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Measurement of regional distribution of atmospheric NO2 and aerosol particles with flashlight long-path optical monitoring
ARTICLE IN PRESS
Atmospheric Environment 39 (2005) 4959–4968 www.elsevier.com/locate/atmosenv
Measurement of regional distribution of atmospheric NO2 and aerosol particles with flashlight long-path optical monitoring Si Fuqia,b, Hiroaki Kuzea,, Yotsumi Yoshiia,c, Masaya Nemotoa, Nobuo Takeuchia, Toru Kimurad, Toyofumi Umekawad, Taisaku Yoshidad, Tadashi Hiokie, Tsuyoshi Tsutsuif, Masahiro Kawasakig a
Center for Environmental Remote Sensing, Chiba University, Inage-ku, Chiba 263-8522, Japan Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China c Toyama National College of Maritime Technology, Shinminato, Toyama 933-0293, Japan d Kyoto Electronics Manufacturing Co., Ltd, Kisshoin-Shinden, Minami-ku, Kyoto 601-8317, Japan e Kyoto Prefectural Comprehensive Center for Small and Medium Enterprises, Shimogyo-ku, Kyoto 600-8813, Japan f Kyoto Prefectural Institute of Hygienic and Environmental Sciences, Fushimi-ku, Kyoto 612-8369, Japan g Department of Molecular Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan b
Received 15 October 2004; accepted 2 May 2005
Abstract We report on an air-pollution monitoring campaign in the urban Kyoto area conducted during December 2003 and January 2004. The method is based on the differential optical absorption spectroscopy (DOAS) using an aviation obstruction light (white flashlight). Three sets of DOAS spectrometers, each consisting of a telescope and a compact charge-coupled device (CCD) spectrometer, were operated to cover the southern part of the city area. Three sites were chosen in the urban Kyoto area, and the average concentrations of NO2 were measured with optical path lengths of 2.7, 4.1, and 7.1 km. The aerosol optical thickness was simultaneously measured for the path length of 2.7 km. It is found that the temporal variations of the retrieved amount of the pollution species agree with each other and they are also consistent with concurrent results of the ground sampling measurements. r 2005 Elsevier Ltd. All rights reserved. Keywords: Flashlight DOAS; Urban atmosphere; Air pollution distribution; Aerosol extinction; Ground sampling
1. Introduction The technique of differential optical absorption spectroscopy (DOAS) is based on the recording of differential absorption, i.e., the difference between local maxima and minima in the absorption spectrum, of the probed gas species (Platt et al., 1979). Compared with conventional methods of point sampling, the DOAS Corresponding author. Fax: +81 43 290 3857.
E-mail address: [email protected] (H. Kuze).
system is more suitable to monitoring horizontally averaged concentrations. For example, Edner et al. (1993) measured the concentrations of NO2, SO2, and O3 with 0.4 km, 1.6 km, and 2.0 km path lengths. The beam-finding servo system and automatic gain control enabled unattended, long-time monitoring. Evangelisti et al. (1995) applied the technique to measure NO2, HNO2, SO2, and O3. Using a path length of 1.7 km, they accomplished sensitivity of 1–10 ppb for these molecular species. More recently, there has been growing interest in advanced techniques to monitor pollutants in the
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.05.002
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atmosphere in view of serious air pollutions in city areas, particularly those in Asian countries. Potentially, the method of DOAS enables simultaneous monitoring of a variety of pollutants found in the atmosphere (Kim and Kim, 2001; Lizamma et al., 2001; Meller and Moortgat, 2000; Stutz and Platt, 1996). In the conventional DOAS approach, light from a broadband xenon high-pressure lamp is transmitted through the atmosphere for distances of several kilometers. The light is then received and analyzed by the combination of a monochromator and a detector such as a photo-multiplier tube, a photodiode array or a charge-coupled device (CCD). In a previous paper, we demonstrated the usefulness of an aviation obstruction light (white flashlight) for DOAS application (Yoshii et al., 2003). Such flashlights are widely used for safety of aviation traffic in many places. In Japan, obstruction lights are installed on tall constructions (above 60 m), and are detectable in every direction from several kilometers away. The optical thickness of NO2 molecules can be retrieved by detecting the transmitted light intensity using a simple telescope and a commercially available, compact CCD spectrometer. Besides, additional knowledge about the spectral intensity of the light source makes it possible to retrieve the optical thickness of the aerosol particles existing in the light path (Yoshii et al., 2003). In the present paper, we describe the application of this flashlight DOAS technique, and show the results of a campaign study conducted in Kyoto district, Japan, during December 2003 and January 2004. Measurements were made using three sets of the DOAS spectrometers, simultaneously monitoring a single light source located at the central part of the city. The temporal variations of NO2 molecules and aerosol particles are retrieved from the data, and compared with the results of conventional measurements at the ground level. The purpose of the present study is twofold. First, this is a first case study in which the flashlight DOAS technique is applied to multisite observations. The resulting area coverage gives us the possibility of understanding the spatial behavior of the air pollution. Second, we compare the long-path result with the point data simultaneously measured at groundbased monitoring stations. This gives us an insight into the vertical homogeneity (or inhomogeneity) of the local atmosphere.
2. Experiment 2.1. DOAS system Fig. 1 shows a schematic of the experimental setup. Since details were given in our previous paper (Yoshii et al., 2003), here we give only a brief outline. An
Telescope
Entrance slit Grating
Linear CCD array USB2000 Spectrometer Notebook PC
Height 67.5m Osaka Gas facility
Fig. 1. Experimental setup for measuring NO2 and SPM.
astronomical telescope (Meade, DS-115), with an aperture diameter of 115 mm, focuses the image of a point light source located at a far distance. The image is formed near the eyepiece location. The eyepiece itself is removed from the telescope, and instead, the entrance slit (5 mm wide) of a CCD spectrometer (Ocean Optics, USB2000) is placed. This spectrometer (89 mm wide 63 mm long 34 mm high) has a mechanically stable, crossed Czerny–Turner design with a fixed grating. The CCD array consists of 2048 elements and has sensitivity in a wavelength range of 200–800 nm. The resulting resolution is 0.3 nm per pixel on average. The CCD gate duration is set at 300 ms in the experiment. Between successive gate periods, there exists a time lag of 7 ms, in which each spectrum is sent to a personal computer (PC) through the universal serial bus. Data acquisition can be attained successfully even when no trigger (synchronous with the flashlight) is applied to the CCD spectrometer, though this relatively long gate time as compared with the flashlight duration (about 0.5 ms) causes somewhat increased amount of the background skylight. For the measurement in the Kyoto district (35.01N, 135.51E), we made use of a xenon strobe (Sanken, FX-7) install at the top of Osaka Gas facility as a light source. The lamp height is 67.5 m above the ground level, and 95.5 m above sea level (ASL). Three lamps are equipped at the same height, each covering a horizontal range of 1201 with a daylight intensity of approximately 300 W sr1. All the lamps flash for approximately 0.5 ms every 1.5 s (40 flashes a min, in accordance with the regulation) in a synchronous manner. According to the regulation, the light intensity is diminished at dusk and dawn, and during the nighttime blinking red lights replace the flashlights. Thus, the DOAS measurement is limited to the daytime, around 7 a.m. to 5 p.m. during the winter months. For the measurement of the regional distribution of atmospheric NO2, three detector setups were situated at
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Fig. 2. Site map. Locations are shown for three light paths for the DOAS measurement and three stations for ground sampling measurement.
different places (Fig. 2). In the southwest direction, a DOAS system was installed in a building of Kyoto University (KU) (about 120 m ASL, and 5.5 km from the source), and a second one at Kyoto Electronics Manufacturing (KEM) (about 34 m ASL, and 2.7 km from the source) in the south direction. A third setup was placed in the southeast direction on the building of Kyoto Prefectural Institute of Hygienic and Environmental Sciences (IHES) (about 51 m ASL, and 7.1 km from the source). The alignment of each telescope system was checked at least once a day (the alignment check for the KU system was sometimes less frequent). During the time interval between 7 December 2003 and 10 January 2004, data were acquired successfully with a probability of 77%. In addition to the misalignment, the failure was brought about also from the rainy weather conditions. The locations of three ground stations are also indicated in Fig. 2. Although the agreements between their locations and the three DOAS optical paths are not always satisfactory, we compare the DOAS data from the KEM site with the Minami ground station, IHES site with the Fushimi station, and KU site with the Nishikyo-ku station in the present analysis. 2.2. Outline of the site Kyoto city, famous for her historical heritage, is situated in a basin, surrounded by mountains except in the southward direction. In the north, Mt. Hieizan (848 m) and Mt. Atagoyama (924 m) are prominent
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peaks, and relatively lower mountain peaks limit the east and west landscape of the area. In winter (December– February) the average temperature is about 6 1C, and northwest to west wind directions are predominant. Because of these wind directions, transportation of the polluted air exhibits only limited effect during the winter months, and consequently, it is likely that most of the NO2 emission originates from sources in the Kyoto basin itself. Since pollutions from industrial activities are rather limited, automobile emissions are considered the main source of the air pollution, including the suspended particulate matter (SPM, referring to aerosols with diameters less than 10 mm). There are several main roads passing through our test area, such as route 1, route 9, route 24, and route 171; besides, the Meishin highway runs between the light source and IHES test site (Fig. 2). The ground sampling stations are operated by municipal governments. In these stations, NO2 concentrations (here shown in units of parts per billion, ppb) are monitored using the chemi-luminescence method, and the SPM amount (in units of mg m3) using the b-ray method. The data are given every hour, and available through the Internet.
3. Analysis of the DOAS spectrum 3.1. NO2 concentration The analysis of the DOAS spectra is based on the Beer–Lambert’s law expressed as IðlÞ ¼ kI 0 ðlÞeLsðlÞn ,
(1)
where IðlÞ is the measured intensity, k is the system constant, I 0 ðlÞ the unattenuated reference intensity, L the path length, sðlÞ the wavelength-dependent absorption cross section, and n the number density of the species averaged over the path length. The dimensionless quantity LsðlÞn represents the optical thickness, denoted as t. In the real atmosphere, both molecules (Rayleigh scattering) and aerosols (Mie scattering) contribute to the radiation extinction. Adding these contributions to Eq. (1) and solving the equation with respect to t, we obtain tðlÞ ¼ ln
kI 0 ðlÞ ¼ L½sðlÞn þ aM ðlÞ þ aA ðlÞ. IðlÞ
(2)
Here aM ðlÞ is the molecular extinction coefficient which is the product of the molecular cross section, sM ðlÞ, and the number density of air molecules, nM. Similarly, the aerosol extinction coefficient, aA ðlÞ, is defined as sA ðlÞnA , where sA ðlÞ is the aerosol cross section, and nA the number density of aerosol particles. Cross sections on the right-hand side of Eq. (2) can be separated into two components, slowly varying one,
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i
þL
X
Diff. Cross Section (10-19 cm2)
sSi ðlÞ, and rapidly varying one, sR i ðlÞ, with wavelength. Both the molecular and aerosol cross sections are slowly varying components of the optical thickness. Hence we obtain X tðlÞ ¼ L sR i ðlÞni ! sSi ðlÞni þ sM ðlÞnM þ sA ðlÞnA ,
ð3Þ
i
1 0 -1
Here DtðlÞ on the left-hand side denotes the peak-topeak variation of the observed optical thickness and DsðlÞ on the right-hand side that of the absorption cross-section data. In the present case, the resolutions of DtðlÞ and DsðlÞ are about 0.3 and 0.017 nm, respectively. Before the spectral matching procedure, the cross-section data are convoluted with the slit function of the CCD spectrometer, which can be determined by measuring the emission spectra of a Hg lamp (Si and Liu, 2002). Comparison between DtðlÞ and DsðlÞ is illustrated in Fig. 3. An example of the actual DOAS spectrum is shown in Fig. 3(c). This was measured at the KEM site on 15 December 2003. The resulting value of the average NO2 concentration is 22.371.0 ppb. In order to evaluate the accuracy of the present spectral matching procedure, a calibration experiment with a standard gas cell was conducted. In this test, diluted NO2 gas (N2 balance) with three different
420 430 440 Wavelength (nm)
450
400
410
420 430 440 Wavelength (nm)
450
0
-1
(b)
Diff. Optical Thickness
(4)
410
1
0.02
In the present analysis of DOAS spectra, the determination of NO2 amount relies on the spectral matching procedure expressed as
400 (a)
3.2. Accuracy of NO2 retrieval
Dt ðlÞ ¼ Ln DsðlÞ.
2
-2
Diff. Cross Section (10-19 cm2)
where subscript i denotes trace gas species such as NO2, O3 or SO2. On the right-hand side of Eq. (3), the first term describes the rapidly varying part, and the second the slowly varying part. The number density for each species is calculated through least square fitting (Gomer et al., 1993; Stutz et al., 2000) after subtracting the slowly varying part from Eq. (3). In the present analysis, we choose a wavelength range of 400–450 nm for the retrieval of the optical thickness due to NO2 molecules. In this wavelength region, the contribution of other trace gas species is negligible as compared with the NO2 contribution. As the cross-section data, sðlÞ in Eq. (2), we employ a laboratory spectrum obtained by Vandaele et al. (1998), with suitable adjustment of the spectral resolution as explained below (Section 3.2). In the conversion of the data from the optical thickness to the concentration in ppb, we postulate a fixed temperature of 280 K.
3
Dec. 15 13:00 KEM
0.01
0.00
-0.01
-0.02 400 (c)
410
420 430 440 Wavelength (nm)
450
Fig. 3. Spectral matching procedure. (a) Cross-section data of NO2 absorption with the original resolution of 0.017 nm. (b) Cross-section data after the convolution with the slit function of the spectrometer. (c) An example of the actual DOAS spectrum measured at KEM site on 15 December 2003. The resulting value of the average concentration is 22.371.0 ppb.
concentrations (256, 510, and 682 ppm) was filled in a 30-cm long glass cell. The result is summarized in Fig. 4, where the ppb values are those expected for a typical
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4. Results and discussion Y=1.06X-0.0026
DOAS (ppb)
40
4.1. Concentration of NO2 from DOAS and ground measurements
R=0.998
30
20
20
30
40
Standard Gas (ppb) Fig. 4. Correlation between the results of the DOAS measurement and the laboratory measurement using standard gas.
DOAS distance of 5 km. These are hypothetical values converted from the gas cell concentrations, giving the same optical thickness as the laboratory experiment. From this calibration measurement, the accuracy of the retrieval of the NO2 concentration is about 74% in a range of 10–50 ppb. 3.3. Aerosol retrieval In a previous paper (Yoshii et al., 2003), we pointed out the possibility of using the DOAS spectrometer as a transmissiometer to derive the aerosol optical thickness along the optical path. Here the method is applied to the DOAS spectra observed in the Kyoto district. In the case of aerosol retrieval, the unattenuated spectrum, I 0 ðlÞ, has to be measured separately. For the aerosol optical thickness, we assume a form of tA ¼ Bðl=550ÞA
(5)
for the wavelength dependence. Here the wavelength l is measured in units of nm. The constants A (Angstrom exponent) and B (aerosol optical thickness at 550 nm) are to be determined by fitting this expression to the optical thickness derived from the DOAS spectra. In the actual analysis, additionally the system constant k in Eq. (1) must also be incorporated in the iteration procedure together with A and B. The inclusion of k takes the temporal variation of experimental conditions (light intensity, detection efficiency, etc.) into account. In order to reduce the correlation among the unknown parameters, A, B, and k, here we obtain the system constant from the data obtained on relatively clean days. Details of this procedure will be explained below (Section 4.2).
The optical thickness of NO2 was measured with DOAS setups at KEM, IHES, and KU sites. The reference spectrum, I 0 ðlÞ in Eq. (1), was obtained from a measurement at a location close to the light source, with an optical path length of about 100 m. Fig. 5 shows data observed for five days between December 2003 and January 2004. For all the cases, a reasonable correlation is found between the result of the long-path measurement (right panels) and the data from a ground station (left panels) near each optical path. Also it is noted that temporal variations observed for the three paths fairly agree with each other, though the concentrations are not always the same. This implies that during the time period, the atmosphere was changing in a consistent way in the region, but the NO2 distribution was not necessarily homogeneous across the region. Among the five DOAS data shown in Fig. 5,(b2), (c2), and (d2) represent typical behavior of the NO2 concentration in winter, in that a high concentration is observed in the morning, and the concentration becomes lower around noon. This is presumably due to the presence and disappearance of the inversion layer near the ground level. Such behavior is not seen in Fig. 5(a2) and (e2). This is probably ascribed to the lack of radiative cooling during the midnight. Fig. 6 shows the correlation of the NO2 concentration between the ground sampling and DOAS measurements. The number of data points in each panel is about 300, corresponding to a total observation time of 300 h (Note that 12 DOAS data obtained in 1 h are averaged in this correlation study). Since the locations of the ground-sampling stations are not exactly below the DOAS paths (Fig. 2), and the DOAS data represent averages over the paths, one cannot expect a full correlation between the two methods. Nevertheless, from Fig. 6, a reasonable correlation is found between the result of the long-path measurement and the data from a ground station near the optical path. The correlation coefficients, however, are in a range of 0.72–0.85, indicating occasional discrepancies between the two methods. In Fig. 6(c), the DOAS concentrations observed at the KU station are close to the ground station values. This is seen from the slope of the correlation fitting (close to 1.0), although the scattering of the data points degrades the accuracy of the comparison between the two values. For the KEM site (Fig. 6(a)), the DOAS values are higher than the ground values, as manifested in the slope of 1.06 in the correlation fitting. On the contrary, the DOAS data for the IHES site (Fig. 6(b)) are smaller
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(a1) 11 Dec
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(b1) 12 Dec. 40
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(c1) 13 Dec.
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(c2) 13 Dec.
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0 (e1) 7 Jan.
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(a2) 11 Dec.
12:00
17:00
(d2) 6 Jan.
DOAS
(e2) 7 Jan.
DOAS
07:00
12:00
17:00
JST Fig. 5. Diurnal change of NO2 concentration observed for 5 days in December 2003–January 2004. Nishikyo-ku station is near the KU site, Fushimi station near the IHES site, and Minami station near the KEM site. (a1 and a2) 11 December 2003, (b1 and b2) 12 December 2003, (c1 and c2) 13 December 2003, (d1 and d2) 6 January 2004, and (e1 and e2) 7 January 2004.
than the ground values, as indicated by the slope of 0.81. In these cases, it is likely that the presence of the heavytraffic roads might have affected either the DOAS data or the ground-sampling data in a significant way. 4.2. Aerosol retrieval In the aerosol retrieval from the DOAS data, we need to reduce the number of unknown parameters. Here we apply a method in which a fixed value is assumed for the system constant k, as obtained from the data with maximum light intensities detected. This method relies
on the fact that within the potential sources of the optical thickness in the DOAS spectra, normally the aerosol part is by far the largest and this part shows the highest variability (an example will be shown below). Thus, the observation of maximum intensity indicates that the aerosol contribution has been minimum (clear day), and at the same time, the system efficiency (including the telescope alignment) has been more or less optimized. When this ‘‘best spectrum’’ is used as a (tentative) reference, it is possible to calculate the difference in the aerosol optical thickness between the turbid and clear days. Then, in a second step, the
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Y=-9.9+1.06X
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KEM (ppb)
R=0.85
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(a)
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Y=2.57+0.81X R=0.72 60 IHES (ppb)
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In(I0/I)
B=0.32
Rayleigh Aerosol
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30 60 Fushimi St. (ppb)
(b)
KU (ppb)
R=0.76
30
0 0 (c)
30 Nishkyo-ku St. (ppb)
600
Fig. 7. Retrieval process of the aerosol concentration. (a) Spectra observed at 11:00 and 12:00 Japan Standard Time on 4 January 2004. Both are obtained by averaging hourly data. The spectrum at 12:00 is used as a (tentative) reference spectrum. (b) Total optical thickness and molecular (Rayleigh) and aerosol components derived from the spectra in (a).
Y=-3.6+0.97X
60
575 Wavelength (nm)
60
Fig. 6. Correlation between the results of the DOAS and ground sampling, point measurements for the NO2 concentration. (a) KEM site (DOAS) and Minami station, (b) IHES site and Fushimi station, and (c) KU site and Nishikyo-ku station.
Angstrom exponent (A in Eq. (5)) is derived from the DOAS spectrum itself. This process of aerosol retrieval is illustrated in Fig. 7. Fig. 7(a) shows the original spectra measured at the KEM site during 10:30–11:30 (local time) and during
11:30–12:30, both on 4 January 2004 (each represents data averaged over one hour). Examination of the dataset on the day indicates that the spectrum during 11:30–12:30 (referred to as 12:00 hereafter) shows the maximum intensity, which we assume to be a tentative reference. For the aerosol retrieval, we make use of the spectral region between 550 and 600 nm, since this region is in effect free from absorption due to NO2, water vapor or ozone. Fig. 7(b) shows the relative optical thickness obtained by comparing the data at 11:00 with the data at 12:00 (reference). The total optical thickness, as well as the Rayleigh and aerosol components, is shown. In this case, the Angstrom exponent, A, is 0.8670.09, and the aerosol optical thickness at 550 nm (turbidity constant), B, is 0.3270.03. Normally the Angstrom exponent takes a value around unity and a value somewhat smaller indicates the dominance of relatively fine particles (Pinker et al., 2004). The value of optical thickness, on the other hand, shows that the average value of aerosol extinction coefficient
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4.3. One month variation of NO2 for three sites For the NO2 concentration observed at the three sites (KEM, IHES, and KU), the change in NO2 concentration during 30 days is shown in Fig. 10, in which a plotted point represents an average over the daytime data. An obvious correlation can be found among the sites. The highest value of NO2 concentration is found at the IHES site, while the KEM site exhibits the smallest value. This is mainly ascribable to the amount of the vehicle traffic that runs below each light path, if the transport of the air pollution is insignificant. For IHES, the light from the source travels a distance of 7.1 km over several main roads before the detector receives it. For the KEM site, the number of main roads is limited
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OT SPM
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(d) Dec. 24
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0.04
2.4
OT SPM
(e) Jan. 7
0.06
0.04
1.6
0.02
0.8
0.00
0.0 07:00 Fig. 8. Diurnal change in the aerosol optical thickness (KEM site) and SPM concentration (Minami station). (a) 4 January 2004 (clear day for reference), (b) 11 December 2003, (c) 12 December 2003, (d) 24 December 2003, and (e) 7 January 2004.
0.04
12:00 Time
17:00
SPM (mg/m3)
is about 1.2 104 m1, a typical value in the urban troposphere. Fig. 8(a) shows the diurnal change of the aerosol optical thickness on 4 January 2004. A reasonable correlation is found between the result of the DOAS measurement at the KEM site and the SPM data from the Minami station. Examination of the ground SPM data indicates that the reported value of SPM concentration changes in the range of 0–0.107 mg m3 during the one-month campaign period. Thus, the data at 12:00 on 4 January 2004 (0 mg m3) are indeed one of the clearest data available. This reference is also used for the analysis of the data on other relatively turbid days. Fig. 8 shows data observed for five days between December 2003 and January 2004. It is seen from these results that the temporal variations are quite similar between the DOAS-derived turbidity constant (aerosol optical thickness) and the ground-based SPM concentration. Also, the positions of the zero values of the two parameters fairly agree with each other. The ratio between the aerosol optical thickness and the aerosol mass concentration gives us the value of aerosol mass extinction efficiency (Lagrosas et al., 2005). This subject will be treated in a separate paper. The change in aerosol optical thickness and SPM concentration during 32 days is shown in Fig. 9, in which a plotted point represents an average over the daytime data. A reasonable correlation is found between the result of the long-path measurement at the KEM site and the SPM data from the Minami ground station. The results in Figs. 8 and 9 prove that if the optimized alignment of the DOAS setup is maintained, the system works well as a transmissiometer, providing us with the data on the aerosol optical thickness.
Aerosol optical thickness
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Seoul for a 13-month period. Using a commercial DOAS system, they found a relationship expressed as
OT
R=0.86
SPM 1.4 0.04 0.7
SPM (mg/m3)
Aerosol optical thickness
0.08 2.1
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0.0 12-09
12-19
12-29 Date
1-08
NO2 concentration (ppb)
KEM KU IHES 40
20
0 12-11
12-20
12-29
NO2 ðGSÞ ¼ ð0:66 0:04Þ NO2 ðDOASÞ þ ð0:74 0:22Þ
where NO2 (GS) and NO2 (DOAS) stand for the concentrations in ppb measured by the conventional, ground-sampling instrument and by DOAS, respectively. The correlation coefficient, R, was about 0.87. Lee et al. (2002) reported a similar campaign, employing an original DOAS system equipped with three gratings. Their result can be expressed as NO2 ðGSÞ ¼ 0:78 NO2 ðDOASÞ þ 3:7
Fig. 9. One-month variation of aerosol optical thickness from the DOAS (KEM site) and the SPM concentration from ground sampling (Minami station). The correlation coefficient is about 0.86 between the two quantities.
60
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with R ¼ 0:85. In our campaign, the relationship is NO2 ðGSÞ ¼ ð0:66 0:02Þ NO2 ðDOASÞ þ ð13:5 0:6Þ with R ¼ 0:80. This refers to all the NO2 data that have been measured at the three test sites. It is pointed out that in the present case, the locations of the groundsampling stations are generally more distant from the DOAS light paths than in the previous cases. On the other hand, relatively few papers have reported on the application of the DOAS system for the aerosol monitoring. Recently, Lee and Kim (2003) measured atmospheric extinction coefficients using a DOAS system, and found that the results were in agreement with those from a long-path transmissiometer. As described above, the present results also show a good correlation (R ¼ 0:86) between the aerosol optical thickness from the DOAS measurement and the SPM concentration measured with a point system.
Fig. 10. One-month variation of NO2 concentration at the three DOAS sites. Note that only 9 points are plotted for the KU case, in which the alignment problem hindered the full operation.
5. Conclusions
because of the short distance (2.7 km) from the source. The KU site is located at 5.5 km from the source, and route 9 runs below the path. Because of the basin landform, the polluted air is hardly diffused out, and this tendency is especially prominent when the wind speed is low. In Fig. 10 a peak concentration is seen on 25 December 2003, after a three-day period of low wind speed (about 1.1 m s1 on average). On the contrary, on 19 December 2003, the concentration reaches its lowest value of 12.1 ppb because of the influence of the relatively high average value of the wind speed (2.7 m s1). For NO2, comparison between the DOAS results and the ground-sampling measurements has been treated in a couple of recent papers. For example, Kim and Kim (2001) made a field-based intercomparison study in
We have applied the pulsed DOAS technique to the measurement of NO2 and SPM. An aviation obstruction light was employed for the simultaneous measurements at three different sites in the Kyoto city area. Through a one-month campaign, a reasonable correlation was found between the long-path and ground-based observations of the NO2 concentrations. Also, a good temporal correlation was found among the data taken using three light paths and the ground data in the region. For aerosol retrieval, a reference obtained on a clear time period works well to derive the aerosol optical thickness (turbidity constant) for other time periods with more turbid atmospheric conditions. Thus, we have demonstrated that the present system is useful to obtain the collocated data of NO2 and SPM concentrations in urban air pollution studies.
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Acknowledgements The authors would like to thank the Pipeline Department of Osaka Gas Co. for the permission to use the obstruction light. This work was financially supported by the Organization for Small and Medium Enterprises and Regional Innovation, Japan.
References Edner, H., Ragnarson, P., Spannare, S., Svanberg, S., 1993. Differential optical absorption spectroscopy (DOAS) system for urban atmospheric pollution monitoring. Applied Optics 32, 327–333. Evangelisti, F., Baronecelli, A., Bonasoni, P., Givanelli, G., Ravegnani, F., 1995. Differential optical absorption spectrometer for measurement of tropospheric pollutants. Applied Optics 34, 2737–2744. Gomer, T., Brauers, T., Heintz, F., Stutz, J., Platt, U., 1993. MFC User Manual, Vers. 1.98. Institut fuer Umweltphysik, University of Heidelberg. Kim, K.-H., Kim, M.-Y., 2001. Comparison of an open path differential optical absorption spectroscopy system and a conventional in situ monitoring system on the basis of long term measurements of SO2, NO2, and O3. Atmospheric Environment 35, 4059–4072. Lagrosas, N., Kuze, H., Takeuchi, N., Fukagawa, S., Bagtasa, G., Yoshii, S., Naito, S., Yabuki, M., 2005. Correlation study between suspended particulate matter and portable automated lidar data. Aerosol Science 36, 439–454. Lee, J.S., Kim, Y.J., 2003. Extinction measurement using a differential optical absorption spectrometer system. Journal of The Korean Physical Society 42, 731–734. Lee, J.S., Kuk, B.J., Kim, Y.J., 2002. Development of a differential optical absorption spectroscopy (DOAS) system for the detection of atmospheric trace gas species; NO2,
SO2, and O3. Journal of The Korean Physical Society 41, 693–698. Lizamma, M., Wu, R., Lo, J.G., 2001. Measurements of sulfur dioxide and formaldehyde in Taipei using a differential optical absorption spectrometer. Journal of Air and Waste Management Association 51, 94–101. Meller, R., Moortgat, G.K., 2000. Temperature dependence of the absorption cross sections of formaldehyde between 223 and 323 K in the wavelength range 225–375 nm. Journal of Geophysical Research 105 (6), 7089–7101. Pinker, R.T., Pandithurai, G., Holben, B.N., Keefer, T.O., Goodrich, D., 2004. Aerosol radiative properties in the semi-arid Western United States. Atmospheric Research 71, 243–252. Platt, U., Perner, D., Patz, H.W., 1979. Simultaneous measurement of atmospheric CH2O, O3, and NO2 by differential optical absorption. Journal of Geophysical Research 84, 6329–6335. Si, F., Liu, J.G., 2002. Standard absorption cross section data processing in differential optical absorption spectroscopy system. Chinese Journal of Quantum Electronics 19 (6), 499–503. Stutz, J., Kim, E.S., Platt, U., Bruno, P., Perrino, C., Febo, A., 2000. UV–VIS absorption cross-section of nitrous acid. Journal of Geophysical Research 105 (D11), 14585–14592. Stutz, J., Platt, U., 1996. Numerical analysis and estimation of the statistical error of differential optical absorption spectroscopy measurements with least-squares method. Applied Optics 35, 6041–6053. Vandaele, A.C., Hermans, C., Simon, P.C., Carleer, M., Colin, R., Fally, S., Merienne, M.F., Jenouvrier, A., Coquart, B., 1998. Measurements of the NO2 absorption cross-section from 42,000 cm1 to 10,000 cm1 (238–1000 nm) at 220 K and 294 K. Journal of Quantitative Spectroscopy and Radiative Transfer 59, 171–184. Yoshii, Y., Kuze, H., Takeuchi, N., 2003. Long-path measurement of atmospheric NO2 with an obstruction flashlight and a charge-coupled-device spectrometer. Applied Optics 42 (21), 4362–4368.
Atmospheric Environment 39 (2005) 4959–4968 www.elsevier.com/locate/atmosenv
Measurement of regional distribution of atmospheric NO2 and aerosol particles with flashlight long-path optical monitoring Si Fuqia,b, Hiroaki Kuzea,, Yotsumi Yoshiia,c, Masaya Nemotoa, Nobuo Takeuchia, Toru Kimurad, Toyofumi Umekawad, Taisaku Yoshidad, Tadashi Hiokie, Tsuyoshi Tsutsuif, Masahiro Kawasakig a
Center for Environmental Remote Sensing, Chiba University, Inage-ku, Chiba 263-8522, Japan Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China c Toyama National College of Maritime Technology, Shinminato, Toyama 933-0293, Japan d Kyoto Electronics Manufacturing Co., Ltd, Kisshoin-Shinden, Minami-ku, Kyoto 601-8317, Japan e Kyoto Prefectural Comprehensive Center for Small and Medium Enterprises, Shimogyo-ku, Kyoto 600-8813, Japan f Kyoto Prefectural Institute of Hygienic and Environmental Sciences, Fushimi-ku, Kyoto 612-8369, Japan g Department of Molecular Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan b
Received 15 October 2004; accepted 2 May 2005
Abstract We report on an air-pollution monitoring campaign in the urban Kyoto area conducted during December 2003 and January 2004. The method is based on the differential optical absorption spectroscopy (DOAS) using an aviation obstruction light (white flashlight). Three sets of DOAS spectrometers, each consisting of a telescope and a compact charge-coupled device (CCD) spectrometer, were operated to cover the southern part of the city area. Three sites were chosen in the urban Kyoto area, and the average concentrations of NO2 were measured with optical path lengths of 2.7, 4.1, and 7.1 km. The aerosol optical thickness was simultaneously measured for the path length of 2.7 km. It is found that the temporal variations of the retrieved amount of the pollution species agree with each other and they are also consistent with concurrent results of the ground sampling measurements. r 2005 Elsevier Ltd. All rights reserved. Keywords: Flashlight DOAS; Urban atmosphere; Air pollution distribution; Aerosol extinction; Ground sampling
1. Introduction The technique of differential optical absorption spectroscopy (DOAS) is based on the recording of differential absorption, i.e., the difference between local maxima and minima in the absorption spectrum, of the probed gas species (Platt et al., 1979). Compared with conventional methods of point sampling, the DOAS Corresponding author. Fax: +81 43 290 3857.
E-mail address: [email protected] (H. Kuze).
system is more suitable to monitoring horizontally averaged concentrations. For example, Edner et al. (1993) measured the concentrations of NO2, SO2, and O3 with 0.4 km, 1.6 km, and 2.0 km path lengths. The beam-finding servo system and automatic gain control enabled unattended, long-time monitoring. Evangelisti et al. (1995) applied the technique to measure NO2, HNO2, SO2, and O3. Using a path length of 1.7 km, they accomplished sensitivity of 1–10 ppb for these molecular species. More recently, there has been growing interest in advanced techniques to monitor pollutants in the
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.05.002
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atmosphere in view of serious air pollutions in city areas, particularly those in Asian countries. Potentially, the method of DOAS enables simultaneous monitoring of a variety of pollutants found in the atmosphere (Kim and Kim, 2001; Lizamma et al., 2001; Meller and Moortgat, 2000; Stutz and Platt, 1996). In the conventional DOAS approach, light from a broadband xenon high-pressure lamp is transmitted through the atmosphere for distances of several kilometers. The light is then received and analyzed by the combination of a monochromator and a detector such as a photo-multiplier tube, a photodiode array or a charge-coupled device (CCD). In a previous paper, we demonstrated the usefulness of an aviation obstruction light (white flashlight) for DOAS application (Yoshii et al., 2003). Such flashlights are widely used for safety of aviation traffic in many places. In Japan, obstruction lights are installed on tall constructions (above 60 m), and are detectable in every direction from several kilometers away. The optical thickness of NO2 molecules can be retrieved by detecting the transmitted light intensity using a simple telescope and a commercially available, compact CCD spectrometer. Besides, additional knowledge about the spectral intensity of the light source makes it possible to retrieve the optical thickness of the aerosol particles existing in the light path (Yoshii et al., 2003). In the present paper, we describe the application of this flashlight DOAS technique, and show the results of a campaign study conducted in Kyoto district, Japan, during December 2003 and January 2004. Measurements were made using three sets of the DOAS spectrometers, simultaneously monitoring a single light source located at the central part of the city. The temporal variations of NO2 molecules and aerosol particles are retrieved from the data, and compared with the results of conventional measurements at the ground level. The purpose of the present study is twofold. First, this is a first case study in which the flashlight DOAS technique is applied to multisite observations. The resulting area coverage gives us the possibility of understanding the spatial behavior of the air pollution. Second, we compare the long-path result with the point data simultaneously measured at groundbased monitoring stations. This gives us an insight into the vertical homogeneity (or inhomogeneity) of the local atmosphere.
2. Experiment 2.1. DOAS system Fig. 1 shows a schematic of the experimental setup. Since details were given in our previous paper (Yoshii et al., 2003), here we give only a brief outline. An
Telescope
Entrance slit Grating
Linear CCD array USB2000 Spectrometer Notebook PC
Height 67.5m Osaka Gas facility
Fig. 1. Experimental setup for measuring NO2 and SPM.
astronomical telescope (Meade, DS-115), with an aperture diameter of 115 mm, focuses the image of a point light source located at a far distance. The image is formed near the eyepiece location. The eyepiece itself is removed from the telescope, and instead, the entrance slit (5 mm wide) of a CCD spectrometer (Ocean Optics, USB2000) is placed. This spectrometer (89 mm wide 63 mm long 34 mm high) has a mechanically stable, crossed Czerny–Turner design with a fixed grating. The CCD array consists of 2048 elements and has sensitivity in a wavelength range of 200–800 nm. The resulting resolution is 0.3 nm per pixel on average. The CCD gate duration is set at 300 ms in the experiment. Between successive gate periods, there exists a time lag of 7 ms, in which each spectrum is sent to a personal computer (PC) through the universal serial bus. Data acquisition can be attained successfully even when no trigger (synchronous with the flashlight) is applied to the CCD spectrometer, though this relatively long gate time as compared with the flashlight duration (about 0.5 ms) causes somewhat increased amount of the background skylight. For the measurement in the Kyoto district (35.01N, 135.51E), we made use of a xenon strobe (Sanken, FX-7) install at the top of Osaka Gas facility as a light source. The lamp height is 67.5 m above the ground level, and 95.5 m above sea level (ASL). Three lamps are equipped at the same height, each covering a horizontal range of 1201 with a daylight intensity of approximately 300 W sr1. All the lamps flash for approximately 0.5 ms every 1.5 s (40 flashes a min, in accordance with the regulation) in a synchronous manner. According to the regulation, the light intensity is diminished at dusk and dawn, and during the nighttime blinking red lights replace the flashlights. Thus, the DOAS measurement is limited to the daytime, around 7 a.m. to 5 p.m. during the winter months. For the measurement of the regional distribution of atmospheric NO2, three detector setups were situated at
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Fig. 2. Site map. Locations are shown for three light paths for the DOAS measurement and three stations for ground sampling measurement.
different places (Fig. 2). In the southwest direction, a DOAS system was installed in a building of Kyoto University (KU) (about 120 m ASL, and 5.5 km from the source), and a second one at Kyoto Electronics Manufacturing (KEM) (about 34 m ASL, and 2.7 km from the source) in the south direction. A third setup was placed in the southeast direction on the building of Kyoto Prefectural Institute of Hygienic and Environmental Sciences (IHES) (about 51 m ASL, and 7.1 km from the source). The alignment of each telescope system was checked at least once a day (the alignment check for the KU system was sometimes less frequent). During the time interval between 7 December 2003 and 10 January 2004, data were acquired successfully with a probability of 77%. In addition to the misalignment, the failure was brought about also from the rainy weather conditions. The locations of three ground stations are also indicated in Fig. 2. Although the agreements between their locations and the three DOAS optical paths are not always satisfactory, we compare the DOAS data from the KEM site with the Minami ground station, IHES site with the Fushimi station, and KU site with the Nishikyo-ku station in the present analysis. 2.2. Outline of the site Kyoto city, famous for her historical heritage, is situated in a basin, surrounded by mountains except in the southward direction. In the north, Mt. Hieizan (848 m) and Mt. Atagoyama (924 m) are prominent
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peaks, and relatively lower mountain peaks limit the east and west landscape of the area. In winter (December– February) the average temperature is about 6 1C, and northwest to west wind directions are predominant. Because of these wind directions, transportation of the polluted air exhibits only limited effect during the winter months, and consequently, it is likely that most of the NO2 emission originates from sources in the Kyoto basin itself. Since pollutions from industrial activities are rather limited, automobile emissions are considered the main source of the air pollution, including the suspended particulate matter (SPM, referring to aerosols with diameters less than 10 mm). There are several main roads passing through our test area, such as route 1, route 9, route 24, and route 171; besides, the Meishin highway runs between the light source and IHES test site (Fig. 2). The ground sampling stations are operated by municipal governments. In these stations, NO2 concentrations (here shown in units of parts per billion, ppb) are monitored using the chemi-luminescence method, and the SPM amount (in units of mg m3) using the b-ray method. The data are given every hour, and available through the Internet.
3. Analysis of the DOAS spectrum 3.1. NO2 concentration The analysis of the DOAS spectra is based on the Beer–Lambert’s law expressed as IðlÞ ¼ kI 0 ðlÞeLsðlÞn ,
(1)
where IðlÞ is the measured intensity, k is the system constant, I 0 ðlÞ the unattenuated reference intensity, L the path length, sðlÞ the wavelength-dependent absorption cross section, and n the number density of the species averaged over the path length. The dimensionless quantity LsðlÞn represents the optical thickness, denoted as t. In the real atmosphere, both molecules (Rayleigh scattering) and aerosols (Mie scattering) contribute to the radiation extinction. Adding these contributions to Eq. (1) and solving the equation with respect to t, we obtain tðlÞ ¼ ln
kI 0 ðlÞ ¼ L½sðlÞn þ aM ðlÞ þ aA ðlÞ. IðlÞ
(2)
Here aM ðlÞ is the molecular extinction coefficient which is the product of the molecular cross section, sM ðlÞ, and the number density of air molecules, nM. Similarly, the aerosol extinction coefficient, aA ðlÞ, is defined as sA ðlÞnA , where sA ðlÞ is the aerosol cross section, and nA the number density of aerosol particles. Cross sections on the right-hand side of Eq. (2) can be separated into two components, slowly varying one,
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i
þL
X
Diff. Cross Section (10-19 cm2)
sSi ðlÞ, and rapidly varying one, sR i ðlÞ, with wavelength. Both the molecular and aerosol cross sections are slowly varying components of the optical thickness. Hence we obtain X tðlÞ ¼ L sR i ðlÞni ! sSi ðlÞni þ sM ðlÞnM þ sA ðlÞnA ,
ð3Þ
i
1 0 -1
Here DtðlÞ on the left-hand side denotes the peak-topeak variation of the observed optical thickness and DsðlÞ on the right-hand side that of the absorption cross-section data. In the present case, the resolutions of DtðlÞ and DsðlÞ are about 0.3 and 0.017 nm, respectively. Before the spectral matching procedure, the cross-section data are convoluted with the slit function of the CCD spectrometer, which can be determined by measuring the emission spectra of a Hg lamp (Si and Liu, 2002). Comparison between DtðlÞ and DsðlÞ is illustrated in Fig. 3. An example of the actual DOAS spectrum is shown in Fig. 3(c). This was measured at the KEM site on 15 December 2003. The resulting value of the average NO2 concentration is 22.371.0 ppb. In order to evaluate the accuracy of the present spectral matching procedure, a calibration experiment with a standard gas cell was conducted. In this test, diluted NO2 gas (N2 balance) with three different
420 430 440 Wavelength (nm)
450
400
410
420 430 440 Wavelength (nm)
450
0
-1
(b)
Diff. Optical Thickness
(4)
410
1
0.02
In the present analysis of DOAS spectra, the determination of NO2 amount relies on the spectral matching procedure expressed as
400 (a)
3.2. Accuracy of NO2 retrieval
Dt ðlÞ ¼ Ln DsðlÞ.
2
-2
Diff. Cross Section (10-19 cm2)
where subscript i denotes trace gas species such as NO2, O3 or SO2. On the right-hand side of Eq. (3), the first term describes the rapidly varying part, and the second the slowly varying part. The number density for each species is calculated through least square fitting (Gomer et al., 1993; Stutz et al., 2000) after subtracting the slowly varying part from Eq. (3). In the present analysis, we choose a wavelength range of 400–450 nm for the retrieval of the optical thickness due to NO2 molecules. In this wavelength region, the contribution of other trace gas species is negligible as compared with the NO2 contribution. As the cross-section data, sðlÞ in Eq. (2), we employ a laboratory spectrum obtained by Vandaele et al. (1998), with suitable adjustment of the spectral resolution as explained below (Section 3.2). In the conversion of the data from the optical thickness to the concentration in ppb, we postulate a fixed temperature of 280 K.
3
Dec. 15 13:00 KEM
0.01
0.00
-0.01
-0.02 400 (c)
410
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450
Fig. 3. Spectral matching procedure. (a) Cross-section data of NO2 absorption with the original resolution of 0.017 nm. (b) Cross-section data after the convolution with the slit function of the spectrometer. (c) An example of the actual DOAS spectrum measured at KEM site on 15 December 2003. The resulting value of the average concentration is 22.371.0 ppb.
concentrations (256, 510, and 682 ppm) was filled in a 30-cm long glass cell. The result is summarized in Fig. 4, where the ppb values are those expected for a typical
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4. Results and discussion Y=1.06X-0.0026
DOAS (ppb)
40
4.1. Concentration of NO2 from DOAS and ground measurements
R=0.998
30
20
20
30
40
Standard Gas (ppb) Fig. 4. Correlation between the results of the DOAS measurement and the laboratory measurement using standard gas.
DOAS distance of 5 km. These are hypothetical values converted from the gas cell concentrations, giving the same optical thickness as the laboratory experiment. From this calibration measurement, the accuracy of the retrieval of the NO2 concentration is about 74% in a range of 10–50 ppb. 3.3. Aerosol retrieval In a previous paper (Yoshii et al., 2003), we pointed out the possibility of using the DOAS spectrometer as a transmissiometer to derive the aerosol optical thickness along the optical path. Here the method is applied to the DOAS spectra observed in the Kyoto district. In the case of aerosol retrieval, the unattenuated spectrum, I 0 ðlÞ, has to be measured separately. For the aerosol optical thickness, we assume a form of tA ¼ Bðl=550ÞA
(5)
for the wavelength dependence. Here the wavelength l is measured in units of nm. The constants A (Angstrom exponent) and B (aerosol optical thickness at 550 nm) are to be determined by fitting this expression to the optical thickness derived from the DOAS spectra. In the actual analysis, additionally the system constant k in Eq. (1) must also be incorporated in the iteration procedure together with A and B. The inclusion of k takes the temporal variation of experimental conditions (light intensity, detection efficiency, etc.) into account. In order to reduce the correlation among the unknown parameters, A, B, and k, here we obtain the system constant from the data obtained on relatively clean days. Details of this procedure will be explained below (Section 4.2).
The optical thickness of NO2 was measured with DOAS setups at KEM, IHES, and KU sites. The reference spectrum, I 0 ðlÞ in Eq. (1), was obtained from a measurement at a location close to the light source, with an optical path length of about 100 m. Fig. 5 shows data observed for five days between December 2003 and January 2004. For all the cases, a reasonable correlation is found between the result of the long-path measurement (right panels) and the data from a ground station (left panels) near each optical path. Also it is noted that temporal variations observed for the three paths fairly agree with each other, though the concentrations are not always the same. This implies that during the time period, the atmosphere was changing in a consistent way in the region, but the NO2 distribution was not necessarily homogeneous across the region. Among the five DOAS data shown in Fig. 5,(b2), (c2), and (d2) represent typical behavior of the NO2 concentration in winter, in that a high concentration is observed in the morning, and the concentration becomes lower around noon. This is presumably due to the presence and disappearance of the inversion layer near the ground level. Such behavior is not seen in Fig. 5(a2) and (e2). This is probably ascribed to the lack of radiative cooling during the midnight. Fig. 6 shows the correlation of the NO2 concentration between the ground sampling and DOAS measurements. The number of data points in each panel is about 300, corresponding to a total observation time of 300 h (Note that 12 DOAS data obtained in 1 h are averaged in this correlation study). Since the locations of the ground-sampling stations are not exactly below the DOAS paths (Fig. 2), and the DOAS data represent averages over the paths, one cannot expect a full correlation between the two methods. Nevertheless, from Fig. 6, a reasonable correlation is found between the result of the long-path measurement and the data from a ground station near the optical path. The correlation coefficients, however, are in a range of 0.72–0.85, indicating occasional discrepancies between the two methods. In Fig. 6(c), the DOAS concentrations observed at the KU station are close to the ground station values. This is seen from the slope of the correlation fitting (close to 1.0), although the scattering of the data points degrades the accuracy of the comparison between the two values. For the KEM site (Fig. 6(a)), the DOAS values are higher than the ground values, as manifested in the slope of 1.06 in the correlation fitting. On the contrary, the DOAS data for the IHES site (Fig. 6(b)) are smaller
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JST Fig. 5. Diurnal change of NO2 concentration observed for 5 days in December 2003–January 2004. Nishikyo-ku station is near the KU site, Fushimi station near the IHES site, and Minami station near the KEM site. (a1 and a2) 11 December 2003, (b1 and b2) 12 December 2003, (c1 and c2) 13 December 2003, (d1 and d2) 6 January 2004, and (e1 and e2) 7 January 2004.
than the ground values, as indicated by the slope of 0.81. In these cases, it is likely that the presence of the heavytraffic roads might have affected either the DOAS data or the ground-sampling data in a significant way. 4.2. Aerosol retrieval In the aerosol retrieval from the DOAS data, we need to reduce the number of unknown parameters. Here we apply a method in which a fixed value is assumed for the system constant k, as obtained from the data with maximum light intensities detected. This method relies
on the fact that within the potential sources of the optical thickness in the DOAS spectra, normally the aerosol part is by far the largest and this part shows the highest variability (an example will be shown below). Thus, the observation of maximum intensity indicates that the aerosol contribution has been minimum (clear day), and at the same time, the system efficiency (including the telescope alignment) has been more or less optimized. When this ‘‘best spectrum’’ is used as a (tentative) reference, it is possible to calculate the difference in the aerosol optical thickness between the turbid and clear days. Then, in a second step, the
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600
Fig. 7. Retrieval process of the aerosol concentration. (a) Spectra observed at 11:00 and 12:00 Japan Standard Time on 4 January 2004. Both are obtained by averaging hourly data. The spectrum at 12:00 is used as a (tentative) reference spectrum. (b) Total optical thickness and molecular (Rayleigh) and aerosol components derived from the spectra in (a).
Y=-3.6+0.97X
60
575 Wavelength (nm)
60
Fig. 6. Correlation between the results of the DOAS and ground sampling, point measurements for the NO2 concentration. (a) KEM site (DOAS) and Minami station, (b) IHES site and Fushimi station, and (c) KU site and Nishikyo-ku station.
Angstrom exponent (A in Eq. (5)) is derived from the DOAS spectrum itself. This process of aerosol retrieval is illustrated in Fig. 7. Fig. 7(a) shows the original spectra measured at the KEM site during 10:30–11:30 (local time) and during
11:30–12:30, both on 4 January 2004 (each represents data averaged over one hour). Examination of the dataset on the day indicates that the spectrum during 11:30–12:30 (referred to as 12:00 hereafter) shows the maximum intensity, which we assume to be a tentative reference. For the aerosol retrieval, we make use of the spectral region between 550 and 600 nm, since this region is in effect free from absorption due to NO2, water vapor or ozone. Fig. 7(b) shows the relative optical thickness obtained by comparing the data at 11:00 with the data at 12:00 (reference). The total optical thickness, as well as the Rayleigh and aerosol components, is shown. In this case, the Angstrom exponent, A, is 0.8670.09, and the aerosol optical thickness at 550 nm (turbidity constant), B, is 0.3270.03. Normally the Angstrom exponent takes a value around unity and a value somewhat smaller indicates the dominance of relatively fine particles (Pinker et al., 2004). The value of optical thickness, on the other hand, shows that the average value of aerosol extinction coefficient
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4.3. One month variation of NO2 for three sites For the NO2 concentration observed at the three sites (KEM, IHES, and KU), the change in NO2 concentration during 30 days is shown in Fig. 10, in which a plotted point represents an average over the daytime data. An obvious correlation can be found among the sites. The highest value of NO2 concentration is found at the IHES site, while the KEM site exhibits the smallest value. This is mainly ascribable to the amount of the vehicle traffic that runs below each light path, if the transport of the air pollution is insignificant. For IHES, the light from the source travels a distance of 7.1 km over several main roads before the detector receives it. For the KEM site, the number of main roads is limited
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0.0 07:00 Fig. 8. Diurnal change in the aerosol optical thickness (KEM site) and SPM concentration (Minami station). (a) 4 January 2004 (clear day for reference), (b) 11 December 2003, (c) 12 December 2003, (d) 24 December 2003, and (e) 7 January 2004.
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17:00
SPM (mg/m3)
is about 1.2 104 m1, a typical value in the urban troposphere. Fig. 8(a) shows the diurnal change of the aerosol optical thickness on 4 January 2004. A reasonable correlation is found between the result of the DOAS measurement at the KEM site and the SPM data from the Minami station. Examination of the ground SPM data indicates that the reported value of SPM concentration changes in the range of 0–0.107 mg m3 during the one-month campaign period. Thus, the data at 12:00 on 4 January 2004 (0 mg m3) are indeed one of the clearest data available. This reference is also used for the analysis of the data on other relatively turbid days. Fig. 8 shows data observed for five days between December 2003 and January 2004. It is seen from these results that the temporal variations are quite similar between the DOAS-derived turbidity constant (aerosol optical thickness) and the ground-based SPM concentration. Also, the positions of the zero values of the two parameters fairly agree with each other. The ratio between the aerosol optical thickness and the aerosol mass concentration gives us the value of aerosol mass extinction efficiency (Lagrosas et al., 2005). This subject will be treated in a separate paper. The change in aerosol optical thickness and SPM concentration during 32 days is shown in Fig. 9, in which a plotted point represents an average over the daytime data. A reasonable correlation is found between the result of the long-path measurement at the KEM site and the SPM data from the Minami ground station. The results in Figs. 8 and 9 prove that if the optimized alignment of the DOAS setup is maintained, the system works well as a transmissiometer, providing us with the data on the aerosol optical thickness.
Aerosol optical thickness
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Seoul for a 13-month period. Using a commercial DOAS system, they found a relationship expressed as
OT
R=0.86
SPM 1.4 0.04 0.7
SPM (mg/m3)
Aerosol optical thickness
0.08 2.1
0.00
0.0 12-09
12-19
12-29 Date
1-08
NO2 concentration (ppb)
KEM KU IHES 40
20
0 12-11
12-20
12-29
NO2 ðGSÞ ¼ ð0:66 0:04Þ NO2 ðDOASÞ þ ð0:74 0:22Þ
where NO2 (GS) and NO2 (DOAS) stand for the concentrations in ppb measured by the conventional, ground-sampling instrument and by DOAS, respectively. The correlation coefficient, R, was about 0.87. Lee et al. (2002) reported a similar campaign, employing an original DOAS system equipped with three gratings. Their result can be expressed as NO2 ðGSÞ ¼ 0:78 NO2 ðDOASÞ þ 3:7
Fig. 9. One-month variation of aerosol optical thickness from the DOAS (KEM site) and the SPM concentration from ground sampling (Minami station). The correlation coefficient is about 0.86 between the two quantities.
60
4967
1-7
Date
with R ¼ 0:85. In our campaign, the relationship is NO2 ðGSÞ ¼ ð0:66 0:02Þ NO2 ðDOASÞ þ ð13:5 0:6Þ with R ¼ 0:80. This refers to all the NO2 data that have been measured at the three test sites. It is pointed out that in the present case, the locations of the groundsampling stations are generally more distant from the DOAS light paths than in the previous cases. On the other hand, relatively few papers have reported on the application of the DOAS system for the aerosol monitoring. Recently, Lee and Kim (2003) measured atmospheric extinction coefficients using a DOAS system, and found that the results were in agreement with those from a long-path transmissiometer. As described above, the present results also show a good correlation (R ¼ 0:86) between the aerosol optical thickness from the DOAS measurement and the SPM concentration measured with a point system.
Fig. 10. One-month variation of NO2 concentration at the three DOAS sites. Note that only 9 points are plotted for the KU case, in which the alignment problem hindered the full operation.
5. Conclusions
because of the short distance (2.7 km) from the source. The KU site is located at 5.5 km from the source, and route 9 runs below the path. Because of the basin landform, the polluted air is hardly diffused out, and this tendency is especially prominent when the wind speed is low. In Fig. 10 a peak concentration is seen on 25 December 2003, after a three-day period of low wind speed (about 1.1 m s1 on average). On the contrary, on 19 December 2003, the concentration reaches its lowest value of 12.1 ppb because of the influence of the relatively high average value of the wind speed (2.7 m s1). For NO2, comparison between the DOAS results and the ground-sampling measurements has been treated in a couple of recent papers. For example, Kim and Kim (2001) made a field-based intercomparison study in
We have applied the pulsed DOAS technique to the measurement of NO2 and SPM. An aviation obstruction light was employed for the simultaneous measurements at three different sites in the Kyoto city area. Through a one-month campaign, a reasonable correlation was found between the long-path and ground-based observations of the NO2 concentrations. Also, a good temporal correlation was found among the data taken using three light paths and the ground data in the region. For aerosol retrieval, a reference obtained on a clear time period works well to derive the aerosol optical thickness (turbidity constant) for other time periods with more turbid atmospheric conditions. Thus, we have demonstrated that the present system is useful to obtain the collocated data of NO2 and SPM concentrations in urban air pollution studies.
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Acknowledgements The authors would like to thank the Pipeline Department of Osaka Gas Co. for the permission to use the obstruction light. This work was financially supported by the Organization for Small and Medium Enterprises and Regional Innovation, Japan.
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