OZONE AND NO2 AIR-MASS FACTORS FOR ZENITH-SKY SPECTROMETERS - INTERCOMPARISON OF CALCULATIONS WITH DIFFERENT RADIATIVE-TRANSFER MODELS

Calculations of air-mass factors (AMFs) for ground-based zenith-sky UV -visible spectrometers are now well developed in laboratories where st ratospheric constituents are measured with this technique. An intercom parison between results from the different radiative transfer models u sed to calculate AMFs at twilight is presented here. The comparison wa s made for ozone AMFs at 510 nm and for NO2 AMFs at 440 nm. Vertical p rofiles were specified. Results are presented firstly for calculations in a pure Rayleigh atmosphere, then including background aerosols. Re lative differences between calculated AMFs from different models cause relative errors in vertical columns of ozone and NO2 measured by zeni th-sky spectrometers. For commonly used averages over solar zenith ang les, these relative errors are +/-2.3% in the vertical column of ozone and +/-1.1% in the vertical column of NO2. Refinements to the calcula tions, suggested by the intercomparison, should reduce these errors to +/-1.0% for ozone and +/-0.5% for NO2.