ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-15-10567-2015NO2 seasonal evolution in the north subtropical free troposphereGil-OjedaM.gilm@inta.esNavarro-ComasM.Gómez-MartínL.https://orcid.org/0000-0002-6655-7659AdameJ. A.Saiz-LopezA.https://orcid.org/0000-0002-0060-1581CuevasC. A.GonzálezY.PuenteduraO.https://orcid.org/0000-0002-4286-1867CuevasE.https://orcid.org/0000-0003-1843-8302LamarqueJ.-F.https://orcid.org/0000-0002-4225-5074KinninsonD.TilmesS.https://orcid.org/0000-0002-6557-3569Instituto Nacional de Técnica Aeroespacial, Torrejón de Ardoz, SpainGroupe de Spectrométrie Moléculaire et Atmosphérique, URM CNRS 7331,
UFR Sciences Exactes et Naturelles, Moulin de la Housse, BP 1039, 51687 Reims CEDEX 2, FranceAtmospheric Chemistry and Climate Group, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, SpainIzaña Atmospheric Research Center, AEMET, Tenerife, SpainAtmospheric Chemistry Division, NCAR, Boulder, CO, USAM. Gil-Ojeda (gilm@inta.es)25September20151518105671057920February201522May20156September20157September2015This 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/15/10567/2015/acp-15-10567-2015.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/15/10567/2015/acp-15-10567-2015.pdf
Three years of multi-axis differential optical absorption spectroscopy
(MAXDOAS) measurements (2011–2013) have been used for estimating the
NO2 mixing ratio along a horizontal line of sight from the high
mountain subtropical observatory of Izaña, at 2370 m a.s.l. (NDACC
station, 28.3∘ N, 16.5∘ W). The method is
based on horizontal path calculation from the O2–O2 collisional
complex at the 477 nm absorption band which is measured simultaneously to
the NO2 column density, and is applicable under low aerosol-loading conditions.
The MAXDOAS technique, applied in horizontal mode in the free troposphere,
minimizes the impact of the NO2 contamination resulting from the
arrival of marine boundary layer (MBL) air masses from thermally forced upwelling breeze during
middle hours of the day. Comparisons with in situ observations show that
during most of the measuring period, the MAXDOAS is insensitive or very
slightly sensitive to the upwelling breeze. Exceptions are found for pollution
events during southern wind conditions. On these occasions, evidence of fast,
efficient and irreversible transport from the surface to the free
troposphere is found.
Background NO2 volume mixing ratio (vmr), representative of the remote free troposphere,
is in the range of 20–45 pptv. The observed seasonal evolution shows an annual
wave where the peak is in phase with the solar radiation. Model simulations
with the chemistry–climate CAM-Chem model are in good agreement with the
NO2 measurements, and are used to further investigate the possible
drivers of the NO2 seasonality observed at Izaña.
Introduction
Nitrogen oxides play an important role in tropospheric chemistry as they
control O3 photochemical catalytic production (Crutzen, 1979), the
abundance of hydroxyl radicals, and they contribute to the formation of nitrate aerosols.
In a background unpolluted atmosphere where NOx
concentrations are low, net ozone loss occurs during photochemically active
periods (Liu et al., 1983;
Isaksen et al., 2005). NOx abundance is
highly variable since it is influenced by non-steady natural and
anthropogenic emissions and its global distribution is still uncertain. Free
troposphere (FT) source inventories indicate that major production comes
from lightning (2–16 Tg N yr-1), followed by NH3 oxidation (0.3–3 Tg N yr-1),
stratospheric intrusion (0.08–1 Tg N yr-1) and
aircraft (0.6-0.7 Tg N yr-1). Contribution from the boundary layer in
remote regions is rare (Bradshaw et al., 2000).
Information about surface NOx in polluted areas is available due to
extended governmental air-quality networks. During the last decade,
satellite instruments have demonstrated a capability for successful
retrieval of tropospheric NO2, identifying enhanced concentrations over
urban and industrial areas in the boundary layer (Richter et al., 2005; Irie
et al., 2005) and tracking the temporal trends (Hilboll et al., 2013; Cuevas
et al., 2014). However, direct NO2 measurements in the background FT
are scarce due to the requirement of observational platforms above,
typically, 2000 m a.s.l., but also due to the low concentrations present at
those levels.
Airborne NO2 measurements have been performed for decades (Ridley et
al., 1988; Carroll et al., 1990); however the need for very short response
times at concentrations close to the instrumental detection limit make the
FT observations a challenging task. Even though well-characterized aircraft
instruments reach detection limits as low as 10 pptv (Heland et al., 2002),
few studies are reported in the literature. Measurements are generally
collected during individual field campaigns associated to specific targets
such as chemistry missions or satellite validations (Jacob et al., 2003;
Bucsela et al., 2008; Boersma et al., 2008; Baidar et al., 2013; Flynn et
al., 2014). These time and space sparse data limit the study of
seasonalities or trends in the FT. Only recently, attempts to obtain global
FT NO2 abundances from a satellite OMI instrument has been performed for
the first time (Choi et al., 2014) by using the cloud-slicing technique
(Ziemke et al., 2001). The method is based on the comparison of cloud and
cloudless scenes to derive the FT mean concentrations. Results show that
valuable information on NO2 large-scale phenomena can be derived from
areas where cloud presence is frequent, but does not provide results in
places such as at east Atlantic subtropical latitudes, where high pressure is
a dominant feature.
Instruments operating in the few high mountain stations existent around the
world are the only alternative to monitor NO2 in the background FT.
However, the in situ measurements are often affected by the “upslope breeze
effect” (Kleissl et al., 2007; Val-Martin et al., 2008; Rodríguez et al.,
2009; Reidmiller et al., 2010; Cuevas et al., 2013). Radiative heating in
the mountain slopes results in air upwelling from the boundary layer,
contaminating the daytime measurements by generally larger values over the
polluted lower layers.
Recently, Gomez et al. (2014) presented a simple method based on a
modified geometrical approximation (MGA) to estimate
concentrations of trace gases at the level of the Izaña observatory from
multi-axis differential optical absorption spectroscopy (MAXDOAS)
measurements. The horizontal path length is obtained from the oxygen
collisional complex (O4, hereafter) simultaneously measured with the
tracer under consideration (NO2 and O3). Gomez et al. (2014) examined a
short summer period to demonstrate the validity of the method. Here we apply
the same technique to data covering 3 full years (2011–2013) to analyze the
seasonal evolution of the NO2 concentrations in volume mixing ratio
(vmr). MAXDOAS present two main advantages with respect to the in situ
instrument at this location, both related to the very long optical path of
the measurements of over 60 km. Firstly, it minimizes the potential
contribution of NO2 that may be upwelled from the marine boundary layer
(MBL). The breeze layer has a limited vertical extension and its relative
contribution to the MAXDOAS long path is small. On the contrary, Izaña
in situ data around noon are strongly influenced by the underlying polluted
MBL (Puentedura et al., 2012; Gomez et al., 2014). Secondly, due to the long
light paths achieved by MAXDOAS in the FT, very low concentrations, of a few
parts per trillion, can be measured.
Section 2 presents the method, its limits and its associated errors. In Sect. 3
the station and data sets are depicted. Section 4 describes the method used
for the retrieval of profiles. The description of the chemical and back
trajectories models is done in Sect. 5. Finally, Sects. 6 and 7 present the
results and discussion, and summary, respectively.
Instrument and methodology
In the year 2010, the DOAS (differential optical absorption spectroscopy) spectrometer, operating in zenith mode at that time,
was upgraded for MAXDOAS measurements. The spectrometer records the sky
spectrum in the visible range at a spectral resolution of 0.55 nm in 10
elevation angles from -1 to 90∘, and 1∘ field of view, covering a
full cycle in 20 min. The number of cycles per day ranges from 26 at winter
solstice to 38 in summer. NO2 is evaluated in the 425–520 nm range in
order to simultaneously retrieve the O4 from the 477 nm absorbing band.
The scanning plane is at 0∘ azimuth (north) to minimize the dependence
of the path with the azimuth (Wittrock et al., 2004). The instrument is part
of the Network for the Detection of the Atmospheric Composition Change
(NDACC) and other settings are those recommended for DOAS-type
spectrometers. NO2 at 294 K temperature from Vandaele et al. (1998) and
O4 from Hermans (2011) cross sections have been used. Details of
the instrument, settings and operational mode can be found in Puentedura et
al. (2012) and Gomez et al. (2014).
The modified geometrical approximation (MGA) described in Gomez et at. (2014)
has been used for the data analysis. NO2 vmr at the level of the
station is obtained by dividing the differential slant column density (DSCD)
measured in the horizontal geometry by the horizontal optical path. The DSCD
is obtained by subtracting the measurement obtained at the zenith
(SZA = 90∘) from the measurement in the horizontal path. In
a first approximation, the slant paths' contributions of 0 and 90∘
geometries are cancelled out, and only the signal of the tracers present in the
horizontal path remains (see Gomez et al., 2014 for details). The method
assumes a quasi-Rayleigh atmosphere, i.e., very low aerosol optical depth
(AOD), and a single scattering before the photon reaches the detector. The
path is obtained from the O4 horizontal column since the amount of
O2 is known from the independent air pressure measurements. The path
length is then corrected to account for the differences in wavelengths
between the O4 and NO2 analysis ranges. In practice, the
scattering of the zenith path does not take place near the instrument but at
a few kilometers above the level of the station. The actual concentration of a
measured species X at the station level is given by
Xvmr=XDSCDO4DSCD[O4]surface⋅f+c,
where XDSCD and O4DSCD are the slant measured columns of the
species X and O4, respectively. [O4]surface is the O4 at the
level of the station, f is the correction factor due to differences in
wavelength absorption ranges of the species under study, with respect to
O4 that can be computed from a radiative transfer model (RTM), and c is
the error of the approach. The latter is a factor accounting for the
dependence with the different vertical distributions of both species and
air mass factors (AMF):
c=h(Rg-R′g′),
where h is the effective scattering height of the vertical ray. R and R′ are the
ratio of the mean concentration of the layer divided by the concentration at
the level of the station of tracer X and O4, respectively, and
g and g′ account for their AMF in the zenith geometry (g= AMF(SZA) - 1), where
SZA stands for solar zenith angle.
The effective scattering height is defined as
h(z)=∑surfacetopI(z)∫surfacetopI(z)dzz,
where I(z)∫surfacetopI(z)dz
represents the normalized contribution of the ray scattered at each
atmospheric layer to the total flux at surface. From radiative transfer
calculations it can be shown that the effective scattering height ranges
between 6.5 and 7.5 km above the station for a solar zenith angle (SZA) below
70∘, which we estimate as the validity limit of the method.
Since both NO2 and O4 are analyzed in the same spectral range, the
difference between the weighted center of the range for NO2, i.e., the
effective wavelength, and that of O4, is small. The value of f is 0.9
for a near-Rayleigh atmosphere.
By using Eq. (2), the error introduced in NO2 vmr due to the
geometrical approximation, if assuming a constant mixing ratio of both
O4 and NO2 with height, is 9.0 % at 70∘ SZA and
2.3 % at 50∘. Since the scattering heights and the AMF
are well known, the data can be corrected. The only uncertainty is due to
the R value related to the vertical distribution of NO2 within the FT.
However, aircraft measurements over the ocean far from large industrial
areas show that the tropospheric vertical distribution is nearly constant
above the MBL (Bucsela et al., 2008).
In the presence of moderate or high aerosol loading at the level of the
observational point, multiple scattering takes place and the method is no
longer valid. Assuming that the aerosol layer is a well-mixed layer, we
estimate an AOD of 0.1 at 500 nm as a safe limit (Gomez et al., 2014).
Since the path length is obtained from O4 measurements, uncertainty in
the magnitude of its absolute cross section is an additional source of
error. It has been reported that paths obtained from O4 are even larger
than that RT computed for a Rayleigh atmosphere (Wagner et al., 2002) when
using the generally accepted cross sections reported in the literature
(Greenblatt, 1990; Hermans, 2011), suggesting that cross sections are
underestimated. There is, however, no agreement in the magnitude of the
correct values. We performed direct Sun measurements on a very clear morning
(aerosol optical density at 500 nm over the observatory was 0.007 ± 0.00077)
at an O4 effective temperature of 250 K, and compared the
retrieved slant columns with the ones calculated from the local
radio sounding of the day (7 October 2014) up to 30 km and the tropical
standard atmosphere from 30 km upwards. Results show an excellent agreement
with no difference at the error level when the retrieval includes O4
cross sections at two temperatures (Fig. 1). In this exercise, the Thalman
and Volkamer (2013) cross sections at 203 and 293 K were used. When
including only the room temperature cross section in the retrieval, the
obtained O4 is 3–5 % too large.
Measured O4 slant column density versus modeled O4 for a pure Rayleigh
atmosphere at the 477 nm band by using cross sections at 203 and 293 K
temperatures (see text for details).
Our results agree with the very recent report by Spinei et al. (2015) who
found a temperature dependence of 9 % for a variation of 44 K and no
pressure dependence based on direct Sun and aircraft MAXDOAS measurements.
The conclusion of their work is that no corrections need to be made for
effective temperatures near 275 K. Since the present method uses only the
horizontal path, the temperature along the path is nearly constant and the
seasonal variability in the subtropical FT is small. Air temperature at the
level of Izaña ranges from 277 K in January–February to 287 K in
July–August. Consequently, no more than 2 % of error is expected due to
this effect.
Typical NO2 slant column density (SCD) root-mean-square error of the fit for horizontal
geometry is of 3×1014 molec. cm-2. These errors
represent 15–20 % of the typical differential SCD. A summary of the
analysis errors is shown in Table 1.
Method uncertainty.
Uncertainty in NO2 due to fit15–20 %Uncertainty in path due to the O4 fit< 1 %Uncertainty of the method (related to unknown vertical distribution of NO2 and actual effective path)2.5–9 % (for sza: 50 to 70∘)Error in horizontal path due to O4 cross sections' temperature dependence2 %OVERALL UNCERTAINTY20–32 %The Izaña observatory and data set
Izaña (28.3∘ N, 16.5∘ W) is a well-known
GAW-NDACC station located at the top of the Izaña Mountain, one of the
peaks of the great crater of the Teide volcano, at 2370 m a.s.l., on
Tenerife Island. The observatory and related meteorology has been
extensively described in previous publications (i.e., Rodríguez et al.,
2009; Cuevas et al., 2013). It is representative of the FT at night. During
daytime it is frequently affected by anabatic winds resulting from heating
of the ground. This upslope breeze carries boundary layer air masses to the
FT. The intensity of the wind peaks near local noon and can extend well
into the afternoon. It can be indirectly quantified using the measurements
of water vapor on the station since air masses from below carry high
humidity to the height of the observatory. The measurements of in situ
NO2 are also useful for this purpose, since the boundary layer (BL) NO2
concentrations in populated areas near the coast are typically more than
1 order of magnitude larger than the background FT.
For the present work, only the horizontal spectra are analyzed. If the SZA
was lower than 10∘, then the 70∘ elevation spectrum was used as
reference to avoid spectral distortions due to integration times that are too short.
In all other cases, the reference was the zenith spectrum of the same cycle.
Data from 3 complete years (2011–2013) have been used after screening
for (a) NO2 RMSE: fit error is limited to 2×1015 molec. cm-2
and a signal to noise ratio of 0.5, which is approximately the detection
limit of the instrument. (b) High SZA: only data corresponding to data below
SZA 70∘ are used in the present analysis to limit the error in the path
calculations. (c) Aerosol loading: measurements on days with aerosol optical
depth (AOD) at 500 nm over 0.1 were rejected. (d) Length of the path:
individual measurements with paths shorter than 30 km were also rejected
(broken clouds or narrow dust layers might cause this effect). (e) Unrealistic
negative values appeared occasionally. Over 15 000 data
passed all filters for the 3-year period (40 % of all possible data).
The Optimal Estimation Method
The Optical Estimation Method (OEM) has been extensively used in the last years
to obtain NO2 vertical profiles of moderate to high polluted environments.
However, on free troposphere background conditions, the concentrations are
near the instrumental detection limit (10–100 pptv), and in these
conditions, the method provides unrealistic profiles. In this work we have used it only
to characterize the vertical distribution of the plume in a particular case
study in which a high NO2 air mass arrived at the station. The mean AOD at 500 nm
on the studied day was only 0.02. Under these conditions the impact of
the aerosol loading in the retrieval is very low. A test with and without
aerosols yielded a mean difference of 1.6 % in the retrieval for heights
between 1 and 5 km. Since uncertainties in the OEM method are larger than
that value, aerosols have not been included in the analysis.
Given a set of measurements y with error covariance Sε,
the OEM (Rodgers, 2000) provides the state
vector x that maximizes the probability that x,
containing the trace gas vertical distribution, belongs to the interval
[x, x+dx]. Following the OEM approach, the
maximum a posteriori solution is calculated as
x^=xa+SaKT(KSaKT+Sϵ)-1(y-Kxa)=xa+Gy(y-Kxa),
where the weighting functions matrix K expresses the sensitivity of the
measurements to variations in the trace gas profile (NO2 in this case).
In this work, K is obtained with the SPSDISORT pseudo-spherical
radiative transfer solver of the libRadtran software package (Mayer and
Kylling, 2005). The xa vector and Sa
matrix correspond to an a priori NO2 profile and
its corresponding error covariance matrix, respectively.
The gain matrix Gy, given by the following expression,
quantifies the sensitivity of the retrieval to the measurements:
Gy=SaKT(KSaKT+Sϵ)-1.
The averaging kernel matrix A, is then obtained as follows:
A=GyK.A expresses the sensitivity of the retrieval to the true state, and it
has an important role in the characterization of the retrieval. The
retrieval at a given altitude is an average of the total profile weighted by
the corresponding row of A, also known as the averaging kernel function
AK. In general, the AKs are functions with a single peak in the
appropriate level, where the measurement provides additional information to
add to the a priori profile. The trace of A provides the number of
independent pieces that can be extracted from the retrieval, usually known
as degrees of freedom (DFS). Typical values of the DFS in our retrievals are
around 1.8.
Another parameter determining the quality of the retrievals is the total
error of the state vector elements. This parameter is the addition of three
contributions: (1) the smoothing of the true profiles given by Sa;
(2) the systematic errors of the measurements, provided
by Sε and (3) systematic errors of the
forward model, mainly provided by the uncertainties in the parameters
characterizing the atmosphere. Average total error (considering the three
mentioned contributions) for all the considered profiles and altitudes
ranging from 1 to 5 km is 0.01 ppb. The choice of the values corresponding
to these three sources of error (Sa, Sε
and the atmosphere parameters) in our study
will be described in the following paragraphs.
The NO2 hourly profiles used as a priori profiles in our calculations
were obtained using a photochemical box model (Denis et al., 2005) derived
from the SLIMCAT 3-D chemical transport model (Chipperfield, 2006). Diagonal
elements of Sa are usually chosen to be a percentage of
xa. In this case, following the L-curve method
described in Schofield et al. (2003), they have been set to 100 % of xa,
and its non-diagonal elements are calculated as follows
(Barret et al., 2002; Friess et al., 2006):
Saij=SaiiSajjexp(-ln(2)((zi-zj)/y)2),
where zi and zj are the altitudes of the altitude grid levels i
and j respectively, and γ is a half of the correlation length. The
functions represented in the last equation are Gaussian correlation
functions which account for correlations between trace gas concentrations at
different altitudes. After several tests on the retrieval, γ value
has been chosen to be 300 m, corresponding to the value (between 0.1 and 1 km)
that maximizes the DFS (trace of A), for the overall retrieval as
well as for the altitudes closer to the station (2.3–2.6 km).
In this work, y represents the differential slant column
densities (DSCD) of NO2 measured with the MAXDOAS spectrometer, and
Sε is set to a diagonal matrix of which diagonal
elements correspond to the molecular errors of the measurements.
Concerning parameters that characterize the atmosphere in our calculations,
vertical profiles of O3, O2, CO2 and H2O have been
obtained from the standard atmosphere for tropical latitudes (Anderson,
1986). Radio-sounding data performed the same day of our calculations
provided the pressure, temperature and air density vertical profiles used in
our retrieval. We have considered layers of 100 m from 0 to 10 km, and with
the same width of those corresponding to the standard atmosphere for
tropical latitudes (Anderson, 1986) for altitudes over 10 km. In this work,
AKs are near zero for altitudes lower than 0.5 km and higher than 5 km (see
Fig. 2). The retrieved profiles have been obtained for altitudes up to 6 km.
Example of NO2 averaging kernels obtained in our profile
retrieval, corresponding to 8 May 2013 at 12:00 UTC.
Model description and trajectory analysis
CAM-Chem (Community Atmosphere Model with chemistry) is a global 3-D
chemistry–climate model fully integrated into the CESM (Community
Earth System Model) framework (Lamarque et al., 2012). In this work, CAM-Chem has been
configured using a horizontal grid resolution of 1.91 latitude × 2.5∘
longitude and 26 hybrid vertical levels from the surface to
approximately 38 km. All simulations have been performed in specified
dynamics (SD), using offline meteorological fields to compute the
atmospheric transport, considering the same high-frequency meteorological
input from a previous CAM-Chem 15-year simulation. This implies that the
model is forced to evolve as if it were a chemical transport model.
The model includes the tropospheric chemistry mechanism of MOZART-4,
implementing also organic and inorganic halogen (chlorine, bromine and
iodine) photochemistry mechanisms, taking into account natural and
anthropogenic sources, heterogeneous recycling and dry and wet deposition
(Saiz-Lopez et al., 2012; Ordoñez et al., 2012; Fernandez et al., 2014).
Anthropogenic emissions due to fossil fuel and biofuel combustions come from
the POET (precursors of ozone and their effects in the troposphere) database
for 2000.
To investigate the air masses reaching the area of study, back trajectories
were computed with the HYSPLIT (Hybrid Single-Particle Lagrangian Integrated
Trajectory) model, developed by NOAA Air Resources Laboratory (ARL)
(Draxler et al., 2009). The ECMWF (European Centre for Medium-Range Weather
Forecasts) meteorological fields were used with a spatial resolution of
0.25∘× 0.25∘, 22 vertical levels from the surface to
250 mb and a time resolution of 6 h. Three-dimensional kinematic back
trajectories were calculated. A daily back trajectory at 12:00 UTC, with a
168 h pathway (7 days) at 2370 m a.s.l., was computed. Following the
HYSPLIT model tools, back trajectories have been grouped into clusters
(Stunder, 1996), arriving at the Izaña observatory.
Results and discussion MAXDOAS vs. in situ
Results of the NO2 comparison can be classified according to three
different meteorological regimes. A period in 2013 has been selected to
illustrate the differences in concentration between the in situ local sampling
and the MAXDOAS long-path average (Fig. 3, upper panel). On days when the
breeze is inhibited, the in situ data are representative of the FT, and the
agreement between instruments is very good (e.g., days 139–145). On days when
anabatic winds are present, NO2 vmr increases are observed in the in
situ measurements, whereas the MAXDOAS signal remains at typical FT levels (e.g.,
days 130–137). Upslope winds cause an air mass mixture with that associated
with the FT synoptic wind. The upslope strength will depend inversely on the
intensity of the zonal synoptic winds (Cuevas et al., 2013). In general,
the depth of the layer is not enough to contaminate the MAXDOAS path. This
situation is the one most commonly observed at Izaña. As expected,
MAXDOAS provides a much better representation of the FT background reactive
gases. A third set of measurements is shown when MAXDOAS data also suffer
large increases (e.g., days 127–129). After excluding thunderstorms with
electric activity and wildfires in the area, it was found that this
situation always takes place in the presence of southern winds. The only
identified NO2 large source upwind is the 980 MW thermal power plant
located 25 km south of Izaña, with NO2 emissions of 4.7 × 106 kg year-1.
As previously reported (Persson and Grazzini,
2007), the thermal local circulations are not captured by atmospheric global
models, even by the ECMWF 0.25∘×0.25∘ used in this
study which predicted trajectories above 2000 m all the way (Fig. 3, lower
right). This orographically forced lifting mechanism has been found to be an
efficient and fast way for the irreversible transport of surface air to the FT.
Pollutants and minor gases of oceanic origin (i.e., IO, BrO) move upslope,
crossing the MBL top in less than 1 h, and then are subsequently mixed with
FT air. The quantification of the amount of air mass transported from the
lower layers of the MBL to the FT is outside of the scope of this work, but
certainly data provide evidence for the existence of an efficient and fast
mechanism to supply halogens and other marine trace gases to the FT. Since
southern wind conditions are common during the summer months (50 % of the
days) and the mountain rift has a NE–SW orientation, and a length of about
30 km, the supply of marine trace gases to the FT might not be negligible,
at least on a local scale.
In situ NO2
volume mixing ratio measurements made every minute versus Izaña
MAXDOAS, for a period of time representative of three different wind situations.
In situ data are smoothed by 50 min running mean (top panel). An expanded
plot is shown for 8 May 2013 (day number 128). A back trajectory ending at Izaña at
15 h of the same day is shown.
Unsmoothed vertical profiles of NO2 vmr (in ppbv) for day
128/2013 obtained by the OEM technique. Each vertical column represents an
individual scanning cycle (for details, see text).
Surprisingly, the very high MAXDOAS vmr reaches as much as a half of the
levels observed by the in situ sensor. This is related to the pointing
direction of the DOAS spectrometer, since the plume propagates northward,
along the same direction of the spectrometer line of sight. The path within
the plume results in an enhancement of the absorption signal. Figure 4 shows
the NO2 vmr vertical cross sections for the day 128/2013, obtained by
using the OEM technique, indicating that the enhancement takes place near
the level of the station, with an upper limit around 4 km. This confirms
that once the air mass passes over the mountain obstacle, it either moves
horizontally or descends again but remains in the FT. Note that the
instrument scanning lowest angle is below the horizon (IEA =-1∘),
thus containing information about the trace gas concentration below the
station.
It is worth mentioning that southern winds are generally related to African
air masses containing Saharan dust and, as previously mentioned, those dusty
days were filtered out from the analysis. The only non-dusty south wind
cases observed are from Atlantic air masses which suffered an abrupt change
in direction when approaching Africa. Consequently the impact of this
effect on the overall data set is small. Only five clear cases have been
identified within the 3-year record. Those cases have been removed for
seasonal evolution studies.
Figure 5a shows the NO2 vmr seasonal evolution
separated by year. The seasonal behavior is similar in all 3 years, with
the maximum in the summer months and the minimum in wintertime. Summer gaps
result from the large number of Saharan dust intrusions during these months.
To explore a possible dependence of the retrieved concentrations with the
SZA, the data have been plotted in colors according the SZA (Fig. 5b). The maxima in summer months are observed, regardless of the SZA,
excluding the possibility of stratospheric contamination or any other SZA-dependent
artifact. The magnitudes of the retrieved concentration are also
independent of the RMSE (Fig. 5c). Sporadic peaks over 100 pptv
are observed with no increase in typical retrieval errors. The scattering
through the day is also large with standard deviations of 10–15 pptv.
Seasonal evolution of the individual data NO2 vmr separated
by (a) years, (b) solar zenith angles and (c) RMSE.
Monthly means clearly show the rapid spring build-up and the autumn decay
(Fig. 6). Mean values range from 20 to 44 pptv throughout the year. A summer
maximum has previously been found in unpolluted continental China in the
boundary layer as a result of soil biogenic NO emissions (Van der A et al.,
2006; Qi, 2015). However, NO2 in long paths over the Atlantic FT
cannot be explained in this way. The output of the CAM-Chem model for the
location and the level of the station shows similar results. The agreement
with observational data is particularly good for the period November to
February. The NO2 summer build-up takes place across the entire FT (not
shown). The winter to summer ratio is largest in the middle troposphere at a
height of 8 km. The peak FT NO2 values occurring in summer were previously
observed by Val-Martin et al. (2008), who attributed the summer maxima in
NO2 to North American biomass burning during this season. The model
results show no seasonal differences in the NO2 chemical formation/loss
channels. However, an increase in the overall reactive nitrogen budget
occurs in this unpolluted FT site, as summer proceeds. We have explored two
possibilities to explain the seasonality of our observations.
NO2 concentration monthly means at the level of Izaña
observatory with their respective standard deviations (open circles and
black lines). CAM-Chem model results for the same level are shown for
comparison (red stars and lines). Individual solid gray circles represent the
3-year diurnal mean.
Contamination by anabatic winds
It has previously been shown that the MAXDOAS-MGA technique minimizes the
unwanted effects of MBL on FT measurements, but in principle, the influence
of a potential seasonal cycle in the intensity of the upwelling wind cannot
be entirely ruled out. Since anabatic winds are driven locally by surface
heating, the depth of the layer of influence is expected to be of only a few
hundreds of meters, but there is not enough information to quantify the size
of this depth and thus the potential contribution of a possible path
contamination from air masses rich in NO2 coming from below. Most of
the works available in the literature refer to the BL, generally with return
flowback. However, cases of upwelling to the FT are reported as well. As
early as 1923, Wenger (1923) observed this situation at the slopes of the
Teide mountain in Tenerife for up to 1500 m a.s.l., but no data were available
above that height. In situ data in Fig. 1 show evidence of MBL nitrogen
oxides transported by anabatic winds up to the level of the station (2370 m a.s.l.).
However, an intensification of the upslope breeze in summer with
respect to winter would result in a larger vertical extension of the
upwelling layer, increasing its relative contribution in the MAXDOAS path.
Out of the few large pollution cases, the concentrations measured are too
low for the OEM technique to be applied. It is nevertheless unlikely that
the summer increase in upwelling can account for the twofold increase in the
background NO2 vmr. For instance, a layer of 200 m with a NO2
load of as high as 600 ppt would represent an increase in the column of
some 5–10 % of the background concentration for a clean day.
One-week HYSPLIT back trajectory clusters arriving at Izaña
observatory for the winter months (DJF), left panel, and for the summer
months (JJA), right panel, for the years 2011–2013.
We have recalculated NO2 monthly means only from the first morning
data, namely data between SZA 70 and 65∘. These SZA values
correspond to fractional days ranging from 0.42 in midwinter to 0.32 in
summer. At this early time of day, the anabatic wind is still under the first
stage of development and the intensity of the upwelling is of only a few
percent of the maximum value after noon. Results show that the seasonality
in the SZA 65 to 70∘ data is almost identical to that when
considering data at all SZA values (ratio > 0.98). We therefore conclude
that the summer increase is not a result of the contamination by high
NO2 upwelled MBL air masses.
Changes in horizontal transport patterns through the season
Val-Martin et al. (2008) analyzed NO2 mountain data from Azores and
reported larger summer concentrations attributed to North American biomass
burning. However, circulation at the lower latitudes of the Canary Islands is
quite different. Attempts to determine the global FT distribution of
NO2, based on the cloud-slicing technique, have recently been made with
OMI data (Choi et al., 2014), but the method does not provide results in
summer over the Sahara region due to lack of cloudiness.
HYSPLIT 7-day back trajectory cluster analysis shows that air masses
arriving at Izaña during the reported period are fundamentally of
Atlantic origin, with a small portion arriving from Africa during the summer
period (Fig. 7), in agreement with the 22-year (1988–2009) backward
trajectory climatology reported by Cuevas et al. (2013). As previously
mentioned, only NO2 observations under no-dust conditions are
considered, therefore days with African trajectories are not included in the
analysis. Winter trajectories are longer than the summer ones and 30 % of
them cross the United States. All trajectory clusters show a steady
descending transport in the last 96 h prior to the arrival at Izaña
and originate at an altitude of 4000–5500 m a.s.l.
The cluster analysis tells us that the origin of the NO2 seasonal
variation has to be searched for in the western Atlantic area at much higher
altitude than the Izaña station. The CAM-Chem model sampled at the 5.9 km level
shows larger summer values over North America and the subtropical Atlantic
than in winter months, both in the range of the observed values (Fig. 8).
The phase of the mid-troposphere seasonal wave is opposite to the BL one
(Lamsal et al., 2010) and is probably due to a combination of seasonality in
convection and lightning. Venting processes from the BL to the FT over US
have been studied (i.e., Parrish et al., 2004; Hudman et al., 2007), finding export of
NOy, mainly in the form of HNO3 and PAN, to the mid-troposphere.
Convection is driven by surface insolation and has a clear seasonal wave.
The same is true for lightning, since thunderstorms mainly occur during the
spring–summer months.
Global distributions of monthly mean NO2 vmr for the level
5.9 km, obtained from the CAM-Chem chemistry–climate model.
NO2 vmr monthly mean vertical profiles from the CAM-Chem model.
The gray band represents the height range where air masses are originated (see
text).
Tropospheric vertical profiles (Fig. 9) show how NO2 vmr are decreasing
in wintertime from the MBL to the mid-FT, whereas in summer the concentration
remains constant up to 6 km and then increases. At the 5 km level, the model
shows differences from 15 to 40 pptv from winter to summer. These calculated
values are in agreement with the 40–50 pptv background NO2 vmr
estimated by Choi et al. (2014) for the summer months FT in an extended
area covering the western Atlantic from subtropics to mid-latitudes.
The build-up is basically due to enhanced NO2 formation via the
NO + O3 reaction under higher concentrations of NO as a result of NOy
reconversion of PAN and HNO3 in the FT. Note that the lifetime of
NOy is long enough for NOx-rich air masses, originated in North
America, to reach the African coast.
In summary, the NO2 seasonal variation obtained from MAXDOAS
measurements can be explained with the help of the back trajectory cluster
analysis and a chemistry–climate model and result from a mixed effect of
long-range transport and free tropospheric subsidence. This is basically the
same conceptual model that explains the origin or relatively high ozone
values recorded at Izaña in summertime described by Cuevas et al. (2013).
The origin of the high summer NO2 values at Izaña is
related to the larger background NO2 vmr found over North America in
the mid-FT, confirming earlier findings from Schultz et al. (1998).
Summary
NO2 volume mixing ratio at the level of the high mountain observatory
of Izaña (2370 m a.s.l.) has been obtained for 3 years of data using the
MAXDOAS technique and the recently reported Modified Geometrical
Approximation (MGA). The method uses the absorption of the O2–O2
collisional complex at 477 nm to obtain the horizontal path and is
applicable in a near-Rayleigh atmosphere. Only data from air masses of
aerosol optical depths below 0.1 were considered, thus removing African
air masses loaded with Saharan dust. Results show that on most of the
observation days, data are representative of the free troposphere.
Exceptions are found when wind blows from the south. On these occasions, we
find evidence that orographically forced surface air masses ascend upslope to
the Izaña observatory, providing a channel for the irreversible transport of
surface origin species to the free troposphere; this might provide an
explanation for the concentrations of halogen oxides found in this region.
The NO2 seasonal evolution shows a well-defined annual cycle in phase
with solar radiation. Mean mixing ratios range from 20 pptv in midwinter
to 42 pptv in summer with a significant day to day variability. By contrast,
we find a small interannual variability during the 3-year observation
period. A number of possible causes to explain the observed seasonality have
been discussed, including seasonal changes in transport and contamination due
to seasonality in the upslope winds (anabatic winds) but individually, they could
not provide an explanation of the observations. The CAM-Chem
chemistry–climate model reproduces the monthly distribution with great accuracy. The
results of the back trajectory cluster together with the model analysis show
that the seasonality in NO2 vmr is related to a combined effect of
long-range transport and subsidence in the free troposphere. Dust-free
trajectories follow North American/North Atlantic pathways, with air masses
coming from the mid-free troposphere between 4000 and 5500 m a.s.l. The
model and previous satellite estimations show a seasonality in NOy and
NO2 in the mid-free troposphere in phase with the MAXDOAS observations
at Izaña. Larger summer values are probably due to a combination of
seasonality in convection and lightning.
Acknowledgements
This work was funded by the Spanish National R+D Funding Agency through the
AMISOC (CGL2011-24891) project and the EU FP7 NORS project (grant agreement
284421). The authors gratefully acknowledge NOAA Air Resources Laboratory
for the provision of the HYSPLIT transport and dispersion model.
Edited by: M. Van Roozendael
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