Development of a passive doas system to retrieve atmospheric pollution columns in the 200 to 355 nm region.

1.9 MB·English

by  Mejia, Ruben Galicia

Galiciaetal.IranianJournalofEnvironmentalHealthSciences&Engineering2013,10:8
IRANIAN JOURNAL OF
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ENVIRONMENTAL HEALTH
SCIENCE & ENGINEERING
RESEARCH ARTICLE Open Access
Development of a passive doas system to retrieve
atmospheric pollution columns in the 200 to 355
nm region
Rubén Galicia Mejía1*, José Manuel de la Rosa Vázquez1, Suren Stolik Isakina1, Edgard Moreno García1
and Gustavo Sosa Iglesias2
Abstract
Inrecent years several techniques have been developed to measure and monitor the pollution ofthe air. Among
thesetechniques, remote sensing using optical methods stands out due to several advantages for air quality
control applications. A Passive Differential Optical Absorption Spectroscopysystem that uses the ultraviolet region
from 200 to 355 nm of thesolar radiation is presented. The developed system is portable; therefore it is practical
for real time and in situ measurements. The enhanced wavelength range of the system is intended to detect the
ultraviolet light penetration intheMexican Valley considering thesolar zenith angle and the altitude. The system
was applied to retrieve atmospheric SO columns emitted either by anthropogenic (power plant) or natural sources
2
(volcano), reaching a detection limit of about 1 ppm. The measurement ofthepenetratingsolar radiation on the
earth surface attheUVC range is presented and the possibility to measure pollution traces ofsome contaminants
as O , NO and aromatic compoundsinrealtime and insitu inthe ultraviolet region is discussed.
3 2
Keywords: Spectroscopy, Pollution,Sulfur dioxide, Ozone, Ultraviolet
Introduction endangerourlivesandenvironments.Theamountsofpo-
The production of thousands of chemicals has contribu- llutantsdischargedbypowerplants,especiallyairpollution
ted to industrial and economic development in many aremorethanassimilationcapacityofnaturebecausenow
parts of the world. This trend however has been asso- it is clear that sustaining and assimilative capacity of the
ciated with the release of new chemicals and possibly biosphere though tremendous, is after all finite. The first
toxic substances into the environment and food chain step is to understand the magnitude of emissions from
and adversely affecting human health in many instances. eachsource[3].
The pollutants associated with the anthropogenic activi- The Differential Optical Absorption Spectroscopy
ties could be inorganic as well organic compounds [1]. (DOAS) has been successfully used for air pollution
The air pollution is very complex dynamic phenomena. detection in several research works [4]. Important
It could be associated with the industrial activities as advantages over other techniques like gas chromatog-
well as with the concentration of the population in large raphy, chemiluminescence, and electrochemical, gravi-
townsandcities [2]. metric, and matrix isolation, are the lower cost and the
Thepowerplantsareusingresourceslikefuelandwater time saving, especially when the sunlight is used as the
to provide electricity that is one of the essential needs for source (passive DOAS). This technique is particularly
sustainable development and life. This activity produ- interesting in the monitoring of pollution emissions by
ces and discharges all different kinds of pollutants such refineriesorthermoelectric powerstations.
as,gaseous,liquid,electromagneticfields,andnoisewhich DuetotheabsorptionbytheO layerabovetheearthin
3
the UVC interval (200 to 290 nm), the DOAS technique
has been based on the UVB-VIS range absorption mea-
*Correspondence:[email protected]
1SEPI-ESIME-Z.InstitutoPolitécnicoNacional,Av.IPNS/N,UPALMEdifZ,3er surements. It has been used to detect chemicals such as
pisocp.,07738,México,D.F.,México
Fulllistofauthorinformationisavailableattheendofthearticle
©2013Galiciaetal.;licenseeBioMedCentralLtd.ThisisanOpenAccessarticledistributedunderthetermsoftheCreative
CommonsAttributionLicense(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,and
reproductioninanymedium,providedtheoriginalworkisproperlycited.
Galiciaetal.IranianJournalofEnvironmentalHealthSciences&Engineering2013,10:8 Page2of9
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CO, SO , O , NO HNO , HCHO, CH , CHO, benzene, column amounts between 240 and 320 DU [13], if there
2 3 2 2 3
toluene,p-andm-Xylene[5]. is a possibility to detect some spectral irradiance in the
The absorption lines of these pollutants interfere with UVC region it could be possible to detect traces absor-
eachother making more laborious the processing of the bing in this wavelength interval. So, in order to prove
measured spectra. Usually, the measurement of SO the usefulness of the passive DOAS technique in the
2
columns by the passive DOAS technique is done using range from 200 to 350 nm in the Mexican Valley, we
the 290 to 330 nm wavelength range, which is well- have developed a portable system to measure pollution
known penetrates at sea level. In this region the SO slant columns in this spectral range of several pollutants
2
absorption lines interfere with the absorption lines of asSO O ,NO ,aromatic compounds,etc.
2, 3 2
lesscommonspecies asHCHO, BrO, O andClO[4]. InthisissuetheSO emissionsaroundtheTula'sIndus-
3 2
In the wavelength range from 230 to 260 nm the trial Complex in the Mexican Valley and around the
ozone is the principal absorber. Benzene and toluene Popocatepetl Volcano are reported. These calculations
have cross sections one order of magnitude lower than wereperformedusingthedevelopedsystem.Somediscus-
O [4]. Ozone atmospheric measurements have been sion is presented regarding the observed well-structured
3
previously reported using active DOAS in the range absorptionspectraintherangeof200to290nm.
from 265 to 345 nm [6,7]. Also, atmospheric O mea-
3
surements using passive DOAS have been done in the Materials and methods
range from 438.5 to 540 nm. However due to the over- The system consists of a fused silica lenses telescope
lapping ofits absorption spectrum with the NO absorp- 84-UV-25 (Ocean Optics) with a 25.4 mm diameter,
2
tion spectrum the concentration of both elements is focal length of 100 mm. This telescope is especially sui-
measured [8]. table for light collimation from a distant source. The
It is very well documented that the solar radiation in collimated light is guided to the spectrometer using an
the 190–280 nm range is strongly absorbed by the optical fiber (QP600-2-SR, Ocean Optics) with a 600 m
stratospheric ozone. Hitherto every report states that diameter core and 2 m long. The spectra measurement
almost all incoming solar UVC and 90% of UVB are was performed using a miniature spectrometer (USB
absorbed by stratospheric ozone [9]. As a result, there 2000, Ocean Optics) with a diffraction grating of 2400
are no published experiments measuring the absorption lines/mm. A cylindrical lens is incorporated to the de-
levels in the UVC range. However, if there is any irradi- tector to increase light collection efficiency focusing a
ance left, once the radiation passes to the troposphere 1 mm high slit onto a considerably shorter (200 μm) de-
the O concentration decreases and it would be possible tector elements. The CCD detector (ILX511A, Sony) is
3
to detect the remaining radiation in UVC with a suffi- coated with an UVanti-reflecting film to reduce the re-
ciently sensitivedetector. flection losses and therefore increasing their sensitivity.
Thesolar irradianceat theearth’ssurface varies greatly The spectral range of the spectrometer is in the 190 to
depending on factors such as latitude, time of the day, 355 nm range with a resolution of 0.5 nm. This system
month of the year, cloud cover, and haze (aerosols). The shows a high signal to dark current random noise ratio
spectral irradiance in the UVC range on the earth sur- in the 190 to 290 nm range (over 3 times for measure-
face also depends on the ozone column, on the altitude ments under pollution measurements and about 12
above sea level and the solar zenith angle. Very sensitive times under clean sky measurements). The system was
measurements from 280 to 300 nm at sea level, 33° checked and no overlapping of different diffraction
North latitude and 25.14° Solar Zenith Angle (SZA), orders and the scattering of the incident light which
using PMT detectors, reported the detection of solar could affect the readings in the UVC region was
spectral irradiance of 3x10-4 Wm-2 nm-1 [10]. Similar detected. To reduce the background noise of the mea-
measurements have reported a value of two orders of surementanaveragingof3to5spectra wasperformed.
magnitude higher at 2500 m altitude, 220 North latitude The coordinates of the measuring spots were deter-
and7.8°SZA[11]. minedwithaGPSlocator(GT37231).Alltheinformation
TheMexicanValleyisahighplateauincentralMexico inputsalaptopwerethedevelopedsoftwareprocessesthe
with a minimum altitude of 2200 m and latitudes bet- data. The coordinates given by the GPS locator were
ween 19o and 20o North Latitude. Under such condi- superimposed on the Google satellite images using the
tions the solar zenith angle is almost 0o at noon. By Google-Earth-KH-80.llb libraries to show the actual mea-
what, these values propitiate a higher probability to de- suringpointsonasatellitephotograph.
tect some irradiance at UVC wavelength range with very The attenuation of the irradiance I (λ) is described
0 R
sensitive detectors as photomultiplier tubes used in by the Beer-Lambert’s Law I(λ)=I (λ)e−σ(λ) c(s)ds. The
0
spectroradiometers or charged coupled devices (CCD) attenuation depends of the concentration c(s) of the
used in minispectrometers [12]. In Mexico City the O different compounds of the light path media [14]. The
3
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absorption cross section σ(λ) defines the light absorption a Hg lamp could be used because, these are narrower
to a specific wavelength. The dependence of the cross (~10 pm) [22] than the typical resolution of the used
sections σ(λ)on the temperature and pressure has not spectrometer (0.1–1 nm). The measured spectrum is a
been considered in this work. The gases columns are good approximation of the spectrometer transference
evaluated by using the mixing ratio method [15]. The function [17]. Mathematically the measured spectrum
light absorption cross section of a specific pollutant can I*(λ)can be expressedasthe convolutionofthe Hg lamp
be obtained from reported data on literature or from la- emission spectrum I(λ') with the spectrometer transfer-
boratorymeasurements. ence function I*(λ)=I(λ0)∗Hsp(λ0), where Hsp(λ') is the
Usually, it is very difficult to obtain separately theRpol- spectrometer transfer function. The STF was measured at
lutant concentrations c(s) and the optical length l= ds. 334.1nmbecauseatthiswavelengththe“cleanest”signalis
Therefore, these parameters are comRbined to form what obtained.
is called slant column density S= c(s)ds expressed in DOASIS software, developed at Heidelberg University
molecules/cm2[16,17]. [23], was used to perform the convolution of the trans-
Actually,themeasuredabsorptioninvolvesallthepresent ference function of our instrument. The high resolution
molecules that absorb in that specific wavelength. The ab- SO2 spectrum is used to obtain our reference spectrum.
sorption of several substances i will be then described by To observe if there is any shift of the SO2 reference
X
the following expression: IðλÞ¼I0ðλÞe(cid:2) i σiðλÞSi, where σi(λ), aspbesoctrrputmionreslpaetectdrutmo twhiethcoanXvoelulatimonp p(Nroecwespso,rtt,hemoSdOe2l
ci(S) and Si are their absorption cross section, its specific 969607) was measured. A constant shift of 0.98 nm was
concentration and its slant column density, respectively.
obtained and it must be considered when the trace gas
ThesunlightspectrumthatreachestheEarthsurfaceshows
concentration calculation is performed. Figure 1 shows
slow and fast variations. The slow variations of the
theSO2referenceandsamplespectra.
spectrum are determined by the light source and by the
A 2.5 mbar, 3 cm long, SO2 cell was used to validate
scattering. The fast variations are determined by the ab-
our DOAS system. We used a Xe-lamp as radiation
sorption of molecules. The spectrum of the cross sections source. The measured column agrees within a +− 4%
canbedividedina slowvariationsσiS(λ)andfastvariations error. To determine the density of a measured column a
partsσiF(λ).Therefore,theabsorptioncrosssectionforeach non-linear adjustment is done by using the Levenberg-
componentwillbeσi(λ)=σiS(λ)+σiF(λ). Marquardt method [24] using the measured I (λ) and
Theslowvariationspartofthemeasuredlightintensity, the reference spectra I (λ). A dark spectrum, pr0eviously
whichdependsonI0(λ),canbeapproximatedbyapolyno- captured, is subtracted0from I(λ) and I (λ).AresidualΔψ
mial. Using some polynomial adjustment method or by 0
is obtained from the fitting process, in our case in the
using high-pass digital filters, for instance the Savitzky-
order of 2.5x1017 particles/cm2, which can be used to
Golay[18]asolutionforfastvariationspartσiF(λ)couldbe calculate the detection limit of our system trough the
obtained[4,19].
relation S ¼Δψ [17]; hence the detection limit is the
For the detection of only one gas component in a min σ0
orderof1ppm.
spectral interval, if the scattering is small compared to
the slow variations of radiation I (λ) the pollutant slant
0 The spectrometer is connected to a laptop. A program
column densitycanbeexpressedas:
was developed in Labview to capture and filter the mea-
(cid:2) (cid:3)
sured spectra to perform the non-linear adjustment and
ln II0ððλλÞÞ DðλÞ to identify the amount of polluting gas. This software
S ¼ ¼
σFðλÞ σFðλÞ consists of several programming stages. First of all, a
driver for the Windows operative system is instal-
Where D(λ) is the Optical Density of the medium led from the spectrometer that comes from the
[4,15]. The cross sections of the gas trace at the atmo- OOIDDrv32.dll (Ocean Optics, 2011) library. Once the
sphere could be considered independent from their hardware is recognized a Sarvitzky-Golay filter program
placement; this means that they are independent from filters the reference I (λ) and measured I(λ) spectra. A
0
theiraltitude[20]. dark spectrum is subtracted from and I(λ) to reduce the
High resolution absorption spectrum for the SO has thermal noise of the instrument itself. Immediately, the
2
been reported [21]. The reference spectrum for the relation of these spectra is calculated and is applied the
evaluation of the measurements is needed and it must logarithm function. Spectra are geographically refe-
consider only the fast variations σiF(λ) and the spectral renced throughout the journey to integrate all the SO2
deformations produced by the spectrometer and deter- measured columns. A mouse-type GPS was used with
mined by the spectrometer transfer function (STF). In the NMEA protocol to link it with the program, to do
practice, to obtain the STF the spectral lines emitted by so, an API for Windows is used which is provided by
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3.0x10-19 Reference 2 Sample
m 1.0x10-2
c
e/
2.0x10-19 ul
c
e
ol 5.0x10-3
1.0x10-19 m
n
o
0.0 pti 0.0
r
o
s
b
-1.0x10-19 al a -5.0x10-3
ntti
-2.0x10-19 e
r
e
Diff -1.0x10-2
-3.0x10-19
280 300 320 280 300 320
Wavelength [nm] Wavelength [nm]
Figure1ReferenceandSamplespectratodeterminetheSO content.
2
Labview under theVISA name, which allows to read the using mixing ratios [27,28]. The SO flow is calculated
2
data from theGPS ([25];Visa,[26]). in tones by day [14], using column densities measured
When the DOAS technique is used, the path length of by the system, which must be multiplied by the distance
light depends on the number of scattering events that of the traverse route using the following formula:
take place until the photon reaches the spectrometer. ton=day¼0:00023ZxxnSO2ðppmÞ∗X∗VsenθdX Here X is
1
Some methods to obtain at least the statistics of photon the traverse distance in meters from the point x1 to xn,V
propagation are based on the simulation Monte Carlo is the wind speed in m/s, θ is the angle between vectors
method [17] to be able to validate statistically what ha- X and V. For the real time in situ measurements around
ppens with a large amount of photons it is necessary to the industrial zone and the volcano, the telescope was
simulate such events unfortunately this process requires fixed on a car for the traverse. Five spectra were ave-
acomputing robust systemandthesimulation process is raged in each data entry to minimize the noise effects.
not performed in real time. That is not practical for The spectra are taken every 3 seconds. The system takes
insitu measurements. Taking this into consideration, in firstly the dark spectrum to reduce the noise produced
this work the column densities are indicated in ppm*m, by the spectrometer and the skylight spectrum Iskl(λ) is
A B
32
Skyligth spectrum ] 120 Clean Skylight
s
s]24 Iskl( ) nit 100
t u
i
n
u e
v 80
rb.16 ati
e [a Spectrum crossing the plume [rel 60
nc I( e
a 8 c 40
di an Crossing the plume
ra di
Ir 0 ra 20 Dark spectrum
Dark spectrum r
I
0
200 240 280 320 200 220 240 260 280
wavelength [nm] wavelength [nm]
Figure2A.cleanskylightspectrumIskl(λ),spectrumwhilecrossingtheplumeI(λ),andthedarkspectruminthewholeworking
wavelengthrange.B.SameopticalIrradiancespectraenlargedintheUVCwavelengthinterval.
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1.4
s]
nit 1.2
u
b. 1.0
ar
y [ 0.8
sit 0.6
n
e
al d 0.4
c
pti 0.2
O
0.0
200 220 240 260 280 300 320 340
Wavelength [nm]
Figure3Opticaldensityobservedfrom200to350nm.
measured. Finally, the sample spectra are taken during band structure related to scattering and absorption in
thetraversearound theindustrialzone. the presence of other molecules in the UVC range can
be observed. Usually, this interval is neglected in most
Results studies. This result shows the importance to develop
The reference and measured spectra used for the deter- some similar algorithms to evaluate the concentration
mination of SO content in the 280–320 nm wavelength of species absorbing in this range, such as tropospheric
2
range are shown in Figure 1. The reference spectrum ozone.
was obtained according to the method described earlier. After the high-pass filter is applied and the spectrum
The absorption bands of SO are very well defined in restricted from 290 to 315 nm range, the differential ab-
2
thisinterval. sorption SO spectrum is obtained and it is presented in
2
Theopticalspectrainthe200-350nmwavelengthinter- Figure4.
val with and without contamination are presented in The developed system was used to measure both,
Figure2A.Theskylightspectrumisdetectedwitha“clear” anthropogenic and natural SO emissions. Two sites
2
sky, which means, without contamination plume. It is with significant SO concentration were chosen near
2
clearly observed the attenuation of the spectral irradiance MexicoCity.
on the Earth surface due to the pollutants absorption.
Morethanthat,itisperfectlyobservedthatacertain“non MeasurementsatTulahidalgorefinery
despicable” irradiance is detected in the UVC region. The InTulaHidalgo,Mexico,twonearbyindustriesarelocated:
UVCwavelengthintervalhasbeenenlargedandpresented “Miguel Hidalgo” PEMEX oil crude refinery and CFE
in Figure 2B where an absorption band structure is ob- thermo power station “Francisco Pérez Ríos”. The satellite
servable. The attenuation in the plume in the range bet- image is presented in Figure 5A. Once the traverse was
ween200to290nmofthemeasuredspectrumI(λ)could completed, the SO columns are calculated using a wave-
2
be related to the O concentration, which is the main lengthintervalfrom290–350nm.Thevariationsinppm*m
3
atmospherecomponentabsorbinginthatregion. canbeobservedineachcolumnandtheresultsareshown
The optical density is presented in Figure 3. The cha- in Figure 5B. Both industries are responsible for the emis-
racteristic SO absorption bands are very well defined in sionofnearly350t/dofSO [29].Inthissense,itisacon-
2 2
the spectrum range between 290 and 320 nm. Besides, a venient place to test the equipment. The wind direction is
n
ptio e 0.08
or ul 0.04
s c
b e
al a mol 0.00
enti m/ 2 -0.04
er c
f
Dif -0.08
290 300 310 320
wavelentgh [nm]
Figure4SO differentialabsorptionspectruminthe290to320nmintervalafterfiltering.
2
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A
B
200
m
*
m
End point 175 p
p
d
150 e
v
e
125 ri
Starting Point et
r
100 ms
Stack u
20.06 75 ol
c
2
20.05 50 O
S
20.04
25
Latitude 20.03 -99.252
Traverse
-99.270
20.02
-99.288 Longitude
20.01 -99.306
Figure5A.SatelliteimagefromMiguelHidalgorefineryinTula.TheSO pollutingchimney,thewinddirection,andthetraverseroute
2
arepresented.B.showsSO measuredcolumnswithpassiveDOASsystemaroundMiguelHidalgoPEMEXrefineryinTulaHidalgo.
2
presented on the satellite image, Figure 5A. Also, the MeasurementsatPopocatepetlvolcano
traverse route is shown on both, the satellite image Popocatepetl volcano is located at the boarding limits
(Figure 5A) and the corresponding SO column distribu- between Estado de Mexico, Puebla and Morelos. Its
2
tion (Figure 5B). The starting and ending points of the coordinatesare19.023°NorthLatitudeand98.627°West
traverse are indicated. The result presented in Figure 5 Longitude. A traverse from San Buenaventura Nealtican
(A and B) corroborates that the distribution of the SO to San Nicolas de los Ranchos in Puebla was done; it is
2
columns with higher concentrations are related directly shown in Figure 6A. It has to be noticed that the tra-
withthewinddirection. verse direction is from east to west, as the starting and
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A
B
300
m
*
250 m
p
p
s
End point 200 mn
u
ol
150 C
e
v
Starting Point e
19.088 100 etri
19.080 R
19.072 50 O2
S
Latitude 19.064
0
19.056 Traverse -98.406
-98.424
19.048 -98.442
-98.460
Longitude
-98.478
Figure6A.SatelliteimagefromPopocatepetlvolcanoinwhichthewinddirectionandthetraverseroutearedepicted.B.showsSO
2
measuredcolumnsatPopocatepetlvolcanowithpassiveDOASsystem.
ending points are indicated. The measured SO columns second part of the traverse the measured values of SO
2 2
are shown in Figure 6B. The first part of the traverse is columnsremainpracticallyconstant.
oriented almost radially to the volcano coordinates. At
the starting point the system was calibrated. As the Discussion
measuring point approaches the volcano the SO co- Usually,themeasurementofSO columnsbythepassive
2 2
lumns readings increase, as it is expected due to the dif- DOAS technique is done using the 290 to 330 nm wave-
fusion andthedilution ofthepollutioncloud. length range. In the wavelength range from 230 to
At some point of the traverse, indicated with an arrow 260 nm the ozone is the principal absorber. Albeit the
on Figure 6A, the direction changes and it becomes arrangement of our spectrometer shows less resolution
nearly perpendicular to the volcano’s direction. In this than a similar one with a range from 290 to 355 nm,
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from Figures 5 and 6 it can be clearly observed that the Authordetails
developedsystemhasstillenoughresolutiontodetermine 1SEPI-ESIME-Z.InstitutoPolitécnicoNacional,Av.IPNS/N,UPALMEdifZ,3er
pisocp.,07738,México,D.F.,México.2DireccióndeInvestigaciónyPosgrado.
theSO pollutantdistributionand,afterFigure3(foralti-
2 InstitutoMexicanodelPetroleo,EjeCenetralLázaroCárdenas,México,D.F.,
tude and latitude conditions as in the Mexican Valley), México.
alsocouldbeusedtosimultaneouslymeasureO columns
3
Received:13December2012Accepted:15December2012
inthe230to260nmrange.
Published:8January2013
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