WMO GLOBAL OZONE RESEARCH AND MONITORING PROJECT
REPORT
No. 13
REVIEW OF THE DOBSON SPECTROPHOTOMETER
AND ITS ACCURACY
by
Reid E. Basher
New Zealansd Meteorological Service
Wellington
Geneva, 1982
TABLE OF CONTENTS Page (from original report) Foreword i Abstract ii 1. Introduction 1 1.1 Purpose and scope 1 1.2 Theory and measurement 2 1.3 Instrument design 7 2. Optics 9 2.1 Introduction 9 2.2 Optical adjustment 10 2.3 Fundamental optical limitations 11 2.4 Summary 14 3. Wedge Calibration 15 3.1 Introduction 15 3.2 Apparatus 16 3.3 Data analysis methods 17 3.4 Error analysis of standard methods 18 3.5 Error analysis of filter methods 20 3.6 Summary 21 4. Stray Light 21 4.1 Introduction 21 4.2 Stray light from atmospheric scattering 22 4.3 Stray light from internal scattering 23 4.4 Non-linearity of log intensity ratios versus airmass 24 4.5 Possible solutions to stray light problems 27 4.6 Summary 28 5. Wavelength Band Uncertainty 28 5.1 Introduction 28 5.2 Basis of Calculations 29 5.3 Routine operation 32 5.4 Case where extraterrestrial constants are determined independently 33 5.5 Case where extraterrestrial constants are established by standard lamp transfer 34 5.6 Case where extraterrestrial constants are established direct intercomparison 34 5.7 General remarks 35 5.8 Summary 36 6. The Bandwidth Effect 37 6.1 Introduction 37 6.2 Theory and calculations 37 6.3 Summary 40 7. Field Operations and Calibrations 40 7.1 Introduction 40 7.2 Organisational factors 40 7.3 Field calibrations and tests 41 7.4 Routine operation 47 7.5 Summary 48 8. Electronics 49 8.1 Accuracy considerations 49 8.2 Summary 51 9. Ozone Absorption Coefficients 51 9.1 Introduction 51 9.2 Laboratory spectral absorption measurements 51 9.3 Investigations using the Dobson instrument 52 9.4 Temperature dependence of absorption coefficients for the Dobson instrument 54 9.5 Absolute accuracy of Dobson absorption coefficients 56 9.6 Summary 57 10. Atmospheric Scattering 58 10.1 Introduction 58 10.2 Rayleigh scattering 58 10.3 Aerosol scattering 59 10.4 Cloud attenuation 61 10.5 Summary 62 11. Interfering Absorption and Emission 63 11.1 Introduction 63 11.2 Interfering absorption 64 11.3 Interfering emission 66 11.4 Summary 66 12. Airmass Calculation 67 12.1 Introduction 67 12.2 Error analysis 68 12.3 Summary 71 13. Solar Constancy 72 13.1 Introduction 72 13.2 Solar spectral irradiance variations 72 13.3 Summary 74 14. Conclusion 75 14.1 Introduction 75 14.2 Summary and discussion of errors 75 14.3 Summary and discussion of recommendations 79 References 81 Appendix A Extract from "Measurements of atmospheric ozone at Moosonee, Canada - July 1 1957 to July 31 1960" by W.D. Komhyr, Toronto, 1960