Measurements of NO2 and O3 vertical column densities over Río

ÓPTICA PURA Y APLICADA. www.sedoptica.es 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: mraponi@citedef.gob.ar (*) 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 Opt. Pura Apl. 45 (4) 397‐403 (2012) ‐ 397 ‐ © Sociedad Española de Óptica ÓPTICA PURA Y APLICADA. www.sedoptica.es. (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 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 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 Opt. Pura Apl. 45 (4) 397‐403 (2012) ‐ 398 ‐ © Sociedad Española de Óptica ÓPTICA PURA Y APLICADA. www.sedoptica.es. 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). 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). 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. 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 Opt. Pura Apl. 45 (4) 397‐403 (2012) ‐ 399 ‐ © Sociedad Española de Óptica ÓPTICA PURA Y APLICADA. www.sedoptica.es. 3. Results reason exist an important difference among the concentrations measured during the twilights. 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. 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. 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 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. Opt. Pura Apl. 45 (4) 397‐403 (2012) ‐ 400 ‐ © Sociedad Española de Óptica ÓPTICA PURA Y APLICADA. www.sedoptica.es. 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. Opt. Pura Apl. 45 (4) 397‐403 (2012) ‐ 401 ‐ © Sociedad Española de Óptica ÓPTICA PURA Y APLICADA. www.sedoptica.es. Fig. 5. O3 and NO2 VCDs retrieved by ERO‐DOAS and SAOZ instruments at Río Gallegos, during September/December 2009. Opt. Pura Apl. 45 (4) 397‐403 (2012) ‐ 402 ‐ © Sociedad Española de Óptica ÓPTICA PURA Y APLICADA. www.sedoptica.es 4. Conclusions 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. 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 Opt. Pura Apl. 45 (4) 397‐403 (2012) 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. ‐ 403 ‐ © Sociedad Española de Óptica