Sun Photometry. Atmospheric Physics Lab Work. Abstract. Institute for Atmospheric and Climate Science IACETH

Transcription

1 Institute for Atmospheric and Climate Science IACETH Atmospheric Physics Lab Work Sun Photometry Abstract This experiment imparts the fundamentals of optical measurement of aerosols and gas concentrations in the atmosphere. First, the total turbidity factor is measured at different wavelengths, then the total water content of the atmosphere is determined by optical means. At the same time, the example of the TEMP notification shows how meteorological observations are coded and transmitted in the international network.

2 Questions to be answered during the reading of the manual (Will be discussed in a small tutorial ahead of the experiment) i) Which terms are neglected in equation 7)? ii) In equation 7), τ Oλ is multiplied with m and not m p. Why? iii) How is equation 13) derived? iv) Why is q 0 independent of the solar distance?

3 Sun Photometry Table of Contents 1. Introduction Absorption and scattering Extinction by water vapor in the atmosphere Determination of PW by radiosonde launching Extinction of ozone in the atmosphere Practical advice The instruments Device 1: Ozone Monitor (S/N: 4703) Device 2: Sun Photometer (S/N: 4088) Handling of the instruments Operating the instruments Geographical longitude and latitude Pressure Performing a measurement Read data from old measurements Exercises Preparation Measurements Data Analysis References Appendix The devices Ozone Monitor and Sun Photometer The menu structure of the instruments The display Output variables Data protocol (print two times for 15 measurements) Page 3

4 Atmospheric Physics Lab Work 1. Introduction 1.1. Absorption and scattering Before radiation from the sun reaches the ground, it is attenuated by different effects in the atmosphere (which is here assumed as cloudless): Scattering by air molecules (without conversion of energy or change in wavelength) Scattering by larger particles (aerosols) Absorption by gases in the atmosphere (mainly by water vapor in the IR, by ozone in UV range) For a given wavelength λ the transmittance T λ is the ratio of the radiation intensity received at the Earth's surface I λ to the incident radiation intensity at the top of the atmosphere I λ,0 : T λ = I λ I λ, 0 (1) The Beer-Lambert law states that the transmittance T λ decays exponentially as a function of the extinction coefficient α λ and the distance l the radiation travels through the material: T λ = I λ I λ,0 =e αλ l =e τλ (2) The product α λ l is called the optical depth or optical thickness τ λ. If the extinction coefficient α λ is not constant along the optical path the optical depth τ λ can be calculated by integration along the path: τ λ = α λ dl (3) In optical experiments in the atmosphere, the measured Intensity I λ depends on how much air mass the radiation has to pass before reaching the detector. Therefore the optical depth τ λ is corrected by a factor called optical air mass: m p =m p p 0 = 1 cos (α) p p 0 (4) The optical air mass m p considers changes in optical path length through the earth atmosphere due to ambient air pressure p and the solar zenith angle α (Figure 1). As a reference point a measurement at sea level ( p 0 =1000 hpa) and normal incidence ( α=0 ) is defined. The ambient air pressure p is a measure of the amount of air that lies above the station. However, equation (4) is not exact because of the curvature of the atmosphere and because a sun ray is curved due to the density of the air which decreases with increasing altitude. Yet, the errors are smaller than 3% for solar zenith angles α<80. Page 4

5 Sun Photometry zenith sun α top of the atmosphere m=1 m= 1 cos Furthermore, the distance between sun and earth is not constant but varies during the year. Therefore, the average extraterrestrial radiation intensity I λ,0 has to be corrected by a factor F: z I λ, 0 =F I λ,0 The factor F for a certain season can be taken from table 1. With all of the above corrections applied, formula (2) becomes: I λ = I λ, 0 F e m p τ The different extinction processes (i.e. Scattering, absorption etc.) do not occur separately, but act at the same time. They also strongly depend on the wavelength. Therefore, the direct solar radiation at a certain wavelength I λ, which is measured with a radiometer directed towards the sun, is described by where earths surface Figure 1: Definition of the optical air mass m I λ = 1 F I λ 0 exp( τ L λ m p τ O λ m τ Aλ m p ) (7) I λ = measured radiation intensity at wavelength λ Ι λo = extraterrestrial radiation intensity at wavelength λ for an average distance between sun and earth F = correction factor for the average distance between sun and earth τ Lλ = optical depth of air related to m p = 1 τ Oλ = optical depth of ozone related to m = 1 τ Aλ = optical depth of aerosols related to m p = 1 Table 1: F values for the different months of the year Month: Jan/ Dec Feb/ Nov March/ Oct Apr/ Sep May/Aug June/ July F: (5) (6) Page 5

6 Atmospheric Physics Lab Work Exact information on turbidity is required for the quantitative determination of the earth s radiation budget. The atmospheric turbidity is the haziness in the atmosphere due to aerosols. (10) The turbidity coefficient B is the optical density of the aerosols at λ=500 nm ( B=τ A,500 ). The optical density is defined as: I λ =10 (10 ) τλ I λ, 0 (8) Note that the bases are different: The optical density τ λ (10) relates to the base 10, whereas the optical depth τ λ relates to the base e. Optical depths τ λ can be converted to optical density τ λ (10) by: τ λ (10) = τ λ (9) (10) Experience has shown that the wavelength dependency of the optical density of aerosols τ A,λ follow a power law: τ 10 Aλ =B(2 λ) γ [λ] = µm (10) However, this relation only holds strictly when the size distribution of the aerosols is given by where dn r d log r r 2 (11) N = number of particles r = particle radius γ = wavelength coefficient For atmospheric haze, this equation is approximately fulfilled at least in the range 0.1 µm r 1 µm. However, exceptional turbidity caused by e.g. forest fires or volcano eruptions can lead to considerable deviations from the power law Extinction by water vapor in the atmosphere Extinction in the atmosphere is most often caused by the absorption bands of polyatomic gases. Therefore, the concentration of a gas can be determined by comparing the measured radiation intensity at an absorbed wavelength with the one at a non-absorbed wavelength. Water vapor absorbs radiation at λ = 936 nm amongst others, but does not attenuate radiation at λ = 870 nm (see Figure 2 and 3). Table 2: Optical densities of air and ozone [atm cm -1 ] λ 440 nm 500 nm 675 nm τ Lλ τ Oλ Page 6

7 Sun Photometry Figure 2: Absorption coefficient of water vapor The transmission factor of water vapor, q, is defined as q(m p )= I 936 I 870 (12) where I λ denotes the intensity at wavelength λ. The total water content of the atmosphere then is given by where PW = K m p log q 0 q (13) PW = total water content of the atmosphere (e.g. in kg m -2 ) q 0 = extraterrestrial value for q (independent of the solar distance) K = instrument constant taking into account the different sensitivity of the instrument to the compared wavelengths and the extinction coefficient of water vapor q 0 is graphically determined for a day where the atmospheric water vapor content is as constant as possible. K is obtained by comparing the optically measured water content of the air with the value determined by a radiosonde. Table 3: Absorption bands of water vapor in the IR range denomination of band spectral region band center position (absorption maximum) α µ ρδτ Φ Ψ Ω χ Page 7

8 Atmospheric Physics Lab Work 1.3. Determination of PW by radiosonde launching In a Stuve diagram / emagram the dewpoints from the TEMP notifications of at least two representative stations are entered (Payerne, Stuttgart, and Munich come into question). For each point, the mixing ratio and from this the absolute humidity are calculated. The atmosphere is then divided into layers such, that each boundary of a layer lies midway between two measuring points. Integration over all layers yields h2 a i dh i a h dh=pw sond (14) h1 where a is the water content in the altitude interval dh Extinction of ozone in the atmosphere Ozone measurements are done with an Ozone Monitor, S/N:4703. For these measurements, the intensities at the following wavelengths are required: 305 nm, 312 nm, and 320 nm. Ozone values are measured twice with different pairs of wavelengths, once with 305 nm and 312 nm, and once with 312 nm and 320 nm. Ozone column thicknesses are reported in Dobson Units (cf. Reference 5): where DU = L 12 = ln ( I λ01 I λ02 ) 1000 [ L 12 ln I 1 I 2 12 m p p 0 ] 12 : extraterrestrial intensities β 12 = (β 1 β 2 ): difference of absorption coefficients for air α 12 = (α 1 α 2 ): difference of absorption coefficients for ozone p 0 = standard pressure: mbar p = Ambient pressure at gaging station The constants L 12, β 12, and α 12 are already set and stored in the device S/N: 4703 and can be controlled under following two equations: -Measurements- -Calibrations- -Calibrations- -Ozone cal.- (15). The factors m and µ can be calculated with the with m=sec SZA sec SZA sec SZA sec SZA 1 3 (16) 1 = R r 2 (17) 1 sin2 SZA R h 2 h = = approx. elevation above sea level of the ozone layer [km] R = radius of the earth: 6371 km r = elevation above sea level of the gaging station [km] φ = latitude of gaging station (Zurich: 47 23'). Page 8

9 Sun Photometry 2. Practical advice 2.1. The instruments Two instruments of the type Microtops II are available. Each has a silicon photovoltaic cell as a sensor to measure the light intensity. Each device is equipped with 5 different filters which can be put in front of the sensor. A built-in targeting device helps in finding the correct orientation towards the sun. The instruments have to be screwed onto a support. Azimuth and elevation are easily adjusted. The elevation scale is located to the right on the head of the support. Prior to each measurement, this scale has to be adjusted by bringing the instrument to a horizontal position with a bubble level and setting the indicator of the elevation scale to zero. To measure the elevation of the sun, the instrument is directed towards the sun such, that the light through the open hole falls onto label M. The elevation can now be read from the scale Device 1: Ozone Monitor (S/N: 4703) This device is used to measure ozone and water vapor columns. 4 gives an overview over the available filters, their labels, and the corresponding wavelengths. For the ozone measurement three lines from the Huggins band are used. Table 4: Wavelengths of the Ozone Monitor (S/N: 4703) UV ozone λ = 305 nm UV ozone λ = nm UV ozone λ = nm IR water vapor λ = 936 nm IR no water vapor λ = 1020 nm Device 2: Sun Photometer (S/N: 4088) This device measures the turbidity coefficients and aerosol absorption. The measured wavelengths are summarized in table 5: 2.2. Handling of the instruments Table 5: Wavelenghts of the Sun Photometer (S/N: 4088) UV blue green red IR λ = 380 nm λ = 440 nm λ = 500 nm λ = 675 nm λ = 870 nm As the instruments are very sensitive to shock, they always have to be kept in the provided box. Do not carry the support with the instruments mounted on it. Since the sensitivity of the photometers depend on temperature, the photometers always have to be kept at room temperature. In an expanded series of measurements, they therefore have to be unscrewed after each single measurement and kept at room temperature between the measurements. Additionally, the offset of the amperemeter has to be checked prior to each measurement. Page 9

10 Atmospheric Physics Lab Work 2.3. Operating the instruments Geographical longitude and latitude 1. Turn on device with On/Off, you are in RDY mode now 2. Press key Menu -Main menu- Enter, the following is displayed: -Clock- (The clock must be always kept at GMT Greenwich Universal Time. To obtain GMT subtract 1 hour from local time in winter and 2 hours in summer.) 3. Press three times until -Main menu- -Location- is displayed. 4. Press Menu Enter 5. Press 6. Press several time until 3:Zuerich is displayed. 7. Press once to select. 8. Press Scan Escape If a different location needs to be entered, select an empty location instead of 3:Zuerich and enter immediately: 1. Press once to store the location selection. 2. Press Scan Escape 3. Press three times until -Location- -Coordinates- is displayed. 4. Enter elevation under -Location- -Altitude-. 5. Entering values is done with the and keys. 6. Get back to RDY mode by pressing Scan Escape three times. The values for ETH Zentrum (roof of CHN tower) are: Longitude: Latitude: Altitude: 47 23' N 8 34' E 504 m above sea level Pressure The devices are not equipped with an internal pressure sensor, hence, the pressure must be set manually. To obtain the current pressure at your position you can use the barometer in the P- floor of the CHN building. 1. Press three times until -Main menu- -Location- is displayed. Page 10

11 Sun Photometry 2. Press once until 3. Enter the pressure. -Location- -Pressure- is displayed Performing a measurement 1. Turn on the device with key on/off while the cap is closed and wait until RDY is displayed. 2. Open the cap. 3. Turn device towards the sun. An image of the sun must be visible in the small window labeled [sun target]. 4. Press under 3. Scan Escape until it beeps twice while the device is kept in the position achieved 5. Individual measured and calculated values can be read out by pushing the and keys Read data from old measurements After turning the device on (key on/off) the data from old measurements stored in the device can be read out. Newer measurements are accessed by the key and older measurements by the key. Individual measured and calculated values can be read out by pushing the and keys. Page 11

12 Atmospheric Physics Lab Work 3. Exercises 3.1. Preparation 1) For initializing the instrument the coordinate and the height above sea level for the roof of the CHN building are needed. Also determine the ambient pressure at the measurement day. (There is a precise barometer at P floor.) 2) Measurements should be taken over a wide range of SZA (0 80 ). Determine at which time on the measurement day SZA = 0 and 80. You can download a Matlabtool from the course homepage for these calculations Measurements 3) Measure on a clear day at least 15 data points with the Ozone Monitor and the Sun Photometer over a SZA range between 0 and 80. 4) Download radiosonde data for the same day from at least two TEMP notifications. ( Data Analysis 5) Determine I λ 0 and τ Aλ for λ = 440 nm, 500 nm and 675 nm. For the optical densities of air and ozone use Table 2: For each wavelength, plot ln (I λ ) versus m p. Use a linear regression to determine I λ 0 and τ Aλ. 6) From the derived optical thickness τ Aλ at λ = 440 nm, 500nm and 675 nm determine the turbidity coefficient B and the power coefficient γ by fitting the three data points to the power law (10). Compare this result for B with the value directly derived from AOT500 (Aerosol optical thickness is a synonym for optical depth). 7) Determine the extraterrestrial value for the transmission factor of water vapor q 0. Using the measurements of I 936 and I 870 plot ln (q) versus m p. Determine q 0 with a linear regression. 8) Determine PW sound from the radiosonde data (cf. equation 14). Compare this value with the direct measured value from the sun photometer. Please note that the sun photometer measures in cm. This value needs to be converted into kg/m 2. 9) Compare the measured Ozone concentration with literature values. Page 12

13 Records to hand in: Short summary of the theory. Complete measurement report. Traceable documentation of the data analysis. Which assumptions are made? Discussion of the results. Are they reasonable? Sun Photometry Discussion of errors. Which systematic errors are made? How can the experiment be improved? 4. References Wallace, J.M., P.V. Hobbs: Atmospheric Science, an Introductory Survey. Academic Press, Inc. San Diego, Liljequist, Gösta H., Konrad Cehak: Allgemeine Meteorologie. Springer, Petty, G.W., A first course in atmospheric radiation, Sindog Publishing, Liou K -N., An introduction to atmospheric radiation, Academic Press, Brasseur G., Solomon S., Aeronomy of the Middle Atmosphere, D. Reidel Publishing Company, Dordrecht, Netherlands 1986 (reprint from 2008 available by Springer). Solar Light Company, Inc. (2000): User's Guide Microtops II, Ozone Monitor & Sun Photometer, Version 2.42, Philadelphia. (available as pdf from the course homepage) Page 13

14 Atmospheric Physics Lab Work 5. Appendix 5.1. The devices Ozone Monitor and Sun Photometer The menu structure of the instruments Ready mode Main menu clock Adjust clock Clock timer Main menu measurement Calibrations Irradiance Ozone calibration Water calibration Pressure calibration Restore calibration Processing Scan length Select top Line frequency Main menu location Saved location Coordinates Altitude Pressure Main menu data logging Clear memory Delete last Main menu baud rate The display R e a d y M o d e O r t I D C o d e R D Y P h i l a.. I D = A u g : 2 4 : 4 5 D a t u m Z e i t G M T Figure 3: Display in ready mode Page 14

15 Sun Photometry Output variables Table 6: Output variables of the Ozone & Sun Photometer (S/N: 4703) O 3 (corr) Corrected ozone column in Dobson Units (DU) O 3 (1/2) Ozone column from channels 1 and 2 O 3 (2/3) Ozone column from channels 2 and 3 water Water column in cm AOT 1020 SZA I 305 I 312 I 320 I 936 I 1020 Aerosol optical thickness at 1020nm Solar zenith angle Intensity at 305nm Intensity at 312nm Intensity at 320nm Intensity at 936nm Intensity at 1020nm Table 7: Output variables of the Sun Photometer (S/N: 4088) AOT 380 AOT 440 AOT 500 AOT 675 AOT 870 I 380 I 440 I 500 I 675 I 870 SZA Aerosol optical thickness at 380nm Aerosol optical thickness at 440nm Aerosol optical thickness at 500nm Aerosol optical thickness at 675nm Aerosol optical thickness at 870nm Intensity at 380nm Intensity at 440nm Intensity at 500nm Intensity at 675nm Intensity at 870nm Solar zenith angle Page 15

16 Atmospheric Physics Lab Work 5.2. Data protocol (print two times for 15 measurements) date: location: coordinates: pressure: operator(s): altitude: No. time (local) GMT Ozone Monitor (4703): O 3 (corr) O 3 (1/2) O 3 (2/3) water AOT 1020 SZA I 305 I 312 I 320 I 936 I 1020 Sun Photometer (4088): AOT 380 AOT 440 AOT 500 AOT 675 AOT 870 water I 380 I 440 I 500 I 675 I 870 SZA Page 16