Vol49_1_2006def 133 ANNALS OF GEOPHYSICS, VOL. 49, N. 1, February 2006 Key words DOAS applications in urban areas – at- mospheric pollutants tomography – aircraft meas- urements of atmospheric trace gases 1. Introduction The growth in human activity over the last century has led to an enormous increase in en- ergy consumption with the consequent emis- sion of substances in quantities greater than those which the atmosphere can absorb. In metropolitan areas wide-gridded net- works of in situ analysers are normally used for the continuous recording of gas pollutant con- centrations. These networks are, however, ex- pensive and the single point measurements are often insufficient to characterise the photo- chemical and transport phenomena even over small areas. Remote sensing systems can play an important role, representing a good compro- mise between spatial resolution and sensitivity of the measurements, providing more extensive coverage. In particular, DOAS (Differential Perspectives of 2D and 3D mapping of atmospheric pollutants over urban areas by means of airborne DOAS spectrometers Giorgio Giovanelli (1), Elisa Palazzi (1), Andrea Petritoli (1), Daniele Bortoli (1)(2), Ivan Kostadinov (1)(3), Federico Margelli (1), Simonetta Pagnutti (1), Margherita Premuda (4), Fabrizio Ravegnani (1) and Giuliano Trivellone (1) (1) Istituto di Scienze dell’Atmosfera e del Clima (ISAC), CNR, Bologna, Italy (2) Geophysics Centre of Évora - University of Évora (CGE-UE), Évora, Portugal (3) Solar-Terrestrial Influences Laboratory (STIL), Bulgarian Academy of Science, Stara Zagora, Bulgaria (4) ENEA, Divisione Fisica Applicata, Centro Ricerche Bologna, Italy Abstract In the field of air quality control, optical remote sensing systems can measure the spatial distribution of gas pollu- tants, offering numerous advantages over conventional networks of in situ analysers. We propose some innovative solutions in the field of DOAS (Differential Optical Absorption Spectroscopy) remote systems, utilizing diffuse so- lar light as the radiation source. We examine the numerous potentialities of minor gas slant column calculations, applying the «off-axis» methodology for collecting the diffuse solar radiation. One of these particular approaches, using measurements along horizontal paths, has already been tested with the spectrometer installed on board the Geophysica aircraft during stratospheric flights up to altitudes of 20 km. The theoretical basis of these new meas- urement techniques using DOAS remote sensing systems are delineated to assess whether low altitude flights can provide 2D and 3D pollution tomography over metropolitan areas. The 2D or 3D trace gas total column mapping could be used to investigate: i) transport and dispersion phenomena of air pollution, ii) photochemical process rates, iii) gas plume tomography, iv) minor gas vertical profiles into the Planetary Boundary Layer and v) minor gas flux divergence over a large area. Mailing address. Dr. Giorgio Giovanelli, Istituto di Scienze dell’Atmosfera e del Clima (ISAC), CNR, Area della Ricerca CNR, Via Gobetti 101, 40129 Bologna, Italy; e-mail: G.Giovanelli@isac.cnr.it 134 Giorgio Giovanelli et al. Optical Absorption Spectroscopy) remote sen- sors open up new perspectives in the field of pollutant analysis and control such as, for ex- ample, the monitoring of a large number of gases simultaneously with a single instrument and a better characterisation of the area exam- ined through the integration of gas concentra- tions over the entire optical path of the meas- urements. In the field of air quality control and moni- toring, one of the current, most important prob- lems is how to measure the three-dimensional distributions of airborne pollutants in the lower atmospheric layers within the Planetary Bound- ary Layer (PBL) over urban and industrial ar- eas, and in a systematic way. Over the past few decades diffusion and chemical modeling tech- niques have been developed which allow calcu- lation of 3D trace gas distributions with a spa- tial resolution of less than 1000 m and the de- tailed control of air quality in urban and indus- trial areas has become a fundamental aim for keeping environmental health within acceptable limits (von Kuhlmann et al., 2003). Passive remote sensing systems, such as spectrometers used in conventional modes, can at best calculate 2D maps of columnar abun- dances of the gaseous species under investiga- tion integrated over the given measurement path (for example, from the ground to the flight altitude with a spectrometer, on board the air- craft, making «nadir» measurements). Even gas profiles obtained using inversion methods (Petritoli et al., 2002a) from a series of zenith- sky DOAS observations during twilight cannot supply the vertical and temporal resolution nec- essary for Chemical Transport Models (CTMs) validation (Finzi et al., 2001). Airborne Differential-LIDAR (Browell et al., 1998) could, theoretically, provide 3D mapping of gases but, besides the «eye-safe» power lim- its of the technique (several tens of mW), there are obvious economic and technical difficulties in reaching a payload compatible with the re- quirements of a local service carrying out at- mospheric analyses and air quality control. Thus, it would seem that there are, at pres- ent, no economically accessible remote sensing techniques available for systematic 3D map- ping of pollutants in the lower atmosphere which could be utilized in widescale national air quality programs and surveys. On the other hand, there is a field to be ex- plored with great attention: that of diffuse solar light measurements, with UV-VIS or IR spec- trometers utilised in non-conventional configura- tions, defined as «off-axis» methods, to distin- guish them from the classic nadir and zenith ob- servations. In this work we will examine the re- sponse of a spectrometric system with zenithal movement of the input optics and estimate the diffuse radiation flux values, collected by the in- put optics of the spectrometer, as a function of the angle from the zenith. The final purpose of these efforts is to formulate a new method for the quasi-real-time reconstruction of the 2D and 3D trace gas distributions in the atmospheric layers closest to ground level. For example, by flying at an altitude between 2000 and 4000 m, it will be possible to measure the vertical distribution of gaseous pollutants, such as NO2, SO2, O3, HNO2, benzene, formaldehyde, toluene and xylene within the PBL. 2. DOAS methodology for pollutant gas measurements in urban areas The DOAS methodology was first introduced by Noxon (1975) in the early 70’s for measure- ments of total atmospheric gas columns, using diffuse solar light along the vertical path as the source of radiation (passive mode). The use of this technique rapidly became widespread in ground-based climatic stations and in airborne (Giovanelli et al., 2000) and satellite systems (Geophysica, ER2 stratospheric aircraft, Gome and Sciamashy) to study the climatic distribution of minor stratospheric gases and ozone loss phe- nomena. Later, Platt and Perner (1980) began to use the method to measure optical depths of trace gases along horizontal trajectories, using a UV lamp as an artificial radiation source (active mode). More recently, the technique has also of- fered a method for environmental monitoring in urban and industrial areas (Brocco et al., 1992; Edner et al., 1993; Evangelisti et al., 1995; Rave- gnani et al., 1997). In fact, the DOAS methodol- ogy permits the simultaneous measurement with 135 Perspectives of 2D and 3D mapping of atmospheric pollutants over urban areas by means of airborne DOAS spectrometers a single instrument of the concentrations of vari- ous gases with distinct absorption bands in the UV/VIS spectral range (conventional analysers are gas-specific) such as O3, NO2, SO2, formalde- hyde CH2O (Platt and Perner, 1980), nitrous acid HNO2 (Platt et al., 1980), the nitrate radical NO3 (Platt et al., 1981), benzene C6H6, toluene C7H8 and other hydrocarbons of general formula CxHy. With the DOAS methodology, gas concen- trations are retrieved from their absorption characteristics in the UV-VIS spectrum. The method is based on the Lambert-Beer Law in its differential form (the low frequency spectra are subtracted from the original ones) (2.1) where I0(m) is the reference spectrum, i.e. the radiance outside the atmosphere, I(m) is the ra- diation intensity after passing through a layer of thickness L, ∆vg is the Differential absorption Cross Section (DCS) of the gth gas at the wave- length m, obtained by subtracting the low fre- quency spectrum from its Absolute absorption Cross Section (ACS). The summation is over all absorbers in the examined spectral range. The result of the equation gives the integral of the gth gas concentration (Cg) along the measure- ment path (L). A more detailed description of DOAS methodology is available in Solomon et al. (1987), Giovanelli et al. (1990). At the ISAC-CNR Institute various types of instruments, known as GASCOD (Gas Absorp- tion Spectrometers Correlating Optical Differ- ences), have been developed for the estimation of gas column quantities and profiles from zenith diffuse solar radiation measurements at ground- based stations. An airborne multi-input version (GASCOD/A4r) has been developed for meas- urement of both gas total columns and actinic flux on board the Geophysica-M55 stratospheric aircraft, together with other remote sensing and in situ equipment. The GASCOD/A4r operated successfully during the APE-THESEO (Seychel- les) and APE-GAIA (Ushuaia, Argentina) cam- paigns in 1999 (Petritoli et al., 1999) and, more recently, in a validation program of the ENVISAT satellite (Kostadinov et al., 2006). ( ) ( ) )(ln ln I I I I C Lgg g n 0 0 1 - = m m m m mv∆ = ] d ] d g n g n / A modified version of this spectrometer has also been used to measure pollution at ground level in urban areas along horizontal paths, em- ploying a xenon lamp as radiation source. Start- ing from this latest prototype, an Italian compa- ny has developed a commercial version for in- stallation in networks of in situ analysers for at- mospheric analyses and air quality control in urban and industrial areas. In conventional ground-based networks ac- tive DOAS systems can be configured as bista- tic, where the measurement path length L is de- fined by the instrument (receiver) and by the projector creating the beam (emitter), or mono- static (fig. 1), where the emitter and the receiv- er are deployed at the same site with the use of retroreflectors for probing of the same optical path examined in bistatic mode, but doubled. Although the most immediate use of DOAS instrumentation is air quality control in an envi- ronmental monitoring network, its potential can be fully exploited when used in less conven- tional ways. There are innovations both in the measurement techniques and in the subsequent data analysis. Often these two aspects become so intertwined that it becomes difficult to draw a distinction. As mentioned above, the DOAS methodolo- gy was first used to obtain slant columns of mi- nor gases in the stratosphere by processing zenith diffuse solar radiation measurements from ground-based stations. The subsequent «off-axis» approach was a logical extension deriving from the need to detect the presence of gases, in the stratosphere, with low absorption coefficients and low concentrations (e.g., total O3 column, nitro- Fig. 1. Monostatic configuration of a DOAS spec- trometer for air pollution measurement. 136 Giorgio Giovanelli et al. gen oxides, chlorine and bromine during ozone depletion processes) (Giovanelli et al., 2001). The most recent application of the off-axis method is the «horizontal view looking» tech- nique, which allows quasi in situ trace gas con- centration measurements (Petritoli et al., 2002b). Its application in «horizontal DOAS tomog- raphy» represents an important methodological innovation in the interpretation of DOAS data to obtain 2D and 3D pollutant distributions over a limited area (Hashmonay et al., 1999; Vogel et al., 2000). However, DOAS tomography in the ground- based configuration, recently proposed by the IUP (Institut für UmweltPhysik) of the Universi- ty of Heidelberg, requires the use of a significant number of spectrometers and retro-reflectors to obtain 2D and 3D pollutant mapping. In the 1980s a similar approach was attempted to ob- tain the bidimensional structure of a stack plume in terms of the optical depth of SO2 using off-ax- is measurements with a Mask Correlation remote sensor (Giovanelli et al., 1979). The reconstruc- tion of plume structure was obtained by making a series of azimuthal scans of diffuse solar radi- ation downwind from the stack, and for each of these zenithal scans were also made. Various Eu- ropean research groups, besides our own, are be- ginning to show increasing interest in off-axis measurements both in ground-based and air- borne configurations, derived from DOAS meth- ods, to obtain 2D or 3D air pollutant mapping in- side the PBL in urban and industrial areas. 3. DOAS measurements with vertical view looking configurations Usually, estimation of the columnar abun- dances of minor atmospheric gases from meas- urements of solar diffuse radiation (at zenith or in other directions) is quite complex, since it re- quires the derivation of a suitable Atmospheric Transfer Equation (ATE). Processing of DOAS measurements in the vertical view looking con- figuration, includes the following steps: i) Derivation of the ATE, which provides the best description of the atmospheric transfer for diffuse solar radiance measurements along the vertical axis. ii) Measurements and retrieval of the slant column of the given gas, using multiple linear re- gression or Single Value Decomposition (SVD). The slant column is the integral of the gas con- Fig. 2. Schematic diagram of path lengths in the atmospheric transfer model of DOAS zenith solar light meas- urement. 137 Perspectives of 2D and 3D mapping of atmospheric pollutants over urban areas by means of airborne DOAS spectrometers centration along the atmospheric slant paths de- fined with respect to the Solar Zenith Angle (SZA) i (fig. 2). iii) Calculation of the Intensity Weighted Optical Path (IWOP), which correlates the opti- cal trajectory of the diffuse radiation with the radiation intensity along that trajectory. iv) Calculation of the «air mass» or «en- hancement» factor (AMF), defined as the ratio between the slant column and the vertical col- umn, defined as the integral of the gas concen- tration along the vertical path. In the ATE model developed in our group (Giovanelli et al., 1990), the atmosphere is sub- divided into L = 120 spherical layers, each ∆z = 1 km thick (fig. 2) and summation is used instead of the integral. The irradiance scattered along the vertical path and collected on the ground can be repre- sented by the following equation: (3.1) where l=1, 2, …, L denotes the lth scattering lev- el and each Il, s(m) is the contribution of the radi- ation flux towards the ground, produced by scat- ( ) ( )I I, ,Ve s l s l L 1 =m m = / tering in the lth layer and defined by the equation (3.2) where I0(m) has the same meaning defined in (2.1), v0(m) represents the scattering cross sec- tion of air molecules, f(i) is the molecular scat- tering function, derived from the Rayleigh scat- tering formula, which defines the fraction of the scattered radiation flux in the ground direction, versus the diurnal variation of SZAi, N0 is the average air molecule density and ∆z is 1 km. The bracketed terms [AVe] and [ASl] are, respec- tively, the absorption along the vertical path and along the slant path. [AVe] is defined as follows: (3.3) where i = 1, 2,…, l is the ith vertical layer, vg(m) is the absorption cross section of the gth ab- sorber with g = 0, 1, 2, …, n (the 0 subscript in- dicates the molecular cross section for Rayleigh scattering), Ni,g(m) is the average molecular -exp=[ ] ( ) ( ) ( )A N z,Ve g i g g n i i l 01 $ $ $v m m ∆ == d n= G// ( ) ( ) ( ) ( ) [ ] [ ]I I f N z A A,l s Ve Sl0 0 0$ $ $ $ $ $=m m v m i ∆ Fig. 3. Computed intensity of downward diffuse solar radiation as function of the scattering point along a verti- cal path. Four curves are plotted for different SZA, considering m= 330 nm and Vis = 5 km. The accumulated prob- ability trends, respectively due to the i= 70° irradiance curve and to the i= 90° irradiance curve are also shown. 138 Giorgio Giovanelli et al. density of the gth absorber in the ith layer. [ASl] is defined by the equation (3.4) where j = (z+1), (z+2), …, L is the jth slant lay- er with z the scattering level, vg(m) and Nj,g(m) have the same meaning defined in (3.3), ng(i) is the gth absorber’s air mass factor, including Mie, Rayleigh and Ring AMFs, computed for the most probable slant path obtained through computation of the IWOP. By applying this atmospheric transfer equa- tion it is possible to compute, for different wave- lengths, the downward scattered radiation re- ceived by the spectrometer input optics, as a func- tion of both the height at which the scattering oc- curs along the vertical direction and the varying SZA. In fig. 3 the flux of zenith diffuse solar ra- diation is plotted for different i values and for m= 330 nm. It is interesting to observe how, in these irradiance curves, the level of the maximum scattering value increases as a function of higher -exp=[ ] ( ) ( ) ( ) ( ) A N s , ( ) Sl g j g g g n j z L j 01 $ $ $ $v m m n i i∆ == + $ d ^ n h = G // SZAs. This can also be observed in the two accu- mulated probability curves of irradiance. 4. DOAS off-axis measurements In order to increase the gas optical depth to measure compounds that do not show strong ab- sorptions in the UV-VIS range, measurements are performed along oblique atmospheric paths, generically called off-axis, by rotating the optical input of the spectrometer from the zenith. The off-axis measurements are divided into several configurations according to the field of application and the various ATEs. The latter have to be developed for an adequate interpre- tation of the flux of diffuse solar radiation along the particular off-axis direction being observed. The off-axis measurements are named accord- ing to the particular angle being examined: if the Line Of Sight (LOS) of the input optics ({) changes from 90° towards 0°, the measurement method is known as oblique-up view looking, whose ATE is briefly described below. There are two other configurations: «horizontal view looking» and «oblique-down view looking». The former is briefly described in the next para- Fig. 4. Schematic diagram of path lengths in the atmospheric transfer model of DOAS solar light measurement at oblique-up view looking. 139 Perspectives of 2D and 3D mapping of atmospheric pollutants over urban areas by means of airborne DOAS spectrometers graph and the latter is still the subject of study and experimental verification. The oblique-up view looking method is used to detect thin optical depth stratospheric minor gases, such as O3 and NO2 in polar re- gion during ozone depletion and denitrification periods and halogen compounds (BrO and OClO), (Giovanelli et al., 2001). By analogy with equations presented in the previous section, the ATE can be derived for oblique paths. The irradiance scattered along the oblique path and collected at ground level can be represented by the following equation: (4.1) where the «Ob» subscript defines the paths of the light collected by the input optics, which should not be confused with the slant paths of the solar radiation before scattering which are defined by the subscript «Sl» (fig. 4) and each IOb,s,l(m) is the contribution of the radiation flux towards the ground, produced by scattering in the lth layer and is defined by the equation (4.2) where I0(m), s0(m), f(i−{), N0 have the same meaning defined in the previous equations, { indicates the spectrometer’s LOS, ∆Ob({) is the thickness of the layer in the LOS direction and the bracketed terms [AOb] and [ASl] are, respec- tively, the absorption along the oblique path and the slant path. As for the case of vertical view looking con- figuration, it is possible to evaluate the trends of the scattered radiation received by the spec- trometer input optics as a function of both the height at which the scattering occurs along the oblique direction and the varying SZA. 5. DOAS measurements with horizontal view looking configuration Measurements with DOAS systems using the horizontal view looking configuration present $( )Ob {∆$N$( )f -i {$( )v m$( )I m=( ) [ ] [ ] I A A , ,Ob s l Ob Sl 0 0 0 $ $ m ( ) ( )I I, , ,Ob s Ob s l l L 1 =m m = / some interesting fields of application both in cli- matic studies as well as in air pollution control. Regarding climatic studies in the strato- sphere, our group has recently developed a nov- el application of horizontal view looking, using a UV-VIS spectrometer on board the Geophysi- ca aircraft (Petritoli et al., 2002b). The measure- ment geometry is very similar to the limb-scan- ning mode used so far by satellite borne instru- ments to measure atmospheric radiance (i.e., in the IR spectral region by MIPAS-E and in the UV-VIS region by Sciamachy for the DOAS methodology, and both together on board the ENVISAT satellite). The scattered solar radia- tion collected along a horizontal trajectory is compared, in terms of the Lambert-Beer Law, with the scattered solar radiation collected along a zenithal path (fig. 5). The differential slant col- umn signal is composed of two contributions from the V and H0,4 path (fig. 5). Radiative trans- fer calculations show that V is negligible with respect to H0,4 so that by calculating H0,4 with the model it is possible to obtain the average concentration of the trace gas along the horizon- tal direction at flight level. This method, which allows quasi in situ gas concentration estimates, combines vertical and horizontal measurements and is known as ACILA (Average Concentration Inside the Layer near the Aircraft). H0,4 is a function of the spectral interval we are measur- ing and of the trace gas we are interested in. For ozone at 330 nm at an average flight altitude of 20 km, H0,4 is about 50-70 km. The method has already been tested with measurements carried out in the frame of APE-GAIA campaign (15 September to 14 October 1999), during which the M55 Geophysica aircraft flew over the Antarctic Peninsula (Carli et al., 2000). In the field of air pollution control the ques- tion is whether a source of artificial radiation is indispensable for horizontal measurements or whether it is possible to operate with diffuse so- lar radiation, thus allowing a greater degree of freedom in the measurements. The latter hypoth- esis is innovative and opens up fields of applica- tion previously considered impossible, or ex- tremely onerous due to the nature of the equip- ment required. The enormous potential of these near ground-level horizontal measurements of dif- 140 Giorgio Giovanelli et al. fuse solar radiation using DOAS lies, especial- ly, in the field of air pollution control and mon- itoring. It will also allow a more rapid use of DOAS instruments, operating in the UV/VIS region, in the new and expanding field of air pollution tomography. The analytical methodology is quite similar to that employed in horizontal measurements in Fig. 5. Measurement geometry used to retrieve ACILA. When spectra from the zenith are used as reference in the DOAS calculation the SV path and S0,4 paths cancel out and what remain in the signal is due to the absorp- tion along V and H0,4. Model calculations give H0,4 > > V so that V can be neglected. Fig. 6. Computed intensity of diffuse solar radiation collected along a horizontal path, as function of the scat- tering distance from the instrument. Four curves are plotted for different SZA and visibility values, considering m= 330 nm. The accumulated probability trends, respectively due to VIS= 23 km and to VIS= 5 km are also shown. 141 Perspectives of 2D and 3D mapping of atmospheric pollutants over urban areas by means of airborne DOAS spectrometers the stratosphere: the value of the given gas slant column is found from the difference between measurements of diffuse solar radiation along a «vertical» (or equivalent) path and those along a «horizontal» path. However, compared to ACI- LA, this method requires the derivation of an ATE which includes simulation of multiple scat- tering (Palazzi, 2003) and a knowledge, during measurement, of additional parameters such as, for example the spectral extinction coefficient of the atmosphere along the horizontal measure- ment path, which is related to the visibility du- ring the measurement time. Figure 6 shows the results of calculations along a near-ground-level horizontal path for two different values both of SZA (50° and 70°) and visibility (5 and 23 km) for a wavelength of 330 nm. It is interesting to observe that, when the visibility is known, one can obtain the integral of the given gas concen- tration along the horizontal measurement path. 6. Conclusions The use of DOAS for the measurement of trace gases in the atmosphere is currently passing through a stage of rapid development. DOAS, as opposed to other remote sensing techniques, does not require absolute irradiance measure- ments since it exploits the differences in nor- malised radiation fluxes over the observed spec- tral region, thus allowing great flexibility and economy in making such measurements. DOAS measures the columnar abundances of trace gas- es such as: NO2, SO2, O3, NO3, BrO, OClO, HNO2, CH2O, benzene, toluene and xylene. DOAS can also measure concentrations of the nitrate radical, NO3 and OClO, which are not ac- cessible using other techniques, as well as pro- viding more accurate and reliable measurements of OH radicals, one of the main initiators of the chain of photochemical processes. The development of inversion algorithms, applied to sets of DOAS measurements for the determination of concentration profiles of trace gases, increases the information content as well as its range of applications, including the im- portant area of the validation of satellite data. For these reasons, the interest in DOAS is shifting from climatic studies in the stratosphere to air quality control and monitoring. In fact, the high information content of DOAS measure- ments allows its use not only in air quality sur- veys or networks, but also in special investiga- tions such as the interpretation of transport and dispersion processes, and the chemical transfor- mations taking place within the PBL. Because the installation of the spectrometer is relatively easy on board an aircraft, this sys- tem can be used to map atmospheric pollutants over large metropolitan areas. By flying at alti- tudes between 2000 and 4000 m, DOAS can de- tect the distribution of gas pollutant concentra- tions within the PBL. To fully exploit the multiplicity and flexibil- ity of DOAS methodology for air quality con- trol and monitoring, the development of a set of measurement procedures, including atmospher- ic transmission models, to be applied according to the particular application, will be crucial. This paper indicates the main areas of study necessary to bring on line the proposed method- ologies, such as horizontal view looking or oblique-down view looking, which are both de- duced from the classical configuration of meas- urement, i.e., the zenith-sky or vertical view looking approach. These new methodologies and their combined use under opportune conditions yields the 2D tomography of atmospheric trace gases. 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