electromagnetic
spectrum on the basis of data from nine summer days in Izaña having low
aerosol levels. The MAX-DOAS observations of that study were performed at a
high-altitude site in the subtropics. The great advantage of the method by
Gomez et al. (2014) is the averaging of trace gas concentrations over a long
horizontal path (on the order of a few tens of kilometers), thus
minimizing local effects of BL air masses that are lifted to the station. A
follow-up study to investigate the seasonal evolution of NO2 in the
free troposphere at the Izaña site was carried out by Gil-Ojeda et al. (2015).
That study reported background NO2 mixing ratios in the range
of 20–45 ppt, with lowest (highest) NO2 amounts in winter (summer).
The aim of the present study is to extend these two studies and provide
estimates of NO2 and HCHO mixing ratios in the FT. For this purpose,
two long-term MAX-DOAS data sets from two high-altitude sites located at
midlatitudes and in the tropics are analyzed. In contrast to Gomez et al. (2014)
and Gil-Ojeda et al. (2015), the retrieval of O4, NO2, and HCHO
DSCDs in our study is performed in the ultraviolet (UV) range of the
electromagnetic spectrum. Here, we obtain monthly means of NO2 and HCHO
mixing ratios during clear-sky conditions at the two measurement sites,
focus on the seasonal variability, and compare our values with values
reported in the literature. To the best of the authors' knowledge, this is
the first study presenting HCHO mixing ratios in the FT based on long-term
MAX-DOAS measurements.
In Sect. 2, the MAX-DOAS system and the two measurement
locations are described. The modified geometrical approach as proposed by
Gomez et al. (2014) to estimate horizontal optical path lengths (hOPLs) and
mixing ratios of NO2 from MAX-DOAS DSCDs is briefly summarized in Sect. 3.
The simulation of synthetic spectra using the radiative transfer model
SCIATRAN is described in Sect. 4. Details about the analysis of measured and
synthetic spectra using the DOAS method are given in Sect. 5. The results of
this study – including a comparison of measured and synthetic O4 DSCDs
and the analysis of hOPLs, NO2, and HCHO mixing ratios for both
locations – are presented in Sect. 6. A short summary and conclusions are
given in Sect. 7.
Instrumentation and location
The MAX-DOAS system – which was operated at the two high-altitude stations
of Zugspitze, Germany (47.5∘ N, 11∘ E, 2650 m a.s.l.), and
Pico Espejo, Venezuela (8.5∘ N, 71∘ W, 4765 m a.s.l.) –
comprised a temperature-stabilized grating spectrometer equipped with a
cooled charge-coupled device (CCD) detector. The spectrometer and CCD detector were housed inside a
building and connected to a telescope unit, which was mounted outdoors. In
addition to the zenith window, a horizon (off-axis) window was implemented
in the telescope to allow for the scanning of different off-axis directions,
which were controlled by a motorized mirror. Scattered sunlight entering the
telescope, either from the zenith or horizon window, is focused by a lens to
reduce the field of view before it reaches the optical fiber mount. During
the instrument's operation at the two measurement stations, a field of view
with an opening angle of ∼1∘ was achieved (Oetjen,
2009; Peters, 2013). Consequently, the signal in the horizontal path
might be slightly affected by the contribution of trace gas absorption at
lower altitudes (up to 500 m below the measurement stations at the end of
the hOPL). Nevertheless, the mixing ratios as presented in Sects. 6.3 and
6.4 for NO2 and HCHO, respectively, are still considered to be
representative of the free troposphere. As a result of the wavelength range
of the spectrometer covering 321–410 nm, HCHO retrieval is possible, and
the retrieval of O4 and NO2 DSCDs is restricted to the UV spectral
region in this study (see Sect. 5).
In February 2003, the MAX-DOAS instrument was temporarily set up at the
midlatitude site Zugspitze, Germany, where it performed measurements until
the end of July 2003. The underlying intention behind choosing Zugspitze as
a measurement location was the fact that MAX-DOAS measurements of NO2
could be used to validate satellite-derived stratospheric NO2 columns
as most of the NO2 above this altitude is assumed to belong to the
stratosphere. The telescope was orientated at 120∘ azimuth
(east-southeast) throughout the measurement period, and elevation angles
(α) of 0, 4, 7, 16, 30, and 90∘ (zenith) were selected for the scanning
sequence. As there were unfortunately no measurements performed in the
horizontal direction (α=0∘) during February and
March, which are needed for the estimation of NO2 and HCHO mixing
ratios based on the modified geometrical approach
(MGA) (see Sect. 3), the analysis is restricted to April,
May, June, and July at this location.
After successful measurements at Zugspitze, the MAX-DOAS instrument was
transferred to Pico Espejo, Venezuela, and put into operation in March 2004.
Measurements were then made until February 2009. Pico Espejo is located in a
tropical region that is generally unperturbed by tropospheric pollution.
However, Hamburger et al. (2013) observed increased aerosol amounts during
the dry season by analyzing in situ measurements from the same station. They
suggested that these aerosols are dominated by Venezuelan savanna fires and
lifted from lower altitudes to the mountain peak. Therefore, these elevated
aerosol loads can be attributed to BL air masses. At the Pico Espejo
station, consecutive viewing directions of 0, 4, 7, 16, and 90∘ were scanned at a constant
azimuth angle of 180∘ (south). In the year 2006, additional
viewing directions were included, and thus fewer data for both α=0∘ and α=90∘ are available from that date on
(see Sect. 6.2). The meteorological conditions at Pico Espejo with permanent
cloud cover during the wet season (from April to November) limit the data
analysis to the dry-season months December, January, February, and March.
Modified geometrical approach
In a recent study by Gomez et al. (2014), a MGA
to estimate long-path-averaged mixing ratios of trace gases (e.g.,
NO2) from mountain MAX-DOAS measurements was proposed.
Briefly, MGA assumes a single-scattering geometry and a scattering point
altitude close to that of the instrument. In order to validate the former
assumption, Gomez et al. (2014) performed radiative transfer model (RTM)
simulations and found that the calculated hOPL for single and
multiple scattering agrees within 5 % for solar zenith angle (SZA) < 70∘. As the
slant paths of the zenith (α=90∘) and horizontal
(α=0∘) measurements in single-scattering
approximation are identical up to the scattering point, they cancel in the
DSCD when using a zenith sky background
spectrum close in time. The column of the trace gas in the horizontal part
of the optical path can thus be determined. The MGA is a useful concept to
derive free-tropospheric trace gas mixing ratios at high altitudes, where
horizontal MAX-DOAS measurements are possible and aerosol loads are usually
low.
The hOPL is obtained in a first step by subtracting the zenith O4 DSCD
from the horizon O4 DSCD, divided by the number density of O4
(nO4) at the level of the station:
hOPL=DSCDO4,0∘-DSCD(O4,90∘)nO4,
where nO4 is easily calculated by
nO4=(nO2)2=(nair×0.20942)2.
In our study, the zenith measurements are interpolated between the SZAs of the horizontal measurements. Thus, no further
correction with respect to the differences in DSCDs due to a difference in
SZA between the sequentially scans is required. The number density of air
(nair) at the altitude of Zugspitze and Pico Espejo is extracted from
the respective midlatitude summer and tropical reference atmospheric model
profiles of the Air Force Geophysics Laboratory (AFGL) standard atmosphere
(Anderson et al., 1986).
Schematic illustration of the MAX-DOAS observation geometry in a
typical urban and/or rural setting (left) and in mountainous terrain
(right). While the altitude of the instrument's horizontal path (hpath)
corresponds to the ground level (hground) of the surrounding area in
the left sketch, hpath is larger than hground in the right graph.
In the second step, the mean mixing ratio of the trace gas (XNO2 and
XHCHO) is estimated for the level of the station by dividing the
NO2 and HCHO DSCD differences by the hOPL and nair:
XNO2,HCHO=DSCDNO2,HCHO,0∘-DSCDNO2,HCHO,90∘hOPLnair.
Recently, Spinei et al. (2015) have shown that too large O4 slant
column densities are obtained when using laboratory-measured O4
cross sections (e.g., Hermans et al., 1999) in low effective-temperature
conditions. They report a temperature dependence in the O4 absorption
cross section from 231 to 275 K at about 9 ± 2.5 % per 44 K rate in
the 335–390 nm spectral window and suggest a small correction factor of
0.94 ± 0.02 at 231 K when using O4 cross sections measured at room
temperature. Following the recommendations made by Spinei et al. (2015), an
overestimation of up to 6 % in hOPL and, thus, an underestimation in
NO2 and HCHO mixing ratios of up to 6 % can be expected in our study
due to the use of a room temperature O4 cross section.
Gomez et al. (2014) provide a detailed discussion about possible errors that
may occur in the MGA. They report an error in the range of 20 % for SZAs
< 70∘, which steeply increases to about 50 % towards
SZA = 80∘. We follow the recommendations made by Gomez et al. (2014)
and estimate hOPL as well as XNO2 and XHCHO only for SZAs
< 70∘.
Simulation of synthetic spectra
In order to determine the effect of altitude settings and surface albedo on
the O4 DSCDs and moreover to compare the measured with simulated
O4 DSCDs at the two high-altitude stations, synthetic spectra
(intensities) are computed for single clear-sky days using the radiative
transfer model SCIATRAN, which is applied in the 1-D case (Rozanov et al.,
2005).
Briefly, the software package SCIATRAN was developed for modeling radiative
transfer processes in the atmosphere and ocean. The simulations can be
performed for observations made by ground-based, satellite-based,
ship-based, and balloon-borne instruments in the spectral range from UV to
thermal infrared. The latest release of SCIATRAN includes multiple-scattering
processes, polarization, thermal emission, and ocean–atmosphere
coupling (Rozanov et al., 2014).
The major advantage of 1-D RTMs is their low computation time and their
ability to reproduce radiative transfer for clear-sky conditions and over
flat terrains quite well. For mountainous terrains, however, factors such as
altitude and surface albedo are more difficult to treat. The computation of
intensities/radiances for ground-based MAX-DOAS instruments using SCIATRAN
requires, amongst other things, information on the height above sea level. For
a typical measurement station in an urban area, for example, the altitude of
the instrument's horizontal path (hpath) does not differ much
from the ground level (hground) of the surrounding area (Fig. 1, left).
Therefore, hpath would be configured by the input for hground
accordingly, and the position of the instrument in SCIATRAN would be set to
“bottom of the atmosphere”. However, for high-altitude stations such as
Zugspitze and Pico Espejo these settings are no longer valid as they are
both located on mountain peaks and information about the atmosphere below
would be lost (Fig. 1, right). Therefore, the altitude settings in SCIATRAN
are adjusted by assuming that the MAX-DOAS instrument is “flying” at the
respective altitude of the two measurement stations and setting the ground
level to 1500 and 150 m for Zugspitze and Pico Espejo, respectively. The
latter values are chosen and optimized in approximate accordance to a
representative altitude of the respective terrain in the immediate vicinity,
or more specifically below the expected respective light path. The lower
value selected for Pico Espejo is a result of the wide Orinoco plains in
central Venezuela. For the simulations performed in this study, a surface
albedo of 0.05 is used for both locations, which seems reasonable for the
analyzed spring and summer season at Zugspitze and the dry season at Pico
Espejo.
Parameter settings used for the computation of synthetic spectra
with the radiative transfer model SCIATRAN.
SCIATRAN inputRTM modeIntensity/radianceRTM typeSpherical atmosphereExtraterrestrial solar fluxSolar atlas (Kurucz et al., 1984)Wavelength range330–410 nm (UV)Wavelength step0.04 nmForward model trace gasesNO2, O3, O4, BrO, HCHO, and SO2AerosolsNo aerosolsCloudsNo clouds“Flying” altitude4765 m (Pico Espejo), 2650 m (Zugspitze)“Ground” altitude150 m (Pico Espejo), 1500 m (Zugspitze)Surface albedo0.05SCIATRAN observation geometrySolar zenith anglesFrom MAX-DOAS observationsSolar azimuthFrom MAX-DOAS observationsElevation angles0, 4, 7, 16, 30, and 90∘ (zenith)
Fit parameters and cross sections included in the spectral fitting
procedure.
Fit parameterSelectionSpectral range346–372 nm (NO2), 338–357 nm (HCHO)Polynomial degree4Wavelength calibrationSolar atlas (Kurucz et al., 1984)ReferenceNoontime spectrum with smallest SZACross sectionTemperatureData sourceO3223 KSerdyuchenko et al. (2014)NO2220 KVandaele et al. (1996)O4296 KHermans et al. (1999)BrO223 KFleischmann et al. (2004)HCHO297 KMeller and Moortgat (2000)Ring–SCIATRAN
In addition to the altitude configurations, an extraterrestrial solar flux
(Kurucz et al., 1984), a wavelength range between 330 and 410 nm, and a
wavelength step of 0.4 nm are selected for the simulations. The forward
model trace gases NO2, O3, O4, BrO, HCHO, and SO2, but
no aerosols and clouds, are implemented in the model runs. Atmospheric
profiles are taken from the U.S. Standard Atmosphere 1976. The input for
SZAs and solar azimuth angles (SAZs) for the selected days is taken from the
MAX-DOAS measurements of the respective days. A summary of the input
parameters used in SCIATRAN is given in Table 1.
DOAS retrieval of O4, NO2, and HCHO differential slant
column densities
The differential optical absorption spectroscopy (DOAS) method is performed
on the measured and synthetic spectra to obtain O4, NO2, and HCHO
DSCDs, which are the parameters used for the MGA-based estimation of hOPLs
as well as XNO2 and XHCHO at the two high-altitude sites.
Briefly, DOAS is based on absorption spectroscopy following Beer–Lambert's
law of absorption and developed for the determination of atmospheric trace
gas concentrations from remote-sensing measurements of light in the UV,
visible, and near-infrared (NIR) spectral range (Platt and Stutz, 2008, and references
therein). After removing the smoothly varying contributions of Mie and
Rayleigh scattering from the signal, DOAS becomes sensitive to variations in
absorption with wavelength. Due to the fact that individual trace gases have
characteristic spectral signatures, they can be separated from the signal
and quantified in terms of trace gas concentration, integrated along the
atmospheric light path.
The trace gases of interest in our study – O4, NO2, and HCHO – have
spectral fingerprints in the UV and visible spectral range. As mentioned
above, the spectrometer of the MAX-DOAS system at Zugspitze and Pico Espejo
collected scattered sunlight between 321 and 410 nm. Therefore, the DOAS
fits performed in our study are restricted to the UV.
The DOAS method is performed on the measured and synthetic spectra in the
spectral range of 346–372 nm, where O4 and NO2 spectral
fingerprints are obvious and interference with other trace gases is rather
low (Platt and Stutz, 2008, and references therein). For the retrieval of
HCHO, the spectral window of the DOAS analysis is shifted to 338–357 nm.
Absorption cross sections of O3 at 223 K, NO2 at 220 K, O4 at 296 K,
BrO at 223 K, and HCHO at 297 K, as well as a ring spectrum and a polynomial
of order 4, are included in the nonlinear least-squares fitting procedure.
For the removal of Fraunhofer lines in the solar spectrum, the measured and
synthetic spectra are divided by a reference spectrum (here, a noontime
spectrum at α=90∘ with the smallest SZA is used). The
resulting output of the DOAS fit is the difference in slant column amounts
of O4, NO2, and HCHO between the measured/synthetic and reference
spectrum, and thus referred to as DSCD. A summary of fit parameters and
cross sections used for the DOAS analysis is summarized in Table 2.
Exemplary fit results of the DOAS analysis in the 346–372 nm
(left) and 338–357 nm (right) fitting window for a horizontal noontime
MAX-DOAS spectrum (α=0∘, SZA = 37.86∘)
as measured on 16 April 2003 at Zugspitze under elevated NO2
(DSCD =1.228×1016 molec cm-2) and HCHO (DSCD =1.737×1016 molec cm-2) pollution. The red (upper panel), green (middle panel), and cyan (middle
panel) lines show the O4, NO2, and HCHO cross sections,
respectively, as scaled to the O4, NO2, and HCHO absorptions
detected in the measured spectrum (black lines). The residuals with
calculated root mean square (rms) errors of 1.2×10-4 (346–372 nm) and
1.3×10-4 (338–357 nm) are also shown (lower panel).
An example of the spectral DOAS analysis in the 346–372 nm fitting window is
shown in Fig. 2 (left) for a single spectrum, which was measured under
elevated NO2 pollution on 16 April 2003 (SZA =37.86∘,
α=0∘) at Zugspitze. For the retrieval of O4 and
NO2 DSCDs on that day, a spectrum in zenith direction (α=90∘) taken at noon (SZA = 37.36∘) is used as a
reference. Fit results of the DOAS analysis for the same measurement, but in
the 338–357 nm fitting window, yield similar values for the residuals and
root mean square (rms) and indicate that HCHO amounts were also higher than usual on that day
(Fig. 2, right).
ResultsMeasured and synthetic O4 differential slant column
densities
In this section, two sensitivity tests are carried out with the RTM SCIATRAN
to determine the effect of varying altitude settings and surface albedo on
the O4 DSCDs at α=0∘ and α=90∘ and, thus, on the estimation of hOPL. The settings used for the
simulation of synthetic spectra are summarized in Table 1.
Two different altitude settings as represented schematically in Fig. 1 are
used as input for SCIATRAN to simulate O4 DSCDs at the Pico Espejo
site. The resultant O4 DSCDs as retrieved in the 346–372 nm fitting
window are shown in Fig. 3 for α=0∘ (red) and α=90∘ (black). While the solid line represents the scenario
that assumes hpath=hground, the dashed line denotes the
modified settings (hpath≫hground).
Clearly, the O4 DSCDs are underestimated when the settings for common
rural and/or urban measurement environments (hpath=hground)
are assumed for the operation of SCIATRAN. This is the result of light path
enhancements due to multiple scattering. For the horizontal viewing
direction (α=0∘), the absolute difference of the two
scenarios is on the order of 400×1040 molec2 cm-5 for SZAs
between 20 and 70∘. The absolute differences are
comparatively low for the vertical viewing direction (α=90∘) but increase towards SZA =70∘ to about 100×1040 molec2 cm-5. Therefore, hOPL is underestimated by about
15 % when the atmosphere below is cut off in the RTM.
Simulated O4 DSCDs (346–372 nm fitting window) at the Pico
Espejo site (left) for α=0∘ (red) and α=90∘ (black), where the solid (hpath=hground) and
dashed (hpath≫hground) lines denote the
two different scenarios as described in the text and shown in Fig. 1. The
absolute differences between the two scenarios are shown in the right panel
for the horizontal and vertical viewing direction.
Simulated O4 DSCDs (346–372 nm fitting window) at the
Zugspitze site for α=0∘ (red) and α=90∘ (black) assuming a surface albedo of 0.05 (solid line) and 0.7
(dashed line). The absolute differences between the two scenarios are shown
in the right panel for the horizontal and vertical viewing direction.
Diurnal evolution of O4 DSCDs for α=0∘
and α=90∘ for single days with clear-sky conditions at
Zugspitze (left) and Pico Espejo (right). Here, O4 DSCDs are retrieved
from measured (red and black lines for α=0∘ and
α=90∘, respectively) and synthetic (orange and gray
lines for α=0∘ and α=90∘,
respectively) spectra in the 346–372 nm fitting window and only shown for
SZAs < 70∘ (see Sect. 3).
As the Zugspitze site is located in the northern Alps at an altitude of
almost 3000 m a.s.l., snow cover at the mountain peak and at lower altitudes
in the immediate vicinity is common during winter and, as a consequence, the
surface albedo can change significantly throughout the season. Therefore,
another sensitivity test with SCIATRAN is performed to evaluate the effect
of a changing surface albedo on the O4 DSCDs and the corresponding hOPL
for this measurement site. The results are shown in Fig. 4 for O4 DSCDs
as retrieved in the 346–372 nm fitting window and simulated for α=0∘ (red) and α=90∘ (black), where the
dashed and solid lines represent a surface albedo of 0.05 and 0.7,
respectively, as selected for the SCIATRAN simulations. For the O4 DSCD
in the horizontal viewing direction (α=0∘), the
absolute differences are up to 400×1040 molec2 cm-5 for
SZAs towards 70∘. Smaller absolute differences of up to 150×1040 molec2 cm-5 are expected for the vertical viewing
direction (α=90∘). Consequently, a decrease in
surface albedo from 0.7 to 0.05 can increase the hOPL as estimated with MGA
by about 5 %. The lower value of hOPL for a surface albedo of 0.7 is the
result of scattering at the ground, which creates much shorter light paths.
These light paths penetrate into lower levels of the atmosphere, and thus a
signal of the BL could contribute to such a measurement. Consequently,
measurements over brighter surfaces are less representative with respect to
the analysis of trace gas amounts in the free troposphere when the
mountain DOAS approach is applied. This has implications for the interpretation of
hOPLs as estimated in mountainous terrains with changing surface albedo
throughout the season.
Horizontal optical path length (hOPL) for two single days at
Zugspitze (left) and Pico Espejo (right). Here, hOPL is obtained from
measured (dotted line) and synthetic (solid line) O4 DSCDs in the
346–372 nm fitting window and only shown for SZA < 70∘
(see Sect. 3).
For the comparison of O4 DSCDs, the synthetic spectra are simulated
with the modified altitude settings, which assume that the MAX-DOAS
instrument is flying at the respective altitude of the two measurement
sites and additionally include an altitude for the ground level to account
for the atmosphere below the horizontal path (see Sect. 4).
The diurnal evolution of O4 DSCDs as retrieved in the 346–372 nm
fitting window is shown for single clear-sky days at Zugspitze (left) and
Pico Espejo (right) in Fig. 5. In general, the pattern of the measured
O4 DSCDs (red and black lines denote α=0∘ and
α=90∘, respectively) can be reproduced by SCIATRAN
(orange and gray lines denote α=0∘ and α=90∘, respectively) to a large extent (within 20 % for SZA
< 70∘) for the zenith geometry at both locations. While
the results show good agreement between measured and simulated horizontal
O4 DSCDs at Pico Espejo, the differences are larger at Zugspitze. The
better agreement at the former station could be explained by the higher
altitude and cleaner atmosphere, which is almost solely affected by Rayleigh
scattering. In contrast, scattering by aerosol could play an increasing role
at the altitude of the latter station. Similar results are obtained for the
338–357 nm fitting window (not shown).
Horizontal optical path length
The estimation of hOPL is based on the O4 DSCDs in the horizontal
(α=0∘) and vertical (α=90∘)
viewing directions as obtained from the MAX-DOAS observations at Zugspitze
(April–July 2003) and Pico Espejo (March 2004–February 2009). As
mentioned in Sect. 2, only the dry-season months (December–March)
are considered for the analysis of hOPL and mixing
ratios of NO2 at the Pico Espejo site to avoid the cloud problem.
In Fig. 6, the estimated hOPL as derived in the 346–372 nm fitting window is
shown for 16 April 2003 at the Zugspitze site and for 25 February 2005
at the Pico Espejo site. While Zugspitze was influenced by elevated NO2
pollution in the afternoon of that day, no significant enhancements in
NO2 DSCDs are observed for Pico Espejo throughout the selected day (not
shown). Clearly, the U-shaped curve of the hOPL as obtained from the
measured O4 DSCDs (dotted line) is reproduced by the simulations (solid
line), and the agreement of both curves is within 10 % for the Pico Espejo
case. In general, one would expect smaller hOPLs from the measurements than
from the simulations as performed with SCIATRAN without the inclusion of
aerosols (see Table 1). One possible explanation for the higher measured
values at Pico Espejo could be an aerosol layer below the measurement site.
The differences are much larger for the case at Zugspitze. While the
differences are about 25 % in the morning, they rise to 70 % in the
afternoon, when NO2 pollution increased as well (see Sect. 6.1).
Again, the better agreement at Pico Espejo might be related to the higher
altitude and cleaner atmosphere with negligible Mie scattering.
Time series of hOPL before (blue) and after (cyan) applying the
filter criteria as defined in Sect. 6.2. Here, hOPL is obtained in the
346–372 nm fitting window and shown for the period April–July 2003 at
Zugspitze (upper panel) and for an exemplary dry season (January–March
2005) at Pico Espejo (lower panel).
Gomez et al. (2014) have simulated the hOPL for a single wavelength in the
visible spectral range (440 nm) and also found a U-shaped curve with higher
values of hOPL towards larger SZAs. However, the distances estimated in
their study are larger by a factor of about 2, although the Izaña
station is located at a lower altitude (2373 m a.s.l.) than the Zugspitze
and Pico Espejo sites. This means that trace gas concentrations are averaged
over a shorter distance in the UV than in the visible, even at higher
altitudes. Consequently, MAX-DOAS measurements in the UV are representative
of free-tropospheric trace gas concentrations to a lesser extent than in the
visible as the influence of BL air masses that are lifted to the station is
larger for the shorter horizontal path. Nevertheless, the hOPL in the UV
during clear-sky conditions is still sufficiently long that NO2 close
to the station is considered to have a negligible influence.
The hOPLs as derived in the 346–372 nm range are illustrated in Fig. 7 for the
entire period at Zugspitze (April–July 2003) and for one dry season at
Pico Espejo (January–March 2005) in the upper and lower panels,
respectively. Clearly, higher hOPL values are obtained from the measurements
at Pico Espejo (> 40 km) than for those at Zugspitze
(> 20 km), which is simply explained by the higher elevation of
the former station. While the highest values are observed during clear-sky
conditions, low hOPLs are generally connected to clouds. Negative values
arise when the retrieved O4 DSCD at α=0∘ is
negative or lower than O4 DSCD at α=90∘, which
can happen due to thick stratus clouds with cloud top heights that overtop
the altitude of the station and, thus, particularly affect the horizontal
MAX-DOAS measurements.
Monthly averaged hOPLs at Zugspitze and multi-year-averaged
(2004–2009) monthly means of hOPLs at Pico Espejo as a function of spatially
averaged aerosol optical depth (AOD) over selected regions (see Table 3).
The standard deviation of daily and monthly means is given for Zugspitze and
Pico Espejo (see Table 3). Here, hOPL is obtained from O4
DSCDs as retrieved for the 346–372 nm fitting window, and AOD measurements at
550 nm from MODIS are used.
Since the estimation of NO2 and HCHO mixing ratios only makes sense
when the sky in the horizontal and vertical viewing directions is neither
influenced by clouds nor by high aerosol amounts, the data are filtered for
clear-sky cases. For further analyses, only those hOPL values are considered
where the rms error of the DOAS fit for α=0∘ is smaller than 5×10-4, O4 and NO2/HCHO DSCDs
in the vertical viewing direction (α=90∘) are
positive, and NO2/HCHO DSCDs at α=90∘ are smaller
than NO2/HCHO DSCDs at α=0∘. As no instruments
with which to obtain information on atmospheric visibility were available at the
stations, we have tested different lower limits of hOPL as a filter
criterion and found that 17.5 and 30 km in the 346–372 nm fitting window
and 12.5 and 22.5 km in the 338–357 nm fitting window are good
compromises for Zugspitze and Pico Espejo, respectively (see Fig. 7). All
data points falling below these thresholds are attributed to high aerosol
amounts and/or clouds and, thus, are removed from the analysis of mixing
ratios. As mentioned in Sect. 3, only data for SZA < 70∘
are included in the analysis. Due the fact that cloudiness is common at
Zugspitze during spring and summer (April–July 2003), the absolute
number of data points is vastly reduced from 11 112 to 366 (320) after
applying these filter criteria in the 346–372 nm (338–357 nm) fitting
window. Nevertheless, the remaining data are of good quality and provide
important insights into the free troposphere at this station with respect to
hOPL, XNO2, and XHCHO. The loss of data is considerably less for
the Pico Espejo site, where the absolute number of data points decreases
from 8364 to 1700 (1682), 10 258 to 944 (354), 2432 to 371 (142), 3019 to 563
(273), and 2081 to 315 (140) in the 346–372 nm (338–357 nm) fitting window
after applying the filter criteria for the dry seasons (December–March)
of the years 2004–2005, 2005–2006, 2006–2007, 2007–2008, and 2008–2009,
respectively.
Arithmetic mean and standard deviation of the parameters hOPL,
XNO2, and XHCHO as estimated from the high-altitude MAX-DOAS
measurements as well as aerosol optical depth (AOD) and fire radiative power
(FRP) from MODIS on board Terra and Aqua satellites. Both AOD and FRP are
averages over monthly means obtained from MODIS on board Terra (10:30 LT)
and Aqua (13:30 LT) satellites. The standard deviation of daily and monthly
means is given for Zugspitze and Pico Espejo, and the numbers
in brackets are latitude and longitude as defined for the regions selected
for spatially averaging AOD and FRP.
The monthly means (April–July 2003) and multi-year-averaged
(2004–2009) monthly means (December–March) of hOPL as derived in the
346–372 nm fitting window for clear-sky conditions as defined in the
paragraph above are shown in Fig. 8 as a function of aerosol optical depth
(AOD) for Zugspitze (left) and Pico Espejo (right). AOD is
defined as the integrated extinction coefficient over a vertical column and,
thus, does not say anything about the vertical profile of the aerosol load.
As for NO2, most of the aerosol load is located in the BL and generally
close to its sources. For example, Gonzi et al. (2015) have estimated
biomass burning injection heights using active fire area and fire radiative
power from MODIS (Moderate Resolution Imaging Spectroradiometer) data in combination with a parameterized plume rise model.
They found that 80 % of the injections remain within the local boundary
layer. Consequently, trace gases and aerosols are only lifted into the free
troposphere when the active fire area and convective heat flux exceed a
certain threshold. For the Pico Espejo site, it is expected that the
possibility of injections reaching the free troposphere increases towards
the end of the dry season with increasing fire activity. Due to a lack of
ground-based measurements of AOD at the two stations that are representative
for the altitude of the station, monthly gridded means of AOD from the MODIS
instrument on board Terra and Aqua satellites have been downloaded from
ftp://ladsweb.nascom.nasa.gov/allData/51/ (Remer et al., 2008) and averaged
over both products and selected regions (see Table 3). The MODIS instruments
have 36 channels covering the spectral range from 410 to 14 400 nm and
representing three different spatial resolutions of 250 m, 500 m, and 1 km.
The AOD product used in our study is retrieved at 550 nm and has a spatial
resolution of 250 m. Again, the MODIS product provides an extinction
coefficient integrated over a vertical column with probably most of the
aerosol load being located below the measurement stations. Nevertheless, the
effect of aerosols on hOPL can still be quantified for the Pico Espejo site
as about 20 % of the aerosol load is expected to be found in the free
troposphere, following the recommendations made by Gonzi et al. (2015).
While the estimated hOPLs increase throughout the season at Zugspitze, the
results show a decrease of hOPLs towards the end of the dry season at Pico
Espejo (Fig. 8). Clearly, AOD seems to be the main driver affecting hOPL in
both cases. The decrease in AOD at Zugspitze might be due to an overall
decrease in residential wood burning towards summertime in that region
(Szidat et al., 2007). On the other hand, the increase in AOD at Pico Espejo
is connected to increased biomass burning towards the end of the dry season
in that region (Hamburger et al., 2013). Interestingly, there is no
relationship between hOPL and AOD observed for the 338–357 nm fitting window
(see Table 3). One reason could be the larger uncertainties in O4
DSCDs, which are used to obtain hOPL.
Long-path-averaged NO2 mixing ratios
The monthly averaged XNO2 as a function of AOD and the
multi-year-averaged monthly means of XNO2 as a function of fire radiative power
(FRP) are shown in Fig. 9 for Zugspitze (left) and Pico Espejo (right).
FRP is defined as the radiant component of energy release from
a fire (Kaufmann et al., 1998).
Monthly averaged values of XNO2 at Zugspitze are 93.5 ± 49.8
and 102 ± 60.5 ppt in the spring months April and May, respectively,
whereas lower values of 59.6 ± 55.6 and 61.8 ± 28.9 ppt are
derived for the summer months June and July. As pollution at the Zugspitze
site is mainly anthropogenically induced, the general co-occurrence of
higher AOD and higher XNO2 is not surprising. Clearly, higher values of
XNO2 are observed during the spring months April and May, which could
be explained by both higher combustion due to residential burning and longer
lifetime of NO2. While the values of the months April, May, and July
are linearly related to a large extent, either AOD is higher or XNO2 is
lower than expected from the linear relation in June. This slight deviation
could be related to the fact that air masses with less NO2 and/or more
aerosols crossed the light path of the instrument in June. However, no
significant differences in the origin of air masses between the different
months could be found by exploring backward trajectories for the days of
measurements that remain after applying the filter criteria to the data (not
shown). The y intercept of the linear least-squares fit (excluding June) on
the order of 20 ppt (not shown) provides some information on the background
levels of XNO2 in this region and at this altitude. In comparison,
Gomez et al. (2014) and Gil-Ojeda et al. (2015) have reported NO2
mixing ratios between 20 and 45 ppt for the clean subtropical FT at a
similar altitude, which is in good agreement with the background value
obtained for Zugspitze, which is located more than 2000 km north of the
Izaña station.
Monthly averaged XNO2 at Zugspitze as a function of AOD and
multi-year-averaged (2004–2009) monthly means of XNO2 at Pico Espejo as
a function of spatially averaged fire radiative power (FRP) over selected
regions (see Table 3). The standard deviation of daily and monthly means is
given for Zugspitze and Pico Espejo (see Table 3).
The multi-year-averaged values of XNO2 for the higher-elevated station
Pico Espejo are somewhat smaller when compared to XNO2 values at
Zugspitze (Table 3). As observed for Zugspitze, there is a positive linear
relationship with AOD (not shown), suggesting a background value of 5.5 ppt.
The multi-year-averaged values of XNO2 as a function of multi-year
averaged FRP is illustrated in Fig. 9. Clearly, XNO2 increases with
increasing fire activity towards the end of the dry season. The calculation
of a linear least-squares fit reveals a y intercept of 4 ppt, which is in
agreement with the y intercept as derived for XNO2 vs. AOD.
Although the long-path-averaged NO2 mixing ratios at Pico Espejo are
estimated at an altitude of almost 5000 m a.s.l. and are thus clearly above
the boundary layer, it seems that the FT is regularly affected by biomass
burning emissions in this region.
Diurnal evolution of XNO2 (for SZA < 70∘
but without applying the other filter criteria as described in Sect. 6.2) on
16 April 2003 at Zugspitze as derived using the 338–357 nm (red) and
346–372 nm (blue) fitting windows.
XNO2 as retrieved in the 346–372 nm fitting window (blue) is compared
with XNO2 as derived in the 338–357 nm fitting window (red) for a
polluted day at Zugspitze (see Fig. 10). In general, the shape of both
curves is similar to a large extent, with lowest (highest) values observed in
the morning (late afternoon). Although the spectral signatures of NO2
and O4 are less pronounced and interference with other gases is
stronger in the 338–357 nm fitting window, mixing ratios of NO2 are
still in good agreement with XNO2 obtained in the 346–372 nm fitting
window.
Long-path-averaged HCHO mixing ratios
The same analysis that was applied to derive NO2 mixing ratios in the
free troposphere was also applied to HCHO. Due to its characteristic
spectral signatures at lower wavelengths, the retrieval of HCHO is shifted
to the spectral range from 338 to 357 nm (see Sect. 5 and Table 2).
Diurnal evolution of HCHO DSCDs for α=0∘ (red) and
α=90∘ (black) on 16 April 2003 at
Zugspitze as retrieved from measured spectra using the 338–357 nm fitting
window.
Diurnal evolution of XHCHO on 16 April 2003 at
Zugspitze without (green) and with (black) applying the filter criteria as
described in Sect. 6.2. The two blue lines indicate SZA = 70∘ on
that day.
Multi-year-averaged (2004–2009) monthly means of XHCHO as a
function of spatially averaged FRP over a selected region (see Table 3)
(left) and multi-year-averaged (2004–2009) monthly means of XHCHO as a
function of XNO2 (right) at Pico Espejo. The standard deviation of
monthly means is given (see Table 3).
In Fig. 11, the diurnal evolution of HCHO DSCDs as retrieved in the
338–357 nm fitting window for α=0∘ (red) and α=90∘ (black) is shown for the same day at Zugspitze. As
observed for NO2, HCHO levels increase in the afternoon. Backward
trajectories as calculated with the Hybrid Single-Particle Lagrangian
Integrated Trajectory (HYSPLIT) online tool (Draxler and Rolph, 2013) using
meteorological data from the NCEP/NCAR 40-year reanalysis project (Kalnay et
al., 1996) indicate that air masses from a more densely populated region at
lower elevation in the northeast moved along the slope of the Bavarian Alps
before crossing the light path of the MAX-DOAS instrument (not shown). While
the increase of HCHO DSCD is obvious for α=0∘ (red),
HCHO DSCD at 90∘ (black) is relatively stable throughout the day,
with slightly negative columns obtained in the morning hours.
The filter criteria, as defined in Sect. 6.2, are applied to obtain
XHCHO during clear-sky conditions. An exemplary diurnal evolution of
XHCHO before (green) and after (black) applying the filter criteria is
shown for the polluted day at Zugspitze in Fig. 12. According to the filter
criteria, the remaining black dots are observed during clear-sky conditions
and are in the range of 400–800 ppt.
Monthly means of XHCHO between April and July are in the range of
500–950 ppt at the Zugspitze site (Table 3). Air sampling of HCHO deduced
from a flight track of the TROPospheric OZone Experiment (TROPOZ-2) above
northern Spain/western France reveals HCHO mixing ratios of ∼400 ppt at a similar altitude, albeit in the winter season (Arlander et al.,
1995). According to the seasonal variability of HCHO at midlatitudes as
obtained from satellite measurements, lowest values of HCHO are observed
during winter months (De Smedt et al., 2015). Therefore, the higher values
observed in our study during spring and summer months are reasonable.
Considerably lower HCHO mixing ratios at Zugspitze are found in April when
compared with values in May, June, and July. This is in good agreement with
HCHO concentrations as obtained from MAX-DOAS measurements at the
high Alpine station Jungfraujoch (Franco et al., 2015). In both cases, the
April values are about 35 % lower than the average of May, June, and July
values.
Multi-year-averaged monthly means of XHCHO as obtained during the dry
season at Pico Espejo are in the range of 255–385 ppt (see Fig. 13 and Table 3) and, thus, well below
the values deduced at Zugspitze. The comparison with HCHO deduced from air
sampling in this region during TROPOZ-2 is quite revealing, with our values
being slightly higher at this altitude. Singh et al. (2001) reported values
of HCHO mixing ratios in the range of 100–200 ppt for this altitude.
However, they used data from a larger region (0–30∘ S,
165∘ E–100∘ W) and focused on the tropospheric
composition over the tropical Pacific Ocean.
Summary and conclusions
In this study, multi-axis differential optical absorption
spectroscopy data sets from two high-altitude stations at midlatitudes
and in the tropics are analyzed for horizontal optical path lengths and
free-tropospheric NO2 and HCHO mixing ratios.
The method is based on the modified geometrical approach, which
assumes a single-scattering geometry and a scattering point altitude close
to the instrument altitude.
For the estimation of hOPL, XNO2, and XHCHO, DSCDs of O4,
NO2, and HCHO in the horizontal (0∘) and vertical
(90∘) viewing directions are obtained with a conventional
differential optical absorption spectroscopy retrieval between 346
and 372 nm for the NO2 and between 338 and 357 nm for the HCHO
retrieval. In order to compare the O4 DSCDs with synthetic data and
moreover to evaluate the effect of a varying surface albedo on the O4
DSCDs, the radiative transfer model SCIATRAN is used. As for the retrieval
of O4 DSCDs from MAX-DOAS measurements, the synthetic spectra are also
analyzed with the DOAS method.
In general, the measured and synthetic vertical O4 DSCDs are in good
agreement (differences < 20%) during clear-sky conditions and for
solar zenith angles smaller than 70∘. While the differences
between measured and synthetic horizontal O4 DSCDs are small at the
higher and “cleaner” Pico Espejo site (< 10 %), they are between
25 and 70 % at Zugspitze. The results of the sensitivity analyses show
that a decrease in surface albedo from 0.7 to 0.05 can increase hOPL by
about 5 %, which can have possible implications for such measurements in
mountainous terrains with seasonally varying snow cover.
Monthly means of hOPL as obtained from the long-term measurements are 19 km
(14 km) at Zugspitze, whereas multi-year-averaged monthly means are 34 km
(26.5 km) at Pico Espejo for the 346–372 nm (338–357 nm) fitting window. At both high-altitude sites, hOPL as derived in the 346-372 nm
fitting window decreases as a function of increasing aerosol amounts. In
contrast, no relationship is found for the 338–357 nm fitting window, which
could be explained by the larger uncertainties in O4 DSCDs.
Long-path-averaged mixing ratios of NO2 are obtained using the
estimated hOPL. The monthly means and multi-year-averaged monthly means of
XNO2 are in the range of 60–100 and 8.5–15.5 ppt at Zugspitze and
Pico Espejo, respectively. A statistically significant linear relationship
between aerosol optical depth and XNO2 is obtained for the
measurements from both measurement sites, suggesting that both NO2 and
aerosols are closely connected to air pollution in these regions and at the
respective altitude level. The results of a linear least-squares fit
performed on the averaged data show that upper limits for the background
free-troposphere NO2 mixing ratios are about 20 and 4.5 ppt for
Zugspitze and Pico Espejo, respectively, where the former value is in good
agreement with background NO2 mixing ratios reported by Gomez et al. (2014)
and Gil-Ojeda et al. (2015) for a similar altitude at the Izaña
station (28.3∘ N, 16.48∘ W, 2373 m a.s.l.). The latter
site on Tenerife is relatively remote, whereas Zugspitze is farther north,
but both are probably probing on average similar free-tropospheric air
masses transported from Tenerife towards Europe. However, the higher values
at midlatitudes in the Northern Hemisphere as opposed to those close to the
Intertropical Covergence Zone (ITCZ) are probably indicating an anthropogenic influence at midlatitudes in
the free troposphere.
The same procedure as for XNO2 is applied to obtain XHCHO, albeit for
an optimized fitting window covering the strong spectral signatures of HCHO.
The monthly means of XHCHO at the Zugspitze site are 500–950 ppt, which
is reasonable when comparing these values with values obtained from air
sampling in winter at similar altitudes and latitudes as well as considering
a seasonal variability in this region. At midlatitudes, the values of HCHO
and NO2 are relatively high for clean conditions. This implies that
there is sufficient NOx for oxidizing hydrocarbons through
photooxidative catalytic cycles. As a consequence, production of some
tropospheric ozone (O3) is occurring at midlatitudes in the free
troposphere. The values of XHCHO are in the range of 255–385 ppt at
Pico Espejo and increase towards the end of the dry season. Moreover,
multi-year-averaged monthly means of XHCHO are linearly correlated with
the multi-year-averaged monthly means of XNO2, further supporting the
notion that emissions from South American biomass burning clearly reach the
free troposphere in this region. This probably also implies some
photooxidation of hydrocarbons and production of O3 is occurring some
of the time in the air masses observed in the free troposphere above Pico
Espejo.
Acknowledgements
This study was funded by the University of Bremen. The setup and operation
of the MAX-DOAS instrument on Zugspitze and Pico Espejo were funded by DLR
Bonn in the framework of SCIAMACHY validation. We would like to thank Thomas
Medeke, Sixten Fietkau, and Astrid Loewe for their work on the DOAS
instruments and data and NASA for the provision of MODIS data.The article processing charges for this open-access publication were covered by the University of Bremen.
Edited by: M. Van Roozendael
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