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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



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.



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.



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.



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.

of NO2 and O3 vertical column densities over Río province ... · M. Raponi, R. Jiménez, E....

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