Lecture 31. Constituent Lidar (3) otational Vibrational-otational (V) aman DIAL Multiwavelength DIAL Comparison of Constituent Lidar Techniques Summary for Constituent Lidar
Conventional aman DIAL for O 3 Measurements in Clouds On- esonance Off- esonance Conventional aman DIAL uses two primary wavelengths transmitted (e.g., 38 and 351 nm) into the atmosphere and ozone is calculated from the differential absorption of the corresponding aman return signals of molecular N 2 (e.g., 332 and 382 nm).
V aman DIAL for O 3 Measurement otational Vibrational-otational aman DIAL transmits single primary wavelength (e.g., 38 nm) into the atmosphere and ozone measurement is based on differential absorption of ozone between the purely rotational aman () return signals from N 2 (37 nm) and O 2 (37 nm) as the onresonance wavelength, and the vibrational-rotational aman (V) return signals from N 2 (332 nm) or O 2 (323 nm) as the off-resonance wavelength.
V aman DIAL
V aman DIAL Equation For the elastic DIAL channel at ON-resonance wavelength 38 nm, [ ] P S ( L,) = P L ( L ) ( aer ( L,) + mol ( L,)) exp 2 IG ( L,r)n IG dr exp 2 A 2 exp 2 ( aer ( L,r) + mol ( L,r))dr abs ( L,r)n c (r)dr [ ( L )G() ]+ P B For the purely rotational aman channels from N 2 and O 2 at 37 nm, [ ] P S (,) = P L ( L ) aman ( L,,) exp exp exp A 2 ( aer ( L,r) + aer (,r) + mol ( L,r) + mol (,r))dr ( IG ( L,r) + IG (,r))n IG dr ( abs ( L,r) + abs (,r))n c (r)dr [ ( )G() ]+ P B The beauty of the V aman DIAL is the utilization of pure rotational aman () scattering so that aerosol backscatter is excluded in the channel.
V aman DIAL Equation Continued For the vibrational-rotational aman channel from N 2 at 332 nm, [ ] P S ( V,) = P L ( L ) aman ( L, V,) exp exp exp A 2 ( aer ( L,r) + aer ( V,r) + mol ( L,r) + mol ( V,r))dr ( IG ( L,r) + IG ( V,r))n IG dr ( abs ( L,r) + abs ( V,r))n c (r)dr [ ( V)G() ]+ P B Certainly, the vibrational-rotational aman (V) scattering channel also excludes aerosol backscatter.
Solution for V aman DIAL From the otational aman () and the Vibrational-otational aman (V) equations, the ozone number density can be derived as ln P S ( V,) P B A P S (,) P B ln ( V ) B ( ) 1 d n c () = abs d ln aman ( V,) C aman (,) aer () D mol () E IG ()n IG F Here, the expressions consist of four terms each, but the outgoing laser wavelength terms are cancelled out = [ ( L ) +( )] [ ( L ) +( V )]= ( ) ( V ) with = abs, aer, mol, IG
Challenge in V aman DIAL Term B is range-independent, so the derivative is zero, Term C is only concerned about molecule aman scattering. Term D will be determined through using the aman V channel and introducing Angstrom exponent. Term E is concerned about molecule ayleigh scattering, so can be be calculated from atmosphere temperature and pressure. Term F can be minimized through choosing proper wavelengths, thus, can be ignored. The main challenge in V aman DIAL is how to sufficiently suppress the elastic scattering at laser wavelength in the pure rotational aman () channel, as the wavelength difference is only 1 nm. Modern interference filter has FWHM of.3 nm, but its wing (or tail) can certainly extend to more than 1 nm. Since ayleigh scattering is about 3 orders of magnitude larger than rotational aman scattering, the influence from ayleigh scattering is not easy to be excluded. Thus, V aman DIAL is not easy to be realized.
Multiwavelength DIAL 1ON 1OFF 2ON 2OFF Multiwavelength DIAL uses three or more wavelengths transmitted into the atmosphere and ozone is calculated from the differential absorption of several pairs of elastic scattering signals from air molecules and aerosols. The influence from differential scattering and extinction of aerosols are dramatically decreased by properly choosing wavelengths.
Multiwavelength DIAL Equation For the elastic DIAL channel at 1ON-resonance wavelength, [( ) ] P S ( 1 ON,) = P L ( 1 ON ) aer ( 1 ON,) + mol ( 1 ON,) exp 2 IG ( ON 1,r)n IG (r)dr exp 2 A 2 exp 2 ( aer ( ON 1,r) + mol ( ON 1,r)) dr [ ] + P B abs ( ON 1,r)n c (r)dr ( ON 1 )G() For the elastic DIAL channel at 1OFF-resonance wavelength, [( ) ] P S ( 1 OFF,) = P L ( 1 OFF ) aer ( 1 OFF,) + mol ( 1 OFF,) exp 2 IG ( OFF 1,r)n IG (r)dr exp 2 A 2 exp 2 ( aer ( OFF 1,r) + mol ( OFF 1,r)) dr [ ] + P B abs( OFF 1,r)n c (r)dr ( OFF 1 )G() For the elastic DIAL channel at 2ON-resonance wavelength, P S ( 2 ON,) = P L ( 2 ON ) aer ( 2 ON,) + mol ( 2 ON,) exp 2 For the elastic DIAL channel at 2OFF-resonance wavelength, [( ) ] P S ( 2 OFF,) = P L ( 2 OFF ) aer ( 2 OFF,) + mol ( 2 OFF,) exp 2 [( ) ] IG ( ON 2,r)n IG (r)dr exp 2 IG ( OFF 2,r)n IG (r)dr exp 2 A 2 A 2 exp 2 exp 2 ( aer ( ON 2,r) + mol ( ON 2,r)) dr [ ] + P B abs ( ON 2,r)n c (r)dr ( ON 2 )G() ( aer ( OFF 2,r) + mol ( OFF 2,r)) dr [ ] + P B abs( OFF 2,r)n c (r)dr ( OFF 2 )G()
3-Wavelength Dual-DIAL for O 3 From above equations, we can derive the following ln P S ( ON 1,) P B P S ( ON 2,) P B P S ( OFF 1,) P B P S ( OFF = ln P L ( ON 1 )( ON 1 ) P L ( ON 2 )( ON 2 ) 2,) P B P L ( OFF 1 )( OFF 1 ) P L ( OFF 2 )( OFF 2 ) + ln aer( ON 1,) + mol ( ON 1,) aer ( ON 2,) + mol ( ON 2,) aer ( OFF 1,) + mol ( OFF 1,) aer ( OFF 2,) + mol ( OFF 2,) 2 [( aer ( ON 1,r) aer ( OFF 1,r)) ( aer( ON 2,r) aer ( OFF 2,r))] dr 2 [( mol ( ON 1,r) mol ( OFF 1,r)) ( mol( ON 2,r) mol ( OFF 2,r))] dr 2 [( IG ( ON 1,r) IG ( OFF 1,r)) ( IG ( ON 2,r) IG ( OFF 2,r))] n IG (r)dr 2 [( abs ( ON 1,r) abs ( OFF 1,r)) ( abs( ON 2,r) abs ( OFF 2,r))] n c(r)dr Three wavelengths form a dual-pair DIAL, with 1OFF = 2ON. Thus, the influence from aerosol extinction and backscatter can be minimized by choosing appropriate pairs of wavelengths.
Solution for Dual-DIAL O 3 Ozone number density can be derived from above equations: n c () = 1 2 abs ln P S ( ON 1,) P B P S ( ON 2,) P B P S ( OFF 1,) P B P S ( OFF 2,) P B +ln P L ( ON 1 )( ON 1 ) P L ( ON 2 )( ON 2 ) P L ( OFF 1 )( OFF 1 ) P L ( OFF 2 )( OFF 2 ) d + ln aer( ON 1,) + mol ( ON 1,) d aer ( OFF 1,) + mol ( OFF ln aer( ON 2,) + mol ( ON 2,) 1,) aer ( OFF 2,) + mol ( OFF 2,) [ ( aer ( ON 1,) aer ( OFF 1,)) ( aer( ON 2,) aer ( OFF 2,))] [ ( mol ( ON 1,) mol ( OFF 1,)) ( mol ( ON 2,) mol ( OFF 2,))] [ ( IG ( ON 1,) IG ( OFF 1,)) ( IG ( ON 2,) IG ( OFF 2,) )] n IG () A B C D E F Where the differential absorption cross-section is defined as abs = ( abs ( ON 1,r) abs ( OFF 1,r)) ( abs( ON 2,r) abs ( OFF 2,r))
Choice of Wavelength for Dual-DIAL = ln aer ( ON 1,) + mol ( ON 1,) aer ( OFF 1,) + mol ( OFF ln aer ( ON 2,) + mol ( ON 2,) 1,) aer ( OFF 2,) + mol ( OFF 2,) aer = ( aer ( ON 1,) aer ( OFF 1,)) ( aer( ON 2,) aer ( OFF 2,)) mol = ( mol ( ON 1,) mol ( OFF 1,)) ( mol ( ON 2,) mol ( OFF 2,)) IG = ( IG ( ON 1,) IG ( OFF 1,)) ( IG ( ON 2,) IG ( OFF 2,)) abs = ( abs ( ON 1,r) abs ( OFF 1,r)) ( abs( ON 2,r) abs ( OFF 2,r)) Different channels interact on the same aerosols and interference gases (IG) but with different wavelengths. The main error in conventional DIAL is caused by the uncertainty of the wavelength dependence and information (like density) of the backscatter, extinction, and interference owing to aerosols and IG. aer () 1 a, aer () 1 a If the wavelength dependence (Angstrom factor) is stable within the detection wavelength range, it is possible to cancel the influence by carefully choosing the wavelengths of two pairs - the difference between two pairs can be minimized.
Solution for Dual-DIAL O 3 By choosing the pairs of wavelengths, terms C-F (now the difference at two pairs of DIAL wavelengths) can be minimized or cancelled out. Thus, the O 3 measurement errors caused by the uncertainties of terms C-F can be dramatically decreased. Notice that the choice of the pairs of wavelengths must satisfy one important condition that the differential absorption cross-section given above must be large enough to meet the requirements of measurements sensitivity and spatial resolution. For the O 3 measurements, the dual-dial can be carried out by three wavelengths chosen at 277.1, 291.8, and 313.2 nm. The middle wavelength 291.8 nm acts as the off-wavelength for the 1st pair, while the onwavelength for the 2nd pair. To minimize the influence from aerosol backscatter and extinction, a constant C can be introduced into above lidar equation. C is approximately determined by the ratio of the wavelength differences between two pairs of DIAL: C = 1 ON 1 OFF 2 ON 2 OFF Here 1 OFF = 2 ON
Solution with Constant C Introduced n c () = 1 2 abs ln P S( ON 1,) P B P S ( ON C 2,) P B P S ( OFF 1,) P B P S ( OFF 2,) P B +ln P L ( ON 1 )( ON 1 ) P L ( ON 2 )( ON C 2 ) P L ( OFF 1 )( OFF 1 ) P L ( OFF 2 )( OFF 2 ) d d + ln aer( ON 1,) + mol ( ON 1,) aer ( OFF 1,) + mol ( OFF C ln aer( ON 2,) + mol ( ON 2,) 1,) aer ( OFF 2,) + mol ( OFF 2,) [ ( aer ( ON 1,) aer ( OFF 1,)) C ( aer( ON 2,) aer ( OFF 2,))] [ ( mol ( ON 1,) mol ( OFF 1,)) C ( mol( ON 2,) mol ( OFF 2,))] [ ( IG ( ON 1,) IG ( OFF 1,)) C ( IG ( ON 2,) IG ( OFF 2,) )] n IG () A B C D E F Where the differential absorption cross-section is defined as abs = ( abs ( ON 1,r) abs ( OFF 1,r)) C ( abs( ON 2,r) abs ( OFF 2,r))
Simulation esults for Dual-DIAL O 3 C =.65
Dual-DIAL for SO 2 Measurements Fukuchi et al., Opt. Eng., 38, 141-145, 1999 Fujii et al., Applied Optics, 4, 949-956, 21
Dual-DIAL for SO 2 Measurements
Comparison of Constituent Lidar Tech Technique esonance Fluorescence Lidar esonance Fluorescence Lidar Signal Source & Trace Gas esonance fluorescence from metal atoms in middle and upper atmosphere esonance fluorescence from He, N 2 +, O in thermosphere Interests Temp, Wind, Density, Wave Density, Temp Wind, etc Conventional DIAL Elastic-scattering from air molecules and aerosols Trace gas absorption in the extinction terms Species, Density aman Lidar Inelastic aman scattering from trace gas and reference N 2 or O 2 (no aerosol scattering) Trace gas scattering in the backscatter terms, Trace gas absorption in the extinction terms Species, Density, Mixing ratio aman DIAL Inelastic aman scattering from N 2 or O 2 Trace gas absorption in the extinction terms Species, Density V aman DIAL Pure rotational aman scattering from N 2 or O 2 and Vibrational- otational aman scattering from N 2 or O 2 Trace gas absorption in the extinction terms Species, Density Multiwavelength DIAL Elastic scattering from air molecules and aerosols Trace gas absorption in the extinction terms Species, Density ange-esolved spatial & temporal distribution of these species, density, temp, wind and waves
Backscatter Cross-Section Comparison Physical Process Mie (Aerosol) Scattering Atomic Absorption and esonance Fluorescence Molecular Absorption Backscatter Cross-Section 1-8 - 1-1 cm 2 sr -1 1-13 cm 2 sr -1 1-19 cm 2 sr -1 Mechanism Two-photon process Elastic scattering, instantaneous Two single-photon process (absorption and spontaneous emission) Delayed (radiative lifetime) Single-photon process Fluorescence From Molecule, Liquid, Solid ayleigh Scattering (Wavelength Dependent) aman Scattering (Wavelength Dependent) 1-19 cm 2 sr -1 1-27 cm 2 sr -1 1-3 cm 2 sr -1 Two single-photon process Inelastic scattering, delayed (lifetime) Two-photon process Elastic scattering, instantaneous Two-photon process Inelastic scattering, instantaneous
Summary for Constituent Lidars To identify species and to measure species, spectroscopy is the key for constituent lidars to infer spectral information of the species. These lidars use atomic resonance fluorescence, molecular absorption, or aman scattering or combination of differential absorption with aman scattering to obtain the specie spectrum. The key is to gain spectral information as much as possible, with multiple frequencies or multiple wavelengths. Lower atmosphere study poses a great challenge to lidar community, as many factors (especially the aerosol backscatter and extinction) are involved with each other and make the derivation of precise information on trace gases a very complicated procedure. Different techniques have been developed or proposed for solving these problems. The main idea is how to minimize the influence from aerosol backscatter and extinction and how to minimize the interference from other gas molecules.
Summary for Constituent Lidars Two major solutions for now: to use aman scattering to avoid aerosol scattering get into the signal channels, or to use multiwavelength selection to cancel out the influence from aerosol and other molecules. Other possible ways are to measure several interference gases simultaneously with multiple channels or to combine the DIAL with Doppler lidar etc to study the dynamic transportation of pollutant. The DIAL and aman lidars are still far from perfection. It could be a growing point in lidar field. The resonance fluorescence lidar for thermosphere species is still under development. The main point is to develop the proper laser transmitter that can give the right wavelength and high enough power. As O resonance frequency is in the far UV range (131 nm), the O lidar has to be operated from the space down-looking to avoid the UV absorption by ozone and atmospheric molecules.