of no2 and o3 vertical column densities over río province ... · m. raponi, r. jiménez, e....
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Opt. Pura Apl. 45 (4) 397‐403 (2012) ‐ 397 ‐ © Sociedad Española de Óptica
Sección Especial: VI Taller de Medidas Lidar en Latinoamérica / Special Section: VI Workshop on Lidar Measurements in Latinamerica
Measurements of NO2 and O3 vertical column densities over Río Gallegos, Santa Cruz province, Argentina, using a portable and
automatic zenith‐sky DOAS system
Mediciones de las densidades en columna vertical de NO2 y O3 sobre Río Gallegos, provincia de Santa Cruz, Argentina, usando un sistema DOAS
cenital portátil y automático
Marcelo Raponi(1,*), Rodrigo Jiménez(2), Elian Wolfram(1),
Jorge O. Tocho(3), Eduardo Quel(1) 1. División LIDAR, CEILAP (CITEDEF‐CONICET), Juan B. de La Salle 4397, B1603ALO, Villa Martelli, Argentina.
2. Universidad Nacional de Colombia, Departamento de Ingeniería Química y Ambiental, Bogotá, D.C. 111321, Colombia.
3. Centro de Investigaciones Ópticas, CIOp (CONICET‐CIC), Buenos Aires, Argentina. (*) Email: [email protected]
Recibido / Received: 13/07/2012. Revisado / Revised: 22/10/2012. Aceptado / Accepted: 24/10/2012.
DOI: http://dx.doi.org/10.7149/OPA.45.4.397
ABSTRACT:
Stratospheric ozone (O3) plays a critical role in the atmosphere due to its capacity to absorb biologically harmful solar UV radiation before it reaches the Earth’s surface. Nitrogen dioxide (NO2) is a key trace gas in the ozone photochemical. The remote sensing of NO2 and other atmospheric minority gases is essential in order to understand the stratospheric O3 destruction and formation processes. A study carried out on the seasonal variation of the O3 and NO2 vertical column densities (VCDs) using a zenith‐sky DOAS (Differential Optical Absorption Spectroscopy) is presented. This system is composed of a spectral analyzer (portable mini‐spectrometer HR4000, Ocean Optics), two optical fibers (400 µm of core, 25 cm and 6 m of longitude) and an automatic mechanical shutter. NO2 and O3 VCDs are derived from zenithal solar spectra acquired during twilights (zenithal angles between 87° and 91°). The data retrieved by our instrument are compared with those coming from the SAOZ spectrometer (Systeme d'Analyse par Observation Zenithale, Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), France). Both systems are located in Rio Gallegos, Santa Cruz province, Argentine (51°36’S; 69°19’W, 15 m asl), in the CEILAP‐RG remote sensing station. We observed that NO2 VCD ranging from 6×1015 molec/cm2 in summer to 1.6×1015 molec/cm2 in winter and early spring. An anticorrelation (shifting approximately 40 days) between NO2 and O3 VCDs was calculated. A good agreement (average relative difference about 13%) among O3 VCD measurements of both instruments was observed. In the case of NO2, a better agreement among results at sunrise than at sunset between SAOZ and ERO‐DOAS data was determined.
Key words: Zenith‐Sky DOAS, NO2, O3, SAOZ.
RESUMEN:
El ozono estratosférico (O3) juega un rol crítico en la atmósfera debido a su capacidad de absorber radiación solar UV biológicamente dañina, antes de arribar a la superficie terrestre. El dióxido de nitrógeno (NO2) es un gas traza clave en la fotoquímica del ozono. El sensado remoto de NO2 y otros gases minoritarios atmosféricos, es esencial para comprender los procesos de destrucción y formación del O3 estratosférico. En este trabajo se presenta un análisis efectuado sobre la variabilidad estacional de la densidad en columna vertical (VCD en inglés) de O3 y NO2, usando un sistema DOAS cenital (Differential Optical Absorption Spectroscopy). Este sistema está compuesto por un analizador espectral (mini‐espectrómetro portátil HR4000 de Ocean Optics), dos fibras ópticas
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(de 25 cm y 6 m de longitud, con un núcleo de 400 µm) y un obturador mecánico automático. Las VCDs de NO2 y O3 son derivadas a partir de espectros solares adquiridos durante los crepúsculos (ángulos cenitales entre 87°‐91°), apuntando al cenit del lugar. Los datos obtenidos por nuestro sistema son comparados con los determinados por el espectrómetro SAOZ (Systeme d'Analyse par Observation Zenithale, Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), France). Ambos sistemas están localizados en Río Gallegos, provincia de Santa Cruz, Argentina (51°36’S; 69°19’O, 15 m snm), en la estación de sensado remoto CEILAP‐RG. Se observaron variaciones de la VCD de NO2 entre 6×1015 molec/cm2 (en el verano) hasta 1.6×1015 molec/cm2 (en el invierno y principios de la primavera). Se calculó una anticorrelación (con defasaje de 40 días aproximadamente) entre las VCDs de NO2 y O3. Se observó un buen acuerdo entre las VCDs de O3 medidas por ambos instrumentos, con una diferencia relativa promedio del orden de 13%. En el caso del NO2, se determinó un mejor acuerdo entre los datos del SAOZ y del ERO‐DOAS medidos al amanecer que al atardecer.
Palabras clave: DOAS cenital, NO2, O3, SAOZ.
REFERENCIAS Y ENLACES / REFERENCES AND LINKS
[1]. D. Fish, R. Jones, “Rotational Raman scattering and ring effect in zenith sky spectra”, Geophys. Res. Lett. 22 (7), 811‐14 (1995).
[2]. M. Gil, M. Yela, L. Gunn, A. Richter, I. Alonso, M. Chipperfield, E. Cuevas, J. Iglesias, M. Navarro, O. Puentedura, S. Rodríguez, “NO2 climatology in the northern subtropical region: diurnal, seasonal and interannual variability”, Atmos. Chem. Phys. Discuss. 7, 15067‐15103 (2007).
[3]. http://www.division‐lidar.com.ar
[4]. M. Raponi, R. Jiménez, E. Wolfram, J. Tocho, E. Quel, “Remote sensing of stratospheric NO2 over the Argentinean Antarctica using a DOAS mini‐spectrometer”, Opt. Pura Apl. 44, 77‐82 (2011).
[5]. U. Platt, J. Stutz, Differential Optical Absorption Spectroscopy. Principles and Applications. Physics of Earth and Space Environments, Springer, Chapter 2, 16‐28 (2008).
[6]. M. Gil, J. Cacho, “NO2 total column evolution during the 1989 spring at Antarctica peninsula”, J. Atmos. Chem. 15, 187‐200 (1992).
1. Introduction
Stratospheric ozone (O3) is one of the most
important gases in our atmosphere due to its
capacity to absorb biologically harmful solar
radiation (completely the UV‐C and partially the
UV‐B radiation) that would otherwise arrive to
the terrestrial surface producing dangerous
effects on different organisms. One of the most
important nitrogen species emitted to the
atmosphere is the nitrous oxide (N2O), which is a
greenhouse gas naturally emitted by earth and
sea bacteria, and also produced by human
activities, mainly agriculture. It is a very stable
molecule which is transported to the
stratosphere. In the middle and upper
stratosphere it is converted to nitric oxide (NO)
by reaction with excited oxygen atoms O (1D)
produced mainly by UV photolysis of O3 [1].
During daylight a balance between the NO
and nitrogen dioxide (NO2) concentrations is
established through the reaction of the former
with O3 and the rapid photolysis and reaction
with atomic oxygen of the latter. At night, NO2 is
converted first to nitrate radical (NO3) and via a
three‐body reaction to dinitrogen pentoxide
(N2O5). This causes a build‐up of N2O5 during the
night followed by a slow release during the
following day through photolysis. The NO2
diurnal variation therefore comprises a
maximum immediately after sunset, followed by
a slow decrease throughout the night and a
sharp drop to minimum at sunrise. As well as the
diurnal variation there is a seasonal variation in
stratospheric NO2 at mid‐latitudes due to the
combined effects of photochemistry and
atmospheric transport [2]. The development of
remote sensing systems for trace gases
monitoring is fundamental to understand the
dynamic processes that occur in the
stratosphere. The LIDAR Division (CEILAP‐
CITEDEF) has in Río Gallegos, Santa Cruz
province (51º36’S; 69º19’W; 15 m asl) a remote sensing station (CEILAP‐RG) where
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systematically are carry out measurements of
several atmospheric parameter, e.g. O3 and NO2
vertical column, O3 profile (LIDAR system),
aerosol optical thickness, solar irradiance (UV‐A,
UV‐B, NIR), etc. [3] It is necessary to highlight
that Río Gallegos city is affected every spring by
a significant decrease of the stratospheric O3 that
produces an increment of the UV solar radiations
that arrive to the surface.
2. Materials and method
The development of a compact atmospheric
remote sensing system able to determine the
VCD (Vertical Column Density) of multiple trace
gases, is presented. A low‐cost and portable
zenith‐sky DOAS system (Fig. 1) ‐hereafter
referred to as ERO‐DOAS‐ composed of a mini‐
spectrometer (HR4000, Ocean Optics), two
optical fibers (400 µm of core, 6 m and 25 cm of
longitude) and a home‐made external shutter,
has been developed [4]. HR4000 allow us to
measure solar spectral irradiance in the UV‐
visible range (290‐650 nm). It is a simple
spectrograph equipped with a fixed diffraction
grating (600 grooves/mm blazed at 400 nm) and
a 3648‐pixel lineal array CCD. An automatic
shutter to determine the dark current of each
measurement and to remove the noise of the
twilight spectra, was developed. A software
development using Labview® controls the start
and the end of spectral measurements, the
retrieval of acquired spectra and the shutter. The
computer’s internal clock is daily updated to
avoid possible time shifts and to maintain
accuracy on zenithal and azimuth angles
calculations.
The software sets the CCD integration time to
maximize signal/noise ratio. The dark current is
obtained with the same integration time than the
twilight spectra measured immediately before.
The instrumental function and the system
resolution were determined using low pressure
lamps spectra provided by the Physics
Laboratory of Instituto Tecnológico de Buenos
Aires (ITBA).
The analysis of visible spectra based on the
DOAS concept presents the advantage of
allowing for simultaneous retrieval of VCDs of
different species, over a wide range of
meteorological conditions. NO2 and O3 VCDs are
retrieved from zenithal solar spectra acquired
on “twilight” conditions (zenithal angle between
87° and 91°) applying the DOAS (Differential
Optical Absorption Spectroscopy) technique. The
analysis is carried out by solving the Beer‐
Lambert (BL) law over an adequate wavelength
range (410‐590 nm). The twilight spectra are
compared with the one taken with the sun near
the zenith (reference) to eliminate the
Fraunhofer’s structure (one order of magnitude
bigger than the absorptions that we wanted to
measure). The slant columns (according to the
trajectory of the twilight rays) are obtained
applying a least squares fitting to the logarithm
of the ratio between the twilight spectra and the
reference spectra [5]. To obtain the VCDs, the
slant column densities (SCDs) are divided by the
Air Mass Factor (AMF). The AMF depends on the
SZA and other parameters like the altitude and
the density profile of the gas in the atmosphere,
etc. [6]. The effects of Rayleigh and Mie
scattering are subtracted out using a high‐pass
polynomial filter (n=4).
Fig. 1. Zenith‐sky DOAS system’s components: a notebook, the software designed using Labview®, the spectrometer (HR4000), an automatic shutter and the optical fibers.
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3. Results
A study on the O3 and NO2 VCDs seasonal
variation at Río Gallegos, is presented. In Fig. 2
we can observe the NO2 VCD seasonal variation
at Río Gallegos during 2004‐2011 retrieved by
OMI/AURA (Ozone Monitoring Instrument/
AURA satellite). The NO2 concentration ranging
from 6×1015 molec/cm2 in summer to 1.6×1015
molec/cm2 in winter and early spring.
In Fig. 3, the O3 and NO2 VCD retrieved by
SAOZ (Système d'Analyse par Observation
Zenithale) at Río Gallegos during 2008‐2009, is
showed. A difference between NO2 VCD at
sunrise and at sunset was observed. In the case
of the O3 VCD, significant differences among the
concentrations at sunrise and at sunset are not
observed, as us we waited. In the case of NO2, to
be a gas with a comparatively short
photochemical time of life, it presents a
significant variability during the day. For this
reason exist an important difference among the
concentrations measured during the twilights.
In Fig. 4 the correlation between NO2 and O3
VCD, obtained by the SAOZ spectrometer during
2008‐2009, is presented. An anticorrelation
(shifting approximately 40 days) between NO2
and O3 VCDs and some days with “ozone hole”
condition (O3 VCD < 220 DU) were observed.
In Fig. 5 the O3 and NO2 VCDs retrieved by
ERO‐DOAS and SAOZ spectrometer (both of
them located in CEILAP‐RG station), during
September/December 2009, are presented.
For the ozone, a good agreement among the
instruments with an average relative difference
about 13%, was observed. In the case of NO2, a
better agreement among results at sunrise than
at sunset between SAOZ and ERO‐DOAS data
was determined.
Fig. 2. NO2 VCD variability at Río Gallegos, Santa Cruz province, Argentina, retrieved by OMI‐AURA, from 2004 to 2011.
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Fig. 3. Seasonal variation of O3 and NO2 VCDs ‐ at sunset and at sunrise ‐ during 2008‐2009 retrieved by SAOZ spectrometer at Río Gallegos.
Fig. 4. Correlation between NO2 and O3 VCD seasonal variation during 2008‐2009.
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Fig. 5. O3 and NO2 VCDs retrieved by ERO‐DOAS and SAOZ instruments at Río Gallegos, during September/December 2009.
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4. Conclusions
Our zenith‐sky DOAS system has the capability
of sensing automatically several chemical
species and the advantage of being portable
(which offers the possibility to move the
instrument to carry out measurements
campaigns). We observe in both ground‐based
instruments a strong daily variability of the NO2
VCDs (sunrise vs. sunset). NO2 is a gas with a
comparatively short photochemical time of life
and it presents a significant variability during
the day. This variability is probably associated
with the NOx vertical distribution, the
temperature in the high layers of the
atmosphere and the variability of other active
species like the tropospheric NO, for example. In
the case of the O3 the daily variability of the gas
is low, reason why the comparison between the
sunrise and sunset data is very good.
Acknowledgements
The authors acknowledge to the Japanese
International Collaboration Agency (JICA) for
funding the acquisition of the HR4000
spectrometer and to SAOZ network and
OMI/AURA for the Río Gallegos data.