**2. Differential optical absorption spectroscopy (DOAS)**

The method of differential optical absorption spectroscopy (DOAS) provides a useful tool for monitoring atmospheric pollutants through the measurement of optical extinction (i.e., the sum of absorption and scattering) over a light path length of a few kilometres (Yoshii et al., 2003, Lee et al., 2009; Si et al., 2005; Kuriyama et al., 2011). The DOAS method in the

Multi-Wavelength and Multi-Direction Remote Sensing of Atmospheric Aerosols and Clouds 281

Fig. 1. Schematic flow of the DOAS analysis. The net radiation from the pulsed light source can be retrieved by subtracting the background due to sky radiation, and an appropriate portion of the spectrum is compared with the molecular cross-section spectrum obtained from laboratory measurement. Then, the "high-frequency" components of the observed optical thickness () and cross-section data () are compared to derive the molecular

Fig. 2. An example of DOAS spectral matching, in which the correlation between the

determine the average volume concentration of NO2 molecules.

differential optical thickness from the DOAS data and differential absorption is examined to

number density along the optical path length, *L*.


Table 1. Various schemes of atmospheric observation discussed in this chapter.

visible spectral region is quite suitable for urban air pollution studies, since both nitrogen dioxide (NO2) and aerosol, the most important pollutants originated from human activities, can directly be measured using a near horizontal light path in the lower troposphere.

Although conventional approach in the DOAS measurement is to install a light source, our group at the Centre for Environmental Remote Sensing (CEReS), Chiba University, has established a unique DOAS approach based on aviation obstruction lights (white flashlights) equipped at tall constructions such as smokestacks (Yoshii et al., 2003, Si et al., 2005; Kuriyama et al., 2011). Since those xenon lamps produce flash pulses every 1.5 s during the daytime, they can easily be recognized with the coverage of the whole visible spectral range. Thus, a simple setup consisting of an astronomical telescope and a compact spectroradiometer can be employed for the measurement of NO2. Also, the stable intensity of the light source makes it possible to retrieve aerosol, or suspended particulate matter (SPM) concentration in the lower troposphere, since the intensity variation of the detected light is mostly ascribable to the aerosol extinction over the light path (Yoshii et al., 2003).

As shown in Figs. 1 and 2, the principle of DOAS analysis of NO2 concentration is based on matching high-pass filtered spectral (wavelength) features between the DOAS-observed optical thickness () and laboratory-observed molecular absorption spectrum (). Because of the Lambert-Beer's law, the optical thickness, , is expressed as

$$
\pi = -\ln(I \;/\; I\_0)\_\prime \tag{1}
$$

where *I* and *I*0 stand for the observed and reference spectrum, respectively. The reference spectrum can be obtained by either operating the DOAS spectrometer at a short distance

Scheme Wavelength Direction Aerosol Trace gas

Nearly horizontal measurement

Solar direction/ any direction including the zenith

Vertical and slant path observations

Nadir or nearnadir directions

visible spectral region is quite suitable for urban air pollution studies, since both nitrogen dioxide (NO2) and aerosol, the most important pollutants originated from human activities,

Although conventional approach in the DOAS measurement is to install a light source, our group at the Centre for Environmental Remote Sensing (CEReS), Chiba University, has established a unique DOAS approach based on aviation obstruction lights (white flashlights) equipped at tall constructions such as smokestacks (Yoshii et al., 2003, Si et al., 2005; Kuriyama et al., 2011). Since those xenon lamps produce flash pulses every 1.5 s during the daytime, they can easily be recognized with the coverage of the whole visible spectral range. Thus, a simple setup consisting of an astronomical telescope and a compact spectroradiometer can be employed for the measurement of NO2. Also, the stable intensity of the light source makes it possible to retrieve aerosol, or suspended particulate matter (SPM) concentration in the lower troposphere, since the intensity variation of the detected light is

can directly be measured using a near horizontal light path in the lower troposphere.

mostly ascribable to the aerosol extinction over the light path (Yoshii et al., 2003).

of the Lambert-Beer's law, the optical thickness, , is expressed as

As shown in Figs. 1 and 2, the principle of DOAS analysis of NO2 concentration is based on matching high-pass filtered spectral (wavelength) features between the DOAS-observed optical thickness () and laboratory-observed molecular absorption spectrum (). Because

where *I* and *I*0 stand for the observed and reference spectrum, respectively. The reference spectrum can be obtained by either operating the DOAS spectrometer at a short distance

0

ln( / ), *I I* (1)

Table 1. Various schemes of atmospheric observation discussed in this chapter.

Measurable through the spectral intensity

Measurable through the spectral intensity

Profiling by solving the lidar equation

Evaluated and removed in the process of atmospheric correction

Measurable (e.g. NO2 around 450 nm)

Measurable (e.g. H2O around 720 nm)

Not applicable unless tunable lasers are employed

Spectral bands are usually too wide to retrieve trace gases

DOAS

solar/skylight

Lidar

UV, VIS, and NIR, with the resolution of array detector

UV and VIS, with the resolution of array detector

Fundamental and harmonics of Nd:YAG laser (1064, 532 and 355 nm)

UV, VIS, and NIR

Satellite Spectral bands in

Fig. 1. Schematic flow of the DOAS analysis. The net radiation from the pulsed light source can be retrieved by subtracting the background due to sky radiation, and an appropriate portion of the spectrum is compared with the molecular cross-section spectrum obtained from laboratory measurement. Then, the "high-frequency" components of the observed optical thickness () and cross-section data () are compared to derive the molecular number density along the optical path length, *L*.

Fig. 2. An example of DOAS spectral matching, in which the correlation between the differential optical thickness from the DOAS data and differential absorption is examined to determine the average volume concentration of NO2 molecules.

Multi-Wavelength and Multi-Direction Remote Sensing of Atmospheric Aerosols and Clouds 283

The analysis of light intensity detected with a DOAS spectrometer can yield information also on aerosol extinction along the light path. The wavelength dependence of each atmospheric component is exemplified in Fig. 4(a), where it is apparent that the contribution from aerosol extinction is much more significant than that from either NO2 or molecular Rayleigh scattering. The optical thickness associated with aerosol extinction can generally be

<sup>0</sup> () ( / ) , *<sup>A</sup>*

where *A*=ang is called the Angstrom exponent and *B* the turbidity constant. The value of *A* changes with the aerosol size distribution in such a way that a smaller value (~ 0.5) indicate the dominance of relatively coarse particles (such as sea salt or dust), while a large value (~ 2) that of relatively fine particles (such as ammonium sulfate or ammonium nitrate). The value of *B*, on the other hand, is equal to the aerosol optical thickness as wavelength 0,

Fig. 4. Aerosol measurement from DOAS data: (a) comparison of contributions of gas (NO2) absorption, aerosol extinction, and molecular extinction (Rayleigh scattering) to DOAS optical thickness, and (b) temporal change of SPM concentration from ground sampling and aerosol extinction coefficient from DOAS during February 1 to 7, 2011 observed in Chiba.

*B* (3)

 

given as

from the light source, or observing the spectrum under very clear atmospheric conditions with minimal aerosol loading. The optical thickness is generally proportional to the product of extinction coefficient, , and the light path length, *L*, i.e.,=*L*. In the case of molecular absorption, the extinction coefficient is equal to the absorption coefficient, which can be given as the product of absorption cross-section, , and the molecular number density, *N*, i.e., =*N*. Although molecular scattering (Rayleigh scattering) and aerosol scattering (Mie scattering) also exist, their contribution can be eliminated by applying the high-pass filtering to both () and () (where is wavelength), since the absorption feature of NO2 is a rapidly varying function with wavelength (see insets in Figs. 1 and 2), while the wavelength dependence of Rayleigh or Mie scattering is much more moderate. Thus, after the high-pass filtering, one obtains

$$
\Delta \tau = (NL)\Delta \sigma.\tag{2}
$$

This indicates that the correlation analysis between the rapidly varying components of the optical thickness and NO2 cross-section in an appropriate wavelength range can lead to the determination of the molecular number density, hence the volume concentration ratio, of NO2 along the DOAS observation light path. An example of the retrieval of NO2 in the Chiba city area is shown in Fig. 3. In this case, the DOAS result shows the average concentration over a light path length of 5.5 km. From Fig. 3, it is seen that the DOAS data show good temporal correlation with the ground sampling data from nearby sampling stations. Note that the temporal resolution (5 min) of the DOAS observation is much better than that of the ground sampling (1 h). The observation of DOAS spectra is limited to daytime, since the white flashlight (Xe light) is replaced with blinking red lamps during night time.

Fig. 3. Comparison of NO2 volume concentration between the DOAS and conventional ground sampling measurements during June 19 - 27, 2011. The DOAS data are based on the measurement at CEReS, Chiba University, using an aviation obstruction flashlamp located around 5.5 km in the north direction. The ground sampling data are from two nearby sampling stations (Chigusadai Elementary School and Miyanogi stations) operated by the municipal government.

from the light source, or observing the spectrum under very clear atmospheric conditions with minimal aerosol loading. The optical thickness is generally proportional to the product of extinction coefficient, , and the light path length, *L*, i.e.,=*L*. In the case of molecular absorption, the extinction coefficient is equal to the absorption coefficient, which can be given as the product of absorption cross-section, , and the molecular number density, *N*, i.e., =*N*. Although molecular scattering (Rayleigh scattering) and aerosol scattering (Mie scattering) also exist, their contribution can be eliminated by applying the high-pass filtering to both () and () (where is wavelength), since the absorption feature of NO2 is a rapidly varying function with wavelength (see insets in Figs. 1 and 2), while the wavelength dependence of Rayleigh or

Mie scattering is much more moderate. Thus, after the high-pass filtering, one obtains

white flashlight (Xe light) is replaced with blinking red lamps during night time.

Fig. 3. Comparison of NO2 volume concentration between the DOAS and conventional ground sampling measurements during June 19 - 27, 2011. The DOAS data are based on the measurement at CEReS, Chiba University, using an aviation obstruction flashlamp located around 5.5 km in the north direction. The ground sampling data are from two nearby sampling stations (Chigusadai Elementary School and Miyanogi stations) operated by the

municipal government.

 (). *NL* 

This indicates that the correlation analysis between the rapidly varying components of the optical thickness and NO2 cross-section in an appropriate wavelength range can lead to the determination of the molecular number density, hence the volume concentration ratio, of NO2 along the DOAS observation light path. An example of the retrieval of NO2 in the Chiba city area is shown in Fig. 3. In this case, the DOAS result shows the average concentration over a light path length of 5.5 km. From Fig. 3, it is seen that the DOAS data show good temporal correlation with the ground sampling data from nearby sampling stations. Note that the temporal resolution (5 min) of the DOAS observation is much better than that of the ground sampling (1 h). The observation of DOAS spectra is limited to daytime, since the

(2)

The analysis of light intensity detected with a DOAS spectrometer can yield information also on aerosol extinction along the light path. The wavelength dependence of each atmospheric component is exemplified in Fig. 4(a), where it is apparent that the contribution from aerosol extinction is much more significant than that from either NO2 or molecular Rayleigh scattering. The optical thickness associated with aerosol extinction can generally be given as

$$\pi(\mathcal{X}) = \mathcal{B}(\mathcal{X}/\mathcal{X}\_0)^{-A},\tag{3}$$

where *A*=ang is called the Angstrom exponent and *B* the turbidity constant. The value of *A* changes with the aerosol size distribution in such a way that a smaller value (~ 0.5) indicate the dominance of relatively coarse particles (such as sea salt or dust), while a large value (~ 2) that of relatively fine particles (such as ammonium sulfate or ammonium nitrate). The value of *B*, on the other hand, is equal to the aerosol optical thickness as wavelength 0,

Fig. 4. Aerosol measurement from DOAS data: (a) comparison of contributions of gas (NO2) absorption, aerosol extinction, and molecular extinction (Rayleigh scattering) to DOAS optical thickness, and (b) temporal change of SPM concentration from ground sampling and aerosol extinction coefficient from DOAS during February 1 to 7, 2011 observed in Chiba.

Multi-Wavelength and Multi-Direction Remote Sensing of Atmospheric Aerosols and Clouds 285

relative contributions of these three basis components as well as the size distribution of each component (Manago et al, 2011). As seen from Fig. 5, the soot component shows remarkably high value of the imaginary part of the refractive index. This indicates that the absorption property is higher (single scattering albedo is lower) for aerosol with more contribution of soot particles. Figure 6 shows an example of the results of the irradiance and radiance observations. Figure 7 shows an example of aerosol optical parameters derived from the TCAM analysis of the data: Fig. 7(a) shows the wavelength dependence of the aerosol extinction coefficient (normalized to the value at 550 nm), (b) single scattering albedo, (c) asymmetry parameter, and (d) scattering phase function at wavelength 550 nm. In Sec. 5 below, we describe the application of these aerosol characteristics to the atmospheric

Fig. 5. Real and imaginary parts of the complex refractive index of the three aerosol components: component 1, 2 and 3 refer to the water soluble, oceanic, and soot aerosol

Fig. 6. Spectra observed around noon on October 16, 2008: (a) direct solar radiation (DSR),

instrument is 5 deg for DSR, 5-20 deg for AUR, and 20 deg for SSR. Simulation curves based on the TCAM best fitting are also shown with data points (circles) used for the fitting.

(b) aureole (AUR), and (c) scattered solar radiation (SSR). Acceptance angle of the

correction of satellite remote sensing data.

types, respectively.

which is chosen to be 550 nm or some appropriate value within the observation wavelength range. Figure 4(b) shows the result of analysis based on eq. (4). As seen from Fig. 4(b) the temporal variation shows good agreement between the DOAS-derived aerosol optical thickness and the SPM mass concentration observed from the ground sampling.
