Measurements of O₃, SO₂, NO₂ and HCHO column amounts using a Brewer spectrometer

Abstract

O₃, SO₂ and NO₂ vertical column amounts and aerosol optical depths at 18 wavelengths from 303 to 363nm were measured daily for the past two years by using a Brewer MK3 double spectrometer in direct-sun mode. The algorithms used are described and compared to the standard algorithm. For O₃ and SO₂ the standard algorithm was modified by using all 6 measured wavelengths instead of only 4 or 5 and by including a correction for the diffuse irradiance entering the instrument's field of view. This reduces the statistical error of the retrievals by 40% and 50%, respectively, for O₃ and SO₂. The NO₂ retrievals are based on a spectral fitting technique using wavelengths between 349 and 363nm. A 'Bootstrap method' has been developed to calibrate the Brewer for NO₂ measurements. This method selects data with lowest NO₂-amounts and uses them to derive the needed extraterrestrial solar spectrum. We discuss possible reasons why an intent to apply to same technique for deriving total HCHO columns failed. The main advantage of direct sun methods compared to Differential Optical Absorption Spectroscopy DOAS is that the uncertainty in the air mass factor is significantly smaller. We think that a better correction for the diffuse irradiance entering the instrument's field of view will further improve the retrievals, especially in the low wavelength range (303 to 320nm) used for O₃ and SO₂.