MONITORING ATMOSPHERIC POLLUTANTS WITH A HETERODYNE RADIOMETER TRANSMITTER-RECEIVER

• US 3,766,380 A
• Filed: 03/24/1972
• Issued: 10/16/1973
• Est. Priority Date: 03/24/1972
• Status: Expired due to Term

• Current Assignee: Robert T. Menzies
• Original Assignee: Robert T. Menzies
• Inventors: Menzies, Robert T.
• Primary Examiner(s): Lawrence, James W.
• Assistant Examiner(s): Dixon, Harold A.
• Application Number:
  US05/237,694
• Time in Patent Office: 571 Days
• Field of Search: 250/43.5R,83.3R
• US Class Current: 250/343
• CPC Class Codes: G01N 2021/1795 Atmospheric mapping of gases
• CPC Class Codes: G01N 21/39 using tunable lasers
• CPC Class Codes: G01N 21/4738 Diffuse reflection precedence is given to G01N21/55 - G01N21/57 if specular component is taken into consideration, e.g. also for testing fluids, fibrous materials
• CPC Class Codes: G01N 2201/0214 Airborne
• CPC Class Codes: G01N 33/0027 concerning the detector

Abstract

The presence of selected atmospheric pollutants can be determined by transmitting an infrared laser beam of proper wavelength through the atmosphere, and detecting the reflections of the transmitted beam with a heterodyne radiometer transmitterreceiver using part of the laser beam as a local oscillator. The particular pollutant and its absorption line strength to be measured are selected by the laser beam wave length. When the round-trip path for the light is known or measured, concentration can be determined. Since pressure (altitude) will affect the shape of the molecular absorption line of a pollutant, tuning the laser through a range of frequencies, which includes a part of the absorption line of the pollutant of interest, yields pollutant altitude data from which the altitude and altitude profile is determined.

Claims

1. An active method for monitoring atmospheric pollutants comprised of the following steps:

directing a laser beam of a selected frequency through the atmosphere to a reflecting surface to excite molecules of a pollutant of interest suspected of being present in the atmosphere between the laser beam source and the reflecting surface, said source being a heterodyne radiometer transmitterreceiver, detecting the absorption of said laser beam transmitted and reflected by said surface using said heterodyne radiometer transmitter-receiver in which a part of the laser beam being transmitted at said frequency is used through a frequency shifter as a heterodyne signal to detect the reflected signal, determining the path length traveled by the laser beam thus transmitted and reflected, and determining the concentration of the pollutant of interest from the measured absorption and the path length, said concentration being proportioned to a ratio of measured absorption to path length, thereby determining the presence and average concentration N of the pollutant of interest over said path length.

2. An active method as defined in claim 1 wherein said average concentration N of the pollutant throughout the absorption path as determined from a measurement of total absorption K at a particular frequency Nu 1 from the equation K( Nu 1) 2/ pi SoN ( Delta Nu / ( Nu 1- Nu o)2 + Delta Nu 2) hM where hM is the maximum altitude of the path length, Nu o is the frequency of peak absorption, So is the average line strength known in advance, and Delta Nu is the average line width also known in advance.

3. An active method as defined in claim 1 including the following further steps tuning said laser transmitter-receIver over a narrow frequency region which includes the absorption line of said pollutant of interest, and at each frequency determining the total absorption to obtain absorption versus frequency data which, when plotted, yields a bell-shaped curve comprised of a composite of Lorentzian shaped curves of various half widths, each corresponding to average absorption in a particular altitude segment, and determining the amplitudes of particular Lorentzian shaped curves that would make up said composite curve, said amplitudes giving average concentrations at respective altitude segments which when plotted as a function of altitude yields an altitude profile of pollutant concentration.

4. An active method for monitoring atmospheric pollutants as defined by claim 3 wherein the last step is effectively carried out by solving the following sets of equations;

K1( Nu

     1) (2 So(h1) N(h1)/ pi ) ( Delta Nu (h1)/( Nu 1- Nu o)2 + ( Delta Nu (h1))2) Delta h C11N(h1) K2( Nu

     1) )2 So(h2) N(h2)/ pi ) ( Delta Nu (h2)/( Nu 1- Nu o)2 + ( Delta Nu (h2))

     2) Delta h C12N(h2) Kn( Nu

     1) (2 So(hn) N(hn)/ pi ) ( Delta Nu (hn)/( Nu 1- Nu o)2 + ( Delta Nu (hn))2) Delta h C1nN(hn) where h1 to hn are average altitudes within respective altitude segments 1 to n, assuming average values for So, Delta Nu and N within each segment, and K( Nu

     1) C11N(h1) + C12N(h2) + . . . . . + C1nN(hn) K( Nu

     2) C21N(h1) + C22N(h2) + . . . . . + C2nN(hn) K( Nu n) Cn1N(h1) + Cn2N(h2) + . . . . . + CnnN(hn) where the number of frequencies Nu 1, Nu 2. . . Nu n used equals the number of altitude segments, hn is the average altitude within the nth altitude segment, K( Nu n) is the total absorption at frequency Nu n, Kn( Nu

     1) is the absorption at frequency Nu 1 in the nth altitude segment, N(hn) is the average concentration in the nth altitude segment, Delta h is the height of each altitude segment, Nu o is the line center frequency, and So and Delta Nu are line strength and line width parameters, respectively.